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 (for instance, reaction of WCl6 and zinc powder under a CO atmosphere) 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 (in liquid ammonia with borohydride and sodium metal, respectively) yields the anionic carbonyl complexes [W2(CO)10]2- & [W(CO)4]4-. Also known are the complexes [W(CO)5]2- & [W3(CO)14]2-.

Substitution of the carbonyl ligand can be facilitated thermally or photochemically, for instance, the reaction with cyclopentadienide to yield [CpW(CO)3]-, 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 W(CO)5 can be photochemically produced. A niche catalysis reaction utilizes the strong Lewis acidity of the W(CO)5 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(CO)6-n(CNR)n (n = 1~3) 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, [W(CN)8]n- (n = 3, 4) are notable for their photochemical and magnetic properties. The face capped cubic cluster compound Mn9[W(CN)8]6•24EtOH, for instance, has the largest known ground state spin value of S = 39/2 (as of 2011). Such complexes can also be used in constructing coordination polymers, such as {(Me3Sn)4[W(CN)8]}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 [WMe7]- or [WMe8]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 (for further details, see the article on hexamethyltungsten). Stabilization of these compounds are possible via dimerization, as in the compound (Me3SiCH2)3W≡W(CH2SiMe3)3. 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 (W(CH2Ph)4).

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*W(NO)(aryl)2 results in the loss of an arene and the formation of aryne complexes (similar reactions are observed for other hydrocarbyl ligands). The aryne complexes are unstable and readily activate other C-H bonds (for instance, in solvent molecules).

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]-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 [CpW(CO)3]- and CH2I2, forming the bridged complex [CpW(CO)3](C≡C)[CpW(CO)3] (along with side products). The main reactivity involves electrophilic attack on the beta-carbon (which forms vinylidene complexes), 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 (as in the case when reacting with allylic halides) 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 (see the synthesis of Fischer carbenes). 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 (containing the metallaallene unit W=C=CHR) can be generated from alkynyl, carbyne, and alkyne tungsten complexes (see the respective sections of each coumpound). 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*W(CO)2(=SiR2) can be produced from the photochemical reaction between Cp*W(CO)3Me and HSiMe2SiMeR2, with the exact structure (monomeric/dimeric) 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 (X indicates a good leaving group) with Lewis acids or alpha-deprotonation/alpha-dehydrogenation of tungsten carbene complexes. Some primary alkyl tungsten complexes (which may be the product of alkene insertion into W-H bonds) 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 (most commonly from W2(t-BuO)6) and an alkyne (see the below section on alkyne metathesis). The reaction cannot proceed when the alkyne is diphenylacteylene due to steric hindrance, and instead forming a mixture of W2(OR)4(μ-PhC≡CPh)2 and W2(OR)4(μ-CPh)2. 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 (e.g. W(CO)4(nbd)) 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 (i.e. of low valent), 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 (i.e. donating between 2 and 4 electrons). 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(CO)6-n(HC≡CR)n (n = 1, 2) can be synthesized via photochemical or thermal displacement of carbonyl ligands on tungsten hexacarbonyl (see above). These complexes are unstable and rearrange into vinylidiene complexes of the form W(CO)5(=C=CHR) 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 (for instance, in the compounds W(CO)2(L-L)(RC≡CR)2, where R is an electron-withdrawing group). This enhances the back-bonding towards the alkyne ligand, while decreasing the electron repulsions (see above paragraph).

One reaction involving tungsten alkyne complexes arises from treating the complex W(L)(PhC≡CPh)3 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 W(C3H5)4 adopts a configuration with S4 symmetry (see molecular 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 (σ coordination). One example is TpW(NO)(PMe3)(η2-DMF), 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 (see below).

Cyclic π complexes