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Tetrakis(trimethylphosphine)tungsten(II) trimethylphospinate hydride

Chemical compound From Wikipedia, the free encyclopedia

Tetrakis(trimethylphosphine)tungsten(II) trimethylphospinate hydride
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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.[1]

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Synthesis

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W(PMe3)4(η2-CH2PMe2)H can be synthesized by treating tungsten hexachloride with trimethylphosphine and sodium.[2][3] WCl6 with excess PMe3 and H2 produces W(PMe3)4(η2-CH2PMe2)H in a 3:1 mixture with W(PMe3)5H2.[4] 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.[5]

W(PMe3)4(η2-CH2PMe2)H is thermodynamically favored relative to W(PMe3)6, as described in the equation:[5][6]

W(PMe3)6 W(PMe3)4(η2−CH2PMe2)H + PMe3 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.[6] Isotopic labeling and the NMR studies indicate that W(PMe3)4(η2-CH2PMe2)H is fluxional such that all methyl groups are equivalenced.[2][4]

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Reactivity

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

W(PMe3)4(η2-CH2PMe2)H reacts with H2 to give W(PMe3)5(H)2 and W(PMe3)4(H)4.[7] With HD, W(PMe3)4(η2-CH2PMe2)H converts to W(PMe3)5HD or W(PMe3)4(η2-HD) in PMe3 solvent.[8]

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Reaction of W(PMe3)4(η2-CH2PMe2)H with H2

W(PMe3)4(η2-CH2PMe2)H adds N2 to give W(PMe3)5(N2).[7]

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Reaction of W(PMe3)4(η2-CH2PMe2)H with N2

In 2 atmospheres of CO, W(PMe3)4(η2-CH2PMe2)H gives fac-W(PMe3)3(CO)3.[3]

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Reaction of W(PMe3)4(η2-CH2PMe2)H with CO

W(PMe3)4(η2-CH2PMe2)H reacts with 3 atmosphere of 1:1 CO2/H2 gas mix to produce W(PMe3)4(κ2-O2CO)H2 and a bimetallacyclic compound.[9][10]

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Reaction of W(PMe3)4(η2-CH2PMe2)H with CO2

W(PMe3)4(η2-CH2PMe2)H reacts with H2 to give W(PMe3)5(H)2 and W(PMe3)4(H)4.[7] With HD, W(PMe3)4(η2-CH2PMe2)H converts to W(PMe3)5HD or W(PMe3)4(η2-HD) in PMe3 solvent.[8]

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Reaction of W(PMe3)4(η2-CH2PMe2)H with H2

The reaction of W(PMe3)4(η2-CH2PMe2)H with SiH4 yields W(PMe3)4(SiH3)2H2.[7] Organosilanes give a variety of products.[11]

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Reactions of W(PMe3)4(η2-CH2PMe2)H with silanes

Acids

HBF4 reacts with W(PMe3)4(η2-CH2PMe2)H in ether to give [W(PMe3)4(OH)2H2][BF4]2.[7] Several derivatives are known: W(PMe3)4H4, W(PMe3)4F2H2, and [W(PMe3)4F(H2O)H2]F.[3][7]

Hydrogen chloride reacts as follows:

W(PMe3)4(η2-CH2PMe2)H + 2 HCl → W(PMe3)4Cl2(H)2 + PMe3

The corresponding dibromide and diiodide form by salt metathesis.[12] Carboxylic acid reacts with W(PMe3)4(η2-CH2PMe2)H to give hydride complexes, e.g., W(PMe3)4(O2CR)H.[13]

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Reactions of W(PMe3)4(η2-CH2PMe2)H with acids

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.[3]

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.[7] 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.[4]

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.[7]

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.[4][8] The crystals of this mixture are yellow and air-sensitive.[4]

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Reactions of W(PMe3)4(η2-CH2PMe2)H with π-system containing hydrocarbons

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.[14]

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Reaction of W(PMe3)4(η2-CH2PMe2)H with quinoxalines

Methanol

W(PMe3)4(η2-CH2PMe2)H, upon addition of methanol in an ethylene atmosphere, can form W(PMe3)4(CO)H2.[15]

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.[16]

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Reaction of W(PMe3)4(η2-CH2PMe2)H with methanol

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].[17]

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Reactions of W(PMe3)4(η2-CH2PMe2)H with germanium

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)}.[18]

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Reactions of W(PMe3)4(η2-CH2PMe2)H with lead

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.[19][20]

The dichalcogenides W(PMe3)4(E)2 (E =S, Se, Te) reversibly bind aldehydes to give W(PMe3)2E2(η2-OCHR) (R = H, Ph).[21][19] [21][19][22][20][23] Related tBuNC complexes have also been produced, e.g., trans, trans, trans-W(PMe3)2(CNtBu)2Se2.[21][19]

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Reactions of W(PMe3)4(η2-CH2PMe2)H with sulfide
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Reactions of W(PMe3)4(η2-CH2PMe2)H with telluride

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.[24]

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Dehydrosulfurization reactions of W(PMe3)4(η2-CH2PMe2)H with thiophenes

C-H bond activation

W(PMe3)4(η2-CH2PMe2)H reacts with phenols forming four- and five-membered oxometallacycles.[25] 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[26]

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Reactions of W(PMe3)4(η2-CH2PMe2)H with phenols

More complex phenols, e.g., 2,2′-methylenebis(4,6-dimethylphenol) and calixarenes, are also reactive toward W(PMe3)4(h2-CH2PMe2)H.[27]

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Reaction of W(PMe3)4(η2-CH2PMe2)H with biphenol

.[28]

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Reaction of W(PMe3)4(η2-CH2PMe2)H with para-tert-butylcalix[4]arene

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.[29]

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Reaction of W(PMe3)4(η2-CH2PMe2)H with aryl halide to generate an alkylidene
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Theoretical work

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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.[14]

Miscione and coworkers – using the B3LYP functional with energy-adjusted pseudopotential[30] 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).[31] 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.[14][31] They also report the reaction as being slightly net endergonic by 3.3 kcal/mol.[31]

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Theoretical mechanisms (transition states not shown) for the cleavage of quinoxaline's aromatic C=C bond by W(PMe3)4(η2-CH2PMe2)H. Sattler and Parkin's original proposed mechanism is highlighted in red.

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.[32]

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.[33]

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.[31] 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.[32] 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.[31][32]

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Theoretical mechanisms (transition states not shown) for the binding of quinoxaline to W(PMe3)4(η2-CH2PMe2)H in an η4-C2N2 fashion.

[14][17][18][24]

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