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

Electrocatalytic properties, X-ray crystallographic studies, and infrared spectroelectrochemistry (IRSEC) of Mn(bpy-tBu)(CO)(3)Br and [Mn(bpy-tBu)(CO)(3)(MeCN)](OTf) are reported. Addition of Bronsted 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 Bronsted acids for catalytic turnover. Current densities up to 30 mA/cm(2) were observed during bulk electrolysis using 5 mM Mn(bpytBu)(CO)(3)Br, 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)(3)Br 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 H-2. 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)(3)Br. These were of the parent Mn(bpy-tBu)(CO)(3)Br complex, the dimer [Mn(bpy-tBu)(CO)(3)](2), and the [Mn(bpy-tBu)(CO)(3)](2), and the [Mn(bpytBu)(CO)(3)](-) anion.