The synthesis of ferrocenyl imidazolium salts and their metal complexes

N-heterocyclic carbenes (NHCs) and their transition metal complexes have been the subject of extensive investigations owing to their successful applications in coordination chemistry and homogeneous catalysis. In the family of NHCs, ferrocenyl-substituted NHCs have received considerable attention because the ferrocenyl groups show unique steric requirement, reversible redox activity, and additional electron donation to adjacent electron deficient centers. A number of NHC transition metal complexes bearing ferrocenyl substituents have been synthesized and used in homogeneous catalysis and electrochemical investigation in recent years. In this study, the ferrocenyl imidazolium chlorides have been synthesized starting from ferrocene according to the literature method. The reduction of acetylferrocene (2) prepared by Friedel-Crafts acylation reaction of ferrocene gave 1-ferrocenylethanol (3). Treatment of 1-ferrocenylethanol with 1-substituted imidazole in acetic acid gave an imidazolium salt with the hydroxide as a counter anion. Subsequently, addition of excess LiCl to the reaction mixture resulted in anion exchange to give ferrocenly imidazolium chlorides (5a-e). In addition, N-butyl substituted imidazolium salts were also obtained by alkylation of N-butyl imidazole (6a-c). The novel PEPPSI-type NHC palladium complexes (8-13) have been prepared by heating their imidazolium salts with PdCl2, K2CO3 in neat pyridine. In addition, deprotonation of 5a and then its reaction with S8 gave imidazol-2-thion (7). The structures of the complexes synthesized were characterized by NMR spectroscopy and besides the structure of 7 and 8 were determined by X-ray analysis.