Synthesis and characterization of the tetrameric, tri-titanium(IV)-substituted Wells–Dawson-substructure polyoxotungstate, [(P2W15Ti3O60.5)4]36−: the significance of ultracentrifugation molecular weight measurements in detecting aggregated, anhydride form

The preparation and crystallization of a tetrameric form of the tri-TiIV-substituted Wells–Dawson-substructure (i.e. [P2W18O62]6−-type) heteropolyoxoanion, [(P2W15Ti3O60.5)4]36−, as its water-soluble potassium salt, are described. This compound has been prepared by a direct reaction of Na12[P2W15O56]·18H2O with 30% aqueous TiIV(SO4)2 solution. The compound with a formula of {K28H8[P2W15Ti3O60.5]4}·0.8KCl·56H2O (1) was obtained in 38% yield as an analytically pure, colorless, powdered solid. The crystallization of 1 was achieved in the presence of KCl in pH 2.0 water to form {K28H8[P2W15Ti3O60.5]4}·46H2O (2). The KCl significantly aids the crystallization of 1, but does not enter into the final composition of crystalline 2. The mixed tetrabutylammonium plus potassium salt {(Bu4N)17H11K8[P2W15Ti3O60.5]4} (3) was successfully prepared as an analytically pure white powder from the reaction of 1 in pH 2.0 water with solid (Bu4N)Br; the resultant 3 is acetonitrile- and DMSO-soluble, but water-insoluble. Compositional and structural characterization of 1–3 was accomplished by complete elemental analyses, TG/DTA, FT-IR, UV–Vis spectroscopy, and solution 31P and 183W NMR spectroscopies, including solid-state GHD/MAS 31P NMR spectroscopy in the case of 2. We also report a scaled-up, 10–12 g preparation of the trimeric {(Bu4N)15H17[P2W15Ti3O61.3]3} (4) (i.e. the tri-TiOTi anhydride-bridged {[P2W15Ti3O61]O[P2W15Ti3O60]O[P2W15Ti3O61]}32−) and its partial deprotonation and partial TiOTi cleavage with Bu4N+OH− to the primarily dimeric {(Bu4N)9H3[P2W15Ti3O∼62]n} (5) (n=1–2) (i.e. the primarily mono-TiOTi anhydride-bridged {[P2W15Ti3O61]O[P2W15Ti3O61]}22−). Most importantly, each of the tri-titanium-substituted Wells–Dawson-type polyoxoanions 1–5 are shown, by solution molecular weight experiments plus appropriate control experiments, to exist in aggregated, TiOTi anhydride forms, with the parent compounds 1, 2 and 3 all existing as tetramers, [(P2W15Ti3O60.5)4]36−. This finding of only TiOTi-bridged, anhydride forms of tri-Ti(IV) substituted Wells–Dawson polyoxoanions is important in that it corrects several previous, erroneous reports in the literature claiming, without molecular weight evidence, that the monomeric ‘[P2W15Ti3O62]12−’ exists. Monomeric ‘[P2W15Ti3O62]12−’ is, however, still unknown.