Studies on palladium-bisphosphine catalyzed alternating copolymerization of CO and ethylene

Abstract

Palladium(II)-bisphosphine catalyzed copolymerization of CO and ethylene was studied in detail. The results showed that DPPPr/Pd(II) mole ratio, CF 3COOH/Pd(II) mole ratio and solvent greatly influence the catalytic activity. Solvent effects were studied and in detail through end group analysis. With high pressure in situ 1 H -NMR technique, the signals of coordinated ethylene at 4.4 ppm and methylene linked with Pd(II) (Pd—CH 2-) at 0.6 ppm were observed. With high-pressure in situ IR technique three palladium carbonyl absorptions were observed generated at 1638 cm −1, 1616 cm −1 and 1970 cm −1, which may be assigned to three intermediates (3) (5) (6). With the above results, the copolymerization mechanisms were discussed. High-pressure in situ 31 P -NMR experimental result showed that only mono-chelate ring complex (1) was produced when DPPPr/Pd(II)=1; only bis-chelate ring complex (2) was produced when DPPPr/Pd(II)=2; equimolar complex (1) and (2) were produced when DPPPr/Pd(II)=1.5. Extended X-ray absorption fine structure (EXAFS) was used to calculate the coordination number (CN) and shell radius ( R, in Å) of complexes (DPPPr)Pd(OCOCF 3) 2 (a), (DPPBu)Pd(OCOCF 3) 2 (b), (DPPEt)Pd(OCOCF 3) 2 (c), (DPPPr) 2Pd(OCOCF 3) 2 (e), and two catalyst solutions of methanol freshly taken before and in the middle of copolymerization (named S1 and S2, respectively). The Pd—P bond length of complexes (a) (b) (c) are 2.25(3) Å, 2.33(3) Å, and 2.38(2) Å, respectively, Pd—O bond length of complexes (a) (b) (c) are 2.07(3) Å, 2.06(2) Å and 2.05(3) Å, respectively. In the order of complexes (a) (b) (c), the catalytic activity increases with Pd—P bond decreasing and Pd—O bond increasing, which may show that the chelate ring is more stable, the anions can go away more easily, and the corresponding catalyst is more efficient. The coordination number of two catalyst solutions S1 and S2 are increased by 1.9 and 0.9 compared with solid complex (a), which may show that 18-electron and five-coordinated pyramidal intermediates were produced and existed in the system besides 16-electron and four-coordinated square planar intermediates.