CRYSTAL STRUCTURES OF DAWSON-TYPE METAL-SUBSTITUTED POLYOXOTUNGSTATES
R. Mattes, P. Moraw, R. Neier, Ph. Schulte
Institute of Inorganic Chemistry, University Muenster,
Wilhelm-Klemm-Str. 8, D-48149 Muenster, Germany.
E-mail: mattes@uni-muenster.de
Keywords: Heteropolytungstates, metal-substituted Dawson-type ions, arsenotungstates
Metal-substituted polyoxoanions are of great interest in catalysis. Important classes of the compounds with overall Dawson-structures are
An example of the first class is [P2W15Nb3O62]9- [1]. Examples of the second class are [M(II)4(H2O)2(P2W15O56)2]16-, M(II) = Co, Cu, Zn [2] and [Fe(III)4(H2O)2 (P2W15O56)2]12- [3].
We have prepared and characterized the following compounds:
Anion 1 belongs to the rare species of twofold
substituted Dawson-ions. It was obtained by attempting the
preparation of a sandwich-type dimer compound. 1 is
disordered with respect to a longitudinal crystallographic mirror
plane, contrary to 2, which is the first example of a
crystallographically not disordered threefold-substituted
Dawson-ion. Here a W3-triplet is replaced by a Al3O10(H2O)3
unit with three rather regular AlO6 octahedra. The
Al-Al distances (307.4 pm) are considerably shorter than the W-W
distances within the W3-triplet (341.0 pm) in the
opposite site of the anion. The crystallographically ordered blue
coloured anion 3 contains a V3-triplet instead
of a W3-triplet. In 1-3 the W-W distances are
smaller in the substituted part and larger in the unsubstituted
part than in the regular Dawson-type polyoxotungstates [4].
Compounds 4a-4d are the first examples of sandwich-type
dimer Dawson-anions with arsenic as the heteroatom. Some of them
have been characterized by X-ray single crystal diffraction.