Mononuclear Fe(I) and Fe(II) Acetylene Adducts and Their Reductive Protonation to Terminal Fe(IV) and Fe(V) Carbynes

Abstract

The activity of nitrogenase enzymes, which catalyze the conversion of atmospheric dinitrogen to bioavailable ammonia, is most commonly assayed by the reduction of acetylene gas to ethylene. Despite the practical importance of acetylene as a substrate, little is known concerning its binding or activation in the iron-rich active site. “Fischer–Tropsch” type coupling of non-native C₁ substrates to higher-order C_≥2 products is also known for nitrogenase, though potential metal–carbon multiply bonded intermediates remain underexplored. Here we report the activation of acetylene gas at a mononuclear tris(phosphino)-silyl-iron center, (SiP₃)Fe, to give Fe(I) and Fe(II) side-on adducts, including S = 1/2 Fe^I(η²-HCCH); the latter is characterized by pulse EPR spectroscopy and DFT calculations. Reductive protonation reactions with these compounds converge at stable examples of unusual, formally iron(IV) and iron(V) carbyne complexes, as in diamagnetic (SiP₃)Fe≡CCH₃ and the paramagnetic cation S = 1/2 [(SiP₃)Fe≡CCH₃]⁺. Both alkylcarbyne compounds possess short Fe–C triple bonds (approximately 1.7 Å) trans to the anchoring silane. Pulse EPR experiments, X-band ENDOR and HYSCORE, reveal delocalization of the iron-based spin onto the α-carbyne nucleus in carbon p-orbitals. Furthermore, isotropic coupling of the distal β-CH₃ protons with iron indicates hyperconjugation with the spin/hole character on the Fe≡CCH₃ unit. The electronic structures of (SiP₃)Fe≡CCH₃ and [(SiP₃)Fe≡CCH₃]⁺ are discussed in comparison to previously characterized, but heterosubstituted, iron carbynes, as well as a hypothetical nitride species, (SiP₃)Fe≡N. Such comparisons are germane to the consideration of formally high-valent, multiply bonded Fe≡C and/or Fe≡N intermediates in synthetic or biological catalysis by iron.

Graphical Abstract

INTRODUCTION

Interest in nominally high valent iron nitrides (N³⁻) and imides (NR²⁻) has grown in the past 15–20 years,¹ and has stemmed from a desire to explore their electronic structures and reactivity patterns, especially as they may pertain to important synthetic and biological transformations. For instance, the intermediacy of an Fe≡N species has been considered in the context of a distal pathway for biological nitrogen fixation by nitrogenase enzymes (Fe−N₂ + 3H⁺/e⁻ → Fe≡N + NH₃).² Relatedly, such an intermediate nitride has been recently characterized within a synthetic iron system that catalyzes N₂-to-NH₃ conversion.³

Nitrogenase enzymes reduce a range of non-native substrates as well, including azide (N³⁻), cyanide (CN⁻), CO, CO₂, and acetylene (C₂H₂).⁴ Indeed, acetylene reduction to ethylene (C₂H₄) is the most convenient and hence most common assay for reductive “nitrogenase” activity.^5–7 It is perhaps surprising, therefore, that acetylene complexes of iron, and their associated reactivity patterns, have not been well-studied to date. Noting this gap, we undertook a study of the reactivity of acetylene with an iron system supported by a tripodal phosphine ligand, in part for comparison with related studies we and others had undertaken in the context of N₂ chemistry. As a result, we uncovered an unexpectedly rich reactivity profile that links acetylene binding at iron(I) to the ultimate generation of nominally high-valent (+4 and +5) iron carbynes via reductive protonation steps. While these transformations do not appear to model the reactivity profile of acetylene reduction by nitrogenase, they are nevertheless fascinating.

Until recently, iron carbynes of any type had little synthetic precedent.^8–12 Reductive activation of CO and CN⁻ at iron recently led to examples of heteroatom-substituted (i.e., Fischer-type) iron carbynes.^10,11 By contrast, the alkylcarbynes featured herein (Fe^IV≡CCH₃ and [Fe^V≡CCH₃]⁺) are unique in that they are not substituted by heteroatoms. In a broader context, iron complexes featuring strong iron-to-carbon multiple bonds are of continuing interest, especially as they may pertain to intermediates in “Fischer–Tropsch” type C–C coupling pathways exhibited by nitrogenase enzymes in the presence of non-native CO, CO₂, and CN⁻ substrates.⁴

Herein, we describe the following:

1. The first well-characterized complexes featuring C₂H₂ bound to a single iron center are described, in the formal +1 and +2 oxidation states (i.e., Fe^I(C₂H₂) and Fe^II(C₂H₂)⁺); the iron(I) derivative is S = 1/2, and pulse EPR data confirm η₂ side-on coordination.

2. Well-characterized examples of Fe^IV≡CCH₃ and Fe^V≡ CCH₃⁺ carbynes are presented, including their solid-state X-ray crystal structures and ⁵⁷Fe Mössbauer spectra. The Fe^V≡CCH₃⁺ species is S = 1/2 and pulse EPR spectroscopy is used to additionally map its electronic structure.

3. Mechanistic studies indicate the Fe^I(C₂H₂) species undergoes initial C–H activation, followed by bimolecular H₂ loss, to generate a terminal Fe^IIC≡CH, which can also be independently prepared. The Fe^V≡CCH₃⁺ species is most conveniently accessed by reductive protonation steps from Fe^IIC≡CH.

