Tungsten Alkylidene Complexes Research Articles

Thermal Formation of Metathesis-Active Tungsten Alkylidene Complexes from Cyclohexene.

A 7-tungstabicyclo[4.3.0]nonane complex forms slowly upon addition of cyclohexene to the ethylene complex, W(NAr)(OSiPh3)2(C2H4), at 22 °C. A single-crystal X-ray study showed its structure to be closest to a square pyramid (τ = 0.23). At 22 °C, loss of cyclohexene or ring contraction of the 7-tungstabicyclo[4.3.0]nonane complex is slow. Above ∼80 °C, cyclohexene is ejected to give W(NAr)(OSiPh3)2(C2H4), but a sufficient amount of 7-tungstabicyclo[4.3.0]nonane complex remains in the presence of cyclohexene and the ring contracts to yield methylenecyclohexane and a methylidene complex or ethylene and a cyclohexylidene complex. Other complexes that have been observed include an 8-tungstabicyclo[4.3.0]nonane complex formed from 1,7-octadiene, a 7-tungstabicyclo[4.2.0]octane complex (formed from a methylidene complex and cyclohexene), and a methylenecyclohexane complex. 13C-Labeling studies show that the exo-methylene group in methylenecyclohexane and the α positions in the 8-tungstabicyclo[4.3.0]nonane come from ethylene. An alternative ring contraction of a tungstacyclopentane made from two molecules of cyclohexene cannot be excluded when concentrations of ethylene are low. A cyclohexylidene complex could also form from two cyclohexenes via a newly proposed "alkyl/allyl" mechanism. The results reported here are the first experimental confirmations that a tungstacyclopentane can ring-contract thermally at a substituted WCα position to form a tungstacyclobutane and therefore metathesis-active alkylidenes.

Molybdenum and Tungsten Alkylidene Complexes That Contain a 2-Pyridyl-Substituted Phenoxide Ligand

In the interest of preparing molybdenum and tungsten alkylidene complexes for olefin metathesis that are longer-lived at high temperatures (∼150 °C or above), we synthesized complexes that contain a phenoxide ligand with a 2-pyridyl in one ortho position and a mesityl (Mes) or 2,4,6-i-Pr3C6H2 (Trip) in the other ortho position ([MesON]− or [TripON]−, respectively). The alkylidene (neophylidene) complexes that were prepared include W(O)(CHCMe2Ph)(Me2Pyr)(RON) (R = Mes or Trip), Mo(NC6F5)(CHCMe2Ph)(RON)Cl, Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(RON)Cl, Mo(N-t-Bu)(CHCMe2Ph)(RON)Cl, and M(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(TripON)(OTf) (M = Mo or W). The reaction between Mo(NAr)(CHCMe2Ph)(TripON)(OTf) and ethylene yielded an ethylene complex, Mo(NAr)(C2H4)(TripON)(OTf)(ether). All neophylidene complexes were essentially unreactive toward terminal olefins at 22 °C and showed modest homocoupling activity (at 80 or 100 °C) and alkane metathesis activity (at 150 and 200 °C). W(O)(CHCMe2Ph)(Me2Pyr)(MesON) also stereoselectively polymerized several substituted norbornadienes at 100 °C.

Activating Thiolate‐Based Imidoalkylidene Tungsten(VI) Metathesis Catalysts by Grafting onto Silica

AbstractIn view of the strong σ‐donor ability of thiolate ligands, a silica‐supported tungsten alkylidene complex was prepared. While the activity of the molecular complex in alkene metathesis was not significant, the activity of the silica‐supported analogue is greatly enhanced, thus illustrating the importance of dissymmetry at the metal center in d0 metathesis catalysts and the specific properties of surface siloxy ligands in activating metathesis catalysts.

