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

Abstract

Controlling the two- versus four-electron oxygen reduction reaction (ORR) remains a central challenge, as the factors that determine whether peroxide is released as H2O2 or further reduced to H2O are often difficult to define at the molecular level. Here we show that axial ligation provides a direct tool for switching ORR selectivity in a flexible dicobalt platform, Co2L1,3 (where L1,3 = 1,3-bis[(bis(2,2′-bipyridin-6-yl)amino)methyl]benzene). In acetonitrile (MeCN), the solvent-bound complex mediates a mixed 2e–/4e– ORR, whereas the presence of additional ligands like triphenylphosphine (PPh3) or 2,6-dicyclohexylimidazole (DCHIm) biases the reaction toward sole H2O2 formation. An anionic thiophenolato coordination, in contrast, redirects the reaction toward a near-quantitative 4e–/4H+ reduction to water. 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.