The EPR data provided for Fe^I(C₂H₂) and Fe^V≡CCH₃⁺ could be of value to mechanistic studies employing related EPR techniques of nitrogenases with C₂H₂,¹³ or to studies of non-native C₁ substrates that lead to Fischer–Tropsch type C–C couplings.⁴

RESULTS AND DISCUSSION

Mononuclear Iron-Acetylene Adducts

Figure 1 outlines the entire reaction manifold discussed herein, where a tris(phosphino)silyl-iron subunit (abbreviated herein as “(SiP₃)Fe” or “Fe”) is common to all of the complexes described. To test the affinity of acetylene for an iron(I) center, the previously reported complex (SiP₃)Fe^IN₂ (1) was exposed to acetylene gas. Displacement of N₂ by 1.2 equiv of acetylene occurs slowly at room temperature in THF to give an unobserved acetylene adduct complex, (SiP₃)Fe^I(η²-HCCH) (2) (see below for further characterization at low T), that proceeds to the red, terminal acetylide complex (SiP₃)-Fe^IICCH (3), along with liberation of 0.5 equiv of H₂ (confirmed by GC).

Figure 1.

Scheme depicting the reaction chemistry discussed herein, along with labeled formal oxidation states and compound numbers.

The acetylide complex 3 can alternatively be prepared by stirring a sodium acetylide slurry with (SiP₃)Fe^IIOTf (4) in the presence of tetramethyl ethylenediamine (TMEDA) in 84% yield. The complex has been characterized in solution and the solid state (see Figure 2) as a trigonal bipyramidal S = 1 iron(II) species featuring the SiP₃ ligand. Complex 3 exhibits a weak C≡C vibration (1902 cm⁻¹) and a terminal C–H stretch (3277 cm⁻¹); isotopic labeling with ¹³C₂-TMED-LiC≡CH (see SI for preparation details) gives ¹³C₂-2 (ν_C≡C = 1832 cm⁻¹) and ν_C–H = 3262 cm⁻¹; Δ_C≡C = 70 cm⁻¹; Δ_C–H = 15 cm⁻¹; Figure S23). Three other terminal iron-acetylide compounds have previously been crystallographically characterized.¹⁴

Figure 2.

Solid-state crystal structures of 3, 6, 7, and 8, with thermal ellipsoids displayed at 50% probability. The hydrogen atoms of (SiP₃) and the counter-anions present in the structures for 6 and 7 have been omitted for clarity. A DFT-minimized structure of (SiP₃)Fe^I(η²-C₂H₂) 2 is also shown, with bond metrics, along with the calculated SOMO orbital and Löwdin spin densities in the primary ligation sphere.

A thermally stable adduct of acetylene is accessible at the iron(II) state. Accordingly, the light-blue cation [(SiP₃)FeN₂]-BArF₂₄ (5)¹⁵ (BArF₂₄= [B(3–5-(CF₃)₂-C₆H₃)₄]⁻) reacts with one equivalent of acetylene gas in fluorobenzene at 0 °C to afford green [(SiP₃)Fe(η²-HCCH)]BArF₂₄ (6) (70% isolated yield). The solid-state X-ray structure of the S = 1 complex shows the η²-HCCH ligand bound side-on in the axial position opposite to the silyl anchor (Figure 2). As stated in the introduction, complex 6 is to our knowledge the only example (other than 2, vide infra) of a well-characterized mononuclear iron complex featuring C₂H₂ as a ligand.¹⁶ Mononuclear adducts of acetylene are highly uncommon for first row metals in general.¹⁷ Fe-to-acetylene backbonding in 6 is modest (C≡C bond length of 1.25 Å; cf. 1.20 Å in free acetylene¹⁸) and the Fe–P and Fe–Si bond are relatively long, in accord with its S = 1 ground state and side-on accommodation of the acetylene ligand.

Because the acetylene adduct Fe^I(η²-HCCH) 2 cannot be observed upon addition of C₂H₂ to Fe^IN₂ 1 at room temperature (owing to slow ligand substitution kinetics), we sought its characterization via the low temperature reduction of [Fe^II(η²-HCCH)]⁺ 6. Thus, addition of stoichiometric cobaltocene (Cp₂Co) to a stirred solution of 6 in 2-MeTHF at −125 °C rapidly generates a new orange species that exhibits first order decay above −80 °C to generate the terminal acetylide Fe^IICCH 3 (t_1/2(−40 °C) ≈ 14 min; monitored by in situ UV–visible spectroscopy; Figure S54).

To more thoroughly characterize Fe^I(η²-HCCH) 2, in particular to assess whether alternative “FeCCH₂” or “Fe^III(H)-(CCH)” isomers might be more appropriate, we undertook continuous wave (CW) and pulse EPR experiments of both the natural abundance and deuterium-labeled isotopologue Fe^I(η²-DCCD) at X-band frequency, coupled with DFT studies. The X-band CW EPR spectrum, shown in Figure 3A and 3B, exhibits a g-tensor with rhombic symmetry (g = [2.114, 2.040, 2.007]) with coupling evident to three distinct phosphorus nuclei. Simulation of the data indicates that one of the P-donors is coupled more strongly than the other two (|A(³¹P)| = [183, 182, 214] MHz; Table 1). These simulated coupling values, and especially the asymmetry with respect to one donor, qualitatively agree with the DFT-predicted ³¹P hyperfine tensors calculated from an optimized structure of 2 (|A(³¹P)| = [201, 203, 242] MHz; TPSS/def2-TZVP/D3ZERO; see SI for details).

