Tungsten Centers Research Articles

Nucleophiles Target the Tungsten Center Over Acetylene in Biomimetic Models.

Inspired by the first shell mechanism proposed for the tungstoenzyme acetylene hydratase, the electrophilic reactivity of tungsten-acetylene complexes [W(CO)(C2H2)(6-MePyS)2] (1) and [WO(C2H2)(6-MePyS)2] (2) was investigated. The biological nucleophile water/hydroxide and tert-butyl isocyanide were employed. Our findings consistently show that, regardless of the nucleophile used, both tungsten centers W(II) and W(IV), respectively, are the preferred targets over the coordinated acetylene. Treatment of 2 with aqueous NaOH led to protonation of coordinated acetylene to ethylene, pointing toward the Brønsted basic character of the coordinated alkyne instead of the anticipated electrophilic behavior. In cases involving isocyanides as nucleophiles, the attack on the W(II) center of 1 took place first, whereas the W(IV) complex 2 remained unchanged. These experiments indicate that the direct nucleophilic attack of W-coordinated acetylene by water, as some computational studies of acetylene hydratase propose, is unlikely to occur.

Dynamics and Coordination of a P2N2 Ligand - from Twisted Conformation to Chelation.

Based on their general spacial flexibility, their Lewis and Brønsted basicity, and ability to mimic second sphere effects the 1,5-diaza-3,7-diphosphacyclooctane ligand family and their complexes have regained substantial scientific interest. It was now possible to structurally analyze a recently reported member of this family with p-tolyl and t-butyl substituents on P and N, respectively, (P2 p-tolN2 tBu). Notably, the ligand crystallizes with a 'twisted' backbone. This compound is the very first of its kind to have been unambiguously characterized with regard to its chemical and molecular structure as being in this conformation. A temperature-dependent NMR study provides insight into the molecular dynamics of two isomers in solution, which are most likely also both twisted, as judged by the observed limited reactivity. Despite the in principle unfavorable conformation of the free ligand, it was successfully chelated to tungsten and molybdenum centers in mononuclear carbonyl complexes. The ligand, a derivative thereof and four new complexes were comprehensively characterized and analyzed in comparison. This includes single crystal XRD molecular structures of P2 p-tolN2 tBu and all four complexes. P2 p-tolN2 tBu, regardless of its twisted conformation, is able to coordinate to metal centers given that enough energy (heat) for a conformational change is provided.

Cooperative tungsten centers in polymeric carbon nitride for efficient overall photosynthesis of hydrogen peroxide

Meticulously designed neighboring WOR and ORR active sites significantly enhance the overall photocatalytic synthesis of hydrogen peroxide.

Interstitial Hydrogen Atom to Boost Intrinsic Catalytic Activity of Tungsten Oxide for Hydrogen Evolution Reaction.

Tungsten oxide (WO3 ) is an appealing electrocatalyst for the hydrogen evolution reaction (HER) owing to its cost-effectiveness and structural adjustability. However, the WO3 electrocatalyst displays undesirable intrinsic activity for the HER, which originates from the strong hydrogen adsorption energy. Herein, for effective defect engineering, a hydrogen atom inserted into the interstitial lattice site of tungsten oxide (H0.23 WO3 ) is proposed to enhance the catalytic activity by adjusting the surface electronic structure and weakening the hydrogen adsorption energy. Experimentally, the H0.23 WO3 electrocatalyst is successfully prepared on reduced graphene oxide. It exhibits significantly improved electrocatalytic activity for HER, with a low overpotential of 33mV to drive a current density of 10mA cm-2 and ultra-long catalytic stability at high-throughput hydrogen output (200 000 s, 90mA cm-2 ) in acidic media. Theoretically, density functional theory calculations indicate that strong interactions between interstitial hydrogen and lattice oxygen lower the electron density distributions of the d-orbitals of the active tungsten (W) centers to weaken the adsorption of hydrogen intermediates on W-sites, thereby sufficiently promoting fast desorption from the catalyst surface. This work enriches defect engineering to modulate the electron structure and provides a new pathway for the rational design of efficient catalysts for HER.

