Publishing type：Research paper (scientific journal)   Publisher：Inorganic Chemistry  

DFT calculations were performed to explore the mechanism underlying the reduction of NO to N2O by a CuI complex. A nitrosyl complex reacts with another NO molecule and the CuI complex, leading to the formation of a dicopper-hyponitrite complex (Cu2N2O2). The first steps follow a common pathway until the formation of the intermediate [CuII-N2O2]+, after which the reaction pathway diverges into three Cu2N2O2 species: κ2-N,N′, κ2-O,O′, and κ3-N,O,O′. These species yield different products along their respective reaction pathways. In the case of the κ2-N,N′ and κ3-N,O,O′ species, the subsequent steps involve a methanol-mediated proton transfer and N-O bond cleavage, resulting in the generation of N2O and [CuII-OH]+. Conversely, for the κ2-O,O′ species, two proton transfers occur without N-O bond cleavage, leading to the formation of H2N2O2 and [CuII]2+. H2N2O2 spontaneously converts into N2O and H2O. These computational results elucidate how the coordination mode of hyponitrite influences reactivity and provide insights into NO reduction via proton transfer. Notably, switching of the N2O2 coordination mode to metal ions from N to O was not required, offering insights for more efficient NO reduction strategies in the future.

DOI： 10.1021/acs.inorgchem.4c03619

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Publishing type：Research paper (scientific journal)   Publisher：Organometallics  

Herein, we demonstrate the significant impact of tetradentate-ligand-coordinated metal complexes, which have not yet been exploited for the direct catalytic hydrogenation of carboxylic acids (CAs). Our previously developed cationic iridium complex coordinated with a PNNP-ligand [(PNNP)Ir] is effective for hydrogenating esters and carboxylic anhydrides generated in situ from CAs but unsuitable for the direct hydrogenation of CAs. In sharp contrast, the corresponding neutral iridium complex with a PNCP-ligand [(PNCP)Ir] developed in this study facilitates the direct hydrogenation of CAs, including biorelevant and pharmaceutical compounds, under not more than 1 MPa of H2. Quantum-chemical calculations indicated that (PNCP)Ir is kinetically a far more competent catalyst than (PNNP)Ir, particularly for the C-H bond formation via hydride transfer from Ir-H to the carbonyl carbon of CA, which was identified as the rate-determining step. While Ir-carboxylates are in resting states throughout the catalytic cycle, CA itself barely interacts with the Ir center during the hydride transfer process.

DOI： 10.1021/acs.organomet.4c00355

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Language：English  

DOI： 10.3390/pharmaceutics16101271

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of the American Chemical Society  

3d-transition metal complexes have been gaining much attention as promising candidates for photocatalytic carbon dioxide (CO2) reduction systems. In contrast to the group 7-12 elements, Cr in group 6 has not yet been investigated as the catalyst of CO2 photoreduction because of its intrinsic disadvantages. Cr has a weak reducing ability due to an insufficient number of d electrons and high Lewis acidity which may deactivate the catalyst by strong coordination with a product formate. To overcome these drawbacks, we rationally designed molecular Cr complexes bearing ferrocenyl PNNP tetradentate ligands (FcCrCy, FcCriPr, FcCrtBu, and FcCrPh). These Cr complexes selectively converted CO2 into formic acid (HCO2H) under photocatalytic conditions and, to our knowledge, represent the first molecular Cr catalysts for CO2 photoreduction. The best catalyst FcCrPh achieved a turnover number of 1180 for HCO2H formation with 86% selectivity after 48 h of light irradiation, with a combined use of an organic photosensitizer. Electrochemical and continuous UV-vis absorption analyses clarified the sequential reaction pathways involving multielectron reduction and protonation of a Cr complex. Moreover, through detailed computational studies, photoinduced electron transfer mediated by ferrocenyl groups and intramolecular proton transfer attributed to hemilabile phosphine ligands would be key to the efficient catalysis that overwhelms the inherent disadvantages of Cr.

DOI： 10.1021/jacs.4c03683

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Chinese Chemical Letters  

Electron-deficient viologens are widely used as ligands or structure-directing agents (SDAs) to synthesize crystalline X-ray induced photochromic materials. Here, a new rational strategy of anion-directed folding a flexible cation (H2imb)2+ ((H2imb)2+ = di-protonated 2,3-bis(imidazolin-2-yl)-2,3-dimethylbutane) has been developed. Electron-donating Cl− and (ZnCl4)2− are used to direct folding a flexible electron-deficient (H2imb)2+ cation. Three complexes (H2imb)(NO3)2 (1), (H2imb)Cl2·H2O (2), and (H2imb)ZnCl4 (3) have been synthesized in which (H2imb)2+ crystallize in an anti-conformation, 88.8°-gauche, and 51.8°-gauche, respectively. In contrary to X-ray silent complex 1, X-ray induced photochromism has been achieved in both complex 2 and 3. An intermolecular charge-transfer mechanism has been elucidated and the anion directed folding of (H2imb)2+ has been validated to be critical to yield colored long-lived charge-separated states.

DOI： 10.1016/j.cclet.2023.109052

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.26434/chemrxiv-2024-mzzcr

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Chemistry - A European Journal  

4,4′-Biazulene is a potentially attractive key component of an axially chiral biaryl compound, however, its structure and properties have not been clarified owing to the lack of its efficient synthesis. We report a breakthrough in the reliable synthesis of 4,4′-biazulene, which is achieved by the access to azulen-4-ylboronic acid pinacol ester and 4-iodoazulene as novel key synthetic intermediates for the Suzuki-Miyaura cross-coupling reaction. The X-ray crystallographic analysis of 4,4′-biazulene confirmed its axial chirality. The enantiomers of 4,4′-biazulene were successfully resolved by HPLC on the chiral stationary phase column. The kinetic experiments and DFT calculations indicate that the racemization energy barrier of 4,4′-biazulene is comparable to that of 1,1′-binaphthyl.

DOI： 10.1002/chem.202400098

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

Polar compounds with switchable polarization properties are applicable in various devices such as ferroelectric memory and pyroelectric sensors. However, a strategy to prepare polar compounds has not been established. We report a rational synthesis of a polar CoGa crystal using chiral cth ligands (SS-cth and RR-cth, cth = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). Both the original homo metal Co crystal and Ga crystal exhibit a centrosymmetric isostructure, where the dipole moment of metal complexes with the SS-cth ligand and those with the RR-cth ligand are canceled out. To obtain a polar compound, the Co valence tautomeric complex with SS-cth in the homo metal Co crystal is replaced with the Ga complex with SS-cth by mixing Co valence tautomeric complexes with RR-cth and Ga complexes with SS-cth. The CoGa crystal exhibits polarization switching between the pseudononpolar state at a low temperature and the polar state at a high temperature because only Co complexes exhibit changes in electric dipole moment due to metal-to-ligand charge transfer. Following the same strategy, the polarization-switchable CoZn complex was synthesized. The CoZn crystal exhibits polarization switching between the polar state at a low temperature and the pseudononpolar state at a high temperature, which is the opposite temperature dependence to that of the CoGa crystal. These results revealed that the polar crystal can be synthesized by design, using a chiral ligand. Moreover, our method allows for the control of temperature-dependent polarization changes, which contrasts with typical ferroelectric compounds, in which the polar ferroelectric phase typically occurs at low temperatures.

DOI： 10.1021/jacs.4c02882

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

Have you ever imagined reactions of alkenes with hydrogen that result in anything other than hydrogenation or hydrogenative C-C coupling? We have long sought to develop not only hydrogenation catalysts that activate H2 as hydride ions but also electron transfer catalysts that activate H2 as a direct electron donor. Here, we report the reductive cyclopropanation of alkenes using an iridium electron storage catalyst with H2 as the electron source without releasing metal waste from the reductant. We discuss the catalytic mechanism with selectivity to give the trans-isomer. These findings are based on the isolation of three complexes and density functional theory calculations.

DOI： 10.1021/jacsau.4c00098

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Journal of Porphyrins and Phthalocyanines  

A saddle-distorted and cationic Zn(II)-porphyrin complex was revealed to exhibit solvatochromism in organic solvents, depending on the donor numbers. The color change was derived from the alteration of the HOMO-LUMO gap caused by the distortion of the porphyrin mean plane due to solvent coordination.

DOI： 10.1142/S108842462450007X

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Bulletin of the Chemical Society of Japan  

Herein, we report a neutral Nazarov reaction using protic solvents instead of strong acids as activators. The key to the success of this reaction lies in the rational design of the divinyl ketone substrates. In particular, the introduction of electron-donating groups (EDGs) at the β- and β′-positions of the carbonyl group in the divinyl ketone increases the Lewis basicity dramatically, an EDG at α-position promotes the cyclization, and the presence of a phenoxy group at β-position enables the irreversible elimination of phenol from the cyclized intermediate, thus shifting the reversible cyclization to the product side. This phenol-releasing reaction can be applied to clip chemistry to target acidic biological environments.

DOI： 10.1093/BULCSJ/UOAD014

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

The catalytic activity of a rhodium(ii) dimer complex, [Rh^II(TMAA)]₂ (TMAA = tetramethyltetraaza[14]annulene), in C–H amination reactions with organic azides is explored.

DOI： 10.1039/d3dt03429a

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：JACS Au  

We have synthesized and characterized a RuII-OH2 complex (2), which has a pentadentate ligand with two pivalamide groups as bulky hydrogen-bonding (HB) moieties in the second coordination sphere (SCS). Complex 2 exhibits a coordination equilibrium through the coordination of one of the pivalamide oxygens to the Ru center in water, affording a η6-coordinated complex, 3. A detailed thermodynamic analysis of the coordination equilibrium revealed that the formation of 3 from 2 is entropy-driven owing to the dissociation of the axial aqua ligand in 2. Complex 2 was oxidized by a CeIV salt to produce the corresponding RuIII(OH) complex (5), which was characterized crystallographically. In the crystal structure of 5, hydrogen bonds are formed among the NH groups of the pivalamide moieties and the oxygen atom of the hydroxo ligand. Further 1e--oxidation of 5 yields the corresponding RuIV(O) complex, 6, which has intramolecular HB of the oxo ligand with two amide N-H protons. Additionally, the RuIII(OH) complex, 5, exhibits disproportionation to the corresponding RuIV(O) complex, 6, and a mixture of the RuII complexes, 2 and 3, in an acidic aqueous solution. We investigated the oxidation of a phenol derivative using complex 6 as the active species and clarified the switch of the reaction mechanism from hydrogen-atom transfer at pH 2.5 to electron transfer, followed by proton transfer at pH 1.0. Additionally, the intramolecular HB in 6 exerts enhancing effects on oxygen-atom transfer reactions from 6 to alkenes such as cyclohexene and its water-soluble derivative to afford the corresponding epoxides, relative to the corresponding RuIV(O) complex (6′) lacking the HB moieties in the SCS.

DOI： 10.1021/jacsau.3c00377

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Organometallics  

For the hydrosilylation of alkenes catalyzed by Co2L8 (L = CO, CNR), the Type II Chalk-Harrod cycle (the CH-II cycle) starting from HCoL4 ([2H]L) and the Type II modified Chalk-Harrod cycle (the mCH-II cycle) from Me3SiCoL4 ([2Si]L) have been proposed as the reaction mechanisms, but neither mechanism is fully consistent with the experimental results. Density functional theory (DFT) calculations verified the reaction pathways of the two catalytic cycles, leading to the conclusion that the behavior of CNR as a ligand is similar to that of CO in catalysis, and a comparison of the activation energies of the rate-determining step of the two cycles suggests that the mCH-II cycle is more favorable. The mCH-II mechanism contradicts the following three experimental results: [2Si]L is not catalytically active without photolysis; vinylsilane is not formed; and facile alkene isomerization occurs. These discrepancies were resolved by assuming that both HCoL3 ([3H]L) and Me3SiCoL3 ([3Si]L) generated from [2H]L were present in the system and acted as catalysts; [3H]L was responsible for the alkene isomerization and thermal generation of [3Si]L, whereas [3Si]L initiated the hydrosilylation of alkenes through the mCH-II cycle. Vinylsilanes could be formed from an intermediate in the mCH-II cycle; however, this pathway is thermodynamically unfavorable.

DOI： 10.1021/acs.organomet.3c00279

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Inorganic Chemistry  

The literature contains numerous reports of copper complexes for nitrite (NO2-) reduction. However, details of how protons and electrons arrive and how nitric oxide (NO) is released remain unknown. The influence of the coordination mode of nitrite on reactivity is also under debate. Kundu and co-workers have reported nitrite reduction by a copper(II) complex [J. Am. Chem. Soc. 2020, 142, 1726-1730]. In their report, the copper(II) complex reduced nitrite using a phenol derivative as a reductant, resulting in NO, a hydroxyl copper(II) complex, and the corresponding biphenol. Also, the involvement of proton-coupled electron transfer was proposed by mechanistic studies. Herein, density functional theory calculations were performed to determine a mechanism for reduction of nitrite by a copper(II) complex. As a result of geometry optimization of an initial complex, two possible structures were obtained: Cu-ONO and Cu-NO2. Two possible reaction pathways initiated from Cu-ONO or Cu-NO2 were then considered. The calculation results indicated that the Cu-ONO pathway is energetically favorable. When changes in the electronic structure were considered, both pathways were found to involve concerted proton-electron transfer (CPET). In addition, an intrinsic reaction coordinate analysis revealed that the two pathways were achieved by different types of CPET. Furthermore, an intrinsic bond orbital analysis clearly indicated that, in the Cu-ONO pathway, the chemical events involved proceeded concertedly yet asynchronously.

DOI： 10.1021/acs.inorgchem.3c01383

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Chemistry - A European Journal  

Herein, we describe Hiyama coupling via intramolecular substituent transfer from silicon on one blade of triptycenes to another to yield 1,8,13-trisubstituted chiral triptycenes. This reaction is attributed to the proximity effect of substituents on triptycene, which plays an important role in not only the formation of the oxy-palladacycle but also the activation of the silyl group to facilitate σ-bond metathesis. After bromination and nucleophilic ring opening, the second intramolecular Hiyama coupling provided various 1,8,13-trisubstituted chiral triptycenes. The optical resolution of 1,8,13-triptycene afforded an optically active form for the first time.

DOI： 10.1002/chem.202300988

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Co(II)-pyrocobester (P-Co(II)), a dehydrocorrin complex, was semisynthesized from vitamin B12 (cyanocobalamin), and its photochemical and electrochemical properties were investigated and compared to those of the cobester (C-Co(II)), the cobalt-corrin complex. The UV-vis absorptions of P-Co(II) in CH2Cl2, ascribed to the π-π* transition, were red-shifted compared to those of C-Co(II) due to the π-expansion of the macrocycle in the pyrocobester. The reversible redox couple of P-Co(II) was observed at E1/2 = −0.30 V vs Ag/AgCl in CH3CN, which was assigned to the Co(II)/Co(I) redox couple by UV-vis, ESR, and molecular orbital analysis. This redox couple was positively shifted by 0.28 V compared to that of C-Co(II). This is caused by the high electronegativity of the dehydrocorrin macrocycle, which was estimated by DFT calculations for the free-base ligands. The reactivity of the Co(I)-pyrocobester (P-Co(I)) was evaluated by the reaction with methyl iodide in CV and UV-vis to form a photosensitive Co(III)-CH3 complex (P-Co(III)-CH3). The properties of the excited state of P-Co(I), *Co(I), were also investigated by femtosecond transient absorption (TA) spectroscopy. The lifetime of *Co(I) was estimated to be 29 ps from the kinetic trace at 587 nm. The lifetime of *Co(I) became shorter in the presence of Ar-X, such as iodobenzonitrile (1a), bromobenzonitrile (1b), and chlorobenzonitrile (1c), and the rate constants of electron transfer (ET) between the *Co(I) and Ar-X were determined to be 2.9 × 1011 M-1 s-1, 4.9 × 1010 M-1 s-1, and 1.0 × 1010 M-1 s-1 for 1a, 1b, and 1c, respectively.

DOI： 10.1021/acs.inorgchem.3c00882

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Chemical Science  

We have demonstrated site-selective radical reactions of the kinetically stable open-shell singlet diradicaloids difluoreno[3,4-b:4′,3′-d]thiophene (DFTh) and difluoreno[3,4-b:4′,3′-d]furan (DFFu) with tributyltin hydride (HSn(n-Bu)3) and azo-based radical initiators. Treatment of these diradicaloids with HSn(n-Bu)3 induces hydrogenation at the ipso-carbon in the five-membered rings, while treatment with 2,2′-azobis(isobutyronitrile) (AIBN) induces substitution at the carbon atoms in the peripheral six-membered rings. We have also developed one-pot substitution/hydrogenation reactions of DFTh/DFFu with various azo-based radical initiators and HSn(n-Bu)3. The resulting products can be converted into substituted DFTh/DFFu derivatives via dehydrogenation. Theoretical calculations unveiled a detailed mechanism of the radical reactions of DFTh/DFFu with HSn(n-Bu)3 and with AIBN, and that the site-selectivity of these radical reactions is controlled by the balance of the spin density and the steric hindrance in DFTh/DFFu.

DOI： 10.1039/d3sc00381g

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Using natural gas as chemical feedstock requires efficient oxidation of the constituent alkanes—and primarily methane1,2. The current industrial process uses steam reforming at high temperatures and pressures3,4 to generate a gas mixture that is then further converted into products such as methanol. Molecular Pt catalysts5–7 have also been used to convert methane to methanol8, but their selectivity is generally low owing to overoxidation—the initial oxidation products tend to be easier to oxidize than methane itself. Here we show that N-heterocyclic carbene-ligated FeII complexes with a hydrophobic cavity capture hydrophobic methane substrate from an aqueous solution and, after oxidation by the Fe centre, release a hydrophilic methanol product back into the solution. We find that increasing the size of the hydrophobic cavities enhances this effect, giving a turnover number of 5.0 × 102 and a methanol selectivity of 83% during a 3-h methane oxidation reaction. If the transport limitations arising from the processing of methane in an aqueous medium can be overcome, this catch-and-release strategy provides an efficient and selective approach to using naturally abundant alkane resources.

DOI： 10.1038/s41586-023-05821-2

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Other Link： https://www.nature.com/articles/s41586-023-05821-2

DOI： 10.1021/jacs.2c131494384

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This paper reports the first example of a reductive C(sp^3)–C(sp^3) homo-coupling of benzyl/allyl halides in aqueous solution by using H_2 as an electron source {turnover numbers (TONs) = 0.5–2.3 for 12 h}. This homo-coupling reaction, promoted by visible light, is catalysed by a water-soluble electron storage catalyst (ESC). The reaction mechanism, and four requirements to make it possible, are also described.

DOI： 10.1021/jacs.2c13149

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The factors that affect acceleration and high trans/cis selectivity in the catalytic cyclopropanation reaction of styrene with ethyl diazoacetate by cobalt N-confused porphyrin (NCP) complexes were investigated using density functional theory calculations. The reaction rate was primarily related to the energy gap between the cobalt–carbene adduct intermediates, A and B, which was affected by the NCP skeletons and axial pyridine ligands more than the corresponding porphyrin complex. In addition, high trans/cis stereoselectivity was determined at the TS1 and, in part, in the isomerization process at the carbon-centered radical intermediates, Ctrans and Ccis.

DOI： 10.3390/molecules27217266

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Antiaromatic compounds have recently received considerable attention because of their novel properties such as narrow HOMO–LUMO gaps and facile formation of mutual stacking. Here, the spontaneous assembly of antiaromatic meso-2-thienyl-substituted 5,15-dioxaporphyrin (DOP-1) is scrutinized at the liquid-solid interface by scanning tunneling microscopy (STM). Polymorphism in monolayers characterized by the orthogonal and parallel assemblies is found at the low concentration of 0.05 mM. The parallel assembly is more stable and dominantly formed at higher concentrations. Aggregation was observed at concentrations >0.2 mM, and the STM images of the aggregates implied the formation of stacked layers. The intrinsic electronic structures of the mutually stacked bilayer generated by applying an electric pulse to the monolayer were probed by scanning tunneling spectroscopy to reveal the narrowing of the HOMO–LUMO gap by about 20 % compared with the monolayer, thus suggesting significant molecular orbital interactions.

DOI： 10.1002/anie.202212726

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

Steric bulkiness – the metric of the robustness of the self-photosensitized, single metal-active-site catalysis, elucidated in CO₂ photoreduction.

DOI： 10.1039/d2cc01701f

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DOI： 10.1038/s42003-022-03464-z

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The oxidation of alkanes with m-chloroperbenzoic acid (mCPBA) catalyzed by the B12 derivative, heptamethyl cobyrinate, was investigated under several conditions. During the oxidation of cyclohexane, heptamethyl cobyrinate works as a catalyst to form cyclohexanol and cyclohexanone at a 0.67 alcohol to ketone ratio under aerobic conditions in 1 h. The reaction rate shows a firstorder dependence on the [catalyst] and [mCPBA] while being independent of [cyclohexane]; Vobs = k2[catalyst][mCPBA]. The kinetic deuterium isotope effect was determined to be 1.86, suggesting that substrate hydrogen atom abstraction is not dominantly involved in the rate-determining step. By the reaction of mCPBA and heptamethyl cobyrinate at low temperature, the corresponding cobalt(III)acylperoxido complex was formed which was identified by UV-vis, IR, ESR, and ESI-MS studies. A theoretical study suggested the homolysis of the O-O bond in the acylperoxido complex to form Co(III)-oxyl (Co-O center dot) and the m-chlorobenzoyloxyl radical. Radical trapping experiments using Ntert-butyl-alpha-phenylnitrone and CCl3Br, product analysis of various alkane oxidations, and computer analysis of the free energy for radical abstraction from cyclohexane by Co(III)-oxyl suggested that both Co(III)-oxyl and the m-chlorobenzoyloxyl radical could act as hydrogen-atom transfer reactants for the cyclohexane oxidation.

DOI： 10.1021/acs.inorgchem.2c01174

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

We have achieved aerobic transformation of methane to C₁ chemicals catalysed by a homogeneous organometallic catalyst with light energy input.

DOI： 10.1039/d2ra01772e

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Dalton Transactions  

A density functional theory study was carried out to investigate the reduction mechanisms of NO to N2O using a dicopper complex reported by Zhang and coworkers (J. Am. Chem. Soc., 2019, 141, 10159-10164). The reaction mechanism consists of three steps: N-N bond formation, isomerization of the resultant N2O2 moiety, and cleavage of the N-O bond.

DOI： 10.1039/d2dt00275b

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Journal of Porphyrins and Phthalocyanines  

A new meso-tetrakis(3,5-di-tert-butylphenyl) substituted porphycene and its cobalt complex were synthesized and characterized. Several meso-tetraarylporphycene cobalt complexes having different substituents showed catalytic activity for the hydrogen gas evolution reaction (HER) in the presence of trifluoroacetic acid (TFA) under electrochemical reductive conditions. The electrochemical potential of the HER was -1.50V vs. Ag/AgCl in THF. The active species of this reaction were determined to be the ligand reduced species of Co(II) porphycenes. The catalytic efficiencies of the cobalt complexes of meso-aryl substituted porphycenes and meso-tetraphenylporphyrin were compared and meso-tetraphenyl porphycene cobalt complex exhibited the best current efficiency of 72%. Based on these experimental results and theoretical calculations, we proposed a possible HER mechanism of this system.

DOI： 10.1142/S108842462250016X

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Chemistry - An Asian Journal  

During the self-assembly of π-conjugated molecules, linkers and substituents can potentially add supportive noncovalent intermolecular interactions to π-stacking interactions. Here, we report the self-assembly behavior of thienopyrrole-fused thiadiazole (TPT) fluorescent dyes that possess ester or ether linkers and dodecyloxy side chains in solution and the condensed phase. A comparison of the self-association behavior of the ester- and ether-bridged compounds in solution using detailed UV-vis, fluorescence, and NMR spectroscopic studies revealed that the subtle replacement of the ether linkers by ester linkers leads to a distinct increase in the association constant (ca. 3–4 fold) and the enthalpic contribution (ca. 3 kcal mol−1). Theoretical calculations suggest that the ester linkers, which are in close proximity to one another due to the π-stacking interactions, induce attractive electrostatic forces and augment self-association. The self-assembly of TPT dyes into well-defined 1D clusters with high aspect ratios was observed, and their morphologies and crystallinity were investigated using SEM and X-ray diffraction analyses. TPTs with ester linkers exhibit a columnar liquid crystalline mesophase in the condensed phase.

DOI： 10.1002/asia.202101341

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Inorganic Chemistry  

Benzene hydroxylation catalyzed by ruthenium-substituted Keggin-type polyoxometalates [RuV(O)XW11O39]n- (RuVOX; X = Al, Ga, Si, Ge, P, As, S; heteroatoms; 3 ≤ n ≤ 6) is investigated using the density functional theory approach. As a possible side reaction, the water oxidation reaction is also considered. We found that the rate-determining step for water oxidation by RuVOX requires a higher activation free energy than the benzene hydroxylation reaction, suggesting that all of the RuVOX catalysts show high chemoselectivity toward benzene hydroxylation. Additionally, the heteroatom effect in benzene hydroxylation by RuVOX is discussed. The replacement of Si by X induces changes in the bond length of μ4O-X, resulting in a change in the activation free energy for benzene hydroxylation by RuVOX. Consequentially, RuVOS is expected to be the most effective catalyst among the (RuVOX) catalysts for the benzene hydroxylation reaction.

DOI： 10.1021/acs.inorgchem.1c02605

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Dalton Transactions  

The reaction of osmium tetroxide (OsO4) and carboxylate anions (acetate: X- = AcO- and benzoate: X- = BzO-) gave 1 : 1 adducts, [OsO4(X)]- (1X), the structures of which were determined by X-ray crystallographic analysis. In both cases, the carboxylate anion X coordinates to the osmium centre to generate a distorted trigonal bipyramidal osmium(viii) complex. The carboxylate adducts show a negative shift of the redox potentials (E1/2) and a red shift of the νOsO stretches as compared to those of tetrahedral OsO4 itself. Despite the negative shift of E1/2, the reactivity of these adduct complexes 1X was enhanced compared to that of OsO4 in benzylic C(sp3)-H bond oxidation. The reaction obeyed the first-order kinetics on both 1X and the substrates, giving the second-order rate constant (k2), which exhibits a linear correlation with the C-H bond dissociation energy (BDEC-H) of the substrates (xanthene, 9,10-dihydroanthracene, fluorene and 1,2,3,4-tetrahydronaphthalene) and a kinetic deuterium isotope effect (KIE) of 9.7 (k2(xanthene-h2)/k2(xanthene-d2)). On the basis of these kinetic data together with the DFT calculation results, we propose a stepwise reaction mechanism involving rate-limiting benzylic hydrogen atom abstraction and subsequent rebound of the generated organic radical intermediate to a remaining oxido group on the osmium centre.

DOI： 10.1039/d1dt03819b

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Publishing type：Research paper (scientific journal)   Publisher：Bulletin of the Chemical Society of Japan  

Interaction of osmium tetroxide (OsO4) with a series of halide ions (X1 = I1, Br1, Cl1, and F1) is examined. Stable 1:1 adducts, [OsO4(X)]1 (1X), are formed in the case of Br1, Cl1, and F1, whereas redox reaction takes place with I1 to give [OsVIIO4]1 and I•. The adduct formation constant (KfX) increases as the basicity of the halide ion increases (Br1 < Cl1 < F1). Upon the adduct formation, the symmetric (¯(Os=O)sym) and asymmetric (¯(Os=O)asym) Os=O stretching vibration energies are lowered as compared with those of OsO4. The X-ray crystallographic analyses of the halide adducts indicate that the structural distortion of the osmium center from tetrahedron to trigonal bipyramid becomes larger as the KfX value becomes larger. 1F shows much higher reactivity compared with 1Br and 1Cl in the oxidation of benzyl alcohol to benzaldehyde, even though 1F has a lower reduction potential compared to 1Br and 1Cl. Mechanistic details of the alcohol oxidation reaction are evaluated by kinetic studies including Hammett analysis and kinetic deuterium isotope effect as well as by DFT calculations.

DOI： 10.1246/bcsj.20210377

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DOI： 10.2355/isijinternational.ISIJINT-2022-197

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Interaction of osmium tetroxide (OsO4) with a series of halide ions (X1 = I1, Br1, Cl1, and F1) is examined. Stable 1:1 adducts, [OsO4(X)]1 (1X), are formed in the case of Br1, Cl1, and F1, whereas redox reaction takes place with I1 to give [OsVIIO4]1 and I•. The adduct formation constant (KfX) increases as the basicity of the halide ion increases (Br1 < Cl1 < F1). Upon the adduct formation, the symmetric (¯(Os=O)sym) and asymmetric (¯(Os=O)asym) Os=O stretching vibration energies are lowered as compared with those of OsO4. The X-ray crystallographic analyses of the halide adducts indicate that the structural distortion of the osmium center from tetrahedron to trigonal bipyramid becomes larger as the KfX value becomes larger. 1F shows much higher reactivity compared with 1Br and 1Cl in the oxidation of benzyl alcohol to benzaldehyde, even though 1F has a lower reduction potential compared to 1Br and 1Cl. Mechanistic details of the alcohol oxidation reaction are evaluated by kinetic studies including Hammett analysis and kinetic deuterium isotope effect as well as by DFT calculations.

