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 O_2 to yield a Mo–quinone complex (3), H_2O, and CO. Si- and B-protected Mo–catecholate complexes also react with O_2 to yield 3 along with (R_2SiO)_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(C_6F_5)_3, to generate a MoI species and a bis(borane)-supported peroxide dianion, [[(F_5C_6)_3B]_2O_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(C_6F_5)_3. Overall, the π-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.
© 2015 American Chemical Society. Received: September 29, 2014; Revised: December 4, 2014; Published: January 10, 2015. We thank Lawrence M. Henling and Dr. Michael Takase for crystallographic assistance, Prof. John E. Bercaw for providing access to a Toepler pump in his laboratory, and Christine Cheng for assistance in organic synthesis. We are grateful to Caltech and the NSF (CHE-1151918 to T.A.; GRFP to J.T.H.) for funding. T.A. is a Sloan, Cottrell, and Dreyfus Fellow. The APEX II X-ray diffractometer was purchased via an NSF CRIF:MU Award to Caltech (CHE-0639094).
Supplemental Material - ja5100405_si_001.cif
Supplemental Material - ja5100405_si_002.pdf