Series of Mixed Uranyl–Lanthanide (Ce, Nd) Organic Coordination Polymers with Aromatic Polycarboxylates Linkers

Three series of mixed uranyl-lanthanide (Ce or Nd) carboxylate
coordination polymers have been successfully synthesized by means
of a hydrothermal route using either conventional or microwave heating
methods. These compounds have been prepared from mixtures of uranyl
nitrate, lanthanide nitrate together with phthalic acid (1,2), pyromellitic acid (3,4), or mellitic acid (5,6) in aqueous solution.
The X-ray diffraction (XRD) single-crystal revealed that the phthalate
complex (UO2)4O2Ln­(H2O)7(1,2-bdc)4·NH4·xH2O (Ln = Ce­(1), Nd­(2); x = 1 for 1, x = 0 for 2), is based on the connection of tetranuclear uranyl-centered
building blocks linked to discrete monomeric units LnO2(H2O)7 via the organic species to generate
infinite chains, intercalated by free ammonium cations. The pyromellitate
phase (UO2)3Ln2(H2O)12(btec)3·5H2O (Ce­(3), Nd­(4)) contains layers of monomeric uranyl-centered
hexagonal and pentagonal bipyramids linked via the carboxylate arms
of the organic molecules. The three-dimensionality of the structure
is ensured by the connection of remaining free carboxylate groups
with isolated monomeric units LnO2(H2O)7. The network of the third series (UO2)2(OH)­Ln­(H2O)7(mel)·5H2O (Ce­(5), Nd­(6)) is built up from dinuclear uranyl
units forming layers through connection with the mellitate ligands,
which are further linked to each other through discrete monomers LnO3(H2O)6. The thermal decomposition of
the various coordination complexes led to the formation of mixed uranium-lanthanide
oxide, with the fluorite-type structure at 1500 °C (for 1, 2) or 1400 °C for 3–6. Expected U/Ln ratio from the crystal structures were observed
for compounds 1–6.