Figure 3.

(A) X-band CW-EPR spectra of (SiP₃)Fe(η²-HCCH) prepared in 2-MeTHF with natural abundance (top), ¹³C-enriched (middle), and ²H-enriched acetylene (bottom) with simulations using parameters in Table 1 overlaid in red. (B) Derivative spectra of X-band CW-EPR for each isotopologue with simulations overlaid in red. Acquisition parameters: temperature = 77 K; MW frequency = 9.375 GHz; MW power = 2 mW; modulation frequency = 100 kHz; modulation amplitude = 0.1 mT; conversion time = 41 ms.

Table 1.

Hyperfine Coupling Constants in MHz Determined for (SiP₃)Fe(η²-HCCH)

nucleus	A1	A2	A3	aiso	Euler angles [α, β, γ]°
31P1	183	183	214	193.3	[0, 0, 0]
31P2	30	28	40	32.7	[0, 0, 0]
31P3	12	0.5	17	9.8	[0, 0, 0]
1H1	9.4	20.2	17.6	15.7	[33, 18, −5]
1H2	8.2	18.4	16.8	14.5	[43, −22, 11]
13C1	17	24	8.6	16.5	[0, 30, 0]
13C2	24	16.6	10.2	16.9	[0, −30, 0]

Complementary X-band electron nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopic data allow for further resolution of the coupling of the paramagnetic iron center with NMR active nuclei. Accordingly, hyperfine coupling of the acetylene ¹H nuclei to the spin on iron can be extracted by comparison of the data for Fe^I(η²-HCCH) with that for Fe^I(η²-DCCD). Field-dependent HYSCORE (Figure 4) and ¹H–²H ENDOR (Figure 5) spectra show weak ¹H hyperfine couplings to two very similar acetylene-derived protons: ¹H₁ (|A(¹H)| = [9.4, 20.2, 17.6] MHz), with Euler rotation angles [α, β, γ] = [33, 18, −5] relative to the g-tensor and ¹H₂(|A(¹H)|= [8.2, 18.4, 16.8] MHz), with Euler rotation angles [α, β, γ] = [43, −22, 11]. These results are also corroborated by the appearance of ²H features in the HYSCORE data for Fe^I(η²-DCCD), which are well simulated by scaling these hyperfine tensors by the ratio of the ¹H/²H gyromagnetic ratios (γ¹H/γ²H = 6.514) (Figures S33–35). These two protons exhibit extremely similar isotropic hyperfine couplings, with a_iso(¹H₁) = 15.7 MHz and a_iso(¹H₂) = 14.5 MHz, and are only differentiated by the respective orientations of their hyperfine tensors relative to the molecular g-tensor coordinate frame, which is resolved in the ¹H–²H difference ENDOR data acquired near the low-field edge of the EPR spectrum (see Figure 5, top trace). This observation strongly disfavors assignment as the “Fe^III(H)-(CCH)” isomer, as a hydride directly bound to an S = 1/2 iron center should exhibit much stronger coupling (a_iso ≈ 45 MHz).^19,20

Figure 4.

(Top) X-band HYSCORE spectrum of natural abundance (SiP₃)Fe(η²-HCCH) acquired at 338.6 mT (g = 2.058). (Bottom) Monochromatic representation of the HYSCORE data with simulations using parameters in Table 1 overlaid: (red) ¹H-1 (blue) ¹H-2, (green) ³¹P-3. Acquisition parameters: temperature = 20 K; microwave frequency = 9.751 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 138 ns, t₁ = t₂ = 100 ns; Δt₁ = Δt₂ = 16 ns; shot repetition time (srt) = 1 ms).

Figure 5.

Field-dependent X-band ¹H and ²H Davies ENDOR difference spectra of (SiP₃)Fe(η²-HCCH) (black) with simulations using parameters in Table 1 overlaid: (red) total ¹H simulation, (blue) ¹H-1, (green) ¹H-2. Acquisition parameters: temperature = 15 K; MW frequency = 9.751 GHz; MW pulse length (π/2, π) = 40 ns, 80 ns; τ = 260 ns; RF pulse length = 15 μs; T_RF = 2 μs; shot repetition time = 5 ms.

To distinguish between the Fe^I(η²-HCCH) and “FeCCH₂” isomers, we prepared the ¹³C-enriched derivative ¹³C₂H₂-2. Consistent fits between the field-dependent ¹³C–¹²C difference HYSCORE (Figure 6) and ENDOR spectra (see SI) give two nearly identical, strongly coupled ¹³C nuclei ¹³C₁(|A(¹³C)| = [17, 24, 8.6]; a_iso = 16.5 MHz; ¹³C₂(|A(¹³C)| = [24, 16.6, 10.2]; a_iso = 16.9 MHz) (Table 1), which also disfavors the “FeCCH₂” isomer, as such a species would be anticipated to show highly inequivalent ¹³C_α and ¹³C_β couplings, with coupling to C_α being much larger. Together, these EPR data are fully consistent with a structure for 2 formulated as an η²-HCCH adduct of iron(I), akin to its one-electron oxidized precursor 6.

