Harnessing Redox-Active Ligands for Low-Barrier Radical Addition at Oxorhenium Complexes

The addition of an [X](+) electrophile to the five-coordinate oxorhenium (V) anion [Re-V(O)(ap(Ph))(2)](-) {[ap(Ph)](2-) = 2,4-di-tert-butyl-6-(phenylamido)phenolate} gives new products containing Re-X bonds. The Re-X bond-forming reaction is analogous to oxo transfer to [Re-V(O)(ap(Ph))(2)](-) in that both are 2e(-) redox processes, but the electronic structures of the products are different. Whereas oxo addition to [Re-V(O)(ap(Ph))(2)](-) yeilds a closed-shell [Re-VII(O)(2)(ap(Ph))(2)](-) product of 2e(-) metal oxidation, [Cl](+) addition gives a diradical Re-VI(O)(ap(Ph))(isq(Ph))Cl product ([isqPh](center dot-) = 2,4-di-tert-butyl-6-(phenylimino)semiquinonate) with le(-) in a Re d orbital and le(-) on a redox-active ligand. The differences in electronic structure are ascribed to differences in the pi basicity of [O](2-) and Cl- ligands. The observation of ligand radicals in Re-IV(O)(ap(Ph))(isq(Ph))X provides experimental support for the capacity of redox-active ligands to deliver electrons in other bond-forming reactions at [Re-V(O)(ap(Ph))(2)](-), including radical additions of O-2 or TEMPO center dot to make Re-O bonds. Attempts to prepare the electron-tranfer series monomers between Re-VI(O)(ap(Ph))(isq(Ph))X and [Re-V(O)(ap(Ph))(2)](-) yeilded a symmetric bis(mu-oxo)dirhenium complex. Formation of this dimer suggested that Re-VI(O)(ap(Ph))(isq(Ph))Cl may be a source of an oxyl metal fragment. The ability of Re-VI(O)(ap(Ph))(isq(Ph))Cl to undergo radical coupling at oxo was revealed in its reaction with Ph3C center dot, which affords Ph3COH and deoxygenated methal products. This reactivity is surprising because Re-VI(O)(ap(Ph))(isq(Ph))Cl is not a strong outer-sphere oxidant or oxo-transfer reagent. We postulate that the unique ability of Re-VI(O)(ap(Ph))(isq(Ph))Cl to effect oxo transfer to Ph3C center dot arises from symmetry-allowed mixing of a populated Re O pi bond with a lignad-centered [isq(Ph)](center dot-) ligand radical, which give oxyl radical character to the oxo ligand. This allows the closed-shell oxo ligand to undergo a net 2e(-) oxo-transfer reaction to Ph3C center dot via kinetically facile redox-active ligand-mediated radical steps. Harnessing intraligand charge transfer for radical reactions at closed-shell oxo ligands is a new strategy to exploit redox-active ligands for small-molecule activation and functionalization. The implications for the design of new oxidants that utilize low-barrier radical steps for selective multielectron transformation are discussed.