Evaluation of mesoionic compound aromaticity using the HOMHED index

Abstract

Mesoionic compounds are a class of heterocyclic compounds whose aromaticity has ignited considerable debate within the scientific community. This study aims to systematically evaluate the aromaticity of 25 mesoionic compounds across eight structural classes, utilizing experimental X-ray diffraction data in conjunction with the HOMHED geometric index to clarify this issue. The findings reveal a wide range of aromaticity (0.494–0.976), influenced by the type of heteroatoms in the heterocycle and exocyclic positions. Compounds containing oxygen atoms typically exhibit lower aromaticity, with HOMHED values below 0.7. In contrast, those containing nitrogen and boron atoms, especially in exocyclic positions, show higher aromaticity, with HOMHED values exceeding 0.9. Remarkably, compounds featuring exocyclic boron atoms displayed the highest aromaticity, with HOMHED values between 0.951 and 0.976. Compound 25 stood out with the lowest aromaticity, having a HOMHED value of 0.494, arising as a missing link, representing an important factor in elucidating the boundaries of the continuum of aromaticity. The study found that, on average, HOMHED values increase in the order of heterocyclic atom (Y): O1 < S1 < N1 and exocyclic atom (X): O6 < S6 ≈ N6 < B6. The research compares two canonical forms, the aromatic structure (AS) and the betaine structure (BS), to assess their impact on mesoionic compounds' overall resonance and aromatic character. Compounds with higher aromaticity demonstrated increased resonance in the D-A-Y and D-E-C-Y fragments, indicating a predominant AS contribution, whereas those with lower aromaticity showed a stronger BS influence. By introducing the AS index (I^AS) and the BS index (I^BS), this study quantitatively distinguishes the contributions of each structure. The findings challenge the binary view of mesoionic compounds as merely aromatic or non-aromatic, suggesting instead that these compounds exhibit varying degrees of aromaticity, as a continuum. The study concludes that the appropriate question is not whether mesoionic compounds are aromatic, but rather, ‘How aromatic are mesoionic compounds?’.