Introduction

Since the pioneering work of Fischer, Schrock, and et al.1,2, transition metal alkylidyne complexes, commonly known as carbyne complexes, have found significant applications in synthetic chemistry and material science3,4,5,6,7,8,9. The exploration of abundant-metal catalysis has spurred considerable interest in synthetic iron complexes with metal-carbon bonds10,11. Previous studies have emphasized the importance of stabilizing the carbyne moiety at the monomeric iron center, typically achieved through the incorporation of heteroatoms in the Fe≡CXR form (where X = O or NR’)12,13,14,15,16. This stabilization often involves the removal of an –OR group from the carbene group of the Fe=C(OR)R’ complex or through electrophilic addition to a cyanide ligand. Notably, Peters reported high valent FeIV ≡ CCH3 and [FeV ≡ CCH3]+ complexes (I, Fig. 1a) resulting from an uncommon sequential reductive protonation process of an iron-acetylene compound17.

Fig. 1: Examples of iron-based carbyne complexes.
figure 1

a Selected examples of iron carbyne complexes. b Transformation of acetylene to carbyne at a Fe-Mo platform.

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In addition to monomeric iron carbyne complexes, Casey and co-workers have synthesized the bridging-methylidyne diiron complex II by performing hydride abstraction from the methylene precursor18,19,20. Marchetti and co-workers have developed a convenient two-step synthetic route using [Cp2Fe2(CO)4] to access diiron bridging amino-carbyne compounds III21,22,23. These complexes have demonstrated intriguing structural and reactivity features, including reactions with alkenes and diazo compounds, as well as the insertion of alkynes into the iron-carbon bonds. In the pursuit of modeling the structure and functionalities of nitrogenases24,25,26,27, recent advancements in iron carbyne chemistry have led to the synthesis of iron-based heteronuclear cubic clusters (IV)28 and the open-shell diiron hydride complex (V)29, featuring bridging carbyne ligands by the group of Agapie.

Bimetallic cooperation strategy is a powerful tool for exploring bond cleavage and formation in inorganic and organometallic chemistry30. Given the crucial role of molybdenum in modulating the electronic properties of the iron–molybdenum cofactor (FeMoco)31,32, bioinspired Fe-Mo complexes are highly promising for the activation of small molecules31,32,33. Particularly, FeMoco effectively facilitate the reduction of various carbon-based unsaturated substrates such as acetylene (C2H2) and carbon monoxide, utilizing protons and electrons (H+/e-)34,35. Investigating the binding and activation of C2H2 with bimetallic Fe-Mo complex could yield a variety of intriguing metal-carbon intermediates such as vinyl and carbyne complexes, offering valuable insights into the interactions and transformations of unsaturated hydrocarbons with abundant metals17,36,37,38,39,40.

Our previous studies demonstrated that molybdenum-acetylene complex Cp*Mo(η2-C2H2)(1,2-PPh2C6H4S) transforms to a cationic vinylthioether complex upon protonation41, while activation of C2H2 by Cp*Fe(1,2-Cy2PC6H4S) results in the formation of ethenylidene and carbene moieties40. In this study, we introduce a bimetallic strategy for the conversion of C2H2 into carbyne (Fig. 1b). By integrating Cp*Fe and Cp*Mo moieties within an unsymmetrical phosphino-thiolate coordination sphere, the cationic FeMo complex, [1-C2H2]BAr4F, binds C2H2 crosswise the Fe---Mo vector in a double side-on fashion. This coordination mode allows for the transformation of C2H2 to a μ-η1:η2-CH = CH2 unsaturated hydrocarbon fragment, which undergoes H-migration catalyzed by H2O to form a μ-carbyne moiety. Further investigations revealed that protonation of either the vinyl or the neutral carbyne compound affords a hydrido carbyne complex.

