Valence Localization in Alkyne and Alkene Adducts of Synthetic [Fe₄S₄]⁺ Clusters

Abstract

Reported herein are alkyne and alkene adducts of synthetic [Fe₄S₄]⁺ clusters that model intermediates and inhibitor-bound states in enzymes involved in isoprenoid biosynthesis. Treatment of the N-heterocyclic carbene-ligated cluster [(IMes)₃Fe₄S₄(OEt₂)][BAr^F₄] (IMes = 1,3-dimesitylimidazol-2-ylidene; [BAr^F₄]⁻ = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) with phenylacetylene (PhCCH) or cis-cyclooctene (COE) results in displacement of the Et₂O ligand to yield the corresponding π-complexes, [(IMes)₃Fe₄S₄(PhCCH)][BAr^F₄] and [(IMes)₃Fe₄S₄(COE)][BAr^F₄]. EPR spectroscopic analysis demonstrates that both clusters are doublets with g_iso > 2, and thus are spectroscopically faithful models of the analogous species characterized in the isoprenoid biosynthetic enzymes IspG and IspH. Structural and Mössbauer spectroscopic analysis reveals that both complexes are best described as [Fe₄S₄]⁺ clusters in which the unique Fe site engages in modest backbonding to the π-acidic ligand. Paramagnetic NMR studies show that, even at room temperature, the alkyne-/alkene-bound Fe centers harbor minority spin, and therefore adopt an Fe²⁺ valence. We propose that such valence localization could likewise occur in Fe–S enzymes that interact with π-acidic molecules.

Graphical Abstract

The synthesis and characterization of [Fe₄S₄]⁺ clusters with alkyne and alkene ligands is reported. These complexes exhibit unusual EPR spectroscopic properties that mirror those observed for alkyne and alkene adducts of [Fe₄S₄]⁺ clusters proposed in enzymatic catalysis. Paramagnetic NMR analysis demonstrates an unusual valence localization pattern in which the Fe bound by the π-acidic ligands maintains Fe²⁺ character, even at ambient temperature.

Introduction

The Fe–S cluster enzymes IspG (GcpE) and IspH (LytB) catalyze the final steps in the isoprenoid biosynthesis pathway used by most bacteria and malaria parasites, the methylerythritol phosphate pathway.^1,2 IspG carries out the two-electron, two-proton reduction of 2-C-methyl-d-erythritol-2,4-cyclodiphosphate (MEcPP) to 4-hydroxy-3-methylbut-2-enyl 1-diphosphate (HMBPP) (Fig. 1A). The latter is further reduced by IspH to yield the key precursors to isoprenoid natural products: isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP).^3–5 IspG and IspH are attractive drug targets because the methylerythritol phosphate pathway is not used by humans,⁶ and as such there has been interest in identifying inhibitors for these enzymes (Fig. 1).^7–11 Approaches to inhibitor design take inspiration from how substrates and products are thought to bind. Specifically, in the native IspG and IspH reactions, several species with alkenes bound to the unique Fe site of the [Fe₄S₄] cluster have been proposed^12–14 (Fig. 1B), and alkyne inhibitors have been designed that likewise feature proposed Fe–π interactions between the unique Fe center of the cluster and the inhibitor.^7,15

Figure 1.

Substrates, products, and inhibitors that form π complexes with [Fe₄S₄] clusters in the methylerythritol phosphate pathway of isoprenoid biosynthesis. A) Reactions catalyzed by IspG and IspH. B) Proposed structures of π complexes formed at the active sites of IspG and IspH.

