Mixed salts containing croconate violet, lanthanide and potassium ions: Crystal structures and spectroscopic characterization of supramolecular compounds

The vibrational spectra and crystal structures of four lanthanide and potassium salts of 3,5-bis(dicyanomethylene)cyclopentane-1,2,4-trionate ( C 1 1 N 4 O 3 2 - ), known as croconate violet (CV), are described in this work. All LnKC22N8O6 (Ln = La+3, Nd+3, Gd+3 and Ho+3) compounds are isostructural, crystallizing in the triclinic P 1 ¯ space group. In each compound the lanthanide ion is acting as both monodentate and chelate metal sites, whereas the potassium presents only monodentate coordination. The crystal structure shows the formation of a periodic 2D structure extended by K–N bonds parallel to the crystallographic [0 0 1] direction; these 2D sheets form hydrogen bonds with water molecules giving rise to a 3D extended arrangement. It is not possible to observe any type of π-interaction and the main forces responsible to stabilize the structures are the hydrogen bonds. The vibrational spectra of all the compounds are very similar, and the most important vibrational markers for the croconate violet ion, namely the ν(CN) and ν(CO) modes, behave differently: the ν(CN) modes are not shifted by the presence of the lanthanide ion species, only showing small band intensity differences, whereas the ν(CO) bands are shifted to higher wavenumbers, due to their coordination to the metal sites.