Three new coordination polymers bearing tetravalent uranium have been isolated with the O‐donor ligands such as isophthalate (1,3‐bdc) or pyromellitate (btec). The compounds 1 and 3 have been solvothermaly synthesized in N,N‐dimethylformamide (DMF). The crystal structure of U(1,3‐bdc)2(DMF) (1) is built up from discrete tricapped trigonal‐primastic UO9 units, for which one carbonyl oxygen atom from DMF is bound to uranium. The connection of the UO9 units with the isophthalate linkers occurs in a chelating and bidentate fashion and gives rise to the formation of a 3D framework, delimiting narrow channels running along the [101] direction. Upon heating, the DMF molecules are removed, generating the second phase U(1,3‐bdc)2 (2) compound, closely related to 1. Indeed, the coordination environment of uranium is reduced to eight with a distorted square‐antiprismatic geometry. This transition induces the relative shrinkage of the network (ΔV = 23 %). The structure of the compound U(btec)(DMF)2 (3) also exhibits a 3D framework composed of an isolated bicapped square‐antiprismatic UO10 unit, for which two carbonyl oxygen atoms from DMF are bonded to uranium. Pyromellitate ensures the connection of the UO10 units through the carboxylate arms in a chelating mode. This results in the formation of a network with diamond‐shaped channels, developed along the c axis and encapsulating the DMF molecules. In contrast to 1, no stable phase is observed upon removing the DMF species by heating.

Five new heterometallic UO-Co 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 [(UO(HO))Co(HO)(mel)]·4HO (1), [UOCo(HO)(Hmel)]·2HO (2), and [(UO(HO))Co(HO)(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 [(UO)(OH)(Co(HO))(mel)]·2HO (3). Due to their 2D framework and the presence of uncoordinated cobalt(II) cations, the compound [(UO)(iso)][Co(HO)]·3(HO) (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 [(UO(HO))Co(HO)(mel)] (1') and [(UO)Co(Hmel)] (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.