Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Yoshihito Shiota Last modified date:2019.06.28

Associate Professor / theoretical chemistry / Department of Fundamental Organic Chemistry / Institute for Materials Chemistry and Engineering


Papers
1. Shin Ichiro Kato, Satoshi Jin, Terutaka Kimura, Naoki Yoshikawa, Daiki Nara, Kenji Imamura, Yoshihito Shiota, Kazunari Yoshizawa, Ryo Katoono, Takeshi Yamanobe, Hiroki Uehara, Yosuke Nakamura, Trithiazolyl-1,3,5-triazines bearing decyloxybenzene moieties
Synthesis, photophysical and electrochemical properties, and self-assembly behavior, Organic and Biomolecular Chemistry, 10.1039/c8ob00471d, 16, 19, 3584-3595, 2018.01, 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..
2. Dachao Hong, Yuto Tsukakoshi, Hiroaki Kotani, Tomoya Ishizuka, Kei Ohkubo, Yoshihito Shiota, Kazunari Yoshizawa, Shunichi Fukuzumi, Takahiko Kojima, Mechanistic Insights into Homogeneous Electrocatalytic and Photocatalytic Hydrogen Evolution Catalyzed by High-Spin Ni(II) Complexes with S2N2-Type Tetradentate Ligands, Inorganic Chemistry, 10.1021/acs.inorgchem.8b00881, 57, 12, 7180-7190, 2018.06, We report homogeneous electrocatalytic and photocatalytic H2 evolution using two Ni(II) complexes with S2N2-type tetradentate ligands bearing two different sizes of chelate rings as catalysts. A Ni(II) complex with a five-membered SC2S-Ni chelate ring (1) exhibited higher activity than that with a six-membered SC3S-Ni chelate ring (2) in both electrocatalytic and photocatalytic H2 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 (NiII-H) species. We also clarified that Ni(II) complexes can act as homogeneous catalysts in the electrocatalytic H2 evolution, in which complex 1 exhibited higher reactivity than that of 2. In the photocatalytic system using [Ru(bpy)3]2+ 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 six-membered chelate ring, although the rates of photoinduced electron-transfer processes were comparable. The Ni-H bond cleavage in the putative NiII-H intermediate should be involved in the rate-limiting step as evidenced by kinetic isotope effects observed in both photocatalytic and electrocatalytic H2 evolution. Kinetic analysis and density functional theory calculations indicated that the difference in H2 evolution activity between the two complexes was derived from that of activation barriers of the reactions between the NiII-H intermediates and proton, which is consistent with the fact that increase of proton concentration accelerates the H2 evolution..
3. Yogesh Kumar Maurya, Katsuya Noda, Kazuhisa Yamasumi, Shigeki Mori, Tomoki Uchiyama, Kazutaka Kamitani, Tomoyasu Hirai, Kakeru Ninomiya, Maiko Nishibori, Yuta Hori, Yoshihito Shiota, Kazunari Yoshizawa, Masatoshi Ishida, Hiroyuki Furuta, Ground-State Copper(III) Stabilized by N-Confused/N-Linked Corroles
Synthesis, Characterization, and Redox Reactivity, Journal of the American Chemical Society, 10.1021/jacs.8b01876, 140, 22, 6883-6892, 2018.06, 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..
4. Shintaro Ida, Kenta Sato, Tetsuya Nagata, Hidehisa Hagiwara, Motonori Watanabe, Namhoon Kim, Yoshihito Shiota, Michio Koinuma, Sakae Takenaka, Takaaki Sakai, Elif Ertekin, Tatsumi Ishihara, A Cocatalyst that Stabilizes a Hydride Intermediate during Photocatalytic Hydrogen Evolution over a Rhodium-Doped TiO
2
Nanosheet, Angewandte Chemie - International Edition, 10.1002/anie.201803214, 57, 29, 9073-9077, 2018.07, 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 TiO
2
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..
5. Takahiko Kojima, Fumiya Ogishima, Takahisa Nishibu, Hiroaki Kotani, Tomoya Ishizuka, Toshihiro Okajima, Shunsuke Nozawa, Yoshihito Shiota, Kazunari Yoshizawa, Hiroyoshi Ohtsu, Masaki Kawano, Takuya Shiga, Hiroki Oshio, Intermediate-Spin Iron(III) Complexes Having a Redox-Noninnocent Macrocyclic Tetraamido Ligand, Inorganic Chemistry, 10.1021/acs.inorgchem.8b00037, 57, 16, 9683-9695, 2018.08, An iron(III) complex having a dibenzotetraethyltetraamido macrocyclic ligand (DTTM
4-
), (NEt
4
)
2
[Fe
III
(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 CH
2
Cl
2
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 [Ru
III
(bpy)
3
](ClO
4
)
3
(bpy = 2,2′-bipyridine) in CH
2
Cl
2
afforded square-pyramidal (NEt
4
)[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 H
ab
= 884 cm
-1
. Two-electron oxidation of 1 by 2 equiv of [(4-Br-Ph)
3
N
·+
](SbCl
6
) in CH
2
Cl
2
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 CH
3
CN 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..
6. M. Haris Mahyuddin, Takahiro Tanaka, Aleksandar Tsekov Staykov, Yoshihito Shiota, Kazunari Yoshizawa, Dioxygen Activation on Cu-MOR Zeolite
Theoretical Insights into the Formation of Cu2O and Cu3O3 Active Species, Inorganic Chemistry, 10.1021/acs.inorgchem.8b01329, 57, 16, 10146-10152, 2018.08, 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..
7. M. Haris Mahyuddin, Yoshihito Shiota, Aleksandar Tsekov Staykov, Kazunari Yoshizawa, Theoretical Overview of Methane Hydroxylation by Copper-Oxygen Species in Enzymatic and Zeolitic Catalysts, Accounts of Chemical Research, 10.1021/acs.accounts.8b00236, 51, 10, 2382-2390, 2018.10, 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..
8. Hiroaki Kotani, Suzue Kaida, Tomoya Ishizuka, Kaoru Mieda, Miyuki Sakaguchi, Takashi Ogura, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Importance of the Reactant-State Potentials of Chromium(V)-Oxo Complexes to Determine the Reactivity in Hydrogen-Atom Transfer Reactions, Inorganic Chemistry, 10.1021/acs.inorgchem.8b02453, 57, 21, 13929-13936, 2018.11, A new chromium(V)-oxo complex, [Cr
V
(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 Cr
V
(O) complexes in a hydrogen-atom transfer (HAT) reaction by comparing it with that of a previously reported Cr
V
(O) complex, [Cr
V
(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 Cr
V
(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 Cr
IV
(OH)/Cr
V
(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 (E
a
= 7.9 kcal mol
-1
for 1 and E
a
= 4.8 kcal mol
-1
for 2), the reactivity of Cr
V
(O) complexes in HAT reactions depends on the energy level of the reactant state rather than the product state..
9. Koichi Hashimoto, Taro Koide, Toru Okawara, Hisashi Shimakoshi, Yuta Hori, Yoshihito Shiota, Kazunari Yoshizawa, Yoshio Hisaeda, Redox behaviour of the β-dihydroporphycene cobalt complex
study on the effect of hydrogenation of the ligand, Dalton Transactions, 10.1039/c8dt03743d, 48, 3, 872-881, 2019.01, 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..
10. Muhammad Haris Mahyuddin, Yoshihito Shiota, Kazunari Yoshizawa, Methane selective oxidation to methanol by metal-exchanged zeolites
A review of active sites and their reactivity, Catalysis Science and Technology, 10.1039/c8cy02414f, 9, 8, 1744-1768, 2019.01, 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..
11. Kazuyuki Takahashi, Kaoru Yamamoto, Takashi Yamamoto, Yasuaki Einaga, Yoshihito Shiota, Kazunari Yoshizawa, Hatsumi Mori, High-temperature cooperative spin crossover transitions and single-crystal reflection spectra of [Fe III (qsal) 2 ](CH 3 OSO 3 ) and related compounds, Crystals, 10.3390/cryst9020081, 9, 2, 2019.01, 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..
12. Kei Ikeda, Yuta Hori, Muhammad Haris Mahyuddin, Yoshihito Shiota, Aleksandar Tsekov Staykov, Takahiro Matsumoto, Kazunari Yoshizawa, Seiji Ogo, Dual Catalytic Cycle of H2 and H2O Oxidations by a Half-Sandwich Iridium Complex
A Theoretical Study, Inorganic Chemistry, 10.1021/acs.inorgchem.9b00307, 58, 11, 7274-7284, 2019.06, While hydrogenase and photosystem II enzymes are known to oxidize H2 and H2O, respectively, a recently reported iridium aqua complex [IrIII5-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..
13. Tokio Yamabe, Koichi Nakamura, Yoshihito Shiota, Kazunari Yoshizawa, Susumu Kawauchi, Mitsuo Ishikawa, Novel aspects of the [1,3] sigmatropic silyl shift in allylsilane, Journal of the American Chemical Society, 10.1021/ja9623525, 119, 4, 807-815, 1997.01, 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 σ-π 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..
14. Kazunari Yoshizawa, Yasunori Yokomichi, Yoshihito Shiota, Takehiro Ohta, Tokio Yamabe, Density functional study on possible peroxo form of non-heme diiron enzyme model, Chemistry Letters, 10.1246/cl.1997.587, 7, 587-588, 1997.01, Dioxygen binding to the active dinuclear iron site of methane monooxygenase (MMO) is theoretically analyzed with a density functional method. The μ-η11-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)..
