AL-27 SOLID-STATE NMR-STUDY OF ALUMINUM COORDINATION-COMPLEXES OF ALIZARIN

The structures of complexes obtained by the reaction of alizarin (C14H 8O4) and Al3+ in basic media depend upon the reaction solvent and the nature of the base. With aqueous or methanolic solutions of sodium hyd roxide 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 eve n in the gas phase by electrospray mass spectrometry. With potassium h ydroxide the monohydrate form is obtained. These two products are dist inguishable 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 a nd 4000 Hz). The chemical shifts of these binuclear complexes or simil ar derivatives are discussed in relation to possible chelation of the four molecules of water with alizarinate entities according to the bul k of the cation. The results are illustrated by two types of structure , named 'closed structure' (with strongly chelated H2O molecules and d ibenzene sandwich Na+) and 'open structure'.