New Rare Earth Metal Bis(alkyl)s Bearing an Iminophosphonamido Ligand. Synthesis and Catalysis toward Highly 3,4-Selective Polymerization of Isoprene

New rare earth metal bis(alkyl) complexes [(NPNPh)Ln(CH2SiMe3)2(THF)] (NPNPh: N(Ph)PPh2NC6H2Me3-2,4,6; Ln = Sc (3a), Ln = Y (3b), Ln = Lu (3c)) and [(NPNPy)Sc(CH2SiMe3)2(THF)] (NPNPy = N(Py)PPh2NC6H2Me3-2,4,6) (3d)) have been prepared via protonolysis reaction between rare earth metal tris(alkyl)s and the corresponding iminophosphonamines. Complexes 3a−d are analogous monomers of THF solvate. Each metal ion coordinates to a η2-chelated NPN ligand and two cis-located alkyl groups, adopting tetrahedron geometry. The scandium complex [(NPNPh)Sc(CH2SiMe3)2(THF)] in combination with [PhNHMe2][B(C6F5)4] and AliBu3 forms the first non-Cp-ligated rare earth metal based catalyst system to provide extremely high activity and 3,4-selectivity for the polymerization of isoprene. The resultant high molecular weight polymer (Mn = 99 × 104) with narrow molecular weight distribution (PDI = 1.55) has a 3,4-regularity up to 94.7% (Tp = –40 °C, toluene, 2 h, 100% yield). The complexes 3b and 3c, with a larger ionic radius, are less regioselective, and 3d, with an electron-donating pyridyl moiety, is inert. The probable initiation species and mechanistic scenario are also presented.