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 MoI species and a bis­(borane)-supported peroxide dianion, [[(F5C6)3B]2O22–], 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.