Synthesis of Molybdenum and Tungsten Alkylidene Complexes That Contain the 2,6-Bis(2,4,6-triisopropylphenyl)phenylimido (NHIPT) Ligand

Molybdenum and tungsten alkylidene complexes that contain the sterically demanding hexaisopropylterphenylimido ligand, N-2,6-(2,4,6-i-Pr3C6H2)2C6H3 (NHIPT), have been prepared from Mo(N-t-Bu)2Cl2(1,2-dimethoxyethane) or W(N-t-Bu)2Cl2(pyridine)2, employing tert-butylimido ligands as sacrificial proton acceptors. These complexes include M(NHIPT)(CH-t-Bu)Cl2 (M = Mo, W), Mo(NHIPT)(CH-t-Bu)(pyrrolide)2, and Mo(NHIPT)(CH-t-Bu)(pyrrolide)(OC6F5)(CH3CN). In all cases only anti alkylidene isomers are observed in solution, as a consequence of the steric demands of the NHIPT ligand. An X-ray structure of W(NHIPT)(CH-t-Bu)Cl2 showed it to be a monomer with a disordered alkylidene that is 86% in the anti configuration and 14% in the syn configuration.

Synthesis of Molybdenum and Tungsten Alkylidene Complexes That Contain Sterically Demanding Arenethiolate Ligands.

Imido alkylidene complexes of Mo and W and oxo alkylidene complexes of W that contain thiophenoxide ligands of the type S-2,3,5,6-Ph4C6H (STPP) and S-2,6-(mesityl)2C6H3 (SHMT = S-hexamethylterphenyl) have been prepared in order to compare their metathesis activity with that of the analogous phenoxide complexes. All thiolate complexes were significantly slower (up to ∼10× slower) for the metathesis homocoupling of 1-octene or polymerization of 2,3-dicarbomethoxynorbornene, and none of them was Z-selective. The slower rates could be attributed to the greater σ-donating ability of a thiophenoxide versus the analogous phenoxide and consequently a higher electron density at the metal in the thiophenoxide complexes.

Syntheses of Tungsten tert-Butylimido and Adamantylimido Alkylidene Complexes Employing Pyridinium Chloride As the Acid

Routes to new tungsten alkylidene complexes that contain tert-butylimido or adamantylimido ligands have been devised that begin with a reaction between WCl6 and 4 equivalents of HNR(TMS) to give [W(NR)2Cl(μ-Cl)(RNH2)]2 (R = t-Bu or 1-adamantyl). Alkylation leads to W(NR)2(CH2R′)2 (R′ = t-Bu or CMe2Ph), which upon treatment with pyridinium chloride yields W(NR)(CHR′)Cl2(py)2 complexes, from which W(NR)(CHR′)(pyrrolide)2 and two W(N-t-Bu)(CHR′)(pyrrolide)(OAr) complexes (OAr = hexamethyl- or hexaisopropylterphenoxide) have been prepared.

Preparation of Tungsten‐Based Olefin Metathesis Catalysts Supported on Alumina

AbstractA new tungsten alkylidene complex, W(NAr)(CHCMe2Ph)(OHIPT‐NMe2)(pyrrolide) {Ar=2,6‐(i‐Pr)2C6H3; HIPT‐NMe2=2,6‐[2,4,6‐(i‐Pr)3C6H2]2‐4‐NMe2‐C6H2}, has been synthesized and shown to be highly selective for Z homocoupling metathesis of selected terminal olefins in pentane, as is W(NAr)(CH2CH2CH2)(OHIPT)(pyrrolide) (5). Both 5 and W(NAr)(CHCMe2Ph)(OHIPT‐NMe2)(pyrrolide) (6) are adsorbed onto calcined alumina. Control experiments and metathesis homocoupling of four substrates lead to the conclusions that 5 is largely adsorbed in a reaction that liberates HIPTOH, while 6 is adsorbed largely through an interaction between the dimethylamino group and an acidic site on the surface. There is no evidence that any adsorbed catalyst can give rise to Z selectivity of a magnitude equal to that found in a homogeneous reaction involving 5 or 6.

