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

The sulfates Nb2O2(SO4)3, MoO2(SO4), WO(SO4)2, and two modifications of Re2O5(SO4)2 have been synthesized by the solvothermal reaction of NbCl5, WOCl4, Re2O7(H2O)2, and MoO3 with sulfuric acid/SO3 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(SO4)2 (monoclinic, P21/n, a = 7.453(1) Å, b = 11.8232(8) Å, c = 7.881(1) Å, β = 107.92(2)°, V = 660.7(1) Å3, Z = 4) and both modifications of Re2O5(SO4)2 (I: orthorhombic, Pba2, a = 9.649(1) Å, b = 8.4260(8) Å, c = 5.9075(7) Å, V = 480.27(9) Å3, Z = 2; II: orthorhombic, Pbcm, a = 7.1544(3) Å, b = 7.1619(3) Å, c = 16.8551(7) Å, V = 863.64(6) Å3, 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 [Re2O5] units, respectively. The cohesion between layers is realized through weak M−O contacts (343−380 pm). Nb2O2(SO4)3 (orthorhombic, Pna21, a = 9.9589(7) Å, b = 11.7983(7) Å, c = 8.6065(5) Å, V = 1011.3(1) Å3, 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 MoO2(SO4) (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) Å3, Z = 24) [MoO2] 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(SO4)2, the metal ions are octahedrally coordinated by monodentate sulfate ions and oxide ligands forming short MO bonds. In WO(SO4)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.