Hybrid open frameworks (MIL-n). Part 3 Crystal structures of the HT and LT forms of MIL-7: a new vanadium propylenediphosphonate with an open-framework. Influence of the synthesis temperature on the oxidation state of vanadium within the same structural type

Abstract

The hydrothermal synthesis and structure determinations of two vanadium propylenediphosphonate compounds are presented. MIL-7, formulated as [(VO) ₂ (OH)(H ₂ O){O ₃ P(CH ₂ ) ₃ PO ₃ }](H ₂ O) ₂ (NH ₄ ), is synthesized at 200°C. Its structure is monoclinic [space group C2/c (no. 15)] with lattice parameters a=14.2928(2) Å, b=10.2440(2) Å,c=18.9901(1) Å, β=96.658(1)°, V=2761.69(7) Å ³ , Z=8. The three-dimensional framework is built up from inorganic V ^IV -P-O layers strongly related by the alkyl chains. Upon heating hydrothermally the same initial mixture but at 170°C one obtains a different phase, formulated as [V ₂ O ₃ (H ₂ O){O ₃ P(CH ₂ ) ₃ PO ₃ }](H ₂ O) ₂ (NH ₄ ), the structure of which is closely related to MIL-7. Its symmetry is monoclinic [space group C2/c (no. 15)] with lattice parameters a=14.8998(8) Å, b=10.2903(6) Å, c=18.515(1)Å,β=101.079(1)°, V=2785.9(3) Å ³ , Z=8. The topology of the inorganic layers is almost identical, but presents a mixed valence V ^IV -V ^V state in a 1:1 ratio.