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 Ag2O 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 [{PdCl2.(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 CD3CN 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 CDCl3 solution two different isomers are observed which have been assigned to isomers of a chloride-bridged dimer (4a) following dissociation of the acetonitrile ligands. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)