Different Types of Coordinated Urea Molecules in its Complexes with Rare-Earth Iodides and Perchlorates

An earlier reported series of the [Ln(Ur)4(H2O)4]I3 (Ln = Y, La, Nd, Eu, Gd, Dy, Ho, Er; Ur = urea) complexes was completed with seven new compounds (Ln = Ce, Pr, Sm, Tb, Tm, Yb, Lu); one of them, [Ce(Ur)4(H2O)4]I3, was studied by X-ray diffraction. The most striking feature of the [Ln(Ur)4(H2O)4]I3 structures is the presence of two types of coordinated urea molecules. There are two planar symmetric and two non-planar asymmetric urea molecules. The LnOC bond angles vary in the ranges 163.06-165.71 degrees and 148.42-152.42 degrees for symmetric and asymmetric urea ligands, respectively, correlating with the ionic mode of urea coordination. To elucidate the role of aqua ligands for the urea coordination mode, two water-free perchlorate complexes, [La(Ur)8](ClO4)3.2Ur and [La(Ur)7(OClO3)](ClO4)2 were synthesized and structurally characterized. In these complexes, all urea molecules are planar symmetric; however, both covalent and ionic types of urea coordination with the LaOC bond angles varying in the 132.4142.3 degrees and 145.5159.1 degrees ranges, respectively can be observed.