Language：English   Publishing type：Research paper (scientific journal)  

Language：Others   Publishing type：Research paper (scientific journal)  

A tin(II) complex coordinated by a sterically demanding o-phenylenediamido ligand is synthesized. The ligand is redox-active to reach a tin(II) complex with the diiminobenzosemiquinone radial anion in the oxidation by AgPF6. The tin(II) complex reacts with a series of nosylazides (x-NO2C6H4–SO2–N3; x = o, m, or p) at −30 °C to yield the corresponding nitrene radical bound tin(II) complexes. The nitrene radical complexes exhibit C(sp3)–H activation and amination reactivity.

DOI： 10.1021/acs.inorgchem.1c02806

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This work reports the synthesis, crystal structures, and electronic properties of structurally constrained S,C,C- and O,C,C-bridged triarylamine derivatives and their persistent radical cations. O,C,C-Bridged triphenylamines and a dinaphthylphenylamine were obtained through a straightforward synthetic protocol. Similar to a previously reported S,C,C-bridged triphenylamine, the O,C,C-bridged triarylamines were easily oxidized to afford the corresponding radical cations, which were obtained as hexachloroantimonate salts. X-ray crystallographic analyses showed almost planar structures for these O,C,C-bridged triarylamine radical cations, which represent new members of the family of planar triarylamine radical cations without substituents on the aryl rings. Detailed investigations of the electronic properties of the S,C,C- and O,C,C-bridged triarylamine radical cations demonstrated that the spin and positive charge are sufficiently delocalized over the planar triarylamine scaffolds. The results provide the following insights into the effects of the bridging unit (sulfur vs oxygen) and the dibenzo-annulation on the spin delocalization in the bridged triarylamine radical cations: (1) An effective decrease of the spin density on the nitrogen atom is observed for the sulfur bridge relative to the oxygen bridge; and (2) a moderate decrease of the spin density on the oxygen atom rather than the nitrogen atom is induced by the dibenzo-annulation.

DOI： 10.1021/acs.joc.1c00969

Language：English   Publishing type：Research paper (scientific journal)  

Pyroelectricity plays a crucial role in modern sensors and energy conversion devices. However, obtaining materials with large and nearly constant pyroelectric coefficients over a wide temperature range for practical uses remains a formidable challenge. Attempting to discover a solution to this obstacle, we combined molecular design of labile electronic structure with the crystal engineering of the molecular orientation in lattice. This combination results in electronic pyroelectricity of purely molecular origin. Here, we report a polar crystal of an [FeCo] dinuclear complex exhibiting a peculiar pyroelectric behavior (a substantial sharp pyroelectric current peak and an unusual continuous pyroelectric current at higher temperatures) which is caused by a combination of Fe spin crossover (SCO) and electron transfer between the high-spin Fe ion and redox-active ligand, namely valence tautomerism (VT). As a result, temperature dependence of the pyroelectric behavior reported here is opposite from conventional ferroelectrics and originates from a transition between three distinct electronic structures. The obtained pyroelectric coefficient is comparable to that of polyvinylidene difluoride at room temperature.

DOI： 10.1038/s41467-021-25041-4

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A Pd-catalyzed domino reaction of 1,8,13-tribromo-9-methoxytriptycenes is reported. Under conventional Suzuki coupling conditions, the triptycenes underwent multiple transformations to give 1,9-bridged triptycenes. Based on mechanistic investigations, a single Pd catalyst functions as Pd0, PdII and PdIV species to catalyze four distinct processes: (1) aryl to alkyl 1,5-Pd migration, (2) intramolecular arylation, (3) homocoupling of phenylboronic acid and (4) Suzuki coupling. DFT calculations revealed that 1,5-Pd migration likely proceeds via both concerted PdII and stepwise PdIV routes. Asymmetric synthesis of the chiral triptycenes, as well as optical resolution, and further transformation are also reported.

DOI： 10.1002/chem.202101728

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Although hydrocarbons are known to act as reductants for the catalytic reduction of nitric oxides (NOx) over copper-based catalysts, the reaction mechanism requires clarification. Herein, density functional theory (DFT) calculations were carried out to investigate the reduction mechanisms of NOx to dinitrogen coupled to the hydroxylation of methane or benzene using the dicopper complex reported by Zhang and co-workers [ J. Am. Chem. Soc. 2019, 141, 10159-10164 ]. The B3LYP functional was used to optimize the (μ-oxo)(μ-nitrosyl)dicopper complex in the quartet state and the (μ-η2:η2-NO2)dicopper complex in the doublet state, the latter of which was found to be the ground state. Then, we investigated the reactivities of the (μ- η2:η2-NO2)dicopper complex toward methane and benzene by considering the conversions of N2O to N2 in the presence and the absence of methane or benzene. In the presence of methane and benzene, the calculated activation energies were 27.0 and 21.0 kcal/mol, respectively, whereas that with N2O alone was prohibitively high (61.9 kcal/mol). Thus, the (μ- η2:η2-NO2)dicopper complex prefers the reactions with methane and benzene to that with N2O. The reaction of the (μ- η2:η2-NO2)dicopper complex with methane or benzene generated the (μ-nitrosyl)dicopper complex. The (μ-nitrosyl)dicopper complex then reacted with N2O to regenerate the (μ- η2:η2-NO2)dicopper complex and N2 with an activation barrier of 31.5 kcal/mol. The overall reactions for methane and benzene hydroxylation were calculated to be exothermic by 41.7 and 54.1 kcal/mol, respectively. These results suggest that the catalytic reduction of NOx using hydrocarbons is feasible at certain operating temperatures. Thus, our calculations provide new insights into the design of catalysts for NOx purification.

DOI： 10.1021/acs.inorgchem.0c03558

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A convergent paired electrolysis catalyzed by a B12 complex for the one-pot synthesis of a tertiary amide from organic trichlorides (R-CCl3) has been developed. Various readily available organic trichlorides, such as benzotrichloride and its derivatives, chloroform, dichlorodiphenyltrichloroethane (DDT), trichloro-2,2,2-trifluoroethane (CFC-113a), and trichloroacetonitrile (CNCCl3), were converted to amides in the presence of tertiary amines through oxygen incorporation from air at room temperature. The amide formation mechanism in the paired electrolysis, which was mediated by a cobalt complex, was proposed.

DOI： 10.1021/acs.joc.1c00161

Language：English   Publishing type：Research paper (scientific journal)  

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A cyclometalated osmium(III) complex, [OsIIICl(phpy)(tpy)]+ (OsIII(phpy); phpy = 2-C6H4-2-C5H4N-κC,N) is synthesized from OsIIICl3(tpy) (tpy = 2,26,2″-terpyridine) and 2-phenylpyridine (H-phpy) in the presence of AgPF6, in which the metalated carbon atom is disposed trans to the chlorido ligand. Oxidation of OsIII(phpy) with oxygen atom transfer oxidants such as N-methylmorpholine N-oxide (NMO), N,N-dimethylaniline N-oxide (DMAO), H2O2, t-BuOOH, and m-chloroperoxybenzoic acid (m-CPBA) in CH3CN or electron transfer type oxidants such as (NH4)2S2O8 and ammonium hexanitratocerate (CAN) in H2O/acetone causes oxygen atom insertion between the osmium(III)-carbon bond to give a phenoxido-osmium(III) complex OsIII(O-phpy). Isotope labeling experiments using H218O and kinetic studies as well as calculation studies indicate formation of an organometallic oxido-osmium(V) complex as the active intermediate.

DOI： 10.1021/acs.organomet.0c00772

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Polarization change induced by directional electron transfer attracts considerable attention owing to its fast switching rate and potential light control. Here, we investigate electronic pyroelectricity in the crystal of a mononuclear complex, [Co(phendiox)(rac-cth)](ClO4)·0.5EtOH (1·0.5EtOH, H2phendiox = 9, 10-dihydroxyphenanthrene, rac-cth = racemic 5, 5, 7, 12, 12, 14-hexamethyl-1, 4, 8, 11-tetraazacyclotetradecane), which undergoes a two-step valence tautomerism (VT). Correspondingly, pyroelectric current exhibits double peaks in the same temperature domain with the polarization change consistent with the change in dipole moments during the VT process. Time-resolved Infrared (IR) spectroscopy shows that the photo-induced metastable state can be generated within 150 ps at 190 K. Such state can be trapped for tens of minutes at 7 K, showing that photo-induced polarization change can be realized in this system. These results directly demonstrate that a change in the molecular dipole moments induced by intramolecular electron transfer can introduce a macroscopic polarization change in VT compounds.

DOI： 10.1038/s41467-020-15988-1

Language：English   Publishing type：Research paper (scientific journal)  

A Ru-II complex, [Ru-II(tpphz)(bpy)(2)](2+) (1) (tpphz = tetrapyridophenazine, bpy = 2,2 '-bipyridine), whose tpphz ligand has a pyrazine moiety, is converted efficiently to [Ru-II(tpphz-HH)(bpy)(2)](2+) (2) having a dihydropyrazine moiety upon photoirradiation of a water-methanol mixed solvent solution of 1 in the presence of an electron donor. In this reaction, the triplet metal-to-ligand charge-transfer excited state ((MLCT)-M-3*) of 1 is firstly formed upon photoirradiation and the (MLCT)-M-3* state is reductively quenched with an electron donor to afford [Ru-II(tpphz(-))(bpy)(2)](+), which is converted to 2 without the observation of detectable reduced intermediates by nano-second laser flash photolysis. The inverse kinetic isotope effect (KIE) was observed to be 0.63 in the N-H bond formation of 2 at the dihydropyrazine moiety. White-light (380-670 nm) irradiation of a solution of 1 in a protic solvent, in the presence of an electron donor under an inert atmosphere, led to photocatalytic H-2 evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its (MLCT)-M-3* state to react with a proton and aldehydes. In photocatalytic H-2 evolution, the H-H bond formation between photoexcited 2 and a proton is involved in the rate-determining step with normal KIE being 5.2 on H-2 evolving rates. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the reaction mechanism of H-2 evolution from the ground and photo-excited states of 2 were performed to have a better understanding of the photocatalytic processes.

DOI： 10.1039/d0dt03546g

Language：English   Publishing type：Research paper (scientific journal)  

We have thoroughly investigated the oxidation of benzyl alcohol (BA) derivatives by a RuIV(O) complex (Ru
IV
(O)) in the absence or presence of Brønsted acids in order to elucidate the proton-coupled electron-transfer (PCET) mechanisms in C-H oxidation on the basis of a kinetic analysis. Oxidation of BA derivatives by Ru
IV
(O) without acids proceeded through concerted proton-electron transfer (CPET) with a large kinetic isotope effect (KIE). In contrast, the oxidation of 3,4,5-trimethoxy-BA ((MeO)3-BA) by Ru
IV
(O) was accelerated by the addition of acids, in which the KIE value reached 1.1 with TFA (550 mM), indicating an alteration of the PCET mechanism from CPET to stepwise electron transfer (ET) followed by proton transfer (PT). Although the oxidized products of BA derivatives were confirmed to be the corresponding benzaldehydes in the range of acid concentrations (0-550 mM), a one-electron-reduction potential of Ru
IV
(O) was positively shifted with increases in the concentrations of acids. The elevated reduction potential of Ru
IV
(O) strongly influenced the PCET mechanisms in the oxidation of (MeO)3-BA, changing the mechanism from CPET to ET/PT, as evidenced by the driving-force dependence of logarithms of reaction rate constants in light of the Marcus theory of ET. In addition, dependence of activation parameters on acid concentrations suggested that an oxidative asynchronous CPET, which is not an admixture of the CPET and ET/PT mechanisms, is probably operative in the boundary region (0 mM < [TFA] < 50 mM) involving a one-proton-interacted Ru
IV
(O)···H+ as a dominant reactive species.

DOI： 10.1021/jacs.0c05738

Language：English   Publishing type：Research paper (scientific journal)  

A dinuclear Ru-II complex, [{(RuCl)-Cl-II(MeBPA)}(2)(mu-bpm)](PF6)(2) (1), exhibited three reversible redox waves at -0.75, + 0.91 and +1.15 V vs Ag/AgCl in acetone at -60 degrees C. The former wave can be assigned to the 1e(-)-reduction process of the bridging bpm ligand and the latter two are assigned to the stepwise 1e(-)-oxidation processes of the two Ru centers. Electrochemical oxidation of 1 afforded a "class I" mixed-valence (MV) state, in which the charge localizes on one of the two Ru centers. On the other hand, electrochemical 1e(-)-reduction of 1 predominantly proceeds on the bridging bpm ligand. The valence-localized MV state should be derived from a largely distorted structure of the Ru-II site to reduce the pi-back bonding from a filled Ru-II d pi orbital to the bpm ligand as suggested by DFT calculations.

DOI： 10.1016/j.inoche.2020.108150

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An FeII complex, 1, having a pentadentate ligand with an NHC moiety catalyzes substrate oxidation to afford 2e-oxidized products with high selectivity by suppression of overoxidation in water. A Bell-Evance-Polanyi plot for the substrate oxidation catalyzed by 1 exhibited an inflection point around 86 kcal mol-1, indicating strong C-H abstraction ability of the reactive species derived from 1.

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Motivated by a recent study on [IrV(η5-C5H5){ppy}(O)]2+ (ppy is 2-phenylpryridine), which is able to cleave the C-H bond of decalin but not of methane due to a high activation energy of 25.7 kcal/mol (Zhou, M. et al. Cp∗ iridium Precatalysts for Selective C-H Oxidation via Direct Oxygen Insertion: A Joint Experimental/Computational Study. ACS Catal. 2012, 2, 208 218), in the present study, we perform density functional theory (DFT) calculations on another kind of an Ir-oxo complex, namely, [IrV(η5-C5Me5){bpy(COOH)2}(O)]2+ or OC, where bpy(COOH)2 is 2,2′-bipyridine-4,4′ dicarboxylic acid, to assess its ability to hydroxylate methane to methanol. OC is formed by light-induced oxidation of the Ir-H2O complex to generate electricity. By hydroxylating methane to methanol and inserting H2O, OC can regenerate the Ir-H2O complex and thus create a catalytic loop similar to a fuel cell. Herein, by utilizing intrinsic bond orbital (IBO) analysis, we show detailed mechanisms of how OC efficiently cleaves the strong C-H bond of methane to form methanol in three possible spin states, namely, the triplet, open-shell singlet, and closed-shell singlet states. In the triplet state, the C-H bond cleavage proceeds with an activation energy of only 11.3 kcal/mol via a hydrogen atom transfer mechanism, where an intermediate species involving •CH3 and Ir-OH• is formed in the same spin state, although a spin change to the open-shell singlet state is likely to occur. A subsequent HO-CH3 recombination occurs without a barrier through direct radical coupling in the open-shell singlet state. In contrast, the C-H bond cleavage in the closed-shell singlet state occurs via a hydride transfer (or oxygen insertion) mechanism, where methanol is directly formed without any intermediate states. Although this reaction mechanism requires a lower C-H activation energy (5.6 kcal/mol), OC in the closed-shell singlet state is less stable by 11.9 kcal/mol than that in the triplet state. These results provide a theoretical prediction of an alternative promising catalyst for the direct conversion of methane to methanol and the methane fuel cell.

DOI： 10.1021/acscatal.0c01610

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We report herein an efficient method to synthesize triptycenes by the reaction of benzynes and anthranoxides, which are electron-rich and readily prepared from the corresponding anthrones. Using this method, 1,9-syn-substituted triptycenes were regioselectively obtained employing 3-methoxybenzynes. This method was also applied to synthesize pentiptycenes. A DFT study revealed that the cycloaddition of lithium anthranoxide and benzyne proceeds stepwise.

DOI： 10.1002/chem.202002065

Language：English   Publishing type：Research paper (scientific journal)  

Orbital angular momentum plays a vital role in various applications, especially magnetic and spintronic properties. Therefore, controlling orbital angular momentum is of paramount importance to both fundamental science and new technological applications. Many attempts have been made to modulate the ligand-field-induced quenching effects of orbital angular momentum to manipulate magnetic properties. However, to date, reported changes in the magnitude of orbital angular momentum are small in both molecular and solid-state magnetic materials. Moreover, no effective methods currently exist to modulate orbital angular momentum. Here we report a dynamic bond approach to realize a large change in orbital angular momentum. We have developed a Co(II) complex that exhibits coordination number switching between six and seven. This cooperative dynamic bond switching induces considerable modulation of the ligand field, thereby leading to substantial quenching and restoration of the orbital angular momentum. This switching mechanism is entirely different from those of spin-crossover and valence tautomeric compounds, which exhibit switching in spin multiplicity.

DOI： 10.1021/jacs.0c02257

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First-principle theoretical study was performed to elucidate the reaction mechanism of the direct conversion of methane to methanol using hydrogen peroxide as an oxidant over Pd and Au/Pd catalysts. We have investigated the effect of Au-Pd alloying and its effect on the reaction steps and energetics and we have provided an understanding of the contribution of surface-exposed Au centers to the overall reaction mechanism. The hydrogen peroxide decomposition reactions to OH groups or surface-adsorbed oxo species and water molecules were compared on the Pd and Au/Pd surfaces as an important preliminary process for the methane oxidation. We have evaluated the C-H bond cleavage energetics assisted by either the OH group or oxo species over Pd and Au/Pd surfaces and compared them to the C-H cleavage energy. Our results show that the products of hydrogen peroxide decomposition significantly reduce the high activation barrier for the hydrogen atom abstraction from methane. The recombination of methyl group and OH group was found to be the rate-determining step for the methanol formation. The proposed reaction pathway in our work provides important insights into the oxidation mechanism by hydrogen peroxide and the fine-tuning of the Pd catalyst by other alloying elements such as Au.

DOI： 10.1021/acs.jpcc.0c03237

Language：English   Publishing type：Research paper (scientific journal)  

DFT calculations are carried out to investigate the geometric effects of the supporting ligands in the relative energies of the (μ-η2:η2-peroxido)CuIICuII complex 1 and the bis(μ-oxido)CuIIICuIII complex 2. The N3-tridentate ligand bearing acyclic propane diamine framework La preferentially provided 1, whereas the N3-tridentate ligand with cyclic diamine framework such as 1,4-diazacycloheptane Lb gave 2 after the oxygenation of the corresponding CuI complexes as reported previously [S. Itoh, et al., Inorg. Chem., 2014, 53, 8786-8794]. Calculations at the B3LYP∗-D3 level of theory can reasonably explain the experimental results in relative energies, structures and harmonic frequencies of 1 and 2. Perturbational effects of the diamine chelates of La and Lb especially on the equilibrium of 1 and 2 are investigated in detail. In the range from 2.30 Å to 3.40 Å of the N-N distance in the diamine moiety, 1 is more stable than 2 by 8.4 kcal mol-1 at the distance of 3.40 Å. Calculated potential energies indicate that the decrease in the N-N distance is associated with a decrease in energy of 2, leading that 2 can be most stabilized at the N-N distance of 2.60 Å. Furthermore, molecular orbitals analyses are performed to explain that the energy gaps between the σ∗ orbital of the O-O bond and the dx2-y2 orbitals of the CuII ions of 1 get small as the diamine moiety is shrunk, leading to facilitate the O-O bond cleavage from 1 to 2.

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A proton–electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi-step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and π electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric-like arrangement.

DOI： 10.1002/anie.202006763

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The composite materials of acidic polymers and imidazole (Im) exhibit good high-proton conduction, making them potentially useful in proton-exchange membrane fuel cells. The proton conduction mechanism must be clearly elucidated for the design of future high proton-conduction materials. This study examines the local hydrogen-bonding structures and dynamics of Im for proton-conducting poly(vinylphosphonic acid)-Im (PVPA-xIm) composites by using molecular dynamics simulations. Radial distribution functions (RDFs) characterize the hydrogen bonds between Im or imidazolium cation (ImH(+)) and phosphonic acid (PA) groups and among Im molecules. RDFs and diffusion coefficients suggest that the intercalation of Im to PVPA reduces Im mobility because of the interaction between Im and PA groups. However, similar to pure Im, the amount of Im with fast rotational motions increases as the amount of Im increases. Our study leads us to conclude that long-range proton transport occurs through the hydrogen-bonding network of Im and that the fast rotational motions of Im enhance proton conduction in PVPA-xIm composites.

DOI： 10.1021/acsapm.9b01222

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1246/BCSJ.20190282

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/ejoc.202000226

Language：English   Publishing type：Research paper (scientific journal)  

An iridium aqua complex [IrIII(Ε5-C5Me5){bpy(COOH)2}(H2O)]2+ under visible light irradiation has been experimentally reported to form an iridium-oxo (Ir-oxo) complex [IrV(Ε5-C5Me5){bpy(COOH)2}(O)]2+, which oxidizes H2O to O2. However, the mechanism for the formation of this Ir-oxo complex remains unclear, due to the difficulties in observing the unstable Ir-oxo complex and computing light-induced systems having different numbers of electrons. In this study, we perform density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to investigate more in detail our previously proposed deprotonation and light-induced oxidation reactions composing the formation of the Ir-oxo complex. In particular, we discuss effects of light irradiation and WO3 support on the formation of the Ir-oxo complex. We suggest two distinct mechanisms, that is, direct and indirect for the light-induced oxidation. In the direct mechanism electrons are directly transferred from the occupied Ï∗ orbitals of IrIII-OH or IrIV=Oâ to the conduction band of the WO3 surface, whereas in the indirect mechanism electrons are first excited from the valence band to the conduction band of the WO3 surface due to the UV light, and then the resultant electron hole oxidizes the Ir complex. In the direct mechanism, in particular, we found that the lowest energy of the anode's conduction band determines the adsorption wavelength of the light irradiation, enabling us to predict alternative semiconductor anodes for more efficient formation of the Ir-oxo complex.

Language：English   Publishing type：Research paper (scientific journal)  

Push-pull fluorenones (FOs) were synthesized by treating a benzopentalenequinone (BPO) derivative with alkynes that bear an electron-rich aniline moietyviaa regioselective [4 + 2] cycloaddition (CA) followed by a [4 + 1] retrocycloaddition (RCA). The resulting FOs were readily converted into dibenzodicyanofulvenes (DBDCFs) by treatment with malononitrile in the presence of TiCl4and pyridine. The FOs and DBDCFs exhibit intramolecular charge-transfer (ICT) that manifests in absorptions at 350-650 nm and amphoteric electrochemical behavior. Furthermore, FOs and DBDCFs that contain a C-C bond react with tetracyanoethylene in a formal [2 + 2] CA followed by a retro-electrocyclization to afford sterically congested tetracyanobutadiene (TCBD) conjugates. The substituent (H or Me) on the aromatic ring adjacent to the butadiene moiety thereby determines whether the butadiene adopts an s-cisor s-transconformation, and thus controls the physicochemical properties of the resulting TCBDs. The TCBD conjugates exhibit ICT absorptions (≤800 nm) together with up to four reversible reduction steps.

Language：English   Publishing type：Research paper (scientific journal)  

The study of transition metal clusters exhibiting fast electron hopping or delocalization remains challenging, because intermetallic communications mediated through bridging ligands are normally weak. Herein, we report the synthesis of a nanosized complex, [Fe(Tp)(CN)3]8[Fe(H2O)(DMSO)]6 (abbreviated as [Fe14], Tp−, hydrotris(pyrazolyl)borate; DMSO, dimethyl sulfoxide), which has a fluctuating valence due to two mobile d-electrons in its atomic layer shell. The rate of electron transfer of [Fe14] complex demonstrates the Arrhenius-type temperature dependence in the nanosized spheric surface, wherein high-spin centers are ferromagnetically coupled, producing an S = 14 ground state. The electron-hopping rate at room temperature is faster than the time scale of Mössbauer measurements (<~10−8 s). Partial reduction of N-terminal high spin FeIII sites and electron mediation ability of CN ligands lead to the observation of both an extensive electron transfer and magnetic coupling properties in a precisely atomic layered shell structure of a nanosized [Fe14] complex.

DOI： 10.1038/s41467-019-13279-y

Language：English   Publishing type：Research paper (scientific journal)  

Much recent attention has been focused on the structure and reactivity of transition-metal superoxide complexes, among which mononuclear copper(II)-superoxide complexes are recognized as key reactive intermediates in many biological and abiological dioxygen-activation processes. So far, several types of copper(II)-superoxide complexes have been developed and their electrophilic reactivity has been explored in C–H and O–H bond activation reactions. Here we demonstrate that a mononuclear copper(II)-(end-on)superoxide complex supported by a N-[(2-pyridyl)methyl]-1,5-diazacyclooctane tridentate ligand can induce catalytic C–C bond formation reaction between carbonyl compounds (substrate) and the solvent molecule (acetone), giving β-hydroxy-ketones (aldol). Kinetic and spectroscopic studies at low temperature as well as DFT calculation studies support a nucleophilic reactivity of the superoxide species toward the carbonyl compounds, providing new insights into the reactivity of transition-metal superoxide species not only in biological oxidation reactions but also in synthetic organic chemistry.

Language：English   Publishing type：Research paper (scientific journal)  

Materials demonstrating unusual large positive and negative thermal expansion are fascinating for their potential applications as high-precision microscale actuators and thermal expansion compensators for normal solids. However, manipulating molecular motion to execute huge thermal expansion of materials remains a formidable challenge. Here, we report a single-crystal Cu(II) complex exhibiting giant thermal expansion actuated by collective reorientation of imidazoliums. The circular molecular cations, which are rotationally disordered at a high temperature and statically ordered at a low temperature, demonstrate significant reorientation in the molecular planes. Such atypical molecular motion, revealed by variable-temperature single crystal X-ray diffraction and solid-state NMR analyses, drives an exceptionally large positive thermal expansion and a negative thermal expansion in a perpendicular direction of the crystal. The consequent large shape change (~10%) of bulk material, with remarkable durability, suggests that this complex is a strong candidate as a microscale thermal actuating material.

DOI： 10.1038/s41467-019-12833-y

Language：English   Publishing type：Research paper (scientific journal)  

In this study, the ring contracting skeletal rearrangement reaction in one-pot under basic conditions from octaethylporphycene to two types of isocorroles, i.e., the meso-formyl and meso-free forms, was confirmed and the reaction mechanism was proposed by the comparative analyses of the experimental and theoretical studies. The electron accepting nature of the porphycene and imine-amine conversion of pyrrole rings make it possible to react with hydroxide ions and the following reaction to afford isocorroles. Based on this reaction, the valence of the compound as a ligand as well as the photochemical and electrochemical properties were dramatically changed.

DOI： https://doi.org/10.1002/ejoc.201901659

Language：English   Publishing type：Research paper (scientific journal)  

The selective oxidation of CH4 using O2 is one of the most attractive subjects as an elusive target reaction. Ohkubo and Hirose recently reported that chlorine dioxide radical (ClO2•), which is generated by mixing NaClO2 and HCl in an aqueous solution, acts as an efficient oxidant in the oxidation of CH4 to CH3OH and HCOOH under photoirradiation in the two-phase system of perfluorohexane and water (Angew. Chem., Int. Ed. 2018, 57, 2126). The reaction system gives CH3OH and HCOOH without further oxidation products. They proposed that methoxy radical (CH3O•) plays an important role as an intermediate in the oxidation of CH4. In the present work, we focus on the reactivity of CH3O• to CH4 in detail to propose a reasonable radical mechanism for the oxidation of CH4 using DFT calculations at the M06-2X/6-311+G** level of theory and UCCSD(T)/6-311+G** calculations. Our reaction analysis suggests that the reaction of CH3O• with CH4 and the disproportionation of CH3O• take place as CH4 + CH3O• ¼ CH3• + CH3OH and 2CH3O• ¼ CH3OH + HCHO, respectively. In contrast, the isomerization from CH3O• to CH2•(OH), suggested by Ohkubo and Hirose, is unlikely to occur under ambient conditions, due to the high activation barrier for this reaction. A better understanding of the well-controlled radical chain reactions is useful for reaction design of the hydroxylation of methane.

DOI： 10.1246/bcsj.20190171

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A helicene compound called AIBTh, wherein two azulene units are fused with isobenzothiophene, has been prepared and characterized by spectroscopic and crystallographic methods. The enantiomers of AIBTh were resolved by HPLC, exhibiting stable optical activity. AIBTh showed two reversible oxidation and one irreversible reduction waves with a HOMO-LUMO gap of 2.07 eV. Upon one-electron oxidation of AIBTh, its air-stable cation radical was isolated and analyzed by EPR as well as X-ray crystallography. Based on the EPR spectrum, the crystal structure, and DFT calculation, it is suggested that favorable resonance structures including aromatic tropylium cation forms and wide delocalization of electronic spin are dominating in the electronic structure of the cation radical.

