Oxidative functionalisation of alkanes: synthesis, molecular structure and catalytic implications of anionic vanadium(V) oxo and peroxo complexes containing bidentate <i>N,O</i> ligands

A mixture of [NBu₄][VO₃] and pyrazine-2-carboxylic acid (Hpca) in acetonitrile catalysed smoothly the reaction of alkanes RH (R = CH₃ or C₆H₁₁) with molecular oxygen (from air) and hydrogen peroxide to give the corresponding alkyl hydroperoxide ROOH as the primary product. The oxo and peroxo anions [VO₂(pca)₂]^– and [VO(O₂)(pca)₂]^–, isolated as the tetrabutylammonium or ammonium salts from acetonitrile solution and fully characterised by single crystal structure analyses, are assumed to be involved in the catalytic process. A screening of different <i>N,O</i> ligands showed Hpca to be the best co-catalyst, while anthranilic acid (Hana) proved to be almost inactive. The isolation and crystal structure analysis of the analogous oxo compound [NBu₄][VO₂(ana)₂] showed a fundamental difference in the co-ordination of the bidentate <i>N,O</i> ligands which might explain the different catalytic activities: while in [VO₂(pca)₂]^– the two pca ligands are bonded through a nitrogen and an oxygen atom (<i>N,O</i> co-ordination), in [VO₂(ana)₂]^– the two ana ligands are co-ordinated <i>via</i> two oxygen atoms of the carboxylato group (<i>O,O</i> co-ordination).