Disilenes solid-state

The stable disilenes are pale yellow to orange-red in the solid state and have electronic absorption maxima in solution between 390 and 480 nm (Table I). The longest wavelength absorptions have been assigned to the ir-ir transition.28... 

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

In 1997, West and coworkers154 reported the solid-state NMR study of the 29Si chemical shift tensors for a series of disilenes with different substitution at the Si=Si double bond... 

After review OW, the molecular structures in the solid state of 13 acyclic di-silenes, 13 endo- and exocyclic disilenes, 4 silicon-based dienes, 6 disilenes with radical, cation, and anion center, and one disilyne have been determined by X-ray analysis. Although the structural characteristics of a number of these new disilenes are summarized in a recent review by Weidenbruch,9 the structural parameters of all new disilenes reported after review OW are listed in Tables I-IV. 

The color of stable disilenes in the solid state ranges from pale yellow to blue. The longest absorption maxima of acyclic disilenes in solution are assignable to the it -mi transitions and are strongly dependent on the electronic and steric effects of substituents as shown in Tables I-IV. The Amax values are 393-493 nm for tetraalkyldisilenes, 394-452 nm for dialkyldiaryldisilenes, 400-460 nm for tetraaryldisilenes, 468 183 nm... 

Chemical Shift Tensors for Disilenes Determined by Solid State NMR Spectroscopy3... 

Stable diaminosilylene 132 (R = /- Bu) does not dimerize to the corresponding tetraaminodisilene but undergoes an insertion reaction giving 131, which further dimerizes to diaminodisilyldisilene 62. Disilene 62 is stable in the solid state but equilibrates with stable silylene 132 (R = /-Bu) via 131 in solution [Eq. (52)].33,34 Intermediacy of 131 is evidenced by the reaction of crystals of 62 with methanol giving a methanol adduct of 131, 133. 

Stable disilenes are usually highly reactive toward oxidation and reduction because the 7i and n levels of disilene are much higher and lower than those of ethylene, as shown theoretically (Section III.A) and by the electrochemical oxidation and reduction potentials (Section III.E). Several stable disilenes are however stable in air for a long period. Disilene 27 undergoes very slow decomposition (half-life ca. 84 h) in wet THF solution at rt,60,61 and air oxidation of disilene 63 is completed in benzene at rt for a week.35 Disilene 27 survives for 4 months in the solid state. Since the oxidations and reductions of disilenes and their mechanisms have been extensively discussed in review OW, we mostly show herein the results of recent studies. 

Heating of /ram-tricyclic disilenes 32 at 110 °C in [d10]-/>-xylene or in the solid state leads to tetracyclic compound 33 in quantitative yield (see Section 2.23.4) <2006AGE6371>. 

The longest wavelength absorptions of the tetraaryldisilenes occur in the region of 400-430 nm. The three tetrasilyldisilenes 16-18 also exhibit absorption bands between 412 and 425 nm that are attributed to tt — tt transitions. Disilene 18, the sterically most heavily overcrowded compound, exhibits an additional absorption band at 480 nm. The surprising color change from yellow in the solid state to deep red in solution suggests that 18 adopts a twisted form in solution in order to reduce the steric strain39. 

Some selected solution and solid state 29Si NMR data of disilenes are presented in Table 3. 

The 29Si NMR chemical shift tensors accessible from solid state NMR spectra also provide important insights into the nature of the chemical bonds in disilenes. After the disilene 962 and some other disilenes63 had been investigated by this method in earlier studies, West and coworkers recently reported the solid state NMR spectra of seven disilenes with widely differing substitution patterns as well as of two solvates of compound 9 (Table 3). These results in combination with MO calculations on model substances support a classical jr-bonding model for the Si=Si bonds64. 

At lower temperature in solution, the aerial oxidation of disilenes initially furnishes the 3,4-disiladioxetanes 119 together with small amounts of the disilaoxiranes 120, both of which react further in the solid state and in solution to give the cyclodisiloxanes 121 (Scheme 4). 

The molecular structures of several disilenes in the solid state have been determined by X-ray crystallography. All of them are characterized by very bulky alkyl or aryl substituents, R. The structures of (tert-butyl)(Mes)SiSi(fert-butyl)(Mes), Mes = phenyl-2,4,6-trimethyl (A), and (Mes)2SiSi(Mes)2 (B) are shown in Eig. 14.7. The central C2SiSiC2 framework of the former is planar as expected for an ethene analogue. The SiSi bond distance is 214 pm, about 9% shorter than the single bond distance in Me3SiSiMe3, 234 pm. (By comparison the CC double bond in ethene is 13% shorter than the single bond in ethane.)... 

The synthesis of a trisilaallyl cation with an allylic cation character is described. The allylic cations are considered important intermediates in organic transformations. The preparation of the trisilaallyl cationic species, cyclotetrasilenylium ion is performed by the reaction of tetrakis(di-tbutylmethylsilyl) disilene (6) with [EtsSifbenzene)]" - TPFPB TPFPB = tetrakisfpenta-fluorophenyl)borate in toluene (Scheme 6.3.1.1). The cyclotetrasilenylium ion is demonstrated to exist as a cation free of anion interaction in the solid state (5). 

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