Tungsten Analogue Research Articles

Neutral Silylyne Complex of Molybdenum: Synthesis, Properties, and Access to Silaiminoacyl Complexes via [2+3] Cycloaddition.

A neutral silylyne complex of molybdenum was synthesized by the stepwise dehydrogenation method and its properties were compared with those of the tungsten analog. The complex takes a dimeric form as crystals but afford a monomer-dimer equilibrium in solution. The replacement of the central metal from W to Mo led to a monomer dominant (~98 %) solution at room temperature. The monomer-dimer dynamics was investigated based on thermodynamic parameters. The molybdenum silylyne complex underwent [2+2] cycloaddition with alkynes much faster than the tungsten analog. The reactions with organic azides led to the formation of the first example of silaiminoacyl complexes through [2+3] cycloaddition. The structures and bonding aspects of the products were clarified by multiple measurements and DFT calculations.

Reactions of Triangular Molybdenum and Tungsten Clusters with Tetrabromocatechol

The reaction of cluster complex (nBu4N)2[Mo3Se7Br6] with tetrabromocatechol (H2Tbc) in the presence of Et3N and Ph4PBr results in the formation of (Ph4P)2[Mo3Se7(Tbc)3]∙2MeOH (Ia∙2MeOH). Under the same conditions, the tungsten analogue (nBu4N)2[W3S7Br6] is completely or partly destroyed to give the complex (Ph4P)(Et3NH)[WO2(Tbc)2] (II). The structures of Ia·2MeOH and II were established by X-ray diffraction analysis (CCDC nos. 2213900 and 2213901, respectively). The redox properties of complex Ia and related (Ph4P)2[Mo3S7(Tbc)3] (Ib) and (Ph4P)2[Mo3S7(Tcc)3] (Ic) (Tcc is tetrachlorocatecholate) were also studied by cyclic voltammetry.

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.

Canopy Catalysts for Alkyne Metathesis: Investigations into a Bimolecular Decomposition Pathway and the Stability of the Podand Cap.

Molybdenum alkylidyne complexes with a trisilanolate podand ligand framework (“canopy catalysts”) are the arguably most selective catalysts for alkyne metathesis known to date. Among them, complex 1 a endowed with a fence of lateral methyl substituents on the silicon linkers is the most reactive, although fairly high loadings are required in certain applications. It is now shown that this catalyst decomposes readily via a bimolecular pathway that engages the Mo≡CR entities in a stoichiometric triple‐bond metathesis event to furnish RC≡CR and the corresponding dinuclear complex, 8, with a Mo≡Mo core. In addition to the regular analytical techniques, 95Mo NMR was used to confirm this unusual outcome. This rapid degradation mechanism is largely avoided by increasing the size of the peripheral substituents on silicon, without unduly compromising the activity of the resulting complexes. When chemically challenged, however, canopy catalysts can open the apparently somewhat strained tripodal ligand cages; this reorganization leads to the formation of cyclo‐tetrameric arrays composed of four metal alkylidyne units linked together via one silanol arm of the ligand backbone. The analogous tungsten alkylidyne complex 6, endowed with a tripodal tris‐alkoxide (rather than siloxide) ligand framework, is even more susceptible to such a controlled and reversible cyclo‐oligomerization. The structures of the resulting giant macrocyclic ensembles were established by single‐crystal X‐ray diffraction.

Synthesis and Reactivity of a Bioinspired Molybdenum(IV) Acetylene Complex.

The isolation of a molybdenum(IV) acetylene (C2H2) complex containing two bioinspired 6-methylpyridine-2-thiolate ligands is reported. The synthesis can be performed either by oxidation of a molybdenum(II) C2H2 complex or by substitution of a coordinated PMe3 by C2H2 on a molybdenum(IV) center. Both C2H2 complexes were characterized by spectroscopic means as well as by single-crystal X-ray diffraction. Furthermore, the reactivity of the coordinated C2H2 was investigated with regard to acetylene hydratase, one of two enzymes that accept C2H2 as a substrate. While the reaction with water resulted in the vinylation of the pyridine-2-thiolate ligands, an intermolecular nucleophilic attack on the coordinated C2H2 with the soft nucleophile PMe3 was observed to give a cationic ethenyl complex. A comparison with the tungsten analogues revealed less tightly bound C2H2 in the molybdenum variant, which, however, shows a higher reactivity toward nucleophiles.

