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 (MwMn 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 (Mw/Mn= 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) Å3, Dcalcd = 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry