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

Phosphine-free NHC-modified Shvo-type complexes catalyze CO 2 hydrogenation to formate in the presence of trimethylamine N -oxide (TMANO) co-catalyst reaching TON>54,000. Combined NMR experiments and DFT calculations confirmed the pivotal role of TMANO and DBU in activating the pre-catalyst and in assisting H-H bond splitting, respectively. • NHC-modified Shvo-type Ru complexes were tested for CO 2 hydrogenation to formate. • TON > 54000 was reached using trimethylamine-N-oxide (TMAO) as co-catalyst. • NMR and DFT confirmed that TMAO and DBU promote key steps of the catalytic cycle. Phosphine-free ruthenium complexes containing a strongly donating 1,3-dimethyl-imidazolyl NHC ligand were tested for homogeneous catalytic CO 2 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 °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.