Organotungsten chemistry

Organotungsten chemistry is the chemistry of chemical compounds with W-C bonds. It shares many similarities with organomolybdenum chemistry, while having more prevalent high oxidation states than the related organochromium chemistry. Notable applications include that in olefin/alkyne metathesis catalysis, and in arene activation. If the compound only has W-C bonds, is it, by definition, a form of tungsten carbide.

Carbonyl & cyanide complexes

Carbonyl complexes

The simplest tungsten carbonyl complex is tungsten hexacarbonyl, most commonly prepared via reductive carbonylation of tungsten halides and similar compounds. Tungsten hexacarbonyl itself is able to catalyze alkene metathesis. Being volatile and easily decomposed, it is also widely used in the electron beam-induced deposition technique to deposit tungsten atoms. Reduction of the hexacarbonyl yields the anionic carbonyl complexes ^2- & ^4-. Also known are the complexes ^2- & ^2-.
Substitution of the carbonyl ligand can be facilitated thermally or photochemically, for instance, the reaction with cyclopentadienide to yield ^-, which can be further derivatized. A roundabout substitution method of first using nitriles to displace the carbonyls and then displacing the nitriles is also viable. Alkane complexes of W₅ can be photochemically produced. A niche catalysis reaction utilizes the strong Lewis acidity of the W₅ fragment, converting thiirane to the sulfur analogs of crown ethers. The other common reactivity of alkyl/aryl containing tungsten carbonyl complexes involve carbonyl insertion.

Isocyanide and cyanide complexes

Isocyanide complexes W_6-n_n are prepared via ligand substitution of tungsten hexacarbonyl, catalyzed by palladium oxide or cobalt dichloride. The reactivity regarding migratory insertion is analogous to that of carbonyl complexes.
Of the cyanide complexes, ^n- are notable for their photochemical and magnetic properties. The face capped cubic cluster compound Mn₉₆•24EtOH, for instance, has the largest known ground state spin value of S = 39/2. Such complexes can also be used in constructing coordination polymers, such as _n. The coordination polymers are held together via cyanide bridges, with carbon coordinating the tungsten atoms while nitrogen coordinating the other central atoms.

Hydrocarbyl complexes

Alkyl complexes

Simple alkyl complexes of tungsten, as those of molybdenum and chromium, are rather unstable. The simplest, hexamethyltungsten, has no molybdenum or chromium analogs. It is extremely reactive, detonating in air or even in vacuum. It is prepared with methylating reagents and WCl₆, and further methylation into ^- or ^2- is possible when using methyllithium. Heteroatoms like oxygen can insert into the W-C bond, performing oxidation. WMe₆ adopts the geometry of distorted trigonal prismatic, which may be attributed to a second-order Jahn-Teller distortion.
Stabilization of these compounds are possible via dimerization, as in the compound ₃W≡W₃. Note that lack of beta hydrogen atoms are necessary to prevent beta-elimination. Neutral mononuclear complexes of different alkyl numbers are known, such as tetrabenzyltungsten.
For electron deficient alkyl tungsten complexes, one example that demonstrates their bonding interactions and reactivity is shown below:

Aryl complexes

As with the alkyl tungsten complexes and most hydrocarbyl organometallics, aryl tungsten complexes can be prepared from tungsten halides and hydrocarbylating agents via transmetallation.
The thermolysis of the complexes Cp*W₂ results in the loss of an arene and the formation of aryne complexes. The aryne complexes are unstable and readily activate other C-H bonds.

Vinyl complexes

Vinyl ligands have two different modes of coordination with tungsten atoms, as depicted:
Synthesis is facilitated via transmetallation, the deprotonation of tungsten alkene complexes, nucleophilic addition to tungsten alkyne complexes, or alkyne insertion into W-H bonds. The isomerization of the η¹ vinyl complexes into carbynes are possible via a 1,2-rearrangement|-hydrogen migration reaction from the alpha carbon, usually via η² vinyl intermediates. Isomerization of the η² vinyl complexes into allyl complexes are also known.

Alkynyl complexes

Alkynyl complexes of tungsten can be prepared via transmetallation or via the deprotonation of alkyne or carbene complexes of tungsten. An exotic method of preparation involves the reaction between ^- and CH₂I₂, forming the bridged complex . The main reactivity involves electrophilic attack on the beta-carbon, as explained in the resonance forms, and it is enhanced with the increasing electron density of the complex. Less common are electrophilic attack on the alpha carbon, which produces alkyne complexes, or electrophilic attack on the tungsten atom to produce allyl tungsten complexes.
Alkynyl tungsten complexes, along with propargyl tungsten complexes, have applications as templates during synthesis of cyclic compounds like lactones. For instance:

Carbene and carbyne complexes

Carbene complexes

The first tungsten carbene complexes were generated from organolithium reagents and tungsten hexacarbonyl. Applications include polymerizing alkynes and cyclopropanation of alkenes. Schrock-type tungsten carbene complexes are usually formed via alpha-deprotonation or alpha-elimination of alkyl tungsten complexes. Another synthesis method involves carbene transfer from carbene sources like Wittig's reagent. Tungsten carbene complexes are active alkene metathesis catalysts.
Tungsten vinylidene complexes can be generated from alkynyl, carbyne, and alkyne tungsten complexes. Vinylidene ligands are of strong π acceptor capabilities, therefore enabling catalytic applications of vinylidene complexes in alkyne polymerization reactions.
On a related note, the silylene complexes Cp*W₂ can be produced from the photochemical reaction between Cp*W₃Me and HSiMe₂SiMeR₂, with the exact structure dependent on the R group.

