Uranyl and/or Rare-Earth Mellitates in Extended Organic–Inorganic Networks: A Unique Case of Heterometallic Cation–Cation Interaction with U^VIO–Ln^III Bonding (Ln = Ce, Nd)

Abstract

A series of uranyl and lanthanide (trivalent Ce, Nd) mellitates (<i>mel</i>) has been hydrothermally synthesized in aqueous solvent. Mixtures of these 4f and 5f elements also revealed the formation of a rare case of lanthanide–uranyl coordination polymers. Their structures, determined by XRD single-crystal analysis, exhibit three distinct architectures. The pure lanthanide mellitate Ln₂(H₂O)₆(<i>mel</i>) possesses a 3D framework built up from the connection of isolated LnO₆(H₂O)₃ polyhedra (tricapped trigonal prism) through the mellitate ligand. The structure of the uranyl mellitate (UO₂)₃(H₂O)₆(<i>mel</i>)·11.5H₂O is lamellar and consists of 8-fold coordinated uranium atoms linked to each other through the organic ligand giving rise to the formation of a 2D 3⁶ net. The third structural type, (UO₂)₂Ln­(OH)­(H₂O)₃(<i>mel</i>)·2.5H₂O, involves direct oxygen bondings between the lanthanide and uranyl centers, with the isolation of a heterometallic dinuclear motif. The 9-fold coordinated Ln cation, LnO₅(OH)­(H₂O)₃, is linked to the 7-fold coordinated uranyl (UO₂)­O₄(OH) (pentagonal bipyramid) via one μ₂-hydroxo group and one μ₂-oxo group. The latter is shared between the uranyl bonding (UO = 1.777(4)/1.779(6) Å) and a long Ln–O bonding (Ce–O = 2.822(4) Å; Nd–O = 2.792(6) Å). This unusual linkage is a unique illustration of the so-called cation–cation interaction associating 4f and 5f metals. The dinuclear motif is then further connected through the mellitate ligand, and this generates organic–inorganic layers that are linked to each other via discrete uranyl (UO₂)­O₄ units (square bipyramid), which ensure the three-dimensional cohesion of the structure. The mixed U–Ln carboxylate is thermally decomposed from 260 to 280 °C and then transformed into the basic uranium oxide (U₃O₈) together with U–Ln oxide with the fluorite structural type (“(Ln,U)­O₂”). At 1400 °C, only fluorite type “(Ln,U)­O₂” is formed with the measured stoichiometry of U_0.63Ce_0.37O₂ and U_0.60Nd_0.40O_2−δ