Kazunari Yoshizawa

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

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+–CH3(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...

Computational Prediction for Singlet- and Triplet-Transition Energies of Charge-Transfer Compounds.

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. (3)CT and locally excited triplet ((3)LE) states are well distinguished and calculated independently.

Lithium intercalation in TiO2 modifications

Lithium intercalation in anatase and TiO2 (B), a synthetic TiO2 polymorph, is analyzed from approximate crystal orbital calculations. As a consequence of lithium incorporation, weak Ti–Ti bonding interactions are formed in both compounds. These bonding interactions lead to one-dimensional zigzag chains in the frameworks of anatase and TiO2 (B). Extended Huckel calculations show that a small distortion of the anatase network is required to achieve Ti–Ti bonds, whereas such a distortion is not necessary in TiO2 (B).

Ruthenium‐Catalyzed Selective and Efficient Oxygenation of Hydrocarbons with Water as an Oxygen Source

The development of methods for the highly selective and efficient conversion of abundant organic resources into valuable products is crucial for a sustainable society. To achieve this goal, extensive studies on the methodology of efficient material conversion with metal complexes as catalysts have been made for a long time. High-valent metal–oxo species are key intermediates in biological oxidations by metalloenzymes (mainly heme and non-heme iron enzymes), which catalyze the oxygenation of hydrocarbons in metabolic and catabolic processes. These oxygenases involve high-valent metal–oxo species as reactive species that arise by reductive activation of molecular oxygen coupled with proton transfer. Peroxides such as hydrogen peroxide can lead to a so-called “peroxide shunt” to perform the catalytic oxygenation; this mechanism is found for cytochrome P450 and methane monooxygenase. Thus, a number of model systems for these enzymatic oxidations have been developed to elucidate the reaction mechanisms and to perform effective catalytic oxygenation of external substrates with metal complexes involving the formation of high-valent metal–oxo species. These systems usually require organic solvents and excess amount of organic or inorganic peroxides as both oxidants and oxygen sources. Moreover, in such cases, the reaction pathways become complicated and give multiple products. Consequently it is difficult to control the product distribution that arises mainly from the inevitably produced radical species. Another strategy to generate a high-valent metal–oxo species has been recognized in the oxygen-evolving complex (OEC) in Photosystem II (PSII) for the photosynthesis to oxidize water to produce dioxygen. At the OEC, a manganese(V)–oxo species has been proposed to be formed by proton-coupled electron transfer (PCET), and the deprotonation of coordinated water and the oxidation of the metal center are thought to occur concertedly. This strategy has been applied to form and isolate high-valent metal–oxo species to perform stoichiometric oxidation reactions; however, it has not been applied to catalytic oxidations with transition-metal complexes as catalysts in water. Inspired by the reactions at the OEC in photosynthesis, we have tried to establish a novel catalytic oxygenation system using water as both the solvent and the oxygen source by virtue of PCET. We report herein the formation of a novel ruthenium(IV)–oxo complex and its reactivity toward highly efficient and selective catalytic oxygenation and oxidation reactions of various hydrocarbons in water, which can be used as an oxygen source. We synthesized a novel bis-aqua Ru complex, [Ru(tpa)(H2O)2](PF6)2 (1; tpa= tris(2-pyridylmethyl)amine) (Figure 1a,b), by the treatment of [RuCl(tpa)]2(PF6)2 [20] with AgPF6 in water. Complex 1 exhibits a reversible twostep deprotonation–protonation equilibrium, and the two pKa values were determined by UV/Vis spectroscopic titration (see Figure S1 in the Supporting Information) in the range of

Catalytic Reduction of Dinitrogen to Ammonia by Use of Molybdenum–Nitride Complexes Bearing a Tridentate Triphosphine as Catalysts

Newly designed and prepared molybdenum-nitride complexes bearing a mer-tridentate triphosphine as a ligand have been found to work as the most effective catalysts toward the catalytic reduction of dinitrogen to ammonia under ambient conditions, where up to 63 equiv of ammonia based on the Mo atom of the catalyst were produced.

