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

Palladium(II)-bisphosphine catalyzed copolymerization of CO and ethylene wa s studied in detail. The results showed that DPPPr/Pd(II) mole ratio, CF3CO OH/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 H-1-NMR technique, the signals of coordinated ethyle ne at 4.4 ppm and methylene linked with Pd(II) (Pd-CH2-) at 0.6 ppm were ob served. With high-pressure in situ IR technique three palladium carbonyl ab sorptions 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 abov e results, the copolymerization mechanisms were discussed. High-pressure in situ P-31-NMR experimental result showed that only mono-chelate ring compl ex(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 pro duced when DPPPr/Pd(II)=1.5. Extended X-ray absorption fine structure (EXAF S) was used to calculate the coordination number (CN) and shell radius (R, in Angstrom) of complexes (DPPPr)Pd(OCOCF3)(2) (a), (DPPBu)Pd(OCOCF3)(2) (b ), (DPPEt)Pd(OCOCF3)(2) (c), (DPPPr)(2)Pd(OCOCF3)(2) (e), and two catalyst solutions or methanol freshly taken before and in the middle of copolymeriz ation (named S1 and S2, respectively). The Pd-P bond length of complexes (a ) (b) (c) are 2.25(3) Angstrom, 2.33(3) Angstrom, and 2.38(2) Angstrom, res pectively, Pd-O bond length of complexes (a) (b) (c) are 2.07(3) Angstrom, 2.06(2) Angstrom and 2.05(3) Angstrom, respectively. In the order of comple xes (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 sta ble, 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 sh ow that 18-electron and five-coordinated pyramidal intermediates were produ ced and existed in the system besides 16-electron and four-coordinated squa re planar intermediates. (C) 2000 Elsevier Science B.V. All rights reserved .