NNN-Tridentate Pyrrolyl Rare-Earth Metal Complexes: Structure and Catalysis on Specific Selective Living Polymerization of Isoprene

The acid–base reactions of NNN-tridentate pyrrolyl
ligands (HL1: 2,5-bis­((pyrrolidin-1-yl)­methylene)-1H-pyrrole; HL2: 2,5-bis­((piperidino)­methylene)-1H-pyrrole) with rare-earth metal tris­(alkyl)­s, Ln­(CH2SiMe3)3(THF)2, afforded the
corresponding bis­(alkyl) complexes L1Ln­(CH2SiMe3)2(THF)x (Ln = Sc, x = 0 (1a); Ln = Y, x = 1
(1b); Ln = Lu, x = 1 (1c)), L2Sc­(CH2SiMe3)2 (2a), and L22Ln2(CH2SiMe3)4 (Ln = Y (2b); Lu (2c)) in moderate to high yields. X-ray diffraction analysis
revealed that the scandium complexes 1a and 2a are THF solvent-free monomers where the ligands coordinate to the
Sc3+ ion in a κ1:κ2 mode,
while the yttrium and lutetium complexes 1b and 1c have the same ligand coordination geometry to that of the
scandium complex but are one-THF solvates; complex 2b, however, is a dimer bridged by two anionic L2 fragments
that coordinate to the two yttrium ions in mixed η5:η5/κ1:κ1 coordination
modes. Upon activation with an organoborate, all these complexes initiated
the controlled polymerization of isoprene. In general, complexes 2a–c, bearing ligand L2, exhibited
higher activity than the analogous complexes 1a–c, attached to the L1 ligand. Complex 2b, in which the L2 ligand adopts the mixed η5/κ1 coordination mode, showed the highest
activity and livingness mode toward the polymerization of isoprene
with high cis-1,4-selectivity (94.1%), and both scandium
complexes 1a and 2a exhibited high 3,4-selectivity
(87%) irrespective of the ligand type; in contrast, the lutetium complexes
initiated the atactic isoprene polymerization. The influences of thell
ligand structural factors, the coordination solvent, and the central
metal ion on the catalytic activity and selectivity are discussed.

NNN-三齿吡咯基配体（HL1：2,5-双((吡咯烷-1-基)亚甲基)-1H-吡咯；HL2：2,5-双((哌啶基)亚甲基)-1H-吡咯）与三(三甲基硅基甲基)稀土配合物(THF)₂（Ln(CH₂SiMe₃)₃(THF)₂）发生酸碱反应，以中等到高产率得到相应的双(烷基)配合物L₁Ln(CH₂SiMe₃)₂(THF)ₓ（Ln=Sc，x=0（1a）；Ln=Y，x=1（1b）；Ln=Lu，x=1（1c））、L₂Sc(CH₂SiMe₃)₂（2a）以及L₂₂Ln₂(CH₂SiMe₃)₄（Ln=Y（2b）；Lu（2c））。X射线衍射分析结果显示，钪配合物1a与2a为无四氢呋喃（THF）溶剂的单体，配体以κ1:κ2配位模式与Sc³+离子配位；而钇配合物1b与镥配合物1c的配体配位几何构型与钪配合物一致，但为单THF溶剂化物。配合物2b则是由两个阴离子型L₂配体片段桥联的二聚体，该配体以混合η5:η5/κ1:κ1配位模式与两个钇离子配位。经有机硼盐活化后，所有上述配合物均可引发异戊二烯的可控聚合。总体而言，携带L₂配体的配合物2a~c的催化活性高于结合L₁配体的类似配合物1a~c。其中，配合物2b的L₂配体采取混合η5/κ1配位模式，在异戊二烯聚合中表现出最高的催化活性与活性聚合特性，且具有高达94.1%的顺式-1,4选择性；两种钪配合物1a与2a均表现出87%的高3,4-选择性，且不受配体类型影响。与之相反，镥配合物则引发异戊二烯的无规聚合。本文探讨了配体结构因素、配位溶剂与中心金属离子对催化活性与选择性的影响。