Radical Control in O-Atom Transfer from an Unusually Oxidized Oxorhenium Complex.

A new S = 1/2 d0 dioxorhenium complex, [Re(O)2(ap)(isq•)] ([ap]2- = 2,4-di-tert-butyl-6-(phenylamido)phenolate, [isq•]- = 2,4-di-tert-butyl-6-(phenylimino)semiquinonate), was prepared by oxidation of the Re(VII) species [Re(O)2(ap)2]-. Solid-state structural and spectroscopic data for [Re(O)2(ap)(isq•)] suggest a strengthening of the Re═O bonding upon oxidation of [Re(O)2(ap)2]-. [Re(O)2(ap)(isq•)] is a weak O-atom donor, H• acceptor, and modest outer-sphere 1e- oxidant, but it cleanly oxidizes the stable triphenylmethyl (Ph3C•) radical, affording Ph3COH and deoxygenated bimetallic μ-oxo dimers. Data support a mechanism of initial C-O radical coupling (RC) at a terminal Re═O bond, followed by net H• transfer from Gomberg's dimer, reversing the steps for classic rebound-type C-H hydroxylation. The closed-shell structural homologue [Re(O)2(ap)2]- has comparable O-atom transfer thermodynamics but is inert to Ph3C•. Computational data show the [isq•]- radical in [Re(O)2(ap)(isq•)] is partially delocalized into the closed-shell metal-oxo group in the ground state, which might permit the net 2e- oxo transfer to Ph3C• to occur via kinetically facile ligand-centered radical steps. Accordingly, a strategy is presented for the preparation of stable oxo-metal complexes that exhibit oxidizing oxyl radical-type reactivity via delocalization of a low-lying, redox-active ligand-centered hole into the terminal M-Ooxo π-bonding manifold. This "masked oxyl" approach establishes design principles for generation of thermodynamically stable oxidants that are kinetically activated for selective odd-electron bond-making and -breaking redox reactions, with broad implications for selective oxidations and energy conversion and storage.