Kinetic and mechanistic studies of the reactions of transition metal-activated oxygen with inorganic substrates

The intermediates generated in the process of oxygen activation at metal centers participate in hydrogen and oxygen atom transfer, electron transfer, substitution, acid–base chemistry, and free radical chemistry. The reactivity and intrinsic lifetimes of such intermediates in aqueous solutions are a strong function of pH as the metal–oxygen interaction adds an extra dimension to the already complex pH dependence of O2 reduction. Acid–base chemistry at “nonparticipating” ligands plays a major role in the kinetics and mechanisms, and can even determine the outcome of some reactions. Superoxometal complexes are subject to homolytic metal–oxygen bond cleavage in acidic solutions, but decompose by heterolysis at higher pH. Reactions of halides by hydroperoxo and peroxo complexes proceed through two major channels – oxidation of halide ions, and catalysis of H2O2 disproportionation – in close resemblance to enzymes haloperoxidases. The combination of the thermodynamics of electron transfer and protonation equilibria make transition metal hydroperoxo complexes both better oxidants and better reductants than the parent H2O2 in 2-electron reactions.