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 Pd(P-P)(L)(2) and have been employed as catalyst precursors for the copolymerization of ethene and carbon monoxide in MeOH under experimental conditions that are comparable to those reported in the relevant literature and patents for dppp-based Pd(II) copolymerization catalysts. It has been found that the introduction of alkyl substituents in the 2-position of the carbon backbone of dppp does not significantly improve the performance of the corresponding 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 with 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 voltammetric studies of the catalyst precursors, it is suggested that the increased productivity provided by the C-1-substituted ligands is both electronic and steric in nature. In situ high-pressure NMR experiments in sapphire tubes equipped with Ti alloy valves showed that the only phosphorus-containing species visible on the NMR time scale in effective copolymerization conditions 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which may either be delivered into the catalysis cycle by action of various reagents (MeOH, H+, H2O, H-2) or be withdrawn after the termination step and watched over deactivation paths.