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.