Complexes of Al(III) with 3'4'-dihydroxy-flavone: characterization, theoretical and spectroscopic study.

Complex formation between aluminium chloride and 3'4'-dihydroxyflavone (3'4'diOHF) in methanol has been studied by UV-visible and Raman spectroscopies combined with quantum chemical calculations. Job's method of continuous variation and the molar ratio method were applied to ascertain the stoichiometry composition of the chelate in pure methanol. A 1:1 complex was indicated by both the methods. Geometry optimizations of free and complexed molecules by AMI and DFT methods show that structural modifications of the ligand, induced by complexation, are minor, and are localized on the chelating site. The good agreement between experimental and theoretical electronic spectra of both 3'4'diOHF and complex confirm the structural models. The great similarities between Raman spectra of the free and complexed form constitute an another proof of the absence of pronounced electronic and geometric changes, and notably demonstrate that the quinoidal form induced by the deprotonation of the two hydroxyl groups does not participate in the 3'4'diOHF complex structure. Whereas no complexation occurs in acidic medium, complexes of high stoichiometry are formed in alkaline medium. (Al(3'4'diOHF)2)- and (Al(3'4'diOHF)3)3- species are observed in methanol in the presence of sodium acetate or sodium methanoate.

Conformational and spectroscopic investigation of 3-hydroxyflavone-aluminium chelates.

3-Hydroxyflavone (3HF), which is the simplest molecule of the flavonol class, possesses chelating properties towards Al(III). Spectrophotometric methods have shown that the 3HF molecule forms an Al(3HF)2 complex in pure methanol. The structure of this complex, obtained by quantum semi-empirical AM1 method, indicated that complexed 3HF adopts a pyronium form. Structural and electronic modifications induced by chelation are illustrated by the important frequency shifts observed between free and complexed 3HF FT-Raman spectra and by the chemical shifts variations in the 13C NMR spectra of the two species. Complexes with the same stoichiometry were formed when AcO- or MeO- are present in the medium. However, in acidic medium the chelate composition is Al2(3HF).