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 forms of polyoxoanions

The preparation and crystallization of a tetrameric form of the tri-Ti-IV-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 Na-12[P2W15O56]. 18H(2)O with 30% aqueous Ti-IV(SO4)(2) solution. The compound with a formula of {K28H8[P2W15Ti3O60.5](4)}. 0.8KCl . 56H(2)O (1) was obtained in 38% yield as an analytically pure, colorless, powdered solid. The crystallization of 1 was achieved in the presence of KCI in pH 2.0 water to form {K28H8[P2W15Ti3O60.5](4)}. 46H(2)O (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)(17)H11K8[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, W-Vis spectroscopy, and solution P-31 and W-183 NMR spectroscopies, including solid-state GHD/MAS P-31 NMR spectroscopy in the case of 2. We also report a scaled-up, 10-12 g preparation of the trimeric {(BU4N)(15)H-17[P2W15Ti3O61.3](3)} (4) (i.e. the tri-Ti-O-Ti anhydride-bridged {[P2W15Ti3O61]-O-[P2W15Ti3O61]-O-[P2W15Ti3O61]}(32-)) and its partial deprotonation and partial Ti-O-Ti cleavage with Bu4N+OH- to the primarily dimeric {(Bu4N)(9)H-3[P2W15Ti3Osimilar to 62]n} (5) (n = 1-2) (i.e. the primarily mono-Ti-O-Ti anhydride-bridged {[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 -)' however, still unknown. (C) 2000 Elsevier Science S.A. All rights reserved.