Combination of Redox-Active Ligand and Lewis Acid for Dioxygen Reduction with π‑Bound Molybdenum−Quinonoid Complexes

A series of π-bound Mo−quinonoid complexes supported by pendant phosphines have been synthesized. Structural characterization revealed strong metal–arene interactions between Mo and the π system of the quinonoid fragment. The Mo–catechol complex (2a) was found to react within minutes with 0.5 equiv of O₂ to yield a Mo–quinone complex (3), H₂O, and CO. Si- and B-protected Mo–catecholate complexes also react with O₂ to yield 3 along with (R₂SiO)_n and (ArBO)₃ byproducts, respectively. Formally, the Mo–catecholate fragment provides two electrons, while the elements bound to the catecholate moiety act as acceptors for the O₂ oxygens. Unreactive by itself, the Mo–dimethyl catecholate analogue reduces O₂ in the presence of added Lewis acid, B­(C₆F₅)₃, to generate a Mo^I species and a bis­(borane)-supported peroxide dianion, [[(F₅C₆)₃B]₂O₂^2–], demonstrating single-electron-transfer chemistry from Mo to the O₂ moiety. The intramolecular combination of a molybdenum center, redox-active ligand, and Lewis acid reduces O₂ with pendant acids weaker than B­(C₆F₅)₃. Overall, the π-bound catecholate moiety acts as a two-electron donor. A mechanism is proposed in which O₂ is reduced through an initial one-electron transfer, coupled with transfer of the Lewis acidic moiety bound to the quinonoid oxygen atoms to the reduced O₂ species.