Enhanced Electrochemical CO2 Reduction by a Series of Molecular Rhenium Catalysts Decorated with Second-Sphere Hydrogen-Bond Donors.

A series of rhenium(I) fac-tricarbonyl complexes containing pendent arylamine functionality in the second coordination sphere have been developed and studied as electrocatalysts for carbon dioxide (CO2) reduction. Aniline moieties were appended at the 6 position of a 2,2'-bipyridine (bpy) donor in which the primary amine was positioned at the ortho- (1-Re), meta- (2-Re), and para- (3-Re) sites of the aniline substituent to generate a family of isomers. The relationship between the catalyst structure and activity was explored across the series, and the catalytic performance was compared to that of the benchmark catalyst Re(bpy)(CO)3Cl (ReBpy). Catalysts 1-Re, 2-Re, and 3-Re outperform the benchmark catalyst both in anhydrous acetonitrile and with added trifluoroethanol (TFE) as an external proton source. In the presence of TFE, the aniline-substituted catalysts convert CO2 to carbon monoxide (CO) with high Faradaic efficiencies (≥89%) and have superior turnover frequencies (TOFs) relative to ReBpy (72.9 s-1), with 2-Re having the highest TOF of the series at 239 s-1, a value that is twice that of the next most active catalyst. TOFs of 123 and 109 s-1 were observed for the ortho- and para-substituted aniline complexes (1-Re and 3-Re), respectively. Indeed, catalytic activities vary widely across the series, showing a high sensitivity to the position of the amine functionality relative to the rhenium active site. IR and UV-vis spectroelectrochemical experiments were conducted on the aniline-substituted systems, revealing important differences between the catalysts and mechanistic insight.

Photochemical CO2 reduction with mononuclear and dinuclear rhenium catalysts bearing a pendant anthracene chromophore.

A series of anthracene-substituted mononuclear and dinuclear rhenium complexes have been studied for photocatalytic CO2 reduction. The effects on catalytic activity of one versus two covalently-linked active sites, their relative proximity to one another, and the pendant organic chromophore are discussed.

Electrocatalytic CO2 Reduction with Cis and Trans 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, cis and trans conformers have been isolated and characterized. Electrochemical studies reveal distinct mechanisms, whereby the cis conformer operates via cooperative bimetallic CO2 activation and conversion and the trans conformer reduces CO2 through well-established single-site and bimolecular pathways analogous to Re(bpy)(CO)3Cl. Higher turnover frequencies are observed for the cis conformer (35.3 s-1) relative to the trans conformer (22.9 s-1), with both outperforming Re(bpy)(CO)3Cl (11.1 s-1). Notably, at low applied potentials, the cis conformer does not catalyze the reductive disproportionation of CO2 to CO and CO32- 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.