Silicon-Containing Formal 4 pi-Electron Four-Membered Ring Systems: Antiaromatic, Aromatic, or Nonaromatic?
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Density functional theory calculations (B3LYP) have been carried out to investigate the 4p-electron systems of 2,4-disila-1,3-diphosphacyclobutadiene (compound 1) and the tetrasilacyclobutadiene dication (compound 2). The calculated nucleus-independent chemical shift (NICS) values for these two compounds are negative, which indicates that the core rings of compounds 1 and 2 have a certain amount of aromaticity. However, deep electronic analysis reveals that neither of these two formal 4p-electron four-membered ring systems is aromatic. Compound 1 has very weak, almost negligible antiaromaticity, and the amidinate ligands attached to the Si atoms play an important role in stabilizing this conjugated 4p-electron system. The monoanionic bidentate ligand interacts with the conjugated p system to cause p-orbital splitting. This ligand-induced p-orbital splitting effect provides an opportunity to manipulate the gap between occupied and unoccupied p orbitals in conjugated systems. Conversely, compound 2 is nonaromatic because its core ring does not have a conjugated p ring system and does not fulfill the requirements of a Huckel system.
CitationCHEMISTRY-A EUROPEAN JOURNAL，2012,18（24）：7516-7524