Selectivity in coordination of 7-substituted-8-hydroxy-5-quinolinesulphonates to aluminium(III)

Abstract

Coordination reaction kinetics and mechanisms of 7-substituted-8-hydroxy-5-quinolinesulphonic acid ligands that have ligating substituents of nitrogen whose nature is different from that at the 1-position, i.e. 7-amino-8-hydroxy-5-quinolinesulphonic acid ( 1, H 2ahqs) and 7-phenylazo-8-hydroxy-5-quinolinesuphonic acid ( 2, H 2phqs), have been investigated spectrophotometrically in an aqueous sodium nitrate solution of constant ionic strength (0.10 mol dm −3) at 25.0°C. These ligands selectively coordinate to aluminium(III) with its 8-quinolinolato moiety through pyridyl nitrogen and phenolato oxygen to form yellow and red complexes, indicating that the donor ability is higher for the aromatic than for the amino and azo nitrogens towards aluminium(III). The complex formation reaction proceeds through two parallel pathways of Al 3+ with Hahqs − and Hphqs − and of Al(OH) 2+ with H 2ahqs and H 2phqs, the coordination of the phenolato oxygen to the central metal atom being the mechanistic rate-determining step. The role of substituent groups at the 7-position of the 8-quinolinolate framework on its coordinating nature as a bidentate ligand towards the central metal atom is emphasized.