Oxomolybdenum Complexes Research Articles

Behavior of Triazolylmolybdenum(VI) Oxide Hybrids as Oxidation Catalysts with Hydrogen Peroxide

A family of molybdenum(VI) oxide hybrid solids with the general compositions [MoO3(tr)], [Mo2O6(tr)] and [Mo2O6(tr)(H2O)2] (tr = 1,2,4-triazole ligands bearing amine, pyrazole, 1H-tetrazole, and 1,2,4-triazole groups) have been shown to promote the epoxidation of cis-cyclooctene, the oxidative dehydrogenation of benzyl alcohol to give benzaldehyde, and the oxidation of benzaldehyde to give benzoic acid, using aqueous H2O2 as oxidant. All materials studied act as pre-catalysts and undergo chemical transformation in the presence of the oxidant to give the active oxidising species which, on the basis of characterisation data for recovered compounds, are either mononuclear or polynuclear oxo(peroxo)molybdenum(VI) complexes. Materials with one-dimensional (1D) structures led to homogeneous catalytic reactions with activities that were generally superior to those obtained with materials possessing more rigid 2D/3D structures containing bitriazole ligands. The latter were proposed to lead to interconverting mononuclear (poorly soluble) and dinuclear (soluble) species of the type [MoO(O2)2(L)] and [{MoO(O2)2(L)}2(μ-O)].

Syntheses and catalytic oxotransfer activities of oxo molybdenum(vi) complexes of a new aminoalcohol phenolate ligand.

The new aminoalcohol phenol 2,4-di-tert-butyl-6-(((2-hydroxy-2-phenylethyl)amino)methyl)phenol (H2L) was prepared by a facile solvent-free synthesis and used as a tridentate ligand for new cis-dioxomolybdenum(vi)(L) complexes. In the presence of a coordinating solvent (DMSO, MeOH, pyridine), the complexes crystallise as monomeric solvent adducts while in the absence of such molecules, a trimer with asymmetric Mo[double bond, length as m-dash]O→Mo bridges crystallises. The complexes can catalyse epoxidation of cis-cyclooctene and sulfoxidation of methyl-p-tolylsulfide, using tert-butyl hydroperoxide as oxidant.

Immobilizing of oxo-molybdenum complex on cross-linked copolymer and its catalytic activity for epoxidation reactions

This work describes the immobilization of molybdenum acetylacetonate oxygen (MoO2(acac)2) on cross-linked porous copolymer support via covalent attachment under mild conditions. The obtained solid product DVB–AA–Mo was fully characterized by FT-IR, TG, CHN elemental analysis, nitrogen adsorption/desorption, and SEM, and was tested for the epoxidation of various alkenes with tert-butyl hydroperoxide (TBHP) as the oxidant. DVB–AA–Mo was proved to be a highly efficient catalyst for epoxidation reactions, it could easily be recovered by filtration and reused for five runs without significant loss in activity.

Does a multiply bonded oxo ligand directly participate in B–H bond activation by a high-valent di-oxo-molybdenum(vi) complex? A density functional theory study

The multiply bonded oxo ligand does not participate in the activation of the B–H bond with organic substrates of amides, amines, and nitriles by the high-valent oxo-molybdenum complex MoO2Cl2.

Synthesis and Spectroscopic Characterization of Mononuclear Oxomolybdenum(IV)O,O′-Ditolyl Dithiophosphate Complexes and Their Adducts With 2,2′-Bipyridyl and 1,10-Phenanthroline

A series of mononuclear oxomolybdenum(IV) O,O′-ditolyldithiophosphate complexes, of the type MoO[S2P(OR)2]2 (R = o-,m-,p-C6H4Me), were synthesized by refluxing of the reaction mixture containing triphenylphosphine and binuclear oxomolybdenum(V)O,O′-ditolyldithiophosphate, Mo2O3[S2P(OR)2]4 (R = o-,m-,p-C6H4Me), in equimolar ratio in dichloromethane. The desired pink colored Mo(IV) complexes were separated out by dissolving in cold n-hexane. The equimolar adducts of mononuclear oxomolybdenum(IV) O,O′-ditolyldithiophosphates with heterocyclic amines, MoO[S2P(OR)2]2.L (R = o-,m-,p-C6H4Me; L = 2,2′-bipyridyl and 1,10-phenanthroline), have been prepared by reacting the corresponding oxomolybdenum(IV) complexes with an excess of 2,2′-bipyridyl and 1,10-phenanthroline in a 1,1 molar ratio. The complexes have been characterized by elemental analysis, IR, 1H. and 31P NMR spectroscopy. Based on these, a square pyramidal structure incorporating bidentate dithiophosphate moieties is suggested and a six-coordinated distorted octahedral structure for the entire base adducts complexes. It appears that oxomolybdenum(IV) is preferentially stabilized by sulfur donors.

