Crystal structures and Raman spectroscopic study of croconate violet salts with alkaline earth ions

In this work are described the crystal structures of two alkaline earth salts as well the vibrational spectra of three alkaline earth salts of 3,5-bis-(dicyanomethylene)cyclopentane-1,2,4-trionate (known as croconate violet – CV2−). The compounds obtained are a mixed salt of potassium and magnesium [MgK2(CV)2], beyond of calcium and barium salts (CaCV and BaCV, respectively). The structures of MgK2(CV)2 and BaCV have been confirmed by single crystal X-ray diffraction and spectroscopic analysis. These compounds crystallize in monoclinic P21/n space group, showing four and five water molecules for MgK2(CV)2 and BaCV, respectively. The dianionic units [MgK2(CV)2(H2O)4] adopt a slight distorted octahedral geometry in which the metallic center is coordinated by six oxygen atoms (four from CV2− and two from water molecules). These units are connected through intermolecular hydrogen bonding giving rise to a supramolecular array in a two-dimensional arrangement (2D). The potassium ion shows monodentate coordination, whereas the magnesium ion presents a chelate coordination in the MgK2(CV)2 salt and for the BaCV, the coordination geometry of metal site is monodentate and chelate coordination; it is straightforward to mention that the barium compound is the only pure divalent salt obtained until now in the literature. In the vibrational spectra, the most important vibrational markers for croconate violet ion are the ν(CN) and ν(CO) modes, which present different behavior due to the modification of chemical. In general way, the bands assigned to ν(CN) and ν(CO) in MgK2(CV)2, CaCV and BaCV present small wavenumber shifts, which can be associated to the different interionic interactions between anion and cations.