Axial Ligand Tuning of the Catalytic Two- versus Four-Electron Reduction of Dioxygen by a Dinuclear Cobalt Complex

Kinetic studies reveal distinct empirical rate laws, indicating a ligand-dependent change in the turnover-limiting sequence. A dicobalt(III)–peroxido intermediate has been identified as the key branching point by spectroscopic and structural characterization of the O₂-derived intermediate. Neutral axial ligands (MeCN, PPh₃ or DCHIm) stabilize this intermediate against O–O bond cleavage, thereby leading to the release of H₂O₂ upon protonation.

In contrast, anionic thiophenolato binding promotes irreversible O–O bond cleavage to form a structurally characterized dicobalt(III) bis-μ-oxido moiety, with the concomitant oxidation of thiophenolato to phenylthiyl radical and the eventual release of water in the presence of a proton donor. These results establish an axial-ligand-controlled mechanism for tuning ORR selectivity. In particular, the thiophenolate-induced cobalt-mediated O–O bond cleavage represents a bioinspired model of the proposed role of cysteine-assisted O–O bond cleavage in cytochrome P450 enzymes.