Crystal Structure and Spectroscopic Properties of Na₂K₆(VO)₂(SO₄)₇

Red-brown crystals of a new mixed alkali oxo sulfato vanadium(V) compound Na2K6(VO)2(SO4)7, suitable for X-ray determination, have been obtained from the catalytically important binary molten salt system M2S2O7−V2O5 (M = 80% K and 20% Na). By slow cooling of a mixture with the mole fraction XV2O5 = 0.24 from 325 °C, i.e., just below the liquidus temperature, to the solidus temperature of around 300 °C, a dark reddish amorphous phase was obtained containing crystals of the earlier described V(V)−V(IV) mixed valence compound K6(VO)4(SO4)8 and Na2K6(VO)2(SO4)7 described here. This compound crystallizes in the tetragonal space group P43212 (No. 96) with a = 9.540(3) Å, c = 29.551(5) Å at 20 °C and Z = 4. It contains a distorted VO6 octahedron with a short V−O bond of 1.552(6) Å, a long one of 2.276(5) Å trans to this, and four equatorial V−O bonds in the range 1.881(6)−1.960(6) Å. The deformation of the VO6 octahedron is less pronounced compared to that of the known oxo sulfato V(V) compounds. Each VO3+ group is coordinated to five sulfate groups of which two are unidentately coordinated and three are bidentate bridging to neighboring VO3+ groups. The length of the S−O bonds in the S−O−V bridges of the two unidentately coordinated sulfato groups are 1.551(6) Å and 1.568(6) Å, respectively, which are unusually long compared to our earlier measurements of sulfate groups in other V(III), V(IV), and V(V) compounds.