Bis(n-heterocyclic carbene) dipalladium complexes: synthesis, solid-state conformational studies and solution behaviour

Abstract

New palladium(ïï) bis(N-heterocyclic carbene) (NHC) complexes have been prepared via metallomacrocyclic AgI Iigand transfer agents. Reaction, of 1,4-bis(3-isopropylimidazolium)xy1ene (la) hexafluorophosphate with Ag2O gives a AgI macrocyclic NHC complex in which two silver ions are sandwiched between two dipodal ligands (2a). The NHC moieties adopt a cisoid conformation relative to one another. Upon transfer to palladium to give [(PdCl2(MeCN)]2(DiSNHC)] (3a) the NHC ligands in the complexes adopt a mixture of cisoid and transoid conformers in the solid state, which have been characterized by X-ray crystallography. In CD 3CN solution the cisoid and transoid conformera interconvert and exhibit resonances for a single averaged form,. DFT calculations indicate an interconversion, barrier of 56kJmOl-1. In CDCl3 solution two different isomers are observed which have been assigned to isomers of a chloridebridged dimer (4a) following dissociation of the acetonitrile ligands. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.