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

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/CI 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. (C) 2013 Elsevier Ltd. All rights reserved.