Barrel-Shaped-Polyoxometalates Exhibiting Electrocatalytic Water Reduction at Neutral pH: A Synergy Effect.

Two copper-based barrel-shaped polyoxometalates (POMs), namely, [{H3O}4{Na6(H2O)22}][{CuI (H2O)3}2{CuII (H2O)}3{B-α-BiIIIWVI9O33}2]·7H2O (NaCu-POM) and Li4[{NH4}2{H3O}3{Li(H2O)5}][{CuII(SH)}{(CuIICuI1.5)(B-α-BiIIIWVI9O33)}2]·9H2O (LiCu-POM) have been synthesized and structurally characterized. The single-crystal X-ray diffraction analyses of NaCu-POM and LiCu-POM reveal the presence of penta- and hexa-nuclear copper wheels per formula units, respectively; these copper wheels are sandwiched between two lacunary Keggin anions {B-α-BiIIIWVI9O33}9- (BiW9) to form the barrel-shaped title POM compounds. In both the compounds NaCu-POM and LiCu-POM, the mixed-valent copper centers are present in their respective penta- and hexa-nuclear copper wheels, established by X-ray photoelectron spectroscopy (XPS) as well as by bond valence sum (BVS) calculations. Compound LiCu-POM additionally shows the presence of a sulfhydryl ligand (SH-), coordinated to one of the copper centers of its {Cu6}-wheel, that is expected to be generated from the in situ reduction of sulfate anion present in the concerned reaction mixture (lithium-ion in ammonia solution may be the reducing agent). Interestingly, the title compounds, NaCu-POM and LiCu-POM exhibit an efficient electrocatalytic hydrogen evolution reaction (HER) by reducing water at neutral pH. Detailed electrochemical studies including controlled experiments indicate that the active sites for this electrocatalysis are the W(VI) centers of the title compounds, not the copper centers. However, a relevant tri-lacunary Keggin cluster anion {PVWVI9O33}7- (devoid of copper ion) does not show comparable HER as shown by the title compounds. The intra-cluster cooperative interactions of the mixed-valent copper centers (CuII/CuI) with the tungsten centers (W6+) make the overall system electrocatalytically active toward water reduction to molecular hydrogen at neutral pH. High Faradaic efficiencies (89 and 92%) and turnover frequencies (1.598 s-1 and 1.117 s-1) make the title compounds NaCu-POM and LiCu-POM efficient catalysts toward electrochemical water reduction to molecular hydrogen.

Lindqvist versus Keggin-Type Polyoxometalates as Catalysts for Effective Desulfurization of Fuels

A correlation between polyoxotungstate structures and their catalytic performance for oxidative desulfurization processes was investigated. Bridged lanthanopolyoxometalates that incorporate identical metallic centers with Keggin- Eu[PW11O39]11− and Lindqvist-type [Eu(W5O18)2]9− structures were used as catalysts for the oxidation of the most representative refractory sulfur compounds. Both compounds were able to desulfurize a multicomponent model diesel under sustainable conditions, i.e., using ionic liquid as an extraction solvent and hydrogen peroxide as an oxidant. However, the Lindqvist catalyst appeared to achieve complete desulfurization faster than the Keggin catalyst while using a lesser amount of catalyst and oxidant. Furthermore, the reusable capacity of the Lindqvist-type [Eu(W5O18)2]9− was confirmed for consecutive oxidative desulfurization processes. The contribution of the lanthanide metallic center for the catalytic performance of these compounds was investigated by studying the analogous [TB(W5O18)2]9− compound. Identical desulfurization efficiency was obtained, even reusing this catalyst in consecutive reaction cycles. These results indicate that the active catalytic center of these compounds is probably related to the octahedral tungsten centers. However, a higher number of tungsten centers in the polyoxometalate structure did not result in higher catalytic activity.

Reversible Photo- and Thermo-Induced Spin-State Switching in a Heterometallic {5d-3d} W2Fe2 Molecular Square Complex.

