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. 2020 Sep 8;59(17):11945-11952.
doi: 10.1021/acs.inorgchem.9b03747. Epub 2020 Mar 5.

Selective Oxidation by H5[PV2Mo10O40] in a Highly Acidic Medium

Affiliations

Selective Oxidation by H5[PV2Mo10O40] in a Highly Acidic Medium

Chandan Kumar Tiwari et al. Inorg Chem. .

Abstract

Dissolution of the polyoxometalate (POM) cluster anion H5[PV2Mo10O40] (1; a mixture of positional isomers) in 50% aq H2SO4 dramatically enhances its ability to oxidize methylarenes, while fully retaining the high selectivities typical of this versatile oxidant. To better understand this impressive reactivity, we now provide new information regarding the nature of 1 (115 mM) in 50% (9.4 M) H2SO4. Data from 51V NMR spectroscopy and cyclic voltammetry reveal that as the volume of H2SO4 in water is incrementally increased to 50%, V(V) ions are stoichiometrically released from 1, generating two reactive pervanadyl, VO2+, ions, each with a one-electron reduction potential of ca. 0.95 V (versus Ag/AgCl), compared to 0.46 V for 1 in 1.0 M aq H2SO4. Phosphorus-31 NMR spectra obtained in parallel reveal the presence of PO43-, which at 50% H2SO4 accounts for all the P(V) initially present in 1. Addition of (NH4)2SO4 leads to the formation of crystalline [NH4]6[Mo2O5(SO4)4] (34% yield based on Mo), whose structure (from single-crystal X-ray diffraction) features a corner-shared, permolybdenyl [Mo2O5]2+ core, conceptually derived by acid condensation of two MoO3 moieties. While 1 in 50% aq H2SO4 oxidizes p-xylene to p-methylbenzaldehyde with conversion and selectivity both greater than 90%, reaction with VO2+ alone gives the same high conversion, but at a significantly lower selectivity. Importantly, selectivity is fully restored by adding [NH4]6[Mo2O5(SO4)4], suggesting a central role for Mo(VI) in attenuating the (generally) poor selectivity achievable using VO2+ alone. Finally, 31P and 51V NMR spectra show that intact 1 is fully restored upon dilution to 1 M H2SO4.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
H5[PV2Mo10O40] (1) in 50% aq H2SO4. Dissolution of 1 (at left) in this acidic medium generates highly reactive pervanadyl ions, VO2+, along with a [Mo2O5]2+-core complex (at right), shown below to be responsible for retention of selectivity. Dilution to 1 M H2SO4 leads to a quantitative reconstitution of intact 1.
Figure 2
Figure 2
Vanadium-51 NMR spectra of 115 mM 1 as [H2SO4] was increased from 0 to 9.4 M (50% v/v). The measurements were carried out using K4[PVW11O40] (0.1 equiv with respect to 1 in 9:1 H2O: D2O; indicated by a hash-tag, #) as an external reference in a coaxial NMR tube. The topmost spectrum was obtained after adding an additional 0.25 equiv of NaVO3 in 9.4 M H2SO4.
Figure 3
Figure 3
(A) Reduction potentials as a function of [H2SO4] for 115 mM 1. Inset: the CV of 1 in 0.1 M H2SO4 (gray curve; for clarity, the y-axis has been suppressed by a factor of 3) and in 9.4 M H2SO4 (red curve). (B) Reduction potentials as a function of [H2SO4] for 230 mM NaVO3 (from 1 to 5 M H2SO4, cathodic-maxima values were used; see Figure S3). Inset: the CV of NaVO3 in pure water (gray curve) and in 9.4 M H2SO4 (red curve).
Figure 4
Figure 4
(A) Amperometric titration of V(V) released from 115 mM 1 (i.e., 230 mM V(V) in 9.4 M H2SO4) by incremental additions of NaVO3 as follows: (a) 0 mM (i.e., 1 alone), (b) 57.5 mM, (c) 115 mM, (d) 172.5 mM, and (e) 230 mM. (B) Plot of cathodic-current maxima relative to that for 1 (i.e., (a) in panel A) as a function of the total concentration of V(V) relative to that from 1. (C) Amperometric titration of 230 mMNaVO3 (in 9.4 M H2SO4) by incremental additions of more NaVO3 to give the following total concentrations: (a) 230 mM, (b) 287.5 mM, (c) 345 mM, (d) 402.5 mM, and (e) 460 mM. (D) Plot of cathodic-current maxima relative to that for 230 mM NaVO3 (i.e., (a) in panel C) as a function of the total concentration of V(V) relative to 230 mM.
Figure 5
Figure 5
Phosphorus-31 NMR spectra of 1 (115 mM) at incrementally larger [H2SO4] values. The measurements were carried out using Na4P2O7 in 90:10 v:v H2O:D2O as a quantitative integration standard (*; 0.33 equiv with respect to 1) in a coaxial NMR tube.
Figure 6
Figure 6
(A) Phosphorus-31 NMR spectra (in 9.4 M H2SO4) of (a) 115 mM H3PO4, (b) 115 mM H3PO4 and 230 mM NaVO3, (c) 115 mM H3PO4 and 1150 mM Na2MoO4, and (d) 115 mM 1. (B) Variable-temperature 31P NMR spectra of 1 in 50% H2SO4, showing coalescence at 333 K.
Figure 7
Figure 7
Conversion and selectivity for oxidations of p-xylene in 9.4 M H2SO4. Species added to the acidic medium, defined in the key at lower left, are abbreviated below the x-axis as equivalents added relative to 115 mM 1, i.e., P = 115 mM H3PO4, 2 V = 230 mM NaVO3, and 10Mo = 1150 mM Na2MoO4. In each case, p-xylene, 1.84 mmol (0.168 g), was layered on top of 8 mL of 9.4 M H2SO4 in a high-pressure vessel and kept at 60 °C for 5 h under N2.
Figure 8
Figure 8
Crystal structure of [NH4]6[Mo2O5(SO4)4] (2). The polyhedra represent six-coordinate MoO6 units, while terminal and bridging SO42– ligands are shown in ball and stick notation, with O in red and S in yellow.
Figure 9
Figure 9
P-31-NMR spectra in 9.4 M H2SO4 of H3PO4 and 2 (1:10 ratio of P to Mo; bottom) and of 1 (top).
Figure 10
Figure 10
H5[PV2W10O40] (1) in 9.4 M H2SO4. The four panels show 31P NMR spectra (A) of 40 mM 1 in pure water (native pH = 0.8), (B) of 115 mM 1 in 9.4 M H2SO4, (C) of the same solution after reduction by p-xylene, (D) after electrochemical reoxidation, and (inset to part A) after dilution of the reoxidized solution to 40 mM, along with addition of NaOH to increase the pH to 0.8.

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