Experimental and theoretical investigation of 1,3-bis(3-hydroxypyridinium)propane dibromide dihydrate, tetrabromozincate hydrate, and tetrabromocuprate

Three gemini hydroxypyridine salts: 1,3-bis(3-hydroxypyridinium)propane dibromide dihydrate (1), 1,3-bis(3hydroxypyridinium)propane tetrabromozincate hydrate (2), and 1,3-bis(3-hydroxypyridinium)propane tetrabromocuprate (3) were synthesized and characterized by X-ray diffraction, FTIR, Raman, NMR, and DFT methods. The dibromide salt (1) crystallizes with two water molecules. One of the two bromide anions is connected to the hydroxyl group of one pyridinium cation moiety through the O(10)-H(10)& sdot;& sdot;& sdot;Br(2)- hydrogen bond of 3.146(2) & Aring;, while the other bromide anion, Br(1)- is surrounded by three water molecules. In 1,3-bis(3hydroxypyridinium)propane tetrabromozincate (2) the water molecule acts as a bridge between the hydroxypyridinium cation and the tetrabromozincate anion, while the tetrabromocuprate anion interacts directly with the hydroxypyridinium cation. The molecular structures of the studied 1,3-bis(3-hydroxypyridinium)propane derivatives 1a, 2a, and 3a were optimized using the APF-D/6-311++G(d,p) method. The optimized structure of salt 1a resembles the crystal structure, while in salts 2a and 3a the structure of 1,3-bis(3-hydroxypyridinium) propane dication is bent to the U-form and the tetrabromomethallate anion is located in a niche. The significant role of the intermolecular interactions and hydrogen bonds was revealed based on IR spectra. The interpretation of 1H and 13C NMR spectra in DMSO-d6 was based on 2D experiments. The quantum theory of atoms in molecules (QTAIM) was employed to classify the strength of hydrogen bonds in salts 1, 2, and 3. Based on the sigma-hole concept, intermolecular interactions in 2 and 3 were described.