Increasing the CO2 Reduction Activity of Cobalt Phthalocyanine by Modulating the σ-Donor Strength of Axially Coordinating Ligands

Axial coordination of pyridyl moieties to CoPc (either exogenous or within poly-4-vinylpyridine polymer) dramatically increases the complex's activity for the CO2 reduction reaction (CO2RR). It has been hypothesized that axial coordination to the Co active site leads to an increase in the Co d(z)2 orbital energy, which increases the complex's nucleophilicity and facilitates CO2 coordination compared to the parent CoPc complex. The magnitude of the energy increase in the Co d(z)2 orbital should depend on the sigma-donor strength of the axial ligand-a stronger sigma-donating ligand (L) will increase the overall CO2RR activity of axially coordinated CoPc(L) and vice versa. To test this, we have studied a series of CoPc(L) complexes where the sigma-donor strength of L is varied. We show that CoPc(L) reduces CO2 with an increased activity as the adonor ability of L is increased. These observed electrochemical activity trends are correlated with computationally derived CO2 binding energy and charge transfer terms as a function of sigma-donor strength. The findings of this study support our hypothesis that the increased CO2RR activity observed upon axial coordination to CoPc is due to the increased energy of the d(z)2 orbital, and highlight an important design consideration for macrocyclic MN4-based electrocatalysts.