We report a family of cobalt complexes based on bidentate phosphine ligands with two, one, or zero pendent amine groups in the ligand backbone. The pendent amine complexes are active electrocatalysts for the formate oxidation reaction, generating CO2 with near-quantitative faradaic efficiency at moderate overpotentials (0.45 – 0.57 V in acetonitrile). These homogeneous electrocatalysts are the first cobalt example and second first-row transition metal example for formate oxidation. Thermodynamic measurements reveal these complexes are energetically primed for formate oxidation via hydride transfer to the cobalt center, followed by deprotonation of the resulting cobalt-hydride by formate acting as a base. The complex with the strongest cobalt- hydride bond, given by its thermodynamic hydricity, is the fastest electrocatalyst in this series, with an observed rate constant for formate oxidation of 135 ± 8 h−1 at 25 °C. Electrocatalytic turnover is not observed for the complex with no pendent amine groups: decomposition of the complex structure is evident in the presence of high formate concentrations.
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A family of neutral cobalt complexes, [CpR'Co(Ropda)] 1-5, based on the redox-active o-phenylenediamide ligand (Ropda) undergo reversible 2e- oxidation revealed by cyclic voltammetry. This multielectron behavior is observed for all complexes regardless of the substituents on the phenylenediamide ligand, enabling redox tuning over more than 0.5 V. These diamagnetic neutral complexes are best described as delocalized systems with covalent bonding across the cobalt-opda metallocycle, consistent with the closed-shell singlet ground-state predicted by density functional theory (DFT) calculations. Two-electron oxidation using chemical oxidants affords the dicationic species, which are formulated as Co(III)-benzoquinonediimine systems with an additional coordinated acetonitrile ligand. DFT calculations also predict an ECE pathway for the 2e- oxidation, in which the first 1e- step is primarily a ligand-based process with redistribution of electron density to the metal. The associated distortion of the coordination geometry and disruption of the metallocycle bonding enable acetonitrile coordination in the intermediate oxidation state, which is critical for favoring the second electron transfer and accessing the potential inversion.
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Thorp, Holden (Ed.)
Ancestral metabolic processes involve the reversible oxidation of molecular hydrogen by hydrogenase. Extant hydrogenase enzymes are complex, comprising hundreds of amino acids and multiple cofactors. We designed a 13–amino acid nickel-binding peptide capable of robustly producing molecular hydrogen from protons under a wide variety of conditions. The peptide forms a di-nickel cluster structurally analogous to a Ni-Fe cluster in [NiFe] hydrogenase and the Ni-Ni cluster in acetyl-CoA synthase, two ancient, extant proteins central to metabolism. These experimental results demonstrate that modern enzymes, despite their enormous complexity, likely evolved from simple peptide precursors on early Earth.