The mechanisms of IspG and IspH have been extensively studied by EPR spectroscopy,^{12–14,16–21} and the key spectroscopic signatures for these π-complexes are their unusual g-tensors. Whereas the S = 1/2 spin states of [Fe₄S₄]⁺ clusters typically give rise to signals with g_iso < 2 (e.g., as observed for clusters ligated by four thiolates),^22,23 the proposed alkene complexes in IspG and IspH feature g_iso > 2.¹² Alkyne inhibitor-bound samples show similar EPR signals with g_iso > 2,¹² suggesting analogous modes of substrate, product, and inhibitor binding. Given that thiolate-ligated [Fe₄S₄]³⁺ clusters display g_iso > 2, one explanation for the unusual g-tensors for the π-complexes of [Fe₄S₄]⁺ clusters is that their electronic structures are dominated by resonance structures consisting of [Fe₄S₄]³⁺ clusters with metallacyclopropene/metallacyclopropane ligands^24,25 (Fig. 2). However, evaluating this and related proposals has been challenging because of a lack of high-resolution crystallographic data for π-complexes of IspG and IspH; the only crystallographically characterized π-complex of either enzyme is IspH bound to a pyridine-based inhibitor that binds in an η² C–N fashion with partial occupancy of the unique Fe site.¹⁴ Given this backdrop, as well as proposals for alkyne binding to synthetic [Fe₄S₄] clusters during alkyne reduction to alkenes^26–28 and the importance of alkyne- and alkene-bound intermediates in reduction of alternative substrates by nitrogenases,^29,30 we set out to synthesize and characterize synthetic [Fe₄S₄]⁺ clusters bound by alkynes and alkenes.

Figure 2.

Limiting resonance structures describing the bonding between [Fe₄S₄]⁺ clusters and alkynes/alkenes.

Experimental

General Considerations

All reactions were performed using standard Schlenk techniques or in an LC Technologies inert atmosphere glove box under an N₂ atmosphere. Glassware was dried in an oven at 160 °C prior to use. Molecular sieves (3 Å) and Celite^® were activated by heating to 300 °C overnight under vacuum prior to storage under an N₂ atmosphere. O-difluorobenzene (DFB) was distilled from CaH₂, C₆D₆ was degassed by three freeze–pump–thaw cycles, and other solvents were degassed by sparging with argon and dried by passing through a column of activated alumina. All solvents were stored under an N₂ atmosphere over 3 Å molecular sieves.

NMR spectra were recorded on Bruker 400 and 500 MHz spectrometers. ¹H chemical shifts are given relative to residual solvent peaks; spectra in Et₂O and DFB are referenced to the triplet of residual n-pentane at 0.89 ppm. Solvent suppression for NMR in protonated solvents was carried out using WET solvent suppression.³¹ FT-IR samples were taken as thin films using a Bruker Alpha Platinum ATR spectrometer with OPUS software in a glovebox under an N₂ atmosphere. EPR spectra were recorded on a Bruker EMX spectrometer at 9.37 GHz as frozen glasses. Simulations were performed using EasySpin³² (5.2.21) in Matlab (R2017b); all spectra were simulated using the ‘pepper’ function with lw = 1 mT. UV-vis spectra were recorded on a Cary 50 spectrometer. Zero-field ⁵⁷Fe Mössbauer spectra were measured with a constant-acceleration spectrometer at 150 K. Isomer shifts are quoted relative to α-Fe foil at room temperature. Mössbauer spectra were simulated with WMOSS v.4.³³ Elemental analyses were performed at Midwest Microlab. X-ray structural determinations were performed at the MIT diffraction facility using a Bruker X8 diffractometer with an APEX II CCD detector or a Bruker D8 Venture diffractometer with a Photon2 CPAD detector. Diffraction data was collected, integrated, and corrected for absorption using Bruker APEX3 software and its associated modules (SAINT, SADABS, TWINABS). Structural solutions and refinements (on F²) were carried out using SHELXT and SHELXL-2018 in ShelXle.³⁴ Ellipsoid plots and figures were made using Mercury 2021.2.0.

(IMes)₃Fe₄S₄Cl,³⁵ Ti(N[^tBu]Ar)₃,³⁶ Na[BAr^F₄],³⁷ and [Cp₂Fe][BAr^F₄]³⁸ were prepared according to literature procedures. [1-OEt₂][BAr^F₄] was prepared by a modification of the literature procedure³⁹ (vide infra). Phenylacetylene (PhCCH) and cis-cyclooctene (COE) were degassed by three freeze-pump-thaw cycles.

