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 [ReV(O)(apPh)2]− {[apPh]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 [ReV(O)(apPh)2]− in that both are 2e– redox processes, but the electronic structures of the products are different. Whereas oxo addition to [ReV(O)(apPh)2]− yields a closed-shell [ReVII(O)2(apPh)2]− product of 2e– metal oxidation, [Cl]+ addition gives a diradical ReVI(O)(apPh)(isqPh)Cl product ([isqPh]•– = 2,4-di-tert-butyl-6-(phenylimino)semiquinonate) with 1e– in a Re d orbital and 1e– on a redox-active ligand. The differences in electronic structure are ascribed to differences in the π basicity of [O]2– and Cl– ligands. The observation of ligand radicals in ReVI(O)(apPh)(isqPh)X provides experimental support for the capacity of redox-active ligands to deliver electrons in other bond-forming reactions at [ReV(O)(apPh)2]−, including radical additions of O2 or TEMPO• to make Re–O bonds. Attempts to prepare the electron-transfer series monomers between ReVI(O)(apPh)(isqPh)X and [ReV(O)(apPh)2]− yielded a symmetric bis(μ-oxo)dirhenium complex. Formation of this dimer suggested that ReVI(O)(apPh)(isqPh)Cl may be a source of an oxyl metal fragment. The ability of ReVI(O)(apPh)(isqPh)Cl to undergo radical coupling at oxo was revealed in its reaction with Ph3C•, which affords Ph3COH and deoxygenated metal products. This reactivity is surprising because ReVI(O)(apPh)(isqPh)Cl is not a strong outer-sphere oxidant or oxo-transfer reagent. We postulate that the unique ability of ReVI(O)(apPh)(isqPh)Cl to effect oxo transfer to Ph3C• arises from symmetry-allowed mixing of a populated ReO π bond with a ligand-centered [isqPh]•– ligand radical, which gives oxyl radical character to the oxo ligand. This allows the closed-shell oxo ligand to undergo a net 2e– oxo-transfer reaction to Ph3C• 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 transformations are discussed.