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

Abstract

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 [MoVIO2(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 and 13C 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 Ophenolate, one Ntripodal, and one Nmorpholine atoms. 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–1 h–1. The formation of the dinuclear intermediates [LMoV–µ‐O‐MoVL] 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 H2O2 to give purpurogallin as the final product. This process follows Michaelis–Menten‐type kinetics with respect to pyrogallol; the kcat values obtained were 394, 300 and 247 h–1 for [MoVIO2(L1)], [MoVIO2(L2)] and [MoVIO2(L3)], respectively.

Dioxidomolybdenum(VI) and -tungsten(VI) complexes with tripodal amino bisphenolate ligands as epoxidation and oxo-transfer catalysts

“…], in refluxing toluene followed by recrystallization from hot acetonitrile. The molecular structure of the complex was found to be identical to the methanol adduct published recently [27].…”

“…The O oxido -M-N angles deviate from 180°, presumably because of ligand constraints; for the central amine nitrogen of the ligand, the O4-M-N1 angle is 157.5 8 Table 2). All O=M=O angles and M=O distances resemble those found in other six-coordinate dioxidomolybdenum(VI) and -tungsten(VI) complexes [27,[29][30][31][32][33][34][35][36]. The M-O phenolate angles are 125.82 (17) and 120.28(16)° for 1 and 122.7(4) and 127.8(4)° for 2, whereas the corresponding M-O phenolate distances are 1.9393 (19) and 1.9673(18) Å for 1 and 1.947(4) and 1.921(5) Å for 2, respectively.…”

“…The synthesis of the tripodal amino bisphenol proligand H 2 L has been reported earlier [27]. In this study, it was prepared by a solvent-free Mannich condensation reaction applying a published procedure [28].…”

“…Such reactivity of complex 1 has been investigated earlier although under different conditions, i.e. heating benzoin in the presence of an excess of DMSO in acetonitrile at 80 °C [27]. The reaction mixtures were characterized by 1 H and 13 C NMR spectroscopy.…”

Cis‐Dioxo‐molybdenum(VI) Complexes with Diaminoguanidinium and Triaminoguanidinium Schiff Bases and Their Catalytic Application for Epoxidation of Cyclohexene

“…[41] The mean Mo=O bond lengths in complexes 1-5 and 7 fall in the range of 1.676 3 , respectively, which are compared with those in the reported di-nuclear molybdenum(VI) complex 6 (82.38(19)°, 80.9(2)°; 70.5(2)°, 71.2(2)°). [31] The Mo(2)À O(6)À Mo(1 A) bond angle of 153.6(4)°in the tetra-nuclear molybdenum(VI) complex 7 is smaller than 422.071 (14) 80.951 1011.872 (11) β (°) γ (°) 99.09 268.505 (8) 64.870 (8) 105.546 (14) V (Å 3 ) 5619 (13) 6415 7 that in the di-nuclear oxo-bridged complex [μ-oxo-(Mo(O) (salalen-t Bu 2 )) 2 ] (180.0°). [42] The intramolecular OÀ H⋅⋅⋅N hydrogen bondings are also observed in the free bidentate Schiff base fragments in complexes 1, 4, 5 and 7 with the O⋅⋅⋅N distance being about 2.610 Å, which may contribute to the stability of the molybdenum(VI) species.…”