Following the complex-as-a-ligand strategy, self-assembly of [W(CN)8]3- and iron(II) with bidentate nitrogen donor ligand bik (bik = bis(1-methyl-1H-imidazol-2-yl)ketone) ligand affords a cyanide-bridged [W2Fe2] molecular square complex [HNBu3]2{[W(CN)8]2[Fe(bik)2]2}·6H2O·CH3OH (1). The complex was characterized by single-crystal X-ray diffraction analyses, (photo)magnetic studies, optical reflectivity, electrochemical studies, and spectroscopic studies. Structural analyses revealed that in the [W2Fe2] square motif tungsten(V) and iron(II) centers reside in an alternate corner of the square and are bridged by the cyanide ligands. Complex 1 exhibits thermo-induced spin crossover (SCO) between {WV (S = 1/2) - FeIILS (S = 0)} and {WV (S = 1/2) - FeIIHS (S = 2)} pairs near room temperature and photoinduced spin-state switching with TLIESST = 70 K under light irradiation at low temperature. To the best of our knowledge, 1 represents the first complex containing iron(II) and [WV(CN)8]3- units exhibiting both SCO and photomagnetic effect.

Multi-Nuclear Rare-Earth-Implanted Tartaric Acid-Functionalized Selenotungstates and Their Fluorescent and Magnetic Properties.

A family of multinuclear rare-earth (RE)-implanted H2tart2--functionalized selenotungstates (STs) [H2N(CH3)2]13H{[W2O5(OH)2(H2tart)2](H2tart){[W3O6RE2(H2O)6][SeW9O33]2}2}·31H2O [RE = Eu3+ (1), Tb3+ (2), Dy3+ (3), Ho3+ (4), Y3+ (5); H4tart = d-tartaric acid] have been afforded by a simple one-pot aqueous reaction and were structurally characterized. Intriguingly, their isomorphous organic-inorganic hybrid anion {[W2O5(OH)2(H2tart)2](H2tart){[W3O6RE2(H2O)6][SeW9O33]2}2}14- includes two sandwich-type {[W3O6[RE2(H2O)6][SeW9O33]2}4- dimeric units with a W-O-RE heterometal core, which are further joined by two H2tart2--decorated dinuclear tungsten-oxo {W2O5(OH)2(H2tart)2} clusters and a bridging H2tart2- ligand, contributing to a surprising Mobius band-like configuration. It is worth emphasizing that three H2tart2- ligands coordinate with tungsten centers rather than RE cations. For all we know, 1-5 delegate the infrequent RE-implanted STs functionalized by triplicate H2tart2- bridges. Furthermore, fluorescent performances of 1-4 as well as magnetic properties of 2-4 have been surveyed. The solid-state fluorescence emission spectra prove that each of them undoubtedly shows the characteristic emission peaks of RE cores, while alternating-current susceptibility measurements suggest field-induced single-molecule magnetic behavior in 3.

Novel porous SiO2@SiC core-shell nanospheres functionalized with an amino hybrid of WO3 as an oxidative desulfurization catalyst

A novel N-doped SiO2@SiC core shell has been synthesized and functionalized with amino groups. Then, amino hybrid of tungsten oxide was supported on the surface of functionalized SiO2@SiC core shell by immobilization of tungsten complex and coordination of ethylenediamine to tungsten centers. Then, the catalytic activities of prepared hybrids were investigated in oxidative desulfurization of model fuels. To evaluate the role of silicon carbide support, silica nanospheres also were functionalized and its catalytic performance was compared. The results indicate that the SiC catalyst shows higher catalytic activity (99.9%, 160 min) compared to the SiO2 analogue (61%, 160 min).

Radiative d–d transitions at tungsten centers in II–VI semiconductors

The luminescence spectra of W impurity centers in II–VI semiconductors, specifically, ZnSe, CdS, and CdSe, are studied. It is found that, if the electron system of 5d (W) centers is considered instead of the electron system of 3d (Cr) centers, the spectral characteristics of the impurity radiation are substantially changed. The electron transitions are identified in accordance with Tanabe–Sugano diagrams of crystal field theory. With consideration for the specific features of the spectra, it is established that, in the crystals under study, radiative transitions at 5d W centers occur between levels with different spins in the region of a weak crystal field.

