Structure-Function Relationship in Ester Hydrogenation Catalyzed by Ruthenium CNN-Pincer Complexes

A series of six pincer ruthenium complexes has been synthesized and applied in the catalytic hydrogenation of esters. The ruthenium complexes have the formula Ru(pincer)HCl(CO), where the CNN-pincer ligands feature N-heterocyclic carbene (NHC), pyridine, and dialkylamino donor groups. Through systematic variation of the steric bulk of the NHC substituent and the amine substituents, a clear structure function relationship emerges. The most active catalysts in this series feature the bulkiest NHC substituent employed, 2,6-diisopropylphenyl. For the dialkylamino group, catalysts substituted with isopropyl or ethyl groups were the most active, while catalysts substituted with methyl groups were significantly less active. The most active catalyst discovered catalyzes the complete hydrogenation of a range of esters at loadings of 0.05-0.2 mol %.