Bis(N‐heterocyclic carbene) Dipalladium Complexes: Synthesis, Solid‐State Conformational Studies and Solution Behaviour

New palladium(II) bis(N‐heterocyclic carbene) (NHC) complexes have been prepared via metallomacrocyclic Ag I ligand transfer agents. Reaction of 1,4‐bis(3‐isopropylimidazolium)xylene ( 1a ) hexafluorophosphate with Ag 2 O gives a Ag I 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 [{PdCl 2 (MeCN)} 2 (bis‐NHC)] ( 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 3 CN solution the cisoid and transoid conformers interconvert and exhibit resonances for a single averaged form.DFT calculations indicate an interconversion barrier of 56 kJ mol –1 . In CDCl 3 solution two different isomers are observed which have been assigned to isomers of a chloride‐bridged dimer ( 4a ) following dissociation of the acetonitrile ligands. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)