Synthesis and structures of bimetallic silicon-containing imido alkylidene complexes of tungsten (R′O) 2(ArN)W [dbnd]CH–SiR 2–CH [dbnd]W(NAr)(OR′) 2 (R = Me, Ph) and (R′O) 2(ArN)W [dbnd]CH–SiMe 2SiMe 2–CH [dbnd]W(NAr)(OR′) 2

Bimetallic alkylidene complexes of tungsten (R′O) 2(ArN)W CH–SiR 2–CH W(NAr)(OR′) 2 (R = Me ( 1), Ph ( 2)) and (R′O) 2(ArN)W CH–SiMe 2SiMe 2–CH W(NAr)(OR′) 2 ( 3) (Ar = 2 , 6 – Pr 2 i C 6 H 3 ; R′ = CMe 2CF 3) have been prepared by the reactions of divinyl silicon reagents R 2Si(CH CH 2) 2 with known alkylidene compounds R′′–CH Mo(NAr)(OR′) 2. (R′′ = Bu t , PhMe 2C) Complexes 1– 3 were structurally characterized. Ring opening metathesis polymerization (ROMP) of cyclooctene using compounds 1– 3 as initiators led to the formation of high molecular weight polyoctenamers with predominant trans-units content in the case of 1 and 3 and predominant cis-units content in the case of 2.

Synthesis and Reactions of Tungsten Alkylidene Complexes That Contain the 2,6-Dichlorophenylimido Ligand

Tungsten alkylidene complexes that contain the 2,6-dichlorophenylimido ligand (NArCl), W(NArCl)(CHCMe3)(ORF6)2 and W(NArCl)(CHCMe3)(Biphen)(THP) (ORF6 = OCMe(CF3)2; Biphen2- = 3,3‘-di-t-Bu-5,5‘,6,6‘-tetramethyl-1,1‘-biphenyl-2,2‘-diolate; THP = tetrahydropyran), have been prepared from W(NArCl)(CHCMe3)(OSO2CF3)2(dme). Reaction of W(NArCl)(CHCMe3)(ORF6)2 with excess ethylene gave the tungstacyclobutane complex W(NArCl)(ORF6)2(C3H6), while reaction of W(NArCl)(CHCMe3)(ORF6)2 with internal olefins gave the dimer, [W(μ-NArCl)(ORF6)2]2. W(NArCl)(CHCMe3)(Biphen)(THP) reacted with excess ethylene to give W(NArCl)(Biphen)(C3H6), which yielded W(NArCl)(Biphen)(C2H4)(THF) upon addition of THF. The tungstacyclopentane complex, W(NArCl)(Biphen)(C4H8), was formed upon treatment of W(NArCl)(Biphen)(C2H4)(THF) with ethylene. W(NArCl)(Biphen)(C3H6) decomposed to yield (inter alia) the dimeric, heterochiral methylidene complex, [W(NArCl)(Biphen)(μ-CH2)]2, which decomposed further to yield homochiral [W(NArCl)(Biphen)]2(μ-CH2CH2). Finally, [W(NArCl)(Biphen)]2(μ-CH2CH2) loses ethylene reversibly to yield [W(NArCl)(Biphen)]2, which could not be isolated in pure form. {W(μ-NArCl)[OC(CF3)2Me]2}2, W(NArCl)(Biphen)(C3H6), W(NArCl)(Biphen)(C2H4)(THF), W(NArCl)(Biphen)(C4H6), [W(NArCl)(Biphen)(μ-CH2)]2, and [W(NArCl)(Biphen)]2(μ-CH2CH2) were characterized through X-ray studies.

Ortho-Selective C−H Activation of Substituted Benzenes Effected by a Tungsten Alkylidene Complex without Substituent Coordination

Ortho-Selective C−H Activation of Substituted Benzenes Effected by a Tungsten Alkylidene Complex without Substituent Coordination

Ortho-Selective C−H Activation of Substituted Benzenes Effected by a Tungsten Alkylidene Complex without Substituent Coordination