Many-body simulation of two-dimensional electronic spectroscopy of excitons and trions in monolayer transition metal dichalcogenides

Indications of coherently interacting excitons and trions in doped transition metal dichalcogenides have been measured as quantum beats in two-dimensional electronic spectroscopy, but the microscopic principles underlying the optical signals of exciton-trion coherence remain to be clarified. Here we present calculations of two-dimensional spectra of such monolayers based on a microscopic many-body formalism. We use a parameterized band structure and a static model dielectric function, although a full ab initio implementation of our formalism is possible in principle. Our simulated spectra are in excellent agreement with experiments, including the quantum beats, while revealing the interplay between excitons and trions in molybdenum- and tungsten-based transition metal dichalcogenides. Quantum beats are confirmed to unambiguously reflect the exciton-trion coherence time in molybdenum compounds, but are shown to provide a lower bound to the coherence time for tungsten analogues due to a destructive interference from coexisting singlet and triplet trions.

Silica‐Supported Molybdenum Oxo Alkylidenes: Bridging the Gap between Internal and Terminal Olefin Metathesis

AbstractGrafting a molybdenum oxo alkylidene on silica (partially dehydroxylated at 700 °C) affords the first example of a well‐defined silica‐supported Mo oxo alkylidene, which is an analogue of the putative active sites in heterogeneous Mo‐based metathesis catalysts. In contrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of a stable off‐cycle metallacyclobutane intermediate, the Mo catalyst shows high metathesis activity for both terminal and internal olefins that is consistent with the lower stability of Mo metallacyclobutane intermediates. This Mo oxo metathesis catalyst also outperforms its corresponding neutral silica‐supported Mo and W imido analogues.

Silica-Supported Molybdenum Oxo Alkylidenes: Bridging the Gap between Internal and Terminal Olefin Metathesis.

Grafting a molybdenum oxo alkylidene on silica (partially dehydroxylated at 700 °C) affords the first example of a well-defined silica-supported Mo oxo alkylidene, which is an analogue of the putative active sites in heterogeneous Mo-based metathesis catalysts. In contrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of a stable off-cycle metallacyclobutane intermediate, the Mo catalyst shows high metathesis activity for both terminal and internal olefins that is consistent with the lower stability of Mo metallacyclobutane intermediates. This Mo oxo metathesis catalyst also outperforms its corresponding neutral silica-supported Mo and W imido analogues.

Use of Single-Metal Fragments for Cluster Building: Synthesis, Structure, and Bonding of Heterometallaboranes.

The synergic property of the CO ligand, in general, can stabilize metal complexes at lower oxidation states. Utilizing this feature of the CO ligand, we have recently isolated and structurally characterized a highly fluxional molybdenum complex [{Cp*Mo(CO)2}2{μ-η2:η2-B2H4}] (2; Cp* = η5-C5Me5) comprising the diborane(4) ligand. Compound 2 represents a rare class of bimetallic diborane(4) complex corresponding to a singly bridged C s structure. In an attempt to isolate the tungsten analogue of 2, [{Cp*W(CO)2}2{μ-η2:η2-B2H4}], we have isolated a rare vertex-fused cluster, [(Cp*W)3WB9H18] (5). Having a structural likeness with the dimolybdenum alkyne complex [{CpMo(CO)2}2C2H2], we have further explored the chemistry of 2 with CO gas that yielded a homoleptic trimolybdenum complex, [(Cp*Mo)3(μ-H)2(μ3-H)(μ-CO)2B4H4] (4). In an attempt to replace the 16-electron {Cp*MoH(CO)2} moiety in 4 with isolobal fragment {W(CO)5}, we treated the intermediate, obtained from the reaction of Cp*MoCl4 and LiBH4, with the monometal carbonyl fragment {W(CO)5·THF}. The reaction indeed yielded two bimetallic clusters, [(Cp*Mo)2B4H8W(CO)4] (7) and [(Cp*Mo)2B4H6W(CO)5] (8), that seem to have been generated by the replacement of one {BH} or {BH3} vertex from [(Cp*Mo)2B5H9], respectively. All of the compounds have been characterized by various spectroscopic analyses and single-crystal X-ray diffraction studies. Electron-counting rules and molecular orbital analyses provided further insight into the electronic structure of all of these molecules.

