Solvothermal synthesis of tetravalent uranium with isophthalate or pyromellitate ligands

Abstract

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. The combination of tetravalent uranium with isophthalic acid and pyromellitic acid leads to the production of two new coordination polymers. Both structures exhibit a 3D framework composed of an isolated tetravalent uranium monomer. In the case of the isophthalate derivative, thermal treatment engenders a single-crystal to single-crystal transformation and the formation of a desolvated structure.