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.

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.

A carbonic anhydrase mimic converting CO2 to carbonic acid, deprotonated under highly basic conditions, and being converted to a carbamate upon reaction with monoethanolamine, a solvent reported for carbon capture reactions.

The capacity of five‐coordinate oxorhenium(V) anions with redox‐active catecholate ligands to homolyze O2 and afford dioxorhenium(VII) products is utilized for the development of new aerobic alcohol oxidation catalysts. The reaction of [ReVII(O)2(cat)2]– with benzyl alcohol (BnOH) affords the expected products of net H2 transfer: [ReV(O)(cat)2]–, benzaldehyde, and presumably H2O. However, mechanistic studies reveal that the formation of the active oxidant requires both the dioxo and monooxo species, so BnOH oxidation by[ReVII(O)2(cat)2]– exhibits an unexpected catalytic dependence on [ReV(O)(cat)2]–. Attempts to oxidize more thermodynamically challenging primary alcohols, which include CH3OH, using the [ReVII(O)2(cat)2]– + [ReV(O)(cat)2]– system did not yield aldehyde products. However, experiments performed in CH3OH allowed the observation of a catalytically active intermediate species, which provides an insight into the mechanism of catalytic action and catalyst degradation. Based on these observations, complexes that contain a more oxidatively robust [Br4cat]2– ligand were shown to exhibit higher catalytic activity as measured by total turnover number. The requirement for a redox‐active ligand for catalyst function has both benefits and limitations that are discussed in the context of aerobic alcohol oxidation catalysis.

Two new small-molecule enzyme mimics of carbonic anhydrase were prepared and characterized. These complexes contain the salen-like ligand bis(hydroxyphenyl)phenanthroline. This ligand is similar to the salen-type ligands previously incorporated into carbonic anhydrase mimics but contains no hydrolyzable imine groups and therefore serves as a promising ligand scaffold for the synthesis of a more robust CO2 hydration catalyst. These homogeneous catalysts were investigated for CO2 hydration in concentrated primary amine solutions through which a dilute CO2 (14%) fluid stream was flowed and showed exceptional activity for increased CO2 absorption rates.