Metalloles Germoles, Siloles

The unique photophysical properties of metalloles (mainly siloles and germoles) forced the development of new preparation methods of these compounds, some of which will be addressed here again. Thus, the reductive cyclization of dialkyn-l-ylsilanes 142 gives first 2,5-dilithiosiloles 143 which possess an enormous synthetic potential, also opening the way to different 2,5-(bisaryl)metalloles (via cross-coupling reactions), known to show outstanding optical properties. 

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. 

Considering the peculiar contribution of the central silicon atom to the --electronic structure of silole derivatives through a -n conjugation, other Group 14 metalloles are also of interest. To elucidate the effects of the central Group 14 elements, we have prepared a series of 2,5-dithienyl-substituted Group 14 metalloles, cyclopentadiene 30, siloles 26m and 27, germole 31 and stannole 32, and compared their photophysical... 

The metallole ring is shown theoretically to be an analogue of cyclopentadiene and can form both rf - and 7 -complexes with transition metals. Thus, siloles and germoles react with Fe(CO)s or Fe2(CO)9 to give... 

The reaction between 1,4-dilithiobutadienes or 2,2 -dilithiodiphenyl is a well-known and successful route to metalloles, although the choice of substituents in the 2-5 positions is somewhat limited. However, various substituents can be introduced at the metal. Thus, dilithiated butadienes 166 and 168 react with halo- or methoxy-derivatives of Si and Ge to give siloles 167 and germoles 169 in high yield <2000JCD1049, 1998JOM(559)73>. 

Digermacyclohexadienes 244 give germoles 245 as sole products by irradiation in benzene <19980M1782>. When silagermacyclohexadiene 246 is treated in the same way, the formation of two metalloles is possible, but germole 247 is formed as a major product (84%) compared with silole 248 (9%). 

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. 

According to structural, thermodynamic, electronic, and magnetic criteria the metallole dianions (and also their dilithio complexes) show indications of cyclic delocalization. The degree of "aromaticity" in silole and germole dianions is of the same magnitude as in the classic aromatic cyclopentadienyl anion... 

---