Tetrakis(trimethylphosphine)tungsten(II) trimethylphospinate hydride

Tetrakis(trimethylphosphine)tungsten(II) trimethylphospinate hydride is the organotungsten compound with the formula W(PMe₃)₄(η²-CH₂PMe₂)H. In this complex, four trimethylphosphine ligands are bonded to tungsten. The remaining ligands are hydride and an ²-CH₂PMe₂. In this complex, the oxidation state of W is usually assigned as 2+, denoted W(II). The complex reacts with many simple reagents.

Synthesis

W(PMe₃)₄(η²-CH₂PMe₂)H can be synthesized by treating tungsten hexachloride with trimethylphosphine and sodium. WCl₆ with excess PMe₃ and H₂ produces W(PMe₃)₄(η²-CH₂PMe₂)H in a 3:1 mixture with W(PMe₃)₅H₂. The co-condensation method produces only W(PMe₃)₄(η²-CH₂PMe₂)H, and the Na(K) alloy method produces a mixture of W(PMe₃)₄(η²-CH₂PMe₂)H and W(PMe₃)₆ only under vast excess of PMe₃.

is thermodynamically favored relative to W(PMe₃)₆, as described in the equation:

<chem><=>></chem> ΔG_rxn = –1.73 kcal mol^{−1}

W(PMe₃)₅, a 16 electron, d⁶ complex, has been proposed as an unstable intermediate between W(PMe₃)₄(η²-CH₂PMe₂)H and W(PMe₃)₆. The rate-determining step from W(PMe₃)₆ is dissociation of PMe₃. Isotopic labeling and the NMR studies indicate that W(PMe₃)₄(η²-CH₂PMe₂)H is fluxional such that all methyl groups are equivalenced.

Reactivity

Small molecule substrates: H₂, CO, N₂, CO₂, SiH₄

reacts with H₂ to give W(PMe₃)₅(H)₂ and W(PMe₃)₄(H)₄. With HD, W(PMe₃)₄(η²-CH₂PMe₂)H converts to W(PMe₃)₅HD or W(PMe₃)₄(η²-HD) in PMe₃ solvent.

adds N₂ to give W(PMe₃)₅(N₂).

In 2 atmospheres of CO, gives fac-W(PMe₃)₃(CO)₃.

reacts with 3 atmosphere of 1:1 CO₂/H₂ gas mix to produce W(PMe₃)₄(κ²-O₂CO)H₂ and a bimetallacyclic compound.

W(PMe₃)₄(η²-CH₂PMe₂)H reacts with H₂ to give W(PMe₃)₅(H)₂ and W(PMe₃)₄(H)₄. With HD, W(PMe₃)₄(η²-CH₂PMe₂)H converts to W(PMe₃)₅HD or W(PMe₃)₄(η²-HD) in PMe₃ solvent.

The reaction of with SiH₄ yields W(PMe₃)₄(SiH₃)₂H₂. Organosilanes give a variety of products.

Acids

HBF₄ reacts with in ether to give [W(PMe₃)₄(OH)₂H₂][BF₄]₂. Several derivatives are known: W(PMe₃)₄H₄, W(PMe₃)₄F₂H₂, and [W(PMe₃)₄F(H₂O)H₂]F.

Hydrogen chloride reacts as follows:

The corresponding dibromide and diiodide form by salt metathesis. Carboxylic acid reacts with to give hydride complexes, e.g., .

π-systems

In 1-2 atmospheres of ethylene at room temperature, W(PMe₃)₄(η²-CH₂PMe₂)H reacts to form trans-W(PMe₃)₄(η²-C₂H₄)₂.

Upon subjecting W(PMe₃)₄(η²-CH₂PMe₂)H to 2 atmospheres of ethylene at 60&nbsp;°C in the presence of light petroleum for a week, W(PMe₃)₂(η²-C₄H₆)₂ is produced. W(PMe₃)₄(η²-CH₂PMe₂)H will ligate to buta-1,3-diene when the latter is in vast excess and in the presence of light petroleum at 50&nbsp;°C to make the same product as ethylene. W(PMe₃)₂(η²-C₄H₆)₂ produces yellow crystals.

Much like with ethylene, propylene (2 atm) also forms C-C bonds upon reaction with W(PMe₃)₄(η²-CH₂PMe₂)H and light petroleum at 70&nbsp;°C. The resultant product is W(PMe₃)₃[η-CH₂=C(Me)CH=C(cis-Me)H]H₂.

