Crystal Structure and Raman Spectroscopy of FeVMoO₇ and CrVMoO₇ with Mo=O Double Bonds

Single crystals of FeVMoO₇ and CrVMoO₇ were grown by a flux method from Fe₂O₃-V₂O₃-MoO₃ and Cr₂O₃V₂O₅-MoO₃ melts, respectively. Single-crystal X-ray diffraction revealed that FeVMoO₇ and CrVMoO₇ are isostructural, in which V⁵⁺ and Mo⁶⁺ ions in the [VMoO₇]^3- units are crystallographically distinct, and one type of oxygen is bonded only to molybdenum, forming a Mo=O double bond. These rare structural features were also studied by infrared and Raman spectroscopy. Raman Mo=O stretching frequencies are consistent with the assignments made in previous surface studies of molybdate catalysts. Crystal data for FeVMoO₇: triclinic, space group P1̄ (No. 2), with a = 5.564(1) Å, b = 6.669(1) Å c = 7.909(1) Å, α = 97.65(1)°, β= 90.33(1)°, γ = 101.32(2)° and Z = 2. Crystal data for CrVMoO₇: triclinic, space group P1̄ (No. 2), with a = 5.5273(9) Å, b = 6.582(1) Å, c = 7.8650(9) Å, α = 96.19(1)°, β= 89.87(1)°, γ = 102.01(1)°, and Z = 2.