UC Davis

Proton relays are known to increase reaction rates for H₂ evolution and lower overpotentials in electrocatalytic reactions. In this report we describe two electrocatalysts, [Fe₄N(CO)₁₁(PPh₃)]^- (1^-) which has no proton relay, and hydroxyl-containing [Fe₄N(CO)₁₁(Ph₂P(CH₂)₂OH)]^- (2^-). Solid state structures indicate that these phosphine-substituted clusters are direct analogs of [Fe₄N(CO)₁₂]^- where one CO ligand has been replaced by a phosphine. We show that the proton relay changes the selectivity of reactions: CO₂ is reduced selectively to formate by 1^- in the absence of a relay, and protons are reduced to H₂ under a CO₂ atmosphere by 2^-. These results implicate a hydride intermediate in the mechanism of the reactions and demonstrate the importance of controlling proton delivery to control product selectivity. Thermochemical measurements performed using infrared spectroelectrochemistry provided pK_a and hydricity values for [HFe₄N(CO)₁₁(PPh₃)]^-, which are 23.7, and 45.5 kcal mol^-1, respectively. The pK_a of the hydroxyl group in 2^- was determined to fall between 29 and 41, and this suggests that the proximity of the proton relay to the active catalytic site plays a significant role in the product selectivity observed, since the acidity alone does not account for the observed results. More generally, this work emphasizes the importance of substrate delivery kinetics in determining the selectivity of CO₂ reduction reactions that proceed through metal-hydride intermediates.