⁵¹V Magic Angle Spinning NMR Spectroscopy of Six-Coordinate Lindqvist Oxoanions:  A Sensitive Probe for the Electronic Environment in Vanadium-Containing Polyoxometalates. Counterions Dictate the ⁵¹V Fine Structure Constants in Polyoxometalate Solids

Abstract

Geometric and electronic environments of vanadium have been addressed by (51)V magic angle spinning NMR spectroscopy of six-coordinated polyoxometalate solids. (C(4)H(9))(4)N(+) and mixed Na(+)/Cs(+) salts of the Lindqvist-type mono- and divanadium-substituted oxotungstates, [VW(5)O(19)](3-) and [V(2)W(4)O(19)](4-), have been prepared as microcrystalline and crystalline solids. The solid-state NMR spectra reflect the details of the local environment of the vanadium site in these hexametalate solids via the anisotropic quadrupolar and chemical shielding interactions. Remarkably, these (51)V fine structure constants in the solid state are dictated by the nature and geometry of the countercations. Electrostatic calculations of the electric field gradients at the vanadium atoms have been performed. Experimental trends are well reproduced with the simple electrostatic model, and explain the sensitivity of the anisotropic NMR parameters to the changes in the cationic environment at the vanadium site.

Effect of Cations in Infrared and Computational Analysis of Vanadium-Containing Six-Coordinate Oxotungstates

“…The inclusion of vanadium into the hexametalate frame causes a series of complex vibrations involving bridge oxygen atom absorption bands to split into an even more complex series, where the exact position of the absorption bands depends on the structure of the salt. It has been shown that (TBA) 4 31 In addition, the inclusion of water enhances the intensity of the group of vibrations around 990 cm -1 with the simple motion of terminal M-O t as the major component. Although the inclusion of additional water molecules may add to the complexity of the theoretically predicted spectrum because of the inhomogeneous broadening of the absorption bands, it is unlikely that any new significant qualitative or quantitative differences will be acquired.…”

“…30 Elemental analysis and X-ray crystallographic characterization of the compounds obtained were performed at Hunter College and are discussed in detail elsewhere. 31 Spectroscopic Methods. Experiments were performed using a Nicolet Magna 560 FTIR spectrometer with a liquid nitrogen cooled MCT detector.…”

Internuclear 31P−51V Distance Measurements in Polyoxoanionic Solids Using Rotational Echo Adiabatic Passage Double Resonance NMR Spectroscopy

“…17,18,27,28 In our recent work, we have explored 51 V and 31 P solid-state NMR spectroscopy for structural characterization of vanadium-and lanthanide-substituted polyoxometalate solids and demonstrated that anisotropic NMR parameters provide a wealth of information about the local geometry and electronic environment in these materials. [29][30][31] In this work, we illustrate that direct P-V distances in a series of vanadium-substituted Keggin and Wells-Dawson solids can be accurately determined by 31 P-51 V REAPDOR spectroscopy. We expect our approach to be particularly useful for structural characterization of non-crystalline polyoxometalate solids.…”

Magic Angle Spinning NMR Spectroscopy: A Versatile Technique for Structural and Dynamic Analysis of Solid-Phase Systems

“…121 Multinuclear MAS NMR spectroscopy has also been commonly applied for the analysis of polyoxometalate based catalysts, where electronic structure of the anions has been investigated as a function of heteroatom substitution and the counterion. 122,123 …”