A new binuclear oxovanadium(v) complex as a catalyst in combination with pyrazinecarboxylic acid (PCA) for efficient alkane oxygenation by H2O2

Abstract

A new binuclear oxovanadium(V) complex [{VO(OEt)(EtOH)}₂L] (1) where H₄L is bis(2-hydroxybenzylidene)terephthalohydrazide has been synthesized and fully characterized. The combination of 1 with pyrazine-2-carboxylic acid (PCA; a cocatalyst) affords a catalytic system for the efficient oxidation of saturated hydrocarbons, RH, with hydrogen peroxide and air in acetonitrile solution at 50 °C to produce alkyl hydroperoxides, ROOH, as the main primary products. Very high turnover numbers (TONs) have been attained in this reaction: for example, after 2220 min, TON = 44 000 and initial TOF (turnover frequency) = 3300 h⁻¹ per molecule of complex 1. The estimated activation energy of the cyclohexane oxygenation in the presence of 1/PCA is E_a = 16 ± 2 kcal mol⁻¹. This value is identical to that obtained for the cyclohexane oxidation with H₂O₂ catalyzed by the (n-Bu₄N)[VO₃]/PCA combination (17 ± 2 kcal mol⁻¹). The dependences of initial oxidation rates W₀ on the initial concentrations of all components of the reaction mixture have been determined. Based on these kinetic data and on the regio- and bond-selectivity parameters measured in the oxidation of linear and branched alkanes a mechanism of the oxidation has been proposed which includes the generation of hydroxyl radicals in the crucial stage.