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

Abstract

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] (<b>1</b>). Higher temperatures and longer reaction times resulted in bromide displacement of the remaining carbonyl ligand and the formation of paramagnetic (^Ph2PPrPDI)­MnBr (<b>2</b>). The molecular structure of <b>1</b> 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 <b>1</b> and <b>2</b> 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 <b>1</b> individually, the addition of H₂O or MeOH to CO₂-saturated acetonitrile solutions of <b>1</b> afforded voltammetric responses featuring increased current density as a function of proton source concentration (<i>i</i>_cat/<i>i</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 <b>1</b> 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 <b>1</b>-mediated H₂ production by lowering apparent pH. While evaluating <b>2</b> 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