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

Abstract

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.