Adaptable coordination of U(IV) in the 2D-(4,4) uranium oxalate network: From 8 to 10 coordinations in the uranium (IV) oxalate hydrates

Crystals of uranium (IV) oxalate hydrates, U(C2O4)2·6H2O (1) and U(C2O4)2·2H2O (2), were obtained by hydrothermal methods using two different U(IV) precursors, U3O8 oxide and nitric U(IV) solution in presence of hydrazine to avoid oxidation of U(IV) into uranyl ion. Growth of crystals of solvated monohydrated uranium (IV) oxalate, U(C2O4)2·H2O·(dma) (3), dma=dimethylamine, was achieved by slow diffusion of U(IV) into a gel containing oxalate ions. The three structures are built on a bi-dimensional complex polymer of U(IV) atoms connected through bis-bidentate oxalate ions forming [U(C2O4)]4 pseudo-squares. The flexibility of this supramolecular arrangement allows modifications of the coordination number of the U(IV) atom which, starting from 8 in 1 increases to 9 in 3 and, finally increases, to 10 in 2. The coordination polyhedron changes from a distorted cube, formed by eight oxygen atoms of four oxalate ions, in 1, to a mono-capped square anti-prism in 3 and, finally, to a di-capped square anti-prism in 2, resulting from rotation of the oxalate ions and addition of one and two water oxygen atoms in the coordination of U(IV). In 1, the space between the ∞2[U(C2O4)2] planar layers is occupied by non-coordinated water molecules; in 2, the space between the staggered ∞2[U(C2O4)2·2H2O] layers is empty, finally in 3, the solvate molecules occupy the interlayer space between corrugated ∞2[U(C2O4)2·H2O] sheets. The thermal decomposition of U(C2O4)2·6H2O under air and argon atmospheres gives U3O8 and UO2, respectively.