A Bifunctional NHC-Aryloxido Titanium Catalyst for the Ring-Opening Polymerization of ε‑Caprolactone and an Unusual Fragmentation of Its Ligand Backbone

N-Heterocyclic carbenes (NHCs) with
a methylene-linked aryloxide
side arm constitute a flexible bidentate ligand platform whose usability
is partly hindered as their alkali metal salts, the primary ligand
transfer agents, are prone to carbene deactivation by 1,2-benzyl migration.
Reacting an imidazolium precursor of this ligand class [HO-4,6-But2-C6H2-2-CH2{CH(NCHCHNAr)}]Br
[LH2Br; Ar = 2,6-Pri2-C6H3 (Dipp)] with Ti(NMe2)4 in 1:1 ratio readily gives the monoligated titanium
complex [(L)Ti(NMe2)2Br] (1). Reacting 1 with an additional 1 equiv of LH2Br shows an interesting fragmentation behavior of L–, distinct from that 1,2-migration, and gives the cationic Ti complex
[(LH)Ti{κ2-(O-4,6-But2-C6H2-2-CH2NMe2)}(NMe2)Br]Br ([2]Br). This shows the vulnerability of the NHC–Ti motif in 1 and presents a rare case in which an imidazolium moiety
acts as a leaving group, departing as an aryl imidazole. The nature
of the NHC–Ti bond in 1 and its conversion into [2]Br are probed by computational analyses. In addition, 1 is established as a catalyst for the ring-opening polymerization
of ε-caprolactone (CL), where it exhibits high activity and
good control over polymer growth under ambient conditions. A kinetic
analysis suggests the classic coordination–insertion mechanism
with a typical first-order dependence on CL concentration, while the
end group characterization indicates a bifunctional nature of the
NHC–Ti combo in which the labile NHC makes the nucleophilic
attack in the ring-opening initiation step.

氮杂环卡宾（N-Heterocyclic carbenes, NHCs）若带有亚甲基连接的芳氧化物侧臂，可构成柔性双齿配体平台，但这类配体的碱金属盐（即主要的配体转移试剂）易因1,2-苄基迁移发生卡宾失活，从而在一定程度上限制了其应用。该配体类别的咪唑鎓前驱体[HO-4,6-二叔丁基-C6H2-2-CH2{CH(NCH=CHNAr)}]Br[LH2Br；Ar = 2,6-二异丙基苯基（2,6-Pri2-C6H3, Dipp）]与Ti(NMe2)4以1:1摩尔比反应，可便捷得到单配体配位的钛配合物[(L)Ti(NMe2)2Br]（1）。将配合物1与额外1当量的LH2Br反应时，观察到配体阴离子L⁻的独特碎裂行为，该行为不同于前述的1,2-迁移过程，最终得到阳离子钛配合物[(LH)Ti{κ²-(O-4,6-二叔丁基-C6H2-2-CH2NMe2)}(NMe2)Br]Br ([2]Br)。这一现象揭示了配合物1中NHC–Ti键合基序的不稳定性，同时提供了一个罕见案例：咪唑鎓单元作为离去基团，以芳基咪唑的形式脱离体系。通过计算分析，对配合物1中的NHC–Ti键本质及其向[2]Br的转化过程进行了探究。此外，配合物1被证实可作为ε-己内酯（ε-caprolactone, CL）开环聚合的催化剂，在室温条件下展现出高催化活性与对聚合物生长的良好可控性。动力学分析表明该聚合反应遵循经典的配位-插入机理，对CL浓度呈现典型的一级动力学依赖；而端基表征则揭示了NHC–Ti复合催化体系的双功能特性：易解离的NHC在开环引发步骤中亲核进攻单体。