Synthesis and structure of dodecamolybdates of tetravalent cerium, thorium, uranium, neptunium, and plutonium

Abstract

A series of isostructural compounds of the composition Na7H[EMo12O42]·12H2O, where E(IV) = Ce, Th, U, Np, or Pu, were synthesized and structurally characterized. In the [EMo12O42]8– heteropolyanion (HPA), the central E(IV) atom is surrounded by six Mo2O9 groups, each constituted by two octahedra sharing a common face. The coordination polyhedron (CP) of the central atom is a weakly distorted icosahedron with the mean E(IV)–О bond lengths of 2.498, 2.529, 2.500, 2.490, and 2.488 A for Ce, Th, U, Np, and Pu, respectively. In the structure of the compounds Na7H[EMo12O42]·12H2O, there are two crystallographically independent sodium atoms: Na(1) and Na(2). The oxygen surrounding of the Na(1) atom is formed by the terminal oxygen atoms of two heteropolyanions adjacent along [001], and its coordination polyhedron is an octahedron. The surrounding of the Na(2) atom (a six-vertex polyhedron) is formed by three terminal oxygen atoms of three Mo2O9 groups belonging to the same HPA and by three water molecules. The coordination polyhedra of the Na(2) atoms are linked with each other via common oxygen atoms of Ow(2) water molecules to form a chain “winding” around the 31 screw axis. The heteropolyanions and Na+ cations in the crystal form a framework constructed in a fashion characteristic of Dexter–Silverton type anions, with the coordination via three terminal oxygen atoms of three Mo2O9 groups. Excess negative charge of HPA is compensated by the proton localized on one of the six bridging O atoms. In the Mo2O9 doubled octahedra, the Mo–O bonds with the О atoms bonded to E(IV) and forming the edge of the common face are sensitive to the kind of the central atom.