A Rhenium Bis-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]⁺.

Alameh, R.; Arteta, S.; Fernández, S.; Barakat, M.; Asempa, E.; Atallah, H.; Durand, N.; Gillis, C.; Assaf, E.; Jakubikova, E.; Miller, A.; Castellano, F. A Rhenium Bis-Tetramethylphenanthroline Catalyst for CO₂ Reduction to Formate, Energy & Fuels, 2025, In Press, https://doi.org/10.1021/acs.energyfuels.5c01212