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, ^Ph2PPrPDI, in THF at 65 °C for 24 h afforded the diamagnetic complex [(^Ph2PPrPDI)Mn(CO)][Br] (1). Higher temperatures and longer reaction times resulted in bromide displacement of the remaining carbonyl ligand and the formation of paramagnetic (^Ph2PPrPDI)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 ^Ph2PPrPDI 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 (i_cat/i_p 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.