σ-Aromaticity in Hexa-Group 16 Atom-Substituted Benzene Dications: A Theoretical Study

Abstract

C6I6(2+) has been reported to have a σ-aromatic character since removal of two σ anti-bonding electrons localized on iodines results in fulfilling Hückel (4n+2) rules for I6(2+) as well as C6 parts. To search for molecules possessing similar character, hexa-group 16 atom-substituted benzene dications C6(ChH)6(2+) (Ch = S, Se, Te) and their derivatives are examined for aromatic character by using nucleus-independent chemical shift (NICS). For these dications, in which iodines in C6I6(2+) are replaced by group 16 atoms, negative NICS values larger in magnitude than for benzene are found when a σ anti-bonding orbital localized on group 16 atoms is unoccupied. To clarify the origin of large negative NICS values, they are decomposed into individual molecular orbitals. It has been shown that both π bonding orbitals on C6 and σ bonding orbitals on Se6 or Te6 contribute to the negative NICS values, indicating that the aromaticity of these dications have a substantial σ character as well as π characters. Aromaticity of group 14 and 15 atom-substituted benzene dications is also discussed.

Double aromaticity arising from σ- and π-rings

“…However, triplet ground states have been predicted for these dications, and accordingly, their isolation as stable entities in the condensed phase should be relatively difficult ( Fig. 1c) [31][32][33] . The two-electron oxidation of hexakis(dimethylamino)benzene (5) afforded the corresponding dication, which revealed a alternation of the C−C bonds in the central benzene ring 34 , indicative of a considerable contribution from a quinoidal resonance structure, which indicates the absence of double aromaticity.…”

Calculations of current densities for neutral and doubly charged persubstituted benzenes using effective core potentials

“…The aromatic character of isoelectronic benzenes with six main-group substituents have been studied computationally by Hatanaka et al to assess whether the corresponding doubly ionised benzenes sustain a σ ring current along the outer edge of the dication. 7 Nucleus independent chemical shift (NICS) calculations showed that some of the investigated dications sustain a stronger ring current than benzene as judge from the calculated NICS values. 7 Ciofina et al investigated computationally the σ delocalisation and degree of aromaticity of C 6 I 6 , C 6 Cl 6 , and the corresponding dications by calculating nu-clear magnetic resonance (NMR) chemical shifts and NICS values.…”

“…7 Nucleus independent chemical shift (NICS) calculations showed that some of the investigated dications sustain a stronger ring current than benzene as judge from the calculated NICS values. 7 Ciofina et al investigated computationally the σ delocalisation and degree of aromaticity of C 6 I 6 , C 6 Cl 6 , and the corresponding dications by calculating nu-clear magnetic resonance (NMR) chemical shifts and NICS values. 5 Havenith et al studied the electron delocalisation and ring currents using current density mappings via the ipsocentric approach and a multi-center delocalisation index.…”

Efficient Synthesis of 1,4‐Bis‐heteroatom‐substituted Tetraselanylbenzenes via 1,4‐Dilithiation of Hexaselanylbenzene and Investigation on Their Electronic Properties

“…The voltammograms of 1 , 5 a and 5 c exhibit reversible one‐ and two‐electron oxidation processes, indicating that the corresponding dicationic species, at least, are stable under the electrochemical conditions. In our previous work, the dication of 1 has been considered to be a σ‐aromatic compound that has a circular σ‐symmetric orbital, and its dicationic charges delocalize on the σ‐orbital . As judged from this MO feature in 1 , the dicationic species of 5 a and 5 c are also predicted to be stable due to similar characteristics found in the HOMOs of the model compounds 8 a and 8 d (Figure ).…”

“…Besides the interest in the benzene core, interactions between the circularly fixed six heteroatoms in the benzene ring have also been studied . We recently reported that hexa‐group‐16 or group‐17 atom‐substituted benzenes would be candidates for precursors of σ‐aromatic compounds (Figure ) . The incorporation of six heteroatoms into benzene induces σ‐type orbital interactions between neighboring heteroatoms, resulting in the formation of σ‐symmetric delocalized orbitals.…”