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

Article abstract of DOI:10.1002/(SICI)1097-458X(199608)34:8<638::AID-OMR926>3.0.CO;2-5

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 [Al₂(μ-OH)₂(C₁₄H₆O ₄)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 ²⁷Al NMR in the solid state by means of their chemical shifts (+23.1 and 0.0 ppm) and their linewidths Δν_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'.