Disilenes molecular structure

After review OW, the molecular structures in the solid state of 13 acyclic di-silenes, 13 endo- and exocyclic disilenes, 4 silicon-based dienes, 6 disilenes with radical, cation, and anion center, and one disilyne have been determined by X-ray analysis. Although the structural characteristics of a number of these new disilenes are summarized in a recent review by Weidenbruch,9 the structural parameters of all new disilenes reported after review OW are listed in Tables I-IV. 

The molecular structures of eleven acyclic disilenes are described in detail in two recent review articles covering the literature up to about the middle of 19956,49a. The number of known compounds has doubled in the subsequent few years49b. These new, structurally characterized disilenes include not only a tetrasilabutadiene and the first molecules with an endocyclic Si=Si double bond, described in separate sections, but also some acyclic disilenes and, in particular, the unusual compound 8. Thus, another brief survey of all the known molecular structures of disilenes reported through the end of 1999 seems to be justified. 

The detailed analysis of the molecular structure of isomer A of trisilaallene 12 revealed the bent-allenic nature with two trans-bent Si==Si double bonds that share a silicon atom the Si -Si -Si bond angle is 136.49 (6)°, the two Si=Si bond lengths are in the range of those for typical stable disilenes, the two terminal silicon atoms are pyramidalized, and the dihedral angles Si -Si -Si -C and Si -Si -Si -C" are around 60°. 

The most informative data should be its Si NMR data for the central triply bonded Si atoms. The resonance of sp-hybridyzed Si atom in RsiSi=SiRsi is observed at 89.9 ppm and is shifted upfield compared with those of the sUyl-substituted disilenes (ca. 142-155ppm).Similar up-field shift is observed in the NMR chemical shifts of the silyl-substituted alkenes (ca. 188-197 ppm) and alkynes (112-114ppm).The molecular structure of the RsiSi=SiRsi was revealed by X-ray crystallographic analysis (Figure 18). It has a trans-bent geometry [Si-Si-Si(Rsi)... 

Summary Three modifications of tetramesityldisilene (1) reported to date (orange unsolvated la and two yellow 1 1 solvates with toluene lb and THF Ic) have been found to readily transform to a new modification, yellow unsolvated Id. According to the spectral data, the new form Id is also a disilene but differs from la-< in both crystal and molecular structure. Id appears to be the most stable conformational polymorph of 1 but can be converted to the orange la upon illumination in the region 514-457 nm. A quasi-/ra i conformation with two aromatic rings nearly orthogonal and the other two nearly coplanar to the double-bond plane is tentatively proposed for Id. 

Tetramesityldisilene (1) was the first Si=Si doubly bonded compound to be synthesized [1] and is undoubtedly the best studied disilene [2, 3]. However, its optical properties, in particular vibrational and electronic spectra, were not studied in detail. We have investigated Raman, pre-resonance Raman, IR, UV-Vis and fluorescence spectra of solid 1, which exists as several modifications. Three different crystalline forms have been reported an unsolvated modification la, orange at room temperature, obtained from solution in hexane [4], a yellow, crystalline 1 1 toluene solvate, (Mes4Si2-C7H8), lb [5], and a similar solvate with one molecule of THF, (Mes4Si2.THF), Ic [6]. The X-ray data of la-c [4-6] show that not only crystal but also molecular structures of la-c differ in several respects (see Fig. I), especially in the aromatic ring orientations. Initially our aim was to obtain and to compare vibrational and electronic spectra of the reported modifications la-c. However, in the course of experiments we discovered the fourth modification of 1 — yellow unsolvated Id. Thus, we had to solve one more problem to determine the spectra and structure of this newly foimd form Id and to study interconversions of la-d. 

The molecular structures of several disilenes in the solid state have been determined by X-ray crystallography. All of them are characterized by very bulky alkyl or aryl substituents, R. The structures of (tert-butyl)(Mes)SiSi(fert-butyl)(Mes), Mes = phenyl-2,4,6-trimethyl (A), and (Mes)2SiSi(Mes)2 (B) are shown in Eig. 14.7. The central C2SiSiC2 framework of the former is planar as expected for an ethene analogue. The SiSi bond distance is 214 pm, about 9% shorter than the single bond distance in Me3SiSiMe3, 234 pm. (By comparison the CC double bond in ethene is 13% shorter than the single bond in ethane.)... 

Disilenyl cation 91 is identified by two characteristic low-field signals in the Si NMR spectrum (8 Si = 54 and 168.8) with the most deshielded signal assigned to the Si atom. The results of computations predict a bent structure for disilenyl cation 92, in contrast to vinyl cations which feature a linear dicoordinated positively charged carbon atom. The molecular structure of NHC-stabilized disilenyl cation 91 differs significantly from the theoretical prediction and closely resembles in many characteristics that of a disilene (Fig. 16). Based on the analysis of theoretical results and spectroscopic data, the authors suggested for disilenyl cation 91 a electronic structure described by resonance between Lewis representations A and B with only minor contributions of the canonical form C (Scheme 29). 

The first of these new synthetic methodologies, the anionic polymerization of masked disilenes developed by Sakurai and co-workers,is elegant and its concept is unique. Almost uniquely, it offers access to a range of polyaUcylsilane structures with narrow molecular weight distributions but involves some procedures that are so painstaking as to cause the principal researcher to caution the uninitiated who might wish to follow a similar path. 

The overall procedures of the methodology are shown in Scheme 11.1. The first reaction depicts the synthesis of masked disilenes of representative structures and the second is a conventional anionic polymerization such as might be applied in the synthesis of polystyrene. Masked disilenes are so-called as they are to all intents and purposes disilenes stabilized by a biphenyl bridge. During the polymerization reaction the bridge is released. Product polymers have been reported to have molecular weights up to 50,000 with... 

---