Electrochemical CO2 reduction catalyzed by Mn catalysts: DFT investigations point to strategies for overpotential reduction and activity improvement

Abstract

Exptl., [(L) Mn(CO)_3]- (where L = bis alkyl- substituted bipyridine) has been obsd. to catalyze the electrochem. redn. of CO_2 to CO in the presence of trifluoroethanol (TFEH). Our DFT calcns. (B3LYP- d3 with continuum solvation) of the free energies of reaction and activation show that the highly exergonic hydrogen bonding between TFEH and TFE (homoconjugation) plays a crit. role in reaction thermodn. and kinetics. The analgous 2,2'- bipyrimidine complex is predicted to have a lower (by ca. 0.5 V) onset overpotential, but at the expense of max. catalytic activity. As a strategy to improve catalytic activity and onset overpotential, a series of 2,2'- and 4,4'- bipyrimidines with tethered alc. or phenol was examd. The major predictions were that: (a) PhOH is not significantly more effective than TFEH at promoting dehydroxylation, due to their similar hydrogen bond acidities; (b) the appended phenols are less effective at promoting catalysis than the appended alc.; and (c) inclusion of one appended phenol arm promotes catalytic turnover by eliminating the entropic penalty of including an external PhOH in the dehydroxylation TS, but inclusion of two phenol arms is detrimental because intramol. hydrogen bonding stabilizes the resting state of the catalytic cycle.