DOI： 10.1246/bcsj.20190219

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Formation of an active oxygen species at the dicopper site of pMMO is studied by using density functional theory (DFT) calculations. The role of the amino acid residues of tyrosine (Tyr374) and glutamate (Glu35) located in the second coordination sphere of the dicopper site is discussed in detail. The phenolic proton of the tyrosine residue is transferred to the Cu2O2 core in a two-step manner via the glutamate residue, and an electron is directly transferred to the Cu2O2 core. These proton- and electron-transfer processes induce the O-O bond cleavage of the μ-ν2:ν2-peroxodicopper(II) species to form the (μ-oxo)(μ-hydroxo)CuIICuIII species, which is able to play a key role of methane hydroxylation at the dicopper site of pMMO (Inorg. Chem. 2013, 52, 7907). This proton-coupled electron-transfer mechanism is a little different from that in tyrosinase in that the proton of substrate tyrosine is directly transferred to the dicopper site (J. Am. Chem. Soc. 2006, 128, 9873) because there is no proton acceptor in the vicinity of the dicopper site of tyrosinase. The rate-determining step for the formation of the (μ-oxo)(μ-hydroxo)CuIICuIII species is determined to be the O-O bond cleavage. These results shed new light on the interpretation of the role of the tyrosine and glutamate residues located in the second coordination sphere of the dicopper site of pMMO.

DOI： 10.1021/acs.inorgchem.9b01752

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Indium-doped zinc oxide (IZO) is an n-type semiconductor and an alternative to indium tin oxide, which is widely used in transparent electrodes. In order to design and prepare optical and electrical functional materials with IZO, it is important to understand the effects of indium (In) doping on zinc oxide (ZnO). In this study, the local structures and electronic properties of IZO were investigated by first-principle calculations. The formation energies of IZO with the substitution of an In atom in the place of a Zn atom were determined to be less than those of IZO with the intercalation of In atoms into interstitial sites in ZnO regardless of the amount of In atoms. From analyzing calculated band structures for the substitution and intercalation models, the relationship between the effect of In dopant and the conductivity of IZO is discussed.

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An important technique to realize novel electron- A nd/or proton-based functionalities is to use a proton-electron coupling mechanism. When either a proton or electron is excited, the other one is modulated, producing synergistic functions. However, although compounds with proton-coupled electron transfer have been synthesized, crystalline molecular compounds that exhibit proton-transfer-coupled spin-transition (PCST) behavior have not been reported. Here, we report the first example of a PCST Fe(II) complex, wherein the proton lies on the N of hydrazone and pyridine moieties in the ligand at high-spin and low-spin Fe(II), respectively. When the Fe(II) complex is irradiated with light, intramolecular proton transfer occurs from pyridine to hydrazone in conjunction with the photoinduced spin transition via the PCST mechanism. Because the light-induced excited high-spin state is trapped at low temperatures in the Fe(II) complex- A phenomenon known as the light-induced excited-spin-state trapping effect-the light-induced proton-transfer state, wherein the proton lies on the N of hydrazone, is also trapped as a metastable state. The proton transfer was accomplished within 50 ps at 190 K. The bistable nature of the proton position, where the position can be switched by light irradiation, is useful for modulating proton-based functionalities in molecular devices.

DOI： 10.1021/jacs.9b07204

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Dissociation of molecular oxygen is an important elementary process in heterogeneous catalysis. Here, we report on a real-space observation of oxygen photolysis on the Ag(110) surface at 78 K by far- and near-field excitation in the ultraviolet-near-infrared range using a low-temperature scanning tunneling microscope (STM) combined with wavelength-tunable laser excitation. The photolysis of isolated oxygen molecules on the surface occurs even by visible light with the cross section of ∼10-19 cm2. Time-dependent density functional theory calculations reveal optical absorption of the hybridized O2-Ag(110) complex in the visible and the near-infrared range which is associated with the oxygen photolysis. We suggest that the photolysis mechanism involves a direct charge transfer process. We also demonstrate that the photolysis can be largely enhanced in plasmonic STM junctions, and the cross section is estimated to be ∼10-17 cm-2 in the visible and the near-infrared range, which appears to be an interesting feature of plasmon-induced reactions from the perspective of photochemical conversion with the aid of solar energy.

DOI： 10.1063/1.5112158

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A RuII - NH3 complex, 2, was oxidized through a proton-coupled electron transfer (PCET) mechanism with a CeIV complex in water at pH 2.5 to generate a RuV=NH complex, 5. Complex 5 was characterized with various spectroscopies, and the spin state was determined by the Evans method to be S = 1/2. The reactivity of 5 in substrate C-H oxidation was scrutinized in acidic water, using water-soluble organic substrates such as sodium ethylbenzene-sulfonate (EBS), which gave the corresponding 1-phenylethanol derivative as the product. In the substrate oxidation, complex 5 was converted to the corresponding RuIII - NH3 complex, 3. The formation of 1-phenylethanol derivative from EBS and that of 3 indicate that complex 5 as the oxidant does not perform nitrogen-atom transfer, in sharp contrast to other high-valent metal-imido complexes reported so far. Oxidation of cyclobutanol by 5 afforded only cyclobutanone as the product, indicating that the substrate oxidation by 5 proceeds through a hydride-transfer mechanism. In the kinetic analysis on the C-H oxidation, we observed kinetic isotope effects (KIEs) on the C-H oxidation with use of deuterated substrates and remarkably large solvent KIE (sKIE) in D2O. These positive KIEs indicate that the rate-determining step involves not only cleavage of the C-H bond of the substrate but also proton transfer from water molecules to 5. The unique hydride-transfer mechanism in the substrate oxidation by 5 is probably derived from the fact that the RuIV - NH2 complex (4) formed from 5 by 1e-/1H+ reduction is unstable and quickly disproportionates into 3 and 5.

DOI： 0.1021/acs.inorgchem.9b01781

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Over the past decade, zeolites (microporous aluminosilicate minerals) have been gaining significant popularity due to their broad applications in catalysis including the dream reaction of selective oxidation (hydroxylation) of methane to methanol at low temperature. In this review, we outline the current main challenges in the development of Fe-, Cu-, Co- and Ni-exchanged zeolites for methane hydroxylation and summarize key findings that have been reported in both spectroscopy and computational studies. Also, using density functional theory (DFT) calculations, we calculate energy diagrams of methane hydroxylation over various structures of metal-oxo active sites in zeolites and discuss some key points that can be improved for achieving higher reactivity. Short outlooks on the future research opportunities are also discussed.

DOI： 10.1039/c8cy02414f

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An iron(II) complex of a quadruply ring-fused porphyrin (QFP), Fe-1, in which four mesityl groups were introduced at the periphery to improve the solubility in organic solvents, has been newly synthesized and characterized. In the pyridine solution, two pyridine molecules bind to the low-spin Fe II center of Fe-1 as axial ligands to make the complex stable even under air. Characterization of Fe-1 was performed using ¹ H NMR spectroscopy, MALDI-TOF-MS spectrometry and single-crystal X-ray diffraction analysis. The ¹ H NMR signals of Fe-1 were observed in the diamagnetic region, reflecting the low-spin state of the Fe II center. In the differential pulse voltammogram of Fe-1, three oxidation waves and four reduction waves were observed in pyridine; the first oxidation wave at -0.08 V vs. Fc/Fc[Formula: see text] can be ascribed to the oxidation process of the Fe II center, Fe II /Fe III , and other six waves can be assigned to the redox processes of the QFP ligand. Furthermore, the ESR measurement of 1e[Formula: see text]-reduced Fe-1 upon controlled-potential bulk electrolysis in pyridine exhibited a signal at [Formula: see text] 2.003 with a well-resolved 45-line hyperfine splitting at room temperature, due to the coupling with four nitrogen nuclei and twelve hydrogen ones of the QFP ligand. This indicates that the ligand radical anion of Fe-1 is stabilized by delocalization of the spin density owing to the peripheral ring fusion and resultant expansion of the [Formula: see text]-conjugation.

DOI： 10.1142/S1088424619500846

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Visible light-driven cross-coupling reactions of alkyl halides with phenylacetylene and its derivatives catalyzed by the cobalamin derivative (B12) with the [Ir(dtbbpy)(ppy)2]PF6 photocatalyst at room temperature are reported. The robust B12 catalyst and Ir photocatalyst provided high turnover numbers of over 33 000 for the reactions.

DOI： 10.1039/c9cc06185a

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While hydrogenase and photosystem II enzymes are known to oxidize H2 and H2O, respectively, a recently reported iridium aqua complex [IrIII(η5-C5Me5){bpy(COOH)2}(H2O)]2+ is able to oxidize both of the molecules and generate energies as in the fuel and solar cells (Ogo et al. ChemCatChem 2017, 9, 4024-4028). To understand the mechanism behind such an interesting bifunctional catalyst, in the present study, we perform density functional theory (DFT) calculations on the dual catalytic cycle of H2 and H2O oxidations by the iridium aqua complex. In the H2 oxidation, we found that the H-H bond is easily cleaved in a heterolytic fashion, and the resultant iridium hydride complex is significantly stabilized by the presence of H2O molecules, due to dihydrogen bond. The rate-determining step of this reaction is found to be the H2O → H2 ligand substitution with an activation energy of 10.7 kcal/mol. In the H2O oxidation, an iridium oxo complex originating from an oxidation of the iridium aqua complex forms a hydroperoxide complex, where an O-O bond is formed with an activation energy of 21.0 kcal/mol. Such a relatively low activation barrier is possible only when at least two H2O molecules are present in the reaction, allowing the water nucleophilic attack (WNA) mechanism to take place. The present study suggests and discusses in detail six reaction steps required for the dual catalytic cycle to complete.

DOI： 10.1021/acs.inorgchem.9b00307

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Isolation and characterisation of RuIV(O) complexes were accomplished to investigate their fundamental electron transfer (ET) and proton-coupled ET (PCET) properties. Reorganisation energies (λ) in electron transfer (ET) and proton-coupled ET (PCET) from electron donors to the isolated RuIV(O) complexes have been determined for the first time to be in the range of 1.70-1.88 eV (ET) and 1.20-1.26 eV (PCET). It was suggested that the reduction of the λ values of PCET in comparison with those of ET should be due to the smaller structural change in PCET than that in ET on the basis of DFT calculations on 1 and 1e--reduced 1 in the absence and presence of TFA, respectively. In addition, the smaller λ values for the RuIV(O) complexes than those reported for FeIV(O) and MnIV(O) complexes should be due to the lack of participation of dσ orbitals in the ET and PCET reactions. This is the first example to evaluate fundamental ET and PCET properties of RuIV(O) complexes leading to further understanding of their reactivity in oxidation reactions.

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New Fe(III) compounds from qsal ligand, [Fe(qsal) 2 ](CH 3 OSO 3 ) (1) and [Fe(qsal) 2 ](CH 3 SO 3 )·CH 3 OH (3), along with known compound, [Fe(qsal) 2 ](CF 3 SO 3 ) (2), were obtained as large well-shaped crystals (Hqsal = N-(8-quinolyl)salicylaldimine). The compounds 1 and 2 were in the low-spin (LS) state at 300 K and exhibited a cooperative spin crossover (SCO) transition with a thermal hysteresis loop at higher temperatures, whereas 3 was in the high-spin (HS) state below 300 K. The optical conductivity spectra for 1 and 3 were calculated from the single-crystal reflection spectra, which were, to the best of our knowledge, the first optical conductivity spectra of SCO compounds. The absorption bands for the LS and HS [Fe(qsal) 2 ] cations were assigned by time-dependent density functional theory calculations. The crystal structures of 1 and 2 consisted of a common one-dimensional (1D) array of the [Fe(qsal) 2 ] cation, whereas that of 3 had an unusual 1D arrangement by π-stacking interactions which has never been reported. The crystal structures in the high-temperature phases for 1 and 2 indicate that large structural changes were triggered by the motion of counter anions. The comparison of the crystal structures of the known [Fe(qsal) 2 ] compounds suggests the significant role of a large non-spherical counter-anion or solvate molecule for the total lattice energy gain in the crystal of a charged complex.

DOI： 10.3390/cryst9020081

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The dihydrogenated porphycene cobalt(ii) complex was synthesized and electrochemical experiments were carried out. The one-electron reduction of the complex proceeded at the central metal to afford the Co(i) species; in contrast, for the non-hydrogenated porphycene cobalt(ii) complex, the one-electron reduction gave the ligand reduced radical anion species. The reactivity of the one-electron reduced species with alkyl halides showed clear differences between the complexes. Hydrogenation of the β-position of the porphycene makes it possible to generate a central cobalt reduced species possessing a higher reactivity than the ligand reduced radical anion species.

DOI： 10.1039/c8dt03743d

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A new chromium(V)-oxo complex, [CrV(O)(6-COO--py-tacn)]2+ (1; 6-COO--py-tacn = 1-(6-carboxylato-2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane), was synthesized and characterized to evaluate the reactivity of CrV(O) complexes in a hydrogen-atom transfer (HAT) reaction by comparing it with that of a previously reported CrV(O) complex, [CrV(O)(6-COO--tpa)]2+ (2; 6-COO--tpa = N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine). Definitive differences of these two CrV(O) complexes were observed in resonance Raman scatterings of the Cr-O bond (ν = 911 cm-1 for 1 and 951 cm-1 for 2) and the reduction potential (0.73 V vs SCE for 1 and 1.23 V for 2); this difference should be derived from that of the ligand bound at the trans position to the oxo ligand, a tertiary amino group in 1, and a pyridine nitrogen in 2. When we employed 9,10-dihydroanthracene as a substrate, the second-order rate constant (k) of 1 was 4000 times smaller than that of 2. Plots of normalized k values for both complexes relative to bond dissociation energies (BDEs) of C-H bonds to be cleaved in several substrates showed a pair of parallel lines with slopes of -0.91 for 1 and -0.62 for 2, indicating that the HAT reactions by the two complexes proceed via almost the same transition states. Judging from estimated BDEs of CrIV(OH)/CrV(O) (85-87 kcal mol-1 for 1 and 92-94 kcal mol-1 for 2) and the activation barrier in the HAT reaction of DHA (Ea = 7.9 kcal mol-1 for 1 and Ea = 4.8 kcal mol-1 for 2), the reactivity of CrV(O) complexes in HAT reactions depends on the energy level of the reactant state rather than the product state.

DOI： 10.1021/acs.inorgchem.8b02453

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Importance of the Reactant-State Potentials of Chromium(V)-Oxo Complexes to Determine the Reactivity in Hydrogen-Atom Transfer Reactions
A new chromium(V)-oxo complex, [Cr(O)(6-COO-py-tacn)] (1; 6-COO-py-tacn = 1-(6-carboxylato-2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane), was synthesized and characterized to evaluate the reactivity of Cr(O) complexes in a hydrogen-atom transfer (HAT) reaction by comparing it with that of a previously reported Cr(O) complex, [Cr(O)(6-COO-tpa)] (2; 6-COO-tpa = N, N-bis(2-pyridylmethyl)- N-(6-carboxylato-2-pyridylmethyl)amine). Definitive differences of these two Cr(O) complexes were observed in resonance Raman scatterings of the Cr-O bond (ν = 911 cm for 1 and 951 cm for 2) and the reduction potential (0.73 V vs SCE for 1 and 1.23 V for 2); this difference should be derived from that of the ligand bound at the trans position to the oxo ligand, a tertiary amino group in 1, and a pyridine nitrogen in 2. When we employed 9,10-dihydroanthracene as a substrate, the second-order rate constant ( k) of 1 was 4000 times smaller than that of 2. Plots of normalized k values for both complexes relative to bond dissociation energies (BDEs) of C-H bonds to be cleaved in several substrates showed a pair of parallel lines with slopes of -0.91 for 1 and -0.62 for 2, indicating that the HAT

DOI： 10.1021/acs.inorgchem.8b02453

Language：English   Publishing type：Research paper (scientific journal)  

ConspectusAs fossil-based energy sources become more depleted and with renewable-energy technologies still in a very early stage of development, the utilization of highly abundant methane as a transitional solution for current energy demands is highly important despite difficulties in transport and storage. Technologies enabling the conversion of methane to liquid/condensable energy carriers that can be easily transported and integrated into the existing chemical infrastructures are therefore essential. Although there commercially exists a two-step gas-to-liquid process involving syngas production, a novel route of methane conversion that can circumvent the high-cost production of syngas should be developed. Among all of the conceptually possible methods for converting methane to methanol, methane hydroxylation (CH4 + 1/2O2 → CH3OH) at low temperature seems to be the most viable since it provides a direct route of conversion and allows a much lower operational cost. However, it is hampered by the fact that the complete oxidation to CO2 is thermodynamically more favored. To overcome this, an effective catalyst that is able to "mildly" oxidize methane and stabilize the resultant methyl radical toward methanol formation is required. Particulate methane monooxygenase (pMMO) and copper-exchanged zeolites are two catalysts known to hydroxylate methane into methanol at low temperature with high selectivity. Having been studied for more than 30 years, these copper-cored catalysts are still relevant topics of discussion since the actual structure of the active sites has not been agreed upon, and thus, the reaction mechanism and factors influencing their reactivity and productivity are yet to be understood. Density functional theory (DFT) has provided us with a powerful computational tool for accomplishing these tasks.This Account presents an overview of the recent progress in the computational elucidation of the catalytic mechanism of methane hydroxylation by mono-, di-, and trinuclear copper sites in pMMO and Cu-exchanged zeolites as well as its correlations to the influencing factors that must be controlled to achieve higher reactivity. First, we briefly introduce the catalytic mechanism of a bare CuO+ cation as the simplest copper-oxo system in methane hydroxylation. The system is then extended to the copper-oxo species in pMMO and zeolites, and the radical and nonradical mechanisms are examined. Investigations of the reactivities of mononuclear and dinuclear copper-oxo species in the pMMO active site suggest that the bis(μ-oxo)CuIICuIII, (μ-oxo)(μ-hydroxo)CuIICuIII, and CuIIIO species are important for the catalytic activity of pMMO. In the case of Cu-exchanged zeolites, as the mono(μ-oxo)CuIICuII and tris(μ-oxo)CuIICuIIICuIII active sites have been fully characterized in experiments, here we discuss the effects of zeolite structures on the geometry and reactivity of the active sites.

DOI： 10.1021/acs.accounts.8b00236

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An iron(III) complex having a dibenzotetraethyltetraamido macrocyclic ligand (DTTM4-), (NEt4)2[FeIII(DTTM)Cl] (1), was synthesized and characterized by crystallographic, spectroscopic, and electrochemical methods. Complex 1 has a square-pyramidal structure in the S = 3/2 spin state. The complex exhibited two reversible redox waves at +0.36 and +0.68 V (vs SCE) in the cyclic voltammogram measured in CH2Cl2 at room temperature. The stepwise oxidation of 1 using chemical oxidants allowed us to observe clear and distinct spectral changes with distinct isosbestic points for each step, in which oxidation occurred at the phenylenediamido moiety rather than the iron center. One-electron oxidation of 1 by 1 equiv of [RuIII(bpy)3](ClO4)3 (bpy = 2,2′-bipyridine) in CH2Cl2 afforded square-pyramidal (NEt4)[Fe(DTTM)Cl] (2), which was in the S = 1 spin state involving a ligand radical and showed a slightly distorted square-pyramidal structure. Complex 2 showed an intervalence charge-transfer band at 900 nm, which was assigned on the basis of time-dependent density functional theory calculations, to indicate that the complex is in a class IIA mixed-valence ligand-radical regime with Hab = 884 cm-1. Two-electron oxidation of 1 by 2 equiv of [(4-Br-Ph)3N·+](SbCl6) in CH2Cl2 afforded two-electron-oxidized species of 1, [Fe(DTTM)Cl] (3), which was in the S = 1/2 spin state; complex 3 exhibited a distorted square-pyramidal structure. X-ray absorption near-edge structure spectra of 1-3 were measured in both CH3CN solutions and BN pellets to observe comparable rising-edge energies for the three complexes, and Mössbauer spectra of 1-3 showed almost identical isomer shifts and quadruple splitting parameters, indicating that the iron centers of the three complexes are intact to be in the intermediate-spin iron(III) state. Thus, in complexes 2 and 3, it is evident that antiferromagnetic coupling is operating between the unpaired electron(s) of the ligand radical(s) and those of the iron(III) center.

DOI： 10.1021/acs.inorgchem.8b00037

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The utilization of low-cost and abundant oxygen (O2) as an oxidant in the activation of copper-exchanged zeolites is highly important for the direct, selective oxidation of methane to methanol at low temperatures. While two motifs of active sites, i.e., the [Cu2(μ-O)]2+ and [Cu3(μ-O)3]2+, have been experimentally observed in mordenite (MOR) zeolite, the mechanisms of their formation from the reaction of Cu-MOR with O2 are still unclear. In this study, we performed density functional theory (DFT) calculations for O2 activation over 2[Cu2]2+-MOR and [Cu3O]2+-MOR zeolites. For the reaction on the dicopper species, we found two possible reaction routes: O-O bond cleavage leading to (1) formation of a [Cu2(μ-O)]2+ active species and a trans-μ-1,2-peroxo-Si2 species and (2) simultaneous formation of two [Cu2(μ-O)]2+ active species neighboring to each other. These routes are both exothermic but require completely different O-O bond activation energies. For the reaction on the tricopper species, we suggest a peroxo-Cu3O species as the intermediate structure with two transition states (TSs) involved in the reaction. The first TS where a significant rearrangement of the tricopper site occurs is found to be rate-determining, while the second TS where the peroxo bond is cleaved results in a smaller activation barrier. This reaction, in contrast to the dicopper case, is slightly endothermic. The present study provides theoretical insights that may help design of better Cu-exchanged zeolite catalysts for methane hydroxylation to methanol.

DOI： 10.1021/acs.inorgchem.8b01329

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The hydrogen evolution reaction using semiconductor photocatalysts has been significantly improved by cocatalyst loading. However, there are still many speculations regarding the actual role of the cocatalyst. Now a photocatalytic hydrogen evolution reaction pathway is reported on a cocatalyst site using TiO2 nanosheets doped with Rh at Ti sites as one-atom cocatalysts. A hydride species adsorbed on the one-atom Rh dopant cocatalyst site was confirmed experimentally as the intermediate state for hydrogen evolution, which was consistent with the results of density functional theory (DFT) calculations. In this system, the role of the cocatalyst in photocatalytic hydrogen evolution is related to the withdrawal of photo-excited electrons and stabilization of the hydride intermediate species; the presence of oxygen vacancies induced by Rh facilitate the withdrawal of electrons and stabilization of the hydride.

DOI： 10.1002/anie.201803214

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Complexation of a RuCp* cation with N-confused tetraarylporphyrins (NCPs) forms directly bound ruthenium(II) pentamethylcyclopentadienyl (Cp*) π-complex on a specific meso-aryl group (e.g., phenyl) neighboring peripheral imino nitrogen of NCPs in high yields. In contrast, in the case of NCPs bearing bulky meso-substituents (e.g., 3,5-di-tert-butylphenyl), new ruthenocenophane-like complex embedded on an N-confused calix[4]phyrin was formed through multiple C−H bond activation of methyl groups of Cp* ligand. The mechanistic insight into the formation of the ruthenocenophane was derived from DFT calculations.

DOI： 10.1002/chem.201801237

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Mechanistic Insights into Homogeneous Electrocatalytic and Photocatalytic Hydrogen Evolution Catalyzed by High-Spin Ni(II) Complexes with S2N2‑Type Tetradentate Ligands
We report homogeneous electrocatalytic and photocatalytic H evolution using two Ni(II) complexes with SN-type tetradentate ligands bearing two different sizes of chelate rings as catalysts. A Ni(II) complex with a five-membered SCS-Ni chelate ring (1) exhibited higher activity than that with a six-membered SCS-Ni chelate ring (2) in both electrocatalytic and photocatalytic H evolution despite both complexes showing the same reduction potentials. A stepwise reduction of the Ni center from Ni(II) to Ni(0) was observed in the electrochemical measurements; the first reduction is a pure electron transfer reaction to form a Ni(I) complex as confirmed by electron spin resonance measurements, and the second is a 1e/1H proton-coupled electron transfer reaction to afford a putative Ni(II)-hydrido (Ni-H) species. We also clarified that Ni(II) complexes can act as homogeneous catalysts in the electrocatalytic H evolution, in which complex 1 exhibited higher reactivity than that of 2. In the photocatalytic system using [Ru(bpy)] as a photosensitizer and sodium ascorbate as a reductant, complex 1 with the five-membered chelate ring also showed higher catalytic activity than that of 2 with the si

DOI： 10.1021/acs.inorgchem.8b00881

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The interesting redox properties and reactivity of metalloporphycene have been studied for decades; however, the detailed experimental investigation on the reactivity and reaction mechanism under inert condition combined with theoretical calculations had not been performed so far. In this study, the novel reactivity of the reduced form of the cobalt porphycene with alkyl halides to form cobalt-carbon (Co-C) bonds was revealed. Under electrochemical reductive conditions, not the central cobalt, but the ligand was reduced and reacted with alkyl halides to afford the cobalt-alkyl complexes under N2 atmosphere in a glovebox. The reaction mechanism was clarified by the combination of experimental and theoretical studies that the porphycene ligand works as a noninnocent ligand and allows the SN2-type Co-C bond formation. This result provides us the possibility of the reaction triggered by the reduction of ligand with macrocyclic π-conjugated system, not by the reduction of metal.

DOI： 10.1021/acsomega.8b00239

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Disilaferra- and disilaruthenacyclic complexes containing mesityl isocyanide as a ligand, 3′ and 4′, were synthesized and characterized by spectroscopy and crystallography. Both 3′ and 4′ showed excellent catalytic activity for the hydrogenation of alkenes. Compared with iron and ruthenium carbonyl analogues, 1′ and 2′, the isocyanide complexes 3′ and 4′ were more robust under the hydrogenation conditions, and were still active even at higher temperatures (∼80 °C) under high hydrogen pressure (∼20 atm). The iron complex 3′ exhibited the highest catalytic activity toward hydrogenation of mono-, di-, tri-, and tetrasubstituted alkenes among currently reported iron catalysts. Ruthenium complex 4′ catalyzed hydrogenation under very mild conditions, such as room temperature and 1 atm of H2. The remarkably high catalytic activity of 4′ for hydrogenation of unfunctionalized tetrasubstituted alkenes was especially notable, because it was comparable to the activity of iridium complexes reported by Crabtree and Pfaltz, which are catalysts with the highest activity in the literature. DFT calculations suggested two plausible catalytic cycles, both of which involved activation of H2 assisted by the metal-silicon bond through σ-bond metathesis of late transition metals (oxidative hydrogen migration). The linear structure of M C≡N - C (ipso carbon of the mesityl group) played an essential role in the efficient hydrogenation of sterically hindered tetrasubstituted alkenes.

DOI： 10.1021/jacs.8b00812

Language：English   Publishing type：Research paper (scientific journal)  

Copper-containing large-pore zeolites, such as Cu-mordenite (Cu-MOR) and Cu-omega (Cu-MAZ), oxidize methane to yield a high amount of methanol. Two distinct active centers in MOR zeolite, namely, [Cu2(μ-O)]2+ and [Cu3(μ-O)3]2+, have been proposed and debated. In particular, the [Cu2(μ-O)]2+ species was experimentally found to be formed on two different Al pair sites with different reactivities toward methane. However, computational attempts based on density functional theory (DFT) have not been able to confirm them. Moreover, the full cycle of the reaction, which includes methane activation, water-assisted methanol desorption, and a second methane reaction with the active species, has not been well understood yet. In this study, we employed DFT calculations based on the Perdew, Burke, and Ernzerhof functional to reasonably calculate all activation energies involved in such a complete reaction over periodic systems of [Cux(μ-O)y]2+-MOR and -MAZ (x, y = 2, 1 and 3, 3) in the high-spin and low-spin states. We found two Al pair sites in MOR zeolite that form two distinct [Cu2(μ-O)]2+ structures able to cleave the C-H bond of methane with activation energies excellently comparable with the experimental values. Our computational results further suggest that the addition of a water molecule helps the reaction to reduce the high methanol desorption energies. We also show that two of the three bridging O atoms in [Cu3(μ-O)3]2+-MOR and -MAZ significantly differ in reactivity toward methane.

DOI： 10.1021/acscatal.7b03389

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A series of [FeII(L)2](BF4)2 compounds were structurally and physically characterized (L = 2,6-bis(2-methylthiazol-4-yl)pyridine). A crystal structure phase transformation from dihydrate compound 1 to anhydrous compound 3 through partially hydrated compounds 2 and 2′ upon dehydration was found. Compounds 1 and 3 exhibited a gradual spin crossover (SCO) conversion, whereas compounds 2 and 2′ demonstrated two-step and one-step abrupt SCO transitions, respectively. An X-ray single-crystal structural analysis revealed that one-dimensional and two-dimensional Fe cation networks linked by π stacking and sulfur-sulfur interactions were formed in 1 and 3, respectively. A thermodynamic analysis of the magnetic susceptibility for 1, 2′, and 3 suggests that the enthalpy differences may govern SCO transition behaviors in the polymorphic compounds 2′ and 3. A structural comparison between 1 and 3 indicates that the SCO behavior variations and crystal structure transformation in the present [FeII(L)2](BF4)2 compounds can be interpreted by the relationship between the lattice enthalpies mainly arising from Coulomb interactions between the Fe cations and BF4 anions as in typical ionic crystals.

