Tungsten Alkylidyne Complexes Research Articles

A case of alkylidyne-imine metathesis.

Reactions between imines and tungsten alkylidyne complexes are studied. The trianionic pincer ligand supported alkylidyne [tBuOCO]WCC(CH3)3(THF)2 (1) reacts with N-(R)-1-phenylmethanimine (PMI-R, R = Me, Ph, Bn, and TMS) yielding [tBuOC(H)O]W(η2-tBuCCPh)N(R) (4-R), products from metathesis reaction. In contrast, the non-pincer alkylidyne (tBuO)3WCC(CH3)3 does not react with PMI-R imines.

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.

Cationic Tungsten Alkylidyne N‐Heterocyclic Carbene Complexes: Synthesis and Reactivity in Alkyne Metathesis

The first cationic and neutral tungsten alkylidyne N‐heterocyclic carbene (NHC) complexes bearing one triflate ligand were synthesized and tested for their reactivity in alkyne metathesis. Both types of tungsten alkylidyne complexes display a higher productivity in alkyne metathesis than the analogous neutral tungsten alkylidyne NHC trisalkoxide complexes. Reaction of W(≡CC6H4OMe)(1,3‐bis(1‐hydroxy‐1,1‐trifluoromethylethyl)‐imidazol‐2‐ylidene)Cl (W18) with AgB(ArF)4 (ArF = 3,5‐bis(trifluoromethyl)phenyl) resulted in the unexpected formation of, to the best of our knowledge, the first cationic ditungstatetrahedrane W2(1,3‐bis(1‐hydroxy‐1,1‐trifluoromethyl‐ethyl)‐imidazol‐2‐ylidene)2(MeCN)(µ‐((Ar)CC(Ar)))+ (B(ArF)4)– (W19, Ar = C6H4OMe), which suggests bimolecular decomposition as a possible decomposition pathway of cationic tungsten alkylidyne NHC complexes. Reaction of the cationic tungsten alkylidyne NHC complex W(≡CC6H4OMe)(1,3‐diisopropylimidazol‐2‐ylidene)(OC(CF3)2Me)2(NCtBu)+ (B(ArF)4)– (W7) with 1‐phenyl‐1‐propyne allowed for the isolation of a cationic tungstacyclobutadiene W(C3(Ph)(Me)(C6H4OMe))(1,3‐diisopropylimidazol‐2‐ylidene)(OC(CF3)2Me)2(NCtBu)+ (B(ArF)4)– (W20). Its formation strongly supports a cationic active species in the alkyne metathesis with tungsten alkylidyne NHC complexes.

Unraveling the Mechanism of 1,3‐Diyne Cross‐Metathesis Catalyzed by Silanolate‐Supported Tungsten Alkylidyne Complexes

The benzylidyne complex [PhC≡W{OSi(OtBu)3 }3 ] (1) catalyzed the cross-metathesis between 1,4-bis(trimethylsilyl)-1,3-butadiyne (2) and symmetrical 1,3-diynes (3) efficiently, which gave access to TMS-capped 1,3-diynes RC≡C-C≡CSiMe3 (4). Diyne cross-metathesis (DYCM) studies with 13 C-labeled diyne PhC≡13 C-13 C≡CPh (3*) revealed that this reaction proceeds through reversible carbon-carbon triple-bond cleavage and formation according to the conventional mechanism of alkyne metathesis. The reaction between 1 and 3* afforded the 3-phenylpropynylidyne complex PhC≡13 C-13 C≡W{OSi(OtBu)3 }3 ] (5*), indicating that alkynylalkylidyne complexes are likely to act as catalytically active species. Attempts to isolate 5* from mixtures of 1 and 3* afforded crystals of the ditungsten 2-butyne-1,4-diylidyne complex [(tBuO)3 SiO}3 W≡13 C-13 C≡13 C-13 C≡W{OSi(OtBu)3 }3 ] (6*), which was additionally characterized by X-ray diffraction analysis. Depolymerization-macrocyclization of a carbazole-butadiyne polymer, obtained from 3,6-diethynyl-9-dodecylcarbazole (7) under copper-catalyzed Hay coupling conditions, was also efficiently catalyzed by 1 and afforded a mixture of mono-, diyne- and triyne-containing tetrameric macrocycles, revealing that diyne disproportionation into monoynes and triynes occurs as a slow side reaction that interferes with a high diyne metathesis selectivity. Potential catalytic pathways were studied by means of quantum-chemical calculations, and kinetic studies were performed to substantiate an α,α-mechanism for the catalytic diyne metathesis reaction, which involves intermediate alkynylalkylidyne and α,α'-dialkynylmetallacyclobutadiene intermediates.