Gentle thermolysis of the bis(neopentyl) complex Cp*W(NO)(CH2CMe3)2 (1) at 70 °C in various substituted benzenes results in the loss of neopentane and the generation of the transient alkylidene complex Cp*W(NO)(CHCMe3) (A), which subsequently effects single C−H bond activations of the benzenes. These activations exhibit a pronounced selectivity for the C−H linkages ortho to the benzene substituents. Thus, thermal reactions of 1 with C6H5X lead to the preferential formation of the corresponding Cp*W(NO)(CH2CMe3)(o-C6H4X) complexes, the ortho-selectivity, i.e., X = F > OMe > Cl > Br > C⋮CPh, diminishing as the steric demands of the substituents increase. Consistently, thermolyses of 1 in o-C6H4X2 afford Cp*W(NO)(CH2CMe3)(2,3-C6H3X2) complexes with 98% selectivity when X = F and 82% selectivity when X = Cl. Similarly, the principal organometallic products resulting from the thermolyses of 1 in m-C6H4X2 are the ortho-para-activated isomers Cp*W(NO)(CH2CMe3)(2,4-C6H3X2) with 84% selectivity when X = F and 89% selectivity when X = Cl. Finally, thermolyses of 1 in para-disubstituted benzenes p-C6H4XY (X = F or OMe, Y = Cl or OMe) again reveal that the ortho-directing abilities of the substituents diminish in the order F > OMe > Cl. Some mechanistic insights into these activation processes have been obtained by monitoring of the early stages of the thermolysis of 1 in chlorobenzene by 1H NMR spectroscopy. This monitoring reveals that the selectivity of the alkylidene intermediate A for forming the ortho-activated isomer is thermodynamic rather than kinetic in nature. Thus, the meta- and para-activated isomers are formed initially, but then convert to the ortho-activated product. The principal organometallic complexes resulting from ortho-C−H activations are formally 16-electron species in which there are no interactions between the tungsten centers and the Lewis-basic benzene substituents. Instead, the Lewis-acidic tungsten centers engage in intramolecular agostic interactions with the methylene C−H bonds of their neopentyl ligands to acquire some additional electron density. Being coordinatively and electronically unsaturated, these tungsten complexes react readily with added Lewis bases. Thus, treatment of Cp*W(NO)(CH2CMe3)(o-C6H4F) with Et4NCN and CO produces [Et4N]+[Cp*W(NO)(CH2CMe3)(CN)(o-C6H4F)]- and Cp*W(NO)(η2-C(O)CH2CMe3)(o-C6H4F), respectively, and the solid-state molecular structure of the latter complex has also been established by a single-crystal X-ray crystallographic analysis.

C−H Activation of Substituted Arenes by Tungsten Alkylidene Complexes: Products, Selectivity, and Mechanism

Thermolyses (70 °C, 40 h) of Cp*W(NO)(CH2CMe3)2 (1) and Cp*W(NO)(CH2CMe3)(CH2C6H5) (2) in xylenes, mesitylene, and α,α,α-trifluorotoluene generate mixtures of the corresponding aryl and/or benzyl products derived from aromatic sp2 and benzylic sp3 C−H bond activations of the solvent molecules by the intermediate alkylidene complexes Cp*W(NO)(CHCMe3)(σ-CMe4) (σ-A) and Cp*W(NO)(CHC6H5)(σ-CMe4) (σ-B), respectively. For instance, the thermolysis of 1 in p-xylene affords products resulting from the activation of one and two molecules of p-xylene. The two products derived from the activation of one solvent molecule are Cp*W(NO)(CH2CMe3)(C6H3-2,5-Me2) (10) and Cp*W(NO)(CH2CMe3)(CH2C6H4-4-Me) (11). The other two complexes derived from the activation of two solvent molecules are Cp*W(NO)(CH2C6H4-4-Me)(C6H4-2,5-Me2) (12) and Cp*W(NO)(CH2C6H4-4-Me)2 (13). The ratio of the four products in the final reaction mixture is 10:11:12:13 = 1.81 ± 0.09:0.44 ± 0.05:1.0:0.15 ± 0.02. All new complexes prepared have been characterized as fully as possible by conventional spectroscopic methods, and the solid-state molecular structures of Cp*W(NO)(CH2C6H5)(C6H3-2,5-Me2) (7) and Cp*W(NO)(CH2C6H5)(CH2C6H3-3,5-Me2) (9) have been established by X-ray diffraction methods. Analyses of the product distributions resulting from the various thermolyses indicate that the benzyl products are increasingly favored over the aryl products as the number of methyl substituents on the solvent molecule increases. Likewise, the movement of the methyl groups from the ortho to meta to para position in the xylenes shifts the aryl vs benzyl product distribution toward the benzyl products. With respect to the aryl product regioselectivities, only the least sterically congested aryl regioisomers are formed in the activations of o- and m-xylene, while the meta and para aryl products are formed preferentially for α,α,α-trifluorotoluene. Finally, the distributions obtained from σ-B are more abundant in the aryl products than are those obtained from σ-A. However, despite these general trends, it is also apparent that the origin of the observed product selectivities is highly dependent on the nature of the substrate, the nature of the C−H activation products, and the intermediate alkylidene complex.

