Sigma-delocalized aromatic species formed from cyclic arrays of hypervalent main-group element species

Abstract

Abstracf -Evidence suggests that the dication formed by oxidation of hexaiodobenzene may reasonably be called a sigma-delocalized aromatic species.A variety of experimental results supports the suggestion that two electrons are removed from the sigma-delocalized HOMO of hexaiodobenzene to form the 10-electron (4n+2) aromatic dication.The higher field chemical shift observed for the 13C NMR singlet upon conversion of hexaiodobenzene to the dication, an upfield shift of 42.6 ppm, is best described by the proposed sigma-delocalized ring-current of the circular array of six iodines, calculation of the graph-theoretical resonance energy for the hexaiodobenzene dication, 0.013 PCC per delocalized sigma electron, provides evidence for sigma-delocalized aromatic stabilizanon for the cyclic array of coplanar overlapped p-orbitals of the six iodines of the dication.The sigma-aromaticity of the dication is similar to the pi-aromaticity of the 10-pi-electron species, azulene (0.015 PCC per electron).The six sigma-delocalized orbitals of the six iodines of the dication are arranged similarly to the pi-delocalized orbitals of benzene but are not as widely separated as are those of benzene.This provides lower frequency absorptions of the dark blue solution of the dication ditriflate with two peaks in the near infrared region (985 nm and 1155 nm).Current evidence supports the suggested 10-electron sigma-delocalized aromatic species, the hexaiodobenzene dication, 1.