Complexes of 2,4,6-trihydroxybenzoic acid: effects of intramolecular hydrogen bonding on ligand geometry and metal binding modes.

This article describes a series of more than 20 new compounds formed by the combination of 2,4,6-trihydroxybenzoic acid (H₄thba) with metal ions in the presence of a base, with structures that include discrete molecular units, chains, and two- and three-dimensional networks. As a result of the presence of two ortho-hydroxy groups, H₄thba is a relatively strong acid (pK_a1 = 1.68). The carboxylate group in H₃thba^- is therefore considerably less basic than most carboxylates with intramolecular hydrogen bonds, conferring a rigid planar geometry upon the anion. These characteristics of H₃thba^- significantly impact upon the way it interacts with metal ions. In s-block metal compounds, where the interaction of the metal centres with the carboxylate O atoms is essentially ionic, the anion bonds to up to three metal centres via a variety of binding modes. In cases where the metal ion is able to form directional coordinate bonds, however, the carboxylate group tends to bond in a monodentate mode, interacting with just one metal centre in the syn mode. A dominant influence on the structures of the complexes seems to be the face-to-face stacking of the aromatic rings, which creates networks containing layers of metal-oxygen polyhedra that participate in hydrogen bonding. This investigation was undertaken, in part, by a group of secondary school students as an educational exercise designed to introduce school students to the technique of single-crystal X-ray diffraction and enhance their understanding of primary and secondary bonding.