Hexa(tert-butoxy)ditungsten(III)

Hexa(tert-butoxy)ditungsten(III) is a coordination complex of tungsten(III). It is one of the homoleptic alkoxides of tungsten. A red, air-sensitive solid, the complex has attracted academic attention as the precursor to many organotungsten derivatives. It an example of a charge-neutral complex featuring a W≡W bond, arising from the coupling of a pair of d3 metal centers.

Synthesis
Hexa(tert-butoxy)ditungsten(III) was first discovered by M. H. Chisholm and M. Extine in 1975. They synthesized hexa(tert-butoxy)ditungsten(III) by reacting tungsten(III) dialkylamides with t-BuOH in organic solvents. They also found that W2(O-t-Bu)6 reacts with carbon dioxide in toluene to form green W2(O-t-Bu)4(O2CO-t-Bu)2 under room temperature. In CO2, these compounds can be separated from cooled toluene purely. Without the presence of CO2, W2(O-t-Bu)4(O2CO-t-Bu)2 is regenerated into W2(O-t-Bu)6 reversibly.

W2(O-t-Bu)6 can also be synthesized by using NaW2Cl7(THF)5 as reactant in THF with addition of NaO-t-Bu under ambient temperature for 18 hours. After the reaction, the solvent is removed, and it becomes a red slurry. Further cooling (-35oC) and decantation or vacuum filtration separate red crystalline W2(O-t-Bu)6. The salt metathesis reaction from the THF complex of ditungsten heptachloride is as follows:

NaW2Cl7(THF)5 + 6 NaO-t-Bu → W2(O-t-Bu)6 + 7 NaCl + 5 THF

Characteristics
These needle-like red crystals are highly unstable under oxygen and water and can be dissolved in most organic solvents such as diethyl ether and pentane. They are found in dimers with two tungsten(III) bond with each other to form triple bonds. These two W(III) form pseudotetrahedral center and adopt a staggered, ethane-like conformation, similar to its dimolybdenum analogue. The structure of the compound was investigated by Chisholm and his team using single crystal X-ray diffraction. The investigation was performed in a C-centered monoclinic crystal. In C2/c space group, there is one half inversion center molecule and one whole molecule in general position. There are several orientations for each position which leads to the length of WW ranging from 1.74 to 2.53 Å. The orientation of t-butyl groups in each W are one direct away from WW (distal) and two over WW (proximal). This arrangement had been calculated as the best to minimize the steric repulsing effect.

This compound can be decomposed into WO2, t-BuOH, and isobutylene, with trace amount of water under 200oC. This compound can react easily with alkynes or nitriles to generate RC≡W(O-t-Bu)3 or both RC≡W(O-t-Bu)3 and N≡W(O-t-Bu)3. With excess amount of nitrile, only N≡W(O-t-Bu)3 are formed along with RC≡CR. RC≡W(O-t-Bu)3 is important catalyst for alkyne metathesis while N≡W(O-t-Bu)3 is a catalyst for nitrogen exchange of nitriles. The C≡W bond in RC≡W(O-t-Bu)3 was concluded to behave as polarized C(-)≡W(+). Thus, the metathesis catalytic reaction starts with tungsten as electrophilic attacker to attack acetylene and followed by alkylidyne carbon as nucleophilic attacker to attack acetylenic carbon atom.

Reactivity
Carbon monoxide adds to W2(O-t-Bu)6 to form W2(O-t-Bu)6(CO). The carbonyl group is a bridging ligand. This compound can further react with i-PrOH to generate W4(μ-CO)2(O-i-Pr)12.
 * W2(O\stert\sBu)6 + CO → W2(O\stert\sBu)6CO

Alkynes
C≡C bonds are cleaved by hexa(tert-butoxy)ditungsten(III) giving a pair of tungsten alkylidyne complexes: Although the reaction applies to many alkynes, PhC≡CPh or Me3SiC≡CSiMe3 do not react.
 * W2(O\stert\sBu)6 + 2 RC≡CR → 2 RC\tW(O\stert\sBu)3 (R = Me, Et, Si(CH3)3)

This reaction includes an alkyne adduct on the μ-perpendicular site to increase both the length of WW bonds and CC (alkyne) bonds. This intermediate can be analogue as a dimetallatetrahedranes and further react into RC≡W(O-t-Bu)3 with internal redox reaction. The resulting RC≡W(O-t-Bu)3 is a catalyst for metathesis reactions. RC≡W(O-t-Bu)3 can react with normal alkynes for metathesis reactions and also with terminal alkynes for both metathesis reactions and polymerizations.

