N-Formylation of Amines with Carbon Dioxide and Hydrogen Catalyzed by Ionic Liquid-Assisted Ru Complexes

The N-formylation reaction of various amines with CO2 is a promising method to utilize CO2 for the production of high value-added chemicals. Herein, an efficient Ru catalyst system consisting of Ru complex ([PPN][Ru(CO)3Cl3]) and the ionic liquid 1-butyl-3-methylimidazolium hydrogen phosphate ([Bmim]2[HPO4]), has been developed for the N-formylation reaction. Activity evaluation demonstrated that the catalyst system exhibited good to excellent yields of formamides under very mild conditions with the assistance of ionic liquids (ILs). Furthermore, the catalyst system could be reused at least five times with a slight loss of activity in N-formylmorpholine production. Further characterization by 1H nuclear magnetic resonance (NMR), 13C NMR, and density functional theory (DFT) calculations showed that the ILs ([Bmim]2[HPO4]) could facilitate the adsorption of CO2 by a concerted action of both cations and anions of the ILs to form phosphorus carbonate intermediate, thus effectively promoting the reaction. DFT calculations demonstrated that the formation of Ru–H was the first step, and then the CO2 absorbed by [Bmim]2[HPO4] inserted directly into Ru–H by cooperating with the Ru sites to form HCOO– species. After that, formate species dissociated from Ru catalysts and dehydrated to form formamides by the catalytic action of Ru complex and the basic IL.