Solution and solid-state studies of a new supramolecular proton transfer salt and its VO2 complex constructed with chelidamic acid and 3,4-diaminopyridine

The proton transfer compound (Hdap)(chelH)·2H 2 O ( 1 ) and its related anionic complex (Hdap) [VO 2 (chel)] ( 2 ), where chelH 2 = 4-hydroxypyridine-2,6-dicarboxylic acid (chelidamic acid) and dap = 3,4-diaminopyridine, were synthesized and characterized by elemental analysis, spectroscopy (IR, UV–Vis), thermal (TG/DTG) analysis and single-crystal X-ray diffraction. Compound 1 resulted from proton transfer between chelH 2 and dap in aqueous solution. In 1, two carboxylic acids of chelH 2 were deprotonated and the protons transferred to the nitrogen atoms of one chelidamate anion and one dap moiety. Compound 2 resulted from complexation of 1 and vanadyl sulfate. In the crystal structure of 2 , the metal ion is five coordinated by one tridentate ligand (chel) 2− and two O 2− anions, with (Hdap) + as a counter cation. In both structures, a complicated hydrogen-bonding network accompanied with π–π, C–O···π and C–H···π stacking interactions leads to formation of a 3D supramolecular network. In the following, solution studies have been performed by means of pH potentiometric titrations method as a power technique. This method was used for determination of protonation constants of chelH 2 and dap in their probable protonated forms and for calculation of equilibrium constants for the chelH 2 –dap proton transfer system and the stoichiometry and stability constants of binary and ternary complexes of this system with VO 2+ ion in aqueous solution. The stoichiometries of the most complex species in solution were compared with the corresponding crystalline complexes in the solid state.