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

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 PO₄^3-, 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.

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

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

(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

(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

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

(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

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

Crystal structure of [NH₄]₆[Mo₂O₅(SO₄)₄] (2). The polyhedra represent six-coordinate MoO₆ units, while terminal and bridging SO₄^2– ligands are shown in ball and stick notation, with O in red and S in yellow.

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

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.