Tungsten Complexes Research Articles

Co-crystals: The neutral molecules in modification of crystal structure of [W(CN)6(bpy)]2− anion salts

Ten new coformers were incorporated into structure of (PPh4)2[W(CN)6(bpy)] salt (bpy = 2,2′-bipyridine), this includes such neutral molecules as: 1,10-phenanthroline, 4-hydroxyacetophenone, 2-amino-5-nitrobenzonitrile, 5-amino-2-pyridinecarbonitrile, 2-nitrobenzonitrile, 4-formylbenzoic acid, 4-pyridinecarbonitrile, 2-pyridinecarbonitrile, 2-hydroxybenzonitrile and 2-naphtol. The composition of cocrystals were confirmed by elemental analysis while the incorporation of coformers was also proved by IR spectra. The single crystal structures were solved and described for salts with 2-naphtol and 4-hydroxyacetophenone. The formation of crystals was caused by formation of strong hydrogen bonds between coformers and cyanido ligands of the anion. The structures were compared and discussed with other similar systems. Additionally, the influence of the coformer on the packing structure of hexacyanido tungsten(IV) complexes was discussed.

Poly(imidazolyliden-yl)borato Complexes of Tungsten: Mapping Steric vs. Electronic Features of Facially Coordinating Ligands.

A convenient synthesis of [HB(HImMe)3](PF6)2 (ImMe = N-methylimidazolyl) is decribed. This salt serves in situ as a precursor to the tris(imidazolylidenyl)borate Li[HB(ImMe)3] pro-ligand upon deprotonation with nBuLi. Reaction with [W(≡CC6H4Me-4)(CO)2(pic)2(Br)] (pic = 4-picoline) affords the carbyne complex [W(≡CC6H4Me-4)(CO)2{HB(ImMe)3}]. Interrogation of experimental and computational data for this compound allow a ranking of familiar tripodal and facially coordinating ligands according to steric (percentage buried volume) and electronic (νCO) properties. The reaction of [W(≡CC6H4Me-4)(CO)2{HB(ImMe)3}] with [AuCl(SMe2)] affords the heterobimetallic semi-bridging carbyne complex [WAu(μ-CC6H4Me-4)(CO)2(Cl){HB(ImMe)3}].

A kinetic study of the reduction of tetrachloroaurate(III) ions by the cyano complexes of iron(II), tungsten(IV) and molybdenum(IV)

The kinetics of the reduction of tetrachloroaurate(III) by the cyano complexes of Fe(II), W(IV) and Mo(IV) have been studied in aqueous acidic medium. The reactions of Fe(CN)64− and W(CN)84− display first-order kinetics in both [AuCl4−] and [M(CN)n4−] (M = Fe and n = 6, W and n = 8) while [H+] has a retarding effect on the reaction rate for both the Fe and W reactions. An acid dissociation constant, Ka1, for the Fe(CN)64− reaction has been established kinetically to be Ka1 = (9.1 ± 0.5) × 10−3 M (pKa1 = 2.04) at µ = 1.2 M while the previously unknown Ka1 for the W(CN)84− reaction was found to be Ka1 = 0.33 ± 0.09 M (pKa1 = 0.48). Alkali metal cations such as Na+ and Cs+ and denoted as (cat+) accelerate the reaction. Some reactions were performed under second order (stoichiometric) conditions while other were performed under pseudo-first order conditions; rate constants utilizing these two techniques were mutually consistent. From the acid dependence and variation of the ionic strength of the reaction medium and the Bronsted-Bjerrum equation it has been established that AuIIICl4− and H(cat+)M(CN)n2− (M = Fe and n = 6 or M = W and n = 8) are the reactive species during the course of the reactions. The reaction between Mo(CN)84− and AuCl− is first order in [AuCl−] and second order in Mo(CN)84−. Reactions were performed under stoichiometric conditions and kinetic data was treated with a third order integrated rate law. The reaction is independent of [H+] and [cat+]. From the retarding effect of Cl− and Mo(CN)83− on the rate of the AuIIICl4− reduction, it is concluded that the molecular formula of the intermediate gold(II) species en route to AuICl2− is AuIICl3−. Activation parameters for all three reactions have been obtained from the Eyring equation.It was concluded that all three reactions follow the same mechanism but that the reverse reaction in the equilibrium between the intermediate gold(II) species, AuIICl3−, and the product MoV(CN)83− is only significant in the MoIV(CN)84− reduction of AuIIICl4−. This was also the cause for Mo(CN)84− showing a second order dependency.Graphical abstract

Advances in Group VI Metal-Catalyzed Homogeneous Hydrogenation and Dehydrogenation Reactions.

