Stable Pervanadyl Cation Encapsulated in Silica: Frivolous Vanadium in the H₄PVMo₁₁O₄₀ Keggin

Abstract

Encapsulation of H 4 PVMo 11 O 40 (1-vanado-11-molybdophosphoric acid) inside silica provided a single-site long-lived pervanadyl cation (VO 2 + ) for oxidation reactions. Protective silica residues allowed vanadium to sit in the secondary Keggin structure without being reduced, contrasting the classical situation for H 4 PVMo 11 O 40 supported on silica. The vanadium of this Keggin molecule is "frivolous", tending to exit the primary structure, with one-electron reduction, even during efflorescence under ambient conditions. Hydrated crystals circumvented this problem, allowing high resolution 51 V MAS NMR conditions and the first quadrupolar and chemical shift anisotropy interaction parameters for this intact Keggin. IntroductionPolyoxometalates (POM's) are nano-sized metal-oxygen cluster anions that self-assemble in aqueous solution. Heteropolyacids, or POM's in the acid form, are widely used to catalyze selective oxidation reactions under homogeneous and heterogeneous conditions. 1-7 Their acidbase and redox properties are finely altered by varying the chemical composition. The most stable POM's have the highly studied Keggin structure. 8 H 4 PVMo 11 O 40 ·xH 2 O is a more active oxidation catalyst than its fully molybdenum analogue (H 3 PMo 12 O 40 ), 4, 5, 9 yet its mode of action is less well understood. This work describes an interesting aspect of chemical behavior for PVMo 11 . POM formulas are abbreviated denoting only the central phosphorus and addenda metal atoms.One major obstacle in using POM's as heterogeneous catalysts is "containing" them in active form on a solid support with high surface area. Conventional wet impregnation on silica (i.e. POM-on-silica) does not provide a highly dispersed active phase with the desired "site isolation" effect. 10, 11 Far more stable and active catalysts are produced by sol-gel encapsulation methods 12 , where a "silica bottle" is built around a "POM ship". 13 Hydro-silanes, derived from silicon alkoxide precursors (e.g. TEOS, tetraethoxysilane), condense around the pre-formed POM to give "POM-in-silica". Preponderant examples exist for PW 12 -in-silica 5,14-19 , followed by PMo 12 20 , SiW 12 21, 22 and GeV 2 W 10 23 encapsulated inside silica. As we show here, little known PVMo 11 -in-silica 24,25, 26 has special properties. In addition to PVMo 11 -in-silica, more hydrophobic catalysts PVMo 11 -in-organosilica (Rsilica), with admixtures of MTOS (methyltriethoxysilane) or DDOS (dimethyldiethoxysilane) and TEOS, were also prepared and examined. The structures of these new catalysts, active in aerobic mercaptans oxidation 26 , are analyzed by multi-nuclear NMR spectroscopy. A technique deemed more effective than IR and Raman for discriminating the molecular species under consideration, especially under hydrated conditions. 27 However, an NMR approach ( 31 P, 51 V) has two shortcomings: 1) poor discrimination of species by 31 P chemical shift and 2) paramagnetic broadening affects from reduced (tetravalent) vanadium nuclei.Another, more intrinsic, obstacle exists fo...

Effect of positional isomerism and vanadium substitution on 51V magic angle spinning NMR Spectra Of Wells-Dawson polyoxotungstates

“…Advancements in theoretical quantum chemical methods have allowed for the investigations of Wells-Dawson (and generally polyoxometalate solids) [16–20], and these studies are expected to aid in the interpretation of the experimental structural and spectroscopic data. We and others have shown that 51 V solid-state NMR spectroscopy can be used as a sensitive reporter of the structural and electronic environment of vanadium sites in a wide variety of inorganic and bioinorganic solids [21–39] including polyoxometalates [40–42]. Specifically, in our previous MAS NMR studies on Lindqvist and Keggin polyoxotungstate solids, we have demonstrated that insights unavailable from X-ray diffraction and other structural methods could be gained, namely that counter cations and the degree of vanadium substitution modulate the electronic structure of the oxoanion as is evident in the 51 V magic angle spinning (MAS) NMR spectra [40, 41] as well as vibrational spectra [16].…”