Highly efficient supercapacitor electrode with two-dimensional tungsten disulfide and reduced graphene oxide hybrid nanosheets

Two-dimensional (2D) nanostructures with their high surface area and large in-plane conductivity have been regarded as promising materials for supercapacitors (SCs). Tungsten disulfide (WS2) is highly suitable for charge accumulation with its abundant active sites in the interspacing between the 2D structures and the intraspacing of each atomic layer, as well as on the tungsten centers with the charges generated by the Faradaic reactions. This study proposes the preparation of well-constructed WS2/reduced graphene oxide (RGO) nanosheets using a simple molten salt process as the electroactive material for SCs, which presents a high specific capacitance (CF) of 2508.07 F g−1 at the scan rate of 1 mV s−1, because of the synergic effect of WS2 with its large charge-accumulating sites on the 2D planes and RGO with its highly enhanced conductivity and improved connections in the WS2 networks. The excellent cycling stability of 98.6% retention after 5000 cycles charge/discharge process and the Coulombic efficiency close to 100% for the entire measurement are also achieved for the WS2/RGO-based SC electrode. The results suggest the potential for the combination of the 2D metal sulfide and carbon materials as the charge storage material to solve the energy problems and attain a sustainable society.

Electrochemistry Investigation of the Monolacunary and Their Transition Metal Substituent Keggin-Type Polyoxometalates

The electrochemical and electrocatalytic behaviors of [XW11O39] n− and [XW11M(H2O)O39] m− (X = B, Si, and P; MII = Mn, Co, and Ni) polyoxometalates were studied with specific emphasis on the tungsten centers by cyclic voltammetry in aqueous solution. Electrochemical studies show the two or three stepwise cathodic/anodic peaks corresponding to the redox processes of WVI centers. The reversibility of redox peaks increases for [SiW11M(H2O)O39]6− and [PW11M(H2O)O39]5− polyoxometalates; however, that of [BW11M(H2O)O39]7− maintains quasi-reversible. The redox waves of the W-O framework for all polyoxometalates are pH- and scan rate-dependent. The tungsten-centered waves for these compound show a classical potential shift as a function of acidity at pH = 2.2–4.2. The electrocatalysis behaviors of these compounds toward the reduction of nitrite ions have been studied in details. The plots of catalytic efficiency versus γ for [XW11O39] n− and [XW11M(H2O)O39] m− (X = B, Si, P; MII = Mn, Co, Ni) show the catalytic efficiency of [BW11O39]9− and [BW11M(H2O)O39]7− (M = Mn, Co, Ni) more than that of the other compounds which are mentioned in this article.

A new sandwich-type polyoxometalates containing mixed transition metals, [Cd2Zn2(H2O)2(P2W15O56)2]16−: Syntheses, spectrochemical characterization and its electrocatalytical activity comparison with [M4(H2O)2(P2W15O56)2]16− (M = Cd2+ and Zn2+) and [P2W15O56]12−

The mixed-metal sandwich-type polyoxometalate [Cd2Zn2(H2O)2(P2W15O56)2]16− (abbreviated Cd2Zn2P4W30) has been synthesized and characterized by FT-IR, 31P NMR, 113Cd NMR spectroscopies, cyclic voltammetry (CV) and elemental analysis. The electrochemical behavior of this sandwich-type complex in aqueous solution has been investigated, and also systematic comparisons of that have been made by [M4(H2O)2(P2W15O56)2]16− (M=Cd2+ and Zn2+) (abbreviated M4P4W30) and [P2W15O56]12− (abbreviated P2W15). All these compounds show successive reduction processes of the tungsten centers in a negative potential range, but in some cases, the electrochemistry involves redox reactions originating from cadmium ions. These sandwich-type complexes have efficient electrocatalytic activities towards the reduction of nitrite. However, there is a significant decrease in efficiency as the number of cadmium centers in the complex increases.

σ-Silane, disilanyl, and [W(μ-H)Si(μ-H)W] bridging silylene complexes via the reactions of W(PMe3)4(η2-CH2PMe2)H with phenylsilanes.