Statement on Compound Purity

The purity of all compounds was assessed by a variety of spectroscopic and analytical methods as detailed below. Compounds [1-Cl]⁺, [1-PhCCH]⁺, and [1-COE]⁺ are air-sensitive but can be isolated as crystalline solids in high purity as determined by NMR, EPR, and Mössbauer spectroscopic analysis (see Fig. 3 and the SI) as well as H and N content from elemental analysis; low C content was obtained by elemental analysis as has been observed for other members of this class of molecules^40,41 and in other contexts.⁴² Elemental analysis results are as follows: [1-Cl][BAr^F₄]: Anal. Found: C, 51.84; H, 4.04; N, 4.19. Calcd. for C₉₅H₈₄N₆Fe₄S₄BF₂₄Cl: C, 52.74; H, 3.91; N, 3.88; [1-PhCCH][BAr^F₄]: Anal. Found: C, 51.09; H, 4.05; N, 3.72. Calcd. for C₁₀₃H₉₀N₆Fe₄S₄BF₂₄: C, 55.47; H, 4.07; N, 3.77; [1-COE][BAr^F₄]: Anal. Found: C, 54.34; H, 4.82; N, 3.92. Calcd. for C₁₀₃H₉₈N₆Fe₄S₄BF₂₄: C, 55.27; H, 4.41; N, 3.75. ¹H NMR spectra for [1-PhCCH]⁺ and [1-COE]⁺ are reported in both DFB and Et₂O because, in DFB, the solvent resonances obscure some of the aryl resonances of [1-PhCCH]⁺ and [1-COE]⁺, whereas in Et₂O, both [PhCCH]⁺ and [1-COE]⁺ exist in equilibrium with [1-OEt₂]⁺ in the absence of excess PhCCH or COE. The ¹H NMR spectra in DFB demonstrate that as-prepared [1-PhCCH]⁺ and [1-COE]⁺ are pure and free of [1-OEt₂]⁺, and the ¹H NMR spectra in Et₂O allow the aryl resonances to be assigned. In combination, the spectra support the purity and NMR assignments of [1-PhCCH]⁺ and [1-COE]⁺.

Figure 3.

X-band EPR spectra (black) and simulations (red) of [1-THF]⁺ (top, 15 K, 63 μW, 9.370 GHz, 10:1 THF/toluene, g-strain = [0.019 0.015 0.018]), [1-PhCCH]⁺ (middle, 15 K, 63 μW, 9.374 GHz, 10:1 Et₂O/toluene, g-strain = [0.035 0.007 0.009]), and [1-COE]⁺ (bottom, 15 K, 16 μW, 9.374 GHz, 10:1 DFB/toluene, g-strain = [0.030 0.005 0.008].).

[(IMes)₃Fe₄S₄Cl][BAr^F₄] ([1-Cl]⁺):

(IMes)₃Fe₄S₄Cl (500 mg, 0.385 mmol) was suspended in 5 mL Et₂O. A solution of [Cp₂Fe][BAr^F₄] (403 mg, 0.385 mmol, 1 equiv.) in 5 mL Et₂O was added dropwise. The mixture was stirred for 5 min and concentrated to 2 mL. The product was precipitated upon addition of n-pentane (15 mL). The solids were collected on a frit and washed thoroughly with n-pentane (3 × 5 mL) to remove Cp₂Fe. Yield: 722 mg (87%). ¹H NMR (400 MHz, Et₂O, 293 K) δ 7.77 (s, 8H, [BAr^F₄]⁻), 7.51 (s, 4H, [BAr^F₄]⁻), 7.00 (s, 6H, backbone CH), 6.91 (s, 12H, Mes m-CH), 2.39 (s, 18H, Mes p-CH₃), 2.16 (s, 36H, Mes o-CH₃). Crystals suitable for X-ray diffraction were grown by layering n-pentane onto a solution of [1-Cl][BAr^F₄] in Et₂O and storage at −35 °C overnight.

[(IMes)₃Fe₄S₄(OEt₂)][BAr^F₄] ([1-OEt₂]⁺):

The preparation reported here is a modification of previously reported procedures.^35,39 [(IMes)₃Fe₄S₄Cl][BAr^F₄] (148 mg, 0.068 mmol) was dissolved in 2 mL Et₂O. A solution of Ti(N[^tBu]Ar)₃ (79 mg, 0.136 mmol) in 2 mL Et₂O was added dropwise. The solution was concentrated to 0.5 mL and n-pentane (10 mL) was added to precipitate the product. The precipitate was collected and recrystallized from Et₂O/n-pentane a second time. Yield: 126 mg (84%). Spectroscopic data were consistent with previous reports.³⁵

