Disilenes solution-observable

Thermochromism was already noted for the first stable disilene l.2a In dialkyl- or bis(trimethylsilyl)diaryldisilenes, the size of the aryl groups affects the thermochromism Tip-substituted ones (9,10) are thermochro-mic,10 while Mes-substituted ones (3,4) are not.9 Interestingly, tetrakisftri-alkylsilyl)disilanes 22 and 23 are highly thermochromic hexane solutions of 22 and 23 are light yellow below 0°C but dark red above 50°C.21 These observations were interpreted in terms of a thermal equilibrium between the bent and twisted conformations. Compound 24, having an absorption band at 480 nm, is dark red in solution even at room temperature. 

Phenyltrimethyldisilene (15) and (E)- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (16) were also observed as transient absorption spectra by laser flash photolysis of the precursors in methylcyclohexanes28. The absorption band at 380 nm, assigned to the disilene 15, reached maximum intensity at ca 10 ns after the excitation and then started to decrease. The half-life assigned to 15 was 700 ns. The logarithm of the decay profile of the transient absorption at 380 nm versus time shows a very good linear relationship, indicating that the decay of the transient absorption fits first-order kinetics. This result shows that intramolecular isomerization or proton abstraction from the solvent is the origin for the decay of the disilene 15, which survives in solution only for several nanoseconds. 

The thermochromic behavior, first observed for the classical disilene 9, has also been found for other disilenes. Thus, the Z- and E-forms of 25 exhibit a darkening of their colors with increasing temperature. In the case of some of the tetrasilyldisilenes, this even occurs in solution at room temperature39. 

The electronic spectrum of 139 clearly reveals that the conjugation between the two halves of the molecule remains intact in solution. The longest wavelength absorption at 518 nm (e = 25 700) is bathochromically shifted by ca 100 nm in comparison to those of other disilenes with similar substitution patterns and thus exhibits a similar red shift to that observed between ethenes and buta-1,3-dienes131. 

Absolute rate constants and Arrhenius parameters have been determined for the thermal E,Z-isomerization of the stable disilene derivatives 92-96 in deuteriated aromatic solvents or THF-ds solution by XH or 29Si NMR spectroscopy133-136. With 1,2-dialkyl- and 1,2-diamino-l,2-dimesityldisilenes such as 92a-94, the (E)-isomers are considerably more stable than the (Z)-isomers, and so rate constants for E,Z-isomerization were determined after first generating mixtures enriched in the (Z)-isomer by UV-irradiation of the (El-isomer, and then monitoring the recovery of the solution to its equilibrium composition. On the other hand, little difference in thermodynamic stability is observed between the (Eland (Z)-isomers of tetraaryldisilenes such as 95a,b, and E,Z-isomerization kinetics were hence determined starting from solutions prepared from the individual, pure (or almost... 

More commonly the disilene is also unstable and undergoes further dimerization or polymerization below room temperature. Attempts to stabilize silylenes sterically have met with only limited success. The best example is the silylene 1, which could be observed in solution but not isolated [17]. For the heavier elements of group 14, dicoordinate compounds are well-known, and in 1991, a stable dicoordinate carbene 2 was reported [18] by Arduengo. The latter compound provided the key to the synthesis of a stable silylene [7]. 

When dry ammonia is passed over the dark red solution of 6, yellow crystals of the twofold 1,2-addition product 19 are formed in high yield. This result is somewhat surprising because attempted additions of ammonia to the disilene 3 were unsuccessful. This observation again reflects the increased reactivity of 6 in comparison to the isolated double bond in 3 and other disilenes. 

Id usually appears as a thin film on a glass surface, and until now we have not succeeded in obtaining a single crystal for the X-ray analysis. However, there are some reasons to speculate about its structure. For disilenes of the type RR Si=SiR R, photochemical cis-tram isomerization in solution was shown to occur [11], the trans isomer being predominant under equilibrium conditions. Of course, symmetrically substituted 1 crumot have real cis—trans isomers, but, by analogy, a similar equilibrium with predominance of a conformer, close to the tram one, seems likely in solution. This assumption is eonfirmed by Raman polarization measurements for a solution of 1 in hexane, because the selection mles observed for the conformer predominant in solution are consistent with C2h symmetry, that is, with a quasi-/ra s structure of this conformer [12]. As both the Raman and UV-Vis absorption and fluorescence spectra of solid Id are similar to those of 1 in hexane solution [1,12], we can suggest for Id also a quasi-/ra/is structure as shown in Fig. I. 

Crystals of the disilene (216 mg, 0.32 mmol) and the powder of tBuLi (25 mg, 0.39 mmol) were placed in a reaction tube inside a glove box. Dry, oxygen-free THF (2 mL) was introduced by vacuum transfer, and the mixture was warmed from -78°C to room temperature over 2 h. A rapid color change from blue to intense red was observed. The solvent was evaporated. The resulting solids were washed with hexane several times. After evaporation of solvent, toluene (1.5 mL) was added, and the solution was... 

---