Physical Properties of Silenes

While infrared characterization of several silenes has been carried out, it has not always been possible to assign the observed bands unequivocally. Table II summarizes the available infrared data. The infrared spectra of reactive silenes, generated by the pyrolysis of suitable precursors, have most often been recorded in an argon matrix at 10 K, whereas the infrared 

Silene IR method temperature (K) IR bands observed0 (cm-1) Calculated Si=C stretching frequency (cm-1) Reference for observed spectrum Reference for calculated spectrum 

The bulky, stable silenes of Brook et al. (104,122-124,168) and Wiberg et al. (166,167) have been the only systems capable of being studied by nuclear magnetic resonance (NMR) spectroscopy to date. Table III lists the 13C and 29Si chemical shifts and the relevant coupling constants of these compounds. 

The larger /(Si=C) and 1J(Sia—Sib) coupling constants of silenes I to V compared to systems having only sp3-hybridized atoms (123) is another indication of -hybridization at silicon b. 

There have now been four experimental determinations of a silicon-carbon double bond length. The first of these was a gas phase electron diffraction study of 1,1-dimethylsilene (173). This study was the subject of much controversy since the experimentally determined bond length, 1.83 A, was much longer than the one predicted by ab initio calculations (1.69-1.71 A, see below) (159). Since the calculations were carried out at a relatively high level of theory and the effects of electron correlation on determining the Si=C bond length were considered, the validity of the data extracted from the electron diffraction study is in serious doubt. 

By 1985 much was known about the geometry and bonding of a wide variety of silenes. The majority of these compounds were very shortlived hence most of this knowledge came from ab initio and other kinds 

The chemical shifts of jp3-hybridized silicon atoms, such as a trimethyl-silyl group attached to the jp2-hybridized silicon in silenes, are normally found from -10 to -18 ppm, as compared to the position at -9.8 ppm for the trimethylsilyl groups of (Me3Si)4Si (the central silicon atom of this compound resonates at -135.5 ppm). On the other hand, a trimethylsiloxy group attached to carbon, either sp2- or sp3-hybridized, normally absorbs in the range from +5 to +20 ppm. 

Details of the ultraviolet absorption maxima for simple silenes, silaaro-matics, and for some relatively stable silenes are known and have been summarized.6 The simplest silenes absorb in the region 245-260 nm, with unknown extinction coefficients but as the substituents become increasingly complex, the Xmax values of the silenes increase until, with the silene (Me3Si)2Si=C(OSiMe3)Ad, the absorption occurs at 340 nm5 with an extinction coefficient of about 7400, consistent with a tt-it transition. A few further studies of interest are summarized below. 

For R = H the silole 51 had a at 278 nm while the silylene 48 absorbed at 250 and 480 nm. The isomeric silenes 49 and 50 absorbed at 2% and 270 nm, respectively. The UV absorptions for these species have been calculated and assigned, and their IR spectra have also been obtained. When R = Me, there was little change in the A.max, the four species absorbing at 280, 255 and 480, 312, and 274 nm, respectively. 

Using nanosecond laser flash photolysis techniques, Leigh80 observed transient absorption spectra which he attributed to the silenes derived from photolysis of various methylphenyldisilylbenzenes. Thus the silenes 52,53, and 54 were found to absorb at 425,460, and 490 nm, respectively, in isooctane, and 55 was also found to absorb at 490 nm.75 In other studies, the silene Ph2Si=CH2 derived by laser flash photolysis was found to absorb at 323 nm.111 

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A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

The facile photochemical sigmatropic 1,3-trimethylsilyl shift in polysilylacylsilanes from silicon to oxygen (equation 33) was utilized historically to prepare the first relatively stable silenes3 86 87. Silenes prepared by isomerization of acylpolysilanes bear, due to the synthetic approach, a trimethylsiloxy group at the sp2-hybridized carbon and relatively stable silenes of this type have in addition also at least one trimethylsilyl group at the silicon. These substituents strongly influence the physical properties and the chemical behaviour of these silenes. This is noticeable in many reactions in which these Brook -type silenes behave differently from simple silenes or silenes of the Wiberg type. 

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