Multinuclear magnetic resonance studies on aluminium chelates. Part i. Chelates with catechol and disodium 4,5-dihydroxybenzene-1,3-disulfonate

Abstract

The complexation of Al³⁺ with catechol and tiron (disodium 4,5-dihydroxybenzene-1,3-disulfonate) has been studied by NMR spectroscopy. The complexes in solution at different pH values were identified by multinuclear (²⁷Al, ¹³C and ¹H) magnetic resonance spectra, and spectrophotometric and potentiometric data. Aluminium formed 1:1, 1:2 and 1:3 chelates with both catechol and tiron depending on the pH and ligand concentration. Disproportionation of the 1:1 chelate with tiron at pH 7 gave a mixture of 1:2 and 1:3 chelates leaving a major amount of aluminium in an uncomplexed form. Both meridional and facial isomers of [AIL₃]^9–(Na₂H₂L = tiron) have been characterised by ¹³C and ¹H NMR. Hydrolysis of the 1:1 Al–L⁴ chelate leading to the formation of [Al(OH)₄] occurs via the formation of the tetrahedral chelate [AIL(OH)₂]^3–(δ 53) as an intermediate. In the Al–catechol (H₂cat) system three complex species coexist with [Al(OH)₄] in a 1:2 mixture at pH 12.5. Unlike in the Al–tiron system, a tetrahedral 1:2 Al–cat chelate (δ 58.5) is also formed during hydrolysis of the octahedral 1:2 Al–cat complex.