Unraveling the Mechanism of 1,3‐Diyne Cross‐Metathesis Catalyzed by Silanolate‐Supported Tungsten Alkylidyne Complexes

AbstractInvited for the cover of this issue is the group of Matthias Tamm at the Technische Universität Braunschweig. The image depicts an ambitious karateka opting to break the carbon–carbon triple bond rather than the single bond. Read the full text of the article at 10.1002/chem.201801651.

Thermodynamic and Structural Factors That Influence the Redox Potentials of Tungsten–Alkylidyne Complexes

The thermodynamic and structural factors that influence the redox properties of an extensive set of tungsten–alkylidyne complexes (W(CR)L4X) are analyzed by combining synthesis, electrochemistry, and computational modeling based on free energy calculations of oxidation potentials at the density functional theory level. The observed linear correlations among oxidation potentials, HOMO energies, and gas-phase ionization energies are found to be consistent with the approximately constant solvation free energy differences between reduced and oxidized species over the complete set. The W–X bond length, trans to the alkylidyne ligand, is found to be a good descriptor of the positioning of the key frontier orbitals that regulate the redox properties of the complexes.

Density Functional Theory Study of the Reaction between d0 Tungsten Alkylidyne Complexes and H2O: Addition versus Hydrolysis.

The reactions of early-transition-metal complexes with H2O have been investigated. An understanding of these elementary steps promotes the design of precursors for the preparation of metal oxide materials or supported heterogeneous catalysts. Density functional theory (DFT) calculations have been conducted to investigate two elementary steps of the reactions between tungsten alkylidyne complexes and H2O, i.e., the addition of H2O to the W≡C bond and ligand hydrolysis. Four tungsten alkylidyne complexes, W(≡CSiMe3)(CH2SiMe3)3 (A-1), W(≡CSiMe3)(CH2tBu)3 (B-1), W(≡CtBu)(CH2tBu)3 (C-1), and W(≡CtBu)(OtBu)3 (D-1), have been compared. The DFT studies provide an energy profile of the two competing pathways. An additional H2O molecule can serve as a proton shuttle, accelerating the H2O addition reaction. The effect of atoms at the α and β positions has also been examined. Because the lone-pair electrons of an O atom at the α position can interact with the orbital of the proton, the barrier of the ligand-hydrolysis reaction for D-1 is dramatically reduced. Both the electronic and steric effects of the silyl group at the β position lower the barriers of both the H2O addition and ligand-hydrolysis reactions. These new mechanistic findings may lead to the further development of metal complex precursors.

Reversible Transformation between Alkylidene, Alkylidyne, and Vinylidene Ligands in High-Valent Bis(phenolate) Tungsten Complexes

A tungsten alkylidyne complex [{(OC6H2tBu2-4,6)2(SCH2CH2S)}W(≡CEt)X] (X = OtBu, 1a) with a bridged bis(phenolate) ligand was prepared from the alkoxy precursor (tBuO)3W≡CEt and the corresponding bis(phenol). The tert-butoxy ligand in 1a was substituted against chloride or trimethylsilylmethyl to give 1b (X = Cl) or 1c (X = CH2SiMe3). X-ray diffraction studies of 1a and 1c showed an octahedral cis α-configuration of the metal atom. Hydride abstraction from the β-position of the n-propylidyne ligand in 1a and 1c with [Ph3C][B(C6F5)4] gave high-valent cationic vinylidene complexes [{(OC6H2tBu2-4,6)2(SCH2CH2S)}W(═C═CHMe)X][B(C6F5)4] (2a: X = OtBu; 2c: X = CH2SiMe3), while protonation of 1a with [PhNMe2H][B(C6F5)4] gave the cationic n-propylidene complex [{(OC6H2tBu2-4,6)2(SCH2CH2S)}W(═CHEt)(OtBu)][B(C6F5)4] (3a). The alkylidyne complex 1a was regenerated from the alkylidene 3a by deprotonation with Ph3P═CH2, whereas 1c was regenerated from 2c by nucleophilic attack by a hydride at the β-carbon of the vinylide...

