Copper and palladium complexes with N-heterocyclic carbene ligands functionalised with carboxylate groups

The new imidazolium salts functionalised with the trimethylsilyl ester group 1a-c, were easily obtained by quaternisation of alkyl- or aryl-imidazoles with trimethylsilyl bromoacetate. Salt 1a was isolated and fully characterised. It reacted with mesityl copper (Cu(5)Mes(5)) under trimethylsilyl abstraction to form the complex 2. Methanolysis of 1a-c gave good yields of the carboxylic acid functionalised imidazolium salts 3a-c. Deprotonation of the latter in liquid ammonia led to the zwitterionic imidazolium carboxylates 4a-c. Reaction of 4a with (Cu(5)Mes(5)) gave solutions from which the insoluble polymeric 5a crystallised slowly. Generation of the carboxylate-functionalised NHC in situ followed by reaction with Pd(OOCCH3)(2) gave the new complex 6a in which the NHC-carboxylate ligand is chelate bidentate. (C) 2008 Elsevier B.V. All rights reserved.