Siloles aromaticity

Reductive cleavage of the Si-Si bond of bis(siloles) with alkali metals resulted in the formation of silole monoanions (Scheme 2.49). Silole monoanions were found to be aromatic on the basis of NMR spectral data and calculations. 

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

Silole anion Bond length (A) <5(29Si) (ppm) S(i3C) (ppm) Aromaticity Ref. 

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... 

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. 

The incorporation of siloles in polymers is of interest and importance in chemistry and functionalities. Some optoelectronic properties, impossible to obtain in silole small molecules, may be realized with silole-containing polymers (SCPs). The first synthesis of SCPs was reported in 1992.21 Since then, different types of SCPs, such as main chain type 7r-conjugated SCPs catenated through the aromatic carbon of a silole, main chain type cr-conjugated SCPs catenated through the silicon atom of a silole, SCPs with silole pendants, and hyperbranched or dendritic SCPs (Fig. 2), have been synthesized.10 In this chapter, the functionalities of SCPs, such as band gap, photoluminescence, electroluminescence, bulk-heterojunction solar cells, field effect transistors, aggregation-induced emission, chemosensors, conductivity, and optical limiting, are summarized. 

Closely related to the Birch reduction, benzene, and other aromatic compounds afforded reduction-silylation products (equation 10). Electron-transfer reactions of bis(phenylethynyl)dimethylsilane gives 2,5-dianion of 3,4-diphenylsilole that is a useful intermediate to variously substituted siloles (equation 11). ... 

Silole dianions, which are of current interest as aromatic species [1 - 3], behave as nucleophiles in simple displacement reactions, generally yielding simple trapping products [4 - 6]. However, the reaction of l,l-dilithio-2,3,4,5-tetraphenylsilole (1) with l,l-dichloro-2,3-diphenylcyclopropene (2) takes a most unexpected course, leading to ring-opening of the cyclopropene ring and ultimate formation of a tetracyclic disilapentalene and a stable bis-silole diradical [7]. 

Silole and Germole Dianions and their Dilithium Derivatives Are they Aromatic ... 

Summary According to structural, thermodynamic, electronic, and magnetic criteria silole and germole dianions show indications of cyclic delocalization. The degree of aromaticity in the silole- l(Si) and germole dianions l(Ge) is of the same magnitude as that in the cyclopentadienyl anion. 

In this paper we report on our studies of the bonding in the free silole and germole dianions and their lithium derivatives using ab initio quantum mechanical methods [5], Several criteria for aromaticity, suggested recently by Schleyer et al. [6], were evaluated to test the degree of delocalization in Group IVa metallole dianions. 

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