Series of Hydrated Heterometallic Uranyl-Cobalt(II) Coordination Polymers with Aromatic Polycarboxylate Ligands: Formation of UOCo Bonding upon Dehydration Process

Five new heterometallic UO22+–Co2+ coordination polymers have been
obtained by hydrothermal reactions of uranyl nitrate and metallic
cobalt with aromatic polycarboxylic acids. Single-crystal X-ray diffraction
reveals the formation of four 3D frameworks with the mellitate (noted
mel) ligand and one 2D network with the isophthalate (noted iso) ligand.
The compounds [(UO2(H2O))2Co­(H2O)4(mel)]·4H2O (1),
[UO2Co­(H2O)4(H2mel)]·2H2O (2), and [(UO2(H2O))2­Co­(H2O)4(mel)] (4) consist of 3D frameworks built up from the connection of mellitate
ligands and mononuclear metallic centers. These three compounds exhibit
two types of geometry for the uranyl cation: pentagonal bipyramidal
environment for 1 and 4, and hexagonal bipyramidal
environment for 2. Using the mellitate ligand, the uranyl
dinuclear unit is isolated in the compound [(UO2)2(OH)2­(Co­(H2O)4)2(mel)]·2H2O (3). Due to their 2D framework
and the presence of uncoordinated cobalt­(II) cations, the compound
[(UO2)­(iso)3]­[Co­(H2O)6]·3­(H2O) (5) is drastically different
than the previous one. The thermal behavior of compounds 1, 2, and 3 has been studied by thermogravimetric
analysis, X-ray thermodiffraction, and in situ infrared.
By heating, the dehydration of compounds 1 and 2 promotes two structural transitions (1 → 1′ and 2 → 2′). The crystal structures of [(UO2(H2O))2Co­(H2O)2(mel)] (1′) and [(UO2)­Co­(H2mel)] (2′) were determined by single-crystal X-ray diffraction; each of them
presents a heterometallic interaction between uranyl bond and the
Co­(II) center. Due to the rarely reported coordination environment
for the cobalt center in compound 2′ (square pyramidal
configuration), the magnetic properties and EPR characterizations
of the compounds 2 and 2′ were also
investigated.