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

The crystal structures and vibrational spectra of two alkaline salts of 3,5-bis-(dicyanomethylene)cyclopentane-1,2,4-trionate (C11N4O32-), known as croconate violet, are described: 1-rubidium and potassium croconate violet (RbKCV), and 2-rubidium croconate violet (Rb2CV). These compounds are isostructural and crystallize in triclinic P1¯ space group. The stoichiometries of the salts show two water molecules to each M2C11N4O3 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 Rb1.4K0.6C11N4O3 · 2H2O. Vibrational spectra of RbKCV and Rb2CV are similar to the spectra of potassium croconate violet (K2CV). In the Raman spectrum of RbKCV some small wavenumber shifts are observed when compared K2CV and Rb2CV 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.

Graphical abstractRbKCV (Rb:K = 3:1) and Rb2CV present the same crystal packing being influenced by cation–anion interaction and hydrogen bonding design. Vibrational spectra of RbKCV and Rb2CV are similar presenting small wavenumber shifts in mixed salt, which is due to the presence of two different cations (Rb+ and K+) that present different interactions to croconate violet anion. However, the exchange between potassium and rubidium ions in the solid state structure does not alter croconate violet anion crystal packing.Figure optionsDownload full-size imageDownload as PowerPoint slide