Synthesis, electrochemistry, and oxygen-atom transfer reactions of dioxotungsten(VI) and -molybdenum(VI) complexes with N₂O₂and N₂S₂tetradentate ligands

A series of N2O2tetradentate ligands with a range of substituents attached to the nitrogen atoms have been prepared (H2L(n)) (n = 1-9). Treatment of these ligands and the N2S2tetradentate ligand H2L10with [WO2Cl2(DME)] (DME = 1,2-dimethoxyethane) in the presence of triethylamine leads to the formation of cis-dioxotungsten(VI) complexes [WO2(L(n))] (n = 1-10). Reaction of the N2O2tetradentate ligands H2L(n) (n = 1, 3-7) with ammonium molybdate tetrahydrate and dilute hydrochloric acid gives the corresponding molybdenum(VI) analogs [MoO2(L(n))] (n = 1, 3-7). These compounds have been spectroscopically characterized and the molecular structures of [WO2(L(n))] (n = 1, 2, 9) and [MoO2(L5)] have been established by X-ray diffraction analysis. These high-valent compounds participate in oxygen-atom transfer reactions and can catalyze the oxidation of benzoin with dimethyl sulfoxide. The complex [WO2(L10)], which contains an S-donor ligand, has lower reduction potential and higher reactivity toward oxo-transfer reactions than analogous tungsten complexes having N2O2ligands. The kinetics of these catalytic processes along with the structure and electrochemistry of these dioxotungsten and -molybdenum complexes are described and compared.