Synthesis, Structure, Bonding, and Reactivity of Metal Complexes Comprising Diborane(4) and Diborene(2): [{Cp*Mo(CO)2}2{μ‐η2:η2‐B2H4}] and [{Cp*M(CO)2}2B2H2M(CO)4], M=Mo,W

The reaction of [(Cp*Mo)2 (μ-Cl)2 B2 H6 ] (1) with CO at room temperature led to the formation of the highly fluxional species [{Cp*Mo(CO)2 }2 {μ-η2 :η2 -B2 H4 }] (2). Compound 2, to the best of our knowledge, is the first example of a bimetallic diborane(4) conforming to a singly bridged Cs structure. Theoretical studies show that 2 mimics the Cotton dimolybdenum-alkyne complex [{CpMo(CO)2 }2 C2 H2 ]. In an attempt to replace two hydrogen atoms of diborane(4) in 2 with a 2e [W(CO)4 ] fragment, [{Cp*Mo(CO)2 }2 B2 H2 W(CO)4 ] (3) was isolated upon treatment with [W(CO)5 ⋅thf]. Compound 3 shows the intriguing presence of [B2 H2 ] with a short B-B length of 1.624(4) Å. We isolated the tungsten analogues of 3, [{Cp*W(CO)2 }2 B2 H2 W(CO)4 ] (4) and [{Cp*W(CO)2 }2 B2 H2 Mo(CO)4 ] (5), which provided direct proof of the existence of the tungsten analogue of 2.

Synthesis, Structure, Bonding, and Reactivity of Metal Complexes Comprising Diborane(4) and Diborene(2): [{Cp*Mo(CO)2}2{μ‐η2:η2‐B2H4}] and [{Cp*M(CO)2}2B2H2M(CO)4], M=Mo,W

AbstractThe reaction of [(Cp*Mo)2(μ‐Cl)2B2H6] (1) with CO at room temperature led to the formation of the highly fluxional species [{Cp*Mo(CO)2}2{μ‐η2:η2‐B2H4}] (2). Compound 2, to the best of our knowledge, is the first example of a bimetallic diborane(4) conforming to a singly bridged Cs structure. Theoretical studies show that 2 mimics the Cotton dimolybdenum–alkyne complex [{CpMo(CO)2}2C2H2]. In an attempt to replace two hydrogen atoms of diborane(4) in 2 with a 2e [W(CO)4] fragment, [{Cp*Mo(CO)2}2 B2H2W(CO)4] (3) was isolated upon treatment with [W(CO)5⋅thf]. Compound 3 shows the intriguing presence of [B2H2] with a short B−B length of 1.624(4) Å. We isolated the tungsten analogues of 3, [{Cp*W(CO)2}2B2H2W(CO)4] (4) and [{Cp*W(CO)2}2B2H2Mo(CO)4] (5), which provided direct proof of the existence of the tungsten analogue of 2.