Transition metal-catalyzed homogeneous hydrogenation and dehydrogenation reactions for attaining plethora of organic scaffolds have evolved as a key domain of research in academia and industry. These protocols are atom-economic, greener, in line with the goal of sustainability, eventually pave the way for numerous novel environmentally benign methodologies. Appealing progress has been achieved in the realm of homogeneous catalysis utilizing noble metals. Owing to their high cost, less abundance along with toxicity issues led the scientific community to search for sustainable alternatives. In this context, earth- abundant base metals have gained substantial attention culminating enormous progress in recent years, predominantly with pincer-type complexes of nickel, cobalt, iron, and manganese. In this regard, group VI chromium, molybdenum and tungsten complexes have been overlooked and remain underdeveloped despite their earth-abundance and bio-compatibility. This review delineates a comprehensive overview in the arena of homogeneously catalysed (de)hydrogenation reactions using group VI base metals chromium, molybdenum, and tungsten till date. Various reactions have been described; hydrogenation, transfer hydrogenation, dehydrogenation, acceptorless dehydrogenative coupling, hydrogen auto transfer, along with their scope and brief mechanistic insights.

Boron Insertion into the N≡N Bond of a Tungsten Dinitrogen Complex.

The 1,3-addition of 1,2-diaryl-1,2-dibromodiboranes (B2Br2Ar2) to trans-[W(N2)2(dppe)2] (dppe = κ2-(Ph2PCH2)2), which is accompanied by a Br-Ar substituent exchange between the two boron atoms, is followed by a spontaneous rearrangement of the resulting tungsten diboranyldiazenido complex to a 2-aza-1,3-diboraallenylimido complex displaying a linear, cumulenic B=N=B moiety. This rearrangement involves the splitting of both the B-B and N=N bonds of the N2B2 ligand, formal insertion of a BAr boranediyl moiety into the N=N bond, and coordination of the remaining BArBr boryl moiety to the terminal nitrogen atom. Density functional theory calculations show that the reaction proceeds via a cyclic NB2 intermediate, followed by dissociation into a tungsten nitrido complex and a linear boryliminoborane, which recombine by adduct formation between the nitrido ligand and the electron-deficient iminoborane boron atom. The linear B=N=B moiety also undergoes facile 1,2-addition of Brønsted acids (HY = HOPh, HSPh, and H2NPh) with concomitant Y-Br substituent exchange at the terminal boron atom, yielding cationic (borylamino)borylimido tungsten complexes.

Ring Expansion Alkyne Metathesis Polymerization.

The synthesis, characterization, and preliminary activity of an unprecedented tethered alkylidyne tungsten complex for ring expansion alkyne metathesis polymerization (REAMP) are reported. The tethered alkylidyne 7 is generated rapidly by combining alkylidyne W(CtBu)(CH2tBu)(O-2,6-i-Pr2C6H3)2 (6) with 1 equiv of an yne-ol proligand (5). Characterized by NMR studies and nuclear Overhauser effect spectroscopy, complex 7 is a dimer. Each metal center contains a tungsten-carbon triple bond tethered to the metal center via an alkoxide ligand. The polymerization of the strained cycloalkyne 3,8-didodecyloxy-5,6-dihydro-11,12-didehydrodibenzo[a,e]-[8]annulene, 8, to generate cyclic polymers was demonstrated. Size exclusion chromatography (SEC) and intrinsic viscosity (η) measurements confirm the polymer's cyclic topology.

