Cationic palladium complexes with mono- and bidentate nitrogen-donor ligands: synthesis, characterization and reactivity in CO/styrene copolymerization reaction

Two series of cationic palladium(II) complexes, [Pd(phen)(L)2][PF6]2 (L=pyridine (1a), 2-picoline (2-pic) (1b), 3-picoline (3-pic) (1c), 4-picoline (4-pic) (1d)) and [Pd(CH3)(L)(phen)][OTf] (L=2-pic (2b), 3-pic (2c) and 4-pic (2d)), with mono and bidentate nitrogen-donor ligands bound to the same metal center have been synthesized and characterized. For the dicationic derivatives the study of the chemical behavior in solution evidences the presence of restricted rotations around the PdN bond of the monodentate ligand, generating syn and anti isomers. Depending on the nature of L the rate of this dynamic process is different on the NMR time scale. On the other hand, only the syn isomer was found by the X-ray analysis in solid state of one of these complexes. The dicationic complexes have been tested as precatalysts in the CO/styrene copolymerization, in comparison with [Pd(phen)2][PF6]2. The main difference between the two kinds of precatalysts is related to the stability of the corresponding active species, being lower for the complexes [Pd(phen)(L)2][PF6]2 than for the bischelated derivative. The insertion reaction of carbon monoxide in the Pdmethyl bond was studied on the monocationic complexes, [Pd(CH3)(L)(phen)][OTf]. In all cases the corresponding Pdacyl species [Pd(COCH3)(L)(phen)][OTf] was formed with no dissociation of the monodentate L ligand. No effect of the nature of L on the rate of the insertion reaction was evidenced.