Regio- and Stereoselective Ring-Opening Metathesis Polymerization of Enantiomerically Pure Vince Lactam

The ring-opening metathesis polymerization (ROMP) of (+)-Vince lactam [(S)-azabicyclo[2.2.1]hept-5-en-3-one] (1) and its N-benzyl, N-trimethylsilyl (TMS), and N-tert-butoxycarbonyl (Boc) derivatives (2a–c) is reported. Highly cis-syndiotactic (st) poly(Vince lactam) was readily accessible by using the cyclometalated ruthenium complex Ru[CH(2-OiPr-Ph)](Piv)(1-mesityl-3-C4H8-imidazol-2-ylidene) (Piv =2,2-dimethylpropanoate) (4); however, small amounts of trans double bonds (ca. 5%) formed. Highly cis-st (>98%) polymers were accessible by the action of the monoaryloxide pyrrolide (MAP) type complexes W(N-2,6-iPr2C6H3)(CHCMe2Ph)(Pyr)(HMTO) (Pyr = pyrrolide, HMTO = 2,6-(2,4,6-Me3C6H2)2C6H3O) (7) and W(O)(CHCMe2Ph)(PMe2Ph)(Me2Pyr)(TPPO) (TPPO = 2,3,5,6-tetraphenylphenolate) (8). Complementary, cis-isotactic (>98% cis-it) polymers were prepared by the action of Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OBiphen) (OBiphen = 3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate) (5) and its tungsten analogue W(N-2,6-Me2C6H3)(CHCMe2Ph)(OBiphen) (6). Notably, none of these Mo- and W-based initiators polymerize unprotected Vince lactam. Deprotection of poly(N-TMS Vince lactam) and poly(N-Boc Vince lactam) with neat trifluoroacetic acid allowed for the isolation of all-cis highly tactic poly(Vince lactam).

Reactions of Silanone(silyl)tungsten and -molybdenum Complexes with MesCNO, (Me2SiO)3, MeOH, and H2O: Experimental and Theoretical Studies

Reactions of silanone(silyl)tungsten and -molybdenum complexes Cp*(OC)2M{O═SiMes2(DMAP)}(SiMe3) (M = W (1a), Mo (1b), Cp* = η5-C5Me5, Mes = 2,4,6-Me3C6H2, DMAP = 4-(Me2N)C5H4N) with MesCNO, (Me2SiO)3, MeOH, and H2O were investigated. No silanone trapping product was detected in the reaction of 1 with MesCNO and (Me2SiO)3, while reactions of 1 with MeOH afforded the addition product Mes2Si(OMe)OH (5). The hydrolysis of molybdenum complex 1b afforded silanediol Mes2Si(OH)2 (8) as a main product with a trace amount of Mes2Si(OH)OSiMe3 (9), while that of the tungsten analogue 1a gave siloxysilanol 9 as the sole product, indicating that product distribution in the hydrolysis strikingly depends on the metal fragments. A theoretical investigation revealed that the product distribution difference between tungsten and molybdenum complexes arises from the difference in electrophilicity of the silicon atom in the silanone ligand and that of the lability for oxidative addition of water to the MII–L fragment.

Comparing spectroscopic and electrochemical properties of complexes of type Cp’M(η3-C3H5)(CO)2 (Cp’ = Cp, Ind, Flu): A complementary experimental and DFT study

A series of allyl complexes of the general formula [Cp'Mo(η3-C3H5)(CO)2], where Cp' = Cp, CpMe, Cp*, Ind, IndMe, IndMe2, Flu, and three tungsten analogues, has been prepared and characterized by 1H, 13C, and 95Mo NMR, cyclic voltammetry, and the structure of [IndMo(η3-C3H5)(CO)2] was determined by X-ray single crystal analysis. Two conformers, corresponding to the two extreme orientations of the allyl ligand (endo and exo), have been identified in solution by 1H and 95Mo NMR for all the complexes, except for [FluMo(η3-C3H5)(CO)2], which only presents an exo conformation in solution.A 95Mo NMR investigation also shows the influence of electron donor capability of the Cp' ligands on the 95Mo chemical shifts of the Mo center. He I and He II photoelectron spectra probe the electron richness of the metal center and the electronic structure of the complexes. Cyclic voltammetry studies show one oxidation process, which is only reversible for the Cp derivatives. DFT calculations provided a reliable way to determine ionization energies, and reflected very well the trends of the other properties, from the 95Mo NMR chemical shifts, to vibration frequencies, and oxidation potentials.

