Synthesis and Structure of Zirconium and Hafnium Polymerisation Catalysts Stabilised by Very Bulky Aminopyridinato Ligands

Mono(aminopyridinato) complexes of the type [ApM(CH2C6H5)(3)] [M = Zr, Hf and Ap = aminopyridinate] were prepared by treating the three different sterically demanding aminopyridines with one equiv. of tetrabenzylzirconium or -hafnium. One of the three benzyl groups is eta(2)-coordinated in the solid state. However all of the three benzyl substituents are equivalent in solution as evidenced by the H-1 NMR spectrum. Treatment of these neutral complexes with B(C6F5)(3) afforded the corresponding zwitterionic dibenzyl complexes. The eta(6)-coordination of the phenyl ring of the B-bound benzyl group to the metal centre was supported by H-1 NMR spectroscopy and confirmed by single-crystal X-ray diffraction analysis. These zwitterionic complexes show very low activity for ethylene polymerisation at low temperature since the coordination site is blocked by the eta(6)-coordinated phenyl ring. At elevated temperature, moderate activity with the formation of high molecular weight polyethylene (PE) was observed. An attempted abstraction of the second benzyl group failed when the zwitterionic complexes were treated with an additional equivalent of B(C6F5)(3). Using one equiv. of [R-2(Me)NH][B(C6F5)(4)] (R = C16H33-C18H37) instead of B(C6F5)(3), moderate activities of ethylene polymerisation were observed. Treatment of the aminopyridinato metal tribenzyls with [R-2(Me)NH][B(C6F5)(4)] (R = C16H33-C18H37) gave active ethylene polymerisation catalysts which produced low molecular weight PE in the case of the zirconium analogues and higher molecular weight PE in the case of the hafnium example. Propylene homopolymerisation under the same conditions failed. Ethylene-propylene copolymers with separated propene units and alternating sequences were observed in the presence of both monomers. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)