NMR, DFT and luminescence studies of the complexation of V(V) oxoions in solution with 8-hydroxyquinoline-5-sulfonate

H-1, C-13 and V-51 NMR spectra, DFT calculations, and UV/visible absorption and luminescence spectra are used to study complexation of 8-hydroxyquinoline-5-sulfonate (8-HQS) with vanadium(v). A full speciation study over the concentration region studied shows that V(V) oxoions form three complexes in the pH range 4-7. Geometries are proposed based on NMR and DFT calculations. Dominant species are 1 : 2 (metal : ligand) mononuclear six-coordinated isomers with an almost octahedral geometry, having the metal centre coordinated to two 8-HQS ligands, and only differing in the arrangement of the donor groups. A minor binuclear complex species was also identified, and possesses two five coordinated metal centres, with distorted trigonal bipyramidal geometry, bridged by two coordinated 8-HQS molecules. Upon binding to V(v), marked changes are seen in UV/visible absorption spectra. However, in contrast to other metals (Al(III), Ga(III), Zn(II)), the complexes are only weakly luminescent; this is suggested to be due to quenching of the emission by low-lying ligand-to-metal (LMCT) states close to the emitting ligand-based excited state.