[(IMes)₃Fe₄S₄(PhCCH)][BAr^F₄] ([1-PhCCH]⁺):

[(IMes)₃Fe₄S₄(OEt₂)][BAr^F₄] (77 mg, 0.035 mmol) was dissolved in 1 mL Et₂O. An excess (ca. 100 mg) of PhCCH was added. The product was precipitated with n-pentane and recrystallized from Et₂O/n-pentane in the presence of excess (ca. 1 mM) PhCCH. Yield: 78 mg (99 %). ¹H NMR (400 MHz, DFB, 293 K) δ 203 (s, 1H, PhCCH), 10.84 (s, 2H, PhCCH o-H), 8.82 (t, 1H, PhCCH p-H), 8.77 (d, 2H, PhCCH m-H), 8.29 (s, 8H, [BAr^F₄]⁻), 7.65 (s, 4H, [BAr^F₄]⁻), 2.47 (s, 18H, Mes p-CH₃), 2.39 (br s, 36H, Mes o-CH₃). IMes aryl resonances are obscured by overlap with DFB resonances. ¹H NMR (400 MHz, Et₂O, 293 K, recorded in the presence of excess PhCCH to prevent formation of [1-OEt₂]⁺) δ 200 (s, 1H, PhCCH), 10.71 (s, 2H, PhCCH o-H), 8.74 (t, 1H, PhCCH p-H), 8.67 (d, 2H, PhCCH m-H), 7.75 (s, 8H, [BAr^F₄]⁻), 7.50 (s, 4H, [BAr^F₄]⁻), 7.28 (s, 6H, backbone CH), 6.98 (s, 12H, Mes m-CH) 2.42 (s, 18H, Mes p-CH₃), 2.37 (br s, 36H, Mes o-CH₃). EPR: g₁ = 2.184, g₂ = 2.017, g₃ = 1.996 (15 K, 63 μW, 9.37 GHz). FT-IR (thin film, cm⁻¹): 1812 (C≡C stretch). This compound was crystallized as the triflate salt by addition of an excess of PhCCH to a solution of [(IMes)₃Fe₄S₄(THF)][OTf]³⁹ in THF followed by vapor diffusion of n-pentane into the THF solution at room temperature overnight.

[(IMes)₃Fe₄S₄(C₈H₁₄)][BAr^F₄] ([1-COE]⁺):

[(IMes)₃Fe₄S₄(OEt₂)][BAr^F₄] (77 mg, 0.035 mmol) was dissolved in 1 mL Et₂O. An excess (ca. 100 mg) of COE was added. The product was precipitated with n-pentane and recrystallized from Et₂O/n-pentane in the presence of excess (ca. 1 mM) COE. Yield: 56 mg (72 %). ¹H NMR (400 MHz, DFB, 293 K, recorded in the presence of excess COE) δ 13.31 (s, 2H, C₈H₁₄), 8.29 (s, 8H, [BAr^F₄]⁻), 7.65 (s, 4H, [BAr^F₄]⁻), 7.44 (s, 2H, C₈H₁₄), 7.29 (s, 6H, backbone CH), 6.52 (s, 2H, C₈H₁₄), 3.17 (s, 2H, C₈H₁₄), 2.78 (s, 2H, C₈H₁₄) 2.50 (s, 18H, Mes p-CH₃), 2.44 (br s, 36H, Mes o-CH₃), 1.82 (s, 2H, C₈H₁₄), −35.8 (s, 2H, C₈H₁₄). IMes m-H resonances are obscured by overlap with DFB resonances. ¹H NMR (400 MHz, Et₂O, 293 K, recorded in the presence of excess cyclooctene, [1-OEt₂]⁺ is present due to an equilibrium between Et₂O and COE coordination in Et₂O solutions) δ 13.36 (s, 2H, C₈H₁₄), 7.74 (s, 8H, [BAr^F₄]⁻), 7.65 (s, 6H, backbone CH), 7.57 (br s, 2H, C₈H₁₄, overlaps with [BAr^F₄]⁻ resonances), 7.49 (s, 4H, [BAr^F₄]⁻), 7.04 (s, 12H, IMes m-H), 6.46 (s, 2H, C₈H₁₄), 3.12 (s, 2H, C₈H₁₄), 2.70 (s, 2H, C₈H₁₄) 2.48 (s, 18H, Mes p-CH₃), 2.40 (br s, 36H, Mes o-CH₃), 1.68 (s, 2H, C₈H₁₄), −36.4 (s, 2H, C₈H₁₄). EPR: g₁ = 2.175, g₂ = 2.011, g₃ = 1.992 (15K, 16 μW, 9.37 GHz). Crystals suitable for X-ray diffraction were grown by layering n-pentane onto a solution of [1-COE][BAr^F₄] in Et₂O and storage at −35 °C overnight.