DOI： 10.1021/acs.inorgchem.7b02721

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The two-electron reduction of a diprotonated dodecaphenylporphyrin derivative by Na2S2O4 gave a corresponding isophlorin (Iph) selectively. Formation of Iph was confirmed by spectroscopic measurements and the isolation of tetramethylated Iph. Further reduction of Iph proceeded to form an unprecedented four-electron-reduced porphyrin (IphH2), which was fully characterized by spectroscopic and X-ray crystallographic analysis. IphH2, with a unique conformation, could be oxidized to reproduce the starting porphyrin, resulting in a proton-coupled four-electron reversible redox system.

DOI： 10.1002/anie.201711058

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We report the synthesis, photophysical properties, redox characteristics, and self-assembly behavior of disk-shaped trithiazolyl-1,3,5-triazines that bear decyloxybenzene moieties. These compounds were synthesized by a Migita-Kosugi-Stille coupling reaction of 1,3,5-trichlorotriazine with three different tributyltin(thiazoles) as the key step. The structure-property relationships, namely the effects of the incorporation of thiazole units into the triazine unit, the conjugation connectivity between the thiazole and triazine units, and the insertion of ethynylene spacers between the thiazole and decyloxybenzene moieties on the properties of the trithiazolyl-1,3,5-triazines were comprehensively investigated. Binding of the triazine core at the 5-position of the thiazole moieties effectively extended the π-conjugation and afforded high fluorescence quantum yields. The ethynylene spacers substantially lowered the LUMO level relative to the HOMO level. The prepared trithiazolyl-1,3,5-triazines self-assembled in solution, and the introduction of thiazole units at the 5-position enhanced this behavior. Detailed thermodynamic studies on the self-association behavior were conducted, and the formation of self-assembled 1D clusters is disclosed.

DOI： 10.1039/c8ob00471d

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A dicopper(II) complex, [Cu2(μ-OH)(6-hpa)]3+, where 6-hpa is 1,2-bis[2-[bis(2-pyridylmethyl)aminomethyl]-6-pyridyl]ethane, generates an oxyl radical of CuIIO• and catalyzes the selective hydroxylation of benzene to phenol. From the structural similarity to methane activation catalysts (e.g., bare CuO+ ion, Cu-ZSM-5, and particulate methane monooxygenase), it is expected to catalyze methane hydroxylation. The catalytic performance for the hydroxylation of methane to methanol by this dicopper complex is investigated by using density functional theory (DFT) calculations. The whole reaction of the methane conversion involves two steps without radical species: (1) C-H bond dissociation of methane by the CuIIO• moiety and (2) C-O bond formation with methyl migration. In the first step, the activation barrier is calculated to be 10.2 kcal/mol, which is low enough for reactions taking place under normal conditions. The activation barrier by the other CuIIO2• moiety is higher than that by the CuIIO• moiety, which should work to turn the next catalytic cycle. DFT calculations show that the dicopper complex has a precondition to hydroxylate methane to methanol. Experimental verification is required to look in detail at the reactivity of this dicopper complex.

DOI： 10.1021/acs.inorgchem.7b02563

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Stable square planar organocopper(III) complexes (CuNCC2, CuNCC4, and CuBN) supported by carbacorrole-based tetradentate macrocyclic ligands with NNNC coordination cores were synthesized, and their structures were elucidated by spectroscopic means including X-ray crystallographic analysis. On the basis of their distinct planar structures, X-ray absorption/photoelectron spectroscopic features, and temperature-independent diamagnetic nature, these organocopper complexes can be preferably considered as novel organocopper(III) species. The remarkable stability of the high-valent Cu(III) states of the complexes stems from the closed-shell electronic structure derived from the peculiar NNNC coordination of the corrole-modified frameworks, which contrasts with the redox-noninnocent radical nature of regular corrole copper(II) complexes with an NNNN core. The proposed structure was supported by DFT (B3LYP) calculations. Furthermore, a π-laminated dimer architecture linked through the inner carbons was obtained from the one-electron oxidation of CuNCC4. We envisage that the precise manipulation of the molecular orbital energies and redox profiles of these organometallic corrole complexes could eventually lead to the isolation of yet unexplored high-valent metal species and the development of their organometallic reactions.

DOI： 10.1021/jacs.8b01876

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We report herein NH tautomerism of a freebase derivative of a quadruply fused porphyrin (H2QFP-Mes, 3), which has one mesityl group at one of the β-positions of the nonfused pyrroles to lower the structural symmetry, allowing us to observe the NH tautomerism with 1H NMR spectroscopy. Compound 3 was revealed to have the two inner NH protons on the two nonfused pyrroles, and the NH tautomerism of 3 was evidenced by variable-temperature (VT) 1H NMR experiments in various deuterated solvents. The VT-NMR studies revealed that the activation barrier for the NH tautomerism of 3 was larger than that of tetraphenylporphyrin. The positive activation entropy (ΔS‡ = 89 J mol-1 K-1), determined for the NH tautomerism, can be explained by dissociation of the π-π stacked dimer structure of 3 in the ground state, as evidenced by the crystal structure and NMR measurements. On the basis of the kinetic studies and density functional theory calculations, the stability of intermediates in the NH tautomerism of 3 and the transition states have been discussed in detail.

DOI： 10.1021/acs.jpcb.7b10945

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A chiral BINOL dimer, (R,R)-3′,3″-BiBINOL, which possesses both rigid (atropos) and dynamic (tropos) axial chiralities, was found to work as an effective NMR chiral solvating reagent for the determination of the enantiomeric purities of various chiral sulfoxides. The unique chiral discrimination mechanism was also revealed by using DFT calculations and X-ray crystallographic analysis.

DOI： 10.1016/j.tetasy.2017.10.008

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While the most likely structure of the active site in iron-containing zeolites has been recently identified as [FeO]2+ (Snyder et al. Nature 2016, 536, 317-321), the mechanism for the direct conversion of methane to methanol over this active species is still debatable between the direct-radical-rebound or nonradical (concerted) mechanism. Using density functional theory on periodic systems, we calculated the two reaction mechanisms over two d4 isoelectronic systems, [FeO]2+ and [MnO]+ zeolites. We found that [FeO]2+ zeolites favor the direct-radical-rebound mechanism with low CH4 activation energies, while [MnO]+ zeolites prefer the nonradical mechanism with higher CH4 activation energies. These contrasts, despite their isoelectronic structures, are mainly due to the differences in the metal coordination number and Oα (oxo) spin density. Moreover, molecular orbital analyses suggest that the zeolite steric hindrance further degrades the reactivity of [MnO]+ zeolites toward methane. Two types of zeolite frameworks, i.e., medium-pore ZSM-5 (MFI framework) and small-pore SSZ-39 (AEI framework) zeolites, were evaluated, but no significant differences in the reactivity were found. The rate-determining reaction step is found to be methanol desorption instead of methane activation. Careful examination of the most stable sites hosting the active species and calculation for N2O decomposition over [Fe]2+-MFI and -AEI zeolites were also performed.

DOI： 10.1021/acs.inorgchem.7b01284

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A new reaction mode for triarylboranes under photochemical conditions was discovered. Photoirradiation of dimesitylboryl-substituted (hetero)arenes produced spirocyclic boraindanes, where one of the C−H bonds in the ortho-methyl groups of the mesityl substituents was formally added in a syn fashion to a C−C double bond of the (hetero)aryl group. Quantum chemical calculations and laser flash photolysis measurements indicated that the reaction proceeds through a [1,6]-sigmatropic rearrangement. This behavior is reminiscent of the photochemical reaction mode of arylalkenylketones, thus demonstrating the isosteric relation between tricoordinate organoboron compounds and the corresponding pseudo-carbocationic species in terms of pericyclic reactions. Despite the disrupted π-conjugation, the resulting spirocyclic boraindanes exhibited a characteristic absorption band at relatively long wavelengths (λ=370—400 nm).

DOI： 10.1002/anie.201706929

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A newly prepared tetraazulene-fused tetracene diimide (TA-fused TDI) showed absorption in the near-IR region owing to the effective extension of the π-conjugated system as well as a large two-photon absorption cross-section (σ(2)=2140 GM) at 950 nm. Four reversible reduction processes and n-type semiconductivity were also confirmed as attractive electronic properties of this compound.

DOI： 10.1002/cplu.201600356

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The hydrogenation of alkenes catalyzed by disilametallacyclic carbonyl complexes of iron, ruthenium or osmium was studied experimentally and theoretically. The disilaruthenacycle 2 with two CO ligands in the trans-configuration was prepared, characterized, and its ability to catalyze hydrogenation was studied. Similar to the corresponding iron analogue 1 in which the CO ligands are in the cis-configuration, 2 contains a H2MSi4 core with SiHSi SISHA (secondary interaction of silicon and hydrogen atoms) and catalyzed the hydrogenation of several alkenes under mild conditions. DFT calculations of 1 and 2 with cis- and trans-CO configurations (cis-1, trans-1, cis-2 and trans-2) revealed that the mechanism of ethylene hydrogenation comprises three catalytic cycles, and a key step involves the H-H bond of H2 being activated by an M-Si bond through oxidative hydrogen migration. These mechanisms are a variety of α-CAM (complex-assisted metathesis) mechanisms. Further calculations suggest that these catalytic cycles can apply to the catalytic hydrogenation of ethylene by osmium analogues of 1 and 2 (cis-3 and trans-3). Some of the elementary reactions in the cycles are dependent on the metal, and the osmium complexes show different performance from the iron and ruthenium analogues due to the characteristic natures of the third-row transition metals.

DOI： 10.1246/bcsj.20170004

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Formation of supramolecular hetero-triads by controlling the hydrogen bonding of conjugate bases with a diprotonated porphyrin based on electrostatic interaction
The thermodynamic stability of diprotonated saddle-distorted dodecaphenylporphyrin (H4DPP(2+)(X(-))2) was controlled by the hydrogen-bonding strength of conjugate bases (X(-)) of strong acids (HX) or acids (R(+)-COOH) having positively charged moieties. The thermodynamic control of H4DPP(2+)(X(-))2 made it possible to achieve selective formation of supramolecular hetero-triads, H4DPP(2+)(X(-))(Cl(-)).

DOI： 10.1039/C7CC03635C

Language：English  

DOI： 10.1039/c7cc90176c

Language：English   Publishing type：Research paper (scientific journal)  

The adhesion between epoxy resin and carbon fiber is investigated by using pair interaction energy decomposition analysis (PIEDA), by which the adhesive interaction energy and adhesive force can be partitioned into the electrostatic, exchangerepulsion, charge-transfer, and van der Waals (dispersion) contributions. The three stabilizing electrostatic, charge-transfer, and dispersion effects are correlated with the destabilizing exchange-repulsion effect. The surface structures of carbon fiber are modeled by the basal face, the armchair-edge structure, and the OH-functionalized armchair-edge structure of graphite. The surface of ?-cristobalite (covered with OH groups), which can be viewed as a good model of a hydrophilic glass surface, is also studied. Adhesive properties of the model interfaces are evaluated on the basis of the binding energy of the resin with the carbon and glass surfaces and the adhesive force acting at the interfaces in terms of energy decomposition. PIEDA calculations demonstrate that only dispersion interactions can substantially work in the hydrophobic surfaces of the basal face and armchair-edge structures. This is a direct consequence of the electrostatic and charge-transfer interactions being cancelled by the exchange-repulsion interactions. On the other hand, both electrostatic and dispersion interactions are significant in the OH-functionalized surfaces of the armchair edge of graphite and ?-cristobalite.

DOI： 10.1246/bcsj.20160426

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Addition of 1 equiv of TFA to an acetone solution containing dodecaphenylporphyrin (H2DPP) in the presence of 10 % MeOH (v/v) resulted in selective formation of a monoprotonated form (H3DPP+), in sharp contrast to protonation of H2DPP directly affording a diprotonated form (H4DPP2+) in acetone in the absence of MeOH. The crucial role of MeOH for selective H3DPP+ formation was interpreted as hydrogen-bonding stabilization of H3DPP+, since a MeOH molecule was found to form hydrogen bonds with an NH proton of H3DPP+ in the crystal. The selectivity of H3DPP+ formation was evaluated by the formation yield of H3DPP+, which increased when elevating the portion of MeOH (0–10 %) in acetone with saturation behavior, suggesting that H3DPP+ is stabilized by hydrogen bonding with MeOH even in solution, together with the thermodynamic parameters determined from a van't Hoff plot based on the spectroscopic titration. Femto- and nanosecond laser flash photolysis allowed us to elucidate the photodynamics of H3DPP+ in intermolecular photoinduced electron transfer (ET) from ferrocene derivatives as one-electron donors to the triplet excited state of H3DPP+ as an electron acceptor. The second-order rate constants of the ET reactions were evaluated in light of the Marcus theory of ET. The reorganization energy of ET was determined to be 1.87 eV, which is slightly larger than that of H4DPP2+ in acetonitrile (1.69 eV), due to larger structural change upon ET than that of H4DPP2+.

DOI： 10.1002/chem.201606012

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A dicopper(II) complex, stabilized by the bis(tpa) ligand 1,2-bis[2-[bis(2-pyridylmethyl)aminomethyl]-6-pyridyl]ethane (6-hpa), [Cu2(μ-OH)(6-hpa)]3+, was synthesized and structurally characterized. This complex catalyzed selective hydroxylation of benzene to phenol using H2O2, thus attaining large turnover numbers (TONs) and high H2O2 efficiency. The TON after 40 hours for the phenol production exceeded 12000 in MeCN at 50 °C under N2, the highest value reported for benzene hydroxylation with H2O2 catalyzed by homogeneous complexes. At 22 % benzene conversion, phenol (95.2 %) and p-benzoquinone (4.8 %) were produced. The mechanism of H2O2 activation and benzene hydroxylation is proposed.

DOI： 10.1002/anie.201702291

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We report herein unique stepwise protonation at inner imino-nitrogen atoms of a freebase derivative of a quadruply fused porphyrin (H2QFP), which has been newly synthesized. H2QFP has been revealed to have the two inner NH protons on the two nonfused pyrroles by X-ray diffraction analysis and 1H NMR spectroscopy. The first protonation at one of the two imino-nitrogen atoms of the fused pyrroles smoothly proceeds with trifluoroacetic acid (TFA) in CH2Cl2 and the equilibrium constant (K1) of the protonation has been determined to be (1.3 ± 0.1) × 105 M-1. In contrast, the second protonation at the other imino-nitrogen atom is hard to occur unless a large excess amount of TFA is used, as reflected on a much smaller equilibrium constant, K2 = 7.3 ± 0.3 M-1. The stepwise protonation is ascribed to the structural rigidity caused by the ring fusion and the resultant steric repulsion among inner NH atoms of the diprotonated form. Electrochemical studies have revealed that protonation at the pyrrole nitrogen atoms caused positive shifts of the reduction potentials of the QFP derivatives. In addition, the ESR spectrum of the electrochemically one-electron-reduced monoprotonated QFP derivative showed well-resolved hyperfine splitting to represent its unsymmetrical electronic structure due to the monoprotonation.

DOI： 10.1021/acs.joc.6b02419

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Atypically anisotropic and large changes in magnetic susceptibility, along with a change in crystalline shape, were observed in a CoIIcomplex at near room temperature. This was achieved by combining oxalate molecules, acting as rotor, and a CoIIion with unquenched orbital angular momentum. A thermally controlled 90° rotation of the oxalate counter anion triggered a symmetry-breaking ferroelastic phase transition, accompanied by contraction–expansion behavior (ca. 4.5 %) along the long axis of a rod-like single crystal. The molecular rotation induced a minute variation in the coordination geometry around the CoIIion, resulting in an abrupt decrease and a remarkable increase in magnetic susceptibility along the direction perpendicular and parallel to the long axis of the crystal, respectively. Theoretical calculations suggested that such an unusual anisotropic change in magnetic susceptibility was due to a substantial reorientation of magnetic anisotropy induced by slight disruption in the ideal D3coordination environment of the complex cation.

DOI： 10.1002/anie.201606165

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Werner type six-coordinate rhodium(iii) complexes coordinated by a planar trianionic ligand and two axial aniline ligands are synthesised. The trianionic ligand behaves as a redox-active ligand to form a ligand radical species upon one-electron oxidation of the complex. The rhodium(iii) complexes catalyse C-H amination of external substrates such as xanthene with tosylazide as the nitrene source. DFT-calculation and kinetic deuterium isotope effects indicate that a di-radical rhodium(iii) complex formed by one-electron transfer from the redox-active ligand to the nitrene group works as a reactive intermediate to induce aliphatic C-H activation.

DOI： 10.1039/c7cc01840a

Language：English   Publishing type：Research paper (scientific journal)  

The thermodynamic stability of diprotonated saddle-distorted dodecaphenylporphyrin (H4DPP2+(X-)2) was controlled by the hydrogen-bonding strength of conjugate bases (X-) of strong acids (HX) or acids (R+-COOH) having positively charged moieties. The thermodynamic control of H4DPP2+(X-)2 made it possible to achieve selective formation of supramolecular hetero-triads, H4DPP2+(X-)(Cl-).

DOI： 0.1039/c7cc03635c

Language：English   Publishing type：Research paper (scientific journal)  

Here we report the resolution of phosphorescent light-emitting iridium(iii) bis[(4,6-difluorophenyl)pyridinato-N,C2]-picolinate into its respective enantiomers by using chiral HPLC and the photo-induced transformation of the isolated enantiomers.

DOI： 10.1039/c7ra04141a

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Photocatalytic syntheses of gem-difluoroolefins were performed using the B12-TiO2 hybrid catalyst during the CC bond reduction of α-trifluoromethyl styrenes with C-F bond cleavage at room temperature under nitrogen. The gem-difluoroolefins were used as synthetic precursors for fluorinated cyclopropanes.

DOI： 10.1039/c7cc04377e

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Recent interest in Cu-exchanged zeolite catalysts for methane hydroxylation has been broadened to small-pore Cu-zeolites such as Cu-SSZ-13 (Cu-CHA), Cu-SSZ-16 (Cu-AFX), and Cu-SSZ-39 (Cu-AEI), which were reported to produce more methanol per copper atom than the medium-pore Cu-ZSM-5 (Cu-MFI) and large-pore Cu-mordenite (Cu-MOR) zeolites do. To elucidate the nature of such fascinating catalytic activities, theoretical investigations based on density functional theory (DFT) were performed on the direct conversion of methane to methanol by [Cu2(μ-O)]2+-exchanged AEI, CHA, AFX, and MFI zeolites in periodic systems. DFT computational results show that the important activation energies for C-H bond dissociation by [Cu2(μ-O)]2+-AEI, -CHA, and -AFX zeolites are lower than those for [Cu2(μ-O)]2+-MFI zeolite. Moreover, the rate-determining methanol desorption and N2O decomposition by [2Cu]2+-AEI zeolite are also found to require low barriers, which renders [Cu2(μ-O)]2+-AEI zeolite highly active for the direct conversion of methane to methanol. Molecular orbital analyses show that AEI, CHA, AFX, and MFI zeolites exert similar confinement effects that stabilize the transition state for C-H bond cleavage. In addition, a decrease in the Cu-O-Cu angle, due to a change in the zeolite ring structure, lowers the acceptor orbital energy of [Cu2(μ-O)]2+-zeolite, which further stabilizes the transition state. We conclude that these two factors play important roles in the activation of methane.

DOI： 10.1021/acscatal.7b00588

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Metal-exchanged zeolites are known to exhibit catalytic activity in the direct conversion of methane to methanol. The influence of different metals on this reaction has been theoretically investigated by using density functional theory (DFT) calculations on a periodic system of MO+-ZSM-5 zeolite (M = Fe, Co, Ni, Cu). The results indicate a high dependence of the reaction on the metals, where the reactivity toward C-H bond dissociation is predicted to increase in the order CoO+-ZSM-5 < NiO+-ZSM-5 < FeO+-ZSM-5 < CuO+-ZSM-5 and the selectivity of methanol is predicted to increase in the order FeO+-ZSM-5 < CoO+-ZSM-5 < NiO+-ZSM-5 < CuO+-ZSM-5. The role of ZSM-5 zeolite in the catalytic activity is also investigated by comparing our calculation results with those reported for the reaction by bare MO+ species in the gas phase. We found that the nanopores of ZSM-5 zeolite exert a confinement effect which destabilizes the adsorption of methane and lowers the activation energy for the C-H bond dissociation. In addition to the conversion of methane, we investigated the direct conversion of ethane to ethanol by FeO+-ZSM-5 and found that this reaction proceeds with a lower C-H bond activation energy and a higher product selectivity in comparison to the conversion of methane to methanol by the same catalyst.

DOI： 10.1021/acscatal.6b01721

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Proton-coupled electron-transfer oxidation of a RuII−OH2complex, having an N-heterocyclic carbene ligand, gives a RuIII−O.species, which has an electronically equivalent structure of the RuIV=O species, in an acidic aqueous solution. The RuIII−O.complex was characterized by spectroscopic methods and DFT calculations. The oxidation state of the Ru center was shown to be close to +3; the Ru−O bond showed a lower-energy Raman scattering at 732 cm−1and the Ru−O bond length was estimated to be 1.77(1) Å. The RuIII−O.complex exhibits high reactivity in substrate oxidation under catalytic conditions; particularly, benzaldehyde and the derivatives are oxidized to the corresponding benzoic acid through C−H abstraction from the formyl group by the RuIII−O.complex bearing a strong radical character as the active species.

DOI： 10.1002/chem.201504883

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Proton transport via dynamic molecules is ubiquitous in chemistry and biology. However, its use as a switching mechanism for properties in functional molecular assemblies is far less common. In this study, we demonstrate how an intra-carboxyl proton shuttle can be generated in a molecular assembly akin to a rack-and-pinion cascade via a thermally induced single-crystal-to-single-crystal phase transition. In a triply interpenetrated supramolecular organic framework (SOF), a 4,4'-azopyridine (azpy) molecule connects to two biphenyl-3,3', 5,5'-tetracarboxylic acid (H4BPTC) molecules to form a functional molecular system with switchable mechanical properties. A temperature change reversibly triggers a molecular movement akin to a rack-and-pinion cascade, which mainly involves 1) an intra-carboxyl proton shuttle coupled with tilting of the azo molecules and azo pedal motion and 2) H4BPTC translation. Moreover, both the molecular motions are collective, and being propagated across the entire framework, leading to a macroscopic crystal expansion and contraction.

DOI： 10.1002/anie.201607886

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Theoretical Study of the Catalytic Hydrogenation of Alkenes by a Disilaferracyclic Complex: Can the Fe-Si sigma-Bond-Assisted Activation of H-H Bonds Allow Development of a Catalysis of Iron?
The mechanisms associated with the hydrogenation of alkenes catalyzed by the iron complex Fe(cis-CO)(2){o-(SiMe2)(2)C6H4}(2)(H)(2) (1) were investigated by DFT calculations. The complex 1 has a structure in which the iron center is bonded to four silicon atoms and two hydrides. Secondary Si center dot center dot center dot H center dot center dot center dot Si interactions were also observed. The exchange of a 1,2-bis(dimethylsilyl)benzene ligand with ethylene and hydrogen gives a disilaferracycle bearing eta(2)-(CH2 = CH2) and eta(2)-H-2 ligands. The catalytic cycle initiated from the disilaferracycle involves cleavage of a H-H linkage assisted by an Fe-Si bond to form Fe-H and n'-(H-Si) moieties (step 1), hydrogen migration from the Fe-H group to the eta(2)-(CH2 = CH2) ligand which accomplishes the insertion of ethylene into the Fe-H bond (step 2), and reaction of the resulting beta-agostic ethyl moiety with the eta(2)-(H-Si) group to form ethane on the iron atom (step 3). The octahedral geometry of 1 as well as the presence of pi-acidic CO ligands and Fe-Si sigma-bonds contributes to all of the catalytic intermediates and the transition states being in the low-spin state. Steps 1 and 3 correspond to the a-complex-assisted metathesis (sigma-CAM) mechanisms proposed by Perutz and Sabo-Etienne, suggesting that these mechanisms can assist in the design of iron-based hydrogenation catalysts operating under mild conditions.

DOI： 10.1021/acs.joc.6b01961

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The polarization switching mechanism is used in various devices such as pyroelectric sensors and memory devices. The change in polarization mostly occurs by ion displacement. The development of materials whose polarization switches via electron transfer in order to enhance operation speed is a challenge. We devised a synthetic and crystal engineering strategy that enables the selective synthesis of a [CrCo] heterometallic dinuclear complex with a polar crystal structure, wherein polarization changes stem from intramolecular charge transfer between Co and the ligand. Polarization can be modulated both by visible-light irradiation and temperature change. The introduction of chiral ligands was paramount to the successful polarization switching in the valence tautomeric compound. Mixing Cr and Co complexes with enantiopure chiral ligands resulted in the selective formation of only pseudosymmetric [CrCo] heterometallic complexes. Furthermore, the left-handed chiral ligands preferentially interacted with their right-handed counterparts, enabling molecules to form a polar crystal structure.

DOI： 10.1021/jacs.6b05089

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A ruthenium(II) complex, [Ru(dmdmp)Cl(MeBPA)] (2) (Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, MeBPA = N-methyl-N,N-bis(pyridylmethyl)amine), having a pterin derivative as a proton-accepting ligand, was synthesized and characterized. Complex 2 shows higher basicity than that of a previously reported RuII-pterin complex, [Ru(dmdmp) (TPA)]+ (1) (TPA = tris(2-pyridylmethyl)amine). On the other hand, 1e--oxidized species of 1 (1OX) exhibits higher electron-acceptability than that of 1e--oxidized 2 (2OX). Bond dissociation enthalpies (BDE) of the two RuII complexes having Hdmdmp as a ligand in proton-coupled electron transfer (PCET) to generate 1OX and 2OX were calculated to be 85 kcal mol-1 for 1OX and 78 kcal mol-1 for 2OX. The BDE values are large enough to perform H atom transfer from C-H bonds of organic molecules to the 1e--oxidized complexes through PCET. The second-order rate constants (k) of PCET oxidation reactions were determined for 1OX and 2OX. The logarithms of normalized k values were proportional to the BDE values of C-H bonds of the substrates with slopes of -0.27 for 1OX and -0.44 for 2OX. The difference between 1OX and 2OX in the slopes suggests that the transition states in PCET oxidations of substrates by the two complexes bear different polarization, as reflection of difference in the electron acceptability and basicity of 1OX and 2OX. The more basic 2OX attracts a proton from a C-H bond via a more polarized transition state than that of 1OX; on the contrary, the more electron-deficient 1OX forms less polarized transition states in PCET oxidation reactions of C-H bonds.

DOI： 10.1021/jacs.6b03785

Language：English   Publishing type：Research paper (scientific journal)  

Coupled-cluster calculations were performed for cyclobutane-1,3-diylidene dicarbenes 2 at the CCSD(T)//CCSD/ cc-pVDZ level of theory, in which the ground-state spin multiplicity and the structures of unique molecules were investigated in detail. The closed-shell singlet state 2(Sσπ ) with a bicyclo-[1.1.0]but-1(3)-ene (BBE) structure found to be the groundstate was much lower in energy than the corresponding singlet dicarbene structure 2(S∗∗ ), the quintet state 2(Q), and the triplet state 2(T), suggesting that the hitherto experimentally unknown BBE structure can be synthesized by the intramolecular dimerization of two carbene units. The energy gap between the BBE structures 2(Ssigma;π) and corresponding quintet states 2(Q) with electron-withdrawing substituents (X = F) at the C2 and C4 positions was found to be larger than that with electrondonating substituents (X = SiH3 ), i.e., ca. 100 kcal mol1 for2b (X = F) > ca. 85 kcal mol1 for 2a (X = H) > ca. 70 kcal mol-1 for 2c (X = SiH3 ). Two unique structures, 2(Tσ ) with a C1σC3 bond and 2(Tπ ) with a C1πC3 bond, were found to be the equilibrium structures for the triplet state of cyclobutane-1,3-diylidene dicarbenes 2.

DOI： 10.1246/bcsj.20160051

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Flexible porous materials generally switch their structures in response to guest removal or incorporation. However, the design of porous materials with empty shape-switchable pores remains a formidable challenge. Here, we demonstrate that the structural transition between an empty orthorhombic phase and an empty tetragonal phase in a flexible porous dodecatuple intercatenated supramolecular organic framework can be controlled cooperatively through guest incorporation and thermal treatment, thus inducing empty shape-memory nanopores. Moreover, the empty orthorhombic phase was observed to exhibit superior thermoelasticity, and the molecular-scale structural mobility could be transmitted to a macroscopic crystal shape change. The driving force of the shape-memory behaviour was elucidated in terms of potential energy. These two interconvertible empty phases with different pore shapes, that is, the orthorhombic phase with rectangular pores and the tetragonal phase with square pores, completely reject or weakly adsorb N-2 at 77 K, respectively.

