Highly 3,4-Selective Polymerization of Isoprene with NPN Ligand Stabilized Rare-Earth Metal Bis(alkyl)s. Structures and Performances
作者：Li, SH，Cui, DM，Li, DF，Hou, ZM
關鍵字：n-heterocyclic carbene; stereospecific polymerization; iminophosphonamide complexes; alkyl; styrene; bearing
論文來源：期刊
具體來源：http://pubs.acs.org/doi/abs/10.1021/om900261n
發表時間：2009年
Deprotonation of Ar(1)NHPPh(2)NAr(2) (H[NPN](n), n = 1 - 10) by Ln(CH(2)SiMe(3))(3)(THF)(2) (Ln = Lu, Y, Sc, Er) generated a series of rare-earth metal bis(alkyl) complexes [NPN](n)Ln(CH(2)SiMe(3))(2)(THF)(2) (1-10), which under activation with [Ph(3)C][B(C(6)F(5))(4)] and AliBu(3) 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 Ar(1) is nonsubstituted or ortho-alkyl-substituted phenyl, adopt trigonal-bipyramidal geometry. The Ar(1) and Ar(2) rings are perpendicular in 1-4 and 8 but parallel in 5. When Ar(1) 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 Sc(3+) 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 Ar(1) and Ar(2), 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 degrees 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%).
