Local Proton Source in Electrocatalytic CO2 Reduction with [Mn(bpy–R)(CO)3Br] Complexes

The electrochemical behavior of fac-[Mn(pdbpy)(CO)3Br] (pdbpy = 4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine), 1, in acetonitrile under Ar and its catalytic performances for CO2 reduction with added water, 2,2’,2’’-trifluoroethanol (TFE) and phenol are discussed in detail. Preparative-scale electrolysis experiments, carried out at –1.5 V vs. SCE in CO2-saturated acetonitrile solutions, reveal that the process selectivity is extremely sensitive to the acid strength, providing CO and formate in different faradaic yields. A detailed spectroelectrochemical (IR and UV-Vis) study under Ar and CO2 atmospheres shows that 1 undergoes fast solvolysis; however dimer formation in acetonitrile is suppressed, providing an atypical reduction mechanism in comparison with other reported MnI catalysts. Spectroscopic evidence of Mn hydride formation supports the existence of different electrocatalytic CO2 reduction pathways. Furthermore, a comparative investigation performed on the new fac-[Mn(ptbpy)(CO)3Br] (ptbpy = 4-phenyl-6-(phenyl-3,4,5-triol)-2,2'-bipyridine) catalyst, 2, bearing a bipyridyl derivative with OH groups in different positions to those in 1, provides complementary information about the role that the local proton source plays during the electrochemical reduction of CO2