Multi-component hydrogen-bonding salts formed between imidazole and aromatic acids: Synthons cooperation and crystal structures

Imidazole base was crystallized with different aromatic carboxylic acids 2,4-dihydroxybenzoic acid, 5-chlorosalicylic acid, and 1,8-naphthalic acid, affording three new binary molecular organic salts of [(C3H5N2+)center dot(C7H5O4 (-))] (1), [(C3H5N2 (+))center dot(C7H4O3Cl-)]-C7H5O3Cl (2), and [(C3H5N2 (+))(-)(C12H7O4 (-))] (3). Proton transfer occurs from the COOH of carboxylic acid to nitrogen of imidazole in all complexes (1-3), leading to the hydrogen bond N-HaEuro broken vertical bar O in all structures. To our knowledge, the recognition pattern between the carboxylic acid group and imidazole (acid-imidazole synthon) is less well-studied so far. The cooperation among COOH, COO- and imidazolium cation functional groups for the observed hydrogen bond synthons is examined in the three structures. Generally, the strong N-HaEuro broken vertical bar O and O-HaEuro broken vertical bar O hydrogen bonds define supramolecular architecture and connectivity within chains, while weaker C-HaEuro broken vertical bar O hydrogen bonds play the dominant role in controlling the interactions between layers in these novel organic salts. Thermal stability of these compounds has been investigated by thermogravimetric analysis (TGA) of mass loss.