15. Kazunari Yoshizawa, Takehiro Ohta, Yoshihito Shiota, Tokio Yamabe, Cleavage of C-H bond of methane on intermediate Q of methane monooxygenase, Chemistry Letters, 10.1246/cl.1997.1213, 12, 1213-1214, 1997.01, 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..
16. Kazunari Yoshizawa, Yoshihito Shiota, Tokio Yamabe, Reaction paths for the conversion of methane to methanol catalyzed by FeO+, Chemistry - A European Journal, 3, 7, 1160-1169, 1997.07, 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. Sextet 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(3r)-type geometry. Each reaction path presented in this paper 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+ -CH3. 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..
17. Kazunari Yoshizawa, Yoshihito Shiota, Songyun Kang, Tokio Yamabe, Possible nitrogen fixation by disilabutadiene, Organometallics, 10.1021/om970553r, 16, 23, 5058-5063, 1997.11, A possible nitrogen fixation based on a Diels-Alder reaction of 1,4-disila-1,3-butadiene and N2 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 N2 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 π* LUMOs of N2 has a good interaction with the high-lying HOMO of disilabutadiene, and this interaction should contribute to significant electron transfer from disilabutadiene to N2. 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 N2 gas are heat-treated in a sealed tube..
18. Kazunari Yoshizawa, Yoshihito Shiota, Tokio Yamabe, Abstraction of the hydrogen atom of methane by iron-oxo species
The concerted reaction path is energetically more favorable, Organometallics, 10.1021/om980067j, 17, 13, 2825-2831, 1998.06, Two kinds of H-atom abstractions from methane by iron-oxo 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-oxo 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..
19. Kazunari Yoshizawa, Yoshihito Shiota, Tokio Yamabe, Reaction pathway for the direct benzene hydroxylation by iron-oxo species, Journal of the American Chemical Society, 10.1021/ja981525i, 121, 1, 147-153, 1999.01, The direct benzene hydroxylation by an iron-oxo 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 η2-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-oxo 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..
20. Kazunari Yoshizawa, Yoshihito Shiota, Tokio Yamabe, Intrinsic reaction coordinate analysis of the conversion of methane to methanol by an iron-oxo species
A study of crossing seams of potential energy surfaces, Journal of Chemical Physics, 10.1063/1.479333, 111, 2, 538-545, 1999.07, 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+(6Σ+)+CH4( 1A1)→OFe+(CH4)( 6A)→TS1(4A′)→HO-Fe+-CH 3(4A)→TS2 (4A)→Fe+(CH3OH)( 4A)→Fe+(6D)+CH3OH( 1A′). 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+-Ch3), 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..
21. Kazunari Yoshizawa, Takashi Yumura, Yoshihito Shiota, Tokio Yamabe, Formation of an iron-oxo species upon decomposition of dinitrogen oxide on a model of Fe-ZSM-5 zeolite, Bulletin of the Chemical Society of Japan, 10.1246/bcsj.73.29, 73, 1, 29-36, 2000.01, The so-called 'a-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 'a- 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-oxo species and N2. 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-oxo 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-oxo complex was also investigated. The activation energy for the oxygen exchange on the 'a-oxygen' is predicted to be 26.8 kcal mol-1. We propose that the bare FeO+ complex and the so-called 'a-oxygen' on Fe-ZSM-5 zeolite involve similar catalytic active centers responsible for similar catalytic functions for methane and benzene..
22. Kazunari Yoshizawa, Yoshihito Shiota, Takashi Yumura, Tokio Yamabe, Direct Methane-Methanol and Benzene-Phenol Conversions on Fe-ZSM-5 Zeolite
Theoretical Predictions on the Reaction Pathways and Energetics, Journal of Physical Chemistry B Materials, 104, 4, 734-740, 2000.02, 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 "α-oxygen", a surface oxygen species responsible for the catalytic reactivities of Fe-ZSM-5 zeolite. Our model involves an iron-oxo 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 "α-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..
23. Kazunari Yoshizawa, Yoshihito Shiota, Yoshihisa Kagawa, Tokio Yamabe, Femtosecond Dynamics of the Methane - Methanol and Benzene - Phenol Conversions by an Iron - Oxo Species, Journal of Physical Chemistry A, 10.1021/jp992464t, 104, 12, 2552-2561, 2000.03, Femtosecond dynamic behavior of the methane - methanol conversion by the bare iron - oxo 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 (kH/kD) 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..
24. Kazunari Yoshizawa, Akiya Suzuki, Yoshihito Shiota, Tokio Yamabe, Conversion of methane to methanol on diiron and dicopper enzyme models of methane monooxygenase
A theoretical study on a concerted reaction pathway, Bulletin of the Chemical Society of Japan, 10.1246/bcsj.73.815, 73, 4, 815-827, 2000.04, 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 doublet state. 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..
25. Yoshihito Shiota, Kazunari Yoshizawa, Methane-to-methanol conversion by first-row transition-metal oxide ions
ScO+, TiO+, VO+, CrO+, MnO+, FeO+, CoO+, NiO+, and CuO+, Journal of the American Chemical Society, 10.1021/ja0017965, 122, 49, 12317-12326, 2000.12, 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..
26. Kazunari Yoshizawa, Yoshihisa Kagawa, Yoshihito Shiota, Kinetic isotope effects in a C-H bond dissociation by the iron-oxo species of cytochrome P450, Journal of Physical Chemistry B Materials, 10.1021/jp001950+, 104, 51, 12365-12370, 2000.12, 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..
27. Kazunari Yoshizawa, Yoshihito Shiota, Y. Kagawa, Energetics for the oxygen rebound mechanism of alkane hydroxylation by the iron-oxo species of cytochrome P450, Bulletin of the Chemical Society of Japan, 10.1246/bcsj.73.2669, 73, 12, 2669-2673, 2000.12, 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-oxo 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..
28. T. Ohta, T. Kamachi, Yoshihito Shiota, Kazunari Yoshizawa, A theoretical study of alcohol oxidation by ferrate, Journal of Organic Chemistry, 10.1021/jo001193b, 66, 12, 4122-4131, 2001.06, The conversion of methanol to formaldehyde mediated by ferrate (FeO4 2-), 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..
29. Kazunari Yoshizawa, T. Kamachi, Yoshihito Shiota, A theoretical study of the dynamic behavior of alkane hydroxylation by a compound I model of cytochrome P450, Journal of the American Chemical Society, 10.1021/ja010593t, 123, 40, 9806-9816, 2001.10, 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 v3 and v4 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..
30. Yoshihito Shiota, M. Kondo, Kazunari Yoshizawa, Role of molecular distortions in the spin-orbit coupling between the singlet and triplet states of the 4π electron systems C4H4, C5H5 +, and C3H3 -, Journal of Chemical Physics, 10.1063/1.1412250, 115, 20, 9243-9254, 2001.11, The SOC between the single and triplet states of cyclobutadiene, cyclopentadienyl cation, and cyclopropenyl anion was studied as a function of certain molecular distortions. Based on the SOC and the distortion energy, it was concluded that the order of the transition probability should lie in the order cyclopropenyl anion>cyclobutadiene>cyclopentadienyl cation..
31. Masakazu Kondo, Yoshihito Shiota, Kazunari Yoshizawa, Possible photoinduced spin transitions in bis(phenylmethylenyl)[2.2]paracyclophanes. A spin-orbit coupling study, Journal of Physical Chemistry A, 10.1021/jp020984+, 106, 34, 7915-7920, 2002.08, 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..
32. Takashi Kamachi, Yoshihito Shiota, Takehiro Ohta, Kazunari Yoshizawa, Does the hydroperoxo species of cytochrome P450 participate in olefin epoxidation with the main oxidant, compound I? Criticism from density functional theory calculations, Bulletin of the Chemical Society of Japan, 10.1246/bcsj.76.721, 76, 4, 721-732, 2003.01, Ethylene epoxidation by an iron(III)-hydroperoxo model of cytochrome 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(IV)-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 cleaved, and the iron(IV)-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..
33. Yoshihito Shiota, Kazunari Yoshizawa, A spin-orbit coupling study on the spin inversion processes in the direct methane-to-methanol conversion by FeO+, Journal of Chemical Physics, 10.1063/1.1557192, 118, 13, 5872-5879, 2003.04, The spin-orbit coupling (SOC) calculations along the reaction pathway from FeO+(CH4) to Fe+(CH3OH) in methane hydroxylation by FeO+ were demonstrated. As such, the analysis of the SOC and the crossing seam will be useful for a deeper understanding of the spin state changes in transition-metal catalyzed reactions. It was also emphasized that the reconstruction of molecular orbitals and the deformation of molecular structures provide a change in the strength of SOC as well as in the potential energy..
34. Yoshihito Shiota, Naoki Kihara, Takashi Kamachi, Kazunari Yoshizawa, A theoretical study of reactivity and regioselectivity in the hydroxylation of adamantane by ferrate(VI), Journal of Organic Chemistry, 68, 10, 3958-3965, 2003.05, The conversion of adamantane to adamantanols mediated by ferrate (FeO4 2-), 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 (3°) and 2-adamantanol by the abstraction of a secondary hydrogen atom (2°). 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 1-adamantanol pathway and 8.4 kcal/mol in the 2-adamantanol pathway, respectively, at the B3LYP/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 (3°)/(2°) for the competing H-atom abstraction reactions is calculated to be 9.30 at 75 °C, which is fully consistent with an experimental value of 10.1..
