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.