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 [Co^II(H₂O)₆]²⁺ reacts with two H₂O₂ molecules to form [(H₂O)₄Co^II(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)₅Co^III(O)]⁺, [(H₂O)₄Co^III(OH)₂]⁺, and [(H₂O)₅Co^III(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)₄Co^II(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.