Tungsten Complexes Research Articles

Green and efficient removal of sulfides using oxo-peroxo tungsten(VI)-MIL-101(Cr) nanoreactor as heterogeneous recyclable catalyst

Efficient removal of sulfide using a modified MIL-101(Cr) nanoreactor by Schiff base complex has been investigated using H2O2 as a green oxidant. In addition, a new oxo-peroxo tungsten(VI) complex has been synthesized successfully by treating WO3 with a tridentate ONO donor Schiff base ligand in the presence of H2O2. Different techniques have been used to characterize the complex, while the crystal structure was investigated using an X-ray crystallographic approach. A seven-coordinated tungsten ion with a deformed pentagonal–bipyramidal geometry was explored using single-crystal X-ray diffraction analysis. The [WO(O2)(L)(CH3OH)] (L: (E)-N'-(5-chloro-2-hydroxybenzylidene) iso nicotinohydrazide) complex has been screened for the sulfoxidation of aryl and alkyl sulfides with H2O2 as a homogeneous catalyst. Furthermore, Chloromethylated MIL-101(Cr) having a mesoporous structure has been used as a support for the immobilization of [WO(O2)(L)(CH3OH)] complex to produce a heterogeneous and recyclable catalyst. This heterogeneous catalyst is novel in its efficiency, as illustrated by its improved activity with reference to the homogeneous catalyst under the same reaction conditions. The heterogeneous structure displayed excellent reusability several times without losing activity due to covalent grafting between –CH2Cl and W(VI) Schiff base, which introduces high stable nanostructure without leaching the complex during the reaction.

A novel method for preparing tungsten and molybdenum peroxy complex solution and its application to tungsten - molybdenum separation

Aiming at the shortcomings of existing separation technology of tungsten (W) and molybdenum (Mo) by hydrogen peroxide (H2O2) complexation, the chemical behaviors of W(VI) and Mo(VI) in aqueous solutions were studied and a new method for preparing W(VI)-Mo(VI)-H2O2 solution was proposed. By using trialkyl phosphine oxide-tributyl phosphate (TRPO-TBP) as a mixed extractant system, the applications of this new method on separation of W and Mo were conducted, including the effect on third phase formation and elimination, satisfactory removal of Mo, and separation efficiency of Mo and W. The results show that the formation and depolymerization of isopoly and heteropoly compounds of W and Mo have an important influence on their separation. The core of the new method lies in the control of pH value when W and Mo begin to complex with H2O2. The formation of metatungstate and heteropoly compounds with high Mo/W atomic ratio could be avoided by adding H2O2 into the mixed solution of tungsten and molybdenum in a high pH range of 5.3–6.5 for complexation. Based on this principle, this method could not only avoid the third phase formation at the source but also effectively improve the satisfactory removal of molybdenum. As the complexation pH value increased from 3.51 to 5.51, after six-stage countercurrent extraction of feed solution containing about 127.8 g/L WO3 and 16.10 g/L Mo, the mass ratio of Mo/WO3 in raffinate decreased from 5.94 × 10−4 to 2.71 × 10−5 and the molybdenum content decreased from 77.50 mg/L to 3.54 mg/L. This study provides a relevant theoretical foundation and feasible improvement for the satisfactory separation of W and Mo from peroxy complex solution system, and is expected to facilitate the industrialization of this technology.

Mathematical model and reaction mechanism of molybdenum and tungsten extraction with TRPO from peroxide solution

To understand the behavior of molybdenum and tungsten extracted by tri-alkyl phosphine oxide (TRPO) from peroxide solution, the extraction mechanism was studied by slope method and Raman and FTIR spectroscopy. The empirical formulas of molybdenum and tungsten extraction distribution ratio (DMo and DW) as functions of equilibrium pH, TRPO concentration and temperature were obtained by establishing mathematical models. Furthermore, the reliability of the empirical formula was verified in the H+−W−Mo−H2O2 solution. The results indicate that the calculated values of DMo or DW were consistent with the experimental values. The apparent extraction equilibrium constants of molybdenum and tungsten were =8.51×103 (0.74≤pHe≤1.7), =99.89×103 (1.7<pHe≤4.62) and =2.65×103 (0.92<pHe<2.16) at 20 °C, respectively. The main extraction complex of molybdenum or tungsten was [H2(Mo or W)2O3(O2)4(H2O)2]·2TRPO. These empirical formulas can be used to analyze and estimate the extraction and separation of Mo and W from low molybdenum and tungsten concentration solutions.