Carbyne complexes

Due to electronic effects, tungsten carbyne complexes often adopt slightly bent geometries. Common methods of synthesis involves treating tungsten carbenes of the form W=CXR with Lewis acids or alpha-deprotonation/alpha-dehydrogenation of tungsten carbene complexes. Some primary alkyl tungsten complexes will spontaneously undergo double alpha-dehydrogenation, yielding tungsten carbynes and dihydrogen. As a result of the reversibility of alpha-dehydrogenation reactions, it's possible to observe the following reaction:
Another preparation method involves the metathesis reaction between the W≡W triple bond and an alkyne. The reaction cannot proceed when the alkyne is diphenylacteylene due to steric hindrance, and instead forming a mixture of W₂₄₂ and W₂₄₂. If the C≡C triple bond is replaced with the C≡N of nitriles, then aside from the carbyne product, a nitrido complex containing W≡N shall yield. The exact reaction conditions are detailed in the article on W₂(t-BuO)₆.
Protonation of the carbynes have been documented, yielding an alpha-agostic cationic carbene complex. For W≡C-H complexes, deprotonation is also possible, forming an anion that can react with nucleophiles to form more complex carbyne complexes. Due to the electron-deficient nature of the center tungsten atom, it's conceivable that beta-hydrogens of the tungsten carbynes also possess some acidity.

Acyclic π complexes

Alkene complexes

The bonding nature of tungsten alkene complexes can be described by the Dewar-Chatt-Duncanson model. Ligand substitution, thermal or photochemical, are most commonly used to prepare such complexes. Norbornadiene complexes ₄) have also been reported.
For complexes with double alkene ligands, it's possible to generate a metallacyclopentane complex via reductive coupling, which can be otherwise generated via intramolecular hydrogen transfer between alkyl and vinyl ligands.

Alkyne complexes

Alkyne ligands can adopt differing coordination modes with tungsten. Namely, when the tungsten atom is of d₆ configuration, the alkyne ligands are usually 2-electron donors; whereas in d₄ & d₂ configuration tungsten complexes, an additional empty d orbital is available for interaction with the alkyne, therefore the alkyne ligands tend to be 4-electron donors.
Some of tungsten alkyne complexes' reactivity arise from alkynes' role as variable electron donors. This is exemplified in the stepwise oxidation reaction shown below:Tungsten complexes can act as templates for the synthesis of chiral alkynes via the reactions of alkyne tungsten complexes followed by alkyne ligand removal.
Tungsten alkyne complexes W_6-n_n can be synthesized via photochemical or thermal displacement of carbonyl ligands on tungsten hexacarbonyl. These complexes are unstable and rearrange into vinylidiene complexes of the form W₅ via hydrogen migration. This is due to the repulsive interactions between the filled tungsten d orbital and the perpendicular alkyne π orbital. The interaction between the alkyne ligand and the tungsten center is best described in the form of metallacyclopropenes.
Electron-withdrawing substituents on alkynes can stabilize alkyne-tungsten complexes ₂. This enhances the back-bonding towards the alkyne ligand, while decreasing the electron repulsions.
One reaction involving tungsten alkyne complexes arises from treating the complex W₃ with excess diphenylacetylene. Reaction products can include tetraphenylcyclobutadiene complexes and pentaphenylcyclopentadienyl complexes as the result of diphenylacetylene coupling:
Related to this is the organotunsgten-catalyzed alkyne trimerization/polymerization, as detailed below in the arene-tungsten complex section.
Tungsten alkyne complexes are susceptible to nucleophilic attack; see the above section on vinyl tungsten complexes.

Allyl complexes

Tunsgten allyl complexes may be prepared via transmetallation between Grignard reagents. Examples of preparation methods involving the nucleophilic attack on allylic halides can be seen in the above section on alkynyl tungsten. The homoleptic allyl tungsten complex W₄ adopts a configuration with S₄ symmetry.
Some allyl tungsten complexes are synthetically useful, as in:

Other notable π complexes

The other, rather unusual π complexes of tungsten involve the π coordination of the carbonyl group, which almost always coordinate through the lone pair of the carbonyl oxygen atom. One example is TpW, where the nitrogen displays a pyramidal geometry and basicity, indicating loss of conjugation. It can be prepared from the corresponding benzene complex via ligand exchange.

Cyclic π complexes

Cyclobutadiene complexes

Tungsten cyclobutadiene complexes can be formed via coupling of alkyne ligands, see the above part on tungsten alkyne complexes.

