Dichlorodioxomolybdenum(VI) complexes bearing oxygen-donor ligands as olefin epoxidation catalysts

Treatment of the solvent adduct [MoO2Cl2(THF)2] with either 2 equivalents of N,N-dimethylbenzamide (DMB) or 1 equivalent of N,N'-diethyloxamide (DEO) gave the dioxomolybdenum(vi) complexes [MoO2Cl2(DMB)2] () and [MoO2Cl2(DEO)] (). The molecular structures of and were determined by single-crystal X-ray diffraction. Both complexes present a distorted octahedral geometry and adopt the cis-oxo, trans-Cl, cis-L configuration typical of complexes of the type [MoO2X2(L)n], with either the monodentate DMB or bidentate DEO oxygen-donor ligands occupying the equatorial positions trans to the oxo groups. The complexes were applied as homogeneous catalysts for the epoxidation of olefins, namely cis-cyclooctene (Cy), 1-octene, trans-2-octene, α-pinene and (R)-(+)-limonene, using tert-butylhydroperoxide (TBHP) as oxidant. In the epoxidation of Cy at 55 °C, the desired epoxide was the only product and turnover frequencies in the range of ca. 3150-3200 mol molMo(-1) h(-1) could be reached. The catalytic production of cyclooctene oxide was investigated in detail, varying either the reaction temperature or the cosolvent. Complexes and were also applied in liquid-liquid biphasic catalytic epoxidation reactions by using an ionic liquid of the type [C4mim][X] (C4mim = 1-n-butyl-3-methylimidazolium; X = NTf2, BF4 or PF6] as a solvent to immobilise the metal catalysts. Recycling for multiple catalytic runs was achieved without loss of activity

Rational synthesis of molybdenum(V) tetramers consisting of [Mo \u3c inf\u3e 2 O \u3c inf\u3e 4 ] \u3c sup\u3e 2+ \u3c/sup\u3e dimers held together by bridging phosphinate ligands and the tungsten(VI) dimer [(CH \u3c inf\u3e 3 O) \u3c inf\u3e 2 (O)W(μ-O)(μ-O \u3c inf\u3e 2 PPh \u3c inf\u3e 2 ) \u3c inf\u3e 2 W(O) (CH \u3c inf\u3e 3 O) \u3c inf\u3e 2 ]: Structural and theoretical considerations

Reacting MoO2(acac)2 with Ph2POOH or Me2POOH in EtOH results in the formation of the tetranuclear molybdenum (V) clusters Mo4(μ 3-O)4(μ- O2PR2)4O4, PR2 = PPh 2, 1, or PMe2, 2, in functional yields (\u3e 90% and 55% respectively). The reaction of WO2(acac)2 with Ph 2POOH in MeOH affords the tungsten dimer [(CH3O) 2(O)W(μ-O)(μ-O2PPh2)2W(O) (CH3O)2], 3. The single crystal X-ray determined structures of complexes 1-3 are reported. In 1 and 2, the four Mo=O units are interconnected by four triply bridging oxygen atoms, resulting in a distorted cubic-like structure for the Mo4(μ 3-O) 4O4 units. Each molybdenum atom forms two additional Mo-O bonds with two oxygen atoms from different adjacent phosphinato ligands. Complex 3, a tungsten dimer, contains packing disorder and consists of bridging oxo and diphenylphosphinato ligands. The bonding of 1 and 2 assessed by density-functional methods showed that bonding between the Mo(V) centers occurs through σ overlap of the d xy orbitals. © 2007 Springer Science+Business Media, LLC

Reexamination of the structure of MoO(O \u3c inf\u3e 2 ) \u3c inf\u3e 2 (H \u3c inf\u3e 2 O)(hmpa), hmpa=hexamethylphosphoramide by crystallographic and theoretical means

The crystal structure of MoO(O2)2(H 2O)(hmpa), hmpa=hexamethylphosphoramide, has been reassessed and corrected as one of the axial parameters (namely the c-axis) was reported incorrectly. This resulted in significant differences in the internal geometry of the molecule, notably an decreased O-O atom distance (≈0.03 Å) in the metal-bonded peroxo ligands. Crystal packing forces and a flat bending potential of the Mo-O-P angle accounts for discrepancies between theory and experimental structures. © 2005 Elsevier B.V. All rights reserved

Tungsten(VI) N-heterocyclic carbene complexes : synthetic, structural, and computational study

The reaction of WOCl4 with 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Idipp) leads to an orange solid whose spectroscopic data are consistent with the 1:1 adduct [WOCl4(Idipp)]. Computational studies at the DFT level further support this formulation. Exposure of this compound to the atmosphere results in rapid hydrolysis to various imidazolium salts. If air diffuses very slowly into solutions of [WOCl4(Idipp)], it also undergoes slow hydrolysis to form [WO2Cl2(Idipp)]. This has been crystallographically characterized and is the first five-coordinate, 1:1 adduct of WO2Cl2. This complex has also been subject to DFT calculations, and its metal-ligand bonding has been explored. The carbene-metal interaction is primarily σ-donor in nature. The mechanism of the hydrolysis has also been probed by computational methods, revealing a plausible, low-energy reaction pathway

Metal oxide-triazole hybrids as heterogeneous or reaction-induced self-separating catalysts

The hybrid metal oxide-triazole materials [MoO3(trz)(0.5)] (1) and [W2O6(trz)] (2) (trz = 1,2,4-triazole) have been hydrothermally synthesized and characterized by different techniques (TGA, SEM, H-1 and C-13 MAS NMR, FT-IR spectroscopy, and structure determination by Rietveld analysis of high resolution synchrotron powder XRD data). Materials 1 and 2 display distinct behaviors when applied as catalysts for oxidation reactions with alcohol, aldehyde, olefin and sulfide substrates, and are more effective with hydrogen peroxide as the oxidant than with tert-butylhydroperoxide. The Mo-VI hybrid 1 transforms into soluble active species during cis-cyclooctene epoxidation with H2O2. When consumption of H2O2 reaches completion, spontaneous reassembly of the 2-dimensional molybdenum oxide network of 1 takes place and the hybrid precipitates as a microcrystalline solid that can be easily separated and recycled. Reaction induced self-separation behavior occurs with 1, H2O2 and other substrates such as methyl oleate and methylphenylsulfide. The W-VI hybrid 2 behaves differently, preserving its structural features throughout the heterogeneous catalytic process. (C) 2016 Elsevier Inc. All rights reserved