Thermal Alkane C−H Bond Activation by a Tungsten Alkylidene Complex: The Reversal of α-Hydrogen Elimination

Thermal Alkane C−H Bond Activation by a Tungsten Alkylidene Complex: The Reversal of α-Hydrogen Elimination

A route to low polydispersity linear and star polyethylenes via ring-opening metathesis polymerization

Abstract Living ring-opening metathesis polymerization of cyclopentene, initiated with a well-defined tungsten alkylidene complex, has been used to prepare low polydispersity linear and star poly(1-pentenylene)s. This was achieved by using di- and trifunctional aldehydes in the Wittig-like capping reactions of the living polymers. The resulting materials were hydrogenated to the corresponding polyethylene structures using a diimide derived from p -toluenesulfonyl hydrazide. An overview of this process, together with characterization data for the resulting materials, is presented.

Preparation of Well-Defined, Metathetically Active Oxo Alkylidene Complexes of Tungsten

The reaction between W(CH-t-Bu)(O)(PMe3)2Cl2 and 2 equiv of KO-2,6-Ph2C6H3 yields yellow crystalline syn-W(CH-t-Bu)(O)(OAr)2(PMe3) (1a; Ar = 2,6-Ph2C6H3) in which the PMe3 is bound to tungsten on the NMR time scale. An analogous PPh2Me complex, syn-W(CH-t-Bu)(O)(OAr)2(PPh2Me) (1b), can be prepared. Both 1a and 1b react readily with 2,3-dicarbomethoxynorbornadiene (2a) and 2,3-bis(trifluoromethyl)norbornadiene (2b) in dichloromethane or toluene to give polymers that are >95% cis and >95% isotactic.

Dihydropyridines in Organometallic Synthesis. Formation of Pyridine and Dihydropyridine-Stabilized Alkylidene Complexes of Tungsten(0) and Chromium(0) from Fischer Carbene Complexes: Structure and Reactivity

1,2- and 1,4-dihydropyridines react with alkoxycarbene complexes of chromium and tungsten to give, upon an unprecedented hydride transfer, alcohol elimination, and pyridine fixation on the carbene carbon, a new class of air-stable pyridinium ylide complexes. These pyridine-protected alkylidene complexes of chromium(0) and tungsten(0) were fully characterized by X-ray crystallography. In the case of (CO)5WC(CH3)(OEt) (5a), besides the pyridinium ylide complex (CO)5W-−C(H)(CH3)(pyridine)+ (7a), the dihydropyridinium complex (CO)5W-−C(H)(CH3)(2,5-dihydropyridine)+ (8a) was also isolated. The intermediate tungstate (CO)5W-−C(H)(CH3)(OEt)(CH3NC5H5)+ could be easily obtained and characterized by using, as reducing agent, N-methyldihydropyridine. Whereas phenyl-substituted pyridinium complexes easily transferred the benzylidene moiety to alkenes, alkyl-substituted complexes appeared more reluctant to such a transfer: satisfactory results were observed in the case of nucleophilic olefins such as enol ethers. However, straightforward transfer of the tungsten(0) alkylidene group took place, even at room temperature, in the case of alkoxycarbene complexes tethered to alkenes, giving access, upon intramolecular cyclopropanation reactions, to polycyclic systems.