W(PMe₃)₄(η²-CH₂PMe₂)H, upon reaction with cyclopentadiene in light petroleum for five days, binds cyclopentadiene and dissociates two PMe₃ ligands to generate W(η⁵-C₅H₅)(PMe₃)₃H, W(PMe₃)₄H₄, W(PMe₃)₃H₆, and trace W(η⁵-C₅H₅)₂H₂. The crystals of this mixture are yellow and air-sensitive.

In the reaction with quinoxaline (Qox^H,HH) and its derivatives 6-methylquinoxaline (Qox^Me,HH) and 6,7-dimethylquinoxaline (Qox^Me,MeH), W(PMe₃)₄(η²-CH₂PMe₂)H forms [κ²-C₂-C₆RR'H₂(NC)₂]W(PMe₃)₄, (η⁴-C₂N₂-Qox^R,R'H)W(PMe₃)₃H₂ (vide infra), and W(PMe₃)₄H₂ (R,R'=H, Me), wherein the first listed product is generated from C-C bond cleavage to form two W=C=B bond motifs. The latter two products are hypothesized to be formed from H₂ generated from the C-C bond cleavage.

Methanol

W(PMe₃)₄(η²-CH₂PMe₂)H, upon addition of methanol in an ethylene atmosphere, can form W(PMe₃)₄(CO)H₂.

W(PMe₃)₄(η²-CH₂PMe₂)H, upon MeOH ligation in an η²-fashion, dissociates PMe₃ and forms W(PMe₃)₄(η²-CH₂O)H₂. This complex undergoes many similar reaction pathways as its precursor retron.

Tungsten-tetrel multiple bonding

W(PMe₃)₄(η²-CH₂PMe₂)H, in pentane and at −20&nbsp;°C, reacts with Ge(C₆H₃-2,6-Trip₂)Cl (Trip=C₆H₂-2,4,6-ⁱPr₃, ⁱPr=CH(CH₃)₂) to dissociate PMe₃ and generate trans-[Cl(H)(PMe₃)₃W{=Ge(C₆H₃-2,6-Trip₂)(CH₂PMe₂)}]. This green, air-sensitive complex can heated at 50&nbsp;°C with toluene or left in ambient conditions with either toluene or pentane to yield the Ge≡C bond-containing complex, trans-[Cl(PMe₃)₄W≡Ge-C₆H₃-2,6-Trip₂]. This brown, air-sensitive complex can also be directly generated from W(PMe₃)₄(η²-CH₂PMe₂)H by heating with toluene and Ge(C₆H₃-2,6-Trip₂)Cl at 50&nbsp;°C. trans-[Cl(PMe₃)₄W≡Ge-C₆H₃-2,6-Trip₂] is, in turn, also a retron for further chemistry by substitution of the labile chloride ligand. Upon addition of lithium iodide in ether, chloride is substituted for iodide, forming red-brown trans-[I(PMe₃)₄W≡Ge-C₆H₃-2,6-Trip₂]. With lithium dimethylamine in THF, the chloride is substituted for a hydride, generating red-brown, air-sensitive trans-[H(PMe₃)₄W≡Ge-C₆H₃-2,6-Trip₂]. With potassium thiocynate in THF, chloride is substituted for thiocynate, forming dark brown trans-[(NCS)(PMe₃)₄W≡Ge-C₆H₃-2,6-Trip₂].

W(PMe₃)₄(η²-CH₂PMe₂)H with 0.5 equivalent of {Pb(Trip)Br₂}₂ and in toluene at 50&nbsp;°C produces (PMe₃)₄BrW{≡Pb(C₆H₃-2,6-Trip₂)}. Upon addition of lithium dimethylamine in THF, Br(PMe₃)₄W{≡Pb(C₆H₃-2,6-Trip₂)} converts to brown, air-sensitive H(PMe₃)₄W{≡Pb(C₆H₃-2,6-Trip₂)}. Alternatively, W(PMe₃)₄(η²-CH₂PMe₂)H, with 0.5 equivalent of {Pb(Trip)NMe₂}₂ (produced from the reaction of {Pb(Trip)Br₂}₂ with lithium dimethylamine) in toluene and at 80&nbsp;°C, also produces H(PMe₃)₄W{≡Pb(C₆H₃-2,6-Trip₂)}.

