Nickel and palladium complexes of pyridine−phosphine ligands and their use in ethene oligomerization

Abstract

New pyridine-phosphine ligands of general structure 2-[2-(diarylphosphino)ethyl]pyridine were developed. The phosphorus substituents in these bidentates are 2-tolyl, 2-anisyl, and mesityl. The ligands could be conveniently synthesized in good yields. The nickel dichloride complexes of the ligands are paramagnetic. The metal centers have a distorted tetrahedral geometry, as was evident from the crystal structures and the magnetic moments in solution. The neutral methylpalladium chloride and the cationic methylpalladium complexes have a distorted square-planar geometry around the metal center. For the complexes of two of the ligands, an anagostic C−H···Pd interaction of a ligand-proton with the palladium atom was observed in the crystal structures and in solution. These interactions probably were related to hindered inversion of the six-membered metallocycle, which was observed in VT-NMR measurements. The complexes of the mesityl-substituted ligand show neither hindered inversion of the metal chelate ring nor a sign of Pd···H interactions. The nickel complexes form active catalysts for the oligomerization of ethene after MAO activation. The bulky 2-tolyl and mesityl groups suppress isomerization of the growing chain, reflected in a high 1-butene selectivity. For the complex made from the ligand with the most bulky (mesityl) substituents, this selectivity was 90%. The anisyl substituents induced a different catalytic behavior of the corresponding nickel complex. Selectivity for 1-butene was lower, but the productivity was higher, with a turnover frequency of 65 × 103 (mol C2H4)·(mol Ni·h)−1. The cationic palladium complexes showed a very low activity in ethene oligomerization. Butenes were the major product, but significant amounts of higher olefins were formed as well.