Crystal structure and vibrational spectra of some metal complexes of pseudo-oxocarbon bis(dicyanomethylene)squarate in its cis and trans forms

A comprehensive analytical and spectroscopic study of the ligands TDCMSQ and CDCMSQ and their coordination complexes with a range of metal ions has been undertaken. Hitherto, only the CDCMSQ and its complex with copper ions have been reported; here, the preparation of the manganese, cobalt, nickel and zinc complexes with the cis and trans ligands is reported for the first time. Only the Co and Zn trans complexes were able to produce single crystals suitable for X-ray diffraction analysis. These compounds are not isostructural, despite their similar molecular arrangement. The metal sites are coordinated to four water molecules and two CN groups in an octahedral geometry. The pseudo-oxocarbon units link the metal sites, giving rise to a unidimensional chain perpendicular to b axis, connected through hydrogen bonds with the water molecules, so extending the dimensionality (2D) of the structure. The powder X-ray diffraction patterns along with electronic absorption spectroscopy, infrared and Raman spectroscopy and thermogravimetry have been sufficient to produce a clear indication of the main structural features. The metal ions are coordinated to the ligand in a 1:1 stoicheiometry and four water molecules complete the first coordination shell. The vibrational spectroscopic data demonstrate the centrosymmetric structure of the TDCMSQ ligand through non-coincidence between the infrared and Raman spectra. This study provides a sound basis for the future expansion of structural and spectroscopic investigations in this interesting group of metal–ligand compounds.