Synthesis, structure and catalytic activity of xylylene-bridged dipalladium complexes with triazolylidene ligands

A series of di-N-heterocyclic carbene (NHC) dipalladium complexes, [PdPyBr2]2(di-NHC) (2a–h), in which di-NHC represents a di-triazolylidene, featuring a 1,4-xylylene spacer between the carbene units, have been prepared from the reactions of the corresponding ditriazolium bromides with PdCl2 and excess NaBr as an additive in the presence of K2CO3 in pyridine. All of the complexes were fully characterized by NMR spectra, IR and elemental analyses. The molecular structure of 2b was determined by X-ray diffraction study, showing the intermolecular hydrogen bonding of Br···H–C. The influences of size of ligands on the catalytic activity have been investigated in the Heck reaction of styrene with bromobenzene. The complex 2g showed the best catalytic activity, and it is active for various aryl bromides with different electronic and steric properties and styrenes with both electron-donating and electron-withdrawing groups.