Carbon Dioxide Promoted H+ Reduction Using a Bis(imino)pyridine Manganese Electrocatalyst

Heating a 1:1 mixture of (CO)5MnBr and the phosphine-substituted pyridine diimine ligand, (Ph2PPr)PDI, in THF at 65 °C for 24 h afforded the diamagnetic complex [((Ph2PPr)PDI)Mn(CO)][Br] (1). Higher temperatures and longer reaction times resulted in bromide displacement of the remaining carbonyl ligand and the formation of paramagnetic ((Ph2PPr)PDI)MnBr (2). The molecular structure of 1 was determined by single crystal X-ray diffraction, and density functional theory (DFT) calculations indicate that this complex is best described as low-spin Mn(I) bound to a neutral (Ph2PPr)PDI chelating ligand. The redox properties of 1 and 2 were investigated by cyclic voltammetry (CV), and each complex was tested for electrocatalytic activity in the presence of both CO2 and Brønsted acids. Although electrocatalytic response was not observed when CO2, H2O, or MeOH was added to 1 individually, the addition of H2O or MeOH to CO2-saturated acetonitrile solutions of 1 afforded voltammetric responses featuring increased current density as a function of proton source concentration (icat/ip up to 2.4 for H2O or 4.2 for MeOH at scan rates of 0.1 V/s). Bulk electrolysis using 5 mM 1 and 1.05 M MeOH in acetonitrile at -2.2 V vs Fc(+/0) over the course of 47 min gave H2 as the only detectable product with a Faradaic efficiency of 96.7%. Electrochemical experiments indicate that CO2 promotes 1-mediated H2 production by lowering apparent pH. While evaluating 2 for electrocatalytic activity, this complex was found to decompose rapidly in the presence of acid. Although modest H(+) reduction activity was realized, the experiments described herein indicate that care must be taken when evaluating Mn complexes for electrocatalytic CO2 reduction.