Preparations of a series of coinage metal complexes with pyridine-based bis(N-heterocyclic carbene) ligands including transmetalation to palladium complexes

A silver complex bearing 2,6-bis(3-tert-butylimidazol-2-ylidene)pyridine (CNC) ligands was easily prepared from the reaction of the CNC ligand precursor with Ag2O. The CNC ligand contains relatively bulky substituents, tert-butyl, on nitrogen atoms. Use of the silver complex as a carbene transfer reagent gave rise to a copper and a gold complex, completing a series of coinage metal complexes [M2(CNC)2](BF4)2 (M = Cu, Ag, Au). In all the coinage metal complexes, the X-ray crystallographic analyses showed that the two CNC ligand strands are intertwined each other and surround the metal ions in a double helical fashion. Moreover, the carbene transfer reactions afforded a monomeric palladium complex [Pd(CNC)Cl]BF4 and a CNC-bridged dipalladium complex [{Pd(η3-C3H5)Cl}2(μ-CNC)]. Facile chloride ligand abstraction from the dipalladium complex yielded a chlorido-bridged dipalladium complex [{Pd(η3-C3H5)}2(μ-CNC)(μ-Cl)]BF4