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

Abstract

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.

Hands-on X-ray Crystallography: A Research-Based Elective Program for Secondary School Students

“…Furthermore, the research provided an insight into factors that influence the formation of networks involving metal cations and organic anions and two peer-reviewed papers describing the results have been accepted for publication in the International Union of Crystallography's journal Acta Crystallographica Section C: Structural Chemistry with the students listed as coauthors. 18,19 Although the reactions performed by the students used reagents that were purposely chosen for their ready commercial availability and simple structures, the compounds they obtained were remarkably different and inherently interesting to scientists working in the areas of crystal engineering and coordination chemistry, with structures that include discrete molecular units and chains, in addition to two-and three-dimensional networks. The structures of two of the compounds are shown in Figure 5.…”

“…While the results from the second year of the program were published in 2022, 18 the report of the results from the first year of the program was published in 2021. 19 The 2021 report has attracted pleasing interest from the scientific community within the first 18 months of its publication in Acta Crystallographica Section C, with over 1000 downloads. Both reports were ranked among the "most read" of the articles in the journal.…”

“…The students are recognized individually in the database for their discoveries. Furthermore, the research provided an insight into factors that influence the formation of networks involving metal cations and organic anions and two peer-reviewed papers describing the results have been accepted for publication in the International Union of Crystallography’s journal Acta Crystallographica Section C: Structural Chemistry with the students listed as coauthors. , …”