Electrocatalytic and Photocatalytic Conversion of CO2 to Methanol using Ruthenium Complexes with Internal Pyridyl Cocatalysts

The ruthenium complexes [Ru(phen)(2)(ptpb alpha)](2+) (Ru alpha) and [Ru(phen)(2)(ptpb beta)](2+) (Ru beta), where phen =1,10-phenanthroline; ptpb alpha = pyrido[2',3':5,6]pyrazino[2,3-f][1,10]phenanthroline; ptpb beta = pyrido[3',4':5,6]pyrazino[2,3-f][1,10]phenanthroline, are shown as electrocatalysts and photocatalysts for CO2 reduction to formate, formaldehyde, and methanol. Photochemical activity of both complexes is lost in water but is retained in 1 M H2O in DMF. Controlled current electrolysis of a solution of Ru beta in CO2 saturated DMF:H2O (1 M) yields predominantly methanol over a 6 h period at similar to -0.60 V versus Ag/AgCl, with traces of formaldehyde. After this time, the potential jumped to -1.15 V producing both methanol and CO as products. Irradiation of Ru beta in a solution of DMF:H2O (1 M) containing 0.2 M TEA (as the sacrificial reductant) yields methanol, formaldehyde, and formate. Identifications of all of the relevant redox and protonated states of the respective complexes were obtained by a combination of voltammetry and differential reflectance measurements. Spectroelectrochemistry was particularly useful to probe the photochemical and electrochemical reduction mechanisms of both complexes as well as the complexes speciation in the absence and presence of CO2.