Navigating structure- activity relationships in molecular electrocatalysts for CO_2 reduction

Abstract

As a route to earth- abundant electrocatalysts for CO_2 redn., we have used quantum mech. calcns. including solvation to uncover relationships between the compn. of known catalysts and their attributes (i.e. rate, product selectivity, onset potential. Among these are Milstein's (PNP)Fe(CO) H_2 and Meyer and Brookhart's (PoCoP)Ir(MeCN)H_2 (demonstrated to electrochem. reduce CO_2 selectively to formate in water. Modeling (PNP)Fe(CO)H (a demonstrated CO_2 hydrogenation catalyst) in an prospective electrochem. cycle generating formate, we find that an impractical potential would be required for the two- electron redn. regenerating the dihydride, and that both hydrides would be prone to hydrogen evolution during such a cycle. A space of earth- abundant metal hydrides characterized by (L_3 ML_2H was explored for free energy surfaces supporting selective, low- overpotential CO_2 redn. Replacing the PNP ligand with a redox- active pincer ligand effectively raises the computed potential required to reduce Fe(II) by 2- electrons over a volt, into an attractive range. Replacing a hydride in the "HFe- H" core with a lesser sigma donor erodes the driving force for hydride transfer, but this can be restored upon substitution of a more nucleophilic metal. We will discuss the influence of compositional degrees of freedom on free energy surfaces and make comparisons to the successful template provided by the iridium catalyst.

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