Crystal structure and vibrational spectra of 2-chloromethyl-5-hydroxy-4H-pyran-4-one and 5-hydroxy-2-methyl-4H-pyran-4-one as potential ligands for Fe(III) complexes

The title compounds form octahedral bidentate complexes with iron(III). Both the ligands and the complexes exhibit biological activity. The complexes may form isomers of fac or mer configuration. To predict the complex configuration, the structures of chlorokojic acid and allomaltol were determined and their vibrational spectra were analysed. The structure of chlorokojic acid, C6H5ClO3, was solved in the polar space group Pna21 (ITC No. 33) and that of allomaltol, C6H6O3, in centrosymmetric space group P1¯ (ITC No. 2). The characteristic features of both structures are molecular ribbons but of different origin. In the case of chlorokojic acid, the ribbons are formed of molecules related by twofold screw axis and linked by H-bonds of OH⋯O type. In allomaltol, the ribbons are built of centrosymmetric dimers joined by the weak interactions of CH⋯O type. The normal modes of vibration have been calculated by means of the density functional theory method B3LYP and the 6-31+G∗∗ basis set. The results were used for the vibrational analysis of both compounds. The theoretical spectra are in agreement with the experimental ones. The detailed inspection of the results obtained led to conclusion that allomaltol should form fac complexes with iron(III) in analogy to kojic acid, whereas for chlorokojic acid the mer isomer might be expected.