HIGHLY-ACTIVE HOMOGENEOUS NICKEL-CATALYSTS FOR ALKENE DIMERIZATION - CRYSTAL-STRUCTURE OF [NI(ETA(3)-C3H5)(PPH(3))BR] AND IN-SITU CHARACTERIZATION OF ALET(3)-ACTIVATED [NI(ETA(3)-C3H5)(PPH(3))BR] BY NUCLEAR-MAGNETIC-RESONANCE AND EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY

In situ NMR and extended X-ray absorption fine structure (EXAFS) spectroscopy have been used to characterise a highly active propene dimerisation catalyst. prepared by the low-temperature addition of AlEt3 to [Ni(eta3-C3H5)(PPh3)Br]. The molecular structure of [Ni(eta3-C3H5)(PPh3)Br] has been determined by X-ray crystallography and variable-temperature NMR spectroscopy. Crystal data: monoclinic, space group P2(1)/n (no. 14), a = 7.951 (1), b = 14.954(1), c = 16.096(3) angstrom, beta = 96.05(1)-degrees, Z = 4, R = 0.045. It has previously been assumed that aluminium interacts with nickel in such catalyst systems via halide-bridging ligands in order to decrease the charge on the nickel, thereby facilitating the co-ordination of electron donors to the nickel centre. EXAFS analysis provided direct structural evidence for a Ni...Al interaction in solution, but questioned the nature of the bridging ligand: the nickel K-edge EXAFS data of a solution of [Ni(eta3-C3H5)(PPh3)Br], AlEt3 (Ni:Al = 1:5) and propene in toluene at -60-degrees-C, showed that the first co-ordination sphere around nickel comprised 3.9 carbons at 1.93 angstrom and 1.0 phosphorus at 2.19 angstrom, with a more distant aluminium being present at 3.21 angstrom. Halide loss from the nickel centre is further confirmed by the bromine K-edge EXAFS data. These results implied Ni...Al interactions via alkyl bridges in the predominant solution species during catalysis. Phosphorus-31 NMR spectroscopy at -60-degrees-C of the activated catalyst however showed at least four different species present. Halide loss from the nickel co-ordination sphere was also observed for the reaction of AlBr3 with [{Ni(eta3-C3H5)Br)2}] in toluene. The nickel K-edge EXAFS was best fitted by 9.0 carbons at 2.01 angstrom, providing strong evidence for the existence of [Ni(eta3-C3H5)(eta6-C6H5Me)]+-AlBr4-.