Bioinspired Nucleophilic Attack on a Tungsten-Bound Acetylene: Formation of Cationic Carbyne and Alkenyl Complexes

Abstract

Inspired by the proposed inner-sphere mechanism of the tungstoenzyme acetylene hydratase, we have designed tungsten acetylene complexes and investigated their reactivity. Here, we report the first intermolecular nucleophilic attack on a tungsten-bound acetylene (C₂H₂) in bioinspired complexes employing 6-methylpyridine-2-thiolate ligands. By using PMe₃ as a nucleophile, we isolated cationic carbyne and alkenyl complexes.

Short abstract

We report the first intermolecular nucleophilic attack on a tungsten-bound C₂H₂ in two bioinspired complexes differing only by the oxidation state of the metal center and one ligand. By using PMe₃ as a nucleophile, we isolated cationic carbyne and alkenyl complexes.

The anaerobic bacterium Pelobacter acetylenicus can grow on acetylene (C₂H₂) as a single carbon and energy source. Its utilization is performed by the tungstoenzyme acetylene hydratase (AH), which catalyzes the hydration of acetylene to acetaldehyde.¹⁻⁴ The coordination sphere of the tungsten(IV) center in the active site consists of four sulfur atoms from two molybdopterin cofactors, a thiolate from cysteine, and a water molecule.⁵ Although several experimental^5,6 and computational studies⁷⁻¹⁰ have been carried out to shed light on the reaction mechanism, it remains unclear where C₂H₂ is located during hydration. Because of lower energetic barriers, density functional theory calculations favor a mechanism where C₂H₂ replaces the coordinated water and subsequently undergoes nucleophilic attack by a hydroxide (Scheme 1).⁷⁻¹⁰ Apart from investigations on [Et₄N]₂[WO(mnt)₂] (mnt = maleonitriledithiolate),¹¹⁻¹³ synthetic approaches to elucidate the mechanism of AH have only included systems with tungsten-coordinated C₂H₂.^14,15 In a recent publication, the S,N-bidentate ligand pyridine-2-thiolate (PyS) was introduced to the tungsten(II) center to model the active site of AH as in Inter1 (Scheme 1).¹⁵ In contrast to previously reported structural model compounds,¹⁴ the coordination of a second C₂H₂ and subsequent insertion into the W–N bond occurred, showing that a second coordinated C₂H₂ is sufficiently activated to undergo a nucleophilic attack. A similar behavior was observed in molybdenum and tungsten complexes, where a nucleophilic attack on one of two coordinated hexafluorobut-2-yne moieties took place, yielding a η²-vinyl complex.¹⁶⁻²⁰ Our aim is to facilitate an intermolecular nucleophilic attack by inhibiting the coordination of a second C₂H₂ and insertion as in the PyS system. Therefore, we anticipated the introduction of a methyl group next to the coordinating nitrogen atom in PyS (6-MePyS) so that the metal center is more shielded against the coordination of a second C₂H₂.

Scheme 1. Proposed Inner-Sphere Mechanism of the Hydration of C₂H₂ Performed by AH: Nucleophilic Attack of a Hydroxide on Coordinated C₂H₂⁸.

