Siloles structure

SCHEME 3.33 Chemical structures of silole compounds as ETMs. 

Siloles, germoles, stannoles, and plumboles have been extensively reviewed.180 The aromaticity of these systems is still under scrutiny. In the study, the cyclopentadienyl anion had the highest aromatic character, with a NICS value of —14 and a Bird index of 100. Structures 138 and 139 also showed high and similar aromaticity indices (Scheme 59). For the planar silacyclopentadienyl anion 138, the NICS value is —10.2 and the Bird index 80, while the values for anion 139 are —10.9 and 77, respectively. 

Table 5 Structures and properties of silole-acetylene n-conjugated systems ...

Figure S Variation in structural parameters of siloles on cr- and -substitution...

TABLE 7. Experimental structural and magnetic properties of silole anions and dianions and conclusions regarding their degree of aromaticity... 

The known silole dianions are coordinated to the metallic counter ions either by rj5,rj5-bonding10a 82 or by r/1, r/ -bondinglob (Figures 8c and 8d). Pyramidality at Si is, of course, not a problem for the silole dianions where the Si is dicoordinated, and the known dianions are all planar (Figures 8c and 8d). The structural and magnetic properties of the silole dianions are consistent with charge delocalization and aromatic character of these species, as shown by the data collected in Table 7. 

NMR studies showed that the only anionic intermediate which is present in solution is dilithiosilole 59115. The marked downfield shift in the 29 Si NMR spectrum (S = +68.4 ppm) of 59 as well as the shielding of the 0 and Cp nuclei indicate, in agreement with calculations113, an aromatic character of this compound. This conclusion is supported by the solid state structure of 59 (Figure 10)116. The silole ring in 59 is planar and exhibits nearly equal C—C distances, as one would expect for a delocalized system. However, in contrast to the computational results113,116, the lithium atoms in 59 are not... 

The reaction of 1-chlorosiloles with alkali metals leads to unstable species that react with methyl iodide, ethyl bromide or chlorotrimethylsilane in the manner expected for the 1-silacyclopentadienide anion. Interestingly, X-ray quality crystals were isolated from the reaction with Li in THF and the molecular structure was shown to be that of a [2 + 2] head-to-tail dimer, which is formed by the 1,5-rearrangement of the anion in the silole ring185 (equation 70). 

The solid-state structure of the silolyl dianion is substantially affected by the nature of the substituents attached to the silole ring and of the metal alkali counterions. Stirring C4Me4SiBr2 with four equivalents of potassium in the presence of 18-crown-6 gives... 

In summary, the anions of group 14 metalloles (C4E rings) have either localized nonaromatic or delocalized aromatic structures, depending of the metal E, Si vs Ge and Sn, and on the substituents, methyl vs phenyl group. As pointed out by West and coworkers198, structural studies of metallole anions and dianions of this kind with different substituents will be of value. Additionally, it should be noted that experimentally observed structures in the solid state for silole and germole dianions are greatly influenced by the nature of the alkali metal counterion. 

A significant contribution by Tilley and coworkers, in addition to material cited earlier187 192, describes the synthesis, structure and electronic properties of various silole... 

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