Nuclear magnetic resonance studies of complexes of aluminium(III), gallium(III), and indium(III) with disulphonated 2,2′-dihydroxyazobenzene ligands in aqueous solution

Abstract

Proton n.m.r. spectra are reported for complexes of Al^III, Ga^III, and In^III with a variety of 5,5′-disulphonated 2,2′-dihydroxyazobenzenes in D₂O solution. Ligand exchange is slow on an n.m.r. time-scale, and both 1 : 1 and 1 : 2 complexes have been characterized. The spectra of the complexes of Al^III and Ga^III show that the metal ions are bound to only one of the nitrogens of the azo groups. In contrast, the 1 : 2 indium(III) complexes of the two symmetric ligands studied are fluxional, and values of ΔG^‡ are given for the process which ‘flips’ the metal ions between the two nitrogen atoms. For the 1 : 1 and 1 : 2 aluminium(III) and the 1 : 2 gallium(III) complexes of asymmetric ligands, the expected isomers have been observed, and the equilibrium isomer ratios are reported. Assignments based on ¹H n.m.r. spectra have been confirmed by the ¹⁵N n.m.r. spectrum of a 1 : 2 gallium(III) complex with a ligand enriched in ¹⁵N. The isomer ratios can be largely rationalized by considering the effect substituents in the 4 and 4′ positions have on the electron density of the remote nitrogen atom in the azo groups. The effect of changes in pD on the isomer ratios and the ¹H chemical shifts has also been studied for the 1 : 2 gallium(III) complexes of a ligand with an ionizable 4′-OH group.