For preparation of the redesigned tungsten complex [W(CO)(C₂H₂)(6-MePyS)₂] (1), which contains the desired 6-MePyS ligands and only one C₂H₂, a previously developed procedure was modified.^14,15,21 The reaction of [WBr₂(CO)₃(NCMe)₂] with 2.1 equiv of Na(6-MePyS) in CH₂Cl₂ followed by stirring under a C₂H₂ atmosphere for 1 h allowed the isolation of 1 in 86% yield after silica gel filtration. The intermediately formed tricarbonyl complex [W(CO)₃(6-MePyS)₂] was characterized by single-crystal X-ray diffraction analysis (see the Supporting Information). Longer reaction times led to the insertion of a second C₂H₂ into the W–N bond forming [W(CO)(C₂H₂)(HCCH-6-MePyS)(6-MePyS)] (2), as previously observed in the unsubstituted analogue [W(CO)(C₂H₂)(HCCH-PyS)(PyS)].¹⁵ However, the additional methyl group significantly decreases intramolecular insertion because even after 24 h and repeated addition of C₂H₂ only partial conversion is observed. Furthermore, in the absence of additional C₂H₂, 2 reacts reversibly to 1 under the elimination of acetylene or polyacetylene depending on the solvent. Nevertheless, we were able to isolate 2 and unambiguously confirm its structure by single-crystal X-ray diffraction analysis (Figure 1) and by spectroscopic means. In CD₂Cl₂, the sterically hindered C₂H₂ protons of 1 resonate at 13.77 and 12.50 ppm and the carbon atoms at 205.73 and 204.14 ppm, suggesting that C₂H₂ acts as a four-electron donor.²² In ¹H NMR spectra of 2 recorded in CD₂Cl₂, the η²-C₂H₂ protons appear as singlets at 12.90 and 12.03 ppm. The protons of the inserted C₂H₂ couple with each other thus appear as doublets (³J = 10.9 Hz) flanked with ¹⁸³W satellites at 7.61 and 6.89 ppm. IR spectra of 1 and 2 show strong C≡O bands at 1891 and 1897 cm^–1, respectively. Single-crystal X-ray diffraction analyses of 1 and 2 revealed almost identical W–C1, W–C2, and C1–C2 bond distances (Figure 1) compared to the literature values of tungsten(II) acetylene complexes.^23,24 The inserted C₂H₂ in 2 is strongly activated and therefore has more ethylene character with a C–C bond length of 1.349(3) Å compared to 1.310(3) Å in η²-acetylene. Compared to the unsubstituted analogue [W(CO)(C₂H₂)(PyS)₂] with W–N distances of 2.161 and 2.212 Å, 1 exhibits slightly longer bonds [2.197(3) and 2.259(4) Å].¹⁵ As was already observed in [MoO₂(6-MePyS)₂], the nitrogen atom in 6-MePyS is not able to bind to the metal center as tightly as it does in the unsubstituted version because of the methyl group in the ortho position.²⁵

Figure 1.

Molecular structures of 1 (left), 2 (middle), and 3 (right) with probability ellipsoids drawn at the 50% probability level.

Considering that the tungsten center in AH is in the oxidation state +IV, we oxidized 1 with pyridine N-oxide to obtain the tungsten(IV) complex [WO(C₂H₂)(6-MePyS)₂] (3) according to Scheme 2. After filtration to remove insoluble byproducts, 3 was crystallized in 84% yield. IR spectra show one strong band indicative of ν(W=O) at 924 cm^–1.^14,26−28 The ¹H NMR spectrum of 3 in CD₂Cl₂ shows two equally sharp singlets flanked with ¹⁸³W satellites for the C₂H₂ protons at 11.23 and 10.99 ppm. Thus, they are shifted upfield compared to 1. The same trend is observed in the ¹³C NMR spectrum, with resonances at 159.69 and 159.05 ppm being characteristic of a two- or three-electron-donor alkyne.²⁹ Single crystals suitable for X-ray diffraction analysis were grown from a CH₂Cl₂/heptane solution. A molecular view of 3 is displayed in Figure 1. The C1–C2 bond [1.279(2) Å] is slightly shorter than that in 1 [1.306(7) Å], while the W–C bonds [W1–C1 of 2.022(5) Å for 1 vs 2.0693(15) Å for 3 and W1–C2 of 2.055(3) Å for 1 vs 2.1027(15) Å for 3] are essentially longer.

Scheme 2. Synthesis of Complexes 1–3.

With compounds 1 and 3 exhibiting related structures but different oxidation states of the metal center, a nucleophilic attack of PMe₃ on the coordinated C₂H₂ was investigated. Of particular interest to us was the potential formation of the intermediate Inter2 (Figure 1). Treatment of a CH₂Cl₂ solution of 1 with 3 equiv of PMe₃ led to an immediate color change from purple to orange-brown and full conversion of the starting material. An X-ray diffraction study on single crystals grown from a CH₂Cl₂/heptane solution revealed the product to be an ion pair consisting of the tungsten carbyne complex [W(CO)(CCH₂PMe₃)(PMe₃)₂(6-MePyS)]⁺ (4) and a chloride deriving from the solvent (Figure 2). Indeed, a nucleophilic attack of PMe₃ at the coordinated C₂H₂ under the formation of a P–C bond had occurred. However, the attack also leads to the cleavage of one 6-MePyS, which reacts with CH₂Cl₂ to form 2-((chloromethyl)thio)-6-methylpyridine (6-MePySCH₂Cl) and Cl^–. The coordination surrounding of tungsten is completed by two PMe₃ ligands preserving the 18e^– character of the complex and explaining the need for 3 equiv of phosphine. After workup, pure 4 was obtained in 50% yield (Scheme 3).

