Highly 3,4-Selective Polymerization of Isoprene with NPN Ligand Stabilized Rare-Earth Metal Bis(alkyl)s. Structures and Performances

Deprotonation of Ar1NHPPh2NAr2 (H[NPN]n, n = 1−10) by Ln(CH2SiMe3)3(THF)2 (Ln = Lu, Y, Sc, Er) generated a series of rare-earth metal bis(alkyl) complexes [NPN]nLn(CH2SiMe3)2(THF)2 (1−10), which under activation with [Ph3C][B(C6F5)4] and AliBu3 were tested for isoprene polymerization. The correlation between catalytic performances and molecular structures of the complexes has been investigated. Complexes 1−5 and 8, where Ar1 is nonsubstituted or ortho-alkyl-substituted phenyl, adopt trigonal-bipyramidal geometry. The Ar1 and Ar2 rings are perpendicular in 1−4 and 8 but parallel in 5. When Ar1 is pyridyl, the resultant lutetium and yttrium complexes 9a and 9b adopt tetragonal geometry with the ligand coordinating to the metal ions in a N,N,N-tridentate mode, whereas in the scandium analogue 9c, the ligand coordinates to the Sc3+ ion in a N,N-bidentate mode. These structural characteristics endow the complexes with versatile catalytic performances. With increase of the steric bulkiness of the ortho-substituents Ar1 and Ar2, the 3,4-selectivity increased stepwise from 81.6% for lutetium complex 1 to 96.8% for lutetium complex 6 and to 97.8% for lutetium complex 7a. However, further increase of the steric bulk of the ligand led to a slight drop of 3,4-selectivity for the attached complex 5 (95.1%). When the smaller scandium ion was employed, the corresponding complex 7c provided 98.1% 3,4-selectivity, which reached 99.4% when the polymerization was performed at −20 °C, and the polymerization had quasi-living characteristics. Complexes 9a and 9b, containing an electron-donating ligand, gave higher 3,4-selectivities (85.0% vs 85.5%) than those attached to electron-withdrawing ligands 9c (33%) and 10 (77%).