β-Diketiminato Rare-Earth Metal Complexes. Structures, Catalysis, and Active Species for Highly cis-1,4-Selective Polymerization of Isoprene

Lithiation of the β-diketimines (2,6-C6H3R2)NH═C(Me)CH═C(Me)N(2,6-C6H3R2) (R = Me (HL1), Et (HL2)) by nBuLi was followed by metathesis reaction with LnCl3(THF)x and Y(BH4)3(THF)2 to afford the corresponding complexes L1LnCl2(THF)2 (Ln = Gd (1), Nd (3), Dy (4), Er (5), Y (6)), L2GdCl2(THF)2 (2), and L1Y(BH4)2(THF) (8), respectively. Treatment of neutral HL1 with Y(CH2SiMe3)3(THF)2 generated the bis(alkyl) complex 7, L1Y(CH2SiMe3)2(THF). Upon activation with [PhNHMe2][B(C6F5)4] and AliBu3, complex 6 showed the highest cis-1,4 selectivity (99.3%, Tp = 0 °C) toward the polymerization of isoprene, while complex 7 had a comparatively low cis-1,4 selectivity, and in contrast, complex 8 was completely inert. The influences of the ortho substituents of the N-aryl rings of the ligands, the types of central metals and cocatalysts, and addition sequence of the catalyst components had been thoroughly investigated. By means of X-ray diffraction and 1H NMR spectroscopy analyses, the intermediates arising from the stoich...