Reversible system for selective oxidation in acidic media

H₅PV₂Mo₁₀O₄₀ (1) polyoxometalate (POM) 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 V¹. Recently, however, Neumann found that in 50% H₂SO₄ the reduction potential of 1 was increased to 1.1 V, 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 this species is responsible for the strongly oxidizing properties of the system while the selectivity is governed by in-situ formed [(Mo₂O₅)²⁺]. 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- and vanadate (as VO₂⁺) as a function of H₂SO₄ concentration were confirmed by ³¹P- and ⁵¹V-NMR respectively. Frozen state EPR spectra of reduced oxidant solution in 50% H₂SO₄ confirmed the presence of isolated vanadyl cation (VO²⁺). Further, substrate-oxidation studies suggested that the actual reactive oxidant is the dioxovanadium cation (VO₂⁺) and the selectivity is enhanced by molybdate cation [(Mo₂O₅)²⁺] which was isolated from the solution of 1 in 50% H₂SO₄ and the structure was solved by single-crystal XRD.

Figure: Reversible System for Selective Oxidation

References:

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