Mechanistic Study of Tungsten Bipyridyl Tetracarbonyl Electrocatalysts for CO 2 Fixation: Exploring the Roles of Explicit Proton Sources and Substituent Effects.

Tungsten bipyridyl tetracarbonyl complexes were shown to reduce CO 2 to CO in acetonitrile [ Chem. Sci ., 2014 , 5 , 1894-1900]. Here, we employ density functional theory (DFT) calculations to investigate the electronic structure and reactivity of a series of tungsten electrocatalysts, [W(bpy-R)(CO) 4 ] (where R = H, CH 3 , t Bu, OCH 3 , CF 3, and CN), for the CO 2 reduction reaction (CO 2 RR). Our proposed mechanism suggests that initial reduction of the starting material by two electrons is required to access the active catalyst upon CO dissociation, which is slightly endergonic, consistent with the slow product release observed experimentally. The doubly reduced species, which has a closed-shell singlet ground state, can bind CO 2 via an η 2 -CO 2 binding mode to yield the metallocarboxylate intermediate. Based on the energy span model, CO 2 addition is the TOF-determining transition state (TDTS) in the presence of water as the proton source. Different substituents at the 4,4'-positions of the bipyridine ligand in [W(bpy-R)(CO) 4 ] (R = H, CH 3 , t Bu, OCH 3 , CF 3, and CN) were considered to comprehend the substituent effects for CO 2 RR. DFT results show that electron-withdrawing substituents, such as CN and CF 3 , do not yield efficient CO 2 reduction catalysts due to the necessity of forming high energy intermediates for the protonation steps, resulting in low TOFs and high overpotentials. Among electron-donating groups, the parent compound and tert -butyl substituted complex are the most active catalysts for CO 2 RR due to higher TOFs at low overpotentials. Overall, based on the energy span model and theoretical Tafel plots, our computational approach provides quantitative information for designing CO 2 reduction electrocatalysts.