Expanding the Scope of Hydroxyl-pyridine Supramolecular Synthon to Design Molecular Solids

Abstract

Supramolecular synthons, i.e., robust noncovalent interactions, are equivalents of the covalent bonds of organic synthesis in supramolecular chemistry. Exploring the novel synthons and further utilizing the extant ones are imperative to develop supramolecular chemistry further. Hydroxyl-pyridine, a neutral interaction, is a serendipitous but robust synthon observed in molecular complexes and cocrystals. Herein, we utilize this intermolecular interaction with a predesign for the development of new molecular complexes [(DHBA-2H)(4,4-BPY)] (1) and [(THBA-3H)(4,4-BPY).H2O] (2), that have been further functionalized by the reaction with 4-iodo aniline to yield [4-I-DHBA-2H)2(4,4-BPY)] (3) and [(4-I-THBA-3H)(4,4-BPY)] (4). Molecular complexes 1 and 2 build upon a centrosymmetric R44(30) and non-centrosymmetric R66(32) synthons to form layered and reticular aggregates, respectively, which are stabilized by CH···π, CH···O, and π–π contacts. The halogen interactions in the iodinated cocrystals 3 and 4 direct the supramolecular aggregation into zigzag and intriguing arc-interwoven three-dimensional solids, respectively. Cocrystals 1–4 provide an opportunity to investigate the influence of halogen atoms/bonding, on the execution of a hydroxyl-pyridine synthon. Incipience of color in cocrystals 1 and 2 and a remarkable color change in 3 and 4 are validated by diffuse reflectance (DR-Uv) studies. Interestingly, 3 exhibits irreversible thermochromism near its melting point which is also accompanied by emission turn-on. 1–4 show enhanced aqueous solubility via-a-vis an acid former, which is strikingly 17, 943% for 3. Intermolecular interactions responsible for supramolecular aggregation are investigated in detail utilizing Hirshfeld, quantum theory of atoms in molecules (QTAIM), and noncovalent interaction index (NCI) analyses. The structural, physicochemical, and computational studies are compared to understand the structure–property relationship.