Theory of chemical bonds in metalloenzymes XVI. Oxygen activation by high-valent transition metal ions in native and artificial systems

Activation mechanisms of oxygen dianion and hydroxide by high-valent transition metal ions for the oxygen-oxygen (O-O) bond formation of water splitting reaction have been investigated on the theoretical grounds, together with experimental results. First of all, broken-symmetry MO formulations are revisited to elucidate the instability of the dπ-pπ bond in high-valent metal-oxo M(X)O (M = Mn, Fe, Ru, etc.; X = IV, V) systems that react with hydroxide anion (or radical) or water to afford hydroperoxide anion (or peroxide). The triplet instability of these bonds entails strong or intermediate diradical characters: •M(X-1)O• and ••M(X-2)O••; the broken-symmetry (BS) molecular orbitals (MO) resulted from strong electron correlation, leading to the concept of electron localizations and local spins. As a continuation of these theoretical results, the BS MO interaction diagrams, namely orbital and spin correlation diagrams, for one-electron and electron-pair transfer mechanisms for the O-O bond formation have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theoretical understanding of mechanisms of oxygenation reactions by p450, methane mono- oxygenase (MMO) and homolytic radical coupling mechanisms by oxygen evolving complex (OEC) of PSII. The spin alignments in high-valent M(X)O systems are found directly corresponding to possible mechanisms of the O-O bond formation between Mn(X)O and hydroxide (OH) anion via one-electron, electron-pair transfer and their superposed (chameleon) processes. The broken-symmetry (BS) UB3LYP calculations of the model systems have been performed to confirm these mechanisms for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms and orbital and spin correlation diagrams. Implications of the theoretical results are discussed in relation to three scenarios of the O-O bond formation for water oxidation: (a) HO-OH, (b) OOH and (c) O-O, and water oxidation of OEC of PSII.