Rhodizonate complexes with transition metal ions: A new vision from an old building block

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In this work a series of metal complexes containing the rhodizonate ion have been synthesized and characterized by means of structural and spectroscopic measurements. Compounds of general formula MC6O6.nH2O, where M = Mn2+, Co2+, Ni2+, Cu2+ and Zn2+) were carefully obtained in an inert atmosphere, and the resulting complexes have been analyzed by powder X-ray technique, showing that manganese, cobalt, nickel and zinc compounds all present the same similar arrangement; the qualitative phase analysis indicate the Pmmm space group for manganese, cobalt, nickel and zinc salts, and P2/m for the copper salt. The unit cell volumes were determined, and they grow approximately with the number of water molecules in the structure measured by elemental and thermogravimetric analysis. The vibrational spectra are in concordance with the X-ray diffraction results since the salts of manganese, cobalt, nickel and zinc show a remarkably similar spectral profile, whereas the copper salt, shows a different spectral profile. It was observed that the number of bands in the Raman spectra of the compounds is greatly increased compared to the free rhodizonate ion. This fact is justified by the lower symmetry due to the complexation of the metal site. The vibrational modes related with the C=O stretching in the 1700–1500 cm−1 region are the most influenced with the metal coordination showing large displacements and band splitting. This last fact strongly supports the loss of symmetry experienced by the oxocarbon ion when coordinated to the metal centers in the complexes.