Results

Bridging acetylene complex

Upon treating a THF solution of Cp*Mo(η2-C2H2)(1,2-PPh2C6H4S)41 (Mo-C2H2) with 0.5 equivalent of [Cp*Fe(μ-Cl)]2 and one equivalent of NaBAr4F at −30 °C, a distinct color change from olive green to deep blue occurred (Fig. 2). The resulting product, [1-C2H2]BAr4F, was isolated in an 88% yield and thoroughly characterized by NMR, Raman spectroscopy, and single crystal X-ray diffraction. The 31P NMR spectrum of [1-C2H2]BAr4F exhibits a singlet signal at δ 79.5, upfield shifted compared to δ 97.5 observed for the parent molybdenum-acetylene adduct. In contrast to the broad 1H resonances at δ 9.48 and 10.49 for the η2-C2H2 ligand in the Mo-C2H2 complex, the two acetylic protons in [1-C2H2]+ display two sharp peaks at δ 9.55 (d, 3JP-H = 13.2 Hz) and 10.48 (s). According to the 1H–13C HSQC studies (Supplementary Fig. 9), the acetylene 13C signals also shifted upfield to δ 162.0 and 160.2 in comparison to Mo-C2H2 (δ 190.3 and 184.9). In the Raman spectrum, [1-C2H2]+ exhibits a sharp νC≡C band at 1470 cm−1, representing a significant red shift compared to free C2H2 (1974 cm−1)42,43,44 and the Mo-C2H2 complex (1507 cm−1, Supplementary Fig. 1). These findings imply a substantial weakening of the C ≡ C triple bond in [1-C2H2]+, suggesting that the heteronuclear constituent plays a more significant role in C≡C bond activation.

Fig. 2
figure 2

Synthesis and reduction of the C2H2-bridged Fe–Mo complex.

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Crystallographic analysis of [1-C2H2]+ revealed that the Cp*Mo(1,2-Ph2PC6H4S) and Cp*Fe+ fragments are linked by the sulfur of the phosphino–thiolate ligand (Fig. 3). A C2H2 molecule bound crosswise the Fe–Mo vector through a μ-η2:η2-coordination fashion with torsion angle of ~85° between the C–C and Fe–Mo vectors. The Fe–Mo distance of 2.4718(6) Å falls within the sum of covalent radii of Fe (low spin, 1.32 Å) and Mo (1.54 Å)45. In comparison to free C2H2 (dC≡C = 1.181(7) Å)44, the C ≡ C bond length in [1-C2H2]+ is elongated to 1.326(6) Å, similar to that of C2H4 (dC=C = 1.337(3) Å)46,47. This C–C bond is much longer than that in (Cp*Fe)2(1,2-SC6H4S)(μ-C2H2) (1.181(7) Å)39. These structural parameters indicate that the C ≡ C triple bond is significantly weakened due to the strong π back-donation from the heterometallic centers to the π* orbitals. The Mo–C lengths are nearly identical, averaging ~2.10 Å. In contrast, the two Fe–C bond lengths differ much, measuring 1.969(4) Å for Fe–C1 and 2.124(4) Å for Fe–C2. The longer C2–Fe bond and the less hindered C2 are likely responsible for the regio-selectivity in the reaction of [1-C2H2]+ with hydride (vide infra).

Fig. 3: Crystal structure of [1-C2H2]+ with 50% probability thermal ellipsoids.
figure 3

For clarity, the counterion [BAr4F]- and hydrogen atoms except the acetylic protons are omitted, and the phenyl groups and Cp* rings are drawn as lines. Selected bond distances (Å) and angles (deg): Mo-P 2.4836(9), Mo–S 2.3281(9), Mo–C1 2.097(4), Mo–C2 2.110(4), C1–C2 1.326(6), Fe–S 2.2448(11), Fe–C1 1.969(4), Fe–C2 2.124(4), Fe–Mo 2.4718(6); Fe–S–Mo, 65.41(3).

