Tetrakis(trimethylphosphine)tungsten(II) trimethylphosphinate hydride

Tetrakis(trimethylphosphine)tungsten(II) trimethylphosphinate hydride is the organotungsten compound with the formula W(PMe3)4(η2-CH2PMe2)H. In this complex, the formal oxidation state of W is 2+, and the tungsten center has bonded four trimethylphosphine ligands. The remaining three ligands are a CH2PMe2, bonded in η2 fashion, and hydride. 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.

Other techniques produce a family of related complexes. Thus excess PMe3 and H2 produces W(PMe3)4(η2-CH2PMe2)H in a 3:1 mixture with W(PMe3)5H2, co-condensation produces only W(PMe3)4(η2-CH2PMe2)H, and reduction with Na-K alloy requires vast PMe3 excess and only produces a mixture of W(PMe3)4(η2-CH2PMe2)H and W(PMe3)6.

is thermodynamically favored relative to W(PMe3)6, as described in the equation: <=>> ΔGrxn = –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:

3 atmosphere of a 1:1 CO2/H2 gas mix to produce W(PMe3)4(κ2-O2CO)H2 and a bimetallacycle:

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 °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 °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 °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 (QoxH,HH) and its derivatives 6-methylquinoxaline (QoxMe,HH) and 6,7-dimethylquinoxaline (QoxMe,MeH), W(PMe3)4(η2-CH2PMe2)H forms [κ2-C2-C6RR'H2(NC)2]W(PMe3)4, (η4-C2N2-QoxR,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 °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 °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 °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 °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 °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(CNtBu)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.'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.'s calculations suggest that the mechanism is exergonic, releasing a net 9.2 kcal/mol of energy.