Silenes matrix isolation

During recent years, fascinating developments have occurred in the area of r 2-silene complexes, which opened up to totally new chemistry. Some of the highlights will be presented in this section. The first investigations of coordination compounds of silenes were carried out by means of matrix isolation techniques at very low temperatures. In particular, photochemical methods proved to be most effective... 

TABLE 3. Matrix isolation of silenes R R2Si=CR3R4 by photolysis of (silyl)diazo compounds according to Scheme 2. 

SCHEME 3. Reactions of matrix-isolated silene 12 on warming... 

Much IR data has been collected for simple silenes in matrix isolation work and these data are summarized in several previous reviews7,8. 

From that value a force constant of k = 5.6 mdynA 1 for the Si=C double bond is deduced255. This frequency is clearly higher than the usual range for Si—C stretch vibrations but substantially less than for C=C stretches, both because Si is heavier than C and because the Si=C bond is weaker than the C=C bond. More suitable for the experimental characterization is the vinylic Si—H stretch vibration which gives rise to a medium band at 2239 cm-1 (25) or 2187 cm-1 (2)29, hypsochromically shifted by around 100 cm-1 relative to the Si—H stretch in simple silanes. A detailed analysis of the vibrational spectra of matrix-isolated MeHSi=CH2 26 using polarized IR spectroscopy established IR transition moment directions relative to the tot -transition moment (Si-C axis) in 26156. These data provide detailed information about the vibrational modes and about the structure of 26156. The bathochromic shift of the Si=C stretch in the isomeric 1,3-silabuta-l,3-dienes 289 and 290 by around 70 cm 1 compared with the Si=C stretch in simple silenes (Table 15), was interpreted as an indication of Si=C—C=C and C=Si—C=C 7r-conjugation159. 

The parent silaethylene 2 could be matrix-isolated by our group in 198117 by pyrolysis of the bicyclic system 1 (equation 1). Later, a new entry into this system was found by photolysis of methyldiazidosilane 318 (equation 1). This simplest representative 2 shows already most aspects of the chemistry of the whole class of silenes, e.g. the photoisomer-izability into the corresponding silylene (see Section VUI). 

Some of the reactions given in the sections above are important routes to reactive organosilicon intermediates such as silenes and silylenes, and because of their tendency to dimerize readily, to disilenes, the latter being formed when matrix-isolated silylenes are warmed up. It appears worthwhile to summarize some of the more useful reactions leading to silenes and silylenes, and their subsequent behaviors. The topics of silylenes97,142 and disilenes97,142,143 have recently been reviewed. 

For comparative study, various silenes were isolated in 3-methylpentane matrix at 77 K by photolysis of the corresponding silyldiazo compounds and their ultraviolet spectra are measured. The results are summarized in Table 1. The introduction of trimethylsilyl group on carbon results in slight red shift compared with the parent silene (H2Si=CH2, 258 nm). As one might expect, considerable bathochromic shifts have been observed for conjugated silenes such as 6c, 6d and 2829,30. 

Oxidation of Silenes and Silylenes Matrix Isolation of Unusual Silicon Species... 

Keywords Matrix Isolation / Oxidation / Silene / Silylene... 

Aromatic compounds containing silicon have attracted considerable interest, both experimental and theoretical, and efforts in this area have intensified since the first synthesis of stable silenes and disilenes19. Despite considerable effort no stable silaaro-matic compounds have yet been isolated, pure or even in a solution. All direct observations to date have been in the gas phase or in matrix isolation. Experimental knowledge about these compounds is therefore quite limited19 (see also below), leaving theory as one of the main sources of reliable information. 

An important step in the process of establishing the existence of silenes and the nature of their bonding was the direct observation of their matrix-isolated IR spectra, accomplished independently by three research groups in the same year97-100. The matrix isolation spectrum of the parent silene was reported subsequently101. A detailed account of the matrix-isolation work is given in Reference 2. 

The 1,2-shift of a hydrogen atom in an alkylsilylene is formally analogous to the above process. It has been utilized in matrix-isolation work for the photochemical conversion of silylenes to silenes129- 133, but it apparently does not proceed thermally with a low enough activation energy to be useful for room-temperature photochemical preparation of silenes from precursors of silylenes. 

Evidence for additional silene-to-silylene isomerizations on simple systems is now available. Thus the pyrolysis of a silabicyclo[2.2.2]octadiene precursor for 1-chlorosil-ene229 produces a mixture of matrix-isolated 1-chlorosilene and chloromethylsilylene. In this instance the thermal rearrangement of the silene to the silylene is probably facilitated by the increased exothermicity of the reaction, since halogenated silylenes are particularly stable (equation 95). 

Unlike thermal silene isomerizations and fragmentations, which were studied primarily on transient silenes involved in pyrolytic processes, photochemical investigations can be performed conveniently only on silenes that are either kept in matrix isolation or are stable in solution or as solids. The number of such investigations is quite limited. 

An alternative method of isolating 1 or 2 without sacrificing their reactivity is the matrix-isolation technique. In a low-temperature inert gas matrix, reactive molecules are immobilized, and thus bimolecular reactions are inhibited. In addition, the low temperatures prevent reactions with activation barriers larger than a few kcal/mol. In most of our experiments, argon matrices at 10 K have been used. Under these conditions, the diffusion of even small molecules like CO or O2 is effectively suppressed. Warming the argon matrix from 10 K to temperatures above 30 K allows small molecules to slowly diffuse. Under these conditions, bimolecular reactions are observed, if the activation barrier is small enough. Thus, reactions of matrix-isolated reactive species such as silenes and silylenes can be effectively controlled by variation of the matrix temperature. 

By far the most important spectroscopic method for this purpose is IR spectroscopy. In combination with DFT or ab initio calculations matrix IR spectroscopy has become a very powerful tool for the reliable identification of reactive and unusual molecules. In addition, isotopic labeling with is frequently used to assign the IR spectra of oxidized species. However, a prerequisite for this technique is the availability of suitable photochemical or thermal precursor molecules of the reactive silicon species. During the last years, we have published details of the oxidation mechanism of alkyl-substituted silenes 2. °In this chapter, our mechanistic studies on the oxidation of silylenes 1 using the matrix-isolation technique are summarized. 

W. Sander, M. Trommer, A. Patyk, Oxidation of Silenes and Silylenes Matrix Isolation of Unusual Silicon Species, Organosilicon Chem. Ill, N. Auner, J. Weis, Wiley-VCH 1998, 86-94. 

Silaacrylate 17 is another example of a matrix-isolated silene. It is formed simultaneously with ketene 18 on irradiation of methyl (pentamethyldisilanyl)diazoacetate... 

The IR and Raman spectra of Cl2HCSiH2CH3 showed that two conformers gauche, trans) were found in fluid phases, but only one trans) in an annealed crystal.IR spectra were reported for matrix-isolated transient silenes, R2Si=CR2, where R = H, D, Cl, CH3, CD3, etc. These were interpreted in terms of ab initio calculations.The nature of conformers present was studied from IR and Raman spectra of ClCH2SiEt and CH2ClSiMe2Cl. ... 

Simple silenes readily couple to yield the head-to-tail dimers, 1,3-disilacyclobuta-nes1,2,15,157. The dimerization is extremely facile and silenes bearing only small alkyl groups dimerize in an argon matrix even at 40 K, i.e., the dimerization proceeds at a diffusion controlled rate. Bulky substituents slow down the dimerization rate and allow the isolation of stable silenes. The head-to-tail dimerization (equation 98) is the predominant dimerization path for silenes, including those of the Auner-Jones 79,80 and Wiberg type72,73,78. 

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