Oxovanadium(IV) complexes of quinoline derivatives.

Complex formation between oxovanadium(IV) and five quinoline derivatives, 8-hydroxyquinoline (8-HQ), 8-hydroxyquinoline-5-sulphonic acid (8-HQS), 2-methyl-8-hydroxyquinoline (8-HQD), 8-hydroxyquinoline-N-oxide (8-HQNO), and 8-quinolinecarboxylic acid (8-QC), was studied in aqueous solution through the combined application of potentiometric and spectroscopic (electronic absorption and EPR) techniques. The results indicate that all the ligands form mono and bis chelated complexes with the VO(IV) ion. The bis chelated complexes are neutral and poorly soluble. In solution 8-HQ and 8-HQS form two cis isomers bearing a water molecule in the equatorial plane. This water molecule deprotonates with a pKa of 8.76 (8-HQ) to give the corresponding EPRactive mono hydroxo species. Four five-coordinated complexes were isolated in the solid state: [VO(8-HQ)2], [VO(8-HQD)2], [VO(8-HQNO)2] and [VO(8-QC)2]. They were characterised by IR spectroscopy and diffuse reflectance electronic absorption, and by solution studies in coordinating (DMSO) or non-coordinating (toluene and CH2Cl2) solvents. Moreover, the corresponding species with Zn2+, GaCl2+ or TiO2+ were doped with different amounts of VO2+. The structures of [VO(8-HQ)2] and [VO(8-HQD)2] show a distortion toward the trigonal bipyramid, whilst those of [VO(8-HQNO)2] and [VO(8-QC)2] are closer to the square pyramid. In coordinating solvents, [VO(8-HQ)2] shows a trans-cis isomerisation and re-originates the structure displayed in aqueous solution. The features of five-coordinated VO(IV) complexes in solution and in solid state are discussed.