Crystal structure and vibrational spectra of cesium salts of bis(dicyanomethylene)croconate (croconate violet) ion

The vibrational spectra and crystal structures of cesium salts of 3,5-bis(dicyanomethylene)cyclopentane-1,2,4-trionate (C11N4O32-), known as croconate violet, are described in this work: 1-cesium and potassium croconate violet (CsKCV), 2-cesium croconate violet (Cs2CV-1), and 3-cesium croconate violet hydrate (Cs2CV-2). Only CsKCV and Cs2CV-2 presented single crystals suitable for X-ray diffraction analysis; these compounds crystallize in triclinic P1¯ and monoclinic P21/a space group, respectively. The stoichiometries of the salts show one water molecule to each M2C11N4O3 unit, which are involved in hydrogen bonds forming dimmers in CsKCV and an extended structure with one-dimensional arrangement (1D) in Cs2CV-2. Two types of coordination modes are observed in both salts: monodentate and chelate. Powder diffraction data show that Cs2CV-1 is isostructural to potassium and rubidium croconate violet salts. Vibrational spectra of cesium salts are almost similar, and the biggest differences can be observed in CV ring stretching modes (1600–1400 cm−1), probably due to small changes in electronic delocalization over pseudo-oxocarbon ring. In despite of different crystal packing in these salts, the CV units are almost planar and are displayed in layers, which are separated by cations and water molecules in CsKCV and Cs2CV-2 crystal structures.