Kinetics and mechanisms of the complex formation reaction of aluminium(III) with 7-(2-carboxyethyl)amino-8-hydroxy-5-quinolinesulphonic acid and related ligands: Evidence for pH-dependent linkage isomerism

Abstract

The formation of the aluminium(III) complexes of new types of bifunctional multidentate ligands, 7-(2-carboxyethyl)amino-8-hydroxy-5-quinolinesulphonic acid ( 1, H 3ceahqs) and 7-(2-carboxymethyl)amino-8-hydroxy-5-quinolinesulphonic acid ( 2, H 3cmahqs), have been investigated in an aqueous sodium nitrate solution of constant ionic strength (0.10 mol dm −3) at 25.0°C. These ligands form yellow bis(ligand)aluminium(III) complexes with two linkage isomers of ONO type that have the coordination mode of an o-(2-carboxyalkyl)aminophenolato moiety, i.e. Al(Hceahqs-O,N,O) 2 − and Al(Hcmahqs-O,N,O) 2 −, below pH 3 and of NO type that have the coordination mode of an 8-quinolinolato moiety, i.e. Al(ceahqs-N,O) 2 3− and Al(cmahqs-N,O) 2 3−, above pH 3, thus exhibiting linkage isomerism controlled by ambient solution acidity. The complex formation reaction mechanism is different in these two isomers: the ONO isomer forms through pathways of Al 3+ with H 3L and H 2L − and of Al(OH) 2+ with H 3L, with the mechanistic rate-determining donation of the imino nitrogen preceded by a rapid ionic association of the central metal atom with the deprotonated carboxylate group, whereas the NO isomer forms through pathways of Al 3+ and HL 2− and of Al(OH) 2+ with H 2L − and HL 2−, with the mechanistic rate-determining donation of the phenolato oxygen to the central metal atom (H 3L  H 3ceahqs or H 3cmahqs). In both of these isomers, the formation of the mono(ligand)aluminium(III) complex is slower than its reaction with another ligand molecule to form the bis(ligand)aluminium(III) complex. Details of the coordination reaction mechanism are described.