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

Abstract

The crystal structures and vibrational spectra of two alkaline salts of 3,5-bis-(dicyanomethylene)cyclopentane-1,2,4-trionate ( C 11 N 4 O 3 2 - ) , known as croconate violet, are described: 1-rubidium and potassium croconate violet (RbKCV), and 2-rubidium croconate violet (Rb 2CV). These compounds are isostructural and crystallize in triclinic P 1 ¯ space group. The stoichiometries of the salts show two water molecules to each M 2C 11N 4O 3 unit. The anions are displayed in layers linked to water molecules by hydrogen bonds, therefore forming an extended structure with two-dimensional arrangement (2D). Crystal structure shows two different metal sites (M1 and M2) that present different types of coordination: monodentate, quelate and bidentate in M1; and monodentate and bidentate in M2. In RbKCV the rubidium cation is predominant in M1 and M2 sites, presenting total occupation of 70% and the molecular formula of this salt is Rb 1.4K 0.6C 11N 4O 3 · 2H 2O. Vibrational spectra of RbKCV and Rb 2CV are similar to the spectra of potassium croconate violet (K 2CV). In the Raman spectrum of RbKCV some small wavenumber shifts are observed when compared K 2CV and Rb 2CV spectra. The structural difference in RbKCV salt is due to the interionic interactions between CV anion and potassium and rubidium cations; this effect could be responsible to spectral differences observed in RbKCV Raman spectra. However, the exchange between potassium and rubidium ions in the solid state structure does not alter croconate violet anion crystal packing, which can be observed in the vibrational spectra.