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FeMo vinyl complex

The activation of C2H2 at the cationic FeMo platform enables it to undergo hydride addition. Treatment of [1-C2H2]+ with one equivalent of KHBPh3 (ΔGH– = 36 kcal·mol–1)48 in THF gradually changes the solution color from deep blue to dark green (Fig. 2). The new 31P NMR signal at 104.6 ppm indicates the formation of a new species in the reaction mixture, from which the vinyl-bridged Fe–Mo complex (1-CHCH2) can be isolated in an 84% yield. In the 1H NMR spectrum, distinct resonances can be observed at δ 12.12 (Ha, apparent t, JH-H = 8.0 Hz), 1.86 (Hb, apparent d, Jab (cis) = 7.8 Hz), and δ 2.57 (Hc, apparent dd, Jac(trans) = 8.9 Hz, 3JP-Hc = 15.1 Hz), which are assigned to the μ-CHCH2 group based on 1H–1H COSY studies (Supplementary Fig. 13). Additionally, the 1H–13C HSQC spectrum confirms the presence of the vinyl ligand, displaying 13C signals at δ 199.0 and 46.4 for the μ-CH and -CH2 units, respectively (Supplementary Fig. 14). In contrast, the mononuclear Mo-C2H2 complex is unreactive toward both KHBPh3 and LiHBEt3, highlighting the reactivity of the cationic FeMo acetylene complex.

In the solid-state structure of 1-CHCH2, the Mo and Fe centers are unsymmetrically bridged by a μ-η1,η2-CH = CH2 ligand (Fig. 4a). The Mo, C1, Fe, and S atoms are coplanar, as indicated by the sum of interior angles (~360°). Furthermore, the vinyl group is π-bonded to the Mo center, with Mo–C distances of 2.107(4) and 2.301(5) Å, comparable to those in other Mo-η2-alkene compounds49,50,51. Upon reduction, the C1–C2 bond lengthens to 1.404(7) Å, resembling a typical C = C double bond47. Compared to 2.4718(6) Å in [1-C2H2]+, the Fe–Mo distance is elongated by 0.06 Å, measuring 2.5313(8) Å in 1-CHCH2, due to the change of the bridging C2H2 ligand from a doubly π-bound alkyne to a σ, π-vinyl.

Fig. 4: Structural characterization and kinetic studies.
figure 4

Solid-state structures of a 1-CHCH2 and b 1-CCH3 with 50% probability thermal ellipsoids. For clarity, only the hydrogen atoms at vinyl and carbyne ligands are shown, and the phenyl groups are drawn as lines. Selected bond distances (Å) and angles (deg): for 1-CHCH2, Fe–Mo 2.5313(8), Mo–P 2.4541(11), Mo–S 2.3397(11), Mo–C1 2.107(4), Mo–C2 2.301(5), C1–C2 1.404(7), Fe–S 2.2193(13), Fe–C1 1.962(5), Fe–S–Mo, 67.39(4), Fe–C1–Mo, 76.85(16); for 1-CCH3, Fe–Mo 2.5167(5), Mo–P 2.4032(8), Mo–S 2.3424(8), Mo–C1 1.905(3), C1–C2 1.470(5), Fe–S 2.2512(8), Fe–C1 1.901(3), Fe–S–Mo, 66.41(2), Fe–C1–Mo, 82.79(13). c Eyring plot of the conversion 1-CHCH2 → 1-CCH3 in d8-toluene with 30 ppm of H2O.

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H2O-promoted carbyne formation

Early studies by Casey demonstrated that the rearrangement of cationic μ-alkylidyne complexes [Cp2(CO)Fe2(μ-CO)(μ-CCH2R)]+ to the μ-alkenyl complexes [Cp2(CO)Fe2(μ-CO)(μ-CHCHR)]+ occurs at elevated temperature, and the alkyl substituent (R) on the β-carbon accelerates the reaction19. Conversely, we discovered that in the presence of catalytic amounts of water, 1-CHCH2 rearranges to a carbyne compound 1-CCH3 through the migration of the proton from the Fe–CH unit to the -CH2 group (Fig. 5). The reaction was conducted in a C6D6 solution containing 30 ppm of H2O at room temperature (see Supplementary Information, Kinetic Studies). As monitored by 1H NMR spectra, the characteristic vinyl proton resonances at δ 12.12 (1H), δ 2.57 (1H), and δ 1.86 (1H) gradually disappear, while a singlet signal for the μ-CCH3 group at δ 4.46 emerges. After 6 h, the 31P NMR spectrum only shows a new signal at δ 93.3, indicating the complete conversion of 1-CHCH2 (31P NMR, δ 104.6) to 1-CCH3. The 1H–13C HSQC and HMBC studies confirmed the formation of the carbyne moiety with 13C chemical shifts at δ 378.6 (2JP-C = 8.9 Hz, μ-CCH3) and δ 41.5 (μ-CCH3), respectively. In contrast, the tautomerization of the vinyl group was not observed in dried solvents such as benzene, toluene, or THF.