Syntheses and Structures of Molybdenum-Oxo Complexes Prepared by the Reactions of [MoII2(OAc)4] with tert-Butyl- or Bromo-Substituted Catechols

Abstract The reactions of [MoII2(OAc)4] (OAc−: acetato) with catecholato derivatives (R-Cat2−), possessing electron-donating t-Bu groups (3,6-DTBCat2−) or electron-withdrawing Br groups (Br4Cat2−), results in the formation of a mononuclear complex [Na(THF)6][MoV(3,6-DTBCat)3] (1) and a dinuclear complex either [Na6(Br4SQ)2][MoIV2(µ-O)2(Br4Cat)4] or [Na6(Br4Cat)2][MoV2(µ-O)2(Br4Cat)4] (3). Complex 3 has a [Mo(µ-O)2Mo] core as part of a dimer of [O=Mom(Br4Cat)2]n− (m = IV or V, n = 2 or 1) moieties assembled in a head-to-tail arrangement, which has been structurally characterized as the a bis-µ-oxo molybdenum dinuclear complex containing catecholato ligands. The bond distance between the Mo atoms (2.4286(17) Å) strongly suggests that a MoIV=MoIV or MoV–MoV bonding character exists in 3. Differences in the substituent groups on the catecholato ligand affect not only the product formed but also the reactivity of both complexes. Complex 3 shows O-atom abstraction reactivity with dimethyl sulfoxide (DMSO) and forms dimethyl sulfide (DMS). These results support a high oxophilicity of the Mo center in 3.

Synthesis, characterization and crystal structure of a dioxomolybdenum(VI) complex derived from <i>N’</i>-(2-hydroxy-4-diethaylaminobenzylidene)-4-hydroxybenzohydrazide

Reaction of [MoO2(acac)2] (where acac = acetylacetonate) with N’-(2-hydroxy-4-diethaylaminobenzylidene)-4-hydroxybenzohydrazide (H2L) in methanol afforded a methanol-coordinated mononuclear molybdenum(VI) oxo complex, [MoO2L(MeOH)]. Crystal and molecular structure of the complex were determined by single crystal X-ray diffraction method. The complex was further characterized by elemental analysis and FT-IR spectra. Single crystal X-ray structural studies indicate that the hydrazone ligand coordinates to the MoO2 core through enolate oxygen, phenolate oxygen and azomethine nitrogen. The Mo atom in the complex is in octahedral coordination. Thermal stability of the complex has also been studied. KEY WORDS: Molybdenum complex, Hydrazone ligand, Crystal structure, X-ray diffraction, Thermal property Bull. Chem. Soc. Ethiop. 2014, 28(3), 409-414.DOI: http://dx.doi.org/10.4314/bcse.v28i3.10

Infrared multiple photon dissociation spectroscopy of a gas-phase oxo-molybdenum complex with 1,2-dithiolene ligands.

Electrospray ionization (ESI) in the negative ion mode was used to create anionic, gas-phase oxo-molybdenum complexes with dithiolene ligands. By varying ESI and ion transfer conditions, both doubly and singly charged forms of the complex, with identical formulas, could be observed. Collision-induced dissociation (CID) of the dianion generated exclusively the monoanion, while fragmentation of the monoanion involved decomposition of the dithiolene ligands. The intrinsic structure of the monoanion and the dianion were determined by using wavelength-selective infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory calculations. The IRMPD spectrum for the dianion exhibits absorptions that can be assigned to (ligand) C=C, C–S, C—C≡N, and Mo=O stretches. Comparison of the IRMPD spectrum to spectra predicted for various possible conformations allows assignment of a pseudo square pyramidal structure with C2v symmetry, equatorial coordination of MoO2+ by the S atoms of the dithiolene ligands, and a singlet spin state. A single absorption was observed for the oxidized complex. When the same scaling factor employed for the dianion is used for the oxidized version, theoretical spectra suggest that the absorption is the Mo=O stretch for a distorted square pyramidal structure and doublet spin state. A predicted change in conformation upon oxidation of the dianion is consistent with a proposed bonding scheme for the bent-metallocene dithiolene compounds [Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc.1976, 98, 1729−1742], where a large folding of the dithiolene moiety along the S···S vector is dependent on the occupancy of the in-plane metal d-orbital.

