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 (H(4)thba) 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, H(4)thba is a relatively strong acid (pK (a1) = 1.68). The car­box­yl­ate group in H(3)thba(−) is therefore considerably less basic than most car­box­yl­ates with intra­molecular hydro­gen bonds, conferring a rigid planar geometry upon the anion. These characteristics of H(3)thba(−) 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.