DOI： 10.1038/ncomms11564

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The Role of Coulomb Interactions for Spin Crossover Behaviors and Crystal Structural Transformation in Novel Anionic Fe(III) Complexes from a pi-Extended ONO Ligand
To investigate the pi-extension effect on an unusual negative-charged spin crossover (SCO) FeIII complex with a weak N2O4 first coordination sphere, we designed and synthesized a series of anionic FeIII complexes from a pi-extended naphthalene derivative ligand. Acetonitrile-solvate tetramethylammonium (TMA) salt 1 exhibited an SCO conversion, while acetone-solvate TMA salt 2 was in a high-spin state. The crystal structural analysis for 2 revealed that two-leg ladder-like cation-anion arrays derived from pi-stacking interactions between pi-ligands of the FeIII complex anion and Coulomb interactions were found and the solvated acetone molecules were in one-dimensional channels between the cation-anion arrays. A desolvation-induced single-crystal-to-single-crystal transformation to desolvate compound 2' may be driven by Coulomb energy gain. Furthermore, the structural comparison between quasi-polymorphic compounds 1 and 2 revealed that the synergy between Coulomb and pi-stacking interactions induces a significant distortion of coordination structure of 2.

DOI： 10.3390/cryst6050049

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Recently, it was shown that μ-oxo-μ-peroxodiiron(III) is converted to high-spin μ-oxodioxodiiron(IV) through O-O bond scission. Herein, the formation and high reactivity of the anti-dioxo form of high-spin μ-oxodioxodiiron(IV) as the active oxidant are demonstrated on the basis of resonance Raman and electronic-absorption spectral changes, detailed kinetic studies, DFT calculations, activation parameters, kinetic isotope effects (KIE), and catalytic oxidation of alkanes. Decay of μ-oxodioxodiiron(IV) was greatly accelerated on addition of substrate. The reactivity order of substrates is toluene<ethylbenzene≈cumene<trans-β-methylstyrene. The rate constants increased proportionally to the substrate concentration at low substrate concentration. At high substrate concentration, however, the rate constants converge to the same value regardless of the kind of substrate. This is explained by a two-step mechanism in which anti-μ-oxodioxodiiron(IV) is formed by syn-to-anti transformation of the syn-dioxo form and reacts with substrates as the oxidant. The anti-dioxo form is 620 times more reactive in the C-H bond cleavage of ethylbenzene than the most reactive diiron system reported so far. The KIE for the reaction with toluene/[D8]toluene is 95 at -30 °C, which the largest in diiron systems reported so far. The present diiron complex efficiently catalyzes the oxidation of various alkanes with H2O2. Strong anti oxidant: A high-spin μ-oxodioxodiiron(IV) species undergoes transformation from the syn-dioxo to the anti-dioxo form, which cleaves strong C-H bonds of alkanes. The high-spin anti-dioxodiiron(IV) species with a sterically less hindered structure (see figure) is a highly reactive and selective oxidant. These results provide insight into the high reactivity of the active species Q of soluble methane monooxygenases and the development of efficient alkane oxidation catalysts.

DOI： 10.1002/chem.201600048

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The reaction of benzopentalenequinone with alkynes bearing an aniline moiety provides access to two classes of push–pull chromophores with interesting optoelectronic properties. The regioselective [4+2] cycloaddition/[4+1] retrocycloaddition sequence gives fluorenone derivatives, and the formal regioselective [2+2] cycloaddition/ring-opening reaction in polar solvents generates hitherto unknown benzazulenequinone derivatives.

DOI： 10.1016/j.tetlet.2016.09.002

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Enzymatic methane hydroxylation is proposed to efficiently occur at the dinuclear copper site of particulate methane monooxygenase (pMMO), which is an integral membrane metalloenzyme in methanotrophic bacteria. The resting state and a possible peroxo state of the dicopper active site of pMMO are discussed by using combined quantum mechanics and molecular mechanics calculations on the basis of reported X-ray crystal structures of the resting state of pMMO by Rosenzweig and co-workers. The dicopper site has a unique structure, in which one copper is coordinated by two histidine imidazoles and another is chelated by a histidine imidazole and primary amine of an N-terminal histidine. The resting state of the dicopper site is assignable to the mixed-valent (CuCuII)-Cu-I state from a computed Cu-Cu distance of 2.62 angstrom from calculations at the B3LYP-D/TZVP level of theory. A mu-eta(2):eta(2)-peroxo-Cu-2(II) structure similar to those of hemocyanin and tyrosinase is reasonably obtained by using the resting state structure and dioxygen. Computed Cu-Cu and O-O distances are 3.63 and 1.46 angstrom, respectively, in the open-shell singlet state. Structural features of the dicopper peroxo species of pMMO are compared with those of hemocyanin and tyrosinase and synthetic dicopper model compounds. Optical features of the mu-eta(2):eta(2)-peroxo-Cu-2(II) state are calculated and analyzed with TD-DFT calculations.

DOI： 10.1021/acs.inorgchem.5b02603

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A bis-hydroxo-bridged dinuclear CoIII-pyridylmethylamine complex (1) was synthesized and the crystal structure was determined by X-ray crystallography. Complex 1 acts as a homogeneous catalyst for visible-light-driven water oxidation by persulfate (S2O82-) as an oxidant with [RuII(bpy)3]2+ (bpy = 2,2′-bipyridine) as a photosensitizer affording a high quantum yield (44%) with a large turnover number (TON = 742) for O2 formation without forming catalytically active Co-oxide (CoOx) nanoparticles. In the water-oxidation process, complex 1 undergoes proton-coupled electron-transfer (PCET) oxidation as a rate-determining step to form a putative dinuclear bis-μ-oxyl CoIII complex (2), which has been suggested by DFT calculations. Catalytic water oxidation by 1 using [RuIII(bpy)3]3+ as an oxidant in a H216O and H218O mixture was examined to reveal an intramolecular O-O bond formation in the two-electron-oxidized bis-μ-oxyl intermediate, prior to the O2 evolution.

DOI： 10.1021/acs.inorgchem.5b02336

Language：English   Publishing type：Research paper (scientific journal)  

Synthesis and Structure of a Water-soluble mu-eta(1):eta(1)-N-2 Dinuclear Ru-II Complex with a Polyamine Ligand
We report the first example of a mu-eta(1):eta(1)-N-2 dinuclear Run complex with a polyamine ligand and elucidate the structure by means of X-ray analysis. The N N stretching vibration has been observed at 1994 cm(-1) by Raman spectroscopy, which is the lowest value of all the known N-2-coordinated Run complexes. This low value strongly suggests the N N. bond is primed for activation.

DOI： 10.1246/cl.151004

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Unprecedented anionic FeIII spin crossover (SCO) complexes involving a weak-field O,N,O-tridentate ligand were discovered. The SCO transition was evidenced by the temperature variations in magnetic susceptibility, Mössbauer spectrum, and coordination structure. The DFT calculations suggested that larger coefficients on the azo group in the HOMO-1 of a ligand might contribute to the enhancement of a ligand-field splitting energy. The present anionic SCO complex also exhibited the light- induced excited-spin-state trapping effect. Photoinduced spin transition: Unprecedented anionic spin crossover (SCO) complexes involving a weak-field FeIIIN2O4 coordination sphere were discovered. The tetramethylammonium salt of the depicted SCO anion exhibited the light-induced excited-spin-state trapping effect, which is quite rare in FeIII SCO complexes (see scheme).

DOI： 10.1002/chem.201504883

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Dinuclear iron carbonyl complex 2, which contains an elongated unsupported Fe-Fe bond, was synthesized by the reaction between Fe 2 (CO) 9 and phosphinyl radical 1. Thermal Fe-Fe bond homolysis led to the generation of a four-coordinate carbonyl-based iron-centered radical, 3, which is stabilized by π-donation. Complex 3 exhibited high reactivity toward organic radicals to form diamagnetic five-coordinate Fe(ii) complexes.

DOI： 10.1039/c5sc02601f

Language：English   Publishing type：Research paper (scientific journal)  

The anionic FeIII complex exhibiting cooperative spin transition with a wide thermal hysteresis near room temperature, K[Fe(5-Brthsa)2] (5-Brthsa-H2=5-bromosalicylaldehyde thiosemicarbazone), is reported. The hysteresis (Δ=69 K in the first cycle) shows a one-step transition in heating mode and a two-step transition in cooling mode. X-ray structure analysis showed that the coexistence of hydrogen bond and cation-π interactions, as well as alkali metal coordination bonds, to give 2D coordination polymer structure. This result is contrary to previous reports of broad thermal hysteresis induced by coordination bonds of FeII spin crossover coordination polymers (with 1D/3D structures), and by strong intermolecular interactions in the molecular packing through π-π stacking or hydrogen-bond networks. As a consequence, the importance, or the very good suitability of alkali metal-based coordination bonds and cation-π interactions for communicating cooperative interactions in spin-crossover (SCO) compounds must be reconsidered.

DOI： 10.1002/chem.201503392

Language：English   Publishing type：Research paper (scientific journal)  

Harnessing molecular motion to reversibly control macroscopic properties, such as shape and size, is a fascinating and challenging subject in materials science. Here we design a crystalline cobalt(II) complex with an n-butyl group on its ligands, which exhibits a reversible crystal deformation at a structural phase transition temperature. In the low-temperature phase, the molecular motion of the n-butyl group freezes. On heating, the n-butyl group rotates ca. 100 degrees around the C-C bond resulting in 6-7% expansion of the crystal size along the molecular packing direction. Importantly, crystal deformation is repeatedly observed without breaking the single-crystal state even though the shape change is considerable. Detailed structural analysis allows us to elucidate the underlying mechanism of this deformation. This work may mark a step towards converting the alkyl rotation to the macroscopic deformation in crystalline solids.

DOI： 10.1038/ncomms9810

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Rh(III) complexes having tris(2-pyridylmethyl)amine (TPA) and its derivative as tetradentate ligands showed reversible deprotonation at a methylene moiety of the TPA ligands upon addition of a strong base as confirmed by spectroscopic measurements and X-ray crystallography. Deprotonation selectively occurred at the axial methylene moiety rather than equatorial counterparts because of the thermodynamic stability of corresponding deprotonated complexes. One-electron oxidation of the deprotonated Rh(III) TPA complex afforded a unique TPA radical bound to the Rh(III) center by a ligand-centered oxidation. This is the first example to demonstrate emergence of the redox-noninnocent character of the TPA ligand.

DOI： 10.1021/jacs.5b06237

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Density-functional-theory (DFT) calculations are performed for the proposal of a plausible mechanism on the reduction of NO to N2O by a dinuclear ruthenium complex, reported by Arikawa and co-workers [J. Am. Chem. Soc. 2007, 129, 14160]. On the basis of the experimental fact that the reduction proceeds under strongly acidic conditions, the role of protons in the mechanistic pathways is investigated with model complexes, where one or two NO ligands are protonated. The reaction mechanism of the NO reduction is partitioned into three steps: reorientation of N2O2 (cis-NO dimer), O-N bond cleavage, and N2O elimination. A key finding is that the protonation of the NO ligand(s) significantly reduces the activation barrier in the rate-determining reorientation step. The activation energy of 43.1 kcal/mol calculated for the proton-free model is reduced to 30.2 and 17.6 kcal/mol for the mono- and diprotonated models, respectively. The protonation induces the electron transfer from the Ru(II)Ru(II) core to the O=N-N=O moiety to give a Ru(III)Ru(III) core and a hyponitrite (O-N=N-O)(2-) species. The formation of the hyponitrite species provides an alternative pathway for the N2O2 reorientation, resulting in the lower activation energies in the presence of proton(s). The protonation also has a marginal effect on the O-N bond cleavage and the N2O elimination steps. Our calculations reveal a remarkable role of protons in the NO reduction via N2O formation and provide new insights into the mechanism of NO reduction catalyzed by metalloenzymes such as nitric oxide reductase (NOR) that contains a diiron active site.

DOI： 10.1021/acs.inorgchem.5b00394

Language：English   Publishing type：Research paper (scientific journal)  

Catalytic activity of [Os-III(OH)(H2O)(L-N4Me2)](PF6)(2) (1: L-N4Me2 = N,N'-dimethyl-2,11-diaza-[3,3](2,6)pyridinophane) in 1,2-cis-aminohydroxylation of alkenes with sodium N-chloro-4-methylbenzenesulfonamide (chloramine-T) is explored. Simple alkenes as well as those containing several types of substituents are converted to the corresponding 1,2-aminoalcohols in modest to high yields. The aminoalcohol products have exclusively cis conformation with respect to the introduced -OH and -NHTs groups. The spectroscopic measurements including cold mass spectroscopic study of the reaction product of complex 1 and chloromine-T as well as density functional theory (DFT) calculations indicate that an oxido-aminato-osmium(V) species [Os-V(O)(NHTs)(L-N4Me2)](PF6)(2) (2) is an active oxidant for the aminohydroxylation. The DFT calculations further indicate that the reaction involves a [3 + 2] cycloaddition between 2 and alkene, and the regioselectivity in the aminohydroxylation of unsymmetrical alkenes is determined by the orientation that bears less steric hindrance from the tosylamino group, which leads to the energetically more preferred product isomer.

DOI： 10.1021/acs.inorgchem.5601083

Language：English   Publishing type：Research paper (scientific journal)  

The gas-phase acidity (GA) of a series of 1,1-bis(trifluoromethanesulfonyl)propane derivatives, Tf2CHCH2CH(R-1)R-2, and Tf2CHCH2Ar (Ar=phenol derivatives) was determined by measuring proton-transfer equilibria using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer and by computing the free energy of the deprotonated carbanions and the corresponding neutrals. The effects of R-1 and R-2 or Ar groups on the acidity were examined. In Tf2CHCH2CH(R-1)R-2, the GA values calculated using the conformers of neutral molecules that are free from the intramolecular hydrogen-bonding interaction between the acidic hydrogen atom of the Tf2CH moiety and R-1 or R-2 group and between the hydrogen atom of the (CHRR2)-R-1 moiety and the SO2CF3 group were correlated in terms of an equation, GA=-17.0 sigma sigma(I)+3.4 sigma sigma+299.5. On the basis of this correlation, it was elucidated that the intramolecular hydrogen bonding or dipole-dipole interaction in the neutral molecule weakens significantly the acidity. In Tf2CHCH2Ar (5, 6 and 7), the GA was strengthened by the strong hydrogen-bonding interaction between the phenolic hydrogen in the aromatic moiety and the SO2CF3 group in the conjugate anion compared with that in the neutral molecule. Copyright (c) 2014 John Wiley & Sons, Ltd.

DOI： 10.1002/poc.3304

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Ruthenium(ii) complexes of PQQTME, a trimethyl ester derivative of redox-active PQQ (pyrroloquinolinequinone), were prepared using a tridentate ligand, 2,2′:6′,2′′-terpyridine (terpy) as an auxiliary ligand. The characterization of the complexes was performed by spectroscopic methods, X-ray crystallography, and electrochemical measurements. In one complex, the pyridine site of PQQTME binds to the [RuII(terpy)] unit as a tridentate ligand, and a silver(i) ion is coordinated by the quinone moiety in a bidentate fashion. In contrast, another complex includes the [RuII(terpy)] unit at the bidentate quinone moiety of the PQQTME ligand. The difference in the coordination modes of the complexes exhibits a characteristic difference in the stability of metal coordination and also in the reversibility of the reduction processes of the PQQTME ligand. It should be noted that an additional metal-ion-binding to the PQQTME ligand largely raises the 1e--reduction potential of the ligand. In addition, we succeeded in the characterization of the 1e--reduced species of the complexes, where the unpaired electron was delocalized in the π-conjugated system of the PQQTME- ligand, using UV-Vis absorption and ESR spectroscopies.

Language：English   Publishing type：Research paper (scientific journal)  

A mononuclear Cr(V)-oxo complex, [CrV(O)(6-COO--tpa)](BF4)2 (1; 6-COO--tpa = N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine) was prepared through the reaction of a Cr(III) precursor complex with iodosylbenzene as an oxidant. Characterization of 1 was achieved using ESI-MS spectrometry, electron paramagnetic resonance, UV-vis, and resonance Raman spectroscopies. The reduction potential (Ered) of 1 was determined to be 1.23 V vs. SCE in acetonitrile based on analysis of the electron-transfer (ET) equilibrium between 1 and a one-electron donor, [RuII(bpy)3]2+ (bpy = 2,2′-bipyridine). The reorganization energy (λ) of 1 was also determined to be 1.03 eV in ET reactions from phenol derivatives to 1 on the basis of the Marcus theory of ET. The smaller λ value in comparison with that of an Fe(IV)-oxo complex (2.37 eV) is caused by the small structural change during ET due to the dπ character of the electron-accepting LUMO of 1. When benzyl alcohol derivatives (R-BA) with different oxidation potentials were employed as substrates, corresponding aldehydes were obtained as the 2e--oxidized products in moderate yields as determined from 1H NMR and GC-MS measurements. One-step UV-vis spectral changes were observed in the course of the oxidation reactions of BA derivatives by 1 and a kinetic isotope effect (KIE) was observed in the oxidation reactions for deuterated BA derivatives at the benzylic position as substrates. These results indicate that the rate-limiting step is a concerted proton-coupled electron transfer (PCET) from substrate to 1. In sharp contrast, in the oxidation of trimethoxy-BA (Eox = 1.22 V) by 1, trimethoxy-BA radical cation was observed by UV-vis spectroscopy. Thus, it was revealed that the mechanism of the oxidation reaction changed from one-step PCET to stepwise ET-proton transfer (ET/PT), depending on the redox potentials of R-BA.

DOI： 10.1039/c4sc02285h

Language：English   Publishing type：Research paper (scientific journal)  

Self-assembly of artificial nanoscale units into superstructures is a prevalent topic in science. In biomimicry, scientists attempt to develop artificial self-assembled nanoarchitectures. However, despite extensive efforts, the preparation of nanoarchitectures with superior physical properties remains a challenge. For example, one of the major topics in the field of molecular magnetism is the development of high-spin (HS) molecules. Here, we report a cyanide-bridged magnetic nanocage composed of 18 HS iron(III) ions and 24 low-spin iron(II) ions. The magnetic iron(III) centres are ferromagnetically coupled, yielding the highest ground-state spin number (S = 45) of any molecule reported to date.

DOI： 10.1038/ncomms6955

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The catalytic conversion of methanol to formaldehyde by three kinds of non-porphyrin Ru complexes, (RuO)-O-IV(TPA) (TPA = tris(2-pyridylmethyl) amine) (1a), (RuO)-O-IV(6-COO-TPA) (6-COO-TPA = 2-(6-carboxyl-pyridyl)methyl-bis(2-pyridylmethyl)amine) (1b), and (RuO)-O-IV(N4Py) (N4Py = N, N-bis(2-pyridyl-methyl)-N-bis(2-pyridyl)methylamine) (1c), is discussed by using density functional theory (DFT) calculations. There are two possible reaction pathways for the oxidation of methanol to formaldehyde with respect to the first hydrogen abstraction from the methyl group (path 1) and the hydroxyl group (path 2). Path 1 and path 2 involve the hydroxymethyl radical (center dot CH2OH) and the methoxyl radical (CH3O center dot), respectively, as an intermediate. DFT calculations demonstrate that the two pathways are energetically comparable in the reactions by the three Ru-IV-oxo complexes. The reactions with 1a and 1c are initiated by the C-H bond dissociation with activation barriers of 22.2 and 21.4 kcal/mol, respectively, while the reaction with 1b is initiated by the O-H bond dissociation with an activation barrier of 18.1 kcal/mol. However, the calculations showed that the rate-determining step is the H-atom abstraction from the CH3 group of methanol in all the pathways. These results are in good agreement with kinetic analysis of the reactions by the Ru-IV-oxo complexes, being useful for considering the mechanism of methanol oxidation.

DOI： 10.1142/S1088424615500285

Language：English   Publishing type：Research paper (scientific journal)  

Many molecular machines with controllable molecular-scale motors have been developed. However, transmitting molecular movement to the macroscopic scale remains a formidable challenge. Here we report a single crystal of a Ni complex whose shape changes abruptly and reversibly in response to thermal changes at around room temperature. Variable-temperature single-crystal X-ray diffraction studies show that the crystalline shape change is induced by an unusual 90 degrees rotation of uniaxially aligned oxalate molecules. The oxalate dianions behave as molecular-scale rotors, with their movement propagated through the entire crystalline material via intermolecular hydrogen bonding. Consequently, the subnanometrescale changes in the oxalate molecules are instantly amplified to a micrometre-scale contraction or expansion of the crystal, accompanied by a thermal hysteresis loop. The shape change in the crystal was clearly detected under an optical microscope. The large directional deformation and prompt response suggest a role for this material in microscale or nanoscale thermal actuators.

DOI： 10.1038/NCHEM.2092

Language：English   Publishing type：Research paper (scientific journal)  

A square-shaped tetranuclear ruthenium(II) complex, [(Ru4Cl5)-Cl-II(bpmpm)(2)](3+) [1; bpmpm = 4,6-bis[[N,N-bis(2'-pyridylmethyl)amino]methyl]pyrimidine], exhibited four reversible and stepwise one-electron-oxidation processes: chemical oxidation of 1 formed three different mixed-valence states, in one of which the charge is partially delocalized on the two Ru centers, to be evidenced by observation of an intervalence charge-transfer absorption band, categorized into the RobinDay class II.

DOI： 10.1021/ic502422u

Language：English   Publishing type：Research paper (scientific journal)  

Many molecular machines with controllable molecular-scale motors have been developed. However, transmitting molecular movement to the macroscopic scale remains a formidable challenge. Here we report a single crystal of a Ni complex whose shape changes abruptly and reversibly in response to thermal changes at around room temperature. Variable-temperature single-crystal X-ray diffraction studies show that the crystalline shape change is induced by an unusual 90 degrees rotation of uniaxially aligned oxalate molecules. The oxalate dianions behave as molecular-scale rotors, with their movement propagated through the entire crystalline material via intermolecular hydrogen bonding. Consequently, the subnanometrescale changes in the oxalate molecules are instantly amplified to a micrometre-scale contraction or expansion of the crystal, accompanied by a thermal hysteresis loop. The shape change in the crystal was clearly detected under an optical microscope. The large directional deformation and prompt response suggest a role for this material in microscale or nanoscale thermal actuators.

DOI： 10.1038/NCHEM.2092

Language：English   Publishing type：Research paper (scientific journal)  

A porphyrin-flavin-linked dyad and its zinc and palladium complexes (MPor-Fl:2-M, M = 2H, Zn, and Pd) were newly synthesized and the X-ray crystal structure of 2-Pd was determined. The photodynamics of 2-M were examined by femto-and nanosecond laser flash photolysis measurements. Photoinduced electron transfer (ET) in 2-H-2 occurred from the singlet excited state of the porphyrin moiety (H(2)Por) to the flavin (Fl) moiety to produce the singlet charge-separated (CS) state (1)(H(2)Por(center dot+)-Fl(center dot-)), which decayed through back ET (BET) to form (3)[H(2)Por]*-Fl with rate constants of 1.2-10(10) and 1.2-10(9) s(-1), respectively. Similarly, photoinduced ET in 2-Pd afforded the singlet CS state, which decayed through BET to form (3)[PdPor]*-Fl with rate constants of 2.1-10(11) and 6.0-10(10) s(-1), respectively. The rate constant of photoinduced ET and BET of 2-M were related to the ET and BET driving forces by using the Marcus theory of ET. One and two Sc3+ ions bind to the flavin moiety to form the Fl-Sc3+ and Fl-(Sc3+)(2) complexes with binding constants of K-1 = 2.2-10(5)m(-1) and K-2 = 1.8-10(3)m(-1), respectively. Other metal ions, such as Y3+, Zn2+, and Mg2+, form only 1:1 complexes with flavin. In contrast to 2-M and the 1:1 complexes with metal ions, which afforded the short-lived singlet CS state, photoinduced ET in 2-Pd center dot center dot center dot Sc3+ complexes afforded the triplet CS state (3[PdPorC(center dot+)-Fl center dot--(Sc3+)(2)]), which exhibited a remarkably long lifetime of tau = 110 ms (k(BET) = 9.1 s(-1)).

DOI： 10.1002/chem.201403960

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The influence of protons on the mechanism of the direct decomposition of NO over adjacent dimeric Cu(I) active sites in zeolite is theoretically investigated by using ONIOM (QM/MM) calculations with two dicopper model systems IT and 2T, where the Cu(I) atoms are separated by one and two SiO4 tetrahedra, respectively. The reaction proceeds through the formation of N2O as a reaction intermediate and further its decomposition into oxygen and nitrogen. The present study shows that the presence of proton plays an important role in the production of N2O from two NO molecules. In the proton-free mechanism, this process requires a large activation barrier of 56.3 and 55.3 kcal/mol on IT and 2T, respectively, while the inclusion of protons reduces it to 31.4 and 17.3 kcal/mol. The significant decrease in the activation barrier is due to the strengthening of the N-N bond of the formed NO dimer upon protonation, which facilitates the formation of N2O. On the other hand, the presence of protons disfavors the decomposition of N2O and needs an activation barrier of 6-9 kcal/mol higher than that of the corresponding reaction in the absence of protons. The stable intermediate Cu-OH+-Cu formed in the proton-assisted mechanism is responsible for the larger activation energy for N2O decomposition. The proton-assisted NO decomposition mechanism is in agreement with the experimental observation that the decomposition of N2O as well as O-2 desorption are the governing reaction steps in the decomposition of NO. The present study explains the role of the Cu-O-Cu species in the NO decomposition reaction. The results disclosed herein will also pave a way to understanding the mechanism of the reductive N-N coupling of NO molecules catalyzed by metalloenzymes and transition-metal catalysts.

DOI： 10.1021/cs500223z

Language：English   Publishing type：Research paper (scientific journal)  

Detailed kinetic studies on the oxidation reactions of organic substrates such as methanol with RuIVO complexes as oxidants, formed electrochemically in water, have been conducted to elucidate the reaction mechanism. The rate constants of the oxidation reactions exhibited saturation behaviours relative to the substrate concentration, regardless of the oxidants and the substrates employed. This indicates the existence of a pre-equilibrium process based on the adduct formation between the RuIVO oxidant and the substrate. Herein, we have experimentally confirmed that the driving force of the adduct formation is the hydrogen bonding between the oxidants and alcohols even in water. In addition, we have investigated the kinetic isotope effects (KIE) on the oxidation reaction using methanol and its deuterated derivatives and as a result observed moderate KIE values for the C-H bond of methanol. We have also revealed the independency of the reaction rates from the bond dissociation enthalpies of the C-H bonds of the substrates. This independency is probably derived from the tightly condensed transition state, whose energy level is strongly controlled by the activation entropy but less sensitive to the activation enthalpy.

DOI： 10.1039/c3sc53002g

Language：English  

Language：English   Publishing type：Research paper (scientific journal)  

Dioxygen activation proceeds via O-O bond scission of peroxodi-iron(iii) to high-spin oxodi-iron(iv) in soluble methane mono-oxygenase (sMMO). Recently, we have shown that reversible O-O bond scission of peroxodi-iron(iii) to high-spin oxodi-iron(iv) is attained with a bis-tpa type dinucleating ligand, 6-hpa. In this study, a new carboxylate-containing dinucleating ligand, 1,2-bis[2-(N-2-pyridylmethyl-N-glycinylmethyl)-6-pyridyl]ethane (H 2BPG2E) and its μ-oxodiaquadi-iron(iii) complexes [Fe2(μ-O)(H2O)2(BPG2E)]X 2 [X = ClO4 (2a) or OTf (2b)] were synthesized to mimic a common carboxylate-rich coordination environment in O2-activating non-heme di-iron enzymes including sMMO. The crystal structures of 2a and 2b revealed that BPG2E prefers a syn-diaqua binding mode. 2b catalyzed the epoxidation of alkenes with H2O2. A new purple species was formed upon reaction of 2b with H2O2, and characterized by the elemental analysis and spectral and kinetic studies. These clearly showed that the purple species was a μ-oxo-μ-peroxodi-iron(iii), and converted to high-spin μ-oxodioxodi-iron(iv) via rate-determining reversible O-O bond scission. In comparison of BPG2E with 6-hpa, it is shown that the carboxylate donor stabilizes the Fe-O-O-Fe structure of the peroxo complex due to the structural effect to retard O-O bond scission. This may shed light on the roles of carboxylate donors in the dioxygen activation of non-heme di-iron enzymes.

DOI： 10.1039/c3sc51541a

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Both spin-crossover complexes and molecular nanomagnets display bistable magnetic states, potentially behaving as elementary binary units for information storage. It is a challenge to introduce spin-crossover units into molecular nanomagnets to switch the bistable state of the nanomagnets through external stimuli-tuned spin crossover. Here we report an iron(II) spin-crossover unit and paramagnetic iron(III) ions that are incorporated into a well-isolated double-zigzag chain. The chain exhibits thermally induced reversible spin-crossover and light-induced excited spin-state trapping at the iron(II) sites. Single-chain magnet behaviour is actuated accompanying the synergy between light-induced excited spin-state trapping at the iron(II) sites and ferromagnetic interactions between the photoinduced high-spin iron(II) and low-spin iron(III) ions in the chain. The result provides a strategy to switch the bistable state of molecular nanomagnets using external stimuli such as light and heat, with the potential to erase and write information at a molecular level.

