Theoretical Study of the Si/C Mixed Benzenes and Their Major Valence Isomers

Changes in the structures and relative stabilities of benzene and the major valence isomers fulvene, Dewar benzene, benzvalene, and prismane on the substitution of the skeletal carbons with silicon atoms in a stepwise manner have been systematically investigated by ab initio molecular orbital methods. The number and arrangement of the formal CC, CSi and SiSi double bonds were found to be considerably important factors controlling the structure and relative stability of a series of the compounds for fulvene, Dewar benzene, and benzvalene analogues. Also, the applicability of “π-electron-holding ability” proposed for the planarity of heavy ethenes or polyenes in our previous study to the benzene analogues was confirmed. On the other hand, the angle strain is the key for the properties of prismane analogues, as expected. The noteworthy point is that benzene-type compounds tend to have a planar structure in comparison to fulvene analogues with a partially conjugated structure, and the stability relative to the other isomers is not extremely lowered even though the skeletal silicon atoms increase. Among them, the Si/C alternately mixed benzene (1,3,5-trisilabenzene) is an attractive “intermediate compound” between benzene and hexasilabenzene, as it has a planar and electronic structure similar to those of benzene. Therefore, in order to provide some useful information for its synthesis and isolation, the thermodynamic and kinetic stabilities were investigated. As a result, the Si/C alternately mixed benzene was found to be highly reactive for addition reactions, especially for a dimerization. However, as a cure for that, some promising cross-linking substituents considerably effective at preventing isomerization and dimerization were finally proposed.