POLYOXOANIONS DERIVED FROM A-BETA-SIW9O(34)10- - SYNTHESIS, SINGLE-CRYSTAL STRUCTURAL DETERMINATION, AND SOLUTION STRUCTURAL CHARACTERIZATION BY 183W NMR AND IR OF A-BETA-SI2W18TI6O(77)14-

The previously unknown Polyoxometalate A-beta-Si2W18Ti6O7714- has been prepared by the reaction of A-beta-HSiW9O349-with Ti(O)(C2O4)22- or Ti(O)SO4 under controlled pH conditions; yield optimization experiments are also reported. Isolation of this new Ti(IV)-substituted polyoxometalate was accomplished both as its water-soluble K+ salt and as its organic-solvent-soluble Bu4N+ salt. The unambiguous characterization of A-beta-Si2W18Ti6O7714-was accomplished by means of elemental analysis, IR and W-183 NMR spectroscopy, and a single-crystal X-ray structural analysis of the Bu4N+/H+ salt, (Bu4N)7H7Si2W18Ti6O77 (Pbcm; a = 19.958(11), b = 36.880(8), c = 28.936(5) angstrom; Z = 4; R = 0.072 for 4614 observed independent reflections). The X-ray diffraction structural analysis reveals that the correct structural formulation is A-beta-(SiW9O37)2(Ti-O-Ti)314-, a condensed aggregate of two hypothetical ''A-beta-SiW9(TiOH)3O377-'' Keggin units that have lost three H2O by forming three Ti-O-Ti bridges. The present synthesis and characterization, plus recent results from the literature, reveal that a new subclass of polyoxoanions of general formula X2M18M'6O77n- can be prepared for M' in at least three different oxidation states, M'(III), M'(IV), and M'(V).