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

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 (PCO= 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.