A Rhenium <i>Bis</i>- tetramethylphenanthroline Catalyst for CO₂ Reduction to Formate

Catalytic CO2 reduction reactions featuring high selectivity toward formate are relatively rare. In some homogeneous molecular CO2-reducing electrocatalysis, using triethylamine (TEA) and isopropanol (IPA) as additives improves catalytic performance in producing formate. In this work, we investigate whether the rhenium(I) bis-diimine dicarbonyl complexes, cis-[Re(N^N)2(CO)2]+, where N^N is 2,2’-bipyridine ([1]+) or 3,4,7,8-tetramethyl-1,10-phenanthroline ([2]+), are capable of electrocatalytically reducing CO2 to formate in acetonitrile containing TEA and IPA. Catalyst [1]+ was ineffective at CO2 reduction, yielding formate quantities comparable to those produced in experiments without the catalyst. Catalyst [2]+, however, is a promising electrocatalyst for the CO2 reduction reaction in the presence of TEA and IPA, with formate being produced in millimolar concentrations (10.5 mM), as detected by 1H NMR spectroscopy after 6 h electrolysis (formate Faradaic efficiency = 11%, with the major balance going to H2). Upon more detailed examination, [2]+ exhibited a turnover frequency (TOF) of 12 s–1 for formate, comparable to other leading molecular catalysts that competently execute this reduction. Combinations of spectroscopy, electrochemistry, and theory were used to better understand the mechanism of CO2 reduction by [2]+. Fourier transform infrared spectroelectrochemical (FTIR-SEC) data provided no evidence for CO ligand dissociation or substitution upon one- and two-electron reduction of [2]+, suggesting that a mechanism distinct from one that is metal-hydride-based is operative in catalysis. Computational studies guide mechanistic investigations toward the proposed formation of a hydrophenanthroline-based intermediate responsible for hydride transfer to CO2 and electrocatalytic formate production from [2]+.