Tungsten-chalcogenide multiple bonding

W(PMe₃)₄(η²-CH₂PMe₂)H forms a variety of brightly colored complexes with terminal W=E bonds (E =2.718 S, Se, Te). H₂Se gives W(PMe₃)₄Se(H)₂, which features a terminal selenide ligand and two hydride ligands. It reacts with H₂S and H₂Se to give W(PMe₃)₄(Se)(S) and W(PMe₃)₄(Se)2, respectively. In related behavior, H₂S reacts with W(PMe₃)₄(η²-CH₂PMe₂)H to give W(PMe₃)₄(SH₂)H₂. The complex can be dehydrogenated to give trans-W(PMe₃)₄S₂. trans-W(PMe₃)₄Te₂, a rare complex with a terminal telluride ligand can be produced as well. Since H₂Te is not easily available, elemental Te in the presence of PMe₃ was used, implicating a role for the phosphine telluride Me₃P=Te.

The dichalcogenides W(PMe₃)₄(E)₂ (E =S, Se, Te) reversibly bind aldehydes to give W(PMe₃)₂E₂(η²-OCHR) (R = H, Ph). Related ^tBuNC complexes have also been produced, e.g., trans, trans, trans-W(PMe₃)₂(CN^tBu)₂Se₂.

Hydrodesulfurization

When treated with thiophenes, benzothiophene, and dibenzothiophene, W(PMe₃)₄(η²-CH₂PMe₂)H inserts into the C-S bonds. All of these complexes react further with H₂, resulting in hydrogenolysis of the C-S bonds. Such reactions are reminiscent of W-catalyzed hydrodesulfurization, a major process in refining petroleum.

C-H bond activation

W(PMe₃)₄(η²-CH₂PMe₂)H reacts with phenols forming four- and five-membered oxometallacycles. With PhOD, the first step is the deuterolysis of the W–C bond, forming W(PMe₃)₄(PMe₂CH₂D)(OPh)H. These phenoxide complexes are further reactive with H₂

More complex phenols, e.g., 2,2′-methylenebis(4,6-dimethylphenol) and calixarenes, are also reactive toward W(PMe₃)₄(h²-CH₂PMe₂)H. .

Alkylidene generation

Upon the addition of bromobenzene, iodobenzene, or para-bromotoluene, W(PMe₃)₄(η²-CH₂PMe₂)H form the cation [W(PMe₃)₄(η²-CHPMe₂)H]⁺ with the corresponding halide anion.

Theoretical work

C-C bond activation mechanism

The novel activation of the aromatic C-C bond in QoxH by W(PMe₃)₄(η²-CH₂PMe₂)H under relatively mundane conditions inspired mechanistic theorizations. In their original publication, Sattler and Parkin suggested a mechanism in QoxH first acts as an L-type ligand from the N lone pair. The Qox ligand then changes its bonding behavior, with the bonding atoms shifting counterclockwise per Qox's numbering scheme. Upon reaching η²-C₂ binding, the complex undergoes reductive elimination of its two hydrides to form H₂. Finally, the complex cleaves its C-C bond to form the two W=C bonds.

Miscione and coworkers – using the B3LYP functional with energy-adjusted pseudopotential and DZVP basis sets — provided the first computational study of the proposed mechanism, wherein they provided a few pathways, building on Sattler and Parkin's work. The first pathway suggests that the hydride moves towards the tucked-in alkyl ligand to form W(PMe₃)₅ before QoxH binds. Upon the loss of a PMe₃ ligand, Qox can then bond in an η²-N,C fashion, forming a hydride which subsequently moves to be trans to Qox. In the second pathway, PMe₃ occurs first, followed by QoxH's ligation. Then, the agostic interaction is transformed into a standard PMe₃ L-type ligand to join the first pathway in following the original proposed mechanism. The third pathway diverges from the first pathway at W(PMe₃)₅, wherein Qox instead interacts at the 2-H site before either bonding in a κ¹-C fashion or losing a PMe₃ to interact with both the 2-H and 3-H sites. Both intermediates then form (along with the loss of PMe₃ in the former complex) a κ¹-C complex with a 3-H interaction, before rejoining the original mechanism at the η²-C₂ complex. Of these paths, path 2 is the least favored due to the ~30–40 kcal/mol energy barrier in breaking the agnostic interaction. Paths 1 and 3 are reported to be of roughly equal thermodynamic favorability with energy barriers mostly around 10–20 kcal/mol, until the maximum of the energy surface, the three-membered ring-containing η²-C₂ intermediate (33.7 kcal/mol higher than W(PMe₃)₄(η²-CH₂PMe₂)H). Miscione and coworker's results substantiate Sattler and Parkin's hypothesis that the ring strain in the η²-C₂ complex facilitates the C-C bond cleavage. They also report the reaction as being slightly net endergonic by 3.3 kcal/mol.

