Russian Chemical Bulletin, International Edition, Vol. 53, No. 4, pp. 780—784, April, 2004 780 1066-5285/04/5304-0780 © 2004 Plenum Publishing Corporation Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 744—748, April, 2004. Determination of composition and instability constants of maltol complexes with iron(III) ions I. A. Antipova, S. A. Mukha, and S. A. Medvedeva  A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 1 ul. Favorskogo, 664033 Irkutsk, Russian Federation. Fax: +7 (395 2) 41 9346. E-mail: msa@irioch.irk.ru Complexation of maltol (MH) with Fe 3+ ions in aqueous solutions was studied. The compositions of [FeMa] 2+ , [FeMa 2 ] + , and [FeMa 3 ] complexes were determined by the method of isomolar series, and their instability constants were calculated. The values of the latter were confirmed by the method of apparent deviation from the Bouger—Lambert—Beer law. An increase in the Ma : Fe 3+ ratio from 1 to 3 decreases the instability constants of the complexes. The [FeMa 3 ] complex can be considered as a basis for the antianemic drug with a prolonged effect. Key words: maltol, iron(III), complexation, instability constants. Maltol (3-hydroxy-2-methyl-4-pyrone, MaH) is a natural substance found in several plants (leaves of Passiflora, 1 Japanese Cercis, 2 Aralia, 3 and others). Maltol is isolated, as a rule, from needles of Siberian fir (Abies sibirica L.), where its content reaches 1—2%. 4 Since the maltol structure contains the hydroxyl and keto groups in the ortho-position, this γ-pyrone is an effi- cient bidentate chelating ligand and can easily be coordinated with bi- and trivalent metal ions to form bis- and trischelate complexes. 5—11 These maltol compounds can be considered as ionophores capable of supplying an organism with necessary microelements. Unlike similar chelating keto-alcohols, maltol has great advantages from the biological point of view, because it is a low-toxicity substance (index LD 50 = 1400 mg kg –1 ), 8 manifests antioxidant properties 12 (due to which it is used in food and fragrance industries), and possesses fungi- static and antibacterial activity. 13 The neutral water-soluble tris(3-hydroxy-2-methyl- 4-pyronato)iron(III) complex is of interest as a potential medicine for treatment of iron-deficient anemia. 10,11 A study of complexation and stability of this com- plex is expected to provide insight into its behavior in the human organism. The purpose of this work is to study the com- plexation of maltol with iron(III) chloride, includ- ing the determination of the compositions and instability constants of the com- plexes formed at different pH of a medium. The reaction was studied by the spectrophotometric method using the absorbance of the resulting complexes at 340—600 nm. Maltol does not absorb in this region of wavelengths, and the absorbance of iron(III) chloride is insignificant. This makes it possible to determine directly the absorbance and, correspondingly, the concentration of the colored maltol complexes. Experimental Maltol was isolated from needles of Siberian fir (Abies sibirica L.) using a procedure proposed by us previously. 14 The following reagents were used: FeCl 3 •6Н 2 О, HСlO 4 (reagent grade), 25% NH 4 OH (analytical grade), glacial AcOH (reagent grade), and bidistilled water. Electronic spectra were recorded on an SF-26 spectropho- tometer. The absorbance of solutions was measured in quartz cells with layer thicknesses of 1 cm and 0.1 cm (in the dilution method) using a buffer with the same pH value as a reference solution. To determine the plots of the absorbance of maltol vs. pH of the medium and pK a , aqueous solutions of the sub- stance with a concentration of 6.87•10 –5 mol L –1 were used. The pH values of solutions were measured with an EV-74 uni- versal pH-meter. To control the pH of the medium, 0.2 М solutions of ammo- nia and perchloric acid were used, because the СlO 4 – anion is much less prone to form complexes with iron compared to an- ions of other acids. The iron salt (FeCl 3 •6H 2 O) is readily hydro- lyzed at pH > 6 to form a precipitate of iron hydroxide. There- fore, an acetate—ammonia buffer was used in some experiments, because the acetate anion of this buffer forms an easily dissoci- ating complex with Fe III , whose weak absorption can be ne- glected compared to that of the iron(III) complexes with maltol.