Bis(dicyanomethylene)squarate squaraines in their 1,2- and 1,3-forms: Synthesis, crystal structure and spectroscopic study of compounds containing alkali metals and tetrabutylammonium ions

In this paper, we describe a new synthetic methodology and the spectroscopic characterization of two squaraines 1,2- and 1,3-substituted, named sodium 1,2-bis(dicyanomethylene)squarate and sodium 1,3-bis(dicyanomethylene)squarate. The squaraine species are derived from 1,2-dihydroxycyclobuten-3,4-dione, also known as squaric acid. A modification in the synthetic route has been developed in order to improve the yield in the synthesis of the 1,3-derivative: the in situ use of the 1,3-dianilinesquarate intermediate. In both cases, an excess basic medium was added to the solution containing all the reactants. These squaraines consist of a cyanine-type chromophore with high thermal stability and several different spectroscopic properties. In addition, it was made a comparative analysis based on the spectroscopic study of the 1,3- and 1,2-compounds, as well as their derivatives with several metal alkalis (K+, Rb+, Mg2+, Ca2+, Sr2+ and Ba2+), besides tetrabutylammonium ion (TBA). The vibrational spectroscopic data demonstrate the centrosymmetric structure in solid state phase of the 1,3-compound due to non-coincidence between the infrared and Raman bands in the vibrational spectra. The crystal structures were also obtained for Ca2+ and Ba2+ ions with the 1,3-isomer. These salts crystallize in different space groups, also showing very distinct coordination properties and intermolecular interactions. This study provides a sound basis for the future expansion of structural and spectroscopic investigations of this interesting group of metal–ligand compounds as building blocks systems in supramolecular chemistry.