Figure 2.

Molecular structures of 4 (top) and 5 (bottom) with probability ellipsoids drawn at the 30% (4) and 50% (5) probability levels, respectively.

Scheme 3. Reaction of 1 and 3 with 3 equiv of PMe₃ in CH₂Cl₂ to Yield 4 and 5, Respectively.

The formation of 4 is also confirmed by ¹H NMR spectroscopy, where the CH₂ protons appear at 3.81 ppm as a doublet of triplets (²J_HP = 19.4 Hz; ⁴J_HP = 5.9 Hz) and the methyl groups of the two tungsten-coordinated PMe₃ molecules at 1.44 ppm and those of the carbon-bound PMe₃ at 2.12 ppm. To confirm that the carbyne carbon atom as well as the adjacent methylene group indeed derive from the coordinated C₂H₂, 1 was resynthesized using C₂D₂ to generate [W(CO)(C₂D₂)(6-MePyS)₂] (1D). Upon reaction of 1D with PMe₃, the signal at 3.81 ppm is absent, confirming a reaction of the coordinated C₂H₂ and no participation of CH₂Cl₂.³⁰ While Cummins et al. reported the reaction of a molybdenum-bound (trimethylsilyl)acetylene with Li[BHEt₃] to a η²-vinyl complex and the subsequent formation of carbyne only after heating to 80 °C for several hours, the η²-vinyl intermediate does not seem to be stable at all in our case.³¹ It is only observed in the ¹H NMR spectrum when a reaction of 1 is carried out with less than 3 equiv of PMe₃ directly in a J. Young NMR tube. The ³¹P{¹H} NMR spectrum of 4 shows a triplet at 19.87 ppm for the carbon-bound PMe₃ and a doublet at −17.94 ppm being flanked with ¹⁸³W satellites (¹J_WP = 277.8 Hz) for the tungsten-bound PMe₃. In the ¹³C NMR spectrum, the methylene carbon resonates at 46.80 ppm (d, ¹J_CP = 49.5 Hz). CO (q at 250.98 ppm) and W≡C (dt at 249.22 ppm) carbon atoms give signals of a similar shift and coupling pattern and could only be distinguished by heteronuclear multiple-bond correlation (HMBC) between the carbyne carbon and methylene protons. The W≡C resonance is similar to that of [W(CCH₃)(PMe₃)₄Cl] (253.3 ppm)³² but shifted upfield compared to those of similar compounds like [W(CCH₂Ph)(CO)₂(dppe)Cl] (276.3 ppm),³³ [Tp′(CO)₂WCCH₂W(CO)(C₂Ph₂)Tp] (312 ppm),³⁴ and [Mo(CCH₂B(C₆F₅)₃)(dppe)₂] (347.0 ppm).³⁵ The byproduct 6-MePySCH₂Cl was identified by ¹H NMR and mass spectroscopy. When the reaction was carried out in CD₂Cl₂, the singlet for the methylene protons was absent. To a lesser extent, also bis((6-methylpyridin-2-yl)thio)methane [(6-MePyS)₂CH₂] was found, which was formed by the reaction of another 6-MePyS with the formerly generated 6-MePySCH₂Cl. The W–C bond length of 1.793(4) Å in 4 confirms the triple-bond character, yet falls toward the shorter end of W≡C distances in recently published carbyne complexes (ca. 1.76–1.86 Å).³⁶⁻⁴² The W1–C1–C2 angle of 179.4(4)° is almost perfectly linear, and the C1–C2 distance of 1.486(6) Å indicates a single bond.⁴³ The P1–C2 bond [1.795(4) Å] is slightly longer than the other three P1–C bonds [1.761(5)–1.779(5) Å] but shorter than the remaining P–C bonds [1.807(6)–1.827(4) Å]. The W–S bond [2.6605(16) Å] is considerably longer than those in 1 [W1–S1 2.5834(12) Å; W1–S2 2.4073(12) Å], indicating a strong trans influence of the carbyne ligand.⁴⁴

