Transition Metal Hydride Catalysts for Sustainable Interconversion of CO2 and Formate: Thermodynamic and Mechanistic Considerations

Abstract

Hydride transfer between transition metal hydride complexes and carbon dioxide is a known reaction, where the thermodynamically favored direction of hydride transfer determines whether CO2 reduction or formate oxidation occurs. Analysis of a growing database of thermodynamic parameters for transition metal hydride complexes now provides clear demarcation between metal hydrides which will function as oxidases and as reductases. The turning point is set at the hydricity of formate (44 kcal/mol in acetonitrile). Here, we utilize hydricity as a framework to reevaluate the catalytic activity and proposed mechanisms for formate oxidation and CO2 reduction with several Ni and Rh P2N2 (P2N2 = 1,5-diaza-3,7-diphosphacyclooctane) complexes, respectively. The series of Ni P2N2 complexes have hydricities between 55–64 kcal/mol and are active catalysts for the electrochemical oxidation of formate. A surprising correlation of increased rate of electrochemical oxidation with decreased overpotential, η, is observed. The ...