In the quest of Hückel–Hückel and Hückel–Baird double aromatic tropylium (tri)cation and anion derivatives

Besides the most common form of aromaticity involving a π-ring, hexaiodobenzene and hexakis(phenylselenyl)benzene dications also present σ-aromaticity in outer ring formed by main group substituents. These two compounds are considered σ- π-double aromatic, their characterization is special interest to fields organic structural chemistry. In this work, we decided explore double substituted tropylium cations for three reasons: (i) seven neutral halogen substituents will, without oxidation, lead 14 σ-electrons (a 4n + 2 Hückel number); (ii) highly stable can be easily generated experimentally; (iii) whereas benzenes distances between optimized structures or X-ray crystals too large allow strong σ-aromaticity, these expected shorter cations. Yet, instead found that geometries puckered, meaning delocalization both π- σ-systems lost. Our results, which include anion trication singlet triplet state, show there need open lone pair hole oxidation generate σ-aromaticity. Among systems studied, only C7Br7+3 with an internal aromatic external incipient Baird Br7 shows This result, however, functional-dependent reveals 3C7Br73+ at borderline onset aromaticity.