35. Kazunari Yoshizawa, Yoshihito Shiota, Takashi Kamachi, Mechanistic proposals for direct benzene hydroxylation over Fe-ZSM-5 zeolite, Journal of Physical Chemistry B Materials, 107, 41, 11404-11410, 2003.10, 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 "α 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 α-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..
36. Akinobu Naka, Hidenobu Ohnishi, Ikuko Miyahara, Ken Hirotsu, Yoshihito Shiota, Kazunari Yoshizawa, Mitsuo Ishikawa, Silicon-carbon unsaturated compounds. 69. Reactions of silenes produced thermally from pivaloyl- and adamantoyltris (trimethylsilyl) silane with silyl-substituted butadiynes and enynes, Organometallics, 10.1021/om034213j, 23, 18, 4277-4287, 2004.08, The thermolysis of pivaloyl- and adamantoyltris(trimethylsilyl)silane (1a and 1b) with bis(trimethylsilyl)butadiyne and bis(dimethylphenylsilyl)butadiyne at 120°C 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 160 °C 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 160 °C, 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 160 °C produced [2 + 2] cycloadducts (7a and 7b). With bis(tert-butyldimethylsilyl)butadiyne at 220 °C, 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(dimethylpnenylsilyl)but-1-en-3-yne and (E)-1,4-bis(pentamethyldisilanyl)-but-1-en-3-yne at 120 °C produced regiospecifically 2trimethylsiloxy-3-[trans-(silyl)ethenyl] -1-silacyclobut-3-ene derivatives (9a, 9b and 10a, 10b). At 200 °C, 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..
37. Yoshihito Shiota, Makoto Yasunaga, Akinobu Naka, Mitsuo Ishikawa, Kazunari Yoshizawa, Theoretical study of thermal isomerization of silacyclobutene to cyclopropene, Organometallics, 10.1021/om0497947, 23, 20, 4744-4749, 2004.09, The reaction pathway and energetics for the conversion of silacyclobutene to cyclopropene was investigated. IRC calculations were performed to trace the 1,2-siloxyl shift in this mechanism. Cyclopropene was obtained experimentally by the thermal isomerization of silacylobutene and by the subsequent trapping of cyclopropane using tert-butyl alcohol. The IRC calculations indicate that the 1,2-siloxyl shift can be divided into two processes: 1,2-siloxyl shift and the formation of the cyclopropene ring..
38. Yoshihito Shiota, Kazunari Yoshizawa, QM/MM study of the mononuclear non-heme iron active site of phenylalanine hydroxylase, Journal of Physical Chemistry B Materials, 10.1021/jp048001r, 108, 44, 17226-17237, 2004.11, 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 (FeIV= 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..
39. Takahiko Kojima, Ken Ichi Hayashi, Yoshihito Shiota, Yoshimitsu Tachi, Yoshinori Naruta, Takuya Suzuki, Kazuya Uezu, Kazunari Yoshizawa, Synthesis and characterization of ruthenium(II)-nitrile complexes with bisamide-tpa ligands (tpa = tris(2-pyridylmethyl)amine), Bulletin of the Chemical Society of Japan, 10.1246/bcsj.78.2152, 78, 12, 2152-2158, 2005, 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 η1-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 CD 3CN solutions to exhibit one mode of thermal motion; it also altered the symmetry of the complexes..
40. Takashi Kamachi, Naoki Kihara, Yoshihito Shiota, Kazunari Yoshizawa, Computational exploration of the catalytic mechanism of dopamine β-monooxygenase
Modeling of its mononuclear copper active sites, Inorganic Chemistry, 10.1021/ic048477p, 44, 12, 4226-4236, 2005.06, Dopamine hydroxylation by the copper-superoxo, -hydroperoxo, and -oxo species of dopamine β-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 α-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 (QM/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 QM region of the whole-enzyme model..
41. Yoshihito Shiota, Kunihiko Suzuki, Kazunari Yoshizawa, Mechanism for the direct oxidation of benzene to phenol by FeO+, Organometallics, 10.1021/om050136b, 24, 14, 3532-3538, 2005.07, 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+-C6H 5, 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 and 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..
42. Kazunari Yoshizawa, Naoki Kihara, Yoshihito Shiota, Hidetake Seino, Yasushi Mizobe, DFT calculations of cubane-type Mo2Ru2S4 clusters. Stability of a possible dinitrogen cluster and an isolable acetonitrile cluster, Bulletin of the Chemical Society of Japan, 10.1246/bcsj.79.53, 79, 1, 53-58, 2006, The electronic properties of cubane-type mixed-metal sulfido clusters with an Mo(V)2Ru(II)2S4 core and possible dinitrogen activation on it are described from DFT computations. The cluster [(CpRu) 2{MoCl2(MeCN)}23-S) 4] (Cp = η5-C5H5), whose Cp* analogue (Cp* = η5-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)23-S)4] cluster. However, upon reduction by two electrons the binding energy for two N2 molecules to the resultant anionic cluster [(CpRu) 2{MoCl2(N2)}23-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) 23-S)4]2- is pointing toward missing ligands and can interact effectively with the πg* orbitals of dinitrogen. This orbital might play a role in the binding of dinitrogen..
43. Kazunari Yoshizawa, Naoki Kihara, Takashi Kamachi, Yoshihito Shiota, Catalytic mechanism of dopamine β-monooxygenase mediated by Cu(III)-oxo, Inorganic Chemistry, 10.1021/ic0521168, 45, 7, 3034-3041, 2006.04, Mechanisms of dopamine hydroxylation by the Cu(II)-superoxo species and the Cu(III)-oxo species of dopamine β-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, 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(III)-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 CUB 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(III)-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(III)-oxo species to activate the benzylic C-H bond of dopamine; the enzymatic reaction can be explained by the so-called oxygen rebound mechanism..
44. Yoshihito Shiota, Kunihiko Suzuki, Kazunari Yoshizawa, QM/MM study on the catalytic mechanism of benzene hydroxylation over Fe-ZSM-5, Organometallics, 10.1021/om0509591, 25, 13, 3118-3123, 2006.06, 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 (FeIII=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 II 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..
45. Kazunari Yoshizawa, Yoshihito Shiota, Conversion of methane to methanol at the mononuclear and dinuclear copper sites of particulate methane monooxygenase (pMMO)
A DFT and QM/MM study, Journal of the American Chemical Society, 10.1021/ja061604r, 128, 30, 9873-9881, 2006.08, 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 CuI species to give a CuII-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 (μ-η22-peroxo)dicopper species, which is then transformed into a bis(μ-oxo)dicopper species. The formation of this species is more favorable in energy than that of the monocopper-oxo species. The reactivity of the CuIII-oxo species is sufficient for the conversion of methane to methanol if it is formed in the protein environment. Since the σ* 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(μ-oxo)dicopper species is also sufficient for the conversion of methane to methanol. The mixed-valent bis(μ-oxo)CuIICuIII species is reactive to methane because the amplitude of the σ* singly occupied MO localized on the bridging oxo moieties plays an essential role in C-H activation..
46. Pawel M. Kozlowski, Yoshihito Shiota, Satomi Gomita, Hidetake Seino, Yasushi Mizobe, Kazunari Yoshizawa, DFT analysis of cubane-type feir3s4 clusters. Dinitrogen binding and activation at the tetrahedral Fe site, Bulletin of the Chemical Society of Japan, 10.1246/bcsj.80.2323, 80, 12, 2323-2328, 2007, 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..
47. Yuko Ito, Hiroki Kondo, Yoshihito Shiota, Kazunari Yoshizawa, Theoretical analysis of the reaction mechanism of biotin carboxylase, Journal of Chemical Theory and Computation, 10.1021/ct700260f, 4, 2, 366-374, 2008.02, 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 CP, followed by collapse of CP into CO 2 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..
48. Takahiko Kojima, Daisuke Noguchi, Tomoko Nakayama, Yuji Inagaki, Yoshihito Shiota, Kazunari Yoshizawa, Kei Ohkubo, Shunichi Fukuzumi, Synthesis and characterization of novel ferrocene-containing pyridylamine ligands and their ruthenium(II) complexes
Electronic communication through hydrogen-bonded amide linkage, Inorganic Chemistry, 10.1021/ic7016038, 47, 3, 886-895, 2008.02, 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)-N,N- bis(2-pyridylmethyl)amine (Fc-TPA; L1) and N,N-bis(6-ferrocenoylamide-2- pyridylmethyl)-N-(2-pyridylmethyl)amine (Fc2-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 dimethyl sulfoxide (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 [RuIIICl(L1 +)(DMSO)]3+ to generate a ferromagnetically coupled triplet state (S = 1) with J = 13.7 cm-1 (H = -JS1S 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..
49. Yuichirou Hirai, Takahiko Kojima, Yasuhisa Mizutani, Yoshihito Shiota, Kazunari Yoshizawa, Shunichi Fukuzumi, Ruthenium-catalyzed selective and efficient oxygenation of hydrocarbons with water as an oxygen source, Angewandte Chemie - International Edition, 10.1002/anie.200801170, 47, 31, 5772-5776, 2008.07, (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..
50. Toshinori Inoue, Yoshihito Shiota, Kazunari Yoshizawa, Quantum chemical approach to the mechanism for the biological conversion of tyrosine to dopaquinone, Journal of the American Chemical Society, 10.1021/ja802618s, 130, 50, 16890-16897, 2008.12, 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 (μ-η22-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 0-0 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..
