NUCLEAR-MAGNETIC-RESONANCE STUDIES OF COMPLEXES OF ALUMINUM(III), GALLIUM(III), AND INDIUM(III) WITH DISULPHONATED 2,2'-DIHYDROXYAZOBENZENE LIGANDS IN AQUEOUS-SOLUTION

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 DELTA-G double-ended-dagger 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 H-1 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 N-15. 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 H-1 chemical shifts has also been studied for the 1:2 gallium(III) complexes of a ligand with an ionizable 4'-OH group.