Besides simple metathesis reactions, W2(O-t-Bu)6 also reacts with 3-hexyne in a 1:1 molar ratio to form a triangular tritungsten complex compound [W3(O-t-Bu)5(μ-O)(μ-CEt)O]2. This reaction takes about 3 days under 75-80 oC in toluene. This reaction has a two steps mechanism; first is the C≡C and W≡W metathesis reaction and follow by formal addition of carbyne (W≡C) to alkoxide (W2):


 * W2(O-t-Bu)6 + RC≡CR → 2[RC≡W(O-t-Bu)3]

W2(O-t-Bu)6 + RC≡W(O-t-Bu)3 → W3(O-t-Bu)5(μ-O)(μ-CEt)O → [W3(O-t-Bu)5(μ-O)(μ-CEt)O]2 W2(O-t-Bu)6 also reacts with EtC≡CC≡CEt to form (t-Bu-O)3W≡CC≡W(O-t-Bu)3:

W2(O-t-Bu)6 + EtC≡CC≡CEt → (t-Bu-O)3W≡CC≡W(O-t-Bu)3 + EtC≡CEt

This compound, however, does not act as a metathesis catalyst.

Nitriles
Similar to the reaction with alkynes, W2(O-t-Bu)6 cleaves nitriles to give the alkyldyne and nitride:
 * W2(O-t-Bu)6 + RC≡N → RC≡W(O-t-Bu)3 + N≡W(O-t-Bu)3

Although W2(O-t-Bu)6 reacts with nitriles, it doesn’t react with nitrogen (N≡N).

When C≡C and C≡N bond both exist, W2(O-t-Bu)6 reacts more rapidly with C≡N than C≡C bond. Here’s an example of W2(O-t-Bu)6 reacting with EtC≡CCN in the presence of quinuclidine:

W2(O-t-Bu)6 + EtC≡CCN + 12quin → EtC≡CC≡W(O-t-Bu)3(quin) + N≡W(O-t-Bu)3

On the other hand, the metathesis catalyst MeC≡W(O-t-Bu)3 reacts more rapidly with C≡C than C≡N bond. Similar reaction with EtC≡CCN and quinuclidine produce different product:

MeC≡W(O-t-Bu)3 + EtC≡CCN + 12quin → NCC≡W(O-t-Bu)3(quin) + EtC≡CMe

Other reactions
W2(O-t-Bu)6 cleaves nitrosobenzene to give [W(O-t-Bu)2(NPh)]2(μ-O)(μ-O-t-Bu)2.

Allenes W2(O-t-Bu)6 can also react with allene (H2C=C=CH2) for adduction. In a ratio of 1:1, allene adduct on W2 to form a v-shape bridge structure:

W2(O-t-Bu)6 + H2C=C=CH2 → W2(O-t-Bu)6(C3H4)

This compound is synthesized under 0oC in hexane and crystallized under -72oC. It decomposes easily in solution at 0oC and in crystalline state at ~25oC but very stable at ~20oC. The bridging allene is parallel to the W2 bond. In a ratio of 1:2, the additional allene will bind to single metal center as typical bonding:

W2(O-t-Bu)6(C3H4) + 2H2C=C=CH2 → W2(O-t-Bu)6(C3H4)2 The product of 1:1 adduction can further react with carbon monoxide to form a similar structure to 1:2 adduction but adducted with carbon monoxide instead of allene:

W2(O-t-Bu)6(C3H4) + 2CO → W2(O-t-Bu)6(C3H4)(CO)2

Reaction using methylallene (MeHC=C=CH2) instead of allene is also feasible forming similar structures.