Carbon dioxide hydrogenation to formate catalyzed by phosphine-free ruthenium(0) cyclopentadienone complexes stabilized by NHC ligands

Phosphine-free ruthenium complexes containing a strongly donating 1,3-dimethyl-imidazolyl NHC ligand were tested for homogeneous catalytic CO2 hydrogenation to formate under various reaction conditions, both with and without the addition of trimethylamine N-oxide (TMANO) as a co-catalyst. Under optimized conditions (80 bar, 100 degrees C), using N-methylpyrrolidone (NMP) as solvent and in the presence of TMANO, turnover numbers (TONs) exceeding 54,000 were achieved in single batch runs. Mechanistic studies by NMR spectroscopy and DFT calculations elucidate key steps and energies associated with the reaction pathway for the best performing catalyst, leading to a proposed mechanism.