51. Jun Li, Yoshihito Shiota, Kazunari Yoshizawa, Metal-ligand cooperation in H2 production and H2O decomposition on a Ru(II) PNN complex
The role of ligand dearomatization- aromatization, Journal of the American Chemical Society, 10.1021/ja905073s, 131, 38, 13584-13585, 2009, (Chemical Equation Presented) The molecular mechanism for H2 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 H2. The metal center and the PNN ligand, which can be dearomatized and aromatized again, play active and synergistic roles in H2 production and the succeeding H2O decomposition. Formation of the cis-dihydroxo complex as the main final product is a result of thermodynamic control..
52. Yoshihito Shiota, Kazunari Yoshizawa, Comparison of the reactivity of bis(μ-oxo)CuIICu III and CuIIICuIII species to methane, Inorganic Chemistry, 10.1021/ic8003933, 48, 3, 838-845, 2009.02, Methane hydroxylation at the dinuclear copper site of particulate methane monooxygenase (pMMO) is studied by using density functional theory (DFT) calculations. The electronic and structural properties of the dinuclear copper species of bis(μ-oxo)CuIICuIII and Cu IIICuIII are discussed with respect to the C-H bond activation of methane. The bis(μ-oxo)CuIICuIII species is highly reactive and considered to be an active species for the conversion of methane to methanol by pMMO, whereas the bis(μ-oxu)CuIIICu III species is unable to react with methane as it is. If a Cu-O bond of the bis(μ-oxo)CuIIICuIII species is cleaved, the resultant CuIIICuIII 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(μ-oxo)Cu IICuIII species. The DFT calculations show that the bis(μ-oxo)CuIICuIII species is more effective for the activation of methane than the bis(μ-oxu)CuIIICuII species. The reactive bis(μ-oxo)CuIICuIII species can be created either from the electron injection to the bis(μ-oxo)Cu IIICuIII species or from the O-O bond cleavage in the μ-η12-peroxoCuICuII species..
53. Hironori Tsutsumi, Yusuke Sunada, Yoshihito Shiota, Kazunari Yoshizawa, Hideo Nagashima, Nickel(II), Palladium(II), and Platinum(II) η3-Allyl Complexes Bearing a Bidentate Titanium(IV) Phosphinoamide Ligand
A Ti←M2 Dative Bond Enhances the Electrophilieity of the π-Allyl, Organometallics, 10.1021/om8011085, 28, 7, 1988-1991, 2009.04, Three Ti-M2 (M2 = Ni, Pd, Pt) heterobimetallic complexes, [η3-methallyl)M2(Ph2PN tBu)2TiCl2](OTf), were synthesized in which a TiIV←M2 interaction was suggested by crystallography and DFT calculations. The TiIV←M2 interaction enhanced electrophilicity of the η3-methallyl ligand of M 2, leading to high reactivity of the η3-methallyl moiety with Et2NH compared with that of the dppp analogue..
54. Hiromasa Tanaka, Yoshihito Shiota, Tsukasa Matsuo, Hiroyuki Kawaguchi, Kazunari Yoshizawa, DFT study on N2 activation by a hydride-bridged diniobium complex. N≡N Bond cleavage accompanied by H2 evolution, Inorganic Chemistry, 10.1021/ic802377p, 48, 8, 3875-3881, 2009.04, 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, {Nbv(μ-N)2Nbv}2-. 22With the assumption of a tetrakis(μ-hydrido)diniobium complex {Nblv(μ-H)4Nblv}2- as an initial complex, the N≡N cleavage on the Nb2 core proceeds in four steps. Dinitrogen is coordinated to the {Nb'''-H)Nb'''} core in a side- on/end-on manner, accompanied by the reductive elimination of H2. The N≡N bond of dinitrogen is activated up to a single bond (formally N 24-) by the two Nb(lll) atoms, once it is bound to the Nb2 core. Two electrons are prepared for the cleavage of the N-N single bond through the μ-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(μ-N)2Nb} 2- is generated after H2 elimination in which the N-bonded H atom is coupled with the remaining μ-H atom. The final H2 elimination is calculated to be the rate-determining step..
55. Takahiko Kojima, Norihisa Hirasa, Daisuke Noguchi, Tomoya Ishizuka, Soushi Miyazaki, Yoshihito Shiota, Kazunari Yoshizawa, Shunichi Fukuzumi, Synthesis and characterization of ruthenium(II)-pyridylamine complexes with catechol pendants as metal binding sites, Inorganic Chemistry, 10.1021/ic902070q, 49, 8, 3737-3745, 2010.04, 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 (Cat2-TPA; L2), and its precursor, N,N-bis[6-{3,4-bis(benzyloxy)-benzamide}-2-pyridyl-methyl]-N-(2-pyridylmethyl) -amine ((Bn2Cat)2-TPA; L1), formed stable ruthenium(II) complexes, [RuCl(L2)]PF6 (2) and [RuCl(L1)]PF6 (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 π-π interactions. The 1H 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 pKa 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(NO3) 2(TMEDA)] (TMEDA = N,N,N′,N′-tetramethylethylenediamine) in methanol allowed us to observe the selective formation of a binuclear complex, [RuCl(L22-){Cu(TMEDA)}]PF6 (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 κ2-O,O′:η2-coordination: it exhibits a novel bridging coordination mode, bis-κ1-O:η1- coordination, to form the binuclear Ru(II)-Cu(II) complex..
56. Yoshihito Shiota, Daisuke Sato, Gergely Juhász, Kazunari Yoshizawa, Theoretical study of thermal spin transition between the singlet state and the quintet state in the [Fe(2-picolylamine)3]2+ spin crossover system, Journal of Physical Chemistry A, 10.1021/jp9122002, 114, 18, 5862-5869, 2010.05, 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 (SQM) were found (ESQ = 6.8 kcal/mol), as well as the MECPs between the triplet and singlet states (STM, EST = 12.9 kcal/mol) and the triplet and quintet states (TQM, ETQ = 12.8 kcal/mol). Although the distortion leading to SQM from the singlet equilibrium geometry is mainly a symmetric expansion of the Fe-N bonds, the distortions leading to STM and SQM 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..
57. Takahiko Kojima, Yuichirou Hirai, Tomoya Ishizuka, Yoshihito Shiota, Kazunari Yoshizawa, Kenichiro Ikemura, Takashi Ogura, Shunichi Fukuzumi, A low-spin ruthenium(IV)-oxo complex
Does the spin state have an impact on the reactivity?, Angewandte Chemie - International Edition, 10.1002/anie.201002733, 49, 45, 8449-8453, 2010.11, 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..
58. Daisuke Sato, Yoshihito Shiota, Gergely Juhász, Kazunari Yoshizawa, Theoretical study of the mechanism of valence tautomerism in cobalt complexes, Journal of Physical Chemistry A, 10.1021/jp107391x, 114, 49, 12928-12935, 2010.12, Valence tautomerism is studied in the [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, 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..
59. Shu Ping Huang, Yoshihito Shiota, Kazunari Yoshizawa, DFT study of the mechanism for methane hydroxylation by soluble methane monooxygenase, Quaternary International, 2011, 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..
60. Hiromasa Tanaka, Yoshiyuki Kondo, Yoshihito Shiota, Akinobu Naka, Mitsuo Ishikawa, Kazunari Yoshizawa, Theoretical study on the formation of silacyclopropene from acylsilane and acetylene via silene-to-silylene rearrangement, Organometallics, 10.1021/om2002393, 30, 11, 3160-3167, 2011.05, 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-[(tertbutyl) bis(trimethylsilyl)methyl]-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 (SidC) intermediate derived from the acylsilane reactswith 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 silacyclopropene. (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..
61. Tomoya Ishizuka, Takuya Sawaki, Soushi Miyazaki, Masaki Kawano, Yoshihito Shiota, Kazunari Yoshizawa, Shunichi Fukuzumi, Takahiko Kojima, Mechanistic insights into photochromic behavior of a ruthenium(II)- Pterin complex, Chemistry - A European Journal, 10.1002/chem.201003522, 17, 24, 6652-6662, 2011.06, The pterin-coordinated ruthenium complex, [RuII(dmdmp)- (tpa)]+ (1) (Hdmdmp=N,N-dimethyl- 6,7-dimethylpterin, tpa=tris(2-pyridylmethyl) 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 (f) 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 dmdmp2+ 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+10+5 s+1. The activation parameters were determined to be DH+=81.8 kJmol+1 and DS+= +49.8 Jmol+1K+1. The negative DS+ 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 firstorder process in acetonitrile were determined to be DH+=59.2 kJmol+1 and DS+=+147.4 kJmol+1K+1. The largely negative DS+ 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 RuII center is a determinant factor in the photoisomerization of the RuII- pterin complex..
