cis-Dioxo-tungsten(VI) and -molybdenum(VI) complexes with N2O2 tetradentate ligands: synthesis, structure, electrochemistry and oxo-transfer properties

Abstract

Treatment of [WO₂Cl₂(dme)] with N₂O₂ tetradentate ligands [H₂Lⁿ (n = 1–7)] in the presence of 2 equivalents of triethylamine gave the corresponding dioxotungsten(VI) complexes [WO₂Lⁿ] (n = 1–7). The molybdenum counterparts [MoO₂Lⁿ] (n = 1–4) were prepared in good yield by treating ammonium molybdate tetrahydrate with the respective ligands and dilute hydrochloric acid. The molecular structures of four of these complexes, namely [WO₂Lⁿ] (n = 1, 5 or 6) and [MoO₂L²], as determined by single crystal X-ray analysis, revealed that the metal centre exhibits distorted octahedral co-ordination with cis-dioxo ligands. These high-valent oxo complexes are active toward oxygen atom transfer reactions and can catalyse the oxidation of benzoin with dmso. Their reactivity and electrochemical properties are discussed and compared.