Direct Observation of Metal Ketenes Formed by Photoexcitation of a Fischer Carbene using Ultrafast Infrared Spectroscopy

Fischer carbenes are commonly used as reagents in the synthesis of new carbon–carbon bonds, a reaction made possible by the unique chemistry of the formal metal–carbon double bond. Nevertheless, the photoinduced reactions of these complexes are relatively poorly understood. For instance, it has been postulated but not confirmed that visible irradiation leads to photocarbonylation, in which a CO ligand inserts into the metal–carbon bond to form a metal ketene intermediate. Here, we report the first direct observation of this intermediate following 400 nm photoexcitation of the model group 6 Fischer carbene Cr(CO)5[CCH3(OCH3)]. Using ultrafast time-resolved infrared spectroscopy (TRIR), we observe the formation of three distinct metal ketene structures, which we assign as a singlet and two isoenergetic triplet excited-state structures. The singlet relaxes to the ground state on a time scale of ∼35 ps, whereas the two triplets are long-lived (>2 ns). TRIR of the tungsten analogue yields no evidence for a metal ketene structure, consistent with the limited reactivity of this complex. The results directly elucidate the fundamental role of triplet metal ketenes in the photoreactivity of Fischer carbene complexes.

Mechanism of Carbon Monoxide Induced N–N Bond Cleavage of Nitrous Oxide Mediated by Molybdenum Complexes: A DFT Study

The detailed mechanism of CO-induced N–N bond cleavage of N2O mediated by molybdenum complexes leading to the nitrosyl isocyanate complex has been investigated via density functional theory (DFT) calculations at the B3LYP level. On the basis of the calculations, we proposed a new reaction mechanism of CO-induced N–N bond cleavage of N2O with an overall free energy barrier of 23.6 kcal/mol, significantly lower than that of the reaction mechanism (42.2 kcal/mol) proposed by Sita et al. The calculations also indicated that CO-induced N–N bond cleavage of N2O is competitive with oxygen atom transfer (OAT) to carbon monoxide due to the comparable free energy barriers. The metal-bound carbonyl complex obtained from OAT can be recycled to give more nitrosyl isocyanate complexes. In addition, we demonstrated why the analogous tungsten complex cannot give the nitrosyl isocyanate complex via CO-induced N–N bond cleavage of N2O. The calculations are consistent with experimental observations.

Two-Dimensional Nanosheets and Layered Hybrids of MoS2 and WS2 through Exfoliation of Ammoniated MS2 (M = Mo,W)

Ammoniated MS2 (M = Mo,W) have been synthesized by reacting LixMS2 with a saturated solution of ammonium chloride. While, largely neutral NH3 is present in the interlayer of ammoniated MoS2, equal amounts of NH3 and NH4+ ions are present in the tungsten analog. The ammoniated MS2 exfoliate readily in a variety of polar solvents with exfoliation being best in water. Ammoniated WS2 forms a more stable colloidal dispersion compared to the Mo analog because the ammonium ions do not deintercalate easily from the layers even on exfoliation. The dispersions are comprised of large nanosheets of MS2 with lateral dimensions in the order of micrometers. The layers could be restacked from the colloidal dispersions by evaporating the solvent. While the colloidal dispersions of ammoniated MoS2 yield NH3-free MoS2 on restacking, the dispersions of ammoniated WS2 yield WS2 intercalated with NH4+ ions. Co-stacking of MoS2 and WS2 nanosheets from a mixture of both the colloidal dispersions results in MoS2–WS2 hybrids in which the MoS2 and WS2 nanosheets are randomly stacked. Photoluminescence measurements of MS2 nanosheets and MoS2–WS2 hybrids indicate phase stability and existence of direct bandgap.