Replacement of Molybdenum by Tungsten in a Biomimetic Complex Leads to an Increase in Oxygen Atom Transfer Catalytic Activity.

Upon replacement of molybdenum by tungsten in DMSO reductase isolated from the Rhodobacteraceae family, the derived enzyme catalyzes DMSO reduction faster. To better understand this behavior, we synthesized two tungsten(VI) dioxido complexes [WVIO2L2] with pyridine- (PyS) and pyrimidine-2-thiolate (PymS) ligands, isostructural to analogous molybdenum complexes we reported recently. Higher oxygen atom transfer (OAT) catalytic activity was observed with [WO2(PyS)2] compared to the Mo species, independent of whether PMe3 or PPh3 was used as the oxygen acceptor. [WVIO2L2] complexes undergo reduction with an excess of PMe3, yielding the tungsten(IV) oxido species [WOL2(PMe3)2], while with PPh3, no reactions are observed. Although OAT reactions from DMSO to phosphines are known for tungsten complexes, [WOL2(PMe3)2] are the first fully characterized phosphine-stabilized intermediates. By following the reaction of these reduced species with excess DMSO via UV–vis spectroscopy, we observed that tungsten compounds directly react to WVIO2 complexes while the Mo analogues first form μ-oxo Mo(V) dimers [Mo2O3L4]. Density functional theory calculations confirm that the oxygen atom abstraction from WVIO2 is an endergonic process contrasting the respective reaction with molybdenum. Here, we suggest that depending on the sacrificial oxygen acceptor, the tungsten complex may participate in catalysis either via a redox reaction or as an electrophile.

Dearomatization [4+2] Cycloaddition of Nonactivated Benzene Derivatives.

Dearomatization reactions have recently emerged as a powerful tool for the rapid buildup of molecular complexity. Here, an unparalleled thermal dearomatization [4+2] cycloaddition reaction between benzene derivatives and a 2H-phosphindole tungsten complex was reported. The unique reactivity of the in situ-generated 2H-phosphindole complex toward benzene was revealed by density functional theory calculations. We thus provide new insights into the dearomatization of nonactivated arenes and pave the way for the manipulation of the dearomatization for further applications.

Luminescence of ZnWO4 crystals under X-ray excitation

Zinc tungstate crystal scintillator is an interesting detector material for double beta decay, dark matter, and rare alpha decay experiments. The luminescence under X-ray excitation in the temperature region from 85 K to 420 K was studied with five samples of ZnWO4 crystals developed by different methods: with a stoichiometric composition, and with excess of Zn or W. It is found that doping a ZnWO4 crystal with 0.2–0.3% Zn and W does not affect the luminescence properties of the material. A theoretical dependence of the sample luminescence intensity on the competing nonradiative recombination centers and traps is obtained. The main center of the recombination luminescence in a ZnWO4 crystal with a spectral maximum at 2.5 eV is the tungsten complex with a structural defect located next to it.

Phenyl Sulfones: A Route to a Diverse Family of Trisubstituted Cyclohexenes from Three Independent Nucleophilic Additions.

