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

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.