Oxygen Activation with Transition-Metal Complexes in Aqueous Solution

Coordination to transition-metal complexes changes both the thermodynamics and kinetics of oxygen reduction. Some of the intermediates (superoxo, hydroperoxo, and oxo species) are close analogues of organic oxygen-centered radicals and peroxides (ROO •, ROOH, and RO • ). Metal-based intermediates are typically less reactive, but more persistent, than organic radicals, which makes the two types of intermediates similarly effective in their reactions with various substrates. The self-exchange rate constant for hydrogen-atom transfer for the couples Cr aq OO 2+ /Cr aq OOH 2+ and L 1 (H 2 O)RhOO 2+ /L 1 (H 2 O)RhOOH 2+ was estimated to be 10 1±1 M −1 s −1 . The use of this value in the simplified Marcus equation for the Cr aq O 2+ /Cr aq OOH 2+ cross reaction provided an upper limit k CrO,CrOH ≤ 10 (−2±1) M −1 s −1 for Cr aq O 2+ /Cr aq OH 2+ self-exchange. Even though superoxo complexes react very slowly in bimolecular self-reactions, extremely fast cross reactions with organic counterparts, i.e., acylperoxyl radicals, have been observed. Many of the intermediates generated by the interaction of O 2 with reduced metal complexes can also be accessed by alternative routes, both thermal and photochemical.