Ligand and solvent effects in the alternating copolymerization of carbon monoxide and olefins by palladium-diphosphine catalysis

The substitution of two hydrogen atoms by methyl groups in the 1,2 positions of 1,2-bis(diphenylphosphino)ethane (dppe) gives meso- and rac-2,3-bis(diphenylphosphino)butane (meso-2,3-dppb and rac-2,3-dppb). The corresponding Pd(II) complexes Pd(OTs)(2)(meso-2,3-dppb) and Pd(OTs)2(rac-2,3-dppb) are effective catalyst precursors for the alternating copolymerization and terpolymerization of carbon monoxide with ethene and ethene/propene in MeOH with productivities that are higher than those of the unsubstituted dppe catalyst Pd(OTs)(2)(dppe) even by a factor of 10 (OTs = p-toluenesulfonate). It has been found that the low productivity of the dppe-based catalyst in MeOH is due to the autoionization of the precursor Pd(OAc)(2)(dppe) in MeOH to give the catalytically inactive bis-chelate species [Pd(dppe)(2)](OAc)(2) and palladium acetate. In an attempt to evaluate and rationalize the effective ligand control on the intrinsic catalytic activity, the methyl complexes [Pd(Me)(MeCN)(PP)]PF6 have been synthesized and employed in CH2Cl2 to catalyze the alternating carbon monoxide/ethene copolymerization. The intrinsic activity of the three precursors decreases in the order [Pd(Me)(MeCN)(meso-2,3-dppb)](+) > [Pd(Me)(MeCN)(rac-2,3-dppb)](+) > [Pd(Me)(MeCN)(dppe)](+). High-pressure NMR experiments and the determination of activation barriers of migratory insertions agree to indicate the relative stability of the beta-chelate ring in [Pd(CH2CH2C(O)Me)(P-P)](+) as the factor that controls the copolymerization rate in aprotic solvents. The impact of the different diphosphines on both productivity and intrinsic catalytic activity has been attributed to the different stereochemical rigidity of the Pd(P-P) five-membered metallarings. The beta-chelate complexes [Pd(CH2CH2C(O)Me)(P-P)]PF6 with diphosphine ligands containing two carbon atoms between the phosphorus donors have been isolated for the first time and employed to study the chain-transfer by protonolysis, which proceeds via the enolate mechanism. It has been shown that the chain-transfer products [Pd(OH)(P-P)](2)(2+) do not represent a dead end for the alternating CO/ethene copolymerization.