Four-Electron Reduction of O2 Using Distibines in the Presence of ortho-Quinones

This study, which aims to identifyatypical platformsfor the reductionof dioxygen, describes the reaction of O-2 with two distibines,namely, 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethylxantheneand 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethyldihydroacridine,in the presence of an ortho-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-2 molecule.As confirmed by O-18 labeling experiments, the two resultingoxo units combine with the ortho-quinone to forman alpha,alpha,beta,beta-tetraolate ligand that bridgesthe two antimony(V) centers. This process, which has been studiedboth experimentally and computationally, involves the formation ofasymmetric, mixed-valent derivatives featuring a stibine as well asa catecholatostiborane formed by oxidative addition of the quinoneto only one of the antimony centers. Under aerobic conditions, thecatecholatostiborane moiety reacts with O-2 to form a semiquinone/peroxoantimonyintermediate, as supported by NMR spectroscopy in the case of thedimethyldihydroacridine derivative. These intermediates swiftly evolveinto the symmetrical bis(antimony(V)) alpha,alpha,beta,beta-tetraolatecomplexes via low barrier processes. Finally, the controlled protonolysisand reduction of the bis(antimony(V)) alpha,alpha,beta,beta-tetraolatecomplex based on the 9,9-dimethylxanthene platform have been investigatedand shown to regenerate the starting distibine and the ortho-quinone. More importantly, these last reactions also produce twoequivalents of water as the product of O-2 reduction.

Automated summary

This study investigates the reaction of O-2 with two distibines in the presence of an ortho-quinone, resulting in the formation of tetraolate complexes. The formation process involves oxidation of the antimony atoms to the +V state and the formation of an alpha,alpha,beta,beta-tetraolate ligand that bridges the two antimony(V) centers. Under aerobic conditions, the tetraolate complexes react with O-2 to form semiquinone/peroxoantimony intermediates and then evolve into bis(antimony(V)) alpha,alpha,beta,beta-tetraolate complexes. The controlled protonolysis and reduction of the bis(antimony(V)) alpha,alpha,beta,beta-tetraolate complexes regenerate the starting distibine and ortho-quinone, producing two equivalents of water as the product of O-2 reduction.