Triniobium Polytungstophosphates. Syntheses, Structures, Clarification of Isomerism and Reactivity in the Presence of H2O2

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• 作者：Mason K. Harrup ; Gyu-Shik Kim ; Huadong Zeng ; Rhoma P. Johnson ; Don VanDerveer ; and Craig L. Hill
• 刊名：Inorganic Chemistry
• 出版年：1998
• 出版时间：October 19, 1998
• 年：1998
• 卷：37
• 期：21
• 页码：5550 - 5556
• 全文大小：97K
• 年卷期：v.37,no.21(October 19, 1998)
• ISSN：1520-510X

文摘

The reaction of K₇[HNb₆O₁₉], H₂O₂ and A-Na₉[PW₉O₃₄] in water followed by treatment with Cs⁺ or (n-Bu₄N)⁺(TBA) affords the corresponding salts of the tris(peroxoniobium) heteropolyanion A,-[(NbO₂)₃PW₉O₃₇]^6- (1) in~60% isolated yields. An X-ray structure of the Cs salt, Cs1 (monoclinic P2/c; a = 16.92360(10) Å, b = 13.5721(2) Å, c = 22.31890(10) Å, = 92.0460(10), and Z = 4) confirms the A-type substitution pattern of the threeNb atoms and clarifies the M₃ rotational (Baker-Figgis) isomerism in the Keggin unit as . The three terminal²-O₂^2- groups on the Nb atoms give 1 an overall symmetry approximating the chiral C₃. These terminal peroxoligands, and these groups only, thermally decompose when either Cs1 or TBA1 is in solution unless additionalH₂O₂ is present. The peroxo groups can be titrated with triphenylphosphine (2.8 ± 0.3 peroxide groups foundper molecule). Refluxing TBA1 in acetonitrile for 24 h in the presence of base generates the parent heteropolyanion,[Nb₃PW₉O₄₀]^6- (2) in 80% yield after isolation. Treatment of 2 with glacial acetic acid in acetonitrile convertsit to [Nb₆P₂W₁₈O₇₇]^6- (3) in ~100% yield, while treatment of TBA3 with hydroxide converts it back to 2 in highyield. Spectroscopic (FTIR, Raman, ¹⁸³W NMR, and ³¹P NMR), titrimetric, mass spectrometric (FABMS), andelemental analysis data are all consistent with these formulas. The addition of TBA1 to solutions of alkenes and33% aqueous peroxide in acetonitrile at reflux results in the generation of the corresponding vicinal diols in highselectivity and yield at high conversion of substrate. Several spectroscopic and kinetics experiments, includinga novel one correlating the incubation time of TBA1 under the reaction conditions with the rates of alkene oxidation,establish that TBA1 functions primarily as a catalyst precursor and that much of the catalytic activity is derivedfrom generation of tungstate under the reaction conditions.