Neutral and Ionic Supramolecular Complexes of Phenanthridine and Some Common Dicarboxylic Acids: Hydrogen Bond and Melting Point Considerations

Supramolecular complexes of phenanthridine have been prepared with various dicarboxylic acids. Cocrystallization of phenanthridine with fumaric acid, succinic acid, and isophthalic acid produced neutral cocrystals. Proton transfer from maleic acid and oxalic acid to the phenanthridine moiety results in salts of these two acids. It was found that neutral cocrystals are formed when the ΔpKa value of the complex is smaller than 2.56, whereas salts are formed when ΔpKa is greater than 3.66. The crystal structures of supramolecular complexes have been determined. The structure of each compound depends on acid geometry, and the compounds may be described as dimers, trimers, or chains. A comparison of the melting points, hydrogen bonds, and densities of each molecular complex and the corresponding dicarboxylic acid is presented. Utilizing the approximate relative bond strengths of hydrogen bonds, a comparison of the number of hydrogen bonds in the acid and the cocrystal is possible.