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

摘要

A mixture of [NBu_4][VO_3] and pyrazine-2-carboxylic acid (Hpca)in acetonitrile catalysed smoothly the reaction of alkanes RH (R=CH_3 or C_6H_(11)) with molecular oxygen (from air) and hydrogen peroxide to give the corresponding alkyl hydroperoxide ROOH as the primary product. The oxo and peroxo anions [VO_2(pca)_2]~- and [VO(O_2)(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 [NBu_4][VO_2(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 [VO_2(pca)_2]~- the tow pca ligands are bonded through a nitrogen and an oxygen atom (N, O co-ordination), in [VO_2(ana)_2]~- the two ana ligands are co-ordinated via two oxygen atoms of the carboxylato group (O, O co-ordination).