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.

Five-coordinate oxorhenium(V) anions with redox-active catecholate and amidophenolate ligands are shown to effect clean bimetallic cleavage of O2 to give dioxorhenium(VII) products. A structural homologue with redox-inert oxalate ligands does not react with O2. Redox-active ligands lower the kinetic barrier to bimetallic O2 homolysis at five-coordinate oxorhenium(V) by facilitating formation and stabilization of intermediate O2 adducts. O2 activation occurs by two sequential Re−O bond forming reactions, which generate mononuclear η1-superoxo species, and then binuclear trans-μ-1,2-peroxo-bridged complexes. Formation of both Re−O bonds requires trapping of a triplet radical dioxygen species by a cis-[ReV(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 O2 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 O2 activation that is atypical of oxorhenium(V) but more typical for oxygenase enzymes and models based on 3d transition metal ions: O2 cleavage occurs by a net 2e− process through a series of 1e− steps. The implications for design of new multielectron catalysts for oxygenase-type O2 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.

Five-coordinate oxorhenium(V) anions with redox-active catecholate ligands deoxygenate stable nitroxyl radicals, including TEMPO•, to afford dioxorhenium(VII) complexes and aminyl radical-derived products. A structural homologue with redox-inert oxalate ligands does not react with TEMPO•. Redox-active ligands are proposed to lower the kinetic barrier to TEMPO• deoxygenation by giving access to 1e− redox steps that are crucial for the formation and stabilization of intermediate species.

New five- and six-coordinate complexes containing the [MnIII(Br4cat)2]− core (Br4cat2− = tetrabromo-1,2-catecholate) have been prepared. Homoleptic [MnIII(Br4cat)3]3− reacts rapidly with O2 to produce tetrabromo-1,2-benzoquinone (Br4bq). The [MnIII(Br4cat)2]− fragment is a robust catalytic platform for the aerobic conversion of catechols to quinones. The oxidase activity apparently derives from the coupling of metal- and ligand-centered redox events.

Square planar cobalt(III) complexes with redox-active amidophenolate ligands are strong nucleophiles that react with alkyl halides, including CH2Cl2, under gentle conditions to generate stable square pyramidal alkylcobalt(III) complexes. The net electrophilic addition reactions formally require 2e− oxidation of the metal fragment, but there is no change in metal oxidation state because the reaction proceeds with 1e− oxidation of each amidophenolate ligand. Although the four-coordinate complexes are very strong nucleophiles, they are mild outer-sphere reductants. Accordingly, addition of alkyl- or phenylzinc halides to the five-coordinate organometallic complexes regenerates the square planar starting materials and extrudes C−C coupling products. The net 2e− reductive elimination reaction also occurs without a oxidation state change at the cobalt(III) center. Together these reactions comprise a complete, well-defined cycle for cobalt Negishi-like cross-coupling of alkyl halides with organozinc reagents.