DOI： 10.1038/ncomms3826

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Our work reveals a high dependence on charge-transfer (CT) amounts for the optimal Hartree-Fock percentage in the exchange-correlation functional of time-dependent density functional theory (TD-DFT) and the error of a vertical transition energy calculated by a given functional. Using these relations, the zero-zero transition energies of the first singlet and first triplet excited states of various CT compounds are accurately reproduced. 3CT and locally excited triplet (3LE) states are well distinguished and calculated independently. © 2013 American Chemical Society.

DOI： 10.1021/ct400415r

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Spin doctor: A mononuclear ferric complex [Fe(H-5-Br-thsa)(5-Br-thsa)] ×H2O (1) (H2-5-Br-thsa=5-bromo-2-hydroxybenzylidene) hydrazinecarbothioamide) was synthesized and its magnetic properties and structure were investigated by DFT calculations. This complex shows unprecedented reversible, six/five-step spin-crossover behavior accompanied by symmetry breaking. More importantly, each step in the multi-step transition was successfully characterized by single-crystal X-ray diffraction. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/chem.201302272

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The thermal and photochemical reactions of a newly synthesized complex, [RuII(TPA)(tpphz)]2+ (1; TPA=tris(2-pyridylmethyl)amine, tpphz=tetrapyrido[3,2-a:2,3-c:3,2-h: 2,3-j]phenazine), and its derivatives have been investigated. Heating a solution of complex 1 (closed form) and its derivatives in MeCN caused the partial dissociation of one pyridylmethyl moiety of the TPA ligand and the resulting vacant site on the RuII center was occupied by a molecule of MeCN from the solvent to give a dissociated complex, [RuII(3-TPA)(tpphz)(MeCN)]2+ (1, open form), and its derivatives, respectively, in quantitative yields. The thermal dissociation reactions were investigated on the basis of kinetics analysis, which indicated that the reactions proceeded through a seven-coordinate transition state. Although the backwards reaction was induced by photoirradiation of the MLCT absorption bands, the photoreaction of complex 1 reached a photostationary state between complexes 1 and 1 and, hence, the recovery of complex 1 from complex 1 was 67%. Upon protonation of complex 1 at the vacant site of the tpphz ligand, the efficiency of the photoinduced recovery of complex 1+H+ from complex 1+H+ improved to 83%. In contrast, dinuclear -tpphz complexes 2 and 3, which contained the RuII(TPA)(tpphz) unit and either a RuII(bpy)2 or PdIICl2 moiety on the other coordination edge of the tpphz ligand, exhibited 100% photoconversion from their open forms into their closed forms (22 and 33). These results are the first examples of the complete photochromic structural change of a transition-metal complex, as represented by complete interconversion between its open and closed forms. Scrutinization by performing optical and electrochemical measurements allowed us to propose a rationale for how metal coordination at the vacant site of the tpphz ligand improves the efficiency of photoconversion from the open form into the closed form. It is essential to lower the energy level of the triplet metal-to-ligand charge-transfer excited state (3MLCT*) of the closed form relative to that of the triplet metal-centered excited state (3MC*) by metal coordination. This energy-level manipulation hinders the transition from the 3MLCT* state into the 3MC* state in the closed form to block the partial photodissociation of the TPA ligand.

DOI： 10.1002/chem.201300437

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Methane hydroxylation at the dinuclear copper site of particulate methane monooxygenase (pMMO) is studied by using density functional theory calculations. The electronic, structural, and reactivity properties of a possible dinuclear copper species (mu-oxo)(mu-hydroxo)(CuCuIII)-Cu-II are discussed with respect to the C-H bond activation of methane. We propose that the tyrosine residue in the second coordination sphere of the dicopper site donates an H atom to the mu-eta(2):eta(2)-peroxoCu(II)Cu(II) species and the resultant (mu-oxo)(mu-hydroxo)(CuCuIII)-Cu-II species can hydroxylate methane. This species for methane hydroxylation is more favorable in reactivity than the bis(p-oxo)(CuCuIII)-Cu-II species. The H-atom transfer or proton-coupled electron transfer from the tyrosine residue can reasonably induce the O-O bond dissociation of the mu-eta(2):eta(2)-peroxoCu(II)Cu(II) species to form the reactive (mu-oxo)(mu-hydroxo)(CuCuIII)-Cu-II species, which is expected to be an active species for the conversion of methane to methanol at the dicopper site of pMMO. The rate-determining step for the methane hydroxylation is the C H cleavage, which is in good agreement with experimental Km values reported so far.

DOI： 10.1021/ic400417d

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Herein, we report the synthesis of a novel heterohexanuclear complex (1) of a heteroaromatic cofactor, pyrroloquinolinequinone (PQQ). The crystal structure of 1 was determined to reveal that two PQQ: bridged (RuAgI)-Ag-II units were linked by two [Ag-I(OTf)(2)](-) units (OTf = CF3SO3-). A solvent-bound (RuAgI)-Ag-II heterodinuclear complex (2) was formed from 1 in a coordinating solvent such as acetone to show an intense metal-to-ligand charge-transfer band at 709 nm.

DOI： 10.1021/ic302617b

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A new bisdehydro[12]annulene derivative having a thiophene-fused structure has been synthesized. This highly twisted pi-conjugated macrocycle with two acetylene moieties in close proximity produces a [2+2]-type alkyne cycloadduct by either photoirradiation or mild heating without any transition metals. Theoretical calculations reveal that the thermal reaction proceeds through successive 8 pi and 4 pi electrocyclic reactions, while the photochemical reaction is an asynchronous concerted [2+2] cycloaddition. The fused structure with the less-aromatic thiophene ring is crucial for achieving this reaction. The cycloadduct, thiophene-fused biphenylene, has significant potential as a new polycyclic pi-scaffold for electronic applications.

DOI： 10.1021/ja3126849

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A novel quadruply-fused porphyrin has been synthesized with a facilely prepared precursor in a high yield. A detailed comparison of the physical properties of a series of fused porphyrins revealed remarkable effects of the ring fusion on lowering LUMO levels rather than HOMO levels.

DOI： 10.1039/c3cc42831a

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The exact structure of the active site of intermediate Q, the methane-oxidizing species of soluble methane monooxygenase (sMMO), and the reaction mechanism of Q with methane molecule are still not fully clear. To gain further insights into the structure and reaction mechanism, five diiron models of Q that differ in shape, oxidation state, spin state, and coordination number of the two iron centers are studied. Different mechanisms in different spin states were explored. Density functional theory (DFT) calculations show that FeIIIFeIV(μ-O)(μ-OH) is more reactive than Fe IV2(μ-O)2 in the oxygen-rich environment and that the reactivity of the active core of sMMO-Q is not enhanced by converting its oxo bridge into a terminal ligand. A four-coordinated diiron model is the most effective for methane hydroxylation. Both radical and non-radical intermediates are involved in the reactions for the four-coordinated diiron model.

DOI： 10.1039/c2dt31304a

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Thermal reactions of a silene derivative (Me3Si)(2)Si=C(OSiMe3)(t-Bu) (1) with silyl-substituted acetylenes, bis(trimethylsilyl)butadiyne, tert-butyldimethylsilylacetylene, and bis(trimethylsilyl)acetylene have been investigated with density functional theory calculations for the understanding of substituent effects in the reactivity of the silene. The first critical reaction step for determining the final product is the formation of a biradical intermediate from 1 and the acetylenes. A stepwise [2 + 2] cycloaddition via the biradical intermediate gives a silacyclobutene derivative, which is the precursor of the final product. The activation energy for this reaction step as well as thermodynamic stability of the biradical intermediate is governed by an interplay between geometric and electronic factors. The biradical intermediate is destabilized by steric hindrance between bulky substituents on 1 and acetylene, while it can be stabilized by delocalization of an unpaired electron in the acetylene moiety. Silacyclobutene derivatives undergo Si-C bond cleavage to give silabutadiene derivatives. The second critical reaction step is the attack of the OSiMe3 group on the silene Si atom in the silabutadienes. If the substituent on one of the acetylene C atoms does not easily migrate, the SiMe3 group of the OSiMe3 group rebounds to the silene C atom to form an oxasilacyclopentene derivative. If this substituent migrates easily, on the other hand, it migrates to the silene C atom with a simultaneous migration of the OSiMe3 group, resulting in the formation of an allene derivative.

DOI： 10.1021/om300310g

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A light change: A linear cyanido-bridged Fe 2Co compound (see picture) exhibits a reversible, thermally induced cooperative charge transfer transition accompanying spin transition and polar-nonpolar transformation in the trinuclear cluster. The change in magnetic properties and polarity could also be induced by irradiation with light.

DOI： 10.1002/anie.201201305

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A heterodinuclear complex based on a Ru-II-TPA [TPA = tris(2-pyridylmethyl)amine] complex having a peripheral Cu-II (bpy)(2) (bpy = 2,2'-bipyridine) group bonded through an amide linkage displayed reversible intramolecular electron transfer between the Ru and Cu complex units that can be controlled by protonation and deprotonation of the bridging amide moiety.

DOI： 10.1021/ja208141b

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Corrigendum: Mechanistic Insights into Photochromic Behavior of a Ruthenium(II)-Pterin Complex

DOI： 10.1002/chem.201102220

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/chem.201102220

Language：English   Publishing type：Research paper (other academic)  

The exact structure of the active site of intermediate Q, the methane-oxidizing species of soluble methane monooxygenase (sMMO), and the reaction mechanism of Q with methane molecule are still not fully clear. To gain further insights into the structure and reaction mechanism, we study different structure models of Q, including several new models not reported earlier, by performing broken-symmetry density functional theory calculations. Different reaction pathways of methane activated by Q with different structure models, including the radical and non-radical mechanism, are explored to evaluate which structure model is the most possible intermediate and the favorable catalytic mechanism. We expect our results to provide new insights into the catalytic oxidation of methane by sMMO and to promote the design of new catalysts.

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The catalytic conversion of 1,2-cyclohexanediol to adipic anhydride by (RuO)-O-IV(tpa) (tpa tris(2-pyridylmethyl)amine) is discussed using density functional theory calculations. The whole reaction is divided into three steps: (1) formation of a-hydroxy cyclohexanone by dehydrogenation of cyclohexanediol, (2) formation of 1,2-cyclohexanedione by dehydrogenation of alpha-hydroxy cyclohexanone, and (3) formation of adipic anhydride by oxygenation of cyclohexanedione. In each step the two-electron oxidation is performed by (RuO)-O-IV(tpa) active species, which is reduced to bis-aqua Ru-II(tpa) complex. The Ru-II complex is reactivated using Ce(IV) and water as an oxygen source. There are two different pathways of the first two steps of the conversion depending on whether the direct H-atom abstraction occurs on a C H bond or on its adjacent oxygen O-H. In the first step, the C-H (O-H) bond dissociation occurs in TS1 (TS2-1) with an activation barrier of 21.4 (21.6) kcal/mol, which is followed by abstraction of another hydrogen pathways. The second process also bifurcates into two reaction pathways. TS3 (TS4 1) is leading to dissociation of the C-H (O-H) bond, and the activation barrier of TS3 (TS4-1) is 20.2 (20.7) kcal/mol. In the third step, oxo ligand attack on the carbonyl carbon and hydrogen migration from the water ligand occur via TS5 with an activation barrier of 17.4 kcal/mol leading to a stable tetrahedral intermediate in a triplet state. However, the slightly higher energy singlet state of this tetrahedral intermediate is unstable; therefore, a spin crossover spontaneously transforms the tetrahedral intermediate into a dione complex by a hydrogen rebound and a C C bond cleavage. Kinetic isotope effects (k(H)/k(D)) for the electronic processes of the C H bond dissociations calculated to be 4.9-7.4 at 300 K are in good agreement with experiment values of 2.8-9.0.

DOI： 10.1021/ic200481n

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The pterin-coordinated ruthenium complex, [Ru-II(dmdmp)-(tpa)](+) (1) (Hdmdmp=N, N-dimethyl-6,7-dimethylpterin, tpa=tris(2-pyridyl-methyl)amine), undergoes photochromic isomerization efficiently. The isomeric complex (2) was fully characterized to reveal an apparent 1808 pseudorotation of the pterin ligand. Photoirradiation to the solution of 1 in acetone with incident light at 460 nm resulted in dissociation of one pyridylmethyl arm of the tpa ligand from the RuII center to give an intermediate complex, [Ru(dmdmp)(tpa)-(acetone)](2+) (I), accompanied by structural change and the coordination of a solvent molecule to occupy the vacant site. The quantum yield (phi) of this photoreaction was determined to be 0.87%. The subsequent thermal process from intermediate I affords an isomeric complex 2, as a result of the rotation of the dmdmp(2-) ligand and the recoordination of the pyridyl group through structural change. The thermal process obeyed first-order kinetics, and the rate constant at 298 K was determined to be 5.83 x 10(-5) s(-1). The activation parameters were determined to be Delta H-not equal = 81.8 kJmol(-1) and Delta S-not equal = -49.8 Jmol K-1 (1). The negative Delta S-not equal value indicates that this reaction involves a seven-coordinate complex in the transition state (i.e., an interchange associative mechanism). The most unique point of this reaction is that the recoordination of the photodissociated pyridylmethyl group occurs only from the direction to give isomer 2, without going back to starting complex 1, and thus the reaction proceeds with 100% conversion efficiency. Upon heating a solution of 2 in acetonitrile, isomer 2 turned back into starting complex 1. The backward reaction is highly dependent on the solvent: isomer 2 is quite stable and hard to return to 1 in acetone; however, 2 was converted to 1 smoothly by heating in acetonitrile. The activation parameters for the first-order process in acetonitrile were determined to be Delta H-not equal = 59.2 kJmol(-1) and Delta S not equal = -147.4 kJmol(-1)K(-1). The largely negative Delta S-not equal value suggests the involvement of a seven-coordinate species with the strongly coordinated acetonitrile molecule in the transition state. Thus, the strength of the coordination of the solvent molecule to the Ru-II center is a determinant factor in the photoisomerization of the Ru-II-pterin complex.

DOI： 10.1002/chem.201003522

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Density functional theory calculations have been performed for a proposal of possible mechanisms of the thermal reaction of an acylsilane, pivaloyltris-(trimethylsilyl)silane, and bis(trimethylsilyl)acetylene yielding a silacyclopropene, 1-[(tert-butyl)bis(trimethylsilyl)ethyl]-1-trimethylsiloxy-2,3-bis(trimethylsilyl)-1-silacycloprop-2-ene. Two reaction pathways in which two different silyl species play a role were considered based on analogous reactions of acylsilane and alkyne: (i) A silene (Si=C) intermediate derived from the acylsilane reacts with the acetylene to yield a silacyclobutene intermediate as a result of a stepwise [2 + 2] cycloaddition, and a ring-opening reaction of the silacyclobutene triggers the formation of the silacydopropene. (ii) The silene intermediate is rearranged to a silylene intermediate, and then [2 + 1] cycloaddition of the acetylene and the silylene gives the silacyclopropene. The high activation energy calculated for the [2 + 2] cycloaddition indicates that the silene would not react with the acetylene, which is consistent with the experimental result that no silacyclobutene intermediate was observed. On the other hand, the second reaction pathway involving the silene-to-silylene rearrangement and the [2 + 1] cycloaddition is more realistic from thermodynamic and kinetic points of view. All the calculated results strongly suggest that the silyl species reacting with bis(trimethylsilyl)acetylene is not silene but silylene.

DOI： 10.1021/om2002393

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Valence tautomerism is studied in the [CoII-HS(sq)(2)(bpy)]/[CoIII-LS(sq)(cat)(bp-y)] mononuclear cobalt complex by using DFT methods (HS, high spin; LS, low spin; cat, catecholate; sq, semiquinone; bpy, 2,2'-bipyridine). Calculations at the B3LYP* level of theory reproduce well the energy gap between the CoII-HS and CoIII-LS forms giving an energy gap of 4.4 kcal/mol, which is comparable to the experimental value of 8.9 kcal/mol. Potential energy surfaces and crossing seams of the electronic states of the doublet, quartet, and sextet spin states are calculated along minimum energy paths connecting the energy minima corresponding to the different spin states. The calculated minimum energy crossing points (MECPs) are located at 8.8 kcal/mol in the doublet/sextet surfaces, at 10.2 kcal/mol in the doublet/quartet surfaces, and at 8.4 kcal/mol in the quartet/sextet surfaces relative to the doublet ground state. Considering the energy of the three spin states and the crossing points, the one-step relaxation mechanism between the CoII-HS and CoIII-LS forms is the most probable. This research shows that mapping MECPs can be a useful strategy to analyze the potential energy surfaces of systems with complex deformation modes.

DOI： 10.1021/jp107391x

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Spin doesn't matter: A ruthenium(II)-aqua complex bearing a pentadentate pyridylamine with a carboxylate group as a ligand affords a seven-coordinate low-spin (S=0) ruthenium(IV)-oxo complex (see structure) by oxidation through proton-coupled electron transfer. Comparison of the reactivity of the low-spin and an intermediate-spin (S=1) RuIV-oxo complexes revealed that the spin state does not affect the reactivity of catalytic oxidation of organic compounds.

DOI： 10.1002/anie.201002733

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The spin transition between the low-spin singlet state and the high-spin quintet state in the [Fe(2-pic)(3)](2+) (2-pic: 2-picolylamine) complex is studied by using density functional theory (DFT) calculations. After careful comparison of density functionals BLYP, B3LYP, and B3LYP* (which has 15% Hartree-Fock exchange compared with 20% for B3LYP), we concluded that the spin-state splitting can be accurately reproduced by using the B3LYP* functional. The potential energy surfaces along minimum energy pathways of the three spin states were calculated at the B3LYP*/6-311+G** level of theory to find minimum energy crossing points (MECPs). The MECPs between the singlet and quintet states (SQ(M)) were found (E-SQ = 6.8 kcal/mol), as well as the MECPs between the triplet and singlet states (STM, E-ST = 12.9 kcal/mol) and the triplet and quintet states (TQ(M), E-TQ = 12.8 kcal/mol). Although the distortion leading to SQ(M) from the singlet equilibrium geometry is mainly a symmetric expansion of the Fe-N bonds, the distortions leading to STM and SQ(M) are asymmetric. Normal mode analysis demonstrates that these geometrical distortions contain a combination of several low-frequency normal modes, and therefore, these modes play a significant role in the intersystem crossing via the crossing seam.

DOI： 10.1021/jp9122002

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A novel tris(2-pyridylmethyl)amine (TPA) derivate having two catechol moieties linked by amide linkages at the 6-positions of two pyridyl groups was synthesized. The ligand, N,N-bis[6-{3,4-(dihydroxy)benzamide}-2-pyridyl-methyl]-N-(2-pyridylmethyl)amine (Cat(2)-TPA; L2), and its precursor, N,N-bis[6-{3,4-bis(benzyloxy)-benzamide}-2-pyridylmethyl]-N-(2-pyridylmethyl)-amine ((Bn(2)Cat)(2)-TPA; L1), formed stable ruthenium(II) complexes, [RuCl(L2)]PF(6) (2) and [RuCl(L1)]PF(6) (1), respectively. The crystal structure of [RuCl(L2)]Cl (2') was determined by X-ray crystallography to show two isomers in terms of the orientation of one catechol moiety. In complex 2, the ligand bearing catechols acts as a pentadentate ligand involving coordination of one of the amide oxygen atoms in addition to that of the tetradentate TPA moiety and two metal-free catechol moieties as metal-binding sites. The coordination of L2 results in the preorganization of the two catechols to converge them to undergo intramolecular pi-pi interactions. The (1)H NMR spectrum of 2 in DMSO-d(6) revealed that only one isomer was present in the solution. This selective formation could be ascribed to the formation of an intramolecular hydrogen-bonding network among the hydroxyl groups of the catechol moieties, as suggested by X-ray analysis. This intramolecular hydrogen bonding could differentiate the pK(a) values of the hydroxy groups of the catechol moieties into three kinds, as indicated by spectroscopic titration with tetramethylammonium hydroxide (TMAOH) in DMF. The complexation of 2 with other metal ions was also examined. The reaction of 2 with [Cu(NO(3))(2)(TMEDA)] (TMEDA = N,N,N',N'-tetramethylethylenediamine) in methanol allowed us to observe the selective formation of a binuclear complex, [RuCl(L2(2-)){Cu(TMEDA)}]PF(6) (3), which was characterized by ESI-MS, UV-vis, and ESR spectroscopies. Its ESR spectrum in methanol suggested that the coordination of the Cu(II)-TMEDA unit to the converged catechol moieties would be different from conventional kappa(2)-O,O':eta(2)-coordination: it exhibits a novel bridging coordination mode, bis-kappa(1)-O:eta(1)-coordination, to form the binuclear Ru(II)-Cu(II) complex.

DOI： 10.1021/ic902070q

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The molecular mechanism for H-2 production and H2O decomposition on an aromatic Ru(II) PNN complex developed by Milstein and co-workers has been elucidated by detailed density functional theory calculations. The rate-determining step is heterolytic coupling of the hydride with a proton transferred from the PNN ligand, which leads to the formation of H-2. The metal center and the PNN ligand, which can be dearomatized and aromatized again, play active and synergistic roles in H-2 production and the succeeding H2O decomposition. Formation of the cis-dihydroxo complex as the main final product is a result of thermodynamic control.

DOI： 10.1021/ja905073s

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DFT Study on N-2 Activation by a Hydride-Bridged Diniobium Complex. N N Bond Cleavage Accompanied by H-2 Evolution
Density functional theory (DFT) calculations have been performed for the investigation of a plausible mechanism of the triple bond cleavage of N-2 in a diniobium complex supported by tridentate aryloxide ligands, {Nb-V(mu-N)(2)Nb-V}(2-22) With the assumption of a tetrakis(mu-hydrido)diniobium complex {Nb-IV(mu-H)(4)Nb-IV}(2-) as an initial complex, the N N cleavage on the Nb-2 core proceeds in four steps. Dinitrogen is coordinated to the {Nb-III(mu-H)(2)Nb-III} core in a side-on/end-on manner, accompanied by the reductive elimination of H-2. The N N bond of dinitrogen is activated up to a single bond (formally N-2(4-)) by the two Nb(III) atoms, once it is bound to the Nb-2 core. Two electrons are prepared for the cleavage of the N-N single bond through the mu-H migration to an N atom, leading to the formation of an Nb-Nb bond. The N-N bond is then dissociated by the two electrons that are shared between the two Nb atoms. Finally, {Nb(mu-N)(2)Nb}(2-) is generated after H-2 elimination in which the N-bonded H atom is coupled with the remaining mu-H atom. The final H-2 elimination is calculated to be the rate-determining step.

DOI： 10.1021/ic802377p

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Nickel(II), Palladium(II), and Platinum(II) eta(3)-Allyl Complexes Bearing a Bidentate Titanium(IV) Phosphinoamide Ligand: A Ti <- M-2 Dative Bond Enhances the Electrophilicity of the pi-Allyl Moiety
Three Ti-M-2 (M-2=Ni, Pd, Pt) heterobimetallic complexes, [(eta(3-)methallyl) M-2((Ph2PNBu)-Bu-t)(2)TiCl2](OTF), were synthesized in which a Ti-IV <- M-2 interaction was suggested by crystallography and DFT calculations. The Ti-IV <- M-2 interaction enhanced electrophilicity of the eta(3)-methallyl ligand of M-2, leading to high reactivity of the eta(3)-methallyl moiety with Et2NH compared with that of the dppp analogue.

DOI： 10.1021/om8011085

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Comparison of the Reactivity of Bis(mu-oxo)(CuCuIII)-Cu-II and (CuCuIII)-Cu-III Species to Methane
Methane hydroxylation at the Binuclear copper site of particulate methane monooxygenase (AMMO) is studied by using density functional theory (DFT) calculations. The electronic and structural properties of the dinuclear copper species of bis(mu-oxo)(CuCuIII)-Cu-II and (CuCuIII)-Cu-III are discussed with respect to the C-H bond activation of methane. The bis(mu-oxo)(CuCuIII)-Cu-II species is highly reactive and considered to be an active species for the conversion of methane to methanol by pMMO, whereas the bis(mu-oxo)(CuCuIII)-Cu-III species is unable to react with methane as it is. If a Cu-O bond of the bis(mu-oxo)(CuCuIII)-Cu-III species is cleaved, the resultant (CuCuIII)-Cu-III species, in which only one oxo ligand bridges the two copper ions, can activate methane. However, its energetics for methane hydroxylation is less favorable than that by the bis(mu-oxo)(CuCuIII)-Cu-II species. The DFT calculations show that the bis(mu-oxo)(CuCuIII)-Cu-II species is more effective for the activation of methane than the bis(mu-oxo)(CuCuIII)-Cu-III species. The reactive bis(mu-oxo)(CuCuIII)-Cu-II species can be created either from the electron injection to the bis(mu-oxo)(CuCuIII)-Cu-III species or from the O-O bond cleavage in the mu-eta(1):eta(2)-peroxoCu(I)Cu(II) species.

DOI： 10.1021/ic8003933

Language：English   Publishing type：Research paper (scientific journal)  

Tyrosinase catalyzes the biological conversion of tyrosine to dopaquinone with dioxygen at the dinuclear copper active site under physiological conditions. On the basis of the recent X-ray crystal structural analysis of tyrosinase (J BioL Chem. 2006, 281, 8981), a possible mechanism for the catalytic cycle of tyrosinase is proposed by using quantum mechanical/molecular mechanical calculations, which can reasonably take effects of surrounding amino-acid residues, hydrogen bonding, and protein environment into account. The (mu-eta(2):eta(2)-peroxo)dicopper(II) species plays a role in a series of elementary processes mediated by the dicopper species of tyrosinase. A stable phenoxyl radical is involved in the reaction pathway. The catalysis has five steps of proton transfer from the phenolic O-H bond to the dioxygen moiety, O-O bond dissociation of the hydroperoxo species, C-O bond formation at an ortho position of the benzene ring, proton abstraction and migration mediated by His54, and quinone formation. The energy profile of the calculated reaction pathway is reasonable in energy as biological reactions that occur under physiological conditions. Detailed analyses of the energy profile demonstrate that the O-O bond dissociation is the rate-determining step. The activation energy for the O-O bond dissociation at the dicopper site is computed to be 14.9 kcal/mol, which is in good agreement with a measured kinetic constant. As proposed recently, the His54 residue, which is flexible because it is located in a loop structure in the protein, would play a role as a general base in the proton abstraction and migration in the final stages of the reaction to produce dopaquinone.

DOI： 10.1021/ja802618s

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(Chemical Equation Presented) Water is not only the solvent but also the sole oxygen source in the smooth and efficient oxidation of organic compounds catalyzed by a RuII-pyridylamine-aqua complex with CeIV as the oxidant. An intermediate-spin RuIV-oxo complex is formed as the reactive species (see scheme; Sub = substrate). This catalytic system is durable and able to gain high turnover numbers for various substrates.

DOI： 10.1002/anie.200801170

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Tris(2-pyridylmethyl)amine (TPA) derivatives with one or two ferrocenoylamide moieties at the 6-position of one or two pyridine rings of TPA were synthesized. The compounds, N-(6-ferrocenoylamide-2-pyridylmethyl)-NN-bis(2-pyridylmethyl)amine (Fc-.TPA; L1) and N,N-bis(6-ferrocenoylamide-2-pyridylmethyl)-N-(2-pyridylmethyl)amine (Fc(2)-TPA; L2), were characterized by spectroscopic methods, cyclic voltammetry, and X-ray crystallography. Their Ru(II) complexes were also prepared and characterized by spectroscopic methods, cyclic voltammetry, and X-ray crystallography. [RuCl(L1)(DMSO)]PF6 (1) that contains S-bound climethyl sulf oxide, (DMSO) as a ligand and an uncoordinated ferrocenoylamide moiety exhibited two redox waves at 0.23 and 0.77 V (vs ferrocene/ferrocenium ion as 0 V), due to Fc/Fc(+) and Ru(II)/Ru(III) redox couples, respectively. [RuCl(L2)]PF6 (2) that contains both coordinated and uncoordinated amide moieties showed two redox waves that were observed at 0.27 V (two electrons) and 0.46 V (one electron), assignable to Ru(II)/Ru(III) redox couples overlapped with the uncoordinated Fc/Fc(+) redox couple and the coordinated Fc/Fc+, respectively. In contrast to 2, an acetonitrile complex, [Ru(L2)(CH3CN)](PF6)(2) (3), exhibited three redox couples at 0.26 and 0.37 V for two kinds of Fc/Fc(+) couples, and 0.83 V for the Ru(II)/Ru(III) couple (vs ferrocene/ferrocenium ion as 0 V). In this complex, the redox potentials of the coordinated and the uncoordinated Fc-amide moieties were discriminated in the range of 0.11 V. Chemical two-electron oxidation of 1 gave [(RuCl)-Cl-III(L1(+))(DMSO)](3+) to generate a ferromagnetically coupled triplet state (S = 1) with J = 13.7 cm(-1) (H = -JS(1)S(2)) which was estimated by its variable-temperature electron spin resonance (ESR) spectra in CH3CN. The electron spins at the Ru(III) center and the Fe(III) center are ferromagnetically coupled via an amide linkage. In the case of 2, its two-electron oxidation gave the same ESR spectrum, which indicates formation of a similar triplet state. Such electronic communication may occur via the amide linkage forming the intramolecular hydrogen bonding.