Liu et al. — using the B3LYP* functional with the LANL2DZ and 6-31G(d,f) basis sets – proposed two mechanisms based on Sattler and Parkin's original proposal. Both pathways start by dissociating both equatorial PMe₃ ligands in the beginning before binding QoxH and generating a κ¹-N QoxH ligand. It then switches to η²-N,C-Qox with a hydride which must move to be trans to Qox. κ¹-N Qox then transitions to κ¹-C Qox, followed by the transformation into η²-N,C Qox. Dissociation of PMe₃ follows suite. Liu et. al.<nowiki/>'s mechanism suggests that the C-C bond is broken at this stage, with a two electron oxidation of tungsten to form a double bond to the already bound carbon and a single bond to the other. The latter carbon's C-H bond forms an agostic interaction with tungsten to account for the lost electron density. The complex then gains its second W–C bond along with a hydride ligand. At this point, the two pathways branch. In the first pathway, an axial PMe₃ moves down to the equatorial plane along with loss of the W=C bonds and reformation of the C-C bond, allowing another PMe₃ to associate and rejoining the original mechanism at the dihydride-containing η²-C₂ Qox complex. The second pathway sees the two hydride ligands move such that they are cis to the W=C bonds before undergoing reductive elimination. PMe₃ then associates, forming the final complex. Liu et. al. claims that the final step to C-C bond cleavage is the concerted, not stepwise, elimination of H₂ and formation W=C bonds. Per their calculations, Sattler and Parkin's mechanism spans a range of 42.0 kcal/mol energy range, in large part due to the aforementioned concerted step. The second pathway was calculated to have energy barriers of ~10 kcal/mol in all steps post-branching, leaving the second PMe₃ dissociation as the highest energy barrier in the mechanism. Liu et al.<nowiki/>'s calculations suggest that the mechanism is exergonic, releasing a net 9.2 kcal/mol of energy.

Li and Yoshizawa – using the B3LYP* functional with the LANL2TZ(f) and 6-31G(d,f) basis sets – also proposed two mechanisms which start with ligand dissociations. Both mechanisms start with the dissociation of an equatorial PMe₃ ligand, before diverging. The first pathway sees the dissociation of the second equatorial PMe₃, leaving the agostic interaction and the hydride. This complex then binds to QoxH, generating a κ¹-N QoxH ligand. Qox then changes its binding to the η²-N,C fashion, as well generating a hydride bond, before breaking the agostic interaction to form a PMe₃ L-type interaction. Another PMe₃ ligates before Qox switches to η²-C₂-type bonding as well as an H₂ ligand. H₂ dissociation, followed by C-C bond cleavage, then leads to the final product. In the second pathway, the agostic bond is broken for a PMe₃ L-type interaction after the first PMe₃ dissociation. QoxH then binds in a κ¹-N fashion before changing to η²-N,C with a hydride bond to tungsten and rejoining pathway 1. Li and Yoshizawa concluded that, between their pathways, pathway 1 is the most thermodynamically favorable. The reformation of PMe₃ after immediately after the first PMe₃ dissociation in pathway 2 has a barrier of 26.3 kcal/mol relative to W(PMe₃)₄(η²-CH₂PMe₂)H. In contrast, the energy maximum of pathway 1 is from the H₂ dissociation step shared by both pathways. Overall, Li and Yoshizawa's work suggest that the C-C bond mechanism is exergonic overall, with the product being 18.5 kcal/mol lower in energy relative to W(PMe₃)₄(η²-CH₂PMe₂)H.

η⁴-C₂N₂ quinoxaline binding

The η⁴-C₂N₂-QoxH ligand is a novel binding behavior discovered from the reaction of W(PMe₃)₄(η²-CH₂PMe₂)H with QoxH. Miscione et al. and Liu et al. also investigated these mechanisms. The former group suggests that upon formation of W(PMe₃)₅ (vide infra), the tungsten undergoes the oxidative addition of H₂, forming hydride bonds. Then, one PMe₃ ligand is dissociated, allowing QoxH to bind, first in a η²-N,C fashion before switching to the final η⁴-C₂N₂ fashion via a 7.3 kcal/mol rearrangement energy barrier. The latter group suggests that one PMe₃ first dissociates, followed by the oxidative addition of H₂, forming an ML₆ complex. One of the axial PMe₃ ligands is lost, allowing QoxH to bind, forming the η⁴-C₂N₂-QoxH ligand. Both sets of calculations agree that the mechanism is net exergonic, with the product being ~20 kcal/mol lower in energy than W(PMe₃)₄(η²-CH₂PMe₂)H.

See also

• Hexakis(trimethylphosphine)tungsten

References