ChemInform Abstract: Alkyne Metathesis by Molybdenum and Tungsten Alkylidyne Complexes

AbstractReview: 33 refs.

Tungsten alkylidyne complexes with ancillary imidazolin-2-iminato and imidazolidin-2-iminato ligands and their use in catalytic alkyne metathesis

The imines 1,3-di-tert-butylimidazolidin-2-imine (10) and 4,5-dimethyl-1,3-di-tert-butylimidazolin-2-imine (14) were prepared starting from the corresponding N-heterocyclic carbenes 8 and 11 and trimethylsilyl azide or tert-butyl azide, respectively. Deprotonation of the imines 10 and 14 with methyl lithium and reaction with an equimolar amount of the tungsten benzylidyne complex [PhCW{OCMe(CF3)2}3(DME)] (16) (DME = 1,2-dimethoxyethane) afforded the complexes [PhCW(L){OCMe(CF3)2}2] (L = 1,3-di-tert-butylimidazolidin-2-iminato, 6; L = 4,5-dimethyl-1,3-di-tert-butylimidazolin-2-iminato, 7), which were characterized by 1H, 13C{1H} and 19F{1H} NMR spectroscopy and by elemental and X-ray diffraction analysis. Complexes 6 and 7 are efficient catalysts for the homocoupling of 3-pentynyl benzyl ether (17) at room temperature in the presence of 5 Å molecular sieve and at low catalyst loadings (1 mol%).

FRET Sensitization of Tungsten–Alkylidyne Complexes by Zinc Porphyrins in Self-Assembled Dyads

The photophysical properties of self-assembled zinc-porphyrin/tungsten-alkylidyne dyads have been investigated with the aim of determining whether the porphyrin S excited state sensitizes the tungsten-alkylidyne (3)[dπ*] state. The luminescent metalloligand W(≡CC(6)H(4)CCpy)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) has been synthesized and shown by electronic and NMR spectroscopy to coordinate axially to ZnTPP and ZnTP(Cl)P (TP(Cl)P = tetra(p-chlorophenyl)porphyrin) via the terminal pyridyl group. Coordination of 1 to ZnPor results in partial quenching of porphyrin S(1) fluorescence and a decrease in the (3)[dπ*] excited-state lifetime of 1. Transient-absorption spectroscopy shows that fluorescence quenching occurs via intramolecular Förster resonance energy transfer from the porphyrin S(1) state to the (1)[dπ*] excited state of 1, which then undergoes rapid singlet-triplet intersystem crossing to produce the (3)[dπ*] excited state. Sensitization of the (3)[dπ*] state occurs with high overall efficiency (φ(EnT) ≈ 80%), thus strongly enhancing light harvesting for the tungsten-alkylidyne compound. The mechanism and rates of the net S(1)→(3)[dπ*] energy transfer are found to differ significantly from those for previously reported zinc-porphyrin/tungsten-alkylidyne dyads that are constructed from similar components but connected instead with covalent bonds at the porphyrin edge. Density functional theory calculations indicate that these differences are due in part to the degree of orbital mixing between the porphyrin and metal-alkylidyne subunits.

Alkyne metathesis by molybdenum and tungsten alkylidyne complexes

Alkyne metathesis by molybdenum and tungsten alkylidyne complexes is now ~45 years old. Progress in the practical aspects of alkyne metathesis reactions with well-defined complexes, as well as applications, in the last decade, guarantees that it is destined to become a useful method for the synthesis of organic molecules.

