Carbon dioxide hydrogenation to formate catalyzed by highly active Ru-tris(pyrazolyl)methane complexes

In the quest for novel, active ruthenium(II) complexes for homogeneous catalytic CO2 hydrogenation to formate, a small library of cationic Ru(κ3-tpm) 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(κ3-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.