SCANNING AND ENERGY-DISPERSIVE EXAFS STUDIES OF NICKEL ALKENE OLIGOMERIZATION CATALYSTS

In situ EDE and scanning EXAFS studies of homogeneous alkene oligomeri sation catalysts are reported. EDE spectra were acquired either on Sta tion 9.3 at the SRS Daresbury Laboratory using a bent Si(220) crystal monochromator (Ni and Br K-edges) or on ID24 at the ESRF using a Si(ll l) monochromator in a Bragg geometry (Ni K-edge). The catalysts studie d were derived from mixtures of Ni(acac)(2) (acac = 2,4-pentanedionato ) and AlEt2(OEt) or NiX2(PEt3)(2) (X = Cl, Br) and AlEt3. For Ni(acac) (2)/AlEt2(OEt), Ni K-edge EDE is consistent with a mean structure simi lar in nature to trimeric Ni(acac)(2) but with partial substitution of acac ligands by alkene and alkyl groups. At 25 degrees C, further bre akdown of the trimer occurs to give alkylated monomeric nickel species . For NiCl2(PEt3)(2)/AlEt3, low temperature Ni K-edge EDE has been use d to monitor alkylation of the Ni centre, and for ambient temperature mixtures of NiBr2(PEt3)(2)/AlEt3, Br K-edge EDE data shows complete lo ss of Br from the metal centre occurring within minutes of adding AlEt 3. Instrumental difficulties (detector and beamline setup)at the curre nt time prevent acquisition of analysable EXAFS in a timescale faster than 0.1-10 s for these types of systems.