Ethene Complexes of Bulky Titanocenes, Their Thermolysis, and Their Reactivity toward 2-Butyne
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Ethene complexes of titanocenes [Ti(II)(η2-C2H4)(Cp′)2] for Cp′ = η5-C5Me5 (1), η5-C5Me4t-Bu (2), η5-C5Me4SiMe3 (3), and η5-C5HMe4 (4) were prepared by reduction of corresponding titanocene dichlorides with magnesium in THF in the presence of ethene. Thermolysis of 1–3 in toluene solution at a maximum of 100 °C resulted in elimination of ethane, affording cleanly doubly tucked-in titanocene compounds 5–7, respectively. Experiments with 2 and 3 in NMR tubes proved that symmetrical isomers 6a and 7a were formed first, and these thermally isomerized to thermodynamically more stable asymmetrical isomers 6b and 7b. The energy difference between 7a and 7b calculated by DFT methods was 15.3 kJ/mol. Thermolysis of 4 in m-xylene required a temperature of 135 °C, affording a mixture of 8b > 8a and “dimeric dehydro-titanocene” 9 as a concurrent product of hydrogen abstraction. In contrast to thermolysis in solvents, heating of 1 and 2 in high vacuum to 135 °C resulted in sublimation of known titanocenes [Ti(C5Me5)2] (10) and [Ti(η5-C5Me4t-Bu)2] (13) (Chirik et al. J. Am. Chem. Soc. 2004, 126, 14688–14689), respectively. The former isomerized in hexane solution to the tucked-in hydride [TiH{C5Me4(CH2)}(C5Me5)] (10A) as described by Bercaw (J. Am. Chem. Soc. 2004, 126, 14688–14689). A mixture of 10/10A decayed within days to give major paramagnetic products [TiH(C5Me5)2] (11) and singly tucked-in titanocene [Ti{C5Me4(CH2)}(C5Me5)] (12) and minor diamagnetic 5 and its so far unknown, less stable isomer [Ti{C5Me4(CH2)}2] (10B), identified by NMR spectra and corroborated by DFT calculations. Solid 3 eliminated ethene at only 80 °C, leaving titanocene 14, whereas compound 4 sublimed at 135 °C mostly without decomposition. Cocrystals of 10 with [TiCl(C5Me5)2] (1:2) (10C) afforded an X-ray single-crystal structure with linear geometry for 10. The ethene complexes 1–4 differed in their reactivity toward but-2-yne: compounds 1 and 4 yielded the respective [Ti(IV)(η1: η1-CH2CH2CMeCMe)(Cp′)2] 2,3-dimethyltitanacyclopent-2-ene complexes 15 and 16, whereas 2 and 3 replaced ethene with but-2-yne, affording the [Ti(II)(η2-MeCCMe)(Cp′)2] complexes 17 and 18, respectively. Crystal structures of 2, 4, 10C, 15, 17, and 18 have been determined by X-ray crystallography.
针对取代基Cp′分别为η⁵-C₅Me₅（1）、η⁵-C₅Me₄t-Bu（2）、η⁵-C₅Me₄SiMe₃（3）以及η⁵-C₅HMe₄（4）的钛茂乙烯配合物[Ti(II)(η²-C₂H₄)(Cp′)₂]，本研究通过在四氢呋喃（THF）中以镁还原相应的二氯化钛茂，并在乙烯存在下完成制备。将配合物1~3的甲苯溶液于最高100℃下进行热解，可消除乙烷，分别得到纯净的双嵌入型钛茂配合物5~7。以核磁共振（NMR）管对2和3开展的实验证实，首先生成对称异构体6a和7a，随后这些异构体经热异构化得到热力学更稳定的非对称异构体6b和7b。通过密度泛函理论（DFT）方法计算得到的7a与7b之间的能量差为15.3 kJ/mol。配合物4在间二甲苯（m-xylene）中的热解则需要135℃，得到8b与8a的混合物，同时伴随脱氢副产物“二聚脱氢钛茂”9。与溶剂中热解不同，将1和2在高真空下加热至135℃，可分别得到已知钛茂配合物[Ti(C₅Me₅)₂]（10）与[Ti(η⁵-C₅Me₄t-Bu)₂]（13）的升华产物（相关研究参见Chirik等，《美国化学会志》，2004年，126卷，14688–14689页）。如Bercaw所述（J. Am. Chem. Soc. 2004, 126, 14688–14689），配合物10在己烷溶液中可异构化为嵌入型氢化物[TiH{C₅Me₄(CH₂)}(C₅Me₅)]（10A）。10与10A的混合物在数天内发生降解，主要生成顺磁性产物[TiH(C₅Me₅)₂]（11）与单嵌入型钛茂[Ti{C₅Me₄(CH₂)}(C₅Me₅)]（12），以及少量反磁性产物5及其迄今尚未见报道、稳定性更差的异构体[Ti{C₅Me₄(CH₂)}₂]（10B），上述产物均通过核磁共振光谱得以鉴定，并经密度泛函理论计算验证。固态3仅需80℃即可消除乙烯，得到钛茂14；而化合物4在135℃升华时基本未发生分解。配合物10与[TiCl(C₅Me₅)₂]按1:2摩尔比形成的共晶（10C）经X射线单晶衍射解析，确认10具有线性几何构型。乙烯配合物1~4与2-丁炔的反应活性存在差异：配合物1和4分别生成相应的[Ti(IV)(η¹:η¹-CH₂CH₂CMe=CMe)(Cp′)₂]型2,3-二甲基钛杂环戊-2-烯配合物15和16；而配合物2和3则以2-丁炔置换乙烯，得到[Ti(II)(η²-MeC≡CMe)(Cp′)₂]型配合物17和18。通过X射线晶体衍射已确定配合物2、4、10C、15、17以及18的晶体结构。
