Synthesis, X-ray crystal structure and solubility of a new zinc-naproxen complex: Potentiometric and thermodynamic studies in methanol + water mixtures

A new hexa-coordinated zinc-complex of naproxen (HL), [Zn(L)2(MeOH)2], has been synthesized and characterized by UV-Vis and FT-IR spectroscopies, elemental analysis and single crystal X-ray diffraction. This complex forms orthorhombic crystals, of a chiral space group P212121, with a = 5.6059(1), b = 8.4418(2), c = 59.081(4) Å and z = 4. Coordination of naproxen to Zn ion increases the solubility of naproxen in water by a factor of 26. The protonation constant of naproxen and the stability constant of its zinc complex were determined by the potentiometric method using the BEST computer program in various concentrations (50-90% v/v) of methanol + water mixtures maintaining an ionic strength of 0.1 mol ⋅ L− 1 adjusted by Me4NCl at three different temperatures (25 °C, 37 °C and 45 °C) in pH range of 2-12. Protonation and stability constants in methanol + water mixtures increase with increasing the amount of methanol due to the electrostatic nature of the interaction between naproxen and zinc ion. Thermodynamic parameters of the protonation of naproxen and its zinc-complex formation, ML2, were also evaluated. The complexation of Zn2 + with naproxen is spontaneous, exothermic and favorable at lower temperatures. Among the various complex species formed in solution, ML2 was found as a main species at pH range of 5-7.