Cobalt(II)\nAqua Complex-Mediated Hydrogen Peroxide\nActivation: Possible Roles of HOOOH and Co(II)–OOOH Intermediates\nin Singlet Oxygen Generation

Density functional theory (DFT) calculations indicate\nthat [Co<sup>II</sup>(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> reacts\nwith two\nH<sub>2</sub>O<sub>2</sub> molecules to form [(H<sub>2</sub>O)<sub>4</sub>Co<sup>II</sup>(OOH)(H<sub>2</sub>O<sub>2</sub>)]<sup>+</sup> reactant complexes, which decompose through three distinct pathways\ndepending on the relative orientation between the coordinated <sup>–</sup>OOH and H<sub>2</sub>O<sub>2</sub> ligands. The reactive\nintermediates produced via these activation pathways include hydroperoxyl\n(<sup>•</sup>OOH)/superoxide (O<sub>2</sub><sup>•–</sup>) radicals, singlet oxygen (<sup>1</sup>O<sub>2</sub>), and Co(III)\nspecies [(H<sub>2</sub>O)<sub>5</sub>Co<sup>III</sup>(O)]<sup>+</sup>, [(H<sub>2</sub>O)<sub>4</sub>Co<sup>III</sup>(OH)<sub>2</sub>]<sup>+</sup>, and [(H<sub>2</sub>O)<sub>5</sub>Co<sup>III</sup>(OH)]<sup>2+</sup>. The Co(III) species display from moderate to strong oxidizing\nabilities that have long been overlooked. Remarkably, our DFT calculations\nreveal the possible formation of hydrogen trioxide (HOOOH) and Co(II)–OOOH\nintermediates during [(H<sub>2</sub>O)<sub>4</sub>Co<sup>II</sup>(OOH)(H<sub>2</sub>O<sub>2</sub>)]<sup>+</sup> decomposition and that the hydrolysis\nof these transient species is a route to <sup>1</sup>O<sub>2</sub> production. Because two of the three activation pathways do not\ninvolve changes in the oxidation state of the Co center, they may\napply to other systems comprising redox-inert metal ions.