Studies of ligand and solvent effects in the alternating copolymerization of carbon monoxide and ethene by palladium-diphosphine catalysis

The Pd(II) methyl complexes [Pd(Me)(MeCN)(P-P)]PF6 are effective catalyst precursors for the alternating copolymerization of carbon monoxide with ethene in CH2Cl2 (P-P = 1,3-bis(diphenylphosphino)propane (dppp), meso-2,4-bis(diphenylphosphino)pentane (meso-bdpp), rac-2,4-bis(diphenylphosphino)pentane (rac-bdpp)). The productivity in high molecular weight polyketones within 30 min follows the ligand order dppp > meso-bdpp > rac-bdpp. All the methyl precursors exhibit comparable values of both intrinsic activity and energy barriers to migratory insertions [Pd(Me)(CO), Pd(COMe)(C2H4)] as well as opening of beta-chelates [Pd-(CH2CH2C(O)Me)(P-P)](+) by CO. It is concluded that the presence and/or stereochemistry of methyl groups in the 1,3-positions of dppp do not exert a significant influence on the propagation rate of the copolymerization reaction in CH2Cl2. High-pressure NMR studies under catalytic conditions show the occurrence of chain transfer by protonolysis with adventitious water to give mu-hydroxo compounds [Pd(mu-OH)(P-P)](2)(2+). With time, the bis-chelates [Pd(P-P)(2)](2+) are the only species visible on the NMR time scale. Independent copolymerization reactions in CH2Cl2 with either mu-OH or bis-chelate precursors show that both resting states can reenter the catalysis cycle to give alternating polyketones with productivities increasing in the orders [Pd(mu-OH)(rac-bdpp)](2)(2+) < [Pd(mu-OH)(meso-bdpp)](2)(2+]) < [Pd(mu-OH)(dppp)](2)(2+), and [Pd(rac-bdpp)(2)](2+) < [Pd(dppp)(2)](2+) < [Pd(meso-bdpp)(2)](2+). On the basis of the batch reactions and in situ NMR experiments it is suggested that both mu-hydroxo and bis-chelate compounds contribute appreciably to determine the overall productivity of the methyl precursors [Pd(Me)(MeCN)(P-P)](+) in CH2Cl2. The bis-chelates are also active catalysts for the CO/ethene copolymerization in MeOH with productivities that increase in the ligand order dppp much less than rac-bdpp < meso-bdpp.