Palladium(II) coordination of N-heterocyclic carbene ligands with a triazole moiety in the nitrogen substituent

A series of N-heterocyclic carbene (NHC) palladium complexes derived from N-methyl or N-mesityl-imidazolium salts bearing a triazole ring as pendant group to the second nitrogen atom, have been synthesized and characterized by NMR spectroscopy, mass spectrometry and single-crystal X-ray diffraction. Deprotonation of the imidazolium salt with palladium(II) acetate in acetonitrile affords square-planar Pd(II) complexes of general formula [PdI2(NHC)2] with trans geometry. The steric hindrance of the NHC ligand substituents allows to characterize in solution both rotation syn and anti isomers. In contrast, in the crystallization process, the cis[PdI2(NHC)2] form was isolated, and its structure was determined by XRD. The stability of the possible geometrical and rotational isomers has been assessed also via DFT calculations (level of theory: COSMO-ZORABLYP(D3(BJ))/TZP). A second product forms along with the mononuclear complex and was identified as a dinuclear complex with the NHC and the nitrogen atom of the triazole ring bridging two trans-PdI2 units. Finally, the nitrogen atom on the triazole ring was methylated affording a pendant triazolium salt, which can also give a carbene ligand upon deprotonation.