Supramolecular associations in bisimidazole derivative/formic acid solvates

Although recrystallisation of a heteromeric solution is commonly used as a method of purification, it may also result in the formation of a salt.Salt formation is a technique often used by the pharmaceutical industry to enhance the physiochemical, formulation, biopharmaceutical, and therapeutic properties of a drug without modifying the basic chemical structure [1].It would be of great benefit to chemists and formulators to be able to predict the effect various salt formers have on a drug.As the properties of the salt are derived from its crystal structure, we have performed a systematic study of solid forms consisting of simple organic molecules with weakly interacting functional groups in order to gain more understanding of the process of salt formation.This study involved reacting a large number of simple nitrogen-containing compounds with α,ω-dicarboxylic acids (diacids), including alkane, alkene and hydroxy diacids.The work presented here describes sixteen salts (twelve synthesised and four obtained from the literature[2]) containing imidazole, 1-methylimidazole, 2-methylimidazole, 4-methylimidazole or 1,2dimethylimidazole.The products were characterised by single crystal X-ray diffraction and the intermolecular interactions between the functional groups in each of the crystal structures were compared.It was found that these sixteen salts all formed similar sheet structures which could be depicted schematically, incorporating the direction of the anion chains, and the orientation of the imidazole methyl group within the sheet.The deprotonation patterns of the diacids, the effect of the methyl group together with its position on the imidazole ring, and the importance of C-H…O hydrogen bonding interactions in the sheet structures are assessed.

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