Four-Electron Reduction of O Using Distibines in the Presence of -Quinones.

This study, which aims to identify atypical platforms for the reduction of dioxygen, describes the reaction of O with two distibines, namely, 4,5-bis(diphenylstibino)-2,7-di--butyl-9,9-dimethylxanthene and 4,5-bis(diphenylstibino)-2,7-di--butyl-9,9-dimethyldihydroacridine, in the presence of an -quinone such as phenanthraquinone. The reaction proceeds by oxidation of the two antimony atoms to the + V state in concert with reductive cleavage of the O molecule. As confirmed by O labeling experiments, the two resulting oxo units combine with the -quinone to form an α,α,β,β-tetraolate ligand that bridges the two antimony(V) centers. This process, which has been studied both experimentally and computationally, involves the formation of asymmetric, mixed-valent derivatives featuring a stibine as well as a catecholatostiborane formed by oxidative addition of the quinone to only one of the antimony centers. Under aerobic conditions, the catecholatostiborane moiety reacts with O to form a semiquinone/peroxoantimony intermediate, as supported by NMR spectroscopy in the case of the dimethyldihydroacridine derivative. These intermediates swiftly evolve into the symmetrical bis(antimony(V)) α,α,β,β-tetraolate complexes via low barrier processes. Finally, the controlled protonolysis and reduction of the bis(antimony(V)) α,α,β,β-tetraolate complex based on the 9,9-dimethylxanthene platform have been investigated and shown to regenerate the starting distibine and the -quinone. More importantly, these last reactions also produce two equivalents of water as the product of O reduction.