Bis-chelated palladium(II) complexes with nitrogen-donor chelating ligands are efficient catalyst precursors for the CO/styrene copolymerization reaction

A series of dicationic bis-chelated palladium(II) complexes [Pd(N-N)(2)][X](2) (N-N = 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), and their substituted derivatives; X = PF6-, BF4- OTf-, OTs-) has been synthesized and completely characterized both in the solid state and in solution. The synthetic procedure involves a simple one-pot reaction between Pd(MeCOO)(2) and [(N-N)H][X]. These compounds are very active precatalysts for the CO/styrene copolymerization yielding perfectly alternating polyketones. The crystal structures of some complexes of the series provide evidence that a distorsion from the ideal square planar geometry toward a twist conformation occurs. In DMSO solution, one of the two nitrogen-donor ligands is involved in a dissociative equilibrium yielding a monochelated complex with two cis coordination sites available for the copolymerization catalytic process. The catalytically active species is very stable in 2,2,2-trifluoroethanol, where its activity was found unaltered for at least 48 h of reaction without apparent decomposition to palladium metal. The addition of 1,4-benzoquinone (BQ) to the catalytic system has a strong influence on the yield and, above all, on the molecular weight of polyketones. The zerovalent palladium complexes [Pd(N-N)(BQ)], which might be formed during the copolymerization process, have been synthesized and characterized. The crystal structure of [Pd(bipy)(BQ)] shows that benzoquinone acts as a mono-olefinic ligand to Pd. In the presence of protons, the Pd(0) complexes are readily oxidized to Pd(II) with the reduction of benzoquinone to hydroquinone. When [(N-N)H][X] is used as the source of protons, the resulting Pd(II) species is the precatalyst and can immediately re-enter the catalytic cycle.