Oxomolybdenum(VI) complexes with glycine bisphenol [O,N,O,O′] ligand: synthesis and catalytic studies

The oxomolybdenum(VI) complex [MoOCl(L)] with a tetradentate glycine bisphenol ligand (H3L) was prepared by reaction of [MoO2Cl2(DMSO)2] with a ligand precursor in hot toluene. The product was isolated in moderate yield as separable cis and trans isomers along with the third minor component, [MoO2(HL)]. The solid-state structure of trans-[MoOCl(L)] was determined by X-ray diffraction. The ligand has tetradentate coordination through three oxygens and one nitrogen, which is located trans to the terminal oxo whereas the sixth coordination site is occupied by a chloride. Both cis and trans isomers of [MoOCl(L)] are active catalysts for epoxidation of cis-cyclooctene and sulfoxidation of tolyl methyl sulfide. The cis isomer gave higher activity in epoxidation and sulfoxidation reactions at room temperature than the trans isomer but they performed identically at 50 °C.

Complexes of cis-dioxomolybdenum(VI) and oxovanadium(IV) with a tridentate ONS donor ligand: Synthesis, spectroscopic properties, X-ray crystal structure and catalytic activity

New cis-dioxomolybdenum(VI) and oxovanadium(IV) complexes of the Schiff base, derived from S-methyl dithiocarbazate and 2,3-dihydroxybenzaldehyde (H2dhsm), have been synthesized. The complexes of the type cis-[MoO2(dhsm)] (1a), cis-[MoO2(dhsm)(D)] (1b–1d) [D=neutral monodentate ligand; EtOH, pyridine (py) or imidazole (imz)], [VO(dhsm)(NN)] (2a, 2b) [NN=2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen)] and [VO(dhsm)] (2c) have been isolated, characterized by 1H NMR, IR, UV–Vis and EPR spectral studies and investigated by cyclic voltammetry. The X-ray crystal structure of cis-[MoO2(dhsm)(EtOH)] (1b) has been determined and shows that the complex has a distorted octahedral geometry in which the H2dhsm behaves as a dianionic ONS tridentate ligand coordinating via phenoxide oxygen, hydrazinic nitrogen and thiolate sulfur. The oxomolybdenum(IV) complex [MoO(dhsm)] (1e) has obtained from dioxomolybdenum(VI) complex (1b) by oxo abstraction with PPh3. The reactivity of the complexes toward catalytic oxidation of alcohols in the presence of H2O2 and t-BuOOH as co-oxidants under solvent free conditions is reported.

Spectral and Thermal Studies of Some Binuclear Dimeric Mo-O-Mo Bridged Oxomolybdenum(V) Complexes with Bis(carboxamides) Derived from 4,4’-Diaminobiphenyl, 1,4-diaminobenzene with Anhydrides

A study of doubly bidentate ligands like 4,4'-bis-(phthalamidyl)biphenyl Bd(PAH)2, 4,4'-bis-(Succinamidyl)biphenyl Bd(SAH)2, 1,4-bis-(phthalamidyl)monophenyl PPD(PAH)2 and 1,4-bis-(succinamidyl)monophenyl PPD(SAH)2 with oxomolybdenum(V) is described. The complexes were prepared by reaction with the precursor (PyH)2(MoOCl5). The coordination behavior of the complexes have been determined with the help of elemental composition, infrared and electronic spectroscopy, cyclicvoltammetric and thermal studies.