Figure 6.

(Top) X-band ¹³C–¹²C difference HYSCORE spectrum of (SiP₃)Fe(η²-HCCH) acquired at 338.6 mT (g = 2.058). (Bottom) Monochromatic representation of the HYSCORE data with ¹³C simulations using parameters in Table 1 overlaid: (blue) ¹³C₁, (red) ¹³C₂. Acquisition parameters: temperature = 20 K; microwave frequency = 9.751 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 138 ns, t₁ = t₂ = 100 ns; Δt₁ = Δt₂ = 16 ns; shot repetition time (srt) = 1 ms).

Several other side-on adducts of alkynes and alkenes at reduced Fe(I) centers have been reported by Holland and co-workers.²¹ These complexes exhibit high-spin S = 3/2 ground state electronic structures and were described with substantial π-backbonding from iron to the coordinated unsaturated ligand. In a study of an α−70^Val mutant nitrogenase that is able to accommodate several substituted-acetylene substrates, the S = 3/2 resting state EPR signal is, by contrast, converted to an iron-localized rhombic S = 1/2 doublet in a freeze-quenched experiment using propargyl alcohol (HC≡CCH₂OH) as the substrate.¹³ This doublet signal closely matches that of a CO-bound/inhibited state. While ¹³C coupling to the doublet signal was measured, it was not possible to distinguish between possible binding modes of the propargyl alcohol.

The transformation of Fe^I(η²-HCCH) 2 to acetylide Fe^IICCH 3 is interesting to consider further. The available data is insufficient to determine the fate of 2 after its first-order decay. However, two distinct paths seem most plausible to us. In one scenario, which we favor, C–H oxidative addition of the coordinated C₂H₂ ligand produces “Fe^III(H)(CCH)”, one of the structural isomers considered above. This intermediate then loses half an equivalent of H₂ in a bimolecular step. Relatedly, we recently reported an example of bimolecular H₂ loss from a well-defined Fe^III-hydride complex that features a sufficiently weak Fe–H bond.²⁰ Additionally, the oxidative addition pathway (Figure 7) is well precedented for a number of previously characterized alkyne-hydride complexes of Co and Fe.²² Of note to the present study, we previously reported reversible activation of phenylacetylene in a structurally related trisphosphine-borane TPBFe system, likely by oxidative addition to a formally Fe(II)-alkyne-hydride/borohydride complex.²³

Figure 7.

A qualitative reaction coordinate for the proposed slow substitution of acetylene for N₂ at (SiP₃)FeN₂, 1, leading to intermediate (SiP₃)Fe(η²-HCCH) 2, which subsequently undergoes sp(C–H)-activation and H₂ release to generate isolable (SiP₃)Fe-(CCH), 3. We presume the H₂ elimination step occurs in a bimolecular fashion via two Fe(III)-hydride species. See text for a discussion of other possible pathways.

Alternatively, another reaction pathway could involve eventual rearrangement of Fe^I(η²-HCCH) 2 or an intermediate Fe^III(H)(CCH) to the end-on “FeCCH₂” isomer, considered above, again followed by bimolecular H₂ loss. However, transformation by a unimolecular 1,2 hydrogen shift of an η²-alkyne adduct to a terminal alkylidene/vinylidene²⁴ is generally thought to have a high barrier and is therefore difficult without an exogenous acid/base catalysts or high temperature.²⁵ Berke and co-workers have reported that chromatography on silylated silica accelerates the formation of terminally stable vinylidene complexes from mixtures of low-valent iron acetylene/alkyne and alkyne-hydride complexes.^22c,26 In the present Fe^I(η²-HCCH) 2 system, deprotonation by solvent (THF) to produce [FeCCH]⁻, followed by reprotonation to produce FeCCH₂, seems highly unlikely considering the low acidity expected for 2 in THF. Finally, while we consider it to be unlikely, a bimolecular reaction between Fe^I(η²-HCCH) and itself, or with Fe^III(H)(CCH), could in principle evolve H₂.

Terminal Iron-Carbyne Complexes

The addition of triflic acid (HOTf) to the terminal Fe-acetylide 3 in diethyl ether at −78 °C precipitates [(SiP₃)Fe≡CCH₃][OTf] (7) as a dark brown solid from solution (40% yield). Use of [H(Et₂O)₂][BArF₂₄] in place of HOTf affords [(SiP₃)Fe≡CCH₃][BArF₂₄] (Figure 1). The alkylcarbyne cation 7 is a nominally iron(V) complex that, to our knowledge, is the only species of its type, formal or otherwise. The complex exhibits an S = 1/2 ground state (vide infra) and is stable as a solid or in THF or acetonitrile solution at room temperature for extended periods. As stated above, there are a handful of previously characterized, terminal, heteroatom-substituted carbynes of iron.^8–12 There are no prior examples of alkylcarbynes of iron, nor examples where the carbyne is substituted instead by H or aryl for that matter. Additionally, paramagnetic examples of carbyne/alkylidyne complexes are rare,^27,28 and, for first row metals, the only others we know of relate to [(SiP₃)FeCNH₂]⁺ and [(SiP₃)FeCNMe₂]⁺.^11,12

The solid-state crystal structure of 7 is shown in Figure 2. As required for an S = 1/2 system, the complex deviates slightly from 3-fold symmetry (∠P–Fe–P angles 114, 117, 120°). Most noteworthy is a characteristically short Fe≡C—CH₃ bond of 1.70 Å and a Fe≡C–CH₃ single bond of 1.42 Å. The Fe–Si bond is long at 2.39 Å, consistent with its position trans to the carbyne ligand. The triflate counteranion exhibits close contacts with the (SiP₃)-isopropyl and carbyne C–H positions.

