Dioxidomolybdenum(VI) Complexes of Tripodal Tetradentate Ligands for Catalytic Oxygen Atom Transfer between Benzoin and Dimethyl Sulfoxide and for Oxidation of Pyrogallol

Article abstract of DOI:10.1002/ejic.201600694

The reactions of the tripodal tetradentate ONNO donor ligands 6,6′-{[(2-morpholinoethyl)azanediyl]bis(methylene)}bis(2,4-di-tert-butylphenol) (H₂L¹), 6,6′-{[(2-morpholinoethyl)azanediyl]bis(methylene)}bis(2,4-dimethylphenol) (H₂L²) and 6,6′-{[(2-morpholinoethyl)azanediyl]bis(methylene)}bis[2-(tert-butyl)-4-methylphenol] (H₂L³) with [Mo^VIO₂(acac)₂] (acac = acetylacetonato) in a 1:1 molar ratio in MeOH gave the corresponding cis-dioxidomolybdenum(VI) complexes [MoO₂(L¹)], [MoO₂(L²)] and [MoO₂(L³)], respectively, in excellent yields. These complexes were characterized by various spectroscopic (IR, UV/Vis,¹H and¹³C NMR), electrochemical, thermogravimetric, single-crystal XRD, and powder XRD (PXRD) studies. In these complexes, the geometry around the cis-[MoO₂]²⁺core is distorted octahedral, and the ligands are tetradentate and coordinate through two O_phenolate, one N_tripodal, and one N_morpholineatoms. One of the oxido groups and the morpholine nitrogen atom occupy the axial sites. These complexes were used for catalytic oxygen atom transfer between benzoin and dimethyl sulfoxide (DMSO) in acetonitrile at 80 °C, and the formation of benzil was followed by HPLC. Detailed kinetic studies revealed a first-order rate in benzoin and catalyst, and the rate constant for the second-order oxygen atom transfer reaction was 0.0162 m^–1h^–1. The formation of the dinuclear intermediates [LMo^V–μ-O-Mo^VL] was established by MALDI-TOF MS and UV/Vis spectroscopy. Its reversible nature was further supplemented by UV/Vis spectroscopy. These complexes also catalyze the oxidation of pyrogallol in a fashion similar to that of transhydroxylases. Under aerobic conditions, the initially formed oxidation product phloroglucinol undergoes further oxidative coupling in the presence of H₂O₂to give purpurogallin as the final product. This process follows Michaelis–Menten-type kinetics with respect to pyrogallol; the k_catvalues obtained were 394, 300 and 247 h^–1for [Mo^VIO₂(L¹)], [Mo^VIO₂(L²)] and [Mo^VIO₂(L³)], respectively.