Organolanthanide Complexes Supported by Thiazole-Containing Amidopyridinate Ligands: Synthesis, Characterization, and Catalytic Activity in Isoprene Polymerization

Neutral
bis­(alkyl)-organolanthanide complexes supported by tridentate
{N–,N,N} monoanionic 5-methylthiazole- or benzothiazole-amidopyridinate
ligands have been prepared and completely characterized: (LThiaMe2)­Ln­(CH2SiMe3)2 [Ln
= Lu3+ (3), Er3+ (7), Yb3+ (8)] and (LBnThMe2)­Lu­(CH2SiMe3)2 (5). Similarly to related Y3+ systems, the nature of the
thiazole unit controls the ultimate catalyst stability in solution.
In the diamagnetic Lu3+ complex 5, a progressive
and complete rearrangement of its metal coordination sphere takes
place through a metal-to-ligand alkyl migration with subsequent benzothiazole
ring-opening and generation of the Lu3+ mono­(alkyl)-arylthiolate
species stabilized by a tetradentate {N–,N,N,S–} dianionic ligand. On the other hand, the 5-methylthiazole-containing
complexes 3, 7, and 8 showed
no evidence of any ligand rearrangement. Complexes 3–8 have been tested as homogeneous catalysts in isoprene (IP)
polymerization, after activation with selected organoborates. Binary
systems 3/TB and 7/TB [TB = tritylium tetrakis­(pentafluorophenyl)­borate]
show the highest activity and living character toward IP polymerization,
affording polymers with relatively high trans-1,4-selectivity
(up to 76.4%), moderate molecular weights (Mn up to 146 000 g/mol), and narrow polydispersities
(Mw/Mn). Depending
on the rare-earth ion of choice, a prevalent trans-1,4 (Lu3+, Er3+, Yb3+; up to 76.4%)
or a dominant 3,4 (Y3+; 92.7%) polymer structure is observed.
The influence of the ligand type, metal ion, and activator(s) on the
ultimate catalyst activity and selectivity is discussed.

本研究合成并全面表征了以三齿{N–,N,N}单阴离子型5-甲基噻唑或苯并噻唑-酰胺吡啶基配体配位的中性双(烷基)有机镧系配合物(organolanthanide complexes)：(LThiaMe2)Ln(CH2SiMe3)2 [Ln = Lu³⁺(3)、Er³⁺(7)、Yb³⁺(8)] 以及(LBnThMe2)Lu(CH2SiMe3)2(5)。与相关的钇(Y³⁺)体系类似，噻唑单元的结构决定了该类催化剂在溶液中的最终稳定性。在抗磁性Lu³⁺配合物5中，其金属配位球会发生渐进且完全的重排：经由金属到配体的烷基迁移，随后引发苯并噻唑环开环，最终生成由四齿{N–,N,N,S–}双阴离子配体稳定的Lu³⁺单(烷基)-芳硫醇盐物种。反观含5-甲基噻唑的配合物3、7和8，则未表现出任何配体重排的迹象。配合物3~8经选定的有机硼酸盐活化后，被用作均相催化剂进行异戊二烯(isoprene, IP)聚合反应测试。其中，二元催化体系3/TB与7/TB [TB = 三苯四(五氟苯基)硼酸盐(tritylium tetrakis(pentafluorophenyl)borate)] 展现出最高的催化活性与活性聚合特性，可制备出具有较高反式1,4-选择性（最高达76.4%）、适中数均分子量（Mn最高可达146000 g/mol）及窄多分散性（Mw/Mn）的聚合物。根据所选用的稀土离子不同，可得到主要为反式1,4结构（Lu³⁺、Er³⁺、Yb³⁺；最高76.4%）或主导3,4结构（Y³⁺；92.7%）的聚合物。本文探讨了配体类型、金属离子及活化剂对催化剂最终活性与选择性的影响。