The conversion of (SiP₃)FeCCH to [(SiP₃)FeCCH₃]⁺ is a net 2-H⁺/1-e⁻ transformation, requiring sacrificial oxidation of half of the starting iron material, for a theoretical 50% yield. Analysis of recovered material shows primarily (SiP₃)FeOTf 4, indicating the net reducing equivalent is likely derived from the acetylide moiety. The propensity for reaction intermediates to cannibalize themselves en route to 7 is consistent with the observed thermodynamic stability of the terminal product.

It is also noted that protonation of 3 by HOTf does not instead lead to its net oxidation (i.e., to form (SiP₃)FeCCH⁺) with concomitant loss of 0.5 equiv of H₂ (eq 1), especially as the cationic [(SiP₃)FeCCH]⁺ is electrochemically accessible (−0.78 V vs Fc in THF; Figure S58).²⁹

$$\text{FeCCH}+\text{HOTf}\to {[\text{FeCCH}]}^{+}+0.5{\text{H}}_2(\text{not}\text{observed})$$ (1)

Alternatively, protonation of acetylide 3 in ether by the weaker, insoluble acid, imidazolium triflate, to our surprise generates the neutral alkyl carbyne derivative, (SiP₃)Fe≡CCH₃ 8 (25% isolated yield), the product of a net 2 H⁺/2 e⁻ reduction, again requiring some amount of sacrificial oxidation of starting material. The neutral carbyne 8 is thus more favorably synthesized in a heterogeneous mixture of triethylammonium chloride and excess sodium metal to balance the proton-coupled reduction (Figure 1). The Fe≡CCH₃⁺/Fe≡CCH₃ redox couple between 7 and 8 occurs at −1.00 V vs Fc in THF (Figure S59); chemical oxidation of 8 by ferrocenium (Cp₂Fe⁺) or a strong acid, like [H(Et₂O)₂]-[BArF₂₄], cleanly generates 7.

Carbyne 8 is diamagnetic with resolved coupling by ¹H, ³¹P, and ¹³C NMR spectroscopy (Figures S10–S16). Isotopic labeling in ¹³C₂-8 reveals a very downfield ¹³C resonance for Cα at 348 ppm^30,31 with coupling to C_β (48 ppm; J_{C(α)–C(β)} = 18 Hz) and three equivalent phosphines (³¹P 107.5 ppm; J_C(α)–P = 18 Hz). By ¹H NMR spectroscopy, the terminal carbyne CH₃ resonance at 2.12 ppm couples to C_α (11 Hz) and C_β (127 Hz). The primary ligation of 8 is contracted with respect to S = 1/2 [Fe≡CCH₃]⁺ 7, with a Fe≡C bond of 1.68 Å, a 2.33 Å Fe–Si bond, and Fe–P bonds between 2.24 and 2.26 Å (Figure 2), presumably consistent with increased backbonding upon reduction.

(SiP₃)Fe≡CCH₃ 8 is quite stable, even in solution at 130 °C over a 24 h time period. Its stability renders it a reaction sink in this system. Indeed, with its diagnostic NMR resonances in hand, it can be identified as a minor side-product of other reactions we have surveyed, including the aforementioned reaction between (SiP₃)FeOTf and excess sodium acetylide (Figure 1).

The terminal stability of both alkylidyne complexes inspired us to further explore their interconnection with the acetylene complexes, described above, by reductive protonation reactions (Figure 1). Interestingly, net hydrogen atom transfer (HAT) reactions can convert the acetylene adducts 2 and 6 to the carbynes 7 and 8. For example, mixing [Fe(η²-HCCH)]⁺ 6 with TEMPO-H in 2-MeTHF generates Fe≡CCH₃⁺ 7. At −100 °C, a relatively stable orange intermediate 9 can be observed by UV–visible spectroscopy, which then decays to 7 on warming (Figure S55). Our tentative assignment of 9 as a cationic alkenyl complex [(SiP₃)Fe(CH=CH₂)]⁺, if correct, would make it a structurally interesting one. Whereas there is precedent for related, substituted iron alkenyl derivatives,^22g the Fe—CH=CH₂ subunit is still, to our knowledge, distinct.

Cationic carbyne 7 can also be generated by the addition of stoichiometric [H(Et₂O)₂][BArF₂₄] to a 2-MeTHF solution of (SiP₃)Fe(η²-HCCH) 2 at −125 °C, presumably via 9 as an intermediate (Figure 1). Under similar conditions, the reaction of 2 with TEMPO-H generates the neutral carbyne 8, along with (SiP₃)FeCCH 2 as side product. There thus appears to be a rich reactivity landscape connecting the nascent acetylene adducts and terminal carbynes described herein.

