Cationic ruthenium(II)-CNC pincer complexes as phosphine-free catalysts for nitrile hydration to amides in aqueous medium

Abstract

We report the synthesis, characterization, and catalytic activity of a series of electron-rich, phosphine-free complexes featuring multiple N-heterocyclic carbene (NHC) ligands, namely [Ru(CNCi-Pr)(CNMe)I]PF6 (1a), [Ru(CNCi-Pr)(CNi-Pr)I]PF6 (1b), [Ru(CNCCy)(CNMe)I]PF6 (2a), [Ru(CNCCy)(CNi-Pr)I]PF6 (2b), [Ru(CNCt-Bu)(CNMe)I]PF6 (3a), and [Ru(CNCt-Bu)(CNi-Pr)I]PF6 (3b) (where CNC = 2,6-bis(alkylimidazol-2-ylidene)-pyridine and CN = 2-(3-alkylimidazol-2-ylidene)-pyridine). Spectroscopic characterization using multinuclear NMR and high-resolution mass spectrometry (HRMS) confirms the composition of all complexes. The molecular structures of 1a and 2a were further confirmed by single-crystal X-ray diffraction. The catalytic activity of these ruthenium(II) complexes in the hydration of nitriles under mild reaction conditions and in the aqueous medium have been explored, with complex 1a exhibiting superior reactivity among the studied complexes. Mechanistic investigations revealed a catalytic pathway initiated by a [Ru-OH] species, facilitated by the hemilability of a pyridine ligand. The versatility of these ruthenium(II) complexes as catalysts for nitrile hydration has been demonstrated through successful conversions of a diverse range of nitriles, containing electron-releasing or electron-withdrawing groups, and heterocyclic nitriles into the corresponding amides with high reactivity and good to excellent yields.