Synthesis, characterization, and spectroscopic analysis of binuclear tungsten(VI) complexes with peroxidase-like activity

Two novel binuclear tungsten(VI) complexes, denoted as 1 and 2, encompasing oxidoperoxido- and dioxido- moieties, have been successfully synthesized and investigated. These complexes, namely [{WO(O2)}2(L)(H2O)2] 1, and [{WO2}2(L)(H2O)2] 2, are formed utilizing the ligand H4L I, obtained through the condensation reaction of 2-hydroxyacetophenone and dihydrazide of isophthalic acid in a 2:1 ratio. The synthetic route for complex 1 entails an in-situ reaction between WO3·H2O and H2O2, executed alongside the ligand in a stoichiometric ratio of 1:2. On the other hand, the synthesis of complex 2 involves the reaction of complex 1 with triphenylphosphine. The synthesized compounds underwent comprehensive analytical investigation through elemental (CHN) analysis, spectroscopic techniques (FT-IR, UV–Vis, 1H, and 13C NMR), and thermal analysis (TGA). Density functional theory (DFT) computations were employed to calculate vibrational frequencies and NMR parameters for both the ligand and the corresponding tungsten complexes. Encouragingly, upon a comparison of the theoretical and experimental data, a remarkable congruence was observed. Furthermore, these complexes displayed noteworthy peroxidase-like activity when subjected to hydrogen peroxide, as evidenced by their catalytic oxidation of dopamine.

Silica-Based Materials Containing Inorganic Red/NIR Emitters and Their Application in Biomedicine.

The low absorption of biological substances and living tissues in the red/near-infrared region (therapeutic window) makes luminophores emitting in the range of ~650-1350 nm favorable for in vitro and in vivo imaging. In contrast to commonly used organic dyes, inorganic red/NIR emitters, including ruthenium complexes, quantum dots, lanthanide compounds, and octahedral cluster complexes of molybdenum and tungsten, not only exhibit excellent emission in the desired region but also possess additional functional properties, such as photosensitization of the singlet oxygen generation process, upconversion luminescence, photoactivated effects, and so on. However, despite their outstanding functional applicability, they share the same drawback-instability in aqueous media under physiological conditions, especially without additional modifications. One of the most effective and thus widely used types of modification is incorporation into silica, which is (1) easy to obtain, (2) biocompatible, and (3) non-toxic. In addition, the variety of morphological characteristics, along with simple surface modification, provides room for creativity in the development of various multifunctional diagnostic/therapeutic platforms. In this review, we have highlighted biomedical applications of silica-based materials containing red/NIR-emitting compounds.

(Decacarbonyl)(1-isopropyl-2-phenyl-1,2,5,6-tetrahydroacenaphtho [5,6-cd][1,2]diphosphole)ditungsten(0)

A binuclear tungsten(0) complex with an asymmetric bridging 1,2-diphosphole ligand was prepared by the photolysis of tungsten hexacarbonyl in the presence of 1-isopropyl-2-phenyl-1,2,5,6-tetrahydroacenaphtho [5,6-cd][1,2]diphosphole. The tungsten complex 1 was characterized by high resolution mass spectrometry, multinuclear NMR spectroscopy (1H, 13C, 31P), and elemental microanalysis. The structure of the trans isomer was resolved by single crystal X-ray diffraction, showing a slight elongation of the P−P bond upon coordination to the W(CO)5 groups.

Expanding the Utility of β-Diketiminate Ligands in Heavy Group VI Chemistry of Molybdenum and Tungsten.

