Synthesis and spectral investigation of Al(III) catechin/β-cyclodextrin and Al(III) quercetin/β-cyclodextrin inclusion compounds

Al-catechin/β-cyclodextrin and Al-quercetin/β-cyclodextrin (β-CD) inclusion compounds were synthesized and characterized by IR, UV–vis, 1H and 13C NMR and TG and DTA analyses. Because quercetin is sparingly soluble in water, the stability constants of the Al-quercetin/β-CD and Al-catechin/β-CD compounds were determined by phase solubility studies. The AL-type diagrams indicated the formation of 1:1 inclusion compounds and allowed calculation of the stability constants. The thermodynamic parameters were obtained from the dependence of the stability constants on temperature and results indicated that the formation of the inclusion compounds is an enthalpically driven process. The thermal decomposition of the solid Al-quercetin/β-CD and Al-catechin/β-CD inclusion compounds took place at different stages, compared with the respective precursors, proving that an inclusion complexation process really occurred.