62. Yoshihito Shiota, Jorge M. Herrera, Gergely Juhász, Takafumi Abe, Shingo Ohzu, Tomoya Ishizuka, Takahiko Kojima, Kazunari Yoshizawa, Theoretical study of oxidation of cyclohexane diol to adipic anhydride by [RuIV(O)(tpa)(H2O)]2+ complex (tpa = Tris(2-pyridylmethyl)amine), Inorganic Chemistry, 10.1021/ic200481n, 50, 13, 6200-6209, 2011.07, The catalytic conversion of 1,2-cyclohexanediol to adipic anhydride by RuIVO(tpa) (tpa = tris(2-pyridylmethyl)amine) is discussed using density functional theory calculations. The whole reaction is divided into three steps: (1) formation of α-hydroxy cyclohexanone by dehydrogenation of cyclohexanediol, (2) formation of 1,2-cyclohexanedione by dehydrogenation of α-hydroxy cyclohexanone, and (3) formation of adipic anhydride by oxygenation of cyclohexanedione. In each step the two-electron oxidation is performed by RuIVO(tpa) active species, which is reduced to bis-aqua RuII(tpa) complex. The RuII 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 with the spin transition in both 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 (kH/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..
63. Tomoya Ishizuka, Takuya Sawaki, Soushi Miyazaki, Masaki Kawano, Yoshihito Shiota, Kazunari Yoshizawa, Shunichi Fukuzumi, Takahiko Kojima, Erratum
Mechanistic Insights into Photochromic Behavior of a Ruthenium(II)-Pterin Complex (Chemistry - A European Journal 17 (2011)), Chemistry - A European Journal, 10.1002/chem.201102403, 17, 36, 9857, 2011.08.
64. Tomoya Ishizuka, Kengo Tobita, Yuichi Yano, Yoshihito Shiota, Kazunari Yoshizawa, Shunichi Fukuzumi, Takahiko Kojima, Proton-coupled electron shuttling in a covalently linked ruthenium-copper heterodinuclear complex, Journal of the American Chemical Society, 10.1021/ja208141b, 133, 46, 18570-18573, 2011.11, A heterodinuclear complex based on a RuII-TPA [TPA = tris(2-pyridylmethyl)amine] complex having a peripheral CuII (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..
65. Tao Liu, Da Peng Dong, Shinji Kanegawa, Soonchul Kang, Osamu Sato, Yoshihito Shiota, Kazunari Yoshizawa, Shinya Hayami, Shuo Wu, Cheng He, Chun Ying Duan, Reversible electron transfer in a linear {Fe 2Co} trinuclear complex induced by thermal treatment and photoirraditaion, Angewandte Chemie - International Edition, 10.1002/anie.201201305, 51, 18, 4367-4370, 2012.04, 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..
66. Hiromasa Tanaka, Yoshihito Shiota, Kazunori Hori, Akinobu Naka, Mitsuo Ishikawa, Kazunari Yoshizawa, Substituent effects in thermal reactions of a silene with silyl-substituted alkynes
A theoretical study, Organometallics, 10.1021/om300310g, 31, 13, 4737-4747, 2012.07, Thermal reactions of a silene derivative (Me 3Si) 2Si=C(OSiMe 3)(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 OSiMe 3 group on the silene Si atom in the silabutadienes. If the substituent on one of the acetylene C atoms does not easily migrate, the SiMe 3 group of the OSiMe 3 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 OSiMe 3 group, resulting in the formation of an allene derivative..
67. Tao Liu, Hui Zheng, Soonchul Kang, Yoshihito Shiota, Shinya Hayami, Masaki Mito, Osamu Sato, Kazunari Yoshizawa, Shinji Kanegawa, Chunying Duan, A light-induced spin crossover actuated single-chain magnet, Nature Communications, 10.1038/ncomms3826, 4, 2013, 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..
68. Shu Ping Huang, Yoshihito Shiota, Kazunari Yoshizawa, DFT study of the mechanism for methane hydroxylation by soluble methane monooxygenase (sMMO)
Effects of oxidation state, spin state, and coordination number, Dalton Transactions, 10.1039/c2dt31304a, 42, 4, 1011-1023, 2013.01, 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 IV 2(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..
69. Aiko Fukazawa, Hiroya Oshima, Yoshihito Shiota, Shouya Takahashi, Kazunari Yoshizawa, Shigehiro Yamaguchi, Thiophene-fused bisdehydro[12]annulene that undergoes transannular alkyne cycloaddition by either light or heat, Journal of the American Chemical Society, 10.1021/ja3126849, 135, 5, 1731-1734, 2013.02, A new bisdehydro[12]annulene derivative having a thiophene-fused structure has been synthesized. This highly twisted π-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 8π and 4π 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 π-scaffold for electronic applications..
70. Hiroumi Mitome, Tomoya Ishizuka, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Heteronuclear RuIIAgI complexes having a pyrroloquinolinequinone derivative as a bridging ligand, Inorganic Chemistry, 10.1021/ic302617b, 52, 5, 2274-2276, 2013.03, 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 RuIIAg I units were linked by two [AgI(OTf)2] - units (OTf = CF3SO3 -). A solvent-bound RuIIAgI 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..
71. Yoshihito Shiota, Gergely Juhász, Kazunari Yoshizawa, Role of tyrosine residue in methane activation at the dicopper site of particulate methane monooxygenase
A density functional theory study, Inorganic Chemistry, 10.1021/ic400417d, 52, 14, 7907-7917, 2013.07, 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 (μ-oxo)(μ-hydroxo)CuIICuIII 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 μ-η22-peroxoCu IICuII species and the resultant (μ-oxo)(μ-hydroxo) CuIICuIII species can hydroxylate methane. This species for methane hydroxylation is more favorable in reactivity than the bis(μ-oxo)CuIIICuIII species. The H-atom transfer or proton-coupled electron transfer from the tyrosine residue can reasonably induce the O-O bond dissociation of the μ-η22- peroxoCuIICuII species to form the reactive (μ-oxo)(μ-hydroxo)CuIICuIII 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 KIE values reported so far..
72. Tomoya Ishizuka, Yuta Saegusa, Yoshihito Shiota, Kazuhisa Ohtake, Kazunari Yoshizawa, Takahiko Kojima, Multiply-fused porphyrins - Effects of extended π-conjugation on the optical and electrochemical properties, Chemical Communications, 10.1039/c3cc42831a, 49, 53, 5939-5941, 2013.07, 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..
73. Takuya Sawaki, Tomoya Ishizuka, Masaki Kawano, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Complete photochromic structural changes in ruthenium(II)-diimine complexes, based on control of the excited states by metalation, Chemistry - A European Journal, 10.1002/chem.201300437, 19, 27, 8978-8990, 2013.07, 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, [RuII3-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 μ-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 (2→2 and 3→3). 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. Ru being served? Photochromic structural changes of RuII/TPA (TPA=tris(2- pyridylmethyl)amine) complexes that contain a diimine ligand are reported. Further metal coordination to the diimine ligand allows interconversion between its open/closed forms (see scheme)..
74. Zhao Yang Li, Jing Wei Dai, Yoshihito Shiota, Kazunari Yoshizawa, Shinji Kanegawa, Osamu Sato, Multi-step spin crossover accompanied by symmetry breaking in an Fe III complex
Crystallographic evidence and DFT studies, Chemistry - A European Journal, 10.1002/chem.201302272, 19, 39, 12948-12952, 2013.09, 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..
75. Shuping Huang, Qisheng Zhang, Yoshihito Shiota, Tetsuya Nakagawa, Kazuhiro Kuwabara, Kazunari Yoshizawa, Chihaya Adachi, Computational prediction for singlet- and triplet-transition energies of charge-transfer compounds, Journal of Chemical Theory and Computation, 10.1021/ct400415r, 9, 9, 3872-3877, 2013.09, 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..
76. Masahito Kodera, Tomokazu Tsuji, Tomohiro Yasunaga, Yuka Kawahara, Tomoya Hirano, Yutaka Hitomi, Takashi Nomura, Takashi Ogura, Yoshio Kobayashi, P. K. Sajith, Yoshihito Shiota, Kazunari Yoshizawa, Roles of carboxylate donors in O-O bond scission of peroxodi-iron(iii) to high-spin oxodi-iron(iv) with a new carboxylate-containing dinucleating ligand, Chemical Science, 10.1039/c3sc51541a, 5, 6, 2282-2292, 2014.01, 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. This journal is.
77. Zi Shuo Yao, Masaki Mito, Takashi Kamachi, Yoshihito Shiota, Kazunari Yoshizawa, Nobuaki Azuma, Yuji Miyazaki, Kazuyuki Takahashi, Kuirun Zhang, Takumi Nakanishi, Soonchul Kang, Shinji Kanegawa, Osamu Sato, Molecular motor-driven abrupt anisotropic shape change in a single crystal of a Ni complex, Nature Chemistry, 10.1038/nchem.2092, 6, 12, 1079-1083, 2014.01, 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. Variableerature single-crystal X-ray diffraction studies show that the crystalline shape change is induced by an unusual 90° 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 subnanometre-scale 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..
78. Tomoya Ishizuka, Shingo Ohzu, Hiroaki Kotani, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Hydrogen atom abstraction reactions independent of C-H bond dissociation energies of organic substrates in water
Significance of oxidant-substrate adduct formation, Chemical Science, 10.1039/c3sc53002g, 5, 4, 1429-1436, 2014.04, 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..
79. P. K. Sajith, Yoshihito Shiota, Kazunari Yoshizawa, Role of acidic proton in the decomposition of NO over dimeric Cu(I) active sites in Cu-ZSM-5 catalyst
A QM/MM study, ACS Catalysis, 10.1021/cs500223z, 4, 6, 2075-2085, 2014.06, 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 1T and 2T, where the Cu(I) atoms are separated by one and two SiO4 tetrahedra, respectively. The reaction proceeds through the formation of N 2O 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 1T 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 N 2O 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 O2 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..
