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

A series of pi-bound Mo-quinonoid complexes supported by pendant phosphines have been synthesized. Structural characterization revealed strong metal-arene interactions between Mo and the p system of the quinonoid fragment. The Mo-catechol complex (2a) was found to react within minutes with 0.5 equiv of O-2 to yield a Mo-quinone complex (3), H2O, and CO. Si- and B-protected Mo-catecholate complexes also react with O-2 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 O-2 oxygens. Unreactive by itself, the Mo-dimethyl catecholate analogue reduces O-2 in the presence of added Lewis acid, B(C6F5)3, to generate a MoI species and a bis(borane)-supported peroxide dianion, [[(F5C(6))(3)B](2)O-2(2-)], demonstrating single-electron-transfer chemistry from Mo to the O-2 moiety. The intramolecular combination of a molybdenum center, redox-active ligand, and Lewis acid reduces O-2 with pendant acids weaker than B(C6F5)(3). Overall, the pi-bound catecholate moiety acts as a two-electron donor. A mechanism is proposed in which O-2 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 O-2 species.