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

摘要

A new nitrile-tethered pyridinium polyoxometalate (POM) was prepared by anion-exchange of the ionic liquid precursor [N-butyronitrile pyridine]CI ([C3CNpy]CI) with the Keggin phosphovanadomolybdic acid H5PMo_(10)V2O_(40) (PMoV2), and the obtained organic POM salt [C3CNpy]4HPMoV2 was characterized by XRD, SEM, TG, ~1H NMR, ~(13)C NMR, ESI-MS, CHN elemental analysis, nitrogen sorption experiment, and melting point measure. When used as a catalyst, [C3CNpy]4HPMoV2 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(O2)]~+ 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.