Living Polymerization of Ethylene Catalyzed by Diene Complexes of Niobium and Tantalum, M(η⁵-C₅Me₅)(η⁴-diene)X2 and M(η⁵-C₆Me₅)(η⁴-diene)₂ (M = Nb and Ta), in the Presence of Methylaluminoxane

Catalyst systems of MX₂(η⁵-C₅R₅)(η⁴-diene) (M = Nb and Ta; R = H and CH₃; X = Cl and CH₃) in a combination of methylaluminoxane (MAO) are the precursors of the living polymerization of ethylene. Mono-diene complexes MCl₂(η⁵-C₅R₅)(η⁴-diene) [M = Nb, R = CH₃ (1); M = Ta, R = CH₃ (2); M = Ta, R = H (3); M = Nb, R = H (4); diene = buta-1,3-diene (a), isoprene (b), 2,3-dimethylbuta-1,3-diene (c)] were prepared by the reaction of MCl₄-(η⁵-C₅R₅) with 2 equiv of the methylated allyl Grignard reagents in THF. Reaction of 2 and 3 with MeMgI afforded dimethyl complexes, Ta(η⁵-C₅R₅)(η⁴-1,3-butadiene)Me₂ [R = CH₃(5); R = H (6)], respectively. The polymerization of ethylene catalyzed by 1/MAO, 2/MAO, and 5/MAO at low temperature (-20 °C) gave polyethylene with very narrow polydispersities (M_wM_n as low as 1.05). The niobium complexes are superior to the tantalum complexes in terms of the catalyst activity and the polydispersity. When the ligand was Cp instead of Cp*, the catalyst activity of 3 for the polymerization of ethylene at -20 °C increased but the polydispersity of the obtained polyethylene broadened (M_w/M_n= 1.40). Protolytic reaction of 5b with 1 equiv of TfOH produces TaCp*(η⁴-isoprene)(CH₃)(OSO₂CF₃) (7), whose structure is determined by single-crystal X-ray diffractometry. Compound 7 crystallizes in the orthorhombic space group P2₁2₁2₁(No. 19) with a = 13.388(6) Å, b = 16.196(4) Å, c = 9.214(4) Å, Z = 4, V = 1998(1) ų, D_calcd = 1.823 g/mL, and R = 0.041 based on 2191 reflections. The molecular structure shows that 7 is not a cationic complex. The tantalum atom is in a chiral center surrounded by four different ligands: η¹-triflate, η⁵-Cp*, η⁴-isoprene, and methyl. A cationic species derived from 5b and B(C₆F₅)₃was detected by ¹H NMR spectroscopy and found to be active in ethylene polymerization. A system of one of the bis-diene complexes of the type M(η⁵-C₅R₅)(η⁴-2,3-dimethyl-1,3-butadiene)₂[M = Nb, R = H (8); M = Nb, R = CH₃ (9); M = Ta, R = CH₃(10)] in the presence of a large excess of MAO was also found to be an active catalyst for the polymerization of ethylene.