Carbon Dioxide Hydrogenation to Formate Catalyzed by Highly Active Ru-tris(pyrazolyl)methane Complexes

In the quest for highly active ruthenium(II) complexes for homogeneous catalytic CO2 hydrogenation to formate, a small library of cationic Ru(k3-tpm) piano-stool complexes [tpm = tris(pyrazolyl)methane], bearing different ancillary ligands, were tested under various reaction conditions, with and without the addition of a Lewis acid co-catalyst. Under optimized conditions (80 bar, 120 °C) and in the presence of LiOTf, TONs > 54000 were obtained in single batch runs with the complex [RuCl(k3-tpm)(PPh3)(CH3CN)]Cl (1). Mechanistic studies using NMR spectroscopy and DFT calculations were also carried out to elucidate key steps and the energies associated with the reaction pathway, which allowed for the proposal of a catalytic mechanism.