Aluminium-27 solid-state NMR study of aluminium coordination complexes of alizarin

The structures of complexes obtained by the reaction of alizarin (C14H8O4) and Al3+ in basic media depend upon the reaction solvent and the nature of the base. With aqueous or methanolic solutions of sodium hydroxide a binuclear complex [Al-2(mu-OH)(2)(C14H6O4)(4)(Na)(4)(H2O)(4)] was obtained, which is stable as the tetrahydrate and is observed even in the gas phase by electrospray mass spectrometry. With potassium hydroxide the monohydrate form is obtained. These two products are distinguishable by Al-27 NMR in the solid state by means of their chemical shifts (+23.1 and 0.0 ppm) and their linewidths Delta nu(1/2) (1200 and 4000 Hz). The chemical shifts of these binuclear complexes or similar derivatives are discussed in relation to possible chelation of the four molecules of water with alizarinate entities according to the bulk of the cation. The results are illustrated by two types of structure, named 'closed structure' (with strongly chelated H2O molecules and dibenzene sandwich Na+) and 'open structure'.