Fig. 5
figure 5

Water-catalyzed rearrangement of 1-CHCH2 to 1-CCH3.

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The role of water in the conversion of 1-CHCH2 to 1-CCH3 was further probed through a deuterium labeling experiment by adding 1 equivalent of D2O to the C6H6 solution. After reaction at room temperature for 30 min, 2H and 1H NMR studies indicated the deuterium atom was unequivocally incorporated into the methyl group of the carbyne ligand (Fig. 5, Supplementary Figs. 26 and 27). This outcome suggests that water serves as a proton relay and participates in the proton migration process52,53. Notably, other protonic reagents such as MeOH, also catalyzed the proton transfer from the vinyl group to a bridging carbyne (see Supplementary Figs. 42 and 43).

Kinetic studies on the isomerization process were conducted with a catalytic amount (30 ppm) of water using 1H NMR spectroscopy (Supplementary Fig. 32). Logarithmic plots reveal first-order kinetics for 1-CHCH2 → 1-CCH3, with a rate constant (k) of 4.09 × 10−3 min−1 at 298 K. Further investigations showed that the temperature significantly affects the rearrangement rate. Independent rate measurements at 45 °C yielded k = 11.0 × 10−3 min−1, approximately 30 times faster than the rate at −5 °C (3.7 × 10−4 min−1). An Eyring plot of ln(k/T) versus T−1 provided the activation parameters: ∆H‡ = 10.8 kcal mol−1 and ∆S‡ = − 41.5 cal·mol−1 K−1. These values yield a ∆G‡ value of 23.2 kcal mol−1 at 298 K (Fig. 4c). We also found that the water concentration has a significant effect on the rate of the tautomerization, and the reaction order in water was determined to be 0.52 at room temperature (Supplementary Fig. 40 and Table 2), suggesting a possible involvement of water in a pre-equilibrium before the turnover-determining step.

The molecular structure of 1-CCH3 was confirmed by X-ray crystallographic analysis (Fig. 4b). In comparison to 1-CHCH2, the Mo and Fe centers in 1-CCH3 are bridged by a methyl-substituted carbyne, rather than the σ, π-vinyl moiety. The Mo–Fe distance of 2.5167(5) Å is slightly shorter than that in 1-CHCH2 (2.5313(8) Å). The Mo–C1 and Fe–C1 bond lengths are 1.905(3) and 1.901(3) Å, respectively, which are comparable to those observed in [CpMoFe(μ-CAr)(CO)5] (Ar = 2,6-C6H3Me2)54. Given that the covalent radius of Mo is much larger compared to Fe, the nearly identical Mo–C and Fe–C bond lengths suggest that the carbon-metal π bond is predominantly situated on the Mo side. Furthermore, the C1–C2 bond length of 1.470(5) Å is intermediate between the typical C–C single bonds (1.54 Å)55 and C = C double bonds (1.339(1) Å)47, and is closer to that of a C–C single bond. It is worth noting that classic alkylidyne ligands lacking hetero-substituents are scarce16,17.

