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

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, ν, is observed. The Rh P₂N₂ complexes have hydricities between 28-34 kcal/mol and function as hydrogenation catalysts for the reduction of CO₂ to formate. Learning from the reactivity of these catalysts, design principles for future metal hydride complexes are presented that focus on the ultimate goal of catalyst optimization for improved energy efficiency (overpotential) with high selectivity (Faradaic efficiency) for both formate oxidation and CO₂ reduction to formate.