Redox-Active Ligands Facilitate Bimetallic O2 Homolysis at Five-Coordinate Oxorhenium(V) Centers

Five-coordinate oxorhenium(V) anions with redox-active catecholate and amidophenolate ligands are shown to effect clean bimetallic cleavage of O-2 to give dioxorhenium(VII) products. A structural homologue with redox-inert oxalate ligands does not react with O-2. Redox-active ligands lower the kinetic barrier to bimetallic O-2 homolysis at five-coordinate oxorhenium(V) by facilitating formation and stabilization of intermediate O-2 adducts. O-2 activation occurs by two sequential Re-O bond forming reactions, which generate mononuclear eta(1)-superoxo species, and then binuclear trans-mu- 1,2-peroxo-bridged complexes. Formation of both Re-O bonds requires trapping of a triplet radical dioxygen species by a cis-[Re-V(O)(cat)(2)](-) anion. In each reaction the dioxygen fragment is reduced by 1e(-), so generation of each new Re-O bond requires that an oxometal fragment is oxidized by 1e(-). Complexes containing a redox-active ligand access a lower energy reaction pathway for the 1e(-) Re-O bond forming reaction because the metal fragment can be oxidized without a change in formal rhenium oxidation state. It is also likely that redox-active ligands facilitate O-2 homolysis by lowering the barrier to the formally spin-forbidden reactions of triplet dioxygen with the closed shell oxorhenium(V) anions. By orthogonalizing 1e(-) and 2e(-) redox at oxorhenium(V), the redox-active ligand allows high-valent rhenium to utilize a mechanism for O-2 activation that is atypical of oxorhenium(V) but more typical for oxygenase enzymes and models based on 3d transition metal ions: O-2 cleavage occurs by a net 2e(-) process through a series of 1e(-) steps. The implications for design of new multielectron catalysts for oxygenase-type O-2 activation, as well as the microscopic reverse reaction, O-O bond formation from coupling of two M=O fragments for catalytic water oxidation, are discussed.