Cobalt(II) Aqua Complex-Mediated Hydrogen Peroxide Activation: Possible Roles of HOOOH and Co(II)–OOOH Intermediates in Singlet Oxygen Generation

Density functional theory (DFT) calculations indicate
that [CoII(H2O)6]2+ reacts
with two
H2O2 molecules to form [(H2O)4CoII(OOH)(H2O2)]+ reactant complexes, which decompose through three distinct pathways
depending on the relative orientation between the coordinated –OOH and H2O2 ligands. The reactive
intermediates produced via these activation pathways include hydroperoxyl
(•OOH)/superoxide (O2•–) radicals, singlet oxygen (1O2), and Co(III)
species [(H2O)5CoIII(O)]+, [(H2O)4CoIII(OH)2]+, and [(H2O)5CoIII(OH)]2+. The Co(III) species display from moderate to strong oxidizing
abilities that have long been overlooked. Remarkably, our DFT calculations
reveal the possible formation of hydrogen trioxide (HOOOH) and Co(II)–OOOH
intermediates during [(H2O)4CoII(OOH)(H2O2)]+ decomposition and that the hydrolysis
of these transient species is a route to 1O2 production. Because two of the three activation pathways do not
involve changes in the oxidation state of the Co center, they may
apply to other systems comprising redox-inert metal ions.