POTENTIOMETRIC AND SPECTROPHOTOMETRIC STUDY OS 3-HYDROXYFLAVONES – La (III) COMPLEXES

FERRARI G.V.; PAPPANO N.B.; DEBATTISTA N. B.; MONTAÑA M.P.

Córdoba, Argentina

Congreso; 9th LATINAMERICAN CONFERENCE ON PHYSICAL ORGANIC CHEMISTRY; 2007

Flavonoids are a large family of natural compounds that includes flavones, flavonols, chalcones, among others. These compounds display varied biological and chemical properties as antioxidant, antiviral activity, metal complexing capacity. In this work La (III) complexes of the following four 3-hydroxyflavones were studied: 3,7-dihydroxyflavone, 3,7,3´-trihydroxyflavone, Quercetin (3,5,7,3´,4´-pentahydroxyflavone) and Morin (3,5,7,2´,4´-pentahydroxyflavone). Ion La (III) have biological applications due to La (III) complexes increase biological properties compared with the ligands by themselves. La (III) - 3-hydroxyflavones complexes were investigated by means of potentiometric and spectrophotometric methods at 26.3 ºC and ionic strength of 0.1 M NaCl in water–dioxane (1/1, v/v). The formation of complexes and their stoichiometry were determined spectrophotometrically, and their formation constants were evaluated by Calvin-Bjerrum and Irving Rossotti methods, which also allow calculating the protonation constants of the ligands. All complexes presented 2:1 ligand-metal stoichiometry, except 3,7-dihydroxyflavone – La (III), which stoichiometry was 1:1 ligand-metal. The evaluated formation constants of the complexes studied were: Complexe K1 K2 b2 3,7-dihydroxyflavone - La (III) 7.656 x 105 - - 3,7,3´-trihydroxyflavone - La (III) 7.261 x 108 2.489 x 106 1.807 x 1015 Morin  - La (III) 6.471 x 106 1.009 x 105 6.529 x 1011 Quercetin - La (III) 7.228 x 107 1.114 x 105 8.052 x 1012 Considering 2:1 ligand-metal complexes, 3,7,3´-trihydroxyflavone - La (III) exhibit the greatest formation constant value. The coordination with the metal ion could involve the OH in the position 3 and the oxygen from carbonyl group, which form a stable fºive-atom ring. Quercetin and Morin present greater stereochemical problems which could justify their lower constant values. The obtained results reveal that 3,7,3´-trihydroxyflavone is the most suitable ligand for Lanthanum (III) complexation. On the other hand, the constant value for 3,7-dihydroxyflavone – La (III) shows that it also might be a very good analytical reagent for this metallic ion.