Phase-transfer hydroxylation of benzene with H₂O₂ catalyzed by a nitrile-functionalized pyridinium phosphovanadomolybdate

A new nitrile-tethered pyridinium polyoxometalate (POM) was prepared by anion-exchange of the ionic liquid precursor [N-butyronitrile pyridine]Cl ([C₃CNpy]Cl) with the Keggin phosphovanadomolybdic acid H ₅PMo₁₀V₂O₄₀ (PMoV₂), and the obtained organic POM salt [C₃CNpy]₄HPMoV₂ was characterized by XRD, SEM, TG, ¹H NMR, ¹³C NMR, ESI-MS, CHN elemental analysis, nitrogen sorption experiment, and melting point measure. When used as a catalyst, [C₃CNpy]₄HPMoV ₂ causes the first example of reaction-controlled phase-transfer hydroxylation of benzene with H₂O₂, showing high activity and stable reusability. Based on spectral characterizations and comparisons of reaction results, plus the reversible color change between fresh and recovered catalyst, a unique reaction mechanism is proposed for understanding the highly efficient [C₃CNpy]₄HPMoV₂-catalyzed phase-transfer catalysis. The formation of dissolvable active species [VO(O ₂)]⁺ is responsible for the phase-transfer behavior, while the intramolecular charge transfer and the protonated nitrile in cations accelerate the reaction and favor a better catalyst recovery rate.