We report the synthesis of 17 molybdenum and tungsten complexes supported by the ubiquitous BDI ligand framework (BDI = β-diketiminate). The focal entry point is the synthesis of four molybdenum and tungsten(V) BDI complexes of the general formula [MO(BDIR)Cl2] [M = Mo, R = Dipp (1); M = W, R = Dipp (2); M = Mo, R = Mes (3); M = W, R = Mes (4)] synthesized by the reaction between MoOCl3(THF)2 or WOCl3(THF)2 and LiBDIR. Reactivity studies show that the BDIDipp complexes are excellent precursors toward adduct formation, reacting smoothly with dimethylaminopyridine (DMAP) and triethylphosphine oxide (OPEt3). No reaction with small phosphines has been observed, strongly contrasting the chemistry of previously reported rhenium(V) complexes. Additionally, the complexes 1 and 2 are good precursors for salt metathesis reactions. While 1 can be chemically reduced to the first stable example of a Mo(IV) BDI complex 15, reduction of 2 resulted in degradation of the BDI ligand via a nitrene transfer reaction, leading to MAD (4-((2,6-diisopropylphenyl)imino)pent-2-enide) supported tungsten(V) and tungsten(VI) complexes 16 and 17. All reported complexes have been thoroughly studied by VT-NMR and (heteronuclear) NMR spectroscopy, as well as UV-vis and EPR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

The double protonation of dihapto-coordinated benzene complexes enables dearomatization using aromatic nucleophiles

Friedel-Crafts Arylation (the Scholl reaction) is the coupling of two aromatic rings with the aid of a strong Lewis or Brønsted acid. This historically significant C–C bond forming reaction normally leads to aromatic products, often as oligomeric mixtures, dictated by the large stabilization gained upon their rearomatization. The coordination of benzene by a tungsten complex disrupts the natural course of this reaction sequence, allowing for Friedel-Crafts Arylation without rearomatization or oligomerization. Subsequent addition of a nucleophile to the coupled intermediate leads to functionalized cyclohexenes. In this work, we show that by coordinating benzene to tungsten through two carbons (dihapto-coordinate), a rarely observed double protonation of the bound benzene is enabled, allowing its subsequent coupling to a second arene without the need of a precious metal or Lewis acid catalyst.

Synthesis and Characterization of Tungsten Amide Complexes for the Deposition of Tungsten Disulfide Thin Films.

To substitute corrosive halogen ligands, we designed and synthesized novel tungsten complexes containing amido ligands, W(DMEDA)3 (1) and W(DEEDA)3 (2) (DMEDA = N,N'-dimethylethylenediamido; DEEDA = N,N'-diethylethylenediamido). Complexes 1 and 2 were characterized by 1H NMR, 13C NMR, FT-IR, and elemental analysis. The pseudo-octahedral molecular structure of 1 was confirmed by single-crystal X-ray crystallography. The thermal properties of 1 and 2 were analyzed by thermogravimetric analysis (TGA), which confirmed that the precursors were volatile and exhibited adequate thermal stability. Additionally, the WS2 deposition test was performed using 1 in thermal chemical vapor deposition (thermal CVD). Further analysis of the surface of the thin films was conducted using Raman spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS).

The influence and mechanism of ligand steric hindrance and electronic effects on the ring-opening metathesis polymerization of polycyclopentene under tungsten complex

Polycyclopentene rubber has attracted attention due to its unique ring-opening metathesis polymerization (ROMP) mechanism, which endows it with a range of excellent properties, including recyclability and structural controllability. In order to facilitate the production of polycyclopentene, a tungsten olefin metathesis catalytic system was developed. The influence of ligand steric hindrance and electronic effects on the reaction was investigated through kinetic experiments and density functional theory (DFT) calculations. The results show that increased ligand steric hindrance enhances reaction stability, while the electro-withdrawing group on the ligand accelerates catalyst activation. Additionally, a ligand “release-return” mechanism for polymerization initiation was proposed. The polycyclopentene's thermal properties and stereoregularity were explored.

Pyridine‐Stabilized Cationic Silanethione Tungsten Complexes: Synthesis, Structures, and Reactivity

AbstractSilanethione compounds, R2Si=S, have been recognized as highly reactive species. One reliable way to stabilize silanethione is its coordination to transition metal fragments to convert silanethione‐coordinated transition metal complexes. Herein, we report the synthesis, structure, and reactivity of a second cationic silanethione tungsten complex [Cp*(OC)3W{S=SiR2(py)}]TFPB (R=Me (5 a), Ph (5 b), Cp*: η5‐C5Me5, py: pyridine, and TFPB−: [B{3,5‐(CF3)2C6H3}4]−). Complex 5 was obtained by H− abstraction from the Si atom in the corresponding silylsulfanyl complex Cp*(OC)3W(SSiR2H) (4) with Ph3CTFPB, followed by the addition of pyridine. The reaction of 5 with PhNCS and PMe3 produced [Cp*(OC)3W{SSiR2N(Ph)C(PMe3)2}]TFPB (R=Me (6 a), Ph (6 b)) via the elimination of pyridine and the addition of the 1,3‐dipolar species PhNC(PMe3)2 (A) to the Si atom.