Treatment of a CH₂Cl₂ solution of 3 with 3 equiv of PMe₃ led to an immediate color change from light yellow to dark green. After workup, the ethenyl complex [WO(CHCHPMe₃)(PMe₃)₂(6-MePyS)]Cl (5; Scheme 3) was isolated in 90% yield as a black-green crystalline powder. Its structure was unambiguously identified by single-crystal X-ray diffraction analysis (Figure 2). Again, a nucleophilic attack at the coordinated C₂H₂ had occurred, forming a P–C bond. In contrast to 4, however, 5 remains with a coordinated ethenyl ligand exhibiting a W–C single bond. Similar reactions were already performed with chromium(0)-bound C₂H₂; however, treatment with PMe₃ predominantly led to an exchange with the coordinated C₂H₂.⁴⁵ The ethenyl protons of 5 are clearly identified by ¹H NMR spectroscopy with resonances at 11.42 and 4.26 ppm and coupling to each other with ³J = 17.5 Hz. Upon reaction of [WO(C₂D₂)(6-MePy_S)₂] (3D) with PMe₃ in CD₂Cl₂, the ethenyl proton resonances are absent. The ³¹P{¹H} NMR spectrum of 5 shows two rather broad singlets at 4.73 and −22.14 ppm, with the latter being flanked with ¹⁸³W satellites. The ethenyl carbon bound to the tungsten center resonates at 222.99 ppm and thus exhibits a significant downfield shift compared to the literature. The other gives a doublet at 96.83 ppm and is thus considerably shifted upfield compared to similar compounds.^46,47 The W–C distance of 2.068(3) Å is shorter than that in rare examples of tungsten ethenyl complexes like [WO₂(CHCH₂)(Tp′)] (2.136 Å)⁴⁶ and [W(Cp)(CHCHC(CH₃)₃)(η²-C(O)NR¹R²)(NO)] (2.161 Å),⁴⁷ while the C1–C2 distance of 1.363(4) Å is slightly longer compared to the aforementioned compounds (1.305 and 1.332 Å). The W1–C1–C2 [135.8(2)°] and C1–C2–P1 [123.4(2)°] angle confirm a slight deviation from sp² hybridization on the carbon atoms. In contrast to 4, the P1–C2 bond [1.745(3) Å] is slightly shorter than the other three P1–C bonds [1.774(3)–1.796(3) Å].

In conclusion, we report the synthesis of tungsten acetylene complexes where intramolecular insertion of C₂H₂ into the W–N bond is prevented by steric adjustment at the ancillary ligand. This allowed the investigation of an intermolecular nucleophilic attack at the solely coordinated C₂H₂ using PMe₃. Starting from the tungsten(II) complex 1, a tungsten ethylidyne complex is formed; hence, the four-electron-donor C₂H₂ converts to the four-electron-donor carbyne when assuming retention of the metal oxidation state. When the carbyne is considered to be a six-electron donor, oxidation to tungsten(IV) is formally taking place. Regarding the oxido ligand as a six-electron donor, the two-electron-donor C₂H₂ in the tungsten(IV) complex 3 converts to a two-electron-donor ethenyl moiety. In both cases, the reactions proceed under preservation of the 18e^– character of the complexes. One 6-MePyS ligand is cleaved under reaction with CH₂Cl₂ to form 6-MePySCH₂Cl and the counterion Cl^– for the cationic tungsten complexes; thus, the presence of CH₂Cl₂ is crucial. We assume that cleavage of the sulfur ligand relieves the charge at tungsten accumulated as a result of the nucleophilic attack. The attack of PMe₃ on the tungsten(IV)-bound C₂H₂ to form an ethenyl ligand resembles the step in which the Inter2 intermediate of the proposed mechanism of AH is formed.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00643.

• Experimental details, syntheses, crystallographic data, and NMR spectra of all compounds (PDF)

Accession Codes

CCDC 2050311–2050316 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Author Contributions

The manuscript was written through contributions of all authors.

Financial support by the Austrian Science Fund (Grant P31583) and NAWI Graz is gratefully acknowledged.

The authors declare no competing financial interest.

Notes

^† Email: madeleine.ehweiner@uni-graz.at.

Notes

^‡ Email: lydia.peschel@uni-graz.at.

Notes

^§ Email: niklas.stix@edu.uni-graz.at.

Notes

^⊥ Email: miljan.corovic@uni-graz.at.

Notes

^∥ Email: ferdinand.belaj@uni-graz.at.