Manganese as a Substitute for Rhenium in CO₂ Reduction Catalysts: The Importance of Acids

Electrocatalytic properties, X-ray crystallographic studies, and infrared spectroelectrochemistry (IR-SEC) of Mn­(bpy-tBu)­(CO)₃Br and [Mn­(bpy-tBu)­(CO)₃(MeCN)]­(OTf) are reported. Addition of Brönsted acids to CO₂-saturated solutions of these Mn complexes and subsequent reduction of the complexes lead to the stable and efficient production of CO from CO₂. Unlike the analogous Re catalysts, these Mn catalysts require the addition of Brönsted acids for catalytic turnover. Current densities up to 30 mA/cm² were observed during bulk electrolysis using 5 mM Mn­(bpy-tBu)­(CO)₃Br, 1 M 2,2,2-trifluoroethanol, and a glassy carbon working electrode. During bulk electrolysis at −2.2 V vs SCE, a TOF of 340 s^–1 was calculated for Mn­(bpy-tBu)­(CO)₃Br with 1.4 M trifluoroethanol, corresponding to a Faradaic efficiency of 100 ± 15% for the formation of CO from CO₂, with no observable production of H₂. When compared to the analogous Re catalysts, the Mn catalysts operate at a lower overpotential and exhibit similar catalytic activities. X-ray crystallography of the reduced species, [Mn­(bpy-tBu)­(CO)₃]⁻, shows a five-coordinate Mn center, similar to its rhenium analogue. Three distinct species were observed in the IR-SEC of Mn­(bpy-tBu)­(CO)₃Br. These were of the parent Mn­(bpy-tBu)­(CO)₃Br complex, the dimer [Mn­(bpy-tBu)­(CO)₃]₂, and the [Mn­(bpy-tBu)­(CO)₃]⁻ anion.