DOI： 10.1021/ic7016038

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A computational approach is taken to clarify the reaction mechanism of biotin carboxylase (BC) by using the B3LYP density functional method. The overall reaction of BC is supposed to consist of two steps: in the first step, carboxyphosphate (CP) is generated from bicarbonate and ATP, and it is subject to nucleophilic attack on its carboxyl group by biotin to form carboxybiotin in the second step. The activation energies for the transition states of the first and second steps are computed to be 46.6 and 7.9 kcal/mol, respectively, demonstrating that the first step limits the overall reaction of BC. In the second step, the ureido moiety of biotin undergoes enolization with the aid of general acid-base catalysis by CID, followed by collapse of CP into CO2 and phosphate. The resulting bent CO2 is highly labile and condenses quickly with enolic biotin to give carboxybiotin. Implicit in this scheme as they are, ingenious proton movements between the two substrates, CP and biotin, dictate all of the succeeding chemical events.

DOI： 10.1021/ct700260f

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The electronic and structural properties of the cubane-type mixed-metal sulfido clusters with a FeIr3S4 core and possible dinitrogen binding and activation were analyzed by density functional theory (DFT) calculations. Five different charges of the cluster and manifold of the electronic states were investigated. For each charge under consideration (+2, +1, 0, -1, -2) systematic analysis of structural and electronic properties was carried out. The DFT calculations show that both Fe - N and N N bond lengths correlate with the total charge of the cluster. The length of the Fe - N bond decreased, whereas the N N bond length increased with the number of added electrons. However, only noticeable elongation of the N N bond was observed when the charge of the cluster became negative. Similar analysis was extended to species that have protonated dinitrogen bond. The results obtained from the DFT analysis are useful in considering the principles of the reduction of dinitrogen to ammonia at a single metal center.

DOI： 10.1246/bcsj.80.2323

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Methane hydroxylation at the mononuclear and dinuclear copper sites of pMMO is discussed using quantum mechanical and QM/MM calculations. Possible mechanisms are proposed with respect to the formation of reactive copper-oxo and how they activate methane. Dioxygen is incorporated into the Cu(I) species to give a Cu(II)- superoxo species, followed by an H-atom transfer from a tyrosine residue near the monocopper active site. A resultant CuII- hydroperoxo species is next transformed into a Cu(III)-oxo species and a water molecule by the abstraction of an H-atom from another tyrosine residue. This process is accessible in energy under physiological conditions. Dioxygen is also incorporated into the dicopper site to form a (mu-eta(2):eta(2)-peroxo)dicopper species, which is then transformed into a bis(mu-oxo) dicopper species. The formation of this species is more favorable in energy than that of the monocopper- oxo species. The reactivity of the Cu(III)-oxo species is sufficient for the conversion of methane to methanol if it is formed in the protein environment. Since the sigma* orbital localized in the Cu-O bond region is singly occupied in the triplet state, this orbital plays a role in the homolytic cleavage of a C-H bond of methane. The reactivity of the bis(mu-oxo) dicopper species is also sufficient for the conversion of methane to methanol. The mixed-valent bis(mu-oxo)Cu(II)Cu(III) species is reactive to methane because the amplitude of the sigma* singly occupied MO localized on the bridging oxo moieties plays an essential role in C-H activation.

DOI： 10.1021/ja061604r

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The direct conversion of benzene to phenol over Fe-ZSM-5 zeolite is investigated using quantum mechanical/molecular mechanical (QM/MM) calculations on a large model consisting of 683 SiO2 units ( 2084 atoms). The active-site model for benzene hydroxylation involves a mononuclear iron-oxo species (Fe-III=O) located at the ion-exchangeable Al site of ZSM-5 zeolite. The proposed catalytic cycle is partitioned into four steps: (i) H atom abstraction, (ii) O atom insertion, (iii) phenyl migration, and (iv) phenol release. The decomposition of nitrous oxide plays an important role in the formation of the iron-oxo species and in the avoidance of the unstable Fe-I state at the active site. The catalytic reaction occurs on the sextet and quartet potential energy surfaces. The quartet state plays a major role in the course of the reaction, whereas the sextet state lies lower in energy in the entrance channel and in the final stages of the reaction. The QM/MM calculations show that the nanopores of the zeolite framework would decrease the activation barriers in the transition states involved in the reaction pathway and accelerate the exchange of phenol and benzene in the final stages of the reaction. The environmental effect from the zeolite framework promotes the conversion of benzene to phenol.

DOI： 10.1021/om0509591

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Catalytic mechanism of dopamine, beta-monooxygenase mediated by Cu(III)-Oxo
Mechanisms of dopamine hydroxylation by the Cu(II)-superoxo species and the Cu(III)-oxo species of dopamine beta-monooxygenase (DBM) are discussed using QM/MM calculations for a whole-enzyme model of 4700 atoms. A calculated activation barrier for the hydrogen-atom abstraction by the Cu(II)-superoxo species is 23.1 kcal/mol, while that of the Cu(III)-oxo, which can be viewed as Cu(II)-O-center dot, is 5.4 kcal/mol. Energies of the optimized radical intermediate in the superoxo- and oxo-mediated pathways are 18.4 and -14.2 kcal/mol, relative to the corresponding reactant complexes, respectively. These results demonstrate that the Cu(Ill)-oxo species can better mediate dopamine hydroxylation in the protein environment of DBM. The side chains of three amino acid residues (His415, His417, and Met490) coordinate to the Cu-B atom, one of the copper sites in the catalytic core that plays a role for the catalytic function. The hydrogen-bonding network between dopamine and the three amino acid residues (Glu268, Glu369, and Tyr494) plays an essential role in substrate binding and the stereospecific hydroxylation of dopamine to norepinephrine. The dopamine hydroxylation by the Cu(Ill)-oxo species is a downhill and lower-barrier process toward the product direction with the aid of the protein environment of DBM. This enzyme is likely to use the high reactivity of the Cu(Ill)-oxo species to activate the benzylic C-H bond of dopamine; the enzymatic reaction can be explained by the so-called oxygen rebound mechanism.

DOI： 10.1021/ic0521168

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The electronic properties of cubane-type mixed-metal sulfido clusters with an Mo(V)(2)Ru(II)(2)S-4 core and possible dinitrogen activation on it are described from DFT computations. The cluster [(CpRu)(2){MoCl2(MeCN)}(2)(mu(3)-S)(4)] (Cp = eta(5)-C5H5), whose Cp* analogue (Cp* = eta(5)-C5Me5) was isolated and fully characterized by X-ray crystallographic analysis, has a closed-shell singlet ground state. Upon release of the acetonitrile ligands, the ground state remains in a closed-shell singlet state, the corresponding open-shell singlet and triplet states lying 2 kcal mol(-1) above the closed-shell singlet state. Dinitrogen can hardly coordinate to this neutral [(CpRu)(2)(MoCl2)(2)(mu(3)-S)(4)] cluster. However, upon reduction by two electrons the binding energy for two N-2 molecules to the resultant anionic cluster [(CpRu)(2){MoCl2(N-2)}(2)(mu(3)-S)(4)](2-) increases to 4 kcal mol(-1). This interaction, which still seems too weak to become isolable, is analyzed in terms of molecular orbitals to increase our understanding of dinitrogen activation by synthetic systems. The HOMO of [(CpRu)(2)(MoCl2)(2)(mu(3)-S)(4)](2-) is pointing toward missing ligands and can interact effectively with the pi(g)* orbitals of dinitrogen. This orbital might play a role in the binding of dinitrogen.

DOI： 10.1246/bcsj.79.53

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Ruthenium(II)-acetonitrile complexes having pentadentate tris(2-pyridylmethyl)amine (tpa) derivatives with coordinated amide CO groups were prepared and characterized by various spectroscopic methods, electrochemical measurements, and DFT calculations. The acetonitrile ligand was indicated to tightly bind to the ruthenium(II) center in an eta(1)-N fashion. The Mulliken charge distribution, obtained by a calculation, indicated that the ruthenium(II)-nitrile bond is more covalent than other coordination bonds of the tpa moiety. The redox potentials of Ru centers and the chemical shifts of methyl groups of the acetonitrile ligands exhibited a linear relationship, indicating that electron density of the Ru center controls that of the acetonitrile ligand. Those complexes showed fluxional behavior in CD3CN solutions to exhibit one mode of thermal motion; it also altered the symmetry of the complexes.

DOI： 10.1246/bcsj.78.2152

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Reaction pathways and energetics for the conversion of benzene to phenol by FeO+ in the gas phase are discussed using density functional theory calculations at the B3LYP/6-311+G** level of theory. Three reaction pathways are available for this reaction. The first one is a nonradical mechanism to form a hydroxo intermediate, HO-Fe+-C6H5 via H atom abstraction with a four-centered transition state, which occurs at a coordinatively unsaturated metal center. The second one is a radical mechanism to form a phenyl radical and an FeOH fragment as an intermediate via H-atom abstraction with a linear C-H-O transition state. The third one is an oxygen-insertion mechanism to form an arenium intermediate via electrophilic aromatic addition. The energies of the transition states with respect to H-atom abstraction (relative to the dissociation limit) increase in the order of the first, third, and second mechanisms. A detailed analysis of the potential energy surfaces shows that the first mechanism is most likely to occur when the metal active site is coordinatively unsaturated. The second mechanism is energetically unlikely. The third pathway is branched into cyclohexadienone and benzene oxide, which are formed by a 1,2-hydrogen migration find a ring closure in the arenium intermediate, respectively. Cyclohexadienone can play a role as an intermediate when the metal active site is coordinatively saturated, whereas the formation of benzene oxide is unlikely to occur under ambient conditions because of its extremely high energy.

DOI： 10.1021/om050136b

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Computational exploration of the catalytic mechanism of dopamine beta-monooxygenase: Modeling of its mononuclear copper active sites
Dopamine hydroxylation by the copper-superoxo, -hydroperoxo, and -oxo species of dopamine P-monooxygenase (DBM) is investigated using theoretical calculations to identify the active species in its reaction and to reveal the key functions of the surrounding amino acid residues in substrate binding. A 3D model of rat DBM is constructed by homology modeling using the crystal structure of peptidylglycine alpha-hydroxylating monooxygenase (PHM) with a high sequence identity of 30% as a template. In the constructed 3D model, the CuA site in domain 1 is coordinated by three histidine residues, His265, His266, and His336, while the CuB site in domain 2 is coordinated by two histidine residues, His415 and His417, and by a methionine residue, Met490. The three Glu268, Glu369, and Tyr494 residues are suggested to play an important role in the substrate binding at the active site of DBM to enable the stereospecific hydrogen-atom abstraction. Quantum mechanical/molecular mechanical (OM/MM) calculations are performed to determine the structure of the copper-superoxo, -hydroperoxo, and -oxo species in the whole-enzyme model with about 4700 atoms. The reactivity of the three oxidants is evaluated in terms of density-functional-theory calculations with small models extracted from the OM region of the whole-enzyme model.

DOI： 10.1021/ic048477p

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The reaction pathway and energetics for the hydroxylation of L-phenylalanine to L-tyrosine are analyzed by a combined quantum mechanical/molecular mechanical (QM/MM) method and a conventional QM method. A reasonable model for the oxygen species responsible for the catalytic reactivity of phenylalanine hydroxylase (PAH) is proposed. The active site model of PAH is a catalytic domain model (including 5176 atoms), which is a truncated form of PAH lacking the N-terminal regulatory and C-terminal tetramerization domains. This model can reasonably treat the protein environment via nonbonding interactions between the QM (iron active site and substrate) and MM (catalytic domain) regions. The QM region involves an iron-oxo species (Fe-IV= O), side-chain ligands of His285, His290, and Glu330, and two water molecules at the binding site of the catalytic active center. Possible reaction pathways are discussed. One is a reaction initiated by a C-H bond cleavage via an H-atom abstraction, followed by a C-O bond formation leading to an L-tyrosine complex. Another is a reaction initiated by an oxygen insertion via electrophilic aromatic addition, followed by the 1,2 hydride shift leading to the keto form (2,4-cyclohexadienone) of the L-tyrosine complex. The structures of the reactant complex, the radical intermediate, and the product complex with the catalytic domain are described. QM/MM calculations tell us that the substrate-binding site in PAH is located at Pro279 and Val379. As a result of QM calculations, the activation energies for the C-H cleavage step and the C-O bond formation in the first mechanism are 24.6 and 9.2 kcal/mol, respectively, while the activation energy for the 1,2 hydride shift in the second one is 9.0 kcal/mol relative to the arenium intermediate. Thus, the QM calculations suggest that the oxygen insertion mechanism is energetically more favorable.

DOI： 10.1021/jp048001r

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The reaction pathway and energetics for the conversion of silacyclobutene to cyclopropene, which plays a role as an intermediate in complicated thermal reactions of silacyclobutene, are discussed using B3LYP/6-31+G(d,p) level calculations. There are two reaction pathways in its thermal isomerization: (1) cyclopropene is formed via silabutadiene that is formed by the ring opening of silacyclobutene in a conrotatory process; (2) cyclopropene is directly formed by a 1,2-siloxyl shift in silacyclobutene. The first mechanism involves two-step processes. Two kinds of ring-opening reactions based on the Woodward-Hoffmann rule result in the formation of cis- and trans-silabutadienes that involve an Si=C double bond. Calculated activation barriers for the formation of cis- and trans-silabutadienes are 36.5 and 21.8 kcal/mol, respectively. Thus, the trans form is energetically more favorable than the cis form in this ring-opening step. This result is in good agreement with experimental observations that the methanol adduct derived from the trans form is detected in the presence of methanol. However, the next transition state with respect to a 1,4-siloxyl shift takes place only in the cis form, due to the orientation of the siloxyl group. A calculated activation barrier for the 1,4-siloxyl shift in cis-silabutadiene is 17.2 kcal/mol. Thus, the first step is the rate-determining step in this two-step mechanism. The second mechanism involves a 1,2-siloxyl shift that leads to the direct formation of cyclopropene. The activation barrier of this reaction is calculated to be 40.0 kcal/mol. IRC calculations are performed to trace this 1,2-siloxyl shift in the second mechanism. Since the activation barrier of the direct formation of cyclopropene is energetically comparable to that of the ring-opening reaction toward the cis-silabutadiene, the B3LYP DFT calculations suggest that both reaction pathways are likely to occur under thermal conditions.

DOI： 10.1021/om0497947

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The thermolysis of pivaloyl- and adamantoyltris(trimethylsilyl)silane (1a and 1b) with bis(trimethylsilyl)butadiyne and bis(dimethylphenylsilyl)butadiyne at 120degreesC gave regiospecifically 2-trimethylsiloxy-3-silylethynyl-1-silacyclobut-3-ene derivatives (2a, 2b and 3a, 3b), arising from the reaction of silenes generated thermally from 1a and 1b with the butadiynes. Similar treatment of 1a and 1b in the presence of bis(trimethylsilyl)butadiyne and bis(dimethylphenylsilyl)butadiyne at 160degreesC gave 2,5-dihydro-1,2-oxasilole derivatives (4a, 4b and 5a, 5b). When 2a and 2b were heated in the presence of methanol at 160degreesC, methanol adducts 6a and 6b were obtained, respectively, as the sole product. The ring-opening processes of 2a and the regiochemistry of [2 + 2] cycloaddition of the silene with bis(silyl)butadiyne and bis(silyl)but-1-en-3-yne are discussed on the basis of theoretical treatment. The thermolysis of 1a and 1b with bis(tert-butyldimethylsilyl)butadiyne at 160degreesC produced [2 + 2] cycloadducts (7a and 7b). With bis(tert-butyldimethylsilyl)butadiyne at 220degreesC, 1a and 1b gave 2,5-dihydro-1,2-oxasilole derivatives (8a and 8b). The reaction of 1a and 1b with (E)-1,4-bis(dimethylphenylsilyl)but-1-en-3-yne and (E)-1,4-bis(pentamethyldisilanyl)but-1-en-3-yne at 120degreesC produced regiospecifically 2-trimethylsiloxy-3-[trans-(silyl)ethenyl]-1-silacyclobut-3-ene derivatives (9a, 9b and 10a, 10b). At 200degreesC, similar treatment of 1a and 1b with silyl-substituted but-1-en-3-ynes afforded 2,5-dihydro-1,2-oxasilole derivatives (11a, 11b and 12a, 12b, respectively). UV-vis absorption and fluorescence spectra have been reported.

DOI： 10.1021/om034213j

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A mechanism of direct benzene hydroxylation over Fe-ZSM-5 zeolite is discussed from B3LYP density functional theory computations. We demonstrate using a mononuclear iron model supported at the AlO4- site of zeolite that Fe-ZSM-5 zeolite reasonably catalyzes the conversion of benzene to phenol with N2O. A key in this catalytic reaction is that benzene hydroxylation and N2O decomposition occur at the same active site in a well-balanced manner. In the initial stages of the reaction, a reactive oxygen species referred to as "alpha-oxygen" is formed upon decomposition of N2O at the iron active site. The first reaction step is the formation of a complex between benzene and the cc-oxygen species. The C-H activation of benzene leads to an intermediate that involves OH and C6H5 groups as ligands. After forming this intermediate, the reaction has a junction that leads to two possible pathways, depending on the concentration of N2O. The recombination between the OH and C6H5 ligands, which leads to a complex of phenol, can occur when the concentration of N2O is low, whereas the decomposition of nitrous oxide can occur on the intermediate involving OH and C6H5 groups when the N2O concentration is sufficiently high. Calculated potential energy diagrams and reaction kinetics show that the latter pathway is energetically more favorable than the former. We propose that benzene hydroxylation over Fe-ZSM-5 zeolite should proceed in the following way: (1) the formation of the benzene complex at the iron active site, (2) the hydrogen abstraction from benzene, (3) the decomposition of N2O at the active site, (4) the recombination between the OH and C6H5 ligands, and (5) the release of product phenol.

DOI： 10.1021/jp030240b

Language：English  

Language：English   Publishing type：Research paper (scientific journal)  

The conversion of adamantane to adamantanols mediated by ferrate (FeO42-), monoprotonated ferrate (HFeO4-), and diprotonated ferrate (H2FeO4) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the C-H bonds of adamantane via two reaction pathways, in which 1-adamantanol is formed by the abstraction of a tertiary hydrogen atom (3degrees) and 2-adamantanol by the abstraction of a secondary hydrogen atom (2degrees). Each reaction pathway is initiated by a C-H bond cleavage via an H-atom abstraction that leads to a radical intermediate, followed by a C-O bond formation via an oxygen rebound step to lead to an adamantanol complex. The activation energies for the C-H cleavage step are 6.9 kcal/mol in the I-adamantanol pathway and 8.4 kcal/mol in the 2-adamantanol pathway, respectively, at the B3LY-P/6-311++G** level of theory, whereas those of the second reaction step corresponding to the rebound step are relatively small. Thus, the rate-determining step in the two pathways is the C-H bond dissociation step, which is relevant to the regioselectivity for adamantane hydroxylation. The relative rate constant (3degrees)/(2degrees) for the competing H-atom abstraction reactions is calculated to be 9.30 at 75 degreesC, which is fully consistent with an experimental value of 10.1.

DOI： 10.1021/jo0207168

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Possible spin inversion processes in the direct conversion of methane to methanol by the bare FeO+ complex are discussed by means of spin-orbit coupling (SOC) calculations. This reaction proceeds via two transition states (TSs) in the following way; FeO++CH4-->FeO+(CH4)-->[TS1]-->HO-Fe+-CH3-->[TS2]-->Fe+(CH3OH)-->Fe++CH3OH. B3LYP density functional theory calculations show that the potential energies in the quartet and sextet states lie close and involve three crossing seams that can provide a chance of spin-forbidden transition. The spin-forbidden transition leads to a significant decrease in the barrier heights of TS1 and TS2 that correspond to the hydrogen atom abstraction and the methyl shift, respectively. To evaluate the spin-forbidden transition in the reaction pathway, the SOC matrix elements are calculated along the intrinsic reaction coordinate of the reaction. The SOC analysis along the IRC is useful to look at how the FeO+/CH4 reacting system changes its spin multiplicity between the sextet and quartet surfaces. The strength of the SOC between the low-lying quartet state and the sextet state is 133.6 cm(-1) in the reactant complex FeO+(CH4), 21.4 cm(-1) in the hydroxo intermediate HO-Fe+-CH3, and 0.3 cm(-1) in the product complex Fe+(CH3OH). Since the SOC value decreases along the oxidation process, the ease of spin inversion probability is the first crossing seam, the second crossing seam, and the third crossing seam, in this order. (C) 2003 American Institute of Physics.

DOI： 10.1063/1.1557192

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Ethylene epoxidation by an iron(III)-hydroperoxo model of cytochroine P450 Fe(OOH)(C20N4H12)(SCH3) is investigated with the B3LYP density functional theory (DFT) method to look at whether or not the iron(III)-hydroperoxo species can participate in olefin epoxidation with the iron(T)-oxo Species (Compound I). The answer is negative. There are two possible entrance channels in the epoxidation reaction, a proximal oxygen transfer mechanism and a distal oxygen transfer mechanism, both mechanisms being stepwise processes involving radical intermediates. In the proximal oxygen transfer mechanism, the homolytic cleavage of the Fe-O bond of the iron(III)-hydroperoxo species initially occurs with a change in the Fe atomic charge from +3 to +2. The proximal oxygen transfer mechanism involves an energetically less stable radical .CH2CH2(OOH). In contrast. in the initial stages of the distal oxygen transfer mechanism, the O-O bond of the hydroperoxo ligand is homolytically cleaned. and the iron(T)-oxo complex thus formed retains a weak interaction with the hydroxy group of the resultant radical intermediate involving .CH2CH2(OH). This mechanism is more realistic, but the activation energy of the transition state of the O-O bond cleavage is comparable to that of the corresponding transition state by hydrogen peroxide. The present result suggests that the mechanism and energetics of ethylene epoxidation remain unchanged by the involvement of the iron-porphyrin complex. The epoxidation of ethylene by a compound I model is also calculated to increase our understanding of olefin epoxidation by P450. The mechanism and energetics of olefin epoxidation by iron(III)-hydroperoxo species are less plausible than epoxidation by compound I.

DOI： 10.1246/bcsj.76.721

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A possible mechanism for the spin transitions in various stacking conformations of bis(phenylmethylenyl)-[2.2]paracyclophanes, which have close-lying lowest singlet, triplet, and quintet spin states, is theoretically investigated by using diphenylcarbene dimers as models. Spin-orbit coupling (SOC) matrix elements, which play an essential role in the spin transition phenomena, are calculated with the effective one-electron spin-orbit Hamiltonian. The SOC between the first excited singlet state and the first excited triplet state and that between the first excited triplet state and the lowest quintet state are strong. The SOC between the first excited quintet state and the first excited triplet state and that between the first excited triplet state and the lowest singlet state are also strong. These results demonstrate that the spin conversion between the low-spin singlet state and the high-spin quintet state can occur via the first excited intermediate-spin triplet state. We propose that possible photoinduced spin-crossover phenomena can be observed in these organic molecular systems.

DOI： 10.1021/jp020984+

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Role of molecular distortions in the spin-orbit coupling between the singlet and triplet states of the 4 pi electron systems C4H4, C5H5+, and C3H3-
How molecular distortions enhance the strength of spin-orbit coupling (SOC) between the singlet and triplet states of cyclobutadiene, cyclopentadienyl cation, and cyclopropenyl anion is described. The crossing region of the two potential energy surfaces of cyclobutadiene is characterized by a Jahn-Teller active vibrational mode that can connect the singlet and triplet structures. The spin inversion from triplet to singlet occurs in cyclobutadiene with a structural change from D-4h to D-2h, but the in-plane distortion along the Jahn-Teller mode cannot directly enhance the strength of SOC. Molecular distortions along some C-H out-of-plane bending modes significantly strengthen the SOC in cyclobutadiene. Also in cyclopentadienyl cation, C-H out-of-plane distortions play an essential role in enhancing the strength of SOC. The out-of-plane motions destroy the planarity of cyclobutadiene and cyclopentadienyl cation, leading to rehybridization of their sigma and pi orbitals. This is a main reason that the strength of SOC is enhanced by the C-H out-of-plane bending distortions in these planar molecules. On the other hand, in cyclopropenyl anion the carbon-ring distortion that can connect the triplet and singlet structures is a main factor that dominates the transition between the two states, due to its nonplanarity. (C) 2001 American Institute of Physics.

DOI： 10.1063/1.1412250

Language：English   Publishing type：Research paper (scientific journal)  

Dynamic aspects of alkane hydroxylation mediated by Compound I of cytochrome P450 are discussed from classical trajectory calculations at the B3LYP level of density functional theory. The nuclei of the reacting system are propagated from a transition state to a reactant or product direction according to classical dynamics on a Born-Oppenheimer potential energy surface. Geometric and energetic changes in both low-spin doublet and high-spin quartet states are followed along the ethane to ethanol reaction pathway, which is partitioned into two chemical steps: the first is the H-atom abstraction from ethane by the iron-oxo species of Compound I and the second is the rebound step in which the resultant iron-hydroxo complex and the ethyl radical intermediate react to form the ethanol complex. Molecular vibrations of the C-H bond being dissociated and the O-H bond being formed are significantly activated before and after the transition state, respectively, in the H-atom abstraction. The principal reaction coordinate that can represent the first chemical step is the C-H distance or the O-H distance while other geometric parameters remain almost unchanged. The rebound process begins with the iron-hydroxo complex and the ethyl radical intermediate and ends with the formation of the ethanol complex, the essential process in this reaction being the formation of the C-O bond. The H-O-Fe-C dihedral angle corresponds to the principal reaction coordinate for the rebound step, When sufficient kinetic energy is supplied to this rotational mode, the rebound process should efficiently take place. Trajectory calculations suggest that about 200 fs is required for the rebound process under specific initial conditions, in which a small amount of kinetic energy (0.1 kcal/mol) is supplied to the transition state exactly along the reaction coordinate. An important issue about which normal mode of vibration is activated during the hydroxylation reaction is investigated in detail from trajectory calculations. A large part of the kinetic energy is distributed to the C-H and O-H stretching modes before and after the transition state for the H-atom abstraction, respectively, and a small part of the kinetic energy is distributed to the Fe-O and Fe-S stretching modes and some characteristic modes of the porphyrin ring. The porphyrin marker modes Of nu (3) and nu (4) that explicitly involve Fe-N stretching motion are effectively enhanced in the hydroxylation reaction. These vibrational modes of the porphyrin ring can play an important role in the energy transfer during the enzymatic process.

DOI： 10.1021/ja010593t

Language：English  

Language：English   Publishing type：Research paper (scientific journal)  

The conversion of methanol to formaldehyde mediated by ferrate (FeO42-), monoprotonated ferrate (HFeO4-), and diprotonated ferrate (H2FeO4) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the O-H and C-H bonds of methanol via two entrance reaction channels: (1) an addition-elimination mechanism that involves coordination of methanol to diprotonated ferrate; (2) a direct abstraction mechanism that involves H atom abstraction from the O-H or C-H bond of methanol. Within the framework of the polarizable continuum model (PCM), the energetic profiles of these reaction mechanisms in aqueous solution are calculated and investigated. In the addition-elimination mechanism, the O-H and C-H bonds of ligating methanol are cleaved by an oxo or hydroxo ligand, and therefore the way to the formation of formaldehyde is branched into four reaction pathways. The most favorable reaction pathway in the addition-elimination mechanism is initiated by an O-H cleavage via a four-centered transition state that leads to intermediate containing an Fe-O bond, followed by a C-H cleavage via a five-centered transition state to lead to formaldehyde complex. In the direct abstraction mechanism, the oxidation reaction can be initiated by a direct H atom abstraction from either the O-H or C-H bond, and it is branched into three pathways for the formation of formaldehyde. The most favorable reaction pathway in the direct abstraction mechanism is initiated by C-H activation that leads to organometallic intermediate containing an Fe-C bond, followed by a concerted H atom transfer from the OH group of methanol to an oxo ligand of ferrate. The first steps in both mechanisms are all competitive in energy, but due to the significant energetical stability of the organometallic intermediate, the most likely initial reaction in methanol oxidation by ferrate is the direct C-H bond cleavage.

DOI： 10.1021/jo001193b

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.14827/jccj1999.13.169

Language：English   Publishing type：Research paper (scientific journal)  

Density-functional-theory calculational results on the reaction pathway for alkane hydroxylation by a compound I model of cytochrome P450 are discussed. Our calculations demonstrate that the transition state for the H-atom abstraction of ethane involves a linear (Fe)O . . .H . . .C array and that the resultant carbon radical is bound to the iron-hydroxo species. This comptational result is partly consistent with the oxygen rebound mechanism in that the direct H-atom abstraction by the iron-ore species takes place in the initial stages of the reaction pathway. However, the iron-hydroxo species cannot be viewed as a stable reaction intermediate in view of the energy diagram. Our results may not be consistent with the model of a free radical species with a finite lifetime and barrier to displacement of the OH group from the iron center that is commonly assumed and typically stated for the oxygen rebound mechanism.

