Effect of Ligand Electronics on the Reversible Catalytic Hydrogenation of CO2 to Formic Acid Using Ruthenium Polyhydride Complexes: A Thermodynamic and Kinetic Study

Hydrogenation of CO2 to formic acid or formates is often carried out using catalysts of the type H4Ru(PR3)(3) (1). These catalysts are also active for the reverse reaction, i.e., the decomposition of formic acid to H-2 and CO2. While numerous catalysts have been synthesized for reactions in both directions, the factors controlling the elementary steps of the catalytic cycle remain poorly understood. In this work, we synthesize a series of compounds of type H4Ru(P(C6H4R)(3))(3) containing both electron-donating and electron-withdrawing groups and analyze their influence on the kinetic and thermodynamic parameters of CO2 insertion and deinsertion. The data are correlated with the catalytic performance of the complexes through linear free-energy relationships. The results show that formic acid dissociation from the catalyst is rate-determining during CO2 hydrogenation, while deinsertion is critical for the decomposition reaction.