New 1,2,3-triazole ligands through click reactions and their palladium and platinum complexes

The new ligands, 1-(4-isopropyl phenyl)-4-(2-pyridyl)-1,2,3-triazole, 1 and 1-(mesityl)-4-(2-pyridyl)-1,2,3-triazole, 2 were prepared by the reactions of the respective azides with 2-ethynylpyridine following the "click method". These ligands together with the reported ligands 1-(phenyl)-4-(2-pyridyl)-1,2,3-triazole, 3 and 1-(benzyl)-4-(2-pyridyl)-1,2,3-triazole, 4 were reacted with palladium and platinum precursors to give mononuclear cis-dichloropalladium and platinum complexes containing the triazole ligands. Structural characterisation of the free ligand 3 shows that the central N-N bond in the triazole ring has double bond character and hence is best described as an "azo-like" N-N double bond. The pyridine ring in 3 has an almost "anti" conformation with respect to the central triazole ring. The metal centers bind to the ligands through the pyridine N and a triazole N atom. The metal-N(triazole) distances are shorter than the metal-N(pyridine) distances. Cyclic voltammograms of the ligands show reduction processes that appear at extreme negative potentials. Coordination of metal centers induces huge anodic shifts of the reduction potentials due to sigma-polarisation by the metal centers. UV/Vis spectra of the ligands and complexes are also discussed. The properties of such chelating triazole ligands towards palladium and platinum centers is being compared and contrasted to the widely used 2,2'-bipyridine ligand.