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 (pKa1 = 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. More than 20 new compounds derived from 2,4,6‐trihydroxybenzoic acid (H4thba) have been synthesized, with structures that include discrete molecular units and chains, in addition to two‐ and three‐dimensional nets. Intramolecular hydrogen bonds between the ortho‐hydroxy groups and the carboxylate group in the H3thba− anion confer a rigid geometry upon the ligand which, when combined with the low basicity of the carboxylate group, limits the variety of metal‐binding modes.