Two mononuclear molybdenum(VI) oxo complexes with tridentate hydrazone ligands: Synthesis, structures, and thermal stability

A reaction of [MoO2(acac)2] (where acac = acetylacetonate) with two hydrazone ligands in methanol yields two mononuclear molybdenum(VI) oxo complexes with the general formula [MoO2L(CH3OH)], where L = L1=(4-nitrophenoxy)acetic acid [1-(5-chloro-2-hydroxyphenyl)methylidene]hydrazide (H2L1) and L = L2=4-dimethylaminobenzoic acid [1-(2-hydroxy-3-methoxyphenyl)methylidene]hydrazide (H2L2). The crystal and molecular structures of the complexes are determined by the single crystal X-ray diffraction method. All of the investigated compounds are further characterized by the elemental analysis, FT-IR spectra, and thermogravimetric analysies. Single crystal X-ray structural studies indicate that hydrazone ligands coordinate to MoO2 cores through enolate oxygen, phenolate oxygen, and azomethine nitrogen atoms. The Mo atoms in both complexes are in octahedral coordination.

Two mononuclear molybdenum(VI) oxo complexes with tridentate hydrazone ligands: Synthesis and crystal structures

Reaction of [MoO2(Acac)2] (Acac = acetylacetonate) with two similar hydrazone ligands in methanol yielded two mononuclear molybdenum(VI) oxocomplexes with general formula [MoO2(L)(CH3OH)], where L = L1 = (4-nitrophenoxy)acetic acid [1-(3-ethoxy-2-hydroxyphenyl)methylidene]hydrazide (H2L1) and L = L2 = (4-nitrophenoxy)acetic acid [1-(5-bromo-2-hydroxyphenyl)methylidene]hydrazide (H2L2). Crystal and molecular structures of the complexes were determined by single crystal X-ray diffraction method. All investigated compounds were further characterized by elemental analysis and FT-IR spectra. Single crystal X-ray structural studies indicate that the hydrazone ligands coordinate to the MoO2 cores through enolate oxygen, phenolate oxygen, and azomethine nitrogen. The Mo atoms in both complexes are in octahedral coordination.

Two Mononuclear Molybdenum(VI) Oxo Complexes with Tridentate Hydrazone Ligands: Synthesis and Crystal Structures

Reaction of [MoO2(Acac)2] (Acac = acetylacetonate) with two similar hydrazone ligands in methanol yielded two mononuclear molybdenum(VI) oxocomplexes with general formula [MoO2(L)(CH3OH)], where L = L 1 = (4 nitrophenoxy)acetic acid [1 (3 ethoxy 2 hydroxyphenyl)meth ylidene]hydrazide (H2L 1) and L = L2 = (4 nitrophenoxy)acetic acid [1 (5 bromo 2 hydroxyphenyl)meth ylidene]hydrazide (H2L 2). Crystal and molecular structures of the complexes were determined by single crystal X ray diffraction method. All investigated compounds were further characterized by elemental analysis and FT IR spectra. Single crystal X ray structural studies indicate that the hydrazone ligands coordinate to the MoO2 cores through enolate oxygen, phenolate oxygen, and azomethine nitrogen. The Mo atoms in both com plexes are in octahedral coordination. DOI: 10.1134/S1070328414020079 1 The article is published in the original.

Application of an indenyl molybdenum dicarbonyl complex in the isomerisation of α-pinene oxide to campholenic aldehyde

The complex [{(η5-Ind)Mo(CO)2(μ-Cl)}2] (1) has been tested for the industrially relevant catalytic isomerisation of α-pinene oxide (PinOx) to campholenic aldehyde (CPA) in the liquid phase. PinOx conversion and CPA selectivity are strongly influenced by the solvent employed. Complete conversion of PinOx was achieved within 1 min at 55 °C or 30 min at 35 °C using 1,2-dichloroethane as solvent, giving CPA in 68% yield. Other products included trans-carveol, iso-pinocamphone and trans-pinocarveol. The stability of 1 under the reaction conditions used was investigated by using FT-IR spectroscopy and electrospray ionisation mass spectrometry (ESI-MS) to characterise recovered solids. In the presence of air/moisture 1 undergoes oxidative decarbonylation upon dissolution to give oxomolybdenum species that are proposed to include a tetranuclear oxomolybdenum(V) complex. Conversely, ESI-MS studies of 1 dissolved in dry acetonitrile show mononuclear species of the type [IndMo(CO)2(CH3CN)n]+. The crystal structure of the ring-slipped dicarbonyl complex [(η3-Ind)Mo(CO)2Cl(CH3CN)2] (2) (obtained after dissolution of 1 in acetonitrile) is reported.

