Substituent effects on the structure and aromaticity of 4-silatriafulvene

The structure and aromaticity of several 4-silatriafulvene derivatives are studied using correlated ab initio MO calculations. Electronegative substituents on silicon have been found to stabilize a nonplanar structure around the formal Si=C double bond, while pi-electron accepters planarize the molecule. To assess the aromaticity, the geometry index Sigma CC, the nucleus-independent chemical shift (NICS), and the Bird indices are employed. Appropriate substituents on 4-silatriafulvene to enhance the aromatic character have been shown to cause the significant decrease of the Si=C ct-bond strength. In cyclopropenylidene-1-silacyclopentadiene derivatives, both the three- and five-membered rings have low aromaticity, irrespective of the pyramidality of the molecular skeleton. Remarkably high aromaticity of the three-membered rings have been found in cyclopropenylidene-2-silaallene and 1-cyclopropenylidene-3-cyclopentadienylidene-2-silaallene.