The synthesis and full characterisation of a range of Mo(VI) and W(VI) dioxo complexes with anionic asymmetric tripodal tetradentate ligands with various combinations of N,O and S donor atoms are described. Their redox chemistry and ability to catalyse the oxidation of primary and secondary allylic alcohols and benzylic alcohols with DMSO without addition of peroxide oxidant are discussed. These oxidations are shown to proceed selectively to the corresponding aldehydes or ketones with Mo complexes with at least two sulfur donors giving the highest yields. A new class of asymmetric N-capped (dianionic/trianionic) tripodal proligands [Hx(Ln)] (x = 2, n = 1-6; x = 3, n = 7, 8) which possess pendant arms with N2OS, N2S2 or NOS2 donor groups and with different chelate ring sizes {5,5,5} or {5,6,5} has been prepared. Treatment of these ligands with [WO2Cl2(dme)] (dme = 1,2-dimethoxyethane) in the presence of base (triethylamine or KOH) leads to the formation of cis-dioxotungsten(VI) complexes of the types [WO2(Ln)] (n = 1-6) and K[WO2(Ln)] (n = 7, 8). Reaction of these tetradentate ligands with [MoO2(acac)2] (acac = acetylacetonate) gives the corresponding Mo(VI) analogues [MoO 2(Ln)] (n = 1-6) and K[MoO2(Ln)] (n = 7, 8). Moreover, a new five coordinate dioxomolybdenum(VI) complex with an NS2 tridentate ligand [MoO2(L9)] has been synthesised using similar procedure. All these compounds have been spectroscopically characterised and the molecular structures of [MoO 2(Ln)] (n = 2, 6) and [WO2(L6)] have been established by X-ray diffraction analysis. The electrochemistry and the catalytic activity for oxidation of allylic and benzylic alcohols of these dioxo complexes have also been investigated. © 2004 Elsevier B.V. All rights reserved.