W(PMe3)4(η(2)-CH2PMe2)H reacts with PhSiH3 to give the first examples of diphenyldisilanyl compounds, W(PMe3)4(SiH2SiHPh2)H3 and W(PMe3)3(SiH2Ph)(SiH2SiHPh2)H4, via a mechanism that is proposed to involve migration of a SiHPh2 group to a silylene ligand. In addition to the formation of the aforementioned mononuclear compounds, the reaction of W(PMe3)4(η(2)-CH2PMe2)H with PhSiH3 also yields a novel dinuclear compound, [W(PMe3)2(SiHPh2)H2](μ-Si,P-SiHPhCH2PMe2)(μ-SiH2)[W(PMe3)3H2], which features a bridging silylene ligand that participates in 3-center-2-electron interactions with both tungsten centers. The bonding within the [W(μ-H)Si(μ-H)W] core can be described by a variety of resonance structures, some of which possess multiple bond character between tungsten and silicon. In this regard, [W(PMe3)2(SiHPh2)H2](μ-Si,P-SiHPhCH2PMe2)(μ-SiH2)[W(PMe3)3H2] possesses the shortest W-Si bond length reported. The corresponding reaction of W(PMe3)4(η(2)-CH2PMe2)H with Ph2SiH2 yields the σ-silane compound, W(PMe3)3(σ-HSiHPh2)H4.

Effect of tungsten coating on the properties of high-resistivity gold-doped silicon

We have studied the effect of tungsten atoms (tungsten coating) on the properties of gold-doped silicon. The coating considerably increased the resistivity of the silicon and markedly reduced the photoconductivity relaxation time. The silicon doped with gold in the presence of tungsten was found to have an energy level with an ionization energy E = E c − (0.23 ± 0.02) eV. In uncoated gold-doped silicon, no energy level with such ionization energy was found. This strongly suggests that the energy level with an ionization energy E = E c − (0.23 ± 0.02) eV is due to tungsten centers.

Electronic, Redox, and Photophysical Consequences of Metal-for-Carbon Substitution in Oligo-Phenylene-Ethynylenes

The electronic structures, redox chemistry, and excited-state properties of tungsten-containing oligo-phenylene-ethynylenes (OPEs) of the form W[C(p-C6H4CC)n-1Ph](dppe)2Cl (n = 1-5; dppe =1,2-bis(diphenylphosphino)ethane) are reported and compared with those of organic analogues in order to elucidate the effects of metal-for-carbon substitution on OPE bonding and electronic properties. Key similarities between the metallo- and organic OPEs that bear on materials-related functions include their nearly identical effective conjugation lengths, reduction potentials, and π* orbital energies and delocalization. In addition to these conserved properties, the tungsten centers endow OPEs with reversible one-electron oxidation chemistry and long-lived emissive triplet excited states that are not accessible to organic OPEs. The electronic similarities and differences between metallo- and organic OPEs can be understood largely on the basis of π/π* orbital energy matching between tungsten and organic PE fragments and the introduction of an orthogonal mid-π/π*-gap d orbital in metallo-OPEs. These orbital energies can be tuned by varying the supporting ligands; this provides a means to rationally implement and control the emergent properties of metallo-OPE materials.

Detection and differentiation of neutral organic compounds by 19F NMR with a tungsten calix[4]arene imido complex.

Fluorinated tungsten calix[4]arene imido complexes were synthesized and used as receptors to detect and differentiate neutral organic compounds. It was found that the binding of specific neutral organic molecules to the tungsten centers induces an upfield shift of the fluorine atom appended on the arylimido group, the extent of which is highly dependent on electronic and steric properties. We demonstrate that the specific bonding and size-selectivity of calix[4]arene tungsten-imido complex combined with (19)F NMR spectroscopy is a powerful new method for the analysis of complex mixtures.

Synthesis and Characterization of Hydrido Carbonyl Molybdenum and Tungsten PNP Pincer Complexes.

