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Keywords:

  • Carbene ligands;
  • Zinc;
  • ­Cations;
  • Ring-opening polymerization;
  • Lactides

Abstract

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 ZnMe2 or (toluene)Zn(C6F5)2. The X-ray molecular structures of derivatives 2b, 4b, and 4c confirmed 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 species 2a and 2b with the methide abstracting reagent B(C6F5)3 in 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)2 were tested as ring-opening polymerization (ROP) initiators of β-butyrolactone (β-BL), rac-lactide (rac-LA), and trimethylene carbonate (TMC). Although cations 3a+ and 3b+ were both inactive ROP initiators, as they appear to decompose prior to any ROP activity, the NHC–Zn(C6F5)2 compounds 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)2 led to efficient, highly controlled, and immortal ROP of TMC and rac-LA with the formation of controlled chain length and narrowly disperse materials.