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.