Mesoionic compounds: the role of intermolecular interactions in their crystalline design

Abstract

The quest for understanding crystal structures using supramolecular cluster demarcation has been applied to various uncharged compounds, and, more recently, it has also been applied to charged compounds in ammonium salts. Given this context, this study sought to expand this approach for intermediate compounds, between compounds with and without localized charges. The selected structures were mesoionic compounds, which have delocalized charges. This study raises some questions: do mesoionic compounds have intermediate characteristics between salts and uncharged molecules? Or are they similar to one of these models? A molecular and supramolecular investigation of mesoionic models was carried out to answer these questions. The study was based on demarcating the supramolecular cluster, enabling us to propose the crystallization mechanisms of twenty-three mesoionic compounds, in which stabilizing and destabilizing energies were observed. In addition, the Cluster Energy Efficiency (CEE) parameter was applied, allowing us to quantitatively evaluate the similarity between compounds containing both stabilizing and destabilizing energies in the crystalline lattice. The CEE data revealed that most mesoionic compounds have CCE = 1000, a characteristic comportment of uncharged compounds. In addition, the compound 2,3-diphenyl-1,3,4-thiadiazolium-5-thiolate (18) was synthesized and characterized to better understand the molecular and supramolecular behavior. Concentration-dependent NMR and LC–MS/MS experiments reveal the first aggregates in solution in the crystallization process of compound 18. The crystallization mechanisms evidenced six different crystallization patterns, and molecular electrostatic potential (MEP) allowed us to evaluate the different patterns of electrostatic potential dispersion of all compounds.