Oxidative functionalisation of alkanes: synthesis, molecular structure and catalytic implications of anionic vanadium(V) oxo and peroxo complexes containing bidentate N,O 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 N,O 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 N,O 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 (N,O co-ordination), in [VO2(ana)2]– the two ana ligands are co-ordinated via two oxygen atoms of the carboxylato group (O,O co-ordination).