Further studies of imido alkylidene complexes of tungsten, well-characterized olefin metathesis catalysts with controllable activity [Erratum to document cited in CA113(17):152665k

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTFurther studies of imido alkylidene complexes of tungsten, well-characterized olefin metathesis catalysts with controllable activity [Erratum to document cited in CA113(17):152665k]Richard R. Schrock, R. T. DePue, J. Feldman, K. B. Yap, D. C. Yang, W. M. Davis, L. Park, M. DiMare, M. Schofield, and Cite this: Organometallics 1990, 9, 10, 2850Publication Date (Print):October 1, 1990Publication History Published online1 May 2002Published inissue 1 October 1990https://doi.org/10.1021/om00160a040RIGHTS & PERMISSIONSArticle Views97Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (80 KB) Get e-Alerts Get e-Alerts

Further studies of imido alkylidene complexes of tungsten, well-characterized olefin metathesis catalysts with controllable activity

An alternative synthesis of W(CH-t-Bu)(NAr)(dme)Cl{sub 2} (Ar = 2,6-C{sub 6}-H{sub 3}-i-Pr{sub 2}) consists of the five steps WCl{sub 6} {yields} W(O)Cl{sub 4} {yields} W(NAr)Cl{sub 4} {yields} W(NAr)(O-t-Bu){sub 2}Cl{sub 2}(THF) {yields} W(NAr)(O-t-Bu){sub 2}(CH{sub 2}-t-Bu){sub 2} {yields} W(CH-t-Bu)(NAr)(dme)Cl{sub 2}, in which tert-butoxide protecting groups are replaced by chlorides in the last step upon addition of PCl{sub 5}. The easiest synthesis to a catalyst precursor consists of the three steps WO{sub 2}Cl{sub 2} {yields} W(NAr){sub 2}Cl{sub 2}(dme) {yields} W(NAr){sub 2}(CH{sub 2}R){sub 2} {yields} W(CHR)(NAr)(OTf){sub 2}(dme) (R = t-Bu, CMe{sub 2}Ph; OTf = OSO{sub 2}CF{sub 3}), in which an imido ligand protecting group is ultimately replaced by two triflate ligands upon addition of triflic acid in the last step. An X-ray study of W(CH-t-Bu)(NAr)(O-t-Bu){sub 2} shows it to be a pseudotetrahedral complex in which the tert-butyl group points toward the imido ligand.

Facile tungsten alkylidene synthesis: alkylidene transfer from a phosphorane to a tungsten imido complex

A number of transition-metal complexes catalyze the ring-opening metathesis polymerization (ROMP) of a variety of cyclic olefins. Notable among these catalysts are the titanacyclobutane derivatives and certain alkylidene complexes of tungsten: molybdenum, tantalum, and rhenium. The highly reactive tungsten alkylidene complexes developed by Schrock, Osborn, and Basset are particularly useful for the synthesis of unsaturated polymers such as novel conducting polymers and soluble precursors and derivatives of polyacetylene. Recent applications of these catalytic systems involve the polymerization of acyclic alkynes and dienes In addition, the use of tungsten alkylidene complexes as Wittig-type reagents in organic synthesis holds considerable promise.

Ring‐opening polymerization of norbornene initiated by tungsten alkylidene complexes. Activation by AlCl3

AbstractAddition of aluminium compounds to the stable metal‐carbene complex tetracarbonyl[4–5‐η‐1‐methoxy‐4‐pentenylidene]tungsten (1), showing an intramolecular coordination of double bond to the transition metal, leads to highly active catalytic systems in the polymerization of norbornene. It is shown that, in optimal conditions, AlCl3 is more efficient than AlEt3 for cyclo‐olefin polymerization. From spectroscopic studies (UV‐visible, 1H and 13C NMR) of the activated system 1‐AlCl3, a mechanism of activation by AlCl3 is proposed involving a modification of the nature of carbene 1 by the Lewis acid.