School of Chemistry, University of Bristol, Cantocks' Close, Bristol BS8 1TS (UK).
Chemistry. 2014 Apr 1;20(14):4077-85. doi: 10.1002/chem.201304396. Epub 2014 Mar 3.
Reactivity studies of dicarba[2]ferrocenophanes and also their corresponding ring-opened oligomers and polymers have been conducted in order to provide mechanistic insight into the processes that occur under the conditions of their thermal ring-opening polymerisation (ROP) (300 °C). Thermolysis of dicarba[2]ferrocenophane rac-[Fe(η(5)-C5H4)2(CHPh)2] (rac-14; 300 °C, 1 h) does not lead to thermal ROP. To investigate this system further, rac-14 was heated in the presence of an excess of cyclopentadienyl anion, to mimic the postulated propagating sites for thermally polymerisable analogues. This afforded acyclic [(η(5)-C5H5)Fe(η(5)-C5H4)-CH2Ph] (17) through cleavage of both a Fe-Cp bond and also the C-C bond derived from the dicarba bridge. Evidence supporting a potential homolytic C-C bond cleavage pathway that occurs in the absence of ring-strain was provided through thermolysis of an acyclic analogue of rac-14, namely [(η(5)-C5H5)Fe(η(5)-C5H4)(CHPh)2-C5H5] (15; 300 °C, 1 h), which also afforded ferrocene derivative 17. This reactivity pathway appears general for post-ROP species bearing phenyl substituents on adjacent carbons, and consequently was also observed during the thermolysis of linear polyferrocenylethylene [Fe(η(5)-C5H4)2(CHPh)2]n (16; 300 °C, 1 h), which was prepared by photocontrolled ROP of rac-14 at 5 °C. This afforded ferrocene derivative [Fe(η(5)-C5H4CH2Ph)2] (23) through selective cleavage of the -H(Ph)C-C(Ph)H- bonds in the dicarba linkers. These processes appear to be facilitated by the presence of bulky, radical-stabilising phenyl substituents on each carbon of the linker, as demonstrated through the contrasting thermal properties of unsubstituted linear trimer [(η(5)-C5H5)Fe(η(5)-C5H4)(CH2)2(η(5)-C5H4)Fe(η(5)-C5H4)(CH2)2(η(5)-C5H4)Fe(η(5)-C5H5)] (29) with a -H2 C-CH2- spacer, which proved significantly more stable under analogous conditions. Evidence for the radical intermediates formed through C-C bond cleavage was detected through high-resolution mass spectrometric analysis of co-thermolysis reactions involving rac-14 and 15 (300 °C, 1 h), which indicated the presence of higher molecular weight species, postulated to be formed through cross-coupling of these intermediates.
为了深入了解热开环聚合（ROP）（300°C）条件下发生的反应机制，我们对二碳[2]费罗伦烷及其相应的开环低聚物和聚合物进行了反应性研究。rac-[Fe(η(5)-C5H4)2(CHPh)2]（rac-14；300°C，1 小时）的热解不会导致热 ROP。为了进一步研究该体系，在 rac-14 存在过量环戊二烯阴离子的情况下加热，以模拟热聚合类似物的假定增长位点。这通过断裂两个 Fe-Cp 键和来自二碳桥的 C-C 键，生成非环[(η(5)-C5H5)Fe(η(5)-C5H4)-CH2Ph]（17）。通过热解 rac-14 的非环类似物，即[(η(5)-C5H5)Fe(η(5)-C5H4)(CHPh)2-C5H5]（15；300°C，1 小时），提供了支持可能发生在不存在环应变的情况下的自由基 C-C 键断裂途径的证据，这也得到了 ferrocene 衍生物 17。这种反应性途径似乎对相邻碳上带有苯基取代基的 ROP 后物种通用，因此在通过光控 ROP 在 5°C 下制备的线性聚二茂铁乙烯[Fe(η(5)-C5H4)2(CHPh)2]n（16；300°C，1 小时）的热解过程中也观察到了这种情况。这通过选择性断裂二碳桥中的-H(Ph)C-C(Ph)H-键，得到了 ferrocene 衍生物[Fe(η(5)-C5H4CH2Ph)2]（23）。这些过程似乎得益于连接体每个碳原子上存在体积大、稳定自由基的苯基取代基，这通过具有-H2 C-CH2-间隔基的未取代线性三聚体[(η(5)-C5H5)Fe(η(5)-C5H4)(CH2)2(η(5)-C5H4)Fe(η(5)-C5H4)(CH2)2(η(5)-C5H4)Fe(η(5)-C5H5)]（29）的对比热性质得到了证明，与类似条件相比，该三聚体明显更稳定。通过 rac-14 和 15（300°C，1 小时）的共热解反应的高分辨率质谱分析检测到通过 C-C 键断裂形成的自由基中间体的证据，这表明存在推测通过这些中间体的交叉偶联形成的更高分子量的物质。
