Copolymerization of carbon monoxide with ethene catalyzed by bis-chelated palladium(II) complexes containing diphosphine and dinitrogen ligands

Abstract

Several bis-chelated palladium(II) complexes, [Pd(P-P)(N-N)_x](PF₆)₂, containing binary combinations of diphosphine and dinitrogen ligands have been prepared and characterized. The diphosphine ligands comprise 1,3-bis(diphenylphosphino)propane (dppp), meso-2,4-bis(diphenylphosphino)pentane (meso-bdpp), rac-2,4-bis(diphenylphosphino)pentane (rac-bdpp) and 2,2′-bis(diphenylphosphinoethyl)pentane (Etdppp), while the dinitrogen ligands are either 2,2′-bipyridine (bipy; x=1) or 1,8-naphthyridine (napy; x=2). The structure of [Pd(meso-bdpp)(N,N′-bipy)](PF₆)₂·CH₂Cl₂ has been determined by an X-ray structural analysis. All the Pd(II) complexes have been tested as catalyst precursors for the copolymerization of carbon monoxide and ethene in methanol solution in either autoclaves or high-pressure sapphire NMR tubes. The combination of meso-bdpp and bipy at palladium, in conjuction with both 1,4-benzoquinone and p-toluenesulfonic acid, has shown the best catalytic performance. The different catalytic activities exhibited by the stereoisomers [Pd(meso-bdpp)(N,N′-bipy)]- (PF₆)₂ and [Pd(rac-bdpp)(N,N′-bipy)](PF₆)₂ has been interpreted in terms of the different spatial distribution of the phenyl rings around the metal center determined by the conformation of the six-membered metallaring.