A novel process is described for the synthesis of di- and trisubstituted cyclohexenes from an arene. These compounds are prepared from three independent nucleophilic addition reactions to a phenyl sulfone (PhSO2R; R = Me, Ph, and NC4H8) dihapto-coordinated to the tungsten complex {WTp(NO)(PMe3)}(Tp = trispyrazolylborate). Such a coordination renders the dearomatized aryl ring susceptible to protonation at a carbon ortho to the sulfone group. The resulting arenium species readily reacts with the first nucleophile to form a dihapto-coordinated sulfonylated diene complex. This complex can again be protonated, and the subsequent nucleophilic addition forms a trisubstituted cyclohexene species bearing a sulfonyl group at an allylic position. Loss of the sulfinate anion forms a π-allyl species, to which a third nucleophile can be added. The trisubstituted cyclohexene can then be oxidatively decomplexed, either before or after substitution of the sulfonyl group. Nucleophiles employed include masked enolates, cyanide, amines, amides, and hydride, with all three additions occurring to the same face of the ring, anti to the metal. Of the 12 novel functionalized cyclohexenes prepared as examples of this methodology, nine compounds meet five independent criteria for evaluating drug likeliness. Structural assignments are supported with nine crystal structures, density functional theory studies, and full 2D NMR analysis.

Synthesis, crystal structure, spectral characterization, theoretical studies, and investigation of catalytic activity in selective oxidation of sulfides by oxo-peroxo tungsten(VI) Schiff base complex

A novel oxo–peroxo tungsten(VI) Schiff base complex, [WO(O2)L(CH3OH)], a homogeneous catalyst for sulfoxidation was synthesized by treating WO3 in the presence of H2O2 with a dibasic tridentate (ONO) Schiff base ligand. The complex was characterized by FT–IR, 1H NMR, and 13C NMR spectroscopy. Moreover, the structure of the crystalline tungsten(VI) complex was further investigated by single crystal X-ray diffraction analysis (SC-XRD). The crystal structure analysis revealed that the complex is hepta-coordinated, and metal ion is surrounded by ONO set of the coordinated doubly deprotonated ligand to adopt a distorted pentagonal–bipyramidal geometry. The theoratical calculations of the geometrical parameters, frontier molecular orbitals (FMOs), molecular electrostatic potential (MEP), and natural bond orbital (NBO) analysis of the synthesized compounds utilizing DFT at the B3LYP level of theory at the Def2-TZVP basis set revealed that the predicted data match the experimental results. The main purpose of the current study is to employ the synthesized W complex for its homogeneous catalytic effectiveness for the oxidation of aryl and alkyl sulfides in ethanol by using 30% aqueous H2O2.

Photoactive Metal Carbonyl Complexes Bearing N-Heterocyclic Carbene Ligands: Synthesis, Characterization, and Viability as Photoredox Catalysts.

This report details the synthesis and characterization of a small family of previously unreported, structurally related chromium, molybdenum, tungsten, manganese, and iron complexes bearing N-heterocyclic carbene and carbonyl supporting ligands. These complexes have the general form [ML(CO)3X] or [ML(CO)3], where X = CO or Br and L = 1-phenyl-3-(2-pyridyl)imidazolin-2-ylidene. Where possible, the solid-state, spectroscopic, electrochemical, and photophysical properties of these molecules were studied using a combination of experiment and theory. Photophysical studies reveal that decarbonylation occurs when these complexes are exposed to ultraviolet light, with the CO ligand being replaced with a labile acetonitrile solvent molecule. To obtain insights into the potential utility, scope, and applications of these complexes in visible-light-mediated photoredox catalysis, their capacity to facilitate a range of photoinduced reactions via the reductive or oxidative functionalization of organic molecules was investigated. These chromium, molybdenum, and manganese catalysts efficiently facilitated atom-transfer radical addition processes. In light of their photolability, these types of catalysts may potentially allow for the development of photoinduced reactions involving less conventional inner-sphere electron-transfer pathways.

1,3,2-Diheterophospholane complexes: access to new tuneable precursors of phosphanoxyl complexes and P-functional polymers.

Synthesis of a testbed of P-H functional diheterophospholane complexes (3 and 6a,b) with no or little steric bulk at the α-position was achieved using [NEt4][WH(CO)5] as a combined reductant and complexation reagent. Reaction with TEMPO leads to P-OTEMP substituted tungsten complexes (4 and 7a,b) possessing different thermostabilities towards N-O bond cleavage. The transient phosphanoxyl complexes obtained were used for the polymerisation of styrene and acrylonitrile. DFT calculations were performed on the formation of various open-shell complexes and Loewdin spin density distributions.