Dihydrogen activation by a tungsten–alkylidyne complex: toward photoredox chromophores that deliver renewable reducing equivalents

The d(1) tungsten-alkylidyne radical [W(CPh)(dppe)2Cl](+) reacts with H2 to give the d(0) hydride [W(CPh)(H)(dppe)2Cl](+), which on deprotonation yields the d(2) photoredox chromophore W(CPh)(dppe)2Cl. This family of reactions results in a cycle by which renewable H2 provides the reducing equivalents for photochemical reductions.

Reactions of d0 tungsten alkylidyne complexes with O2 or H2O. Formation of an oxo siloxy complex through unusual silyl migrations

(Me3SiCH2)3(Me3SiC≡)W←O=PMe3 (1), an adduct between (Me3SiCH2)3W≡CSiMe3 (2) and O=PMe3, reacts with O2 to give O=W(OSiMe3)(CH2SiMe3)3 (3) and CO2. Reaction of 2 with H2O yields 3 and the trimer [(μ-O)W(CH2SiMe3)2(=O)(THF)]3 (4). In the reaction of D2O with 2, 3-d(n) and methane isotopologues CH2D2, CHD3 and CD4 have been observed.

Unusually stable tungstenacyclobutadienes featuring an ONO trianionic pincer-type ligand

This report presents the synthesis of the first neutral trianionic ONO pincer-type tungsten alkylidyne complex, [CF(3)-ONO]W≡C((t)Bu)(OEt(2)) (5) {where CF(3)-ONO = (MeC(6)H(3)[C(CF(3))(2)O])(2)N(3-)}. Treating 5 with 1-phenylpropyne, 4,4-dimethyl-2-pentyne, and cyclooctyne yields the corresponding tungstenacyclobutadiene complexes [CF(3)-ONO]W[κ(2)-C((t)Bu)C(Me)C(Ph)] (6), [CF(3)-ONO]W[κ(2)-C((t)Bu)C(Me)C((t)Bu)] (7), and [CF(3)-ONO]W[κ(2)-C((t)Bu)C(CH(2))(6)C] (8). Complexes 6, 7, and 8 do not undergo retro-[2 + 2]-cycloaddition even at 200 °C or in the presence of PMe(3). DFT methods to elucidate the electronic structure of complexes 5 and 6 reveal important electronic factors that contribute to the lack of reactivity for the tungstenacyclobutadienes. An important bonding combination between the pincer N-atom lone pair and the W[triple bond, length as m-dash]C bond within 5, termed an inorganic enamine, provides an explanation for the lack of retro-[2 + 2]-cycloaddition from 6, 7, and 8. (15)N NMR spectroscopy was used to confirm the computational finding of an inorganic enamine bonding combination. Single crystal X-ray analysis of 5, 6, 7, and 8 provides insight into possible steric inadequacies within the CF(3)-ONO(3-) ligand to promote catalytic metathesis.

Reactions of a tungsten alkylidyne complex with mono-dentate phosphines: Thermodynamic and theoretical studies

Addition of mono-dentate phosphines PMe3 and PMe2Ph to the W(VI) alkyl alkylidyne complex W(CH2SiMe3)3(CSiMe3) (1) is reversible, each reaching equilibrium. Thermodynamic studies of the equilibria have been conducted, giving ΔH°=−10.0(1.1)kcal/mol and ΔS°=−23(4)eu for the addition of PMe3 and ΔH°′=−3.0 (0.7)kcalmol−1 and ΔS°′=−6(3)eu for the addition of PMe2Ph, indicating that the addition is exothermic. The experimental measurement allows a benchmarking study to select a proper DFT method to describe the current system. Of the DFT methods tested, M06 has demonstrated superior performance in calculating binding energy of a bimolecular reaction. The calculated reaction pathways show that W(CH2SiMe3)3(CSiMe3) (1) reacts with PR3 to form W(CH2SiMe3)3(CSiMe3)(PR3) (PR3=PMe3, 3a; PMe2Ph, 3b), and the adduct then undergoes α-H migration to form W(CH2SiMe3)2(CHSiMe3)2(PR3) (4a, 4b). 4a and 4b are found to be thermodynamically and kinetically stable intermediates. The calculations also suggest a pathway in the formation of the alkyl alkylidene alkylidyne complex W(CH2SiMe3)-(CHSiMe3)(CSiMe3)(PR3)2 (5a).