Results and Discussion

To synthesize alkyne and alkene adducts of [Fe₄S₄]⁺ clusters, we used the (IMes)₃Fe₄S₄ platform (IMes = 1,3-dimesitylimidazol-2-ylidene),³⁵ which features three bulky IMes ligands that enforce 3:1 site-differentiation of the cluster and allow for the installation of labile, neutral ligands at the unique Fe site. We first prepared the previously reported cluster³⁵ [(IMes)₃Fe₄S₄(OEt₂)][BAr^F₄] ([1-OEt₂]⁺; Ar^F = 3,5-bis(trifluoromethyl)phenyl) by treatment of [(IMes)₃Fe₄S₄Cl][BAr^F₄] ([1-Cl]⁺) with Ti(N[^tBu]Ar)₃ (Ar=3,5-dimethylphenyl)³⁶ in diethyl ether (Et₂O). Subsequent treatment of [1-OEt₂]⁺ with an excess of PhCCH or COE in Et₂O afforded the alkyne and alkene adducts [(IMes)₃Fe₄S₄(PhCCH)][BAr^F₄] ([1-PhCCH]⁺) or [(IMes)₃Fe₄S₄(COE)][BAr^F₄] ([1-COE]⁺), respectively, as black, microcrystalline solids after crystallization from Et₂O and n-pentane (Scheme 1). Both complexes possess C_3v symmetry on the NMR timescale, consistent with relatively fast rotation of the PhCCH and COE ligands around the apical Fe site. The ¹H NMR spectrum of [1-COE]⁺ reveals that the methylene protons on the COE ligand are diastereotopic, indicating that the COE ligand does not dissociate and recoordinate on the NMR timescale. The alkyne and alkene ligands are weakly bound to the clusters and can be displaced by Et₂O. In Et₂O solutions, both [1-PhCCH]⁺ and [1-COE]⁺ exist in equilibrium with [1-OEt₂]⁺ in the absence of excess PhCCH or COE, respectively; even in the presence of excess COE, [1-COE]⁺ is observed in equilibrium with [1-OEt₂]⁺ in Et₂O (Fig. S5). However, no equilibrium with a solvent adduct is observed for [1-PhCCH]⁺ or [1-COE]⁺ in o-difluorobenzene (DFB), which is more weakly coordinating than Et₂O (Fig. 3, S1, and S2).

Scheme 1.

Synthesis of [1-PhCCH]⁺ and [1-COE]⁺. [BAr^F₄] anions omitted for clarity.

Both [1-PhCCH]⁺ and [1-COE]⁺ have S = 1/2 ground spin states as determined by EPR spectroscopy (Fig. 3) with g_iso = 2.066 for [1-PhCCH]⁺ and g_iso = 2.059 for [1-COE]⁺. Comparatively, the other reported [Fe₄S₄]⁺ clusters with IMes ligands^35,39 have g_iso ~ 2 (cf. g_iso = 1.990 for [1-THF]⁺).³⁵ Thus, the unusually high g_iso values for [1-PhCCH]⁺ and [1-COE]⁺ mirror those observed for proposed alkyne and alkene adducts of the [Fe₄S₄] clusters in IspH and IspG (vide supra),^12,13 and on this basis we conclude that [1-PhCCH]⁺ and [1-COE]⁺ are appropriate models for π-complexes of protein-bound [Fe₄S₄] clusters.

