Aerobic oxidation of aldehydes catalyzed by ε-Keggin type polyoxometalates [Mo₁₂^VO₃₉(μ₂-OH)₁₀H₂{X^II(H₂O)₃}₄] (X = Ni, Co, Mn and Cu) as heterogeneous catalysts

Polyoxometalates of the ε-Keggin structure [Mo₁₂^VO₃₉(μ₂-OH)₁₀H₂{X^II(H₂O)₃}₄] (X = Ni, Co, Mn and Cu), have been synthesized. While the nickel substituted compound was known, the cobalt, manganese and copper analogs are new. These ε-Keggin polyoxometalates are insoluble and, except for the nickel analog, could not be obtained in crystalline form. The IR spectra indicate that for cobalt and manganese substitution the compounds are isostructural to the nickel derivative. The ε-Keggin polyoxometalates were used as heterogeneous catalysts for the aerobic oxidation of aldehydes to carboxylic acids. The selectivity of the reaction was highly dependent on the identity of the aldehyde. Aliphatic linear aldehydes, e.g. octanal, and benzaldehyde react quantitatively and selectively to the respective carboxylic acids. 2-Methyl-undecanal reacted quite selectively yielding as minor product 2-undecanone by decarboxylation. 2-Phenylpropanal showed more significant β-methyl cleavage. Substrates with a variety of functional moieties such as myrtenal and 5-norbornene-2-carboxaldehyde yielded considerable amounts of formates via a Dakin type reaction and allylic oxidation products.