Activation of Enamido Zirconium Complexes for Ethylene Polymerization: Electrophilic Addition versus Electrophilic Abstraction Reaction

Deprotonation of the α-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 N,N‘-(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)-κ2N,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 methyl-abstracted 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.