Comparison of the Reactivity of Bis(μ-oxo)Cu^IICu^III and Cu^IIICu^III 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(μ-oxo)CuIICuIII and CuIIICuIII 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-oxo)CuIIICuIII 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)CuIICuIII species. The DFT calculations show that the bis(μ-oxo)CuIICuIII species is more effective for the activation of methane than the bis(μ-oxo)CuIIICuIII species. The reactive bis(μ-oxo)CuIICuIII species can be created either from the electron injection to the bis(μ-oxo)CuIIICuIII species or from the O−O bond cleavage in the μ-η1:η2-peroxoCuICuII species

Conversion of Methane to Methanol at the Mononuclear and Dinuclear Copper Sites of Particulate Methane Monooxygenase (pMMO): A DFT and QM/MM Study

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 CuIII−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-η2:η2-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

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 (μ-η2:η2-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

Mechanism for the Direct Oxidation of Benzene to Phenol by FeO⁺

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

Methane−Methanol Conversion by MnO⁺, FeO⁺, and CoO⁺: A Theoretical Study of Catalytic Selectivity

The entire reaction pathway for the gas-phase methane−methanol conversion by late transition-metal-oxide ions, MnO+, FeO+, and CoO+, is studied using an ab initio hybrid (Hartree−Fock/density-functional) method. For these oxo complexes, the methane−methanol conversion is proposed to proceed via two transition states (TSs) in such a way MO+ + CH4 → OM+(CH4) → [TS1] → HO−M+−CH3 → [TS2] → M+(CH3OH) → M+ + CH3OH, where M is Mn, Fe, and Co. A crossing between high-spin and low-spin potential energy surfaces occurs both at the entrance channel and at the exit channel for FeO+ and CoO+, but it occurs only once near TS2 for MnO+. The activation energy from OMn+(CH4) to HO−Mn+−CH3 via TS1 is calculated to be 9.4 kcal/mol, being much smaller than 22.1 and 30.9 kcal/mol for FeO+ and CoO+, respectively. This agrees with the experimentally reported efficiencies for the reactions. The excellent agreement between theory and experiment indicates that HO−M+−CH3 plays a central role as an intermediate in the reaction between MO+ and methane and that the reaction efficiency is most likely to be determined by the activation energy from OM+(CH4) to HO−M+−CH3 via TS1. We discuss in terms of qualitative orbital interactions why MnO+ (d4 oxo complex) is most effective for methane C−H bond activation. The activation energy from HO−M+−CH3 to M+(CH3OH) via TS2 is computed to be 24.6, 28.6, and 35.9 kcal/mol for CoO+, FeO+, and MnO+, respectively. This result explains an experimental result that the methanol-branching ratio in the reaction between MO+ and methane is 100% in CoO+, 41% in FeO+, and +. We demonstrate that both the barrier heights of TS1 and TS2 would determine general catalytic selectivity for the methane−methanol conversion by the MO+ complexes

Metal−Ligand Cooperation in H₂ Production and H₂O Decomposition on a Ru(II) PNN Complex: The Role of Ligand Dearomatization−Aromatization

Metal−Ligand Cooperation in H2 Production and H2O Decomposition on a Ru(II) PNN Complex: The Role of Ligand Dearomatization−Aromatizatio

A Theoretical Study of the Dynamic Behavior of Alkane Hydroxylation by a Compound I Model of Cytochrome P450

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 ν3 and ν4 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

Theoretical Investigation into Selective Benzene Hydroxylation by Ruthenium-Substituted Keggin-Type Polyoxometalates

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

Mechanisms of Co₂L₈ (L = CO, CNR)-Catalyzed Hydrosilylation of Alkenes: A Theoretical Study

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

Mechanisms of Co₂L₈ (L = CO, CNR)-Catalyzed Hydrosilylation of Alkenes: A Theoretical Study

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