Solvothermal Synthesis of Tetravalent Uranium with Isophthalate or Pyromellitate Ligands

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.