Hydrido carbyne complex

Surprisingly, both the vinyl complex and the carbyne complex can be protonated to afford a hydrido carbyne complex, [H1-CCH3]+. When an Et2O solution of 1-CHCH2 or 1-CCH3 was treated with H(Et2O)2BAr4F at −30 °C, 31P NMR studies indicated that the two reactions gave the same product, displaying a common signal at δ 81.8. The 1H NMR spectrum showed a characteristic hydride signal at δ − 2.78 (doublet, 2JP-H = 75 Hz), indicating the formation of a Mo–H species. The μ-CCH3 proton signal was observed at δ 5.09 (s), and the bridging carbon μ-CCH3 displayed a 13C resonance at δ 404.5. HRMS studies confirmed the formation of [H1-CCH3]+ with m/z = 745.1613 (Cald. 745.1618).

Deuterium labeling experiments were also performed to gain deeper insights into the protonation reactions (Fig. 6). Upon treatment with D(Et2O)2BAr4F, 1-CHCH2 yielded [H1-CCH2D]+ as evidenced by 1H and 2H NMR studies (Supplementary Figs. 26 and 27). This suggests that protonation occurs at the β-vinyl carbon, followed by oxidative addition of the α-C–H onto molybdenum. In contrast, the reaction of 1-CCH3 with D(Et2O)2BAr4F led to protonation occurring at the molybdenum center, giving rise to a deuteride species [D1-CCH3]+ (Supplementary Figs. 28–30).

Fig. 6
figure 6

Protonation of 1-CHCH2 and 1-CCH3 with D(Et2O)2BAr4F.

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The crystallographic analysis of [H1-CCH3][BAr4F] consistently revealed disordered structures. However, when replacing the counter anion from [BAr4F]− to [B(C6F5)4]−, we were able to obtain X-ray-quality single crystals. The solid-sate structure of [H1-CCH3]+ exhibits a similar skeleton to the neutral μ-carbyne compound (Fig. 7). The bond lengths of C1–C2 (1.461(8) Å) and Fe–C1 (1.845(6) Å) are close to those observed in 1-CCH3. However, the Mo–C1 bond shows noticeable elongation, i.e. 1.959(6) Å for [H1-CCH3]+ vs. 1.905(3) Å for 1-CCH3. The hydride position was refined, revealing a Mo–H bond length of 1.67(6) Å, consistent with the previously reported Mo(VI)–H species56,57. Note that [H1-CCH3]+ represents the first heteronuclear hydrido μ-carbyne complex. Spectroscopic studies have shown that the active states of FeMoco feature hydride ligands58,59,60, and metal-hydride intermediates have been proposed to be involved in the reduction of acetylene by nitrogenase34,61,62. Therefore, [H1-CCH3]+ can serve as a promising synthetic entry point for the development of FeMoco models.

Fig. 7: Solid-state structure of [H1-CCH3]+ with 50% probability thermal ellipsoids.
figure 7

For clarity, the counterion [B(C6F5)4]- and most hydrogen atoms are omitted, and the two phenyl groups at the phosphorus site are drawn as lines. Selected bond distances (Å) and angles (deg): Mo–H 1.67(6), Mo–P 2.4672(14), Mo–S 2.3803(14), Mo–C1 1.959(6), C1–C2 1.461(8), Fe–S 2.2212(14), Fe–C1 1.845(6), Mo–Fe 2.5469(8); Fe–S–Mo, 67.11(4); Fe–C1–Mo, 84.0(2).

