THE NEW OPEN FRAMEWORK Na_x[(V^IVO₂)₂(V^V(O,OH))_0.5 PO₄](PO₄)_y(OH)_x-1.5-3y . ZH₂O (WITH x=1.5-2.0), y=0-0.17, z=3-.4). M. Schindler, W. H. Baur, Inst. für Kristallographie, Goethe-Universität, Senckenberganlage 30, 60054 Frankfurt am Main, Germany

The title compound (short: FVP-1) is one of a new class of frameworks that are more open than all aluminosilicates and Al- or Ga-phosphates known so far. These frameworks exchange ions and their H₂O can be reversibly removed. FVP-1 was grown hydrothermally in the Na/V/P/-H₂O/tetraethylamine system (473K). It crystallizes in space group Im3m with a=16.025Å. It contains V^IV₄V^VO₉(PO₄)_4/2 groups made up of 5 square pyramidal coordinations of O atoms around V in the shape of a spiked helmet, where the spike is the V=O group of the central V^V co-ordination polyhedron. These molecules are statistically arranged around mirror planes. In contrast to the Müller-Döring-type (1) Keggin molecule, e.g. V₁₈O₄₂(SO₄), the tetrahedral groups are placed here outside of the shell of square pyramidally coordinated V atoms and thus are able to form bridges to neighboring molecules. We call them anti-Keggin molecules. The framework formed by the V^IV₄V^VO₉(PO₄)_4/2 molecules is best visualized by recalling the net on which the crystal structure of NbO is based [Wells, (2)]. The mean free channel diameters measure about 4.1Å and the framework density is 10.2 V and P atoms/1000 Å. Thus FVP-1 and two other recently found microporous frameworks (3), are the most open framework described so far. FVP-1 does not collapse when dehydrated, and is stable up to 623 K. The size of the pores in the framework is large, because its building unit, V^IV₄V^VO₉(PO₄)_4/2, is of low symmetry and already large in itself. V^IV/V^V compounds are known to be potentially useful as catalysts. It can be expected that other phases based on the same principle can be synthesized under similar conditions.

(1) A. Müller and J. Döring, Z. anorg. allg. Chem., 1991, 595, 251.

(2) A. F. Wells, 'Structural Inorg. Chem. '' Oxford Univ. Press, 1993.

(3) M. I. Khan et al., Chem. Mater., 1996, 8, 43.