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

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Hydrogenation of CO 2 to formic acid or formates is often carried out using catalysts of the type H 4 Ru(PR 3 ) 3 ( 1 ). These catalysts are also active for the reverse reaction, i.e., the decomposition of formic acid to H 2 and CO 2 . 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 H 4 Ru(P(C 6 H 4 R) 3 ) 3 containing both electron-donating and electron-withdrawing groups and analyze their influence on the kinetic and thermodynamic parameters of CO 2 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 CO 2 hydrogenation, while deinsertion is critical for the decomposition reaction.