Synthesis, Structure, and Reactivity of a Gallylene Derivative Bearing a Phenalenyl-Based Ligand.

Phenalenyl-based N,N-bidentate ligand-stabilized monovalent gallium(I) complex 1 was synthesized and characterized by NMR spectroscopies, single-crystal X-ray diffraction, and theoretical calculations. In solution, complex 1 has a high thermal stability at 80 °C, with an absorption maximum at 505 nm. Complex 1 promotes the oxidative addition of I-I, Si-Cl, C-I, and S-S bonds and oxidative cyclization with various π components. Complex 1 can also coordinate to a tungsten complex to form a Ga-W bond.

Dioxygen Activation by a Bioinspired Tungsten(IV) Complex.

An increasing number of discovered tungstoenzymes raises interest in the biomimetic chemistry of tungsten complexes in oxidation states +IV, +V, and +VI. Bioinspired (sulfur-rich) tungsten(VI) dioxido complexes are relatively prevalent in literature. Still, their energetically demanding reduction directly correlates with a small number of known tungsten(IV) oxido complexes, whose chemistry is not well explored. In this paper, a reduction of the [WO2(6-MePyS)2] (6-MePyS = 6-methylpyridine-2-thiolate) complex with PMe3 to a phosphine-stabilized tungsten(IV) oxido complex [WO(6-MePyS)2(PMe3)2] is described. This tungsten(IV) complex partially releases one PMe3 ligand in solution, creating a vacant coordination site capable of activating dioxygen to form [WO2(6-MePyS)2] and OPMe3. Therefore, [WO2(6-MePyS)2] can be used as a catalyst for the aerobic oxidation of PMe3, rendering this complex a rare example of a tungsten system utilizing dioxygen in homogeneous catalysis. Additionally, the investigation of the reactivity of the tungsten(IV) oxido complex with acetylene, substrate of a tungstoenzyme acetylene hydratase (AH), revealed the formation of the tungsten(IV) acetylene adduct. Although this adduct was previously reported as an oxidation product of the tungsten(II) acetylene carbonyl complex, here it is obtained via substitution at the sulfur-rich tungsten(IV) center, mimicking the initial step of the first shell mechanism for AH as suggested by computational studies.

Improved Synthesis of (TBA)2[W6Br14] Paving the Way to Further Study of Bromide Cluster Complexes.

Octahedral cluster complexes of molybdenum and tungsten, [M6X8Y6]n- (M = Mo, W; X, Y = Cl, Br, I), are promising active components in various fields, including biomedicine and solar energy. Cluster complexes draw considerable attention due to their X-ray opacity, red/near-IR luminescence, and ability to convert triplet molecular oxygen to active singlet oxygen under UV and visible irradiation. Among the octahedral cluster complexes of molybdenum and tungsten, compounds with a {W6Br8}4+ core are the least studied. There are only a few examples of compounds with substituted terminal ligands, and their properties are not well understood. Among other things, this is due to more labor-intensive and expensive methods for obtaining the starting compounds in comparison with molybdenum counterparts. In this paper, we describe the synthesis of an octahedral cluster complex, (TBA)2[W6Br14] (TBA+ = tetrabutylammonium), in gram quantities, starting from simple substances─W, Br2, and Bi─in 70% yield. The formation of pentanuclear tungsten cluster complexes was recorded as a byproduct. Compounds with substituted terminal ligands (TBA)2[W6Br8Y6] (Y = NO3, Cl, I) were obtained. We also discuss the instability of (TBA)2[W6Br8(NO3)6] under light exposure, the optical properties of a series of compounds (TBA)2[W6Br8Y6] (Y = Cl, Br, I), and the effect of terminal ligands on the chemical shifts in 183W NMR spectra in dimethyl sulfoxide-d6. The presented approach to the synthesis of one of the main precursors of various bromide cluster complexes on a gram scale can stimulate the study of their properties and development of new functional materials based on them.

