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

Abstract

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