Catalytic transformation of dinitrogen into ammonia and hydrazine by iron-dinitrogen complexes bearing pincer ligand

Synthesis and reactivity of iron-dinitrogen complexes have been extensively studied, because the iron atom plays an important role in the industrial and biological nitrogen fixation. As a result, iron-catalyzed reduction of molecular dinitrogen into ammonia has recently been achieved. Here we show that an iron-dinitrogen complex bearing an anionic PNP-pincer ligand works as an effective catalyst towards the catalytic nitrogen fixation, where a mixture of ammonia and hydrazine is produced. In the present reaction system, molecular dinitrogen is catalytically and directly converted into hydrazine by using transition metal-dinitrogen complexes as catalysts. Because hydrazine is considered as a key intermediate in the nitrogen fixation in nitrogenase, the findings described in this paper provide an opportunity to elucidate the reaction mechanism in nitrogenase.

Catalytic formation of ammonia from molecular dinitrogen by use of dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing PNP-pincer ligands: remarkable effect of substituent at PNP-pincer ligand.

A series of dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing 4-substituted PNP-pincer ligands are synthesized by the reduction of the corresponding molybdenum trichloride complexes under 1 atm of molecular dinitrogen. In accordance with a theoretical study, the catalytic activity is enhanced by the introduction of an electron-donating group to the pyridine ring of PNP-pincer ligand, and the complex bearing 4-methoxy-substituted PNP-pincer ligands is found to work as the most effective catalyst, where 52 equiv of ammonia are produced based on the catalyst (26 equiv of ammonia based on each molybdenum atom of the catalyst), together with molecular dihydrogen as a side-product. Time profiles for the catalytic reactions indicate that the rates of the formation of ammonia and molecular dihydrogen depend on the nature of the substituent on the PNP-pincer ligand of the complexes. The formation of ammonia and molecular dihydrogen is complementary in the reaction system.

Metal-ligand cooperation in H2 production and H2O decomposition on a Ru(II) PNN complex: The role of ligand dearomatization- aromatization

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

Iron-catalysed transformation of molecular dinitrogen into silylamine under ambient conditions

Although stoichiometric transformations using transition metal-N(2) complexes have been well investigated towards the goal of nitrogen fixation under mild reaction conditions, only a few examples of the catalytic transformations of N(2) using transition metal-N(2) complexes as catalysts have been reported. In almost all the catalytic systems, the use of Mo is essential to realize the catalytic transformation of N(2), where Mo-N(2) complexes are considered to work as effective catalysts. Here we show the first successful example of the Fe-catalysed transformation of N(2) into N(SiMe(3))(3) under ambient conditions, in which iron complexes such as iron pentacarbonyl [Fe(CO)(5)] and ferrocenes have been found to work as effective catalysts. A plausible reaction pathway is proposed, where Fe(II)-N(2) complex bearing two Me(3)Si groups as ancillary ligands has an important role as a key reactive intermediate, with the aid of density-functional-theory calculations.

A spin–orbit coupling study on the spin inversion processes in the direct methane-to-methanol conversion by FeO+

Possible spin inversion processes in the direct conversion of methane to methanol by the bare FeO+ complex are discussed by means of spin–orbit coupling (SOC) calculations. This reaction proceeds via two transition states (TSs) in the following way; FeO++CH4→FeO+(CH4)→[TS1]→HO–Fe+−CH3→[TS2]→Fe+(CH3OH)→Fe++CH3OH. B3LYP density functional theory calculations show that the potential energies in the quartet and sextet states lie close and involve three crossing seams that can provide a chance of spin-forbidden transition. The spin-forbidden transition leads to a significant decrease in the barrier heights of TS1 and TS2 that correspond to the hydrogen atom abstraction and the methyl shift, respectively. To evaluate the spin-forbidden transition in the reaction pathway, the SOC matrix elements are calculated along the intrinsic reaction coordinate of the reaction. The SOC analysis along the IRC is useful to look at how the FeO+/CH4 reacting system changes its spin multiplicity between the sextet and quartet su...