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.