Structure determination of the anhydrous form of MIL-3K: a vanadodiphosphonate with a 3D hybrid framework

K(VVO(H2O))(VIVO)O{O3P–(CH2)2–PO3} (labelled MIL-3K) is a vanado-ethylenediphosphonate whose previously reported hybrid framework reversibly dehydrates. Its anhydrous form (KVIV/V2O3{O3P–(CH2)2–PO3} or MIL-3Kan) has been determined from X-ray powder diffraction data in the centrosymmetric triclinic P (no. 2) space group with lattice parameters: a = 7.2375(1) , b = 8.1852(1), c = 9.8862(2) A, α = 83.2209(9), β = 65.223(1), γ = 76.492(1)°, V = 516.90(2), Z = 2. RF2 = 0.0920 and Rwp = 0.0157. The two structures of MIL-3K and MIL-3Kan are quite similar, just the square pyramidal coordination of one vanadium atom of the hydrate transforms into a VVO4 tetrahedron in the dehydrated form. This change induces the opening of eight-membered channels delimited by phosphonate tetrahedra and vanadium polyhedra in strict alternation.