Jutzi Peter, Mix Andreas, Neumann Beate, Rummel Britta, Schoeller Wolfgang W, Stammler Hans-Georg, Rozhenko A B
Faculty of Chemistry, University of Bielefeld, 33615 Bielefeld, FRG.
J Am Chem Soc. 2009 Sep 2;131(34):12137-43. doi: 10.1021/ja902153u.
The salt (eta(5)-pentamethylcyclopentadienyl)silicon(II) tetrakis(pentafluorophenyl)borate (5) reacts at -78 degrees C with lithium bis(trimethylsilyl)amide in dimethoxyethane (DME) as solvent to give quantitatively the compound [bis(trimethylsilyl)amino][pentamethylcyclopentadienyl]silicon(II) 6A in the form of a colorless viscous oil. The reaction performed at -40 degrees C leads to the silicon(IV) compound 7, the formal oxidative addition product of 6A with DME. Cycloaddition is observed in the reaction of 6A with 2,3-dimethylbutadiene to give the silicon(IV) compound 8. Upon attempts to crystallize 6A from organic solvents such as hexane, THF, or toluene, the deep yellow compound trans-1,2-bis[bis(trimethylsilyl)amino]-1,2-bis(pentamethylcyclopentadienyl)disilene (6B), the formal dimer of 6A, crystallizes from the colorless solution, but only after several days or even weeks. Upon attempts to dissolve the disilene 6B in the described organic solvents, a colorless solution is obtained after prolonged vigorous shaking or ultrasound treatment. From this solution, pure 6A can be recovered after solvent evaporation. This transformation process can be repeated several times. In a mass spectroscopic investigation of 6B, Si=Si bond cleavage is observed to give the molecular ion with the composition of 6A as the fragment with the highest mass. The X-ray crystal structure analysis of the disilene 6B supports a molecule with a short Si=Si bond (2.168 A) with efficiently packed, rigid sigma-bonded cyclopentadienyl substituents and silylamino groups. The conformation of the latter does not allow electron donation to the central silicon atom. Theoretical calculations at the density functional level (RI-BP86 and B3LYP, TZVP basis set) confirm the structure of 6B and reveal for silylene 6A the presence of an eta(2)-bonded cyclopentadienyl ligand and of a silylamino group in a conformation that prevents electron back-donation. Further theoretical calculations for the silicon(II) compound 6A, the disilene 6B, and the two species 11 and 11* derived from 6A (which derive from Si=Si bond cleavage) support the experimental findings. The reversible phase-dependent transformation between 6A and 6B is caused by (a) different stereoelectronic and steric effects exerted by the pentamethylcyclopentadienyl group in 6A and 6B, (b) some energy storage in the solid state structure of 6B (molecular jack in the box), (c) a small energy difference between 6A and 6B, (d) a low activation barrier for the equilibration process, and (e) the gain in entropy upon monomer formation.
盐（η⁵-五甲基环戊二烯基）硅（II）四（五氟苯基）硼酸盐（5）在-78℃下于二甲氧基乙烷（DME）作为溶剂中与双（三甲基硅基）氨基锂反应，定量生成无色粘性油状的化合物[双（三甲基硅基）氨基][五甲基环戊二烯基]硅（II）6A。在-40℃下进行的反应生成硅（IV）化合物7，它是6A与DME的形式上的氧化加成产物。在6A与2,3-二甲基丁二烯的反应中观察到环加成，生成硅（IV）化合物8。当试图从己烷、四氢呋喃或甲苯等有机溶剂中结晶6A时，深黄色化合物反式-1,2-双[双（三甲基硅基）氨基]-1,2-双（五甲基环戊二烯基）乙硅烯（6B），即6A的形式上的二聚体，从无色溶液中结晶出来，但这仅在几天甚至几周后才会发生。当试图将乙硅烯溶解在所述有机溶剂中时，经过长时间剧烈振荡或超声处理后可得到无色溶液。从该溶液中，蒸发溶剂后可回收纯的6A。这个转化过程可以重复几次。在对6B的质谱研究中，观察到Si=Si键断裂，生成组成为6A的分子离子作为质量最高的碎片。乙硅烯6B的X射线晶体结构分析表明，该分子具有短的Si=Si键（2.168 Å），五甲基环戊二烯基取代基和甲硅烷基氨基基团排列紧密且刚性，后者的构象不允许向中心硅原子提供电子。密度泛函理论水平（RI-BP86和B3LYP，TZVP基组）的理论计算证实了6B的结构，并揭示了硅烯6A中存在一个η²键合的环戊二烯基配体和一个处于阻止电子回授构象的甲硅烷基氨基基团。对硅（II）化合物6A、乙硅烯6B以及由6A衍生的两个物种11和11*（它们源自Si=Si键断裂）的进一步理论计算支持了实验结果。6A和6B之间可逆的相依赖转化是由以下原因引起的：（a）6A和6B中五甲基环戊二烯基基团施加的不同立体电子和空间效应；（b）6B固态结构中的一些能量储存（分子盒中玩偶）；（c）6A和6B之间的能量差较小；（d）平衡过程的活化能垒较低；（e）单体形成时熵的增加。