Electronic Structure of Iron-Carbyne Complexes

The Mössbauer parameters for carbynes 7 and 8 compare well with those of the previously reported and structurally characterized iron carbynes featuring the same (SiP₃)Fe subunit: (SiP₃)Fe-(COSiMe₃) and [(SiP₃)Fe(CNMe₂)]^0/+ (Table 2). The isomer shifts are relatively close to 0 mm s⁻¹ across the series, reflecting strong covalency in the short Fe–C triple bond, with Fe≡CCH₃⁺ 7 and Fe≡CCH₃ 8 having the smallest δ values.^2b,3,32–34 By comparison, [(SiP₃)FeCNMe₂]⁺ has an isomer shift of 0.19 mm s⁻¹, indicating less Fe≡C covalency, possibly consistent with a [Fe=C=NMe₂]⁺ resonance form and its slightly longer Fe–C bond (1.74 Å). The measured quadrupole splittings for this series of carbynes are rather narrow, especially by comparison to the very large ΔE_Q values reported for other formally high-valent oxide and nitride Fe complexes in C₃ symmetry.^1–3,33,34 Frontier orbital manifolds whereby the electrons populate only orbitals of d(xy) and d(x²–y²) parentage, as for the d⁴ and d³ configurations described for the latter systems, is predicted to give large quadrupole splittings, >5 mm s⁻¹, due to the strongly asymmetric, equatorially disposed electric field gradient at Fe.³² By contrast, the comparatively small ΔE_Q values observed for the (SiP₃)Fe-carbyne series reflect the additional presence of substantial electron density along the z-axis (parallel to the Fe≡C–R vector). Inspection of the molecular orbitals of the carbyne series shows that the nominally high-lying Fe–C σ*-antibonding orbital is highly stabilized by mixing with Si(σ),³⁵ resulting in a heavily mixed orbital of a₁ symmetry energetically below the filled, primarily nonbonding orbitals of d(xy) and d(x²–y²) parentage (Figure 8). This observation bears relevance to a contributing resonance structure we have previously considered, with electron density being polarized between a cationic R₃Si⁺ anchor and a nominally reduced Fe center (Figure 9).^9,36

Table 2.

Mössbauer Parameters of (SiP₃)Fe Carbynes

Fe≡CX	OSiMe3d	NMe2e	NMe2+e	CH3	CH3+
Fe≡Ca	1.67	1.71	1.74	1.68	1.70
Fe—Sia	2.30	2.31	2.35	2.33	2.39
δb	0.061	0.058	0.19	−0.029	0.0097
ΔEQb	1.12	1.12	1.51	0.72	1.33
13Cαc	250.3	279.6	–	348.4	–

^aApproximate measurements in Å.

^bMeasured in mm s⁻¹.

^cMeasured in ppm in C₆D₆.

^dRef 9.

^eRef 10.

Figure 8.

Simplified molecular orbital diagram for the metal–ligand bonding of a (SiP₃)Fe≡CR carbyne in C₃ symmetry. Approximate orbital energies and compositions shown for (SiP₃)FeCCH₃ (TPSS/Def2-TZVP/D3ZERO). (Inset) DFT-predicted spin density map for [(SiP₃)FeCCH₃]⁺ (isovalue: 0.005 e⁻/ų).

Figure 9.

(Top) Representative chemical depictions of the related amino- and alkylcarbynes (SiP₃)Fe(CNMe₂)⁺ and (SiP₃)Fe(CCH₃)⁺, along with formal iron valence assignments. The true relative state of oxidation at iron is presumed to vary little due to strong covalency. The primary bonding of the carbyne to iron is indicated as x₃, x₂, or L, with backbonds from iron into empty p-orbitals on carbon indicated by the green arrows. Note that the formal oxidation states indicated have not been adjusted for the backbonds. A half arrow is used to indicate hyperconjugation stabilization for the C-centered radical Fe(IV) form of (SiP₃)Fe(CCH₃)⁺. (Bottom) DFT-optimized structure of the hypothetical molecule (SiP₃)Fe^IV(N), with a simplified MO diagram and lobal representations for the Si–Fe–N bonding in C₃ symmetry.

To further probe the electronic structure of paramagnetic [Fe≡CCH₃]⁺ 7 we undertook CW and pulse EPR studies. The doublet ground state of 7 can only be observed by EPR at very low temperatures due to fast electronic relaxation and line broadening at 77 K. At 10 K, an axial EPR spectrum exhibiting significant anisotropy between g-parallel at 2.61 and g-perpendicular at 1.96–1.93 is resolved (Figure 10), consistent with a largely iron-centered spin. Electron spin inversion recovery experiments show a strong temperature dependence for T₁ spin relaxation (Figure S41). This behavior is likely consistent with a low-lying, thermally populated doublet excited state or efficient electronic relaxation through coupling of low-energy vibrations (<100 cm⁻¹) with the bath. If an Orbach mechanism of electronic relaxation from the excited state is assumed,³⁷ a small energy difference (Δ) between the ground state and first excited state as low as Δ = 24 cm⁻¹ can be roughly calculated from the temperature dependence of T₁.³⁸

Figure 10.

X-band CW-EPR spectrum of [(SiP₃)FeCCH₃]OTf prepared in 7:1 2-MeTHF/MeCN. Acquisition parameters: temperature = 10 K; MW frequency = 9.637 GHz; MW power = 200 μW; modulation frequency = 100 kHz; modulation amplitude = 0.4 mT; conversion time = 82 ms.

