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

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.