Halocarbon matrices

In the case of halocarbon matrices, the SOMO is of the a type and the cleavage is facilitated by the antibonding nature of the SOMO. In other ions, a or a orbitals of scissile bonds interact with a ti or 7i orbital, causing them to be weakened. Accordingly, radical anions of benzyl halides may generate benzylic radicals with loss of a halide ion. Conversely, benzylsilane radical cations may form benzylic radicals with loss of a silyl cation (Fig. 6.15). 

In contrast to the radical cations of strained-ring cycloalkanes, the cyclopentane radical cation, c-CsHio , formed by electron transfer to radiolysis-induced holes in halocarbon matrices, had a simpler spectrum. A triplet with uh = 2.5 mT (2H) was attributed to a localized species with Cj symmetry. The unpaired electron was assigned to a W-shaped cr-orbital, involving C5-C1-C2, and the two equatorial protons at C5 and C2 [80, 88, 89]. At temperatures above 77 K, all ring protons become equivalent, most probably as a result of processes such as ring inversion, or pseudo-rotation around the C5-axis [89]. 

Takemura and Shida54 prepared the allene radical ion by /-radiolysis of halocarbon solid solution of allene at low temperatures and showed that the radical cation has a lower D2 structure than the precursor with a skew angle of 30-40°. Kubonzo and coworkers55 56 produced by /-radiolysis in a low-temperature halocarbon matrix several derivatives of the allene radical cation, i.e. the radical cations of 1,2-butadiene, 3-methyl-1,2-butadiene,... 

In addition to rearrangements, many radical ions undergo cleavage reactions. We have already encountered such a reaction Electron attachment to halocarbon matrix molecules results in fragmentation to a halide ion and a free radical. 

A recent and quite original approach to the synthesis of RejX, cluster complexes involves the cocondensation of rhenium atoms, generated from a positive hearth electron-gun furnace, and reactive halocarbons such as 1,2-dibromo- (or dichloro-) ethane. Extraction of the reaction matrix at room temperature with THF gives Re3X9(THF)3 (X = Cl or Br) in yields of 90%.185 This strategy will likely see further developments in the future. 

Summary The stable silylene 1 reacts with halocarbons like dichloro-, trichloro-, and tetrachloromethane at room temperature under formation of the disilane 3. The mechanism of this reaction is still unknown. Results of recent ab initio calculations do not agree with the experimental observations. In this investigation we have studied the photochemical behavior of the stable silylene 1 in a diluted ai]gon-tetrachloromethane matrix at 10 K as well as the thermal properties of 1 in pure solid CCU in the temperature range of 10 - 200 K. The formation of a trichloromethyl radical in both types of experiments indicates that electron transfer may play an important role in these processes. 

In the Giessen group, the spectral properties and reactivity of Lewis type complexes between halomethylsilylenes and methyl halides, generated in co-condensation experiments between silicon atoms and methyl halides, have been studied [5]. These less stabilized silylenes add the methyl halides under transformation into the stable dihalodimethylsilanes. Encouraged by these results, we believed that matrix isolation spectroscopy may also be a very powerful tool to give new insights into the surprising behavior of the stable silylene 1 with halocarbons. 

New Insights into the Halophilic Reaction of a Stable Silylene with Halocarbons A Matrix -... 

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