Yoshihito Shiota

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.

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

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.

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...

Comparison of the Reactivity of Bis(μ-oxo)CuIICuIII and CuIIICuIII Species to Methane

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

Reversible Electron Transfer in a Linear {Fe2Co} Trinuclear Complex Induced by Thermal Treatment and Photoirraditaion

Bistable materials possess two close-lying states, which can be reversibly interchanged by external stimuli such as temperature, light, pressure, and guest molecules. These materials offer attractive opportunities for the realization of energyefficient, switchable, molecule-based data storage in electronic devices. A current topic for research in this field is the preparation of switchable multifunctional molecules in which more than two properties coexist or interact synergistically. An important multifunctional compound shows a significant change in both magnetic properties and charge distribution (polarization) at the molecular level. Tunable magnetic molecules, such as spin-crossover complexes, are important for magnetic recording devices. Furthermore, switchable polarity is an essential feature for regulating nonlinear optical and ferroelectric properties. In particular, the study of electronic ferroelectricity, in which an electronic charge order without inversion symmetry is responsible for the electric polarization, has recently attracted significant attention. Thus, it is important to design new compounds in which the spin state and charge distribution can be reversibly controlled by external stimuli. To this end, the development of compounds that consist of paramagnetic donors and acceptors is attracting considerable interest because lattice distortion and charge-transfer processes in such compounds involve not only concomitant spin-state changes but also changes in their dielectric properties. 9] Significant changes in the magnetic susceptibility and the dielectric constant were observed near the neutral–ionic phase transition temperature of chargetransfer complexes. Furthermore, dimerization of the donor and the acceptor induces the formation of a polar structure from a nonpolar structure because of the breaking of the inversion center as a result of molecular charge transfer. Moreover, it has been reported that a dinuclear cobalt complex with a dioxolene bridging ligand exhibits charge transfer between the bridging ligand (donor/acceptor) and cobalt (acceptor/donor) induced by a temperature change and light irradiation. This transfer is accompanied by magnetization change and the formation of a polar structure in the cluster, which is a molecular-level representation of the interconversion of both magnetization and electric polarization similar to that in the aforementioned charge-transfer complex. With a rational design, various discrete multinuclear complexes have been synthesized by using different building blocks such as cyanometallates. However, until now, only the cobalt-dioxolene system has been reported to show such magnetization changes and polar–nonpolar transformation through charge transfer in response to both thermal and optical stimuli. Therefore, the preparation of new compounds with such properties remains a challenge. In this work, we aimed at synthesizing linear bimetallic trinuclear clusters with centrosymmetrical structures that are capable of charge transfer between the metal in the center and a metal ion on the edge. The charge-transfer process was expected to induce a change in magnetization because of the change in spin multiplicity. Furthermore, charge transfer in a cluster with an inversion center also induces the formation of a polar structure from a nonpolar one. To synthesize such a centrosymmetric bimetallic trinuclear cluster, we choose [FeTp(CN)3] (Tp = hydrotris(pyrazolyl)borate) as the building block to treat with [Co(Meim)4] 2+ (Meim = N-methylimidazole). One cyanide bridge of the [FeTp(CN)3] unit is thought to coordinate with the Co ion, which tunes the redox potential required for charge transfer. Moreover, the terminal cyanide ligands are thought to form potential hydrogen-bonding interactions with noncoordinated solvent molecules, stabilizing the bistable state through intermolecular cooperative interactions. In fact, we recently synthesized an Fe2Co trinuclear cluster {[FeTp(CN)3]2Co(Meim)4}·6 H2O (1), in which the cobalt ion is sandwiched between two iron building blocks (Scheme 1). Compound 1 exhibited thermally induced, reversible electron transfer with a thermal hysteresis and photoinduced electron transfer by excitation of the charge-transfer band. Single-crystal X-ray diffraction (XRD) analysis revealed that 1 crystallizes in a P 1 space group. The crystal structure comprises neutral {[FeTp(CN)3]2Co(Meim)4} trinuclear clusters (Figure 1 a) with noncoordinated water molecules located between the clusters (Figure 1b). Within the neutral trinu[*] Prof. T. Liu, Dr. D.-P. Dong, Prof. S. Wu, Prof. C. He, Prof. C.-Y. Duan State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Rd., 116024 Dalian (China) E-mail: liutao@dlut.edu.cn cyduan@dlut.edu.cn

Quantum chemical approach to the mechanism for the biological conversion of tyrosine to dopaquinone.

Tyrosinase catalyzes the biological conversion of tyrosine to dopaquinone with dioxygen at the dinuclear copper active site under physiological conditions. On the basis of the recent X-ray crystal structural analysis of tyrosinase (J. Biol. Chem. 2006, 281, 8981), a possible mechanism for the catalytic cycle of tyrosinase is proposed by using quantum mechanical/molecular mechanical calculations, which can reasonably take effects of surrounding amino-acid residues, hydrogen bonding, and protein environment into account. The (mu-eta2:eta2-peroxo)dicopper(II) species plays a role in a series of elementary processes mediated by the dicopper species of tyrosinase. A stable phenoxyl radical is involved in the reaction pathway. The catalysis has five steps of proton transfer from the phenolic O-H bond to the dioxygen moiety, O-O bond dissociation of the hydroperoxo species, C-O bond formation at an ortho position of the benzene ring, proton abstraction and migration mediated by His54, and quinone formation. The energy profile of the calculated reaction pathway is reasonable in energy as biological reactions that occur under physiological conditions. Detailed analyses of the energy profile demonstrate that the O-O bond dissociation is the rate-determining step. The activation energy for the O-O bond dissociation at the dicopper site is computed to be 14.9 kcal/mol, which is in good agreement with a measured kinetic constant. As proposed recently, the His54 residue, which is flexible because it is located in a loop structure in the protein, would play a role as a general base in the proton abstraction and migration in the final stages of the reaction to produce dopaquinone.

A ferromagnetically coupled Fe 42 cyanide-bridged nanocage

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.

Multi-step spin crossover accompanied by symmetry breaking in an Fe III complex: Crystallographic evidence and DFT studies

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.