Synthesis, structures, electrochemistry and properties of dioxo-molybdenum(VI) and -tungsten(VI) complexes with novel asymmetric N2OS, and partially symmetric N2S2, NOS2 N-capped tripodal ligands

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 [MoO2(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 [MoO2(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.