Amide functionalized aminobisphenolato MoO2 and WO2 complexes: Synthesis, characterization, and alkene epoxidation catalysis

The use of dioxidomolybdenum(vi) and -tungsten(vi) complexes supported by a variety of structurally different tri- and tetradentate aminobisphenolato ligands as pre-catalysts in the epoxidation of alkenes is well established. However, under the widely used standard 1 mol-% catalyst loadings these types of complexes generally show modest activity only. Recently, amide functionalities in the ligand design of various aminomonophenolato MoO2 complexes have been shown to lead to heightened catalytic activity in alkene epoxidation. In this paper we show that similar ligand amide functionalization can lead to significant enhancement in the alkene epoxidation activity of aminobisphenolato MoO2 complexes. Although the W variants showed much lower performance in comparison, the epoxidation activity of the Mo congeners is generally ca. two orders of magnitude higher than previously reported for structurally related aminobisphenolato complexes. An interesting phenomenon dubbed as “dilution effect” was discovered, wherein pre-catalyst loadings as low as 0.01 mol-% may be realized without significantly reduced impact in activity. Moreover, the [pre-catalyst]:[oxidant] molar ratio – an often overlooked reaction parameter in the literature – was found to be critical for optimal catalytic performance.

N2 Reduction versus H2 Evolution in a Molybdenum- or Tungsten-Based Small-Molecule Model System of Nitrogenase.

Molybdenum dinitrogen complexes have played a major role as catalytic model systems of nitrogenase. In comparison, analogous tungsten complexes have in most cases found to be catalytically inactive. Herein, a tungsten complex was shown to be supported by a pentadentate tetrapodal (pentaPod) phosphine ligand, under conditions of N2 fixation, primarily catalyzes the hydrogen evolution reaction (HER), in contrast to its Mo analogue, which catalytically mediates the nitrogen-reduction reaction (N2 RR). DFT calculations were employed to evaluate possible mechanisms and identify the most likely pathways of N2 RR and HER activities exhibited by Mo- and W-pentaPod complexes. Two mechanisms for N2 RR by PCET are considered, starting from neutral (M(0) cycle) and cationic (M(I) cycle) dinitrogen complexes (M=Mo, W). The latter was found to be energetically more favorable. For HER three scenarios are treated; that is, through bimolecular reactions of early M-Nx Hy intermediates, pure hydride intermediates or mixed M(H)(Nx Hy ) species.

Stabilization of propene molybdenum and tungsten half-sandwich complexes by intramolecular coordination of a thioether function.

This study reports the stabilizing effect of an intramolecularly coordinated thioether function in propene complexes of the general formula [{η5:κS-C5H4(CH2)2SR}M(CO)2(η2-C2H3Me)][BF4] (M = Mo, W; R = Et, Ph). They are formed by protonation of allyl analogues [{η5-C5H4(CH2)2SR}M(CO)2(η3-C3H5)] by tetrafluoroboric acid in non-coordinating solvents. In contrast to analogues with unsubstituted Cp ligands, these propene complexes are isolable in a pure form and characterized by NMR spectroscopy. The molybdenum compounds are stable at low temperature and the propene ligand can easily be exchanged by thioethers or acetonitrile. Several representatives of the reaction products were characterized by X-ray structure analysis. The stabilization effect in tungsten complexes [{η5:κS-C5H4(CH2)2SR}W(CO)2(η2-C2H3Me)][BF4] (R = Et, Ph) was unusually high. The compounds are long-term stable at room temperature and do not undergo ligand exchange reactions even with strong chelators such as 1,10-phenanthroline. The molecular structure of the tungsten propene complex was confirmed by X-ray diffraction analysis on a single crystal.