Bis(functionalized NHC) Palladium(II) Complexes via a Postmodification Approach
journal contributionposted on 26.01.2016, 12:09 by Qiaoqiao Teng, Daniel Upmann, Sheena Ai Zi Ng Wijaya, Han Vinh Huynh
A series of 20 bis(functionalized NHC) Pd(II) complexes have been conveniently synthesized through postmodification reactions of the common parent NHC complexes trans-[PdBr2(C3Br-bimy)2] (1a) and trans-[PdBr2(C3Br-imy)2] (1b) (C3Br-bimy = 1-benzyl-3-(3-bromopropyl)benzimidazolin-2-ylidene and C3Br-imy = 1-benzyl-3-(3-bromopropyl)imidazolin-2-ylidene) with an N-C3Br tether. Depending on the nature of the nucleophiles added, competing bromido ligand displacements also occurred. In comparison to the conventional access to functionalized NHC complexes, which involves metalation of individually prefunctionalized azolium salts, this highly modular method proves more effective in cost and time savings. The tetraalkylammonium-functionalized complex trans-[PdBr2(C3NEt3-bimy)2]Br2 (14) as a product of the second-generation postmodification of complex 1a was also tested for its catalytic activity in Mizoroki–Heck coupling reactions to study and model the effect of a catalytic amount of ammonium salt additive in Pd-catalyzed C–C coupling reactions. The ammonium functionalities in this complex exhibit a positive effect on the catalysis in comparison to its parent complexes.