New atropisomeric bidentate nitrogen-donor compounds as potential stereocontrollers in mild CO–styrene copolymerisation catalysed by palladium(II) salts

Abstract

Two new atropisomeric bidentate nitrogen-donor chelating ligands, namely (–)-(S,S)-3,3′-(1,2-dimethylethylenedioxy)-2,2′-bipyridine L¹ and (+)-(R)-3,3′-(1-methylethylenedioxy)-2,2′-bipyridine L², have been synthesised and characterised. The crystal structure of L² confirmed its atropisomeric nature, the dihedral angle between the planes containing the pyridine rings being 50.1(1)°. The interaction of the new compounds, with palladium(II) salts led to the corresponding monochelated palladium complexes, [PdL¹(O₂CCF₃)₂]1 and [PdL²(O₂CCF₃)₂]2, the crystal structures of which have been determined. Both complexes crystallised with two independent molecules in the unit cell. In the two molecules of 1 the ligand L¹ is in the same conformation; while for 2 the two independent molecules correspond to two different atropisomers. The co-ordination geometry around palladium is square planar in all the molecules. In the two independent molecules of 1 and 2 the dihedral angle between the pyridine rings is considerably smaller than that observed in free L². The complexes are very active catalyst precursors in CO–styrene copolymerisation under mild reaction conditions (P_CO= 1 atm, 30 °C). The inhibiting role of carbon monoxide is evidenced. A low asymmetric induction was observed together with short isotactic sequences in the copolymer chain.