80. Takahiko Kojima, Ryosuke Kobayashi, Tomoya Ishizuka, Shinya Yamakawa, Hiroaki Kotani, Tatsuaki Nakanishi, Kei Ohkubo, Yoshihito Shiota, Kazunari Yoshizawa, Shunichi Fukuzumi, Binding of Scandium Ions to Metalloporphyrin-Flavin Complexes for Long-Lived Charge Separation, Chemistry - A European Journal, 10.1002/chem.201403960, 20, 47, 15518-15532, 2014.11, A porphyrin-flavin-linked dyad and its zinc and palladium complexes (MPor-Fl: 2-M, M=2 H, 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-H2 occurred from the singlet excited state of the porphyrin moiety (H2Por) to the flavin (Fl) moiety to produce the singlet charge-separated (CS) state 1(H2Por.+-Fl.-), which decayed through back ET (BET) to form 3[H2Por]∗-Fl with rate constants of 1.2×1010 and 1.2×109 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 2.1×1011 and 6.0×1010 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=2.2×105 M -1 and K 2=1.8×103 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 Sc3+ complexes afforded the triplet CS state (3[PdPor.+-Fl.--(Sc3+)2]), which exhibited a remarkably long lifetime of τ=110 ms (k BET=9.1 s-1). More than transient: A porphyrin-flavin-linked dyad and its zinc and palladium complexes were synthesized and the X-ray crystal structure of the palladium complex was determined. The Pd-Sc3+ complex afforded a remarkably long lifetime of 110 ms of the charge-separated state (see figure)..
81. Shingo Ohzu, Tomoya Ishizuka, Hiroaki Kotani, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Tetranuclear ruthenium(II) complex with a dinucleating ligand forming multi-mixed-valence states, Inorganic Chemistry, 10.1021/ic502422u, 53, 24, 12677-12679, 2014.12, A square-shaped tetranuclear ruthenium(II) complex, [RuII 4Cl5(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 Robin-Day class II..
82. Yusuke Sunada, Shintaro Ishida, Fumiya Hirakawa, Yoshihito Shiota, Kazunari Yoshizawa, Shinji Kanegawa, Osamu Sato, Hideo Nagashima, Takeaki Iwamoto, Persistent four-coordinate iron-centered radical stabilized by π-donation, Chemical Science, 10.1039/c5sc02601f, 7, 1, 191-198, 2015, Dinuclear iron carbonyl complex 2, which contains an elongated unsupported Fe-Fe bond, was synthesized by the reaction between Fe2(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..
83. Min Zhang, Takaaki Sonoda, Yoshihito Shiota, Masaaki Mishima, Hikaru Yanai, Masaya Fujita, Takeo Taguchi, Gas-phase acidity of 1,1-bis(trifluoromethanesulfonyl) propane derivatives and related compounds
Experimental and theoretical studies, Journal of Physical Organic Chemistry, 10.1002/poc.3304, 28, 3, 181-186, 2015, The gas-phase acidity (GA) of a series of 1,1-bis(trifluoromethanesulfonyl)propane derivatives, Tf2CHCH2CH(R1)R2, 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 R1 and R2 or Ar groups on the acidity were examined. In Tf2CHCH2CH(R1)R2, 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 R1 or R2 group and between the hydrogen atom of the CHR1R2 moiety and the SO2CF3 group were correlated in terms of an equation, GA=-17.0∑σl + 3.4∑σα + 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..
84. Yoshihito Shiota, Shoya Takahashi, Shingo Ohzu, Tomoya Ishizuka, Takahiko Kojima, Kazunari Yoshizawa, Mechanistic study of methanol oxidation by RuIV-oxo complexes, Journal of Porphyrins and Phthalocyanines, 10.1142/S1088424615500285, 19, 1-3, 417-426, 2015.01, The catalytic conversion of methanol to formaldehyde by three kinds of non-porphyrin Ru complexes, RuIVO(TPA) (TPA = tris(2-pyridylmethyl)amine) (1a), RuIVO(6-COO-TPA) (6-COOTPA = 2-(6-carboxyl-pyridyl)methyl-bis(2-pyridylmethyl)amine) (1b), and RuIVO(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 (•CH2OH) and the methoxyl radical (CH3O•), respectively, as an intermediate. DFT calculations demonstrate that the two pathways are energetically comparable in the reactions by the three RuIV-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 RuIV-oxo complexes, being useful for considering the mechanism of methanol oxidation..
85. Soonchul Kang, Hui Zheng, Tao Liu, Kohei Hamachi, Shinji Kanegawa, Kunihisa Sugimoto, Yoshihito Shiota, Shinya Hayami, Masaki Mito, Tetsuya Nakamura, Motohiro Nakano, Michael L. Baker, Hiroyuki Nojiri, Kazunari Yoshizawa, Chunying Duan, Osamu Sato, A ferromagnetically coupled Fe 42 cyanide-bridged nanocage, Nature Communications, 10.1038/ncomms6955, 6, 2015.01, 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..
86. Hiroaki Kotani, Suzue Kaida, Tomoya Ishizuka, Miyuki Sakaguchi, Takashi Ogura, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Formation and characterization of a reactive chromium(V)-oxo complex
Mechanistic insight into hydrogen-atom transfer reactions, Chemical Science, 10.1039/c4sc02285h, 6, 2, 945-955, 2015.02, 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..
87. Hiroumi Mitome, Tomoya Ishizuka, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Controlling the redox properties of a pyrroloquinolinequinone (PQQ) derivative in a ruthenium(ii) coordination sphere, Dalton Transactions, 10.1039/c4dt03358b, 44, 7, 3151-3158, 2015.02, 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. This journal is.
88. Hideki Sugimoto, Akine Mikami, Kenichiro Kai, P. K. Sajith, Yoshihito Shiota, Kazunari Yoshizawa, Kaori Asano, Takeyuki Suzuki, Shinobu Itoh, Cis -1,2-Aminohydroxylation of Alkenes Involving a Catalytic Cycle of Osmium(III) and Osmium(V) Centers
OsV(O)(NHTs) Active Oxidant with a Macrocyclic Tetradentate Ligand, Inorganic Chemistry, 10.1021/acs.inorgchem.5b01083, 54, 14, 7073-7082, 2015.07, Catalytic activity of [OsIII(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 [OsV(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..
89. Tatsuya Suzuki, Hiromasa Tanaka, Yoshihito Shiota, P. K. Sajith, Yasuhiro Arikawa, Kazunari Yoshizawa, Proton-Assisted Mechanism of NO Reduction on a Dinuclear Ruthenium Complex, Inorganic Chemistry, 10.1021/acs.inorgchem.5b00394, 54, 15, 7181-7191, 2015.08, 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 - Ni = 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..
90. Hiroaki Kotani, Takumi Sugiyama, Tomoya Ishizuka, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Redox-Noninnocent Behavior of Tris(2-pyridylmethyl)amine Bound to a Lewis Acidic Rh(III) Ion Induced by C-H Deprotonation, Journal of the American Chemical Society, 10.1021/jacs.5b06237, 137, 35, 11222-11225, 2015.09, 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..
91. Sheng Qun Su, Takashi Kamachi, Zi Shuo Yao, You Gui Huang, Yoshihito Shiota, Kazunari Yoshizawa, Nobuaki Azuma, Yuji Miyazaki, Motohiro Nakano, Goro Maruta, Sadamu Takeda, Soonchul Kang, Shinji Kanegawa, Osamu Sato, Assembling an alkyl rotor to access abrupt and reversible crystalline deformation of a cobalt(II) complex, Nature Communications, 10.1038/ncomms9810, 6, 2015.11, 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° 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..
92. Soonchul Kang, Yoshihito Shiota, Akira Kariyazaki, Shinji Kanegawa, Kazunari Yoshizawa, Osamu Sato, Frontispiece
Heterometallic FeIII/K Coordination Polymer with a Wide Thermal Hysteretic Spin Transition at Room Temperature, Chemistry - A European Journal, 10.1002/chem.201680262, 22, 2, 532, 2016, The FeIII anionic 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 shows a one-step transition in heating mode and a two-step transition in cooling mode. X-ray structure analysis shows the coexistence of hydrogen bond and cation-π interactions, along with metal coordination bonds, to give 2D coordination polymer structure..
93. You Gui Huang, Yoshihito Shiota, Sheng Qun Su, Shu Qi Wu, Zi Shuo Yao, Guo Ling Li, Shinji Kanegawa, Soonchul Kang, Takashi Kamachi, Kazunari Yoshizawa, Katsuhiko Ariga, Osamu Sato, Thermally Induced Intra-Carboxyl Proton Shuttle in a Molecular Rack-and-Pinion Cascade Achieving Macroscopic Crystal Deformation, Angewandte Chemie - International Edition, 10.1002/anie.201607886, 55, 47, 14628-14632, 2016.01, 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..
94. Koji Yoshimoto, Takeshi Yatabe, Takahiro Matsumoto, Andrew Robertson, Hidetaka Nakai, Hiromasa Tanaka, Takashi Kamachi, Yoshihito Shiota, Kazunari Yoshizawa, Koichiro Asazawa, Hirohisa Tanaka, Seiji Ogo, Synthesis and structure of a water-soluble μ-η1
η1-N2 dinuclear RuII complex with a polyamine ligand, Chemistry Letters, 10.1246/cl.151004, 45, 2, 149-151, 2016.01, We report the first example of a μ-η11-N2 dinuclear RuII 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 N2-coordinated RuII complexes. This low value strongly suggests the N=N bond is primed for activation..