Cyclopentadienyl complexes

Monomeric tungstenocene is highly unstable and polymerize above 10 kelvin to form a red-brown solid. It's generated via the photolysis of Cp₂WH₂ or the thermolysis of Cp₂WH and readily inserts into C-H, O-H and B-B bonds. Similarly, the complex WH releases methane when heated in benzene, forming the compound WH via a σ-alkane tungsten complex intermediate. The decaphenyl derivative of tungstenocene, W₂, can be generated by the coupling of diphenylacteylene ligands, as detailed in the above section on alkyne tungsten complexes. Oxidation into the mono- and di-valent cations are possible.
Cp₂WH₂ is a strong Lewis base, forming adducts like BF₄. It can be chlorinated with chloroform to form Cp₂WCl₂, and the reverse reaction is possible with lithium aluminum hydride. Cp₂WCl₂ itself is made from cyclopentadienide and WCl₄ and can be further oxidized into ²⁺. Many other tungstenocene derivatives can be produced from Cp₂WCl₂, like alkyl derivatives using halide metathesis. Cp₂W can participate in cycloaddition reactions.
Non-tunsgtenocene derived cyclopentadienyl tungsten complexes like Cp*WF₅, Cp*WMe₄, and amide derivatives can be prepared from precursors like Cp*WCl₄. Trichalcogenide cyclopentadienyl complexes are also known, and notable ones include the chiral complex ^- and the complex ^-.
Of great importance is the applications of cyclopentadienyl tungsten complexes in C-H bond activation. One example is shown below :

Arene complexes

The dibenzenechromium analog is a yellow-green substance that has not been extensively studied due to difficult preparation. It is easily oxidized into ⁺, and can undergo protonation. Photolysis of tunsgeten hexacarbonyl in acetylene forms benzene and the complex W₃. Related complexes can catalyze the cyclotrimerization and polymerization of alkynes. Arene complexes deriving from intramolecular ligand interactions are known. For instance, the complex on the right can be generated by reducing the corresponding chloro-complex in the presence of the pyridine ligand.
In the unique arene-tungsten complex TpW, the benzene adopts a η² coordination mode. Such allows the unusual Diels-Alder reaction of the benzene ligand, which is normally extremely inert in this aspect. The reason behind this activation is related with the strong π backbonding to the arene ligand, localizing the electrons while rendering the bond carbon atoms unreactive. Pyridines can be activated in such manners as well, forming isoquinuclidine centers that are biologically significant. The arenes are also activated towards hydrogenation and electrophilic addition. It's worth noting that the tungsten in this case is rather π basic depite the acidic NO⁺ ligand, and can form complexes with electron-deficient arenes that are not typically coordinating, such as fluorobenzenes.
A synthesis reaction utilizing this type of activation is exemplified below, note that the tungsten can be removed from the substrate oxidatively:
Fullerene can also adopt this curious η² coordination pattern in complexes like W₃, where L-L denotes a bidentate ligand.

Cycloheptatrienyl complexes

The complex ⁺ can be generated from the corresponding tropylium ion and the complex fac-. The CO ligand can be displaced by iodide ions. It is worth noting that the same reaction but with the ⁺ cation would yield a η⁵ complex instead due to repulsion between the methyl groups that hinders planarity of the ligand.

As metathesis catalysts

Alkene metathesis

The general structure of tungsten-based alkene metathesis catalysts is shown in the image. R₁ is typically methyl or isopropyl, R₂ is usually a bulky group, most commonly -CMe₂, while R₃ is typically a bulky alkyl group, like t-butyl or -CMe₂Ph. They belong to Schrock catalysts, and their activities can be tuned by varying the alkoxide group. It is also possible for the catalysts to act as stoichiometric olefinating reagents on hydroxy ketones.

Alkyne metathesis

Many tungsten-based alkyne metathesis catalysts are of the general type . Activity is manipulated by the ligands. A typical route to such catalysts entails treatment neopentyl Grignard reagent to tungsten precursor followed by net alcoholysis of the alkyl ligands. Complex 3 can undergo a ligand exchange with lithium salts to generate Schrock type catalysts. Another way to make complex 4 is via cleavage of internal alkyne by W complex, such as 5. Complex 2, as well as 3, is unable to metathesize internal alkynes, the related pathway is shown right. In detail, compound 6 will react with two equivalent alkynes to form complex 7. Complex 7 will undergo an "associative path" to generate a metallabenzene complex 8. It will decompose to polymerized compounds or a cyclopentadienyl complex with a formally reduced tungsten center. Tungstenocenes, or tungsten-containing metallocenes, may be formed from these cyclopentadienyl complexes.
The formal 12-electron count of the W center in Schrock catalyst represents an appreciable Lewis acidity, which seriously limits the scope of these catalysts. For example, Schrock catalyst is unable to metathesize substrates containing donor or basic sites such as amines, thio ethers or crown ether segments. Acid-sensitive groups such as acetals can be destroyed. Replacement of tert-butoxide ligands by fluorinated alkoxides increase the Lewis acidic character. To reach a balance, it is proposed that a heteroleptic push/pull environment around the tungsten center will work. For example, complex 13 is highly active and compatible with many functional groups.