N-heterocyclic carbene and cyclic (alkyl)(amino)carbene complexes of molybdenum(IV) and tungsten(IV).

The synthesis and characterization of N-heterocyclic carbene (NHC) and cyclic (alkyl)(amino)carbene (cAAC) complexes of molybdenum(IV) and tungsten(IV) chloride is reported. Reaction of two equivalents of the NHCs IMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), IDipp (1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) and IiPrMe (1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene) with [MCl4(SEt)2] (M = Mo, W) in toluene afforded the bis-NHC complexes [MCl4(NHC)2] (M = Mo: NHC = IMes 1, NHC = IMes 2; M = W: NHC = IMes 3, NHC = IDipp 4, NHC = IiPrMe5). Mono-carbene complexes [MCl4(NHC)(SEt2)] 6-9 (M = Mo: NHC = IMes 6, NHC = IMes 7; M = W: NHC = IMes 8, NHC = IDipp 9) are available via ligand dismutation of 1-4 with [MCl4(SEt)2]. Complexes 1-9 were characterized by using elemental analysis, IR- and multinuclear NMR spectroscopy and X-ray diffraction for 1-8. The reactivity of the mono-NHC complexes 6-9 towards the cyclic (alkyl)(amino)carbene cAACMe as well as the two electron donor ligands trimethylphosphine, triphenylphosphine, pyridine (py) and N,N-dimethylpyridin-4-amine (DMAP) was studied, which led to the synthesis of the mixed substituted complexes [MCl4(NHC)(cAACMe)] (M = Mo: NHC = IMes 10, NHC = IDipp 11; M = W: NHC = IMes 12, NHC = IDipp 13), [MCl4(NHC)(PR3)] (M = W: NHC = IMes, R = Me 14, NHC = IMes, R = Ph 15, NHC = IDipp, R = Me 16, NHC = IDipp, R = Ph 17), [MCl4(NHC)(py)] (M = Mo: NHC = IMes 18, NHC = IDipp 20; M = W: NHC = IMes 22, NHC = IDipp 24) and [MCl4(NHC)(DMAP)] (M = Mo: NHC = IMes 19, NHC = IDipp 21; M = W: NHC = IMes 23, NHC = IDipp 25).

On metal coordination of neutral open-shell P-ligands focusing on phosphanoxyls, their electron residence and reactivity.

This feature article highlights the discovery and development of phosphanoxyl complex chemistry starting from (neutral) low-coordinate phosphorus radicals and the quest of metal ligation effects. We describe synthesis and reactions of precursors, namely 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) substituted phosphane tungsten(0) complexes. Trapping reactions of transient phosphanoxyl complexes, formed via thermal homolytic N-O bond cleavage, as well as their use in radical polymerisations are illustrated, thus revealing an interesting reactivity dichotomy. DFT calculations provide insight into thermal stabilities of precursors and the resulting spin density distributions (SDDs) in these reactive intermediates. Systematic studies on the dependance of the electron delocalisation in phosphanoxyl complexes have been performed examining different substitution pattern at phosphorus and different co-ligand combinations at the tungsten(0) center. Preliminary results on Mn and Fe complexes are reported.

Tungsten and molybdenum dinitrogen complexes supported by a pentadentate tetrapodal phosphine ligand: comparative spectroscopic, electrochemical and reactivity studies.

The tungsten dinitrogen complex [W(N2)(PMe2PPPh2)] (2) (PMe2PPPh2 = [2-({bis[3-(diphenylphosphino)propyl]-phosphino}methyl)-2-methylpropane-1,3-diyl]bis(dimethylphosphine)) is synthesized and characterized by X-ray diffraction as well as IR and NMR spectroscopies, showing strong analogies to its molybdenum analogue [Mo(N2)(PMe2PPPh2)] (1). Whereas cyclic voltammetry studies indicate very similar redox potentials, detailed electrochemical and IR-spectroelectrochemical investigations reveal characteristic differences between 1 and 2 upon electrochemical oxidation in THF. Protonation of 2 with HBArF (BArF = tetrakis(3,5-bis(trifluoromethyl)-phenyl)borate) leads to the hydrazido(2-) derivative 3 which is spectroscopically characterized as well. In the presence of SmI2/H2O slightly overstoichiometric conversion of N2 to ammonia (2.75 equiv.) is observed. Although this is far below the activity of the Mo-complex 1, it renders 2 the first W complex to produce more than 2 equivalents of NH3 from N2 upon addition of protons and reductant.

