Substituted silylenes

The effect of the substituents R on the stabilities of the singlet and triplet silylenes can be evaluated by the isodesmic bond separation equations 34 and 35, respectively. The MP4SDTQ/6-31G //3-21G results are presented in Table 32 together with the data for 

RSiH (singlet) + SiH4--------- H3SiR + H2Si (singlet) (34) 

TABLE 32. Stabilization energies (kcalmol 1) for singlet and triplet substituted silylenes RSiH and substituted carbenes RCH (MP4SDTQ/6-31G //3-21G)b as calculated by equations 34-37 

Two features are immediately obvious from the data in Table 32. The singlet and triplet states respond differently to substitution and the effects are mostly larger (especially in the singlet state) in the carbenes than in the silylenes. Let us examine first the substituent effects in the singlet state. Most substituents stabilize the singlet silylene, but the most effective stabilization is achieved with 7t-donors and it decreases with the decrease in the 

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Photochemical transformations of cyclic and short chain polysilane oligomers have been intensively investigated (39). Irradiation of these materials in the presence of trapping reagents, such as silanes or alcohols, has suggested that substituted silylenes and silyl radicals are primary reactive intermediates. The former have been... 

In summary, the production of substituted silylenes and silyl radicals upon exhaustive irradiation at 254 nm of polysilane high polymers suggested that the polymer photochemistry resembled that previously reported for short chain acyclic and cyclic oligomers (39). More recent experiments, however, have suggested that the photochemical mechanism for the degradation of the high polymers is more complex than first envisioned (vide infra) (48). 

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. 

In spite of its long-assumed intermediacy in several reactions, no carbon-substituted silylene was directly observed for many years. In 1979, however, Drahnak et al. detected a broad ultraviolet (UV) absorption band (3-niax = 450 nm) after the photolysis of dodecamethylcyclohexasilane (6) in 3-methylpentane. This band was assigned to dimethylsilylene (5). Many different approaches to this intermediate, either photochemicaUy or thermally, were examined (Scheme 14.6). ... 

As mentioned above, there have been no examples of stable diarylsilylene although dialkyl- or diamino-substituted silylenes are now available as stable crystalline compounds. However, Tokitoh et al. found that the overcrowded diarylsilylene-isonitrile complexes 139 act as a free silylene 138 in solution even at room temperature. Since there is a severe limitation in a study concerning the reactivities of silylenes under the conventional generating conditions, the reactivity of silylene-isonitrile complexes 139 as a masked silylene is of great interest. 

Carbene type radical anions have been often postulated as reactive intermediates. An tetratrimethylsilyl substituted silylene (16) was reduced with several alkali metals in dimethoxyethane to yield a persistent silylene radical anion.169... 

As seen from Table IV, the feature most characteristic of the formation of a matrix silylene-Lewis base complex is the hypsochromic shift of the visible absorption band of the silylene on complexation. At this point it is worthwhile to mention that as early as 1982 it was reported28 that the absorption maxima of methyl-substituted silylenes in a nitrogen matrix are shifted to significantly higher energies than those of the same silylenes in an inert argon matrix. However, the cause of the shift difference was not addressed in this publication. 

Levin et al.55 observed, in line with the qualitative results of Gillette et al. 6 that less bulkily substituted silylene 13 is stabilized when coordinated by Lewis bases. Dimethylsilylene 13 was generated photochemically from 32 in cyclohexane solutions at 296 K, and it was shown by means of UV spectroscopy that the lifetime of this silylene is 0.4 fjs under the conditions... 

It was impossible to avoid a certain amount of overlap between the sections of this review, especially because of the easy conversion of substituted silylenes into species with multiply bonded silicon and vice versa. But we tried to reduce duplications of this type as far as possible. 

Silicon substituted silylenes attract the chemists interest for a special reason. According to ab initio calculations 85 137 176], substituents acting as cr-donors should induce a relatively large red-shift of the UV maximum or, in other words, the n(Si) —> 3p(Si) transition energy should be relatively small. Therefore these species are potential candidates for the long-sought triplet ground state silylenes, especially if this electronic effect is supported by a steric one. Nevertheless, reports on the matrix isolation of silicon substituted silylenes are comparatively scarce. 

The first isolation of an oxygen substituted silylene was reported by West and coworkers169. The generation of (t-butoxy)mesitylsilylene, (t-BuO)MesSi , upon... 

TABLE 3. n(Si)—>-3p(Si) absorption maxima of substituted silylenes (RR Si) in hydrocarbon matrices... 

FIGURE 8. Schematic orbital interaction diagram for silylenes having (a) n donors and (b) n groups as substituents. AEq, AE and AE2 denote the vertical excitation energies in the parent, an n-donor-substituted and a -substituted silylenes, respectively. Modified from Reference 176... 

The reaction of a nitrogen-substituted silylene (88) with a 2H-azaphosphirene (87) gave a bicyclic compound (89), which contains an Si-Si bond and a tungsten-bound carbene.96 Compound (89) was isolated in 62% yield and its crystal structure was described. However, when an extra phosphorus atom was inserted into the ring as in (90), the monocyclic (91) was isolated in 90% yield. A crystal structure for (91) was also reported. 

The preference of the cleavage (II) with a-phenylated disilanes is due to the higher stability of a phenyl-substituted silylene anion. The alkoxy ion primarily attacks the 3 silicon atom. 

Mixed substituted silylenes occur, such as Sime(Ome), SiHBr, SiHCl, SiHme. 

Inorganic substituted silylenes are formed in the reaction of silicon with silicon chloride527 540, 543> ... 

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