title: Redox-active ligands facilitate bimetallic O2 homolysis at five-coordinate oxorhenium(V) centers.
abstract: 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.
authors.affiliation: School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
authors.forename: Cameron A
authors.initials: CA
authors.lastname: Lippert
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Arnstein
authors.forename: C David
authors.initials: CD
authors.lastname: Sherrill
authors.forename: Jake D
authors.initials: JD
authors.lastname: Soper
mesh_terms: D008024:Ligands; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D011789:Quantum Theory; D012211:Rhenium / Q000737:chemistry*
chemical_list: D008024:Ligands; C015428:perrhenate; D012211:Rhenium; D010100:Oxygen