EPR analysis of a paramagnetic carbyne/alkylidyne has previously been limited to a tungsten methylidyne cation ([(dmpe)₂(Cl)W^V≡CH]⁺; g_iso = 2.026; |A(¹³⁸W)| = 221 MHz; |A(³¹P)| = 149 MHz; |A(¹³C)| = 34 MHz; dmpe = bis(dimethylphosphino)ethane) and to examples with the (SiP₃)Fe-system.^27c,12 Interestingly, [(SiP₃)FeCNMe₂]⁺ and [Fe≡CCH₃]⁺ 7 both exhibit nearly identical EPR signatures.¹¹ Löwdin spin population and spin density analysis of 7 by DFT (TPSS/def2-TZVP/D3ZERO) predicts the majority of unpaired spin on iron (0.99 e⁻), with 0.07 e⁻ of opposite spin polarization on C_α (Figure 8) (cf. [(SiP₃)FeCNMe₂]⁺: 0.97 e⁻ on Fe, oppositely polarized to 0.03 e⁻ on C_α and 0.01 e⁻ on N_β). This prediction is in agreement with the large anisotropy between g-parallel and g-perpendicular and the strong temperature dependence observed for T₁ (Figure S41).

This predicted spin delocalization compares very well with that for a terminal iron(V)-nitride reported by Smith and co-workers.³³ A follow-up study on this system³⁹ invoked a dynamic “pseudo-Jahn–Teller” distortion for a C₃-symmetric iron center. Related electronic structure descriptions have been offered for ostensibly low-valent S = 1/2 (SiP₃)Fe(H₂) and TPBCo(H₂) complexes.⁴⁰ In Smith’s iron(V)-nitride complex, vibronic coupling to a partially filled, degenerate d(x²–y²)/d(xy) pair gives rise to significant g-anisotropy, and mixing with the unfilled d(xz)/d(yz) pair—termed “e–e mixing”—yields some spin delocalization onto the terminal nitride p-orbitals.³⁹ The electronic structure of [Fe≡CCH₃]⁺ 7 can likely be understood by a similar model: a nearly degenerate S = 1/2 ground state leads to large g-anisotropy, and mixing of unoccupied d(xz)/d(yz) Fe–C π*-bond orbitals with partially filled d(xy)/d(x²–y²) orbitals delocalizes modest spin onto C_α of the paramagnetic carbyne (Figure 8).⁴¹

As a direct probe of this hypothesis, pulse X-band HYSCORE and ENDOR spectroscopy of 7 was undertaken (Figures 11 and 12). From these complementary methods, comparison of data from (¹³C₂-7) and its natural abundance isotopologue shows hyperfine couplings for the ¹³C nuclei of C_α and C_β with the spin on iron. Simulation of the data gives an anisotropic hyperfine tensor for ¹³C_α|A(¹³C)| = [18, 33, 47] MHz; a_iso = 32.7 MHz; T = 14.5 MHz (Table 3), indicative of strong coupling to the ¹³C_α nuclear spin. Coupling to ¹³C_β is weaker and nearly isotropic (|A(¹³C)| = [5.8, 5.0, 4.0] MHz; a_iso = 4.9 MHz; T = 0.9 MHz). Following the analysis of Hoffman and co-workers,³⁹ the ¹³C_α hyperfine coupling tensor can be decomposed into isotropic and anisotropic terms; the anisotropic component allows estimation of spin density of approximately 0.06 electrons at C_α in the 2p(x) and 2p(y) orbitals, orthogonal to the Fe≡C—R bond vector. This estimate agrees well with the aforementioned Löwdin spin population analysis.

Figure 11.

(top) X-band HYSCORE spectrum of ¹³C₂-[(SiP₃)-FeCCH₃]OTf measured at 280 mT (g = 2.48). (Bottom) Monochromatic representation of the HYSCORE data with simulations of hyperfine couplings overlaid using simulations parameters in Table 3: (green) ³¹P, (red) ¹³C_α, (blue) ¹³C_β. Acquisition parameters: temperature = 6.5 K; microwave frequency = 9.715 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 168 ns, t₁ = t₂ = 100 ns; Δt₁ = Δt₂ = 16 ns; shot repetition time (srt) = 1 ms.

Figure 12.

(A) (black) X-band ¹³C–¹²C difference ENDOR spectra of (¹³C₂-7) and (¹²C₂-7); (red) ¹³C_α simulated fit; (blue) ¹³C_β simulated fit. (B) (black) X-band ¹H–²H difference ENDOR spectra of (¹H₃C-7) and (²H₃C-7); (red) ¹H₃C simulated fit 1; (blue) ¹H₃C simulated fit 2. Acquisition parameters: temperature = 6.5 K; MW frequency = 9.715 GHz; MW pulse length (π/2, π) = 40 ns, 80 ns; τ = 190 ns; RF pulse length = 15 μs; T_RF = 2 μs; shot repetition time = 5 ms.

Table 3.

