Synthesis and structural characterization of N-heterocyclic carbene silver complexes derived from N-ferrocenylmethyl-N′-(pyridylmethyl)imidazolium iodides

Abstract

A series of N-heterocyclic carbene (NHC) silver complexes derived from N-ferrocenylmethyl-N′-(pyridylmethyl)imidazolium iodides have been synthesized, which show remarkable structural diversity. Reaction of N-ferrocenylmethyl-N′-(2-pyridylmethyl)imidazolium iodide with Ag2O yielded stair-like NHC–Ag complexes, whose assembly modes significantly depended on experimental conditions. Reaction of N-ferrocenylmethyl-N′-(4-pyridylmethyl)imidazolium iodide with Ag2O yielded a chain NHC–Ag complex with a repeating 16-membered macrocyclic unit. However, similar reaction of N-ferrocenylmethyl-N′-(3-pyridylmethyl)imidazolium iodide with Ag2O only gave a dimeric NHC–Ag complex through bridging iodide. In these NHC–Ag complexes, 2- and 4-pyridyl-substituted NHCs act as bridging bidentate ligands through the pyridyl nitrogen and carbene carbon atoms, while 3-pyridyl-substituted NHC only acts as a monodentate ligand through the carbene carbon atom. An I/Cl exchange reaction was observed upon recrystallization of these NHC silver iodides in CH2Cl2. These NHC–Ag complexes exhibited highly effective catalytic activity in the three-component coupling reaction of alkyne, aldehyde and amine to generate propargylamines.