DOI： 10.1246/bcsj.73.2669

Language：English   Publishing type：Research paper (scientific journal)  

The reaction pathway and energetics for methane-to-methanol conversion by first-row transition-metal oxide ions (MO(+)s) are discussed from density functional theory (DFT) B3LYP calculations, where M is Sc. Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. The methane-to-methanol conversion by these MO+ complexes is proposed to proceed in a two-step manner via two transition states: MO+ + CH4 --> OM+(CH4) --> [TS] --> OH-M+-CH3 --> [TS] --> M+(CH3OH) --> M+ + CH3OH. Both high-spin;and low-spin potential energy surfaces are characterized in detail. A crossing between the high-spin and the low-spin potential energy surfaces occurs once near the exit channel for ScO+, TiO+, VO+ CrO+, and MnO+, but it occurs twice in the entrance and exit channels for FeO+, CoO+, and NiO+. Our calculations strongly suggest that spin Inversion can occur near a crossing region of potential energy surfaces and that it can play a significant role in decreasing the barrier heights of these transition states. The reaction pathway from methane to methanol is uphill in energy on the early MO+ complexes (ScO+, TiO+, and VO+); thus, these complexes are not good mediators for the formation of methanol. On the other hand, the late MO+ complexes (FeO+, NiO+, and CuO+) are expected from the general energy profiles of the reaction pathways to efficiently convert methane to methanol, Measured reaction efficiencies and methanol branching ratios for MnO+, FeO+, CoO+, and NiO+ are rationalized from the energetics of the high-spin and the low-spin potential energy surfaces. The energy diagram for the methane-to-methanol conversion by CuO+ is downhill toward the product direction, and thus CuO+ is likely to be an excellent mediator for methane hydroxylation.

DOI： 10.1021/ja0017965

Language：English   Publishing type：Research paper (scientific journal)  

Kinetic isotope effects (KIEs) in the electronic process of the H-atom abstraction from substrate ethane by a compound I model of cytochrome P450 are discussed at the B3LYP level of density functional theory. Our calculations demonstrate that the transition State for the H-atom abstraction involves a linear (Fe)O . .H . . .C array and that, a resultant radical species with a spin density of nearly one is bound to an iron-hydroxo complex, followed by recombination and release of product ethanol. Although the reacting system involves this carbon radical species in the course of the hydroxylation, in view of the energy profile of the reaction pathway it cannot be viewed as a stable reaction intermediate with a finite lifetime. The KIEs calculated with transition state theory are significantly dependent on temperature and substituents, falling in a range of 7-13 at 300 K.

DOI： 10.1021/jp001950+

Language：English   Publishing type：Research paper (scientific journal)  

We present theoretical analyses for the conversion of methane to methanol on a diiron model of soluble methane monooxygenase (sMMO) and on dicopper models of particulate methane monooxygenase (pMMO) using the hybrid density-functional-theory B3LYP method. Methane is proposed to be reasonably converted into methanol in a two-step concerted manner on the dinuclear enzyme models. The first step in our proposal is concerted H atom abstraction of methane via a four-centered transition state (TS1) and the second step is concerted methyl migration via a three-centered transition state (TS2). The general features of the electronic process are identical to those of the gas-phase process for the methane-methanol conversion by the bare FeO+ complex. The concerted H atom abstraction and the direct H atom abstraction via a transition state with a linear C-H-O(Fe) array are compared using the dinuclear models. The transition state for the direct H atom abstraction (TSd) on the diiron model is found in the spin undecet state; however, that on the dicopper models is found in the doubler stare. Kinetic isotope effects (k(H)/k(D)) are calculated and analyzed for the concerted and the direct H atom abstraction mechanisms using the transition state theory. Calculated k(H)/k(D) values for the concerted process and the direct process are 9 and 14, respectively, at 300 K.

DOI： 10.1246/bcsj.73.815

Language：English   Publishing type：Research paper (scientific journal)  

Femtosecond dynamic behavior of the methane-methanol conversion by the bare iron-ore complex (FeO+) is presented using the B3LYP density-functional-theory (DFT) method. We propose that the reaction pathway for the direct methane-methanol conversion is partitioned into the H atom abstraction via a four-centered transition state and the methyl migration via a three-centered transition state. It is demonstrated that both the H atom abstraction and the methyl migration occur in a concerted manner in a time scale of 100 fs. The concerted H atom abstraction and the direct H atom abstraction via a transition state with a linear C-H-O(Fe) array are compared. The direct H atom abstraction of methane is predicted to occur in a time scale of 50 fs. Isotope effects on the concerted and the direct H(D) atom abstractions are also computed and analyzed in the FeO+/methane system. Predicted values of the kinetic isotope effect (k(H)/k(D)) for the H(D) atom abstraction of methane are 9 in the concerted mechanism and 16 in the direct abstraction mechanism at 300 K. Dynamics calculations are also carried out on the benzene-phenol conversion by the FeO+ complex. The general profile of the electronic process of the benzene-phenol conversion is identical to that of the methane-methanol conversion with respect to essential bonding characters. It is demonstrated that the concerted H atom abstraction and the phenyl migration require 200 and 100 fs to be completed, respectively, in the FeO+/benzene system.

DOI： 10.1021/jp992464t

Language：English  

The reaction pathways and the energetics for the direct methane-methanol and benzene-phenol conversions that occur on the surface of Fe-ZSM-5 zeolite are analyzed from B3LYP DFT computations. We propose a reasonable model for "alpha-oxygen", a surface oxygen species responsible For the catalytic reactivities of Fe-ZSM-5 zeolite. Our model involves an iron-ore species on the AlO4 surface site of the zeolite as a catalytic active center and as a source of oxygen. The essential features of the reaction pathways for the methane-methanol and benzene-phenol conversions are identical, especially in bonding characters, In the initial stages of each reaction, methane or benzene comes into contact with the active iron site of the "alpha-oxygen" model, leading to the reactant (methane or benzene) complex. After the initial complex is formed, each reaction takes place in a two-step concerted manner, via neither radical species nor ionic intermediates. The concerted reaction pathway for the methane (benzene) hydroxylation involves an H atom abstraction and a methyl (phenyl) migration at the iron active center. From computed energetics for the reaction pathways, we predict that the benzene hydroxylation should be energetically more favorable than the methane hydroxylation.

DOI： 10.1021/jp991844b

Language：English   Publishing type：Research paper (scientific journal)  

The so-called "alpha-oxygen" on Fe-ZSM-5 zeolite, a surface oxygen species responsible for high reactivity in oxidation of methane and of benzene, has been investigated. We present from density-functional-theory (DFT) calculations how such a reactive surface species is generated upon decomposition of dinitrogen oxide (N2O) and propose a possible form of "alpha-oxygen" on Fe-ZSM-5 zeolite. In the initial stages of the reactions, a complex involving an Fe(ON2) moiety is formed, followed by dissociation into an iron-ore species and N-2. The activation energy for the decomposition of N2O on a possible iron active site model of Fe-ZSM-5 zeolite is predicted to be 2.4 kcal mol(-1) at the B3LYP level of theory. Therefore the decomposition of N2O is expected to take place easily at a coordinatively unsaturated iron active center supported on zeolite. The iron-ore species thus formed should play an essential role in the direct hydroxylation of methane, benzene, and other hydrocarbons if it involves a coordinatively unsaturated iron. The oxygen exchange on the iron-ore complex was also investigated. The activation energy for the oxygen exchange on the "alpha-oxygen" is predicted to be 26.8 kcal mol(-1). We propose that the bare FeO+ complex and the so-called "alpha-oxygen" on Fe-ZSM-5 zeolite involve similar catalytic active centers responsible for similar catalytic functions for methane and benzene.

DOI： 10.1246/bcsj.73.29

Language：English   Publishing type：Research paper (scientific journal)  

Crossing seams between the potential energy surfaces and possible spin inversion processes for the direct conversion of methane to methanol by the bare FeO+ species are discussed by means of the intrinsic reaction coordinate (IRC) approach. There are three crossing seams between the sextet and the quartet potential energy surfaces, and spin inversion should occur twice in the entrance and the exit channels; FeO+((6)Sigma(+)) + CH4((1)A(1)) --> OFe+(CH4)((6)A) --> TS1((4)A') --> HO-Fe+-CH3((4)A) --> TS2 ((4)A) --> Fe+(CH3OH)((4)A) --> Fe+(D-6) + CH3OH((1)A'). The first crossing seam exists in prior to TS1, a four-centered transition state for the cleavage of a C-H bond of methane. This crossing seam is the most important aspect in this reaction pathway because the molecular system should change its spin multiplicity from the sextet state to the quartet state near this crossing region, leading to a significant decrease in the barrier height of TS1 from 31.1 to 22.1 kcal/mol at the B3LYP level of density functional theory. The second crossing seam occurs in the vicinity of the hydroxy intermediate (HO-Fe+-Ch(3)), but this crossing seam would not play a significant role because the quartet IRC valley always lies below the sextet one in this region of reaction coordinate and accordingly the molecular system would preferentially move on the quartet potential energy surface. The third crossing seam exists in the exit channel in which the elimination of methanol occurs from the product complex. This crossing seam will again lead to spin inversion from the quartet to the sextet state, by which the elimination energy can be decreased from 57.2 to 37.4 kcal/mol in the FeO+/CH4 system. (C) 1999 American Institute of Physics. [S0021-9606(99)01926-1].

DOI： 10.1063/1.479333

Language：English   Publishing type：Research paper (scientific journal)  

The direct benzene hydroxylation by an iron-ore species is discussed from density-functional-theory (DFT) calculations. The proposed reaction pathway is FeO+ + C6H6 --> OFe+(C6H6) --> [TS1] --> HO-Fe+-C6H5 --> [TS2] --> Fe+(C6H5OH) --> Fe+ + C6H5OH, in which TS means transition state. This reaction is initiated by the formation of the reactant complex, OFe+(C6H6), exhibiting an eta(2)-C6H6 binding mode; benzene C-H bonds are activated on this complex due to significant electron transfer from the benzene to the iron-oxo species. The reaction should proceed in a concerted manner, neither via the formation of radical species nor ionic intermediates. The reaction mechanism is quite similar to the two-step concerted mechanism that we have proposed originally for the direct methane hydroxylation by an iron-ore species. The quartet potential energy surface affords a low-cost reaction pathway for the benzene hydroxylation, spin inversion being unimportant in contrast to the methane hydroxylation in which crossing between the sextet and quartet potential energy surfaces plays an important role. We suggest that our two-step concerted mechanism should be widely applicable to hydrocarbon hydroxylations catalyzed by transition-metal oxides if coordinatively unsaturated metal oxides are responsible for such important catalytic reactions.

DOI： 10.1021/ja981525i

Language：English   Publishing type：Research paper (scientific journal)  

Two kinds of H-atom abstractions from methane by iron-ore complexes with different charges are discussed from density functional theory calculations. A concerted H-atom abstraction via a four-centered transition state is shown to be energetically more favorable than a direct H-atom abstraction via a transition state with a linear C-H-O array. Iron-(IV)-oxo complexes appear to be the most effective for the cleavage of the C-H bond of alkanes in the concerted mechanism, which is rationalized from qualitative orbital interaction analyses. The results of this paper support the establishment of the two-step concerted mechanism that we have proposed for alkane hydroxylations by iron-ore species. The proposed reaction mechanism may have relevance to our understanding of some catalytic and enzymatic processes concerning alkane hydroxylations if coordinatively unsaturated transition-metal oxides are responsible for such important chemical reactions.

DOI： 10.1021/om980067j

Language：English   Publishing type：Research paper (scientific journal)  

The entire reaction pathway for the gas-phase methane-methanol conversion by late transition-metal-oxide ions, MnO+, FeO+, and CoO+, is studied using an ab initio hybrid (Hartree-Fock/density-functional) method. For these oxo complexes, the methane-methanol conversion is proposed to proceed via two transition states (TSs) in such a way MO+ + CH4 --> OM+(CH4) --> [TS1] --> HO-M+-CH3 --> [TS3] --> M+(CH3OH) --> M+ + CH3OH, where M is Mn, Fe, and Co. A crossing between high-spin and low-spin potential energy surfaces occurs both at the entrance channel and at the exit channel for FeO+ and CoO+, but it occurs only once near TS2 for MnO+. The activation energy from OMn+(CH4) to HO-Mn+-CH3 via TS1 is calculated to be 9.4 kcal/mol, being much smaller than 22.1 and 30.9 kcal/mol for FeO+ and CoO+, respectively. This agrees with the experimentally reported efficiencies for the reactions. The excellent agreement between theory and experiment indicates that HO-M+-CH3 plays a central role as an intermediate in the reaction between MO+ and methane and that the reaction efficiency is most likely to be determined by the activation energy from OM+(CH4) to HO-M+-CH3 via TS1. We discuss in terms of qualitative orbital interactions why MnO+ (d(4) oxo complex) is most effective for methane C-H bond activation. The activation energy from HO-M+-CH3 to M+(CH3OH) via TS2 is computed to be 24.6, 28.6, and 35.9 kcal/mol for CoO+, FeO+, and MnO+, respectively. This result explains an experimental result that the methanol-branching ratio in the reaction between MO+ and methane is 100% in CoO+, 41% in FeO+, and < 1% in MnO+. We demonstrate that both the barrier heights of TS1 and TS2 would determine general catalytic selectivity for the methane-methanol conversion by the MO+ complexes.

DOI： 10.1021/ja971723u

Language：English   Publishing type：Research paper (scientific journal)  

A possible nitrogen fixation based on a Diels-Alder reaction of 1,4-disila-1,3-butadiene and N-2 is discussed. The activation energy from the reactants to the product, a six-membered ring of C2Si2N2, was computed to be only 5.1 kcal/mol at the B3LYP/6-311G** level of density functional theory. QCISD(T)/6-31G**//CASSCF/6-31G** calculations also gave 8.0 kcal/mol comparable to the B3LYP value. These computed activation energies are smaller than that of the well-known Diels-Alder reaction of butadiene and ethylene, 24.8 kcal/mol at the B3LYP/6-31G* level (Goldstein, E.; Beno, B.; Houk, K. N. J. Am. Chem. Soc. 1996, 118, 6036), and consequently, this unique reaction would proceed under mild conditions if this reactive species, disilabutadiene, is prepared. The triple bond of N-2 is partially cleaved in a reductive manner to lead to the formation of an N=N double bond. Orbital interaction analyses suggest that one of the degenerate pi* LUMOs of N-2 has a good interaction with the high-lying HOMO of disilabutadiene, and this interaction should contribute to significant electron transfer from disilabutadiene to N-2. 1,4-Disila-1,3-butadiene, could be prepared by thermal ring opening of 1,2-disilacyclobutene; the activation barrier for a symmetry-allowed conrotatory process of the two silylene groups was calculated to be 40.9 kcal/mol at the B3LYP/6-311G** level and 47.6 kcal/mol at the QCISD(T)/6-31G**//CASSCF/6-31G** level. We, thus, expect the artificial nitrogen fixation to occur when 1,2-disilacyclobutene and N-2 gas are heat-treated in a sealed tube.

DOI： 10.1021/om970553r

Language：English   Publishing type：Research paper (scientific journal)  

We propose possible theoretical reaction paths for the conversion of methane to methanol catalyzed by FeO+. The geometric and electronic structures for the reactant. product. intermediates. and transition states were calculated and analyzed in detail by means of a hybrid Hartree-Fock/density functional method. Sestet and quartet spin states were taken into consideration in the analysis of the reaction paths. The conversion of methane to methanol was shown to proceed through basic concerted hydrogen- and methyl-shift reactions. A fragment molecular orbital analysis for the Formation of the reactant complex, OFe+-CH4. which plays an important role in the initial stage of methane activation. was carried out in order to understand the nature of the interesting Fe-C bond. The five-coordinate methane in the reactant complex was calculated to have a C-3v-type geometry. Each reaction path presented in this gaper includes an important insertion species. HO-Fe+-CH3 or H-Fe+-OCH3, and two transition states. Thus. there are several kinds of reaction paths. if the high-spin sextet and low-spin quartet states are taken into consideration. A reaction towards the hydroxy intermediate, HO - Fe+ - CH3, was found to be more favorable in both the sextet and quartet spin states from the viewpoint of activation energy, and this intermediate is extremely stable. It was found from intrinsic reaction coordinate (IRC) analyses that two basic reactions coexist, namely hydrogen or methyl migration between the reactant and the methoxy intermediate, H-Fe+-OCH3. This transition state is interesting, because the two transition states resulting from C-H bond cleavage and methyl migration are located in the same region of space on the potential energy surfaces. IRCs are partially shown for the complicated first halves of the total reaction paths.

DOI： 10.1002/chem.19970030722

Language：English   Publishing type：Research paper (scientific journal)  

The phase transitions of ATiO(3)-type perovskite structures, mainly those of barium titanate (BaTiO3), aro studied theoretically in terms of orbital interactions and vibrational modes. Vibrational analyses for fundamental TiO6 clusters, up to Ti8O36, are performed with a hybrid (Hartree-Fock/density functional) method. Important structural distortions from the high symmetry cubic phase are rationalized by a second-order perturbational treatment of quantum chemistry. The orbital symmetry argument based on group theory indicates that the transition density around frontier molecular orbitals plays an important role in the motions of ions at the transition temperatures. Our orbital interaction and vibrational mode analyses of the phase transitions are consistent with the soft mode theory of Cochran.

DOI： 10.1039/a607475h

Language：English   Publishing type：Research paper (scientific journal)  

Dioxygen binding to the active dinuclear iron site of methane monooxygenase (MMO) is theoretically analyzed with a density functional method. The μ-η1:η1-O2 mode was calculated to be the most favorable binding mode of dioxygen to a supposed active site of MMO that contains two five-coordinate irons(II).

Language：English   Publishing type：Research paper (scientific journal)  

Ab initio molecular orbital calculations of the C-CCSi torsion and the [1,3] sigmatropic silyl shift in allylsilane (SiC3H8, 1) have been carried out. It is clarified that the skew conformer of 1 is more stable than other conformers. Its torsional energy depends on the sigma-pi hyperconjugation as compared with that of 1-butene (2). In the [1,3] silyl shift, the reaction coordinate of the silyl-group migration in the early and final stages is congruent with that of the C=CCSi torsion near the skew conformer. Divergence points on the inversion and retention paths are discussed from the viewpoint of the degree of nonplanarity of the migrating silyl group. The activation energy along the retention path is calculated to be lower than that along the inversion path at various levels of theory, which is in remarkable contrast to 2, in which the inversion path is more favorable. From the energetical viewpoint, the [1,3] shift of the silyl group is expected to proceed with retention of the silicon configuration at the migrating center. Although this looks like an exception to the Woodward-Hoffmann rules, this may be caused by the nearly degenerate HOMO and (HOMO-1) in the transition state on the inversion path in 1.

DOI： 10.1021/ja9623525

Language：English   Publishing type：Research paper (scientific journal)  

A possible reaction pathway for the C-H bond cleavage of methane on intermediate Q of methane monooxygenase is studied theoretically. We propose that the activation of methane C-H bond by Q, suggested to involve a high valent Fe2(μ-O)2 diamond core, should occur through the formation of the Q(CH4) complex with an Fe-C bond and Fe-H bonds, followed by a concerted H atom abstraction via a four-centered transition state.

Event date： 2020.9 - 2020.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2020.9 - 2020.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2020.3 - 2021.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：奈良   Country：Japan  

Event date： 2018.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京   Country：Japan  

Event date： 2017.9

Language：Japanese   Presentation type：Oral presentation (invited, special)  

Venue：オンライン   Country：Japan  

Event date： 2013.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：京都   Country：Japan  

Event date： 2013.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：京都   Country：Japan  

Event date： 2013.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：京都   Country：Japan  

Event date： 2013.3 - 2012.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：滋賀   Country：Japan  

Event date： 2013.3 - 2012.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：滋賀   Country：Japan  

Event date： 2013.3 - 2012.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：滋賀   Country：Japan  

Event date： 2012.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：富山   Country：Japan  

Event date： 2012.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：福岡   Country：Japan  

Event date： 2012.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：滋賀   Country：Japan  

Event date： 2011.11

Presentation type：Oral presentation (general)  

Venue：名古屋   Country：Japan  

Event date： 2011.11

Presentation type：Oral presentation (general)  

Venue：大阪   Country：Japan  

Event date： 2011.9

Presentation type：Oral presentation (general)  

Venue：北見   Country：Japan  

メタンモノオキシゲナーゼ(pMMO)はX線構造解析や最近の実験的研究から二核銅及び単核銅の活性点を持つことが明らかとなったものの、その活性化の仕組みや反応機構は分子レベルでは未だによく分かっていない。我々は活性中心の二核銅酸素錯体の電子状態が+1価と+2価の混合原子価のときにメタンと効率よく反応することをDFT計算から明らかした。今回、我々はチロシン残基からの水素原子の付加により、bis(μ-Oxo)Cu(III)Cu(III)からメタンと反応可能な活性種を生じる可能性について理論的に検討した

Event date： 2011.9

Presentation type：Oral presentation (general)  

Venue：大阪   Country：Japan  

Event date： 2011.9

Presentation type：Oral presentation (general)  

Venue：東京   Country：Japan  

I. Introduction
Valence tautomeric (VT) complexes have attracted much attention recently because of their possible application as new molecular devices like memory or display devices. VT complexes show a ligand-metal electron transfer coupled with a spin change on the central metal ion, which can be switched by external stimuli like light, heat, or pressure. We use a model for the cobalt bis(quinone) complex [CoIII(3,5-dbcat)(3,5-dbsq)(bpy)] (cat: catecholate, sq: semiquinon, and bpy: 2,2’-bipyridine), which was first reported by Buchanan and Pierpont in 1980. The complex is in the CoIII-LS form (LS: low-spin doublet state) below 273 K, and the population of the CoII-HS form (HS: high-spin sextet state) starts with increasing temperature.
II. Method of calculation
Energy calculations for the [CoII-HS(sq)2(bpy)] and [CoIII-LS(sq)(cat)(bpy)] complexes in the doublet, quartet, and sextet spin states were carried out by using the B3LYP* functional (with 15% Hartree–Fock exchange) implemented in the program packages Gaussian 03 and GAMESS. For the Co atom, the (14s9p5d)/[9s5p3d] primitive set of Wachters-Hay with one polarization f-function (α = 1.117) and for the H, C, N, and O, the 6-311G** basis set were used.
III. Results and discussions
Valence tautomerism is studied in [CoII-HS(sq)2(bpy)]/[CoIII-LS(sq)(cat)(bpy)] mononuclear cobalt complex by using DFT methods. Calculations at the B3LYP* level of theory reproduce well the energy gap between the CoII-HS and CoIII-LS forms, giving an energy gap of 4.4 kcal/mol, which is comparable to an experimental value of 8.9 kcal/mol. Potential energy surfaces and crossing seams of the electronic states of the doublet, quartet, and sextet spin states are calculated along minimum energy paths connecting the energy minima corresponding to the different spin states. Calculated minimum energy crossing points (MECPs) are located 8.8 kcal/mol in the doublet/sextet surface, 10.2 kcal/mol in the doublet/quartet surface, and 8.4 kcal/mol in the quartet/sextet surface relative to the doublet ground state. Considering the energy of the three spin states and the crossing points, the one-step relaxation between the CoII-HS and CoIII-LS forms is the most probable. This research shows that mapping MECPs can be a useful strategy to analyze the potential energy surfaces of systems with complex deformation modes.

Event date： 2011.3

Presentation type：Oral presentation (general)  

Venue：東京   Country：Japan  

Valence tautomerism is studied in [CoII-HS(sq)2(bpy)]/[CoIII-LS(sq)(cat)(bpy)] mononuclear cobalt complex by using DFT methods (HS: high spin, LS: low spin, cat: catecholate, sq: semiquinon, and bpy: 2,2'-bipyridine). Calculations at the B3LYP* level of theory reproduce well the energy gap between the CoII-HS and CoIII-LS forms giving an energy gap of 4.4 kcal/mol, which is comparable to the experimental value of 8.9 kcal/mol.

Event date： 2011.1

Presentation type：Oral presentation (general)  

Venue：札幌   Country：Japan  

A new mechanism for methane hydroxylation by particulate methane monooxygenase (pMMO) is proposed by using QM/MM calculations on the basis of the X-ray crystal structure analysis of pMMO. Important roles of the dicopper active site for methane hydroxylation are discussed. In the initial stages of the reaction substrate methane comes into contact with the mixed-valent bis(μ-oxo)CuIICuIII active center and then a C-H bond of methane is cleaved by one of the oxo species with the formation of an O-H bond. At the same time, the resulting methyl radical species is bound to one of the copper ions. The migration of the methyl ligand to the other bridging oxo ligand occurs in the next step, instead of the direct CH3 and OH recombination that is expected to occur in the oxygen rebound mechanism. Finally methanol is formed as a result of the recombination of the hydroxo hydrogen and the methoxy ligand. The initial H-atom abstraction step is the rate-determining step in the computed reaction pathway. The computational result is consistent with observed kinetic isotope effects.

Event date： 2011.1

Presentation type：Oral presentation (general)  

Venue：福岡   Country：Japan  

Event date： 2010.9

Presentation type：Oral presentation (general)  

Venue：大阪   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：静岡   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：静岡   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：静岡   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：沖縄   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：沖縄   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：富山   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：富山   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：富山   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：沖縄   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：大阪   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：福岡   Country：Japan  

Presentation type：Symposium, workshop panel (public)  

Venue：福岡   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：福岡   Country：Japan  

Presentation type：Symposium, workshop panel (public)  

Venue：東京   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：名古屋   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：大阪   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：札幌   Country：Japan  

Language：English  

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Theoretical study of methane activation catalyzed by transition metal-oxo species Yoshihito SHIOTA (Institute for materials chemistry and engineering, Kyushu University, Motooka, Nishi-ku, Fukuoka 819-0395, Japan)
Reaction pathways and energetics for the conversion of methane to methanol by the bare FeO+ ion, the FeO+ species over Fe-ZSM-5, and the dicopper species of pMMO are discussed using density-functional-theory calculations. Computed reaction pathway is a non-radical mechanism to form a hydroxo intermediate, HO–M+–CH3, via H-atom abstraction with a four-centered transition state, which occurs at a coordinatively unsaturated metal center. A detailed analysis of the potential energy surfaces shows that crossing between high-spin and low-spin potential energy surfaces occurs twice in the entrance channel and in the exit channel in FeO+.

Language：English  

Language：English  

Language：English  

The entire reaction pathway for the gas-phase methane-methanol conversion by late transition-metal-oxide ions, MnO+, FeO+, and CoO+, is studied using an ab initio hybrid (Hartree-Fock/density-functional) method. For these oxo complexes, the methane-methanol conversion is proposed to proceed via two transition states (TSs) in such a way MO+ + CH4 --> OM+(CH4) --> [TS1] --> HO-M+-CH3 --> [TS3] --> M+(CH3OH) --> M+ + CH3OH, where M is Mn, Fe, and Co. A crossing between high-spin and low-spin potential energy surfaces occurs both at the entrance channel and at the exit channel for FeO+ and CoO+, but it occurs only once near TS2 for MnO+. The activation energy from OMn+(CH4) to HO-Mn+-CH3 via TS1 is calculated to be 9.4 kcal/mol, being much smaller than 22.1 and 30.9 kcal/mol for FeO+ and CoO+, respectively. This agrees with the experimentally reported efficiencies for the reactions. The excellent agreement between theory and experiment indicates that HO-M+-CH3 plays a central role as an intermediate in the reaction between MO+ and methane and that the reaction efficiency is most likely to be determined by the activation energy from OM+(CH4) to HO-M+-CH3 via TS1. We discuss in terms of qualitative orbital interactions why MnO+ (d(4) oxo complex) is most effective for methane C-H bond activation. The activation energy from HO-M+-CH3 to M+(CH3OH) via TS2 is computed to be 24.6, 28.6, and 35.9 kcal/mol for CoO+, FeO+, and MnO+, respectively. This result explains an experimental result that the methanol-branching ratio in the reaction between MO+ and methane is 100&#37; in CoO+, 41&#37; in FeO+, and < 1&#37; in MnO+. We demonstrate that both the barrier heights of TS1 and TS2 would determine general catalytic selectivity for the methane-methanol conversion by the MO+ complexes.

DOI： 10.1021/ja971723u

Language：English  

The phase transitions of ATiO(3)-type perovskite structures, mainly those of barium titanate (BaTiO3), aro studied theoretically in terms of orbital interactions and vibrational modes. Vibrational analyses for fundamental TiO6 clusters, up to Ti8O36, are performed with a hybrid (Hartree-Fock/density functional) method. Important structural distortions from the high symmetry cubic phase are rationalized by a second-order perturbational treatment of quantum chemistry. The orbital symmetry argument based on group theory indicates that the transition density around frontier molecular orbitals plays an important role in the motions of ions at the transition temperatures. Our orbital interaction and vibrational mode analyses of the phase transitions are consistent with the soft mode theory of Cochran.

DOI： 10.1039/a607475h

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Number of participants：100

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Number of participants：50

Type：Competition, symposium, etc. 

Number of participants：50

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Number of participants：1,500

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Number of participants：50

Type：Competition, symposium, etc.