Structural control in the nanoassembly of the tungsten and molybdenum dithiolene complex analog

A strategy for precisely tuning the self-assembly of tungsten and molybdenum dithiolene complexes to nanoflowers and nanopolyhedra is put forward.

CO2 Cleavage Reaction Driven by Alkylidyne Complexes of Group 6 Metals and Uranium: A Density Functional Theory Study on Energetics, Reaction Mechanism, and Structural/Bonding Properties.

Designing novel catalysts is essential for the efficient conversion of metal alkylidyne into metal oxo ketene complexes in the presence of CO2, which to some extent resolves the environmental concerns of the ever-increasing carbon emission. In this regard, a series of metal alkylidyne complexes, [b-ONO]M≡CCH3(THF)2 ([b-ONO] = {(C6H4[C(CF3)2O])2N}3-; M = Cr, Mo, W, and U), have been comprehensively studied by relativistic density functional theory calculations. The calculated thermodynamics and kinetics unravel that the tungsten complex is capable of catalyzing the CO2 cleavage reaction, agreeing with the experimental findings for its analogue. Interestingly, the uranium complex shows superior catalytic performance because of the associated considerably lower energy barrier and larger reaction rate constant. The M≡C moiety in the complexes turns out to be the active site for the [2 + 2] cyclic addition. In contrast, complexes of Cr and Mo could not offer good catalytic performance. Along the reaction coordinate, the M-C (M = Cr, Mo, W, and U) bond regularly transforms from triple to double to single bonds; concomitantly, the newly formed M-O in the product is identified to have a triple-bond character. The catalytic reactions have been extensively explained and addressed by geometric/electronic structures and bonding analyses.

The Effect of Pyridine-2-thiolate Ligands on the Reactivity of Tungsten Complexes toward Oxidation and Acetylene Insertion.

Intending to deepen our understanding of tungsten acetylene (C2H2) chemistry, with regard to the tungstoenzyme acetylene hydratase, here we explore the structure and reactivity of a series of tungsten acetylene complexes, stabilized with pyridine-2-thiolate ligands featuring tungsten in both +II and +IV oxidation states. By varying the substitution of the pyridine-2-thiolate moiety with respect to steric and electronic properties, we examined the details and limits of the previously reported intramolecular nucleophilic attack on acetylene followed by the formation of acetylene inserted complexes. Here, we demonstrate that only the combination of high steric demand and electron-withdrawing features prevents acetylene insertion. Nevertheless, although variable synthetic approaches are necessary for their synthesis, tungsten acetylene complexes can be stabilized predictably with a variety of pyridine-2-thiolate ligands.

Metal-doped titanium oxide films for suppressing potential-induced degradation of photovoltaic modules

Titanium oxide films were prepared from peroxotitanium complex aqueous solution containing a peroxo complex of vanadium, molybdenum or tungsten of 2, 5, 10 mol % by heating at 200 °C for the purpose of suppressing the photocatalytic activities that may be developed by crystallization during long-term use as a prevention film against potential-induced degradation (PID) of crystalline silicon (c-Si) photovoltaic (PV) module. The films containing those dopants crystallized to anatase by hydrothermal treatment at 100 °C for 15 h as an acceleration test, but hardly showed any photocatalytic activity. According to the lattice constants, X-ray photoelectron spectroscopy analyses and UV–visible light reflection spectra, it was estimated that vanadium (V), molybdenum (VI) and tungsten (VI) were substituted with titanium (IV) in titanium oxide heated at low temperature of 200 °C. The titanium oxide films containing the dopants of 2 mol % were coated at 200 °C onto the rear side of front cover glass with 200 nm thickness. No significant PID was observed in the c-Si PV modules based on the film-coated glass by a PID test by applying −1000 V at 85 °C for 2 h.