Synthesis, characterization and oxo-transfer kinetics of oxomolybdenum(IV) complexes with two tridentate ONS donor Schiff base ligands derived from benzoylacetone and S-benzyl/S-methyl dithiocarbazate

Synthesis, characterization and oxo-transfer kinetics of oxomolybdenum(IV) complexes with two tridentate ONS donor Schiff base ligands derived from benzoylacetone and S-benzyl/S-methyl dithiocarbazate

Iron–molybdenum-oxo complexes as initiators for olefin autoxidation with O2

The reaction between [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) yields the novel tetranuclear complex [(TPA)Fe(μ-Cp*MoO3)]2(OTf)2, 1, with a rectangular [Mo-O-Fe-O-]2 core containing high-spin iron(ii) centres. 1 proved to be an efficient initiator/(pre)catalyst for the autoxidation of cis-cyclooctene with O2 to give cyclooctene epoxide. To test, which features of 1 are essential in this regard, analogues with zinc(ii) and cobalt(ii) central atoms, namely [(TPA)Zn(Cp*MoO3)](OTf), 3, and [(TPA)Co(Cp*MoO3)](OTf), 4, were prepared, which proved to be inactive. The precursor compounds of 1, [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) as well as Cp2*Mo2O5, were found to be inactive, too. Reactivity studies in the absence of cyclooctene revealed that 1 reacts both with O2 and PhIO via loss of the Cp* ligands to give the triflate salt 2 of the known cation [((TPA)Fe)2(μ-O)(μ-MoO4)](2+). The cobalt analogue 4 reacts with O2 in a different way yielding [((TPA)Co)2(μ-Mo2O8)](OTf)2, 5, featuring a Mo2O8(4-) structural unit which is novel in coordination chemistry. The compound [(TPA)Fe(μ-MoO4)]2, 6, being related to 1, but lacking Cp* ligands failed to trigger autoxidation of cyclooctene. However, initiation of autoxidation by Cp* radicals was excluded via experiments including thermal dissociation of Cp2*.

Mechanistic insights into B–H bond activation with the high-valent oxo-molybdenum complex MoO2Cl2

The ionic mechanistic model involving the heterolytic cleavage of the B–H bond is slightly energetically favorable than the [2+2] addition mechanism for the high-valent oxo-molybdenum complex MoO2Cl2activating the B–H bond.

A dinuclear oxo-bridged molybdenum(VI) complex containing a bidentate pyrazolylpyridine ligand: Structure, characterization and catalytic performance for olefin epoxidation

Abstract The oxo-bridged dinuclear complex [Mo2O4(μ2-O)Cl2(pypzEA)2] (2) (pypzEA = ethyl[3-(pyridin-2-yl)-1H-pyrazol-1-yl]acetate) was obtained upon the treatment of [MoO2Cl2(pypzEA)] (1) with ethanol. The molecular structure of 2 consists of two highly distorted {MoClN2O3} octahedra sharing a corner oxygen atom, with a Mo O Mo “kink” angle of 163.2(2)° and a Mo⋯Mo separation of 3.7029(7) A. Catalytic results with 2 for the epoxidation of cis-cyclooctene using tert-butylhydroperoxide as oxidant compare favorably with those reported for related oxomolybdenum(VI) complexes. The reaction is homogeneous in nature, and a fraction of 2 dissolves and is converted to other types of soluble active species, one of which was isolated and identified as the oxodiperoxo complex [MoO(O2)2(pypzEA)] (3).

Two new scorpionate oxomolybdenum(vi)–poly(pyrazolyl)borate complexes: synthesis, structure, and catalytic performance in the oxidation of cyclohexane

Two new oxomolybdenum(VI) complexes [(TpMoO(2))(2)(μ-O)]·H(2)O (1) and (Tp(4-I)MoO(2))(2)(μ-O) (2) were synthesized and characterized by elemental analysis, IR spectra, UV-Vis spectroscopy, powder X-ray diffraction, single-crystal X-ray diffraction and thermogravimetric analyses (TG). In addition, the catalytic performances of complexes 1 and 2 were studied firstly on the oxidation reaction of cyclohexane at room temperature.