Palladium Complexes with Pyrimidine-Functionalized N-Heterocyclic Carbene Ligands: Synthesis, Structure and Catalytic Activity

A series of novel pyrimidine functionalized palladium(II)(NHC) complexes with aryl and alkyl substituents [1-(2-Pyrimidyl)-3-(aryl or alkyl)imidazoline-2-ylidene palladium(II) chlorides] was synthesized by transmetalation via the corresponding silver complexes. All compounds have been fully characterized by 1H and 13C NMR spectroscopy, elemental analysis, and in two cases by X-ray single crystal structures. Different solid state structures were observed for aryl and alkyl substituted ligands: for the sterically less demanding methyl substituent, a complex is formed, where two ligands are coordinated to one metal center [Pd(L)2Cl]+, while in the mesityl case a [Pd(L)Cl2] structure was observed. They show good catalytic activity in the CH activation of methane as well as in the Mizoroki-Heck reaction, where especially the methyl substituted complex shows a remarkably high activity combined with a very high selectivity.