Role of ancillary ligands in selectivity towards acceptorless dehydrogenation versus dehydrogenative coupling of alcohols and amines catalyzed by cationic ruthenium(II)-CNC pincer complexes

Abstract

An unexpected reversal in catalytic activity for acceptorless dehydrogenative coupling compared to acceptorless alcohol dehydrogenation has been observed using a series of cationic Ru(II)-CNC pincer complexes with different ancillary ligands. In continuation of our study of cationic Ru(II)-CNC pincer complexes 1a-6a, new complexes with bulky N-wingtips [Ru(CNC^iPr)(CO)(PPh₃)Br]PF₆ (1b), [Ru(CNC^Cy)(CO)(PPh₃)Cl]PF₆ (1c), [Ru(CNC^Cy)(CO)(PPh₃)H]PF₆ (2c), [Ru(CNC^iPr)(PPh₃)₂Cl]PF₆ (3b), [Ru(CNC^Cy)(PPh₃)₂Cl]PF₆ (3c), [Ru(CNC^iPr)(PPh₃)₂H]PF₆ (4b), [Ru(CNC^Cy)(PPh₃)₂H]PF₆ (4c), [Ru(CNC^iPr)(DMSO)₂Cl]PF₆ (6b), and [Ru(CNC^Cy)(DMSO)₂Cl]PF₆ (6c) [CNC^R = 2,6-bis(1-alkylimidazol-2-ylidene)-pyridine] have been synthesized and the catalytic activities of the new complexes have been compared with their N-methyl analogues for transfer hydrogenation of cyclohexanone and acceptorless dehydrogenation of benzyl alcohol. Furthermore, all complexes have been utilized as catalysts in the dehydrogenative coupling reaction of benzyl alcohol with amines. While the catalytic activities of the new complexes for transfer hydrogenation and acceptorless alcohol dehydrogenation were found to be in line with the previously observed trend based on the ancillary ligands (CO > COD > DMSO > PPh₃), for the acceptorless dehydrogenative coupling reaction, complexes containing PPh₃ and DMSO ligands performed better compared to complexes containing CO and COD ligands. Based on NMR and mass investigation of catalytic reactions, a plausible mechanism has been suggested to explain the difference in catalytic activity and its reversal during the dehydrogenative coupling reaction. Furthermore, the substrate scope for the dehydrogenative coupling reaction of benzyl alcohol with a wide range of amines has been explored, including synthesizing some pharmaceutically important imines. All new complexes have been characterized by various spectroscopic techniques, and the structures of 4b and 6b have been confirmed by the single-crystal X-ray diffraction technique.