In the present study the Mo(0) and W(0) complexes [M(PNP)(CO)3] as well as seven-coordinate cationic hydridocarbonyl Mo(II) and W(II) complexes of the type [M(PNP)(CO)3H]+, featuring PNP pincer ligands based on 2,6-diaminopyridine, have been prepared and fully characterized. The synthesis of Mo(0) complexes [Mo(PNP)(CO)3] was accomplished by treatment of [Mo(CO)3(CH3CN)3] with the respective PNP ligands. The analogous W(0) complexes were prepared by reduction of the bromocarbonyl complexes [W(PNP)(CO)3Br]+ with NaHg. These intermediates were obtained from the known dinuclear complex [W(CO)4(μ-Br)Br]2, prepared in situ from W(CO)6 and stoichiometric amounts of Br2. Addition of HBF4 to [M(PNP)(CO)3] resulted in clean protonation at the molybdenum and tungsten centers to generate the Mo(II) and W(II) hydride complexes [M(PNP)(CO)3H]+. The protonation is fully reversible, and upon addition of NEt3 as base the Mo(0) and W(0) complexes [M(PNP)(CO)3] are regenerated quantitatively. All heptacoordinate complexes exhibit fluxional behavior in solution. The mechanism of the dynamic process of the hydrido carbonyl complexes was investigated by means of DFT calculations, revealing that it occurs in a single step. The structures of representative complexes were determined by X-ray single-crystal analyses.

Mixed addenda polyoxometalate “solutions” for stationary energy storage

A series of redox flow batteries utilizing mixed addenda (vanadium and tungsten), phosphorus-based polyoxometalates (A-α-PV3W9O40(6-), B-α-PV3W9O40(6-), and P2V3W15O62(9-)) were prepared and tested. Cyclic voltammetry and bulk electrolysis experiments on the Keggin compounds (A-α-PV3W9O40(6-) and B-α-PV3W9O40(6-)) established that the vanadium centers of these compounds could be used as the positive electrode (PV(IV)3W(VI)9O40(9-)/PV(V)3W(VI)9O40(6-)), and the tungsten centers could be used as the negative electrode (PV(IV)3W(VI)9O40(9-)/PV(IV)3W(V)3W(VI)6O40(12-)) since these electrochemical processes are separated by about 1 V. The results showed that A-α-PV3W9O40(6-) (where A indicates adjacent, corner-sharing vanadium atoms) had coulombic efficiencies (charge in divided by charge out) above 80%, while the coulombic efficiency of B-α-PV3W9O40(6-) (where B indicates adjacent edge-sharing vanadium atoms) fluctuated between 50% and 70% during cycling. The electrochemical yield, a measurement of the actual charge or discharge observed in comparison with the theoretical charge, was between 40% and 50% for A-α-PV3W9O40(6-), and (31)P NMR showed small amounts of PV2W10O40(5-) and PVW11O40(4-) formed with cycling. The electrochemical yield for B-α-PV3W9O40(6-) decreased from 90% to around 60% due to precipitation of the compound on the electrode, but there were no decomposition products detected in the solution by (31)P NMR, and infrared data on the electrode suggested that the cluster remained intact. Testing of P2V3W15O62(9-) (Wells-Dawson structure) suggested higher charge density clusters were not as suitable as the Keggin structures for a redox flow battery due to the poor stability and inaccessibility of the highly reduced materials.

Preparation, characterization and electrochemical investigation of a new inorganic–organic hybrid material based on Keggin-type polyoxometalate and organic imidazole cation

A new soluble inorganic–organic hybrid based on polyoxotungstate, [Himi]4[SiW12O40].H2O 1 (imi = Imidazole), has been successfully synthesized and characterized by using elemental analysis, infrared spectrum, 1H NMR technique and single-crystal X-ray diffraction. X-ray crystallography reveals that 1 displays an organic–inorganic hybrid frameworks constructed by [SiW12O40]4−α-Keggin-type polyoxoanion and four {(Himi)}+ monocationic hydrogen-bonded fragments. Redox activities for the tungsten centers have been observed using cyclic voltammetry. It was found that 1 presents good electrocatalytic activities for the reduction of NO2− rather than H4SiW12O40 heteropolyacid.