Synthesis, Structure, Gas‐Phase Reactivity, and Catalytic Relevance of Trinuclear Mo3S4 Clusters Bearing Terminal Hydroxo and Hydrosulfido Groups

AbstractMolybdenum(IV) hydroxo [Mo3S4(dmpe)3(OH)3]+ (1+) and hydrosulfido [Mo3S4(dmpe)3(SH)3]+ (2+) [dmpe = 1,2‐bis(dimethylphosphanyl)ethane] trimetallic cuboidal cluster complexes have been isolated in high yields by treating their chloride precursors with sodium hydroxide or sodium hydrosulfide, respectively. The crystal structures of [1]BPh4 and [2]PF6 confirm that –QH (Q = O, S) groups are coordinated to metal centers. Both hydroxo and hydrosulfido Mo3S4 cluster complexes are fully characterized by spectroscopic, mass spectrometric, and X‐ray techniques. A comparative study of the gas‐phase dissociation of 1+ and 2+ cations using ESI tandem mass spectrometry is presented, and the results are compared with those already reported for the hydroxo tungsten analogue. The gas‐phase reactivity of 1+ and 2+ species towards ethanol and 1‐penthanethiol have been explored. The gas‐phase‐generated [Mo3S4(dmpe)2(O)(OH)]+ cation activates ethanol molecules through a similar mechanism to that proposed for its tungsten congener. The main differences in aldehyde elimination under collision‐induced dissociation (CID) conditions between molybdenum and tungsten cluster sulfides are discussed.

Mono‐oxo‐bis‐dithioveratrol‐molybdate – in Solution a Model for Arsenite Oxidase and in the Solid State a Coordination Polymer with Unprecedented Binding Motifs

AbstractMono‐oxo‐bis‐dithioveratrol‐molybdate was synthesized, structurally characterized, and investigated with respect to its oxo‐transfer activity. The latter was compared with the activity of the analogous tungsten complex. The title complex is a structural model for molybdopterin bearing arsenite oxidase and both complexes catalyze oxo‐transfer reactions successfully to 100 % conversion. With the dithioveratrol ligand the oxidation of triphenylphosphine proved to be faster for the tungsten complex, which is untypical. The solid‐state structure of the molybdenum complex exhibits an unexpected and very unusual polymeric structural motif consisting of the complex anion mono‐oxo‐bis‐dithioveratrol‐molybdate, sodium cations, and methanol. Infinite double‐decker strands are formed with sodium bridges between the ether functions of two ligands in one strand and the Mo=O moiety of the second strand.

Accessing yttrium–polyoxometalate–carboxylate hybrids from a versatile arsenotungstate(III) precursor

Exploration of the reactivity of the versatile polyoxometalate precursor [As2W19O67(H2O)]14− with yttrium salts and organic proligands derived from 2-picolinic acid: 2,5-pyridinedicarboxylic acid (pdcH2) and 5-(methoxycarbonyl)-2-pyridinecarboxylic acid (mcpcH), has afforded the three new hybrid complexes [Y{AsW8O30}2(AsO)2{WO2(pdc)}2]13− (1) and [{Y(H2O)3}2{As2W19O68}{WO2(L)}2]8− (L−=pdcH− (2) or mcpc− (3)). The structurally-related and organic ligand free species [{Y(H2O)3}4{Y(H2O)6}2{As2W19O68}2{WO3(H2O)}4(AsO)2]20− (4) was also obtained. Structural characterization of the mixed salts of the four complexes, K8H5[1]·9H2O, K2.5H5.5[2]·37H2O, K2.5H5.5[3]·34H2O and K6Na2H4[4]·30H2O, has revealed that five membered N,O-chelated {WO2(L)} units play a key structural role in complexes 1–3. With no picolinate-based ligand present in 4, additional arsenite fragments bind to the analogous tungsten coordination sites.