Electrocatalytic CO 2

Anthracene-bridged dinuclear rhenium complexes are reported for electrocatalytic carbon dioxide (CO) reduction to carbon monoxide (CO). Related by hindered rotation of each rhenium active site to either side of the anthracene bridge, cis and trans conformers have been isolated and characterized. Electrochemical studies reveal distinct mechanisms, whereby the cis conformer operates via cooperative bimetallic CO activation and conversion and the trans conformer reduces CO through well-established single-site and bimolecular pathways analogous to Re(bpy)(CO)Cl. Higher turnover frequencies are observed for the cis conformer (35.3 s) relative to the trans conformer (22.9 s), with both outperforming Re(bpy)(CO)Cl (11.1 s). Notably, at low applied potentials, the cis conformer does not catalyze the reductive disproportionation of CO to CO and CO in contrast to the trans conformer and mononuclear catalyst, demonstrating that the orientation of active sites and structure of the dinuclear cis complex dictate an alternative catalytic pathway. Further, UV-vis spectroelectrochemical experiments demonstrate that the anthracene bridge prevents intramolecular formation of a deactivated Re-Re-bonded dimer. Indeed, the cis conformer also avoids intermolecular Re-Re bond formation

Electrocatalytic CO₂ Reduction with <i>Cis</i> and <i>Trans</i> Conformers of a Rigid Dinuclear Rhenium Complex: Comparing the Monometallic and Cooperative Bimetallic Pathways

Anthracene-bridged dinuclear rhenium complexes are reported for electrocatalytic carbon dioxide (CO₂) reduction to carbon monoxide (CO). Related by hindered rotation of each rhenium active site to either side of the anthracene bridge, <i>cis</i> and <i>trans</i> conformers have been isolated and characterized. Electrochemical studies reveal distinct mechanisms, whereby the <i>cis</i> conformer operates via cooperative bimetallic CO₂ activation and conversion and the <i>trans</i> conformer reduces CO₂ through well-established single-site and bimolecular pathways analogous to Re­(bpy)­(CO)₃Cl. Higher turnover frequencies are observed for the <i>cis</i> conformer (35.3 s^–1) relative to the <i>trans</i> conformer (22.9 s^–1), with both outperforming Re­(bpy)­(CO)₃Cl (11.1 s^–1). Notably, at low applied potentials, the <i>cis</i> conformer does not catalyze the reductive disproportionation of CO₂ to CO and CO₃^2– in contrast to the <i>trans</i> conformer and mononuclear catalyst, demonstrating that the orientation of active sites and structure of the dinuclear <i>cis</i> complex dictate an alternative catalytic pathway. Further, UV–vis spectroelectrochemical experiments demonstrate that the anthracene bridge prevents intramolecular formation of a deactivated Re–Re-bonded dimer. Indeed, the <i>cis</i> conformer also avoids intermolecular Re–Re bond formation

Bioinspired design of redox-active ligands for multielectron catalysis: effects of positioning pyrazine reservoirs on cobalt for electro- and photocatalytic generation of hydrogen from water.

Mononuclear metalloenzymes in nature can function in cooperation with precisely positioned redox-active organic cofactors in order to carry out multielectron catalysis. Inspired by the finely tuned redox management of these bioinorganic systems, we present the design, synthesis, and experimental and theoretical characterization of a homologous series of cobalt complexes bearing redox-active pyrazines. These donor moieties are locked into key positions within a pentadentate ligand scaffold in order to evaluate the effects of positioning redox non-innocent ligands on hydrogen evolution catalysis. Both metal- and ligand-centered redox features are observed in organic as well as aqueous solutions over a range of pH values, and comparison with analogs bearing redox-inactive zinc(ii) allows for assignments of ligand-based redox events. Varying the geometric placement of redox non-innocent pyrazine donors on isostructural pentadentate ligand platforms results in marked effects on observed cobalt-catalyzed proton reduction activity. Electrocatalytic hydrogen evolution from weak acids in acetonitrile solution, under diffusion-limited conditions, reveals that the pyrazine donor of axial isomer 1-Co behaves as an unproductive electron sink, resulting in high overpotentials for proton reduction, whereas the equatorial pyrazine isomer complex 2-Co is significantly more active for hydrogen generation at lower voltages. Addition of a second equatorial pyrazine in complex 3-Co further minimizes overpotentials required for catalysis. The equatorial derivative 2-Co is also superior to its axial 1-Co congener for electrocatalytic and visible-light photocatalytic hydrogen generation in biologically relevant, neutral pH aqueous media. Density functional theory calculations (B3LYP-D2) indicate that the first reduction of catalyst isomers 1-Co, 2-Co, and 3-Co is largely metal-centered while the second reduction occurs at pyrazine. Taken together, the data establish that proper positioning of non-innocent pyrazine ligands on a single cobalt center is indeed critical for promoting efficient hydrogen catalysis in aqueous media, akin to optimally positioned redox-active cofactors in metalloenzymes. In a broader sense, these findings highlight the significance of electronic structure considerations in the design of effective electron-hole reservoirs for multielectron transformations