Unusual reaction of a tungsten alkylidyne complex with water. Formation, characterization, and crystal structures of oxo trimers

The reaction of the alkyl alkylidyne W(CH2CMe3)3(≡CSiMe3) (1) with H2O in THF and with D2O in benzene-d6 gave two new trimeric oxo complexes: W3O3(µ=O)3(CH2CMe3)6(THF)3 (2) and [W3O3(µ=O)3(CH2CMe3)6(D2O)3]·2benzene-d6 (3·2benzene-d6), respectively, each containing two alkyl ligands per W atom. This is in contrast to the dimer W2O2(µ-O)(CH2CMe3)6, containing three alkyl ligands per W atom, from the previously reported reaction of the alkyl alkylidyne analog W(CH2CMe3)3-(≡CCMe3) with H2O. In the reaction of 1 with D2O in THF, an unstable intermediate W2O2(µ-O)(CD2SiMe3)2(CH2CMe3)4 (4-d4) was identified. Kinetic studies of the reactions of 1 with excess H2O and D2O in THF-d8, yielding 4 and 4-d4, respectively, give a kinetic isotope effect (KIE) of 3.46(3) at 298(1) K, suggesting that the disappearance of 1 is a rate-determining step.

Phosphoraneiminato tungsten alkylidyne complexes as highly efficient alkyne metathesis catalysts

The tungsten neopentylidyne complexes [Me 3CC W(NPR 3){OCMe(CF 3) 2} 2] ( 3a, R = Cy; 3b, R = iPr) were prepared in excellent yields by the reaction of the solvate complex [Me 3CC W{OCMe(CF 3) 2} 3(DME)] (DME = 1,2-dimethoxyethane) with equimolar amounts of the corresponding phosphoraneiminato lithium species R 3PNLi (R = Cy, iPr) at room temperature. The products were characterized by 1H, 13C{ 1H}, 19F{ 1H} and 31P{ 1H} NMR spectroscopy and elemental analysis. Single crystals of both complexes were obtained from diethyl ether solutions at −35 °C, and the molecular structures were determined by X-ray diffraction analysis. Complexes 3a and 3b are able to efficiently catalyse alkyne cross-metathesis (ACM) of 3-pentynyl benzyl ether ( 4) and ring-closing alkyne metathesis (RCAM) of bis(3-pentynyl)adipate ( 6) at room temperature and with low catalyst loadings (1 or 2 mol%) to afford the diether 5 and the cyclic diester 7 in virtually quantitative yields. The reactions were carried out in the presence of molecular sieve 5 Å to adsorb 2-butyne during the metathesis.

Recent advances in the development of alkyne metathesis catalysts.

The number of well-defined molybdenum and tungsten alkylidyne complexes that are able to catalyze alkyne metathesis reactions efficiently has been significantly expanded in recent years.The latest developments in this field featuring highly active imidazolin-2-iminato- and silanolate–alkylidyne complexes are outlined in this review.

Ring‐Opening Metathesis Polymerization of Cyclooctyne Employing Well‐Defined Tungsten Alkylidyne Complexes

AbstractSummary: The ring‐opening metathesis polymerization of cyclooctyne was studied was studied in the presence of catalytic amounts of the tungsten alkylidyne complexes [RCW(NImtBu){OCMe(CF3)2}2] (1a: R = CMe3, 1b: R = Ph). The resulting polymers show relatively narrow polydispersities with the PDI ranging from 1.2 to 2.4. Treatment of 1b with cyclooctyne in dilute toluene or hexane solution afforded only low molecular weight oligomers. The mass spectra of these oligomers indicate the existence of macrocycles of the formula [C(CH2)6C]n (n = 3–9). In contrast, reactions at high substrate concentration led to mixtures of cyclic oligomers and linear polymers, which is probably a result of ring‐chain equilibria, established in agreement with the Jacobson‐Stockmayer theory of macrocyclization. In contrast, treatment of neat cyclooctyne with a catalytic amount of the catalyst produced medium molecular weight polymers in good yields.