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Abstract

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 H₂ 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.