Musgrave Rebecca A, Hailes Rebekah L N, Annibale Vincent T, Manners Ian
School of Chemistry , University of Bristol , Bristol BS8 1TS , UK . Email:
Department of Chemistry , University of Victoria , Victoria , BC V8W 3V6 , Canada.
Chem Sci. 2019 Sep 10;10(42):9841-9852. doi: 10.1039/c9sc02624j. eCollection 2019 Nov 14.
Ring-opening polymerisation (ROP) of strained [1]- and [2]metallocenophanes and related species is well-established, and the monomer ring-strain is manifest in a substantial tilting of the cyclopentadienyl ligands, giving angles of ∼14-32°. Surprisingly, tetracarba[4]nickelocenophane [Ni(η-CH)(CH)] () undergoes ROP (pyridine, 20 °C, 5 days) to give primarily insoluble poly(nickelocenylbutylene) [Ni(η-CH)(CH)] (), despite the lack of significant ring-tilt. The exoenthalpic nature of the ROP was confirmed by DFT calculations involving the cyclic precursor and model oligomers (Δ0ROP = -14 ± 2 kJ mol), and is proposed to be a consequence of torsional strain present in the bridge of . The similarly untilted disila-2-oxa[3]nickelocenophanes [Ni(η-CH)(SiMe)O] () and [Ni(η-CH)(SiMePh)O] () were found to lack similar torsional strain and to be resistant to ROP under the same conditions. In contrast, 1-methyltricarba[3]nickelocenophane {Ni(η-CH)(CH)[CH(CH)]} () with a significant tilt angle ( ∼ 16°) was found to undergo ROP to give soluble polymer {Ni(η-CH)(CH)[CH(CH)]} (). The reversibility of the process in this case allowed for the effects of temperature and reaction concentration on the monomer-polymer equilibrium to be explored and thereby thermodynamic data to be elucidated (Δ0ROP = -8.9 kJ mol, Δ0ROP = -3.1 kJ mol). Compared to the previously described ROP of the unsubstituted analogue [Ni(η-CH)(CH)] () (Δ0ROP = -10 kJ mol, Δ0ROP = -4.0 kJ mol), the presence of the additional methyl substituent in the bridge appears to marginally disfavour ROP and leads to a corresponding decrease in the equilibrium polymer yield.
环张力[1]-和[2]茂金属环的开环聚合反应（ROP）以及相关物种已得到充分证实，并且单体环张力表现为环戊二烯基配体的显著倾斜，倾斜角度约为14 - 32°。令人惊讶的是，四碳[4]镍茂环[Ni(η-C₅H₄)(C₄H₈)]（）在ROP反应中（吡啶，20 °C，5天）主要生成不溶性的聚（镍茂基亚丁基）[Ni(η-C₅H₄)(C₄H₈)] （），尽管其环倾斜不明显。通过涉及环状前体和模型低聚物的密度泛函理论（DFT）计算证实了ROP反应的放热性质（Δ₀ROP = -14 ± 2 kJ/mol），并认为这是由于中桥存在扭转应变所致。发现类似未倾斜的二硅-2-氧杂[3]镍茂环[Ni(η-C₅H₄)(SiMe₂)O]（）和[Ni(η-C₅H₄)(SiMePh)O]（）缺乏类似的扭转应变，并且在相同条件下对ROP反应具有抗性。相比之下，具有显著倾斜角（约16°）的1-甲基三碳[3]镍茂环{Ni(η-C₅H₄)(C₄H₈)[CH(CH₃)]}（）发生ROP反应生成可溶性聚合物{Ni(η-C₅H₄)(C₄H₈)[CH(CH₃)]} （）。这种情况下反应的可逆性使得能够探索温度和反应浓度对单体 - 聚合物平衡的影响，从而阐明热力学数据（Δ₀ROP = -8.9 kJ/mol，Δ₀ROP = -3.1 kJ/mol）。与先前描述的未取代类似物[Ni(η-C₅H₄)(C₄H₈)]（）的ROP反应（Δ₀ROP = -10 kJ/mol，Δ₀ROP = -4.0 kJ/mol）相比，桥中额外甲基取代基的存在似乎略微不利于ROP反应，并导致平衡聚合物产率相应降低。
