Role of torsional strain in the ring-opening polymerisation of low strain [n]nickelocenophanes.
Role of torsional strain in the ring-opening polymerisation of low strain [n]nickelocenophanes.
复制标题
扭转应变在低应变[n]二茂镍开环聚合中的作用。
DOI：
10.1039/c9sc02624j
复制
发表时间：
2019
期刊：
影响因子：
8.4
通讯作者：
Musgrave RA
中科院分区：
文献类型：
--
作者：
Musgrave RA
应变[1]-和[2]-烯吩和相关物种的开环聚合（ROP）是公认的，并且单体环应变表现为α配体的显著倾斜，得到α 14-32°的α角。令人惊讶的是，四碳[4]镍茂烷[Ni（η5-C5 H4）2（CH 2）4]（2）经历ROP（吡啶，20 °C，5天），得到主要不溶的聚（镍茂基丁烯）[Ni（η5-C5 H4）2（CH 2）4]n（12），尽管缺乏显著的环倾斜。通过涉及环状前体和模型低聚物的DFT计算（Δ H 0 ROP = −14 ± 2 kJ mol−1）证实了ROP的外旋性质，并提出这是2的柄桥中存在的扭转应变的结果。发现类似的未倾斜的二硅杂-2-氧杂[3]镍茂烷[Ni（η5-C5 H4）2（SiMe 2）2 O]（13）和[Ni（η5-C5 H4）2（SiMePh）2 O]（14）缺乏类似的扭转应变并且在相同条件下抵抗ROP。相反，发现具有显著倾斜角（α = 16°）的1-甲基三碳[3]镍茂烷{Ni（η5-C5 H4）2（CH 2）2[CH（CH 3）]}（15）经历ROP以得到可溶性聚合物{Ni（η5-C5 H4）2（CH 2）2[CH（CH 3）]}n（18）。在这种情况下，该过程的可逆性允许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（Δ H 0 ROP = −8.9 kJ mol−1，Δ G 0 ROP = −3.1 kJ mol−1）。与先前描述的未取代类似物[Ni（η5-C5 H4）2（CH 2）3]（1）的ROP（Δ H 0 ROP = −10 kJ mol−1，Δ G 0 ROP = −4.0 kJ mol−1）相比，在柄桥中存在额外的甲基取代基似乎略微不利于ROP，并导致平衡聚合物产率的相应降低。
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 α angles of ∼14–32°. Surprisingly, tetracarba[4]nickelocenophane [Ni(η5-C5H4)2(CH2)4] (2) undergoes ROP (pyridine, 20 °C, 5 days) to give primarily insoluble poly(nickelocenylbutylene) [Ni(η5-C5H4)2(CH2)4]n (12), 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 (ΔH0ROP = −14 ± 2 kJ mol−1), and is proposed to be a consequence of torsional strain present in the ansa bridge of 2. The similarly untilted disila-2-oxa[3]nickelocenophanes [Ni(η5-C5H4)2(SiMe2)2O] (13) and [Ni(η5-C5H4)2(SiMePh)2O] (14) were found to lack similar torsional strain and to be resistant to ROP under the same conditions. In contrast, 1-methyltricarba[3]nickelocenophane {Ni(η5-C5H4)2(CH2)2[CH(CH3)]} (15) with a significant tilt angle (α ∼ 16°) was found to undergo ROP to give soluble polymer {Ni(η5-C5H4)2(CH2)2[CH(CH3)]}n (18). 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 (ΔH0ROP = −8.9 kJ mol−1, ΔG0ROP = −3.1 kJ mol−1). Compared to the previously described ROP of the unsubstituted analogue [Ni(η5-C5H4)2(CH2)3] (1) (ΔH0ROP = −10 kJ mol−1, ΔG0ROP = −4.0 kJ mol−1), the presence of the additional methyl substituent in the ansa bridge appears to marginally disfavour ROP and leads to a corresponding decrease in the equilibrium polymer yield.