Quantum Chemical Studies on Dioxygen Activation and Methane Hydroxylation by Diiron and Dicopper Species as well as Related Metal–Oxo Species

Abstract

Quantum chemical studies by the author and co-workers on dioxygen activation and methane hydroxylation by diiron and dicopper enzyme species as well as related metaloxo species are reviewed. The activation of the OO bond of dioxygen at the mono-and dimetal sites of metalloenzymes is an essential process in the initial catalytic stages of naturally occurring oxygenation reactions. A way of orbital thinking about dioxygen activation at diiron and dicopper enzymes is developed at the extended Hückel level of theory. Two different reaction pathways that lead from dimetal peroxo complexes with¯-η , and CuO + are systematically analyzed on the basis of density functional theory (DFT) calculations. An important feature in the reaction is the spin crossover between the highspin and low-spin potential energy surfaces in particular in the CH activation process. The spin inversion from the highspin state to the low-spin state effectively decreases the barrier height of CH activation. Another feature in the reaction is that no radical species is involved in the course of the hydroxylation because the methyl species formed as a result of the CH bond cleavage can directly coordinate to the metal active site. The coupling of the OH and CH 3 ligands occurs to produce methanol at the metal active center. The reaction profiles obtained from DFT calculations are similar to those of the Gif chemistry proposed by D. H. R. Barton, in particular the involvement of the HOMCH 3 species as an intermediate in the hydroxylation of methane. The nonradical mechanism is extended to methane hydroxylation by the diiron and dicopper species of methane monooxygenase (MMO). This mechanism is possible when the metal active center is coordinatively unsaturated to have a space for the coordination of the OH and CH 3 groups as ligands. Although compelling discussion to support the involvement of oxygen-and carbon-centered radicals is provided, the nonradical mechanism still seems to be applicable for methane hydroxylation from the viewpoint of reaction selectivity. Kinetic isotope effects (KIEs) in the CH activation process by the bare FeO + complex and diiron and dicopper enzyme models are compared with respect to the radical and nonradical mechanisms by using transition state theory.

Theoretical Investigation of Methane Hydroxylation over Isoelectronic [FeO]2+- and [MnO]+-Exchanged Zeolites Activated by N2O

“…These high barriers are due to the formation of Mn I , which is a rather unstable oxidation state for manganese. Previously, our group suggested that, to avoid the formation of such unstable species, N 2 O decomposition possibly occurs in the hydroxo intermediate when the concentration of N 2 O is high . For the same reason, the [Mn­(CH 3 OH)] + -MFI and -AEI product complexes also lie in high levels (8.6 and 6.6 kcal/mol, respectively).…”

“…Previously, our group suggested that, to avoid the formation of such unstable species, N 2 O decomposition possibly occurs in the hydroxo intermediate when the concentration of N 2 O is high. 73 For the same reason, the [Mn(CH 3 OH)] + -MFI and -AEI product complexes also lie in high levels (8.6 and 6.6 kcal/ mol, respectively). The formed methanol molecule is bound to the Mn atom through its O atom with Mn−O bond lengths of 1.999 and 2.009 Å, respectively.…”

Possible Peroxo State of the Dicopper Site of Particulate Methane Monooxygenase from Combined Quantum Mechanics and Molecular Mechanics Calculations

“…The experimental observations are reasonable because the bis­(μ-oxo)­Cu III 2 species is essentially a closed-shell system with limited reactivity toward stronger C–H bonds. The spin density of oxo species is highly correlated with the ability of the activation of the methane C–H bond (BDE = 104 kcal/mol) . Therefore, it is necessary to consider a more reactive dicopper-dioxygen species that can activate the methane C–H bond.…”

Simulation vs. Understanding: A Tension, in Quantum Chemistry and Beyond. Part B. The March of Simulation, for Better or Worse

“…Yoshizawa group on dioxygen activation and methane hydroxylation. [112] Notice how far these studies are from "brute force" approaches.Inthem enters much knowledge of biochemistry, the weighing of sometimes contradictory experimental results,and the design of an appropriate computational strategy. This is real science.Even as we worry about what the tools of AI will do the future,a nd to the grand enterprise of understanding on which our Greek, Arab and Chinese forefathers embarked, we revel in what we (or at least our students,orgiven our age,their students) can do.…”