title: Sulfates of the refractory metals: crystal structure and thermal behavior of Nb2O2(SO4)3, MoO2(SO4), WO(SO4)2, and two modifications of Re2O5(SO4)2.
abstract: The sulfates Nb(2)O(2)(SO(4))(3), MoO(2)(SO(4)), WO(SO(4))(2,) and two modifications of Re(2)O(5)(SO(4))(2) have been synthesized by the solvothermal reaction of NbCl(5), WOCl(4), Re(2)O(7)(H(2)O)(2), and MoO(3) with sulfuric acid/SO(3) mixtures at temperatures between 200 and 300 °C. Besides the X-ray crystal structure determination of all compounds, the thermal behavior was investigated using thermogravimetric studies. WO(SO(4))(2) (monoclinic, P2(1)/n, a = 7.453(1) Å, b = 11.8232(8) Å, c = 7.881(1) Å, β = 107.92(2)°, V = 660.7(1) Å(3), Z = 4) and both modifications of Re(2)O(5)(SO(4))(2) (I: orthorhombic, Pba2, a = 9.649(1) Å, b = 8.4260(8) Å, c = 5.9075(7) Å, V = 480.27(9) Å(3), Z = 2; II: orthorhombic, Pbcm, a = 7.1544(3) Å, b = 7.1619(3) Å, c = 16.8551(7) Å, V = 863.64(6) Å(3), Z = 4) are the first structurally characterized examples of tungsten and rhenium oxide sulfates. Their crystal structure contains layers of sulfate connected [W═O] moieties or [Re(2)O(5)] units, respectively. The cohesion between layers is realized through weak M-O contacts (343-380 pm). Nb(2)O(2)(SO(4))(3) (orthorhombic, Pna2(1), a = 9.9589(7) Å, b = 11.7983(7) Å, c = 8.6065(5) Å, V = 1011.3(1) Å(3), Z = 4) represents a new sulfate-richer niobium oxide sulfate. The crystal structure contains a three-dimensional network of sulfate connected [Nb═O] moieties. In MoO(2)(SO(4)) (monoclinic, I2/a, a = 8.5922(6) Å, b = 12.2951(6) Å, c = 25.671(2) Å, β = 94.567(9)°, V = 2703.4(3) Å(3), Z = 24) [MoO(2)] units are connected through sulfate ions to a three-dimensional network, which is pervaded by channels along [100] accommodating the terminal oxide ligands. In all compounds except WO(SO(4))(2), the metal ions are octahedrally coordinated by monodentate sulfate ions and oxide ligands forming short M═O bonds. In WO(SO(4))(2), the oxide ligand and two monodentate and two bidentate sulfate ions build a pentagonal bipyramid around W. The thermal stability of the sulfates decreases in the order Nb > Mo > W > Re; the residues formed during the decomposition are the corresponding oxides.
authors.affiliation: Institute for Pure and Applied Chemistry, Carl von Ossietzky University of Oldenburg, Oldenburg, Germany.
authors.forename: Ulf
authors.initials: U
authors.lastname: Betke
authors.forename: Mathias S
authors.initials: MS
authors.lastname: Wickleder