The influence of Gd2O3 on shielding, thermal and luminescence properties of WO3–Gd2O3–B2O3 glass for radiation shielding and detection material

The physical, structural, optical, shielding, thermal and luminescence properties of WO3-Gd2O3–B2O3 glasses were studied in this work. The effect of WO3 replacement by Gd2O3 on radiation shielding and radiation detection potential was evaluated. The glass density increases with increasing Gd2O3 concentration and the maximum value is 5.97 g/cm3. The FTIR and XRD result confirm that the trigonal BO3, tetrahedral BO4 and tungsten complex are the main chemical groups and coorperate into the amorphous structure of glass. The optical absorption spectra exhibit the absorbance of glasses in ultraviolet and visible light region. The shielding parameters of glasses, such as mass attenuation coefficient, effective atomic number, electron density and half value layer, were investigated theoretically and experimentally. They perform a small degradation of shielding property by increasing Gd2O3 content. There is no obvious dependence of Gd2O3 concentration on glass thermal conductivity and expansion. The strong photoluminescence of [WO6]6- group with 524 nm occurred under its charge transfer excitation and under the energy transfer from excited Gd3+ ion. In the radioluminescence, WGB glasses emitted the visible light with 562 nm based on the mixed emission from [WO6]6- group and oxygen vacancy. The glass possesses the X-ray detection efficiency around 18.65% of BGO crystal. The WO3–Gd2O3–B2O3 glass with 17.5 and 27.5 mol% of Gd2O3 is a promising γ-ray shielding material at room temperature and high temperature circumstance, respectively and one with 25 mol% of Gd2O3 owns the attractive potential for the X-ray radiation detection applications.

Synthesis of [(CO)5MS=CFcCH3] and exploration of the nature of M-S vs. M-O bonds in [(CO)5ME=CFcCH3]; (M = Cr, Mo, W and E = O, S) complexes

Photolysis of a methanol solution containing substituted alkyne, group VI metal hexacarbonyl and P4S10 at low temperature under an inert atmosphere leads to metal-sulfur coordinated complexes of chromium, molybdenum, and tungsten. The structure of pentacarbonylthioacetylferrocene-S-chromium (1) is confirmed by single crystal x-ray diffraction study and both the complexes (1 and 2) are confirmed using IR and NMR spectral studies. The sulfur-coordinated metal carbonyl complexes are thought to be formed by the initial activation of metal carbonyl with acetylenic ligand which then rearranges into metal coordinated thioacetylferrocene by dithiophosphoryl radical (formed in situ by reaction of P4S10 and methyl alcohol under photolytic condition). The stability of pentacarbonylthioacetylferrocene of group VI metal complexes are further compared with their acetylferrocene derivatives of Group VI metal complexes using DFT studies. Density functional theory was performed on six complexes [(CO)5ME=CFcCH3] (where M = Cr, Mo and W; while E = O and S) to assess their geometry and electronic structure aspects. Natural bonding orbital (NBO) and energy-decomposition analysis (EDA) were carried out to probe the nature and strength of the M-E interactions. NBO and EDA analysis predict M-E bond strength in the following order: Cr-E < Mo-E < W-E. The M-E bond covalency increases as we move down the group from Cr, Mo to W, which rationalizes the observed trends in the M-E bonds. While comparing the nature of the E (O and S) linkage in bonding, we found that -S analogue shows exceptionally robust σ and π interactions compared to the -O analogue. The reasonably strong M-S interactions resulted in the isolation of these -thio analogues.

Cover Feature: NIR‐Emissive Chromium(0), Molybdenum(0), and Tungsten(0) Complexes in the Solid State at Room Temperature (Chem. Eur. J. 51/2021)

Cover Feature: NIR‐Emissive Chromium(0), Molybdenum(0), and Tungsten(0) Complexes in the Solid State at Room Temperature (Chem. Eur. J. 51/2021)