Polyoxoanions derived from tungstosilicate (A-.beta.-SiW9O3410-): synthesis, single-crystal structural determination, and solution structural characterization by tungsten-183 NMR and IR of titanotungstosilicate (A-.beta.-Si2W18Ti6O7714-)

Abstract

The previously unknown polyoxometalate A-[beta]-Si[sub 2]W[sub 18]Ti[sub 6]O[sub 77][sup 14[minus]] has been prepared by the reaction of A-[beta]-HSiW[sub 9]O[sub 34][sup 9[minus]] with Ti(O)(C[sub 2]O[sub 4])[sub 2][sup 2[minus]] or Ti(O)SO[sub 4] under controlled pH conditions; yield optimization experiments are also reported. Isolation of this new Ti[sup IV]-substituted polyoxometalate was accomplished both as its water-soluble K[sup +] salt and as its organic-solvent-soluble Bu[sub 4]N[sup +] salt. The unambiguous characterization of A-[beta]-Si[sub 2]W[sub 18]Ti[sub 6]O[sub 77][sup 14[minus]] was accomplished by means of elemental analysis, IR and [sup 183]W NMR spectroscopy, and a single-crystal X-ray structural analysis of the Bu[sub 4]N[sup +]/H[sup +] salt, (Bu[sub 4]N)[sub 7]H[sub 7]Si[sub 2]W[sub 18]Ti[sub 6]O[sub 77] (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]-(SiW[sub 9]O[sub 37])[sub 2](Ti-O-Ti)[sub 3][sup 14[minus]], a condensed aggregate of two hypothetical [open quotes]A-[beta]-SiW[sub 9](TiOH)[sub 3]O[sub 37][sup 7[minus]][close quotes] Keggin units that have lost three H[sub 2]O 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 X[sub 2]M[sub 18]M[prime][sub 6]O[sub 77][supmore » n[minus]] can be prepared for M[prime] in at least three different oxidation states, M[prime](III), M[prime](IV), and M[prime](V).« less