An Efficient Keim-Type Catalyst Based on an Electron-Poor P,O-Chelate; Tuning the Selectivity of Ethylene Oligomerisation towards Short α-Olefins



The tertiary phosphane Ph2P-pzONa, in which the phosphorus atom is substituted by an electron-withdrawing pyrazol-onato unit, reacts with trans-[NiPhCl(PPh3)2] to afford quan-titatively trans-P,P'-[NiPh(Ph2P-pzO)(PPh3)] (4), the first Keim-type catalyst derived from an amide. In the absence of any cocatalyst, complex 4 converts ethylene in high selectivity and activity into α-olefins. The product distribution is drastically shifted towards lower oligomers when compared to the outcome of the reaction carried out with the conventional SHOP catalyst. Thus, when operating at 1 bar, over 99 wt.-% of the oligomers formed are C4–C16 oligomers. In ethylene-free solvents, 4 was shown to slowly undergo reductive elimination to produce the stabilised phosphorus ylide Ph3P-pzO (5). Both compounds (4 and 5) were characterised by single-crystal X-ray diffraction analyses. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)