95. Shin ichiro Kato, Tomokazu Kijima, Yoshihito Shiota, Toshitada Yoshihara, Seiji Tobita, Kazunari Yoshizawa, Yosuke Nakamura, Push–pull fluorenones and benzazulenequinones
regioselective [4+2] and [2+2] cycloadditions of benzopentalenequinone derivative and alkynes bearing an aniline moiety, Tetrahedron Letters, 10.1016/j.tetlet.2016.09.002, 57, 41, 4604-4607, 2016.01, 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..
96. Soonchul Kang, Yoshihito Shiota, Akira Kariyazaki, Shinji Kanegawa, Kazunari Yoshizawa, Osamu Sato, Heterometallic FeIII/K Coordination Polymer with a Wide Thermal Hysteretic Spin Transition at Room Temperature, Chemistry - A European Journal, 10.1002/chem.201503392, 22, 2, 532-538, 2016.01, 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..
97. Yoshihiro Shimoyama, Tomoya Ishizuka, Hiroaki Kotani, Yoshihito Shiota, Kazunari Yoshizawa, Kaoru Mieda, Takashi Ogura, Toshihiro Okajima, Shunsuke Nozawa, Takahiko Kojima, A Ruthenium(III)–Oxyl Complex Bearing Strong Radical Character, Angewandte Chemie - International Edition, 10.1002/anie.201607861, 55, 45, 14041-14045, 2016.01, 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) �. 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..
98. Kazuyuki Takahashi, Kiko Kawamukai, Mitsunobu Okai, Tomoyuki Mochida, Takahiro Sakurai, Hitoshi Ohta, Takashi Yamamoto, Yasuaki Einaga, Yoshihito Shiota, Kazunari Yoshizawa, A New Family of Anionic FeIII Spin Crossover Complexes Featuring a Weak-Field N2O4 Coordination Octahedron, Chemistry - A European Journal, 10.1002/chem.201504883, 22, 4, 1253-1257, 2016.01, 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)..
99. Tomoya Ishizuka, Atsuko Watanabe, Hiroaki Kotani, Dachao Hong, Kenta Satonaka, Tohru Wada, Yoshihito Shiota, Kazunari Yoshizawa, Kazuaki Ohara, Kentaro Yamaguchi, Satoshi Kato, Shunichi Fukuzumi, Takahiko Kojima, Homogeneous Photocatalytic Water Oxidation with a Dinuclear CoIII-Pyridylmethylamine Complex, Inorganic Chemistry, 10.1021/acs.inorgchem.5b02336, 55, 3, 1154-1164, 2016.02, 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 (S2O8 2-) 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 H2 16O and H2 18O 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..
100. Shuhei Itoyama, Kazuki Doitomi, Takashi Kamachi, Yoshihito Shiota, Kazunari Yoshizawa, Possible Peroxo State of the Dicopper Site of Particulate Methane Monooxygenase from Combined Quantum Mechanics and Molecular Mechanics Calculations, Inorganic Chemistry, 10.1021/acs.inorgchem.5b02603, 55, 6, 2771-2775, 2016.03, 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 CuICuII state from a computed Cu-Cu distance of 2.62 Å from calculations at the B3LYP-D/TZVP level of theory. A μ-η22-peroxo-CuII2 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 Å, 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 μ-η22-peroxo-CuII2 state are calculated and analyzed with TD-DFT calculations..
101. Masahito Kodera, Shin Ishiga, Tomokazu Tsuji, Katsutoshi Sakurai, Yutaka Hitomi, Yoshihito Shiota, P. K. Sajith, Kazunari Yoshizawa, Kaoru Mieda, Takashi Ogura, Formation and High Reactivity of the anti-Dioxo Form of High-Spin μ-Oxodioxodiiron(IV) as the Active Species That Cleaves Strong C-H Bonds, Chemistry - A European Journal, 10.1002/chem.201600048, 22, 17, 5924-5936, 2016.04, 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 toluene8]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..
102. Suguru Murata, Kazuyuki Takahashi, Takahiro Sakurai, Hitoshi Ohta, Takashi Yamamoto, Yasuaki Einaga, Yoshihito Shiota, Kazunari Yoshizawa, The role of coulomb interactions for spin crossover behaviors and crystal structural transformation in novel anionic fe(III) complexes from a π-extended ono ligand, Crystals, 10.3390/cryst6050049, 6, 5, 2016.05, To investigate the π-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 π-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 π-stacking interactions between π-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 π-stacking interactions induces a significant distortion of coordination structure of 2..
103. You Gui Huang, Yoshihito Shiota, Ming Yan Wu, Sheng Qun Su, Zi Shuo Yao, Soonchul Kang, Shinji Kanegawa, Guo Ling Li, Shu Qi Wu, Takashi Kamachi, Kazunari Yoshizawa, Katsuhiko Ariga, Mao Chun Hong, Osamu Sato, Superior thermoelasticity and shape-memory nanopores in a porous supramolecular organic framework, Nature Communications, 10.1038/ncomms11564, 7, 2016.05, 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..
104. Yoshiki Fujita, Manabu Abe, Yoshihito Shiota, Tatsuya Suzuki, Kazunari Yoshizawa, Computational Study of Cyclobutane-1,3-diylidene Dicarbenes
Ground-State Spin Multiplicity and New Strategy toward the Synthesis of Bicyclo[1.1.0]but-1(3)-enes, Bulletin of the Chemical Society of Japan, 10.1246/bcsj.20160051, 89, 7, 770-778, 2016.07, 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..
105. Hiroumi Mitome, Tomoya Ishizuka, Hiroaki Kotani, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Mechanistic Insights into C-H Oxidations by Ruthenium(III)-Pterin Complexes
Impact of Basicity of the Pterin Ligand and Electron Acceptability of the Metal Center on the Transition States, Journal of the American Chemical Society, 10.1021/jacs.6b03785, 138, 30, 9508-9520, 2016.08, 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..
106. Atsushi Tahara, Hiromasa Tanaka, Yusuke Sunada, Yoshihito Shiota, Kazunari Yoshizawa, Hideo Nagashima, Theoretical Study of the Catalytic Hydrogenation of Alkenes by a Disilaferracyclic Complex
Can the Fe-Si σ-Bond-Assisted Activation of H-H Bonds Allow Development of a Catalysis of Iron?, Journal of Organic Chemistry, 10.1021/acs.joc.6b01961, 81, 22, 10900-10911, 2016.11, The mechanisms associated with the hydrogenation of alkenes catalyzed by the iron complex Fe(cis-CO)2{o-(SiMe2)2C6H4}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···H···Si interactions were also observed. The exchange of a 1,2-bis(dimethylsilyl)benzene ligand with ethylene and hydrogen gives a disilaferracycle bearing η2-(CH2=CH2) and η2-H2 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 η1-(H-Si) moieties (step 1), hydrogen migration from the Fe-H group to the η2-(CH2=CH2) ligand which accomplishes the insertion of ethylene into the Fe-H bond (step 2), and reaction of the resulting β-agostic ethyl moiety with the η2-(H-Si) group to form ethane on the iron atom (step 3). The octahedral geometry of 1 as well as the presence of π-acidic CO ligands and Fe-Si σ-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 σ-complex-assisted metathesis (σ-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..
107. Shinji Kanegawa, Yoshihito Shiota, Soonchul Kang, Kazuyuki Takahashi, Hajime Okajima, Akira Sakamoto, Tatsuya Iwata, Hideki Kandori, Kazunari Yoshizawa, Osamu Sato, Directional Electron Transfer in Crystals of [CrCo] Dinuclear Complexes Achieved by Chirality-Assisted Preparative Method, Journal of the American Chemical Society, 10.1021/jacs.6b05089, 138, 43, 14170-14173, 2016.11, 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..
108. M. Haris Mahyuddin, Aleksandar Tsekov Staykov, Yoshihito Shiota, Kazunari Yoshizawa, Direct Conversion of Methane to Methanol by Metal-Exchanged ZSM-5 Zeolite (Metal = Fe, Co, Ni, Cu), ACS Catalysis, 10.1021/acscatal.6b01721, 6, 12, 8321-8331, 2016.12, 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. (Chemical Equation Presented)..
109. Konoka Hoshi, Atsushi Tahara, Yusuke Sunada, Hironori Tsutsumi, Ryoko Inoue, Hiromasa Tanaka, Yoshihito Shiota, Kazunari Yoshizawa, Hideo Nagashima, α-CAM mechanisms for the hydrogenation of alkenes by cis- and trans- disilametallacyclic carbonyl complexes (M = Fe, Ru, Os)
Experimental and theoretical studies, Bulletin of the Chemical Society of Japan, 10.1246/bcsj.20170004, 90, 5, 613-626, 2017, 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..
110. Tomokazu Tsuji, Antonius Andre Zaoputra, Yutaka Hitomi, Kaoru Mieda, Takashi Ogura, Yoshihito Shiota, Kazunari Yoshizawa, Hiroyasu Sato, Masahito Kodera, Specific Enhancement of Catalytic Activity by a Dicopper Core
Selective Hydroxylation of Benzene to Phenol with Hydrogen Peroxide, Angewandte Chemie - International Edition, 10.1002/anie.201702291, 56, 27, 7779-7782, 2017, 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..
