The Nature of H₅PV₂Mo₁₀O₄₀ Oxidants in Strongly Acidic Solvents

H₅PV₂Mo₁₀O₄₀ (1) polyoxometalates are electron transfer (ET) and electron transfer−oxygen transfer (ET−OT) catalysts for selective oxidation of hydrocarbons. In many organic solvents, the scope of these reactions was limited by the oxidation potential of 1, which is ca. 0.4−0.45 V versus saturated calomel electrode (SCE).¹ Recently, however, Neumann found that in 50% H₂SO₄ the reduction potential of 1 was increased to 1.1 V versus SCE, and as a result, able to oxidize C-H bonds of toluene² and benzene³ to benzaldehyde and phenol respectively. To better understand the nature of 1 in H₂SO₄, we (in collaboration with the Neumann group) have been investigating the solution-state chemistry of 1 as a function of H₂SO₄ concentration. Recent findings suggest that 1 rearranges in 50% H₂SO₄ so as to release V(V) (VO₂⁺ in the presence of H₂SO₄)⁴, and that this species might be responsible for the strongly oxidizing properties of the system. In cyclic voltammograms (CVs) of 1 in 50% H₂SO₄, for example, an electrochemically reversible redox couple is observed at 1.2 V, effectively identical to those obtained when NaVO₃ was dissolved in 50% H₂SO₄. Next, the release of PO₄^3- as a function of H₂SO₄ concentration was investigated by ³¹P-NMR spectroscopy, which confirmed the presence of free PO₄^3-. Similar indications of V(V) release were supported by UV-vis spectra for PV₂Mo₁₀O₄₀^5- obtained as a function of H₂SO₄ concentration. Additional analytical and substrate-oxidation studies currently in progress are designed to provide a more detailed understanding of the nature and reactivity of PV₂Mo₁₀O₄₀^5- in strongly acidic solvents.

References:

1. Neumann, R., Inorg. Chem., 2010, 49, 3594−3601
2. Sarma B. B., Efremenko I., and Neumann R., J. Am. Chem. Soc., 2015, 137, 5916–5922.
3. Sarma B. B., Carmieli R., Collauto A., Efremenko I., Martin M. L. Jan and Neumann R., ACS Catal. 2016, 6, 6403−6407.
4. Freund M. S. and Lewis N.S., Inorg. Chem. 1994, 33, 1638-1643.