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

Lithium derivatives of substituted cyclopentadiene ligands reacted with CrCl3(THF)3 in THF solution to afford homodinuclear complexes of the type [{(η5-RCp)CrCl(μ-Cl) }2] [R=SiMe3 (1), CH2C(Me)CH2 (2)]. Complex 1 reacts with pyrazole (C3H4N2) to yield the mononuclear half-sandwich complex [(η5-Me3SiCp)CrCl2(pyrazole)] (3). The similar complex [Cp*CrCl2(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 [{(η5-CH2C(Me)CH2Cp)CrCl2 }2(μ-L-L)] [L-L=Ph2PCH2CH2PPh2 (5), trans-Ph2P(O)CHCHP(O)Ph2 (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.