Aluminium carboxylates in aqueous solutions. Part 2. Metal speciation in the AlIII–lactate–OH––H2O system

Abstract

The tris(hydroxyl–carboxyl) chelated complex Al(lact)₃(lact = lactate) dissolves in water to give stable acidic (pH ca. 3) solutions, in which free and metal-co-ordinated lactate ligands are detected by IR spectra in D₂O. Proton NMR spectra (90 MHz) reveal that ligand exchange between metal-bonded and free lactate ligands occurs, also at +4 °C. The separate resonances due to metal-bonded and free lactate are clearly detected at 400 MHz. When the pH of an Al(lact)₃ solution (analytical concentration 0.1 mol dm^–3) is adjusted to 7.5 with NaOD the resulting ¹H and ¹³C NMR spectra are very similar to those exhibited by a 0.3 mol dm^–3 Na(lact) solution. The IR spectrum of the 0.1 mol dm^–3 aluminium(III) solution displays two major bands at 1585 and 1420 cm^–1 due to the unco-ordinated lactate ligands. A 0.1 mol dm^–3 neutralised solution is (meta)stable for months and the whole of the data at pH 7.5 can be interpreted in terms of the release of most of the lactate from the co-ordination sphere of Al^III, with concomitant formation of metastable species including possibly [Al(OH)₃(H₂O)₃].