Silylenes spectra

In contrast to diazido compounds [102] and [104], which throw off two azido groups and form silylene and germylene, photodecomposition of silyl azide [105] led to the generation of aminosilylene [107] via isomerization of initially formed nitrene [106] (Maier et al., 1989b). The IR spectrum of the... 

The product 33 (Amax = 345 nm) was formed as a film (estimated by the optical density of the UV spectrum to be several tens of nanometres thick) on a target quartz plate, the surface of which had been treated with 10 wt.% NaOH solution, followed by dichlorodimethylsilane and finally LiAlH4 to afford a surface terminated with -OSiMe2H groups. On the basis of trapping experiments (to confirm the formation of silylenes) and photo-CVD in the presence of styrene (to confirm the absence of radical species), a mechanism based on silylene formation and insertion into the surface Si-H bond was shown to be likely, as outlined in Scheme 19. 

The presently known silicon chemical shift range is 990 ppm. This includes the Dsd form of decamethylsilicocene 28 (5 Si = —423 (solid state)), which is the most shielded resonance reported to date and the alkyl-substituted silylene 45, which presently defines the high-frequency end of the spectrum at 5 Si = 567. Most silicon chemical shifts occur, however, in a much smaller range from 5 Si = +50 to —190. This includes hexa-, penta- and tetracoordinated silicon compounds and for trivalent, positively charged silicon a significant low-field shift compared to comparable tetravalent silicon species is expected. 

FIGURE 1. Mesityl(neopentyl)silylene and its dimerization ------denotes electronic spectrum of... 

Thus, the formation of two types of paramagnetic complexes of atomic hydrogen with silylene sites is related to the difference in the structure of surface SC. For surface SC, whose optical absorption spectrum is shifted to the short-wave region, the addition of the H atom results in the formation of a site of type F, whereas type 1 is characteristic of long-wave SC. 

Molecular hydrogen is one of the few compounds that can be introduced into the bulk of vitreous silica. Chemical processes with the participation of molecular hydrogen are used for modifying the structure of this widely used material. The defects of vitreous silica play an important role in these processes. The exposure of =Si-N -H radicals to an atmosphere of H2 at room temperature is accompanied by the chemisorptions of the gas in an amount that is comparable with the number of radicals and by the disappearance of the EPR spectrum of these radicals (radiospectroscopic measurements were performed at 77 K (see Section 12.1)). When diamagnetic centers of the silylene type ((=Si-0)2Si ) also occurred on the sample surface in addition to the PC =Si-N -H, the decay of radicals in an atmosphere of H2 was accompanied by the quantitative formation of new PCs >Si -H. The appearance of the > Si -H radicals in the system suggests that H atoms... 

The coordination of stable silylenes to metal complexes was also reported to produce transition metal silylenoids.40,41 Exposure of unsaturated silylene 16 to Ni(CO)4 resulted in the formation of the disilylene-substituted tetrahedral nickel complex 17 (Scheme 7.3).42,43 Similarly, mixing Ph PAuCI with decamethylsilico-cene 18 produced the silylgold complex 19.44 The 29 Si H NMR spectrum of 19 (8 78 ppm) revealed its silylenoid character. In addition to nickel and gold, other metals, including tungsten,41 platinum,45 iron,40 and ruthenium,46 have been utilized to form silylmetal complexes of stable silylenes. 

The silylene species generated during photolysis are thought to polymerize in the absence of a trapping agent. The polymeric substances thus obtained react with bromine very violently, suggesting the presence of silicon-silicon bonds. The IR spectrum of the polymeric substances shows a weak absorption at 2090 cm-1 due to the Si—H stretching vibration. 

The h NMR spectrum provides some evidence for aromatic stabilization in 5. The important number is the chemical shift for the ring C-H protons, which falls at 6.75 ppm. This is significantly deshielded compared with the same protons in the precursor 4, 5.73 ppm, or the corresponding dihydride (LSiH2), 6.00 ppm. Theoretical calculations also support the idea that 5 is an aromatic molecule, with aromatic resonance energy of 12 s kcal mol . Particularly convincing, however, is a comparison of the properties of 5 with those of its saturated analog [22]. Silylene 13 was made by a reaction sequence similar to that for 5 in Eq. 4 and Eq. 5, except that for the final step it was necessary to use the dibromo rather than the dichloro compound (Eq. 13). For 13 an X-ray crystal structure could be determined it is shown in Fig. 5. 

The success of the GIAO calculations in predicting correctly the NMR spectrum of 4a and 5a suggests that this method can be used to study also the NMR spectra of transient silylenes which cannot yet be studied experimentally. Such studies can provide important ftmdamental information on the electronic structure of silylenes. 

If the concentration of methyl chloride is raised (6b Ar = 1 10), no IR bands of n-adduct T-5b or silylene S-lb can be detected. But T-5b has to be present in the matrix since in the UV spectrum the band at 260 (strong) is observable, together with a second absorption at 330 nm (weak), which we also attribute to n-adduct T-Sb. Upon irradiation with 330 nm the band at 260 nm disappears. At the same time a new absorption, again at 330 nm, can be detected. If one compares with the IR results (see below), it has to be concluded that this band belongs to adduct 7b, which is formed from the primarily generated uncomplexed silylene S-lb. Upon irradiation of the matrix with 330 nm light the IR bands of dichlorodimethylsilane 8b appear. Obviously, during irradiation a... 

In our first experiments we have recorded the IR and UVA is spectra of the matrix isolated silylene 1. The measured IR bands of 1 fit the calculated IR spectrum very well. 

Complicated reactions occur in the absence of trapping agents. Many signals can be observed besides those of aminosilanes in the Si spectrum, possibly oligosilanes by insertion of silylenes into Si-N bonds [8]. More investigations are necessary. 

The Si=Sn double bond of 56 is rather strong, but this species does not dissociate in solution into a silylene and a stannylene, judging from the spectrum and the reactivity. 

---