Tridentate, bis-phenolate N-heterocyclic carbenes (NHCs) are among the ligands giving the most selective and active group 4-based catalysts for the copolymerization of cyclohexene oxide (CHO) with CO2. In particular, ligands based on imidazolidin-2-ylidene (saturated NHC) moieties have given catalysts which exclusively form polycarbonate in moderate-to-high yields even under low CO2 pressure and at low copolymerization temperatures. Here, to evaluate the influence of the NHC moiety on the molecular structure of the catalyst and its performance in copolymerization, we extend this chemistry by synthesizing and characterizing titanium complexes bearing tridentate bis-phenolate imidazol-2-ylidene (unsaturated NHC) and benzimidazol-2-ylidene (benzannulated NHC) ligands. The electronic properties of the ligands and the nature of their bonds to titanium are studied using density functional theory (DFT) and natural bond orbital (NBO) analysis. The metal–NHC bond distances and bond strengths are governed by ligand-to-metal σ- and π-donation, whereas back-donation directly from the metal to the NHC ligand seems to be less important. The NHC π-acceptor orbitals are still involved in bonding, as they interact with THF and isopropoxide oxygen lone-pair donor orbitals. The new complexes are, when combined with [PPN]Cl co-catalyst, selective in polycarbonate formation. The highest activity, albeit lower than that of the previously reported Ti catalysts based on saturated NHC, was obtained with the benzannulated NHC-Ti catalyst. Attempts to synthesize unsaturated and benzannulated NHC analogues based on Hf invariably led, as in earlier work with Zr, to a mixture of products that include zwitterionic and homoleptic complexes. However, the benzannulated NHC-Hf complexes were obtained as the major products, allowing for isolation. Although these complexes selectively form polycarbonate, their catalytic performance is inferior to that of analogues based on saturated NHC.

中文翻译：

---

钛和铪的不饱和和苯环化 N-杂环卡宾配合物：对环己烯氧化物与 CO2 共聚催化剂结构和性能的影响

三齿、双酚 N-杂环卡宾 (NHC) 是配体之一，为氧化环己烯 (CHO) 与 CO2 的共聚提供了最具选择性和活性的第 4 组催化剂。特别是，基于咪唑啉-2-亚基（饱和NHC）部分的配体提供的催化剂即使在低CO2压力和低共聚温度下也能以中等至高产率专门形成聚碳酸酯。在这里，为了评估 NHC 部分对催化剂分子结构及其在共聚中的性能的影响，我们通过合成和表征带有三齿双酚咪唑 - 2-亚基（不饱和 NHC）和苯并咪唑 - 2-亚基（苯并NHC）配体。使用密度泛函理论 (DFT) 和自然键轨道 (NBO) 分析来研究配体的电子性质及其与钛的键的性质。金属-NHC 键距和键强度受配体-金属 σ-和 π-供体控制，而直接从金属到 NHC 配体的回供似乎不太重要。NHC π 受体轨道仍参与键合，因为它们与 THF 和异丙醇氧孤对供体轨道相互作用。当与 [PPN]Cl 助催化剂结合时，新配合物在聚碳酸酯形成中具有选择性。使用苯并环化 NHC-Ti 催化剂获得最高活性，尽管低于先前报道的基于饱和 NHC 的 Ti 催化剂的活性。尝试合成基于 Hf 的不饱和和苯并环 NHC 类似物总是导致，与早期使用 Zr 的工作一样，包括两性离子和均质复合物的产品混合物。然而，得到了苯并环化 NHC-Hf 复合物作为主要产品，允许进行分离。尽管这些配合物选择性地形成聚碳酸酯，但它们的催化性能不如基于饱和 NHC 的类似物。