Copolymerization of carbon monoxide with ethene catalyzed by palladium(II)complexes of 1,3-bis(diphenylphosphino)propane ligands bearing different substituents on the carbon backbone

In this work are described the syntheses of several new dppp-like ligands ( dppp = 1,3-bis(diphenylphosphino)propane) bearing different substituents on the carbon backbone and of their palladium (II) complexes with acetate or trifluoroacetate coligands (L). The complexes exhibit the general formula P d(P-P)(L)(2) and have been employed as catalyst precursors for the copolyme rization of ethene and carbon monoxide in MeOH under experimental condition s that are comparable to those reported in the relevant literature and pate nts for dppp-based Pd(II) copolymerization catalysts. It has been found tha t the introduction of alkyl substituents in the 2-position of the carbon ba ckbone of dppp does not significantly improve the performance of the corres ponding catalyst precursors (highest productivity value 6.2 kg of copolymer (g of Pd h)(-1) vs 5.4 kg of copolymer (g of Pd h)(-1) for Pd(dppp)(L)(2)) . In contrast, the productivity increases remarkably when methyl groups are introduced in both 1-positions of the diphosphine ligand, particularly wit h R,S (S,R) stereochemistry as in meso-CH2(CH3CHPPh2)(2) (productivity of 8 .0 kg of copolymer (g of Pd h)(-1)). On the basis of NMR and cyclic voltamm etric studies of the catalyst precursors, it is suggested that the increase d productivity provided by the C-1-substituted ligands is both electronic a nd steric in nature. In situ high-pressure NMR experiments in sapphire tube s equipped with Ti alloy valves showed that the only phosphorus-containing species visible on the NMR time scale in effective copolymerization conditi ons are Pd(II) complexes with the formula Pd(diphosphine)X-2 (X = p-toluene -sulfonate, trifluoroacetate, or MeOH). It has been proposed that these Pd( II) complexes act as a reservoir of [Pd(diphosphine)](2+) moieties which ma y either be delivered into the catalysis cycle by action of various reagent s (MeOH, H+, H2O, H-2) or be withdrawn after the termination step and watch ed over deactivation paths.