A variety of N-heterocyclic carbene (NHC) zinc adducts of the type NHC–ZnMe2[2a, NHC = 1,3-dimesitylimidazol-2-ylidene;2b, NHC = 1,3-(2,6-iPr2Ph)2-imidazol-2-ylidene;2c, NHC = 1,3-di-tert-butylimidazol-2-ylidene] and NHC–Zn(C6F5)2[4a, NHC = 1,3-dimesitylimidazol-2-ylidene;4b, NHC = 1,3-(2,6-iPr2Ph)2-imidazol-2-ylidene;4c, NHC = 1,3-di-tert-butylimidazol-2-ylidene] were synthesized by reaction of the appropriate NHC ligand with ZnMe2or (toluene)Zn(C6F5)2. The X-ray molecular structures of derivatives2b,4b, and4cconfirmed their adduct nature, and their bonding and geometrical parameters are influenced by the nature of the Zn–X group. The ionization reaction of the dimethyl Zn species2aand2bwith the methide abstracting reagent B(C6F5)3in the presence of tetrahydrofuran (THF) afforded the NHC–Zn organocations (1,3-dimesitylimidazol-2-ylidene)Zn(Me)(THF)2+(3a+) and [1,3-(2,6-iPr2Ph)2-imidazol-2-ylidene]Zn(Me)(THF)+(3b+) as dissociated MeB(C6F5)3–salts, as deduced from NMR spectroscopic data. All of these NHC–Zn compounds and BnOH/Zn(C6F5)2were tested as ring-opening polymerization (ROP) initiators of β-butyrolactone (β-BL),rac-lactide (rac-LA), and trimethylene carbonate (TMC). Although cations3a+and3b+were both inactive ROP initiators, as they appear to decompose prior to any ROP activity, the NHC–Zn(C6F5)2compounds readily mediate the ROP of β-BL,rac-LA, and TMC to afford the corresponding polymeric materials with a moderate level of control, as deduced from NMR, size-exclusion chromatography (SEC), and mass spectrometric MALDI-TOF data. In contrast, the use of the alcohol BnOH (instead of a NHC moiety) as a nucleophile for association with (toluene)Zn(C6F5)2led to efficient, highly controlled, and immortal ROP of TMC andrac-LA with the formation of controlled chain length and narrowly disperse materials.
- Carbene ligands
- Ring-opening polymerization