Hyperfine Coupling Constants in MHz Determined for [(SiP₃)FeCCH₃]⁺ and Computed Spin Densities

nucleus	A1	A2	A3	aiso	spin density
Fe					0.99
31P	18	14	25	19.0	0.016
					0.008
					0.015
13Cα	18	33	47	32.7	−0.067
13Cβ	5.8	5.0	4.0	4.9	−0.006
1H3C	8.6	12.5	8.6	9.9	−0.004
	2.3	6.3	6.3	5.0	−0.001
					−0.0002

Interestingly, despite weak coupling to C_β, proton hyperfine coupling to the −CH₃ substituent of [Fe≡CCH₃]⁺ 7 is detected by both ENDOR and HYSCORE. Comparison of ²H-7 with its natural abundance isotopologue and simulation of the ENDOR difference spectrum (Figure 12) provides two hyperfine tensors for ¹H₁(CH₃) (|A(¹H)| = [8.6, 12.5, 8.6] MHz; a_iso = 9.9 MHz; T = 1.3 MHz) and ¹H₂(CH₃) (|A(¹H)| = [2.3, 6.3, 6.3] MHz; a_iso = 5.0 MHz; T = 1.3 MHz). Importantly, the isotropic coupling components for the carbyne −CH₃ substituent to the unpaired spin are correlated with a “hyperconjugation” interaction between the methyl Hatoms and the spin active Fe and C_α orbitals.^42–44

A hyperconjugation effect indicates, to some extent, delocalization of spin/hole or cationic character on Fe≡C_α to the hydrogen s-orbitals on the terminal methyl (see Figure 9 for this and other chemical depictions). This exaggerated resonance description is thus juxtaposed with the electronic structures typically invoked in heteroatom-substituted carbyne complexes, like our previously reported (SiP₃)FeCOSiMe₃, (DPB)Fe(COSiMe₃)₂, [(SiP₃)FeCNMe₂]^0/+, and [(SiP₃)-FeCNH₂]⁺.^20,21,23 In each of these cases, the Fe–C and C–O/N bonding is qualified by the free “lone pair” on the distal atom, capable of delocalizing bonding across the Fe–C–O/N unit and decreasing the formal valency at iron. The CNR₂ moiety of the amino carbynes are even planar (Σ∠ ≈ 360°), consistent with conjugation between nitrogen and the Fe–C π-bond. However, hyperconjugation and delocalization in [(SiP₃)FeCCH₃]⁺ should be appreciated as a subtle effect, as the spectroscopic data and DFT calculations determine a majority of unpaired spin to be located on iron (Fe: +0.99 e⁻ vs C_α: −0.07 e⁻). Furthermore, the strong similarity between the EPR data for [(SiP₃)FeCNMe₂]⁺ and [(SiP₃)FeCCH₃]⁺, the comparably modest calculated spin density across the amino carbyne (Fe: +0.97 e⁻ vs C_α: −0.03 e⁻ and N_β: −0.01 e⁻), and the close congruence between the Mössbauer parameters for all of the (SiP₃)Fe carbynes, both alkyl- and heteroatom-substituted, suggests that a “Fischer-like” resonance description (the L-type carbyne donor resonance forms in Figure 9) may in fact be of limited importance to the structural and electronic stabilities of this series of Fe≡C triply bonded species.

The latter point speaks to the potential relevance of other hypothetical iron-ligand multiply bonded species in the (SiP₃) Fe platform, namely terminal Fe≡C(H) carbide/methylidyne or Fe≡N nitride complexes (Figure 9, bottom), the latter possibly arising during the limited nitrogen reduction catalysis mediated by (SiP₃)FeN₂;⁴⁵ the former have yet to be directly evidenced from cleavage of CO, CN⁻, or C₂H₂ at iron. We have extensively explored the reactivity and thermochemistry of the early stages of reductive protonation on FeN₂ compounds. In the trisphosphine-borane TPBFeN₂, the N–N bond is cleaved to yield NH₃ and a spectroscopically characterized [(TPB)Fe^IV≡N]⁺ nitride cation.³ Observation of an iron-N₂H₄ hydrazine cation⁴⁶ derived from (SiP₃)FeN₂ questions the accessibility of any (SiP₃)Fe≡N species through functionalization of dinitrogen. However, the stable bonding in the [(SiP₃)FeCCH₃]^+/0 complexes, and the aforementioned implication for the electronic structures of the heteroatomsubstituted relatives, support the potential viability of highvalent, multiply bonded Fe≡C or Fe≡N intermediates in the chemistry of this system, and structurally related synthetic or biological iron sites.

CONCLUSION

To close, we have described the activation of acetylene gas at a mononuclear iron site, along with reductive protonation reactions that converge at stable and structurally unusual Fe(IV) and Fe(V) alkylcarbyne complexes. X-ray crystallography, pulse X-band EPR, and other spectroscopies have allowed us to characterize the first Fe(I) and Fe(II) adducts of acetylene, and to discriminate their side-on binding forms from alternative structural isomers (e.g., Fe=CCH₂). Characterization of their electronic structures, through Mössbauer, DFT, and pulse EPR experiments, reveal strong covalency in the Fe≡C triple bonds of the carbyne species and moderate spin delocalization in the cationic S = 1/2 congeners. Congruence between the structural, spectroscopic, and computed parameters of the heteroatom-substituted iron carbynes with those of the newly explored alkylcarbynes described here establishes the viability of a highly covalent Fe-to-C triple bond in trigonal symmetry without a requirement for heteroatom resonance stabilization. More generally, the complexes and spectroscopic signatures reported here may guide the ongoing investigation of reactive nitrogenase intermediates with non-native substrates, particularly acetylenic adduct species, or possible carbyne intermediates of Fischer–Tropsch type C–C couplings with C₁ substrates.