Substituted cyclopentadienylchromium(III) complexes containing neutral donor ligands. Synthesis, crystal structures and reactivity in ethylene polymerization

Abstract

Lithium derivatives of substituted cyclopentadiene ligands reacted with CrCl 3(THF) 3 in THF solution to afford homodinuclear complexes of the type [{(η 5-RCp)CrCl(μ-Cl) } 2] [R=SiMe 3 ( 1), CH 2C(Me)CH 2 ( 2)]. Complex 1 reacts with pyrazole (C 3H 4N 2) to yield the mononuclear half-sandwich complex [(η 5-Me 3SiCp)CrCl 2(pyrazole)] ( 3). The similar complex [Cp*CrCl 2(pyrazole)] ( 4) was synthesised by reaction of [{Cp*CrCl(μ-Cl)} 2] with pyrazole. Complex 2 reacts with bidentate ligands to give binuclear complexes of the type [{(η 5-CH 2C(Me)CH 2Cp)CrCl 2 } 2(μ-L-L)] [L-L=Ph 2PCH 2CH 2PPh 2 ( 5), trans-Ph 2P(O)CHCHP(O)Ph 2 ( 6)]. All complexes were structurally characterised by X-ray diffraction. After reaction with methylaluminoxane these complexes are active in the polymerization of ethylene. At 25 °C and 4 bar of ethylene, complex 3 yields polyethylene with a bimodal molecular weight distribution centred at 155,000 and 2000 g/mol. Complex 4 shows similar activity, yielding only the low molecular weight fraction. On the other hand, the binuclear complexes 5 and 6 under the same conditions were three times more active than mononuclear complexes. The melting point of the polymers indicates the formation of linear polyethylene.