NMR, DFT and luminescence studies of the complexation of Zn(II) with 8-hydroxyquinoline-5-sulfonate

Multinuclear (H-1, C-13) magnetic resonance spectroscopy, DFT calculations and luminescence techniques have been used to study 8-hydroxyquinoline-5-sulfonate (8-HQS) and its complexes with Zn(II), in aqueous solution. The study combines the high sensitivity of luminescence techniques, the selectivity of multinuclear NMR spectroscopy with the structural details accessible through DFT calculations, and aims to obtain a detailed understanding of the complexation between the Zn2+ ion and 8-HQS. In addition to a complete assignment of the H-1 and C-13 NMR signals of 8-HQS, a full speciation study has been performed. Over the concentration region studied, Zn2+ metal ion forms only one significant complex species with 8-HQS in aqueous solution in the pH range 6-8. Job's method shows that this species has a 1: 2 (metal: ligand) stoichiometry. The geometry around the metal centre, according to structural optimization using DFT calculations, is suggested to be square bipyramidal, with two coordinated water molecules mutually trans, and the remaining positions occupied by the donor groups of the two coordinated 8-HQS ligands. On binding to Zn(II), 8-HQS shows a marked fluorescence compared with the weakly-luminescent free ligand. In addition, as previously noted, there are marked changes in the absorption spectra, which support the use of 8-HQS as a sensitive fluorescent sensor to detect Zn2+ metal ion in surface waters, biological fluids, etc. Based on results of the structural studies, suggestions are made of ways for enhancing fluorescence sensitivity.