Metalloradical intermediates in electrocatalytic reduction of CO₂ to CO: Mn versus Re bis-N-heterocyclic carbene pincers

This work examines the relative reactivities of Re-I and Mn-I tricarbonyl pyridine-2,6-bis-N-heterocyclic carbene pincers M(CO)(3)(CNCX)-X-Bn (M = Re, Mn and X = Cl and Br) towards catalysis for the electrochemical conversion of CO2 to CO. Unlike prior well-studied group VII catalysts, Mn(CO)(3)(CNCX)-X-Bn is extraordinarily active, while the new Re(CO)(3)(CNCX)-X-Bn complex surprisingly does not exhibit catalytic response. DFT calculations shed light on this puzzling behavior and show that the redox-active pyridine-2,6-bis-N-heterocyclic carbene ligand facilitates the reduction of the ground-state complexes; however, the extent of electronic delocalization in the reduced intermediates differs in the degree of metalloradical character. The highly-active Mn(CO)(3)(CNCX)-X-Bn complex proceeds through an intermediate with nucleophilic metalloradical character in which 66% of the unpaired electron spin resides on Mn. In contrast, Re(CO)(3)(CNCX)-X-Bn reduction proceeds through an intermediate with less metalloradical character in which only 38% of the unpaired spin is localized on Re with the remainder delocalized over the ligand. The energetic penalty of the electron delocalization of an electron on the ligand affects the M-CO bond strengths and related kinetic barriers. We discuss these observations in the context of turnover-enabling effects in CO2 reductions mediated by group VII NHC pincer molecular electrocatalysts.