Palladium (II) complexes chelated by 1-substituted-4-pyridyl-1H-1,2,3-triazole ligands as catalyst precursors for selective ethylene dimerization

A series of neutral as well as cationic palladium methyl complexes bearing 1-substituted-4-pyridyl-1H-1,2,3-triazole ligands were prepared and fully characterized by a range of analytical techniques. Conventional and 2D NMR spectroscopy as well as single-crystal X-ray diffraction analysis unambiguously determined the molecular structure of the complexes. The neutral complexes activated by methylaluminoxane were found to be effective catalysts in the ethylene dimerization reaction. The catalyst performance of the in-situ-generated active species was compared with the discrete cationic complexes of the same ligand scaffold. Activities and selectivities for the two systems were remarkably similar, pointing to similarities in the nature of the active species. Both catalytic systems showed a strong correlation of activity and selectivity with the nature of the ligand scaffold. Highest activities were attained when electron-withdrawing groups were incorporated into the triazole ring, while increasing steric bulk in the ortho-position on the pyridyl ring of the ligand led to the almost exclusive dimerization of ethylene with selectivities up to 94% observed toward 1-butene.