Electronic Effects Drive Selectivity in CO2 Reduction Catalysis by Heptacoordinated Cobalt Complexes

Controlling selectivity in CO2 reduction catalysis is essential for minimizing the formation of H2 byproduct. We recently reported that the cobalt and iron complexes of the 1-([2,2′-bipyridin]-6-yl)-N-([2,2′-bipyridin]-6-ylmethyl)-N-(pyridin-2-ylmethyl) methanamine ligand (L, DBPy-PyA) are efficient catalysts to selectively generate H2 and CO, respectively. Herein we demonstrate that selectivity can be controlled not only by exchanging the metal center but also adjusting the electronic properties of the ligand. Under electrochemical conditions in acetonitrile, with 2,2,2-trifluoroethanol as a proton source, the unsubstituted CoL and the electron-rich CoLOMe predominantly produce H2 (selectivity of 82% and 55%, respectively). In contrast, the electron-deficient CoLCF3 favors CO formation with a selectivity up to 87%. DFT calculations show that formation of the metal-hydride is favored in the case of CoL and CoLOMe, whereas it is substantially endergonic in the case of CoLCF3. Concurrently, the binding of CO2 and the evolution of the resulting intermediates in CoLCF3 can benefit from ligand-assisted proton transfer, as confirmed by microkinetic modeling. Catalytic tests were then conducted under photochemical conditions, affording syngas with tunable CO/H2 ratios that follow the electronic effects of the chosen catalyst. Overall, the results underscore how ligand tuning can be a powerful handle for controlling selectivity in molecular CO2RR.