111. Naoki Ando, Aiko Fukazawa, Tomokatsu Kushida, Yoshihito Shiota, Shuhei Itoyama, Kazunari Yoshizawa, Yasunori Matsui, Yutaro Kuramoto, Hiroshi Ikeda, Shigehiro Yamaguchi, Photochemical Intramolecular C-H Addition of Dimesityl(hetero)arylboranes by a [1,6]-Sigmatropic Rearrangement, Angewandte Chemie - International Edition, 10.1002/anie.201706929, 2017, 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 by 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)..
112. Hui Tian, Hisashi Shimakoshi, Kenji Imamura, Yoshihito Shiota, Kazunari Yoshizawa, Yoshio Hisaeda, Photocatalytic alkene reduction by a B12-TiO2 hybrid catalyst coupled with C-F bond cleavage for
Gem -difluoroolefin synthesis, Chemical Communications, 10.1039/c7cc04377e, 53, 68, 9478-9481, 2017, 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..
113. Daiki Fujita, Hideki Sugimoto, Yoshihito Shiota, Yuma Morimoto, Kazunari Yoshizawa, Shinobu Itoh, Catalytic C-H amination driven by intramolecular ligand-to-nitrene one-electron transfer through a rhodium(III) centre, Chemical Communications, 10.1039/c7cc01840a, 53, 35, 4849-4852, 2017, 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..
114. Wataru Suzuki, Hiroaki Kotani, Tomoya Ishizuka, Kei Ohkubo, Yoshihito Shiota, Kazunari Yoshizawa, Shunichi Fukuzumi, Takahiko Kojima, Thermodynamics and Photodynamics of a Monoprotonated Porphyrin Directly Stabilized by Hydrogen Bonding with Polar Protic Solvents, Chemistry - A European Journal, 10.1002/chem.201606012, 23, 19, 4669-4679, 2017.01, 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+..
115. Kazunari Yoshizawa, Takayuki Semoto, Seiji Hitaoka, Chisa Higuchi, Yoshihito Shiota, Hiromasa Tanaka, Synergy of electrostatic and van der waals interactions in the adhesion of epoxy resin with carbon-fiber and glass surfaces, Bulletin of the Chemical Society of Japan, 10.1246/bcsj.20160426, 90, 5, 500-505, 2017.01, 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..
116. Yue Wang, Takunori Harada, Yoshihito Shiota, Kazunari Yoshizawa, Heng Wang, Sheng Wang, Xichong Ye, Masamichi Ogasawara, Tamaki Nakano, Isolation and phototransformation of enantiomerically pure iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2]picolinate, RSC Advances, 10.1039/c7ra04141a, 7, 47, 29550-29553, 2017.01, 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..
117. Wataru Suzuki, Hiroaki Kotani, Tomoya Ishizuka, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Formation of supramolecular hetero-triads by controlling the hydrogen bonding of conjugate bases with a diprotonated porphyrin based on electrostatic interaction, Chemical Communications, 10.1039/c7cc03635c, 53, 47, 6359-6362, 2017.01, 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-)..
118. Zi Shuo Yao, Shu Qi Wu, Yasutaka Kitagawa, Sheng Qun Su, You Gui Huang, Guo Ling Li, Zhong Hai Ni, Hiroyuki Nojiri, Yoshihito Shiota, Kazunari Yoshizawa, Soonchul Kang, Shinji Kanegawa, Osamu Sato, Anisotropic Change in the Magnetic Susceptibility of a Dynamic Single Crystal of a Cobalt(II) Complex, Angewandte Chemie - International Edition, 10.1002/anie.201606165, 56, 3, 717-721, 2017.01, 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..
119. Yuta Saegusa, Tomoya Ishizuka, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Acid-Base Properties of a Freebase Form of a Quadruply Ring-Fused Porphyrin - Stepwise Protonation Induced by Rigid Ring-Fused Structure, Journal of Organic Chemistry, 10.1021/acs.joc.6b02419, 82, 1, 322-330, 2017.01, 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..
120. M. Haris Mahyuddin, Aleksandar Tsekov Staykov, Yoshihito Shiota, Mayuko Miyanishi, Kazunari Yoshizawa, Roles of Zeolite Confinement and Cu-O-Cu Angle on the Direct Conversion of Methane to Methanol by [Cu2(μ-O)]2+-Exchanged AEI, CHA, AFX, and MFI Zeolites, ACS Catalysis, 10.1021/acscatal.7b00588, 7, 6, 3741-3751, 2017.06, 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..
121. Taro Koide, Moritaka Takesue, Toshihiro Murafuji, Koichiro Satomi, Yasutaka Suzuki, Jun Kawamata, Kengo Terai, Mitsuharu Suzuki, Hiroko Yamada, Yoshihito Shiota, Kazunari Yoshizawa, Fumito Tani, An Azulene-Fused Tetracene Diimide with a Small HOMO–LUMO Gap, ChemPlusChem, 10.1002/cplu.201600356, 82, 7, 1010-1014, 2017.08, 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..
122. M. Haris Mahyuddin, Yoshihito Shiota, Aleksandar Tsekov Staykov, Kazunari Yoshizawa, Theoretical Investigation of Methane Hydroxylation over Isoelectronic [FeO]2+- and [MnO]+-Exchanged Zeolites Activated by N2O, Inorganic Chemistry, 10.1021/acs.inorgchem.7b01284, 56, 17, 10370-10380, 2017.09, 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..
123. Toshio Kawanami, Kentaro Ishizuka, Hiroshi Furuno, Yoshihito Shiota, Kazunari Yoshizawa, Junji Inanaga, Efficient 1H NMR chiral discrimination of sulfoxides caused by the dynamic nature of (R,R)-3′,3″-biBINOL, Tetrahedron Asymmetry, 10.1016/j.tetasy.2017.10.008, 28, 11, 1587-1590, 2017.11, 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..
124. Takaaki Yamamoto, Koki Mitsuno, Shigeki Mori, Shuhei Itoyama, Yoshihito Shiota, Kazunari Yoshizawa, Masatoshi Ishida, Hiroyuki Furuta, Two Discrete RuCp* (Cp*=Pentamethylcyclopentadienyl) Binding Modes of N-Confused Porphyrins
Peripheral π Complex and Sitting Atop Ruthenocenophane Complex by Skeletal Transformation, Chemistry - A European Journal, 10.1002/chem.201801237, 2018.01, 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..
125. Yuta Saegusa, Tomoya Ishizuka, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, NH Tautomerism of a Quadruply Fused Porphyrin
Rigid Fused Structure Delays the Proton Transfer, Journal of Physical Chemistry B, 10.1021/acs.jpcb.7b10945, 122, 1, 316-327, 2018.01, 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..
126. Taro Koide, Isao Aritome, Tatsuya Saeki, Yoshitsugu Morita, Yoshihito Shiota, Kazunari Yoshizawa, Hisashi Shimakoshi, Yoshio Hisaeda, Cobalt-Carbon Bond Formation Reaction via Ligand Reduction of Porphycene-Cobalt(II) Complex and Its Noninnocent Reactivity, ACS Omega, 10.1021/acsomega.8b00239, 3, 4, 4027-4034, 2018.01, 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..
127. Yuta Hori, Yoshihito Shiota, Tomokazu Tsuji, Masahito Kodera, Kazunari Yoshizawa, Catalytic Performance of a Dicopper-Oxo Complex for Methane Hydroxylation, Inorganic Chemistry, 10.1021/acs.inorgchem.7b02563, 57, 1, 8-11, 2018.01, 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..
128. M. Haris Mahyuddin, Takahiro Tanaka, Yoshihito Shiota, Aleksandar Tsekov Staykov, Kazunari Yoshizawa, Methane Partial Oxidation over [Cu2(μ-O)]2+ and [Cu3(μ-O)3]2+ Active Species in Large-Pore Zeolites, ACS Catalysis, 10.1021/acscatal.7b03389, 8, 2, 1500-1509, 2018.02, 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..
129. Wataru Suzuki, Hiroaki Kotani, Tomoya Ishizuka, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima, Formation and Isolation of a Four-Electron-Reduced Porphyrin Derivative by Reduction of a Stable 20π Isophlorin, Angewandte Chemie - International Edition, 10.1002/anie.201711058, 57, 7, 1973-1977, 2018.02, 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..
130. Kazuyuki Takahashi, Mitsunobu Okai, Tomoyuki Mochida, Takahiro Sakurai, Hitoshi Ohta, Takashi Yamamoto, Yasuaki Einaga, Yoshihito Shiota, Kazunari Yoshizawa, Hisashi Konaka, Akito Sasaki, Contribution of Coulomb Interactions to a Two-Step Crystal Structure Phase Transformation Coupled with a Significant Change in Spin Crossover Behavior for a Series of Charged FeII Complexes from 2,6-Bis(2-methylthiazol-4-yl)pyridine, Inorganic Chemistry, 10.1021/acs.inorgchem.7b02721, 57, 3, 1277-1287, 2018.02, 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..
131. Yusuke Sunada, Hajime Ogushi, Taiji Yamamoto, Shoko Uto, Mina Sawano, Atsushi Tahara, Hiromasa Tanaka, Yoshihito Shiota, Kazunari Yoshizawa, Hideo Nagashima, Disilaruthena- and Ferracyclic Complexes Containing Isocyanide Ligands as Effective Catalysts for Hydrogenation of Unfunctionalized Sterically Hindered Alkenes, Journal of the American Chemical Society, 10.1021/jacs.8b00812, 140, 11, 4119-4134, 2018.03, 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..