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Mechanism studies

To elucidate the mechanism of water-promoted proton transfer process, DFT studies were performed (Fig. 8). According to the calculations, the reaction is proposed to start with the coordination of H2O onto the iron center of 1-CHCH2, leading to the formation of Int1 (3.5 kcal mol−1). This coordination enhances the acidity of H2O, facilitating the turnover-determining proton transfer to the CH2 group to give Int2 (7.1 kcal mol−1) via TS1 (24.4 kcal mol−1). We also measured the kinetic isotope effect (KIE) by comparing the initial reaction rates in the presence of H2O and D2O (29.2 mol%) in independent runs at 298 K. The normal primary KIE value (kH/kD = 3.70) is consistent with a turnover-determining protonation step. Int2 features a bridging alkylidene between the Mo and Fe centers and β–agostic interaction between the CH3 group and the Mo center. The subsequent reorientation of the bridging alkylidene gives Int3 (7.2 kcal mol−1) via TS2 (18.1 kcal mol−1). Such an isomerization brings the α-proton of the bridging alkylidene and the iron-bound OH to the syn configuration with respect to the C–Fe bond for the facile proton transfer via TS3 (11.4 kcal·mol−1) to afford Int4 (−1.8 kcal mol−1). The dissociation of H2O from the iron center of Int4 gives the final product 1-CCH3 (−9.6 kcal mol−1). The well-ordered rate-determining transition state TS1 can also account for the negative experimental ∆S‡. The computed energetic span of 24.4 kcal mol−1 is similar to the experimental result (23.2 kcal mol−1). In contrast, a few alternative routes involving water were ruled out for thermodynamic reasons (See Supplementary Information, DFT Calculations). The computed direct isomerization of 1-CHCH2 to 1-CCH3 has a free energy barrier of 38.6 kcal mol−1, which is consistent with the lack of reactivity in thoroughly dried solvents at ambient temperature and the barrier (>31.0 kcal mol−1) estimated by Casey19.

Fig. 8: DFT studies.
figure 8

a Proposed mechanism for water-catalyzed isomerization of 1-CHCH2. b Computed Gibbs free energy profile (in kcal mol−1, at 298 K, 1 mol L−1 concentration). All structures were optimized in full (M11L, def2tzvp, PCM). For clarity, the drawings of all MoFe complexes were simplified by omitting the P,S-ligand. For computational details, see the DFT Calculations in Supplementary Information.

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In summary, we have demonstrated the transformation of C2H2 into a bridging carbyne ligand on a bioinspired FeMo platform. The incorporation of Cp*Fe and Cp*Mo moieties into an unsymmetrical phosphino-thiolate coordination environment enables the activation of C2H2 crosswise the Fe–Mo vector and its subsequent reduction by a hydride, resulting in the formation of a vinyl group (μ-η1,η2-CH = CH2). Mechanism studies reveal that water plays a significant role as a proton shuttle in the tautomerization reaction, converting the vinyl group into a carbyne. This water-promoted C–H bond cleavage and formation are reminiscent of biological C–H bond activation processes observed in the enzymes such as cytochrome P45063,64. Intriguingly, both the vinyl and the carbyne complexes undergo protonation, leading to the same hydrido carbyne complex. Deuterium labeling experiments reveal that the vinyl protonation occurs at the β-carbon, whereas the molybdenum site in the carbyne complex is more prone to protonation compared to the bridging carbyne carbon (Mo-C(CH3)-Fe). In general, this MoFe system offers a synthetic method for the incorporation of a bridging carbon and hydride ligands into an Fe-S-Mo framework starting from common, simple organic substrates.

Methods

General procedures for kinetic studies on the conversion of 1-CHCH2 to 1-CCH3

The solvent d8-toluene was dried over 3 Å molecular sieves for 3 days (residual water content 0.9 ± 0.3 ppm)65. A d8-toluene solution (contain 30 ppm H2O) was then prepared by adding 3 μL H2O to 100 mL predried d8-toluene. A 26.9 mM solution of 1-CHCH2 was prepared by dissolving 1-CHCH2 (10.0 mg, 0.013 mmol) in 500 μL d8-toluene (contain 30 ppm H2O, 4.38 mol% with respect to 1-CHCH2). The solution was transferred to a J. Young NMR tube and placed into a temperature-calibrated NMR probe (−5 to 45 °C). 1H NMR spectra were collected until ~30% conversion to 1-CCH3 completed. No side products were evidenced by 1H NMR spectroscopy. The concentrations of 1-CHCH2 and 1-CCH3 were determined by the integration of the vinyl signal –CHCH2 at δ 12.03 and the methyl signal μ–CCH3 at δ 4.42, respectively (SiEt4 as internal standard). The resulting data was fit to a first-order kinetics plot. The rate constants at −5, 5, 15, 25, 35, and 45 °C were determined from the time dependence plots of ln[1-CHCH2].