Supramolecular salts assembled by melamine and two organic hydroxyl acids: synthesis, structure, hydrogen bonds, and luminescent property

The reaction of 1,3,5-triazine-2,4,6-triamine (melamine, MA) with 3-hydroxy-2-naphthoic acid (3H2NA) and 6-hydroxy-2-naphthoic acid (6H2NA) provided two ionic supramolecular salts, I and II. These supramolecular salts were constructed based on N–H⋯N, N–H⋯O and O–H⋯O hydrogen bonding interactions and confirmed by single-crystal X-ray diffraction. These structures revealed the typical monoprotonated propensity of MA when reacting with organic hydroxyl acids, and formed assembled aggregates, such as chain and reticular structures. The analysis of weak interaction networks in supramolecular salts reflected the relative approach of the –COOH group toward the N atom in the triazine ring. The assembly pattern and geometric structure were explored by density functional theory (DFT) analysis. As predicted by molecular electrostatic potential analysis (MESP), the binding mode and stable configuration were obtained. The proton-transfer phenomenon was predicted using the pKa method. The quantum theory of atoms in molecules (QTAIM) suggested that the hydrogen bonds presented in salts I and II were moderate in nature except for N5–H5A⋯O2 in salt II. Although classical hydrogen bonds dominate for the synthon formation, weak dispersive forces cannot be ignored. The dnorm-mapped Hirshfeld surface of MA established the strong characteristic features of weak interactions, which were reflected in corresponding 2D fingerprint plots (FPs). The UV-vis absorption and luminescent property were investigated, and the as-obtained salts exhibited different behaviors from that of their precursors. In addition, thermal behaviors of salts I and II were investigated by TGA analysis in detail.