Push or Pull? Proton Responsive Ligand Effects in Rhenium Tricarbonyl CO2 Reduction Catalysts

Proton responsive ligands offer control of catalytic reactions through modulation of pH-dependent properties, second coordination sphere stabilization of transition states, or by providing a local proton source for multiproton, multielectron reactions. Two fac-[Re(I)(α-diimine)(CO)3Cl] complexes with α-diimine = 4,4'- (or 6,6'-) dihydroxy-2,2'-bipyridine (4DHBP and 6DHBP) have been prepared and analyzed as electrocatalysts for the reduction of carbon dioxide. Consecutive electrochemical reduction of these complexes yields species identical to those obtained by chemical deprotonation. An energetically feasible mechanism for reductive deprotonation is proposed in which the bpy anion is doubly protonated followed by loss of H2 and 2H(+). Cyclic voltammetry reveals a two-electron, three-wave system owing to competing EEC and ECE pathways. The chemical step of the ECE pathway might be attributed to the reductive deprotonation but cannot be distinguished from chloride dissociation. The rate obtained by digital simulation is approximately 8 s(-1). Under CO2, these competing reactions generate a two-slope catalytic waveform with onset potential of -1.65 V vs Ag/AgCl. Reduction of CO2 to CO by the [Re(I)(4DHBP-2H(+))(CO)3](-) suggests the interaction of CO2 with the deprotonated species or a third reduction followed by catalysis. Conversely, the reduced form of [Re(6DHBP)(CO)3Cl] converts CO2 to CO with a single turnover.