Structural variability in uranyl–lanthanide heterometallic complexes with DOTA and oxalato ligands

Four novel 4f–5f heterometallic complexes could be obtained from the reaction of uranyl and lanthanide nitrates with DOTA (H₄L) under hydrothermal conditions. In all cases, as in the previous examples reported, additional oxalato ligands are formed in situ. Variations in the stoichiometry of the final products and the presence of hydroxo ions in some cases appear to result in a large structural variability. In the two isomorphous complexes [(UO₂)₂Ln₂(L)₂(C₂O₄)] with Ln = Sm (1) or Eu (2), the lanthanide ion is located in the N₄O₄ site and is also bound to a carboxylate oxygen atom from a neighbouring unit, to give zigzag chains which are further linked to one another by [(UO₂)₂(C₂O₄)]²⁺ dications, resulting in the formation of a 3D framework. In [(UO₂)₄Gd₂(L)₂(C₂O₄)₃(H₂O)₆]·2H₂O (3), 2D bilayer subunits of the “double floor” type with uranyl oxalate pillars are assembled into a 3D framework by other, disordered uranyl ions. [(UO₂)₂Gd(L)(C₂O₄)(OH)]·H₂O (4) is a 2D assembly in which cationic {[(UO₂)₂(C₂O₄)(OH)]⁺}_n chains are linked to one another by the [Gd(L)]⁻ groups. The most notable feature of this compound is the environment of the 4f ion, which is eight-coordinate and twisted square antiprismatic (TSA′), instead of nine-coordinate monocapped square antiprismatic (SA), as generally observed in DOTA complexes of gadolinium(III) and similarly-sized ions.