Selective Oxidation by H(5)[PV(2)Mo(10)O(40)] in a Highly Acidic Medium

[Image: see text] Dissolution of the polyoxometalate (POM) cluster anion H(5)[PV(2)Mo(10)O(40)] (1; a mixture of positional isomers) in 50% aq H(2)SO(4) 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) H(2)SO(4). Data from (51)V NMR spectroscopy and cyclic voltammetry reveal that as the volume of H(2)SO(4) in water is incrementally increased to 50%, V(V) ions are stoichiometrically released from 1, generating two reactive pervanadyl, VO(2)(+), 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 H(2)SO(4). Phosphorus-31 NMR spectra obtained in parallel reveal the presence of PO(4)(3–), which at 50% H(2)SO(4) accounts for all the P(V) initially present in 1. Addition of (NH(4))(2)SO(4) leads to the formation of crystalline [NH(4)](6)[Mo(2)O(5)(SO(4))(4)] (34% yield based on Mo), whose structure (from single-crystal X-ray diffraction) features a corner-shared, permolybdenyl [Mo(2)O(5)](2+) core, conceptually derived by acid condensation of two MoO(3) moieties. While 1 in 50% aq H(2)SO(4) oxidizes p-xylene to p-methylbenzaldehyde with conversion and selectivity both greater than 90%, reaction with VO(2)(+) alone gives the same high conversion, but at a significantly lower selectivity. Importantly, selectivity is fully restored by adding [NH(4)](6)[Mo(2)O(5)(SO(4))(4)], suggesting a central role for Mo(VI) in attenuating the (generally) poor selectivity achievable using VO(2)(+) alone. Finally, (31)P and (51)V NMR spectra show that intact 1 is fully restored upon dilution to 1 M H(2)SO(4).