On the reaction products of lanthanide chlorides with biurete

The synthesis and results of IR spectroscopy and X-ray diffraction analysis of new complexes of biurete NH₂CONHCONH₂ (BU) with the composition LnCl₃ · 2BU · 4H₂O, where Ln = La (I), Pr (II), Ho (III), Er (IV), and Lu (V), are presented. Crystals of complexes I–V include complex cations [Ln(H₂O)₄(BU)₂]³⁺ and uncoordinated chloride ions. The coordination mode of biurete molecules is bidentate through the oxygen atoms, and upon coordination the BU molecules are transformed from the initial trans to cis configuration. Water molecules are also coordinated through the oxygen atom (the shape of the polyhedron of the Ln atoms is a two-capped trigonal prism). The oxygen atoms of both BU molecules and the oxygen atoms of the first and second water molecules form a trigonal prism, whereas the oxygen atoms of the third and fourth water molecules form two caps of the coordination polyhedron. The coordinated BU molecules are joined with the chloride ions and water molecules of the adjacent complex cations by hydrogen bonds. The degree of conversion of trans-BU to cis-BU in the lanthanide series of complexes of this type is discussed.