Activation of enamido zirconium complexes for ethylene polymerization: Electrophilic addition versus electrophilic abstraction reaction

摘要

Deprotonation of the alpha-diimine compound {2,6-(CHMe2)(2)-C6H3}N=C(CH3)C(CH3)=N{2,6-(CHMe2)(2)-C6H3} with excess KH in THF gives dipotassium NN'-(1,2-dimethylene-1,2-ethanediyl)bis(2,6-diisopropylanilide) (1) in 75% yield. Reaction of 1 with bis(2-picolyl)-zirconium dichloride generated in situ by the reaction of ZrCl4 and 2 equiv of (2-picolyl)potassium affords [N,N'-(1,2-dimethylene-1,2-ethanediyl)bis(2,6-diisopropylanilido)-kappa(2) N,N']bis(2-picolyl)zirconium(IV) (2) in 35% yield. Similar reaction of 1 with LMCl3 (L = Cp, Cp*; M = Zr, Hf) gives the desired complexes {2,6-(CHMe2)(2)-C6H3}NC=(CH2)C=(CH2)N-{2,6-(CHMe2)(2)-C6H3}MLCl (L = Cp*, M = Zr, 4; L = Cp*, M = Hf, 5; L = Cp, M = Zr, 6) in 79%, 94% and 54% yields, respectively. Reaction of the chloride complexes with MeMgBr affords the corresponding methyl complexes {2,6-(CHMe2)(2)-C6H3}NC=(CH2)C=(CH2)N{2,6-(CHMe2)(2)-C6H3}MLMe (L = Cp*, M = Zr, 7; L = Cp*, M = Hf, 8; L = Cp, M = Zr, 9) in 63%, 69% and 90% yields, respectively. The solid-state structure of 7 was determined. When 2, 7, or 9 is treated with 1 equiv of B(C6F5)(3), one observes formation of the picolyl- or methylabstracted ion-paired complex in NMR spectra. When Al(C6F5)(3) is added to 2, 8, or 9, the aluminum atom is coordinated by the methylene functionality of the enamide ligand to form zwitterionic complexes. The solid structure of a zwitterionic complex generated by the addition of Al(C3F5)(3) to 8 was determined and confirms the molecular connectivity. The zwitterionic complexes are active to ethylene polymerization, while the ion-paired complexes are sluggish. Complexes 2 and 6 are highly active to the ethylene polymerization when they are activated with MAO. [References: 46]