COORDINATION SELECTIVITY IN THE REACTION OF IRON(II) WITH 8-HYDROXY-7-(8-QUINOLYL)AZO-5-QUINOLINESULFONIC ACID

The formation of an iron(II) complex with 8-hydroxy-7-(8-quinolyl)azo-5-quinolinesulfonic acid (QHQS, H2qhqs) has been investigated on the basis of the electronic absorption spectra, the equilibrium, and the stopped-flow kinetics in a 0.10 mol dm-3 aqueous sodium chloride solution at 25°C with a view to elucidating the coordination selectivity of the ligand. A 1:2 (metal:ligand) complex forms in an acidic medium with a selective coordination of QHQS to iron(II), with its 2-(8-quinolylazo)phenol moiety acting as a terdentate ligand. The complex formation reaction proceeds through a single pathway of Fe2+ with Hqhqs-, a ligand species singly protonated at the phenolate oxygen, with the rate constant (k21) of k21=(3.16±0.20) x 105 mol-1 dm3s-1, to form a mono-ligand complex as the rate-determining step. The coordination of the ligand by 8-quinolyl nitrogen, followed by a rate-determining bonding of phenolato oxygen, was suggested as the reaction mechanism.