As noted above, the unusual g_iso values observed for alkene- and alkyne-bound [Fe₄S₄]⁺ clusters have raised interest in the electronic structures of these adducts, particularly as to what extent they are best described as [Fe₄S₄]³⁺ clusters featuring metallacyclopropene/metallacyclopropane ligands.¹³ Structural analysis of the Fe–C and C–C (alkyne/alkene) distances as well as the metrics of the [Fe₄S₄] core could speak to this issue, with short Fe–C bonds, long C–C bonds, and contracted [Fe₄S₄] cores being the hallmarks of a dominant [Fe₄S₄]³⁺ resonance structure. However, analysis of such parameters for enzymatic alkyne and alkene adducts of [Fe₄S₄] clusters is hampered by a lack of high-resolution X-ray crystallographic data for these species, and we therefore undertook structural studies of [1-PhCCH]⁺ and [1-COE]⁺.

Compound [1-PhCCH]⁺ was crystallized as its triflate salt (prepared as described in the SI) by diffusion of n-pentane into a THF solution, and [1-COE]⁺ was crystallized with a [BAr^F₄] anion by layering n-pentane onto an Et₂O solution. Their structures were determined by single-crystal X-ray diffraction (Fig. 4), which verified their identities as π-complexes of [Fe₄S₄] clusters. The C–C distance in [1-PhCCH]⁺ (1.253(3) Å) is between that of a free C≡C bond (1.17–1.20 Å)⁴³ and that of a C=C bond (1.31–1.34 Å),⁴³ and the C–C distance in [1-COE]⁺ (1.390(2) Å) is closer to that of a free C=C bond than that of a C–C bond (1.53–1.55 Å).⁴³ Both complexes can also be compared to structurally characterized high-spin Fe¹⁺–alkyne and –alkene complexes,^44–54 which show average C–C bond lengths of 1.271(7) Å (alkyne) and 1.40(2)Å (alkene). High-spin Fe¹⁺ compounds would be expected to have a greater resonance contribution from the metallacyclopropene/metallacyclopropane structures than the formally Fe²⁺/Fe³⁺ centers in an [Fe₄S₄]⁺ cluster, though even the former are typically dominated by the neutral alkyne/alkene resonance structure.⁴⁸ That the C–C distances in [1-PhCCH]⁺ and [1-COE]⁺ are similar to or shorter than those in the analogous Fe¹⁺ compounds supports the description of the cluster complexes as neutral alkyne and alkene adducts of [Fe₄S₄]⁺ clusters. Consistent with the structural analysis of the C–C distances, the IR spectrum of [1-PhCCH]⁺ reveals that its PhCCH ligand is less activated than that in analogous Fe¹⁺ compounds (v(C≡C) = 1812 cm⁻¹ in [1-PhCCH]⁺ compared to 1730(10) cm⁻¹ on average for Fe¹⁺–(PhCCH) compounds^44,45,50,52). Furthermore, the relatively long average Fe–C(alkyne/alkene) distances in [1-PhCCH]⁺ (2.003(3) Å) and [1-COE]⁺ (2.128(2) Å) compared to the corresponding distances in high-spin Fe¹⁺–alkyne (1.96(2) Å) and –alkene (2.04(3) Å) complexes demonstrate that the clusters engage in weaker backbonding to the π-accepting ligand.

Figure 4.

Single-crystal X-ray diffraction structures of [1-PhCCH]⁺ (top) and [1-COE]⁺ (bottom). Thermal ellipsoids shown at 50% probability with carbon (gray), iron (orange), sulfur (yellow), and nitrogen (blue). IMes ligands are shown as sticks and solvent molecules, anions and H-atoms except for the alkynyl and alkenyl H-atoms are omitted for clarity.

Additionally, the core Fe–S bond metrics indicate that the [Fe₄S₄] cores of [1-PhCCH]⁺ and [1-COE]⁺ are more reduced than would be expected for an [Fe₄S₄]³⁺ cluster. Although no crystal structures of IMes-supported [Fe₄S₄]³⁺ clusters have been reported, the Fe–S bond metrics of [1-PhCCH]⁺ and [1-COE]⁺ can be compared to those of [1-OEt₂]⁺ (an [Fe₄S₄]⁺ cluster)³⁵ and [1-Cl]⁺ (an [Fe₄S₄]²⁺ cluster, whose structure is reported here). The average Fe–S distance in [1-Cl]⁺ is 2.275(5) Å, which is 0.007 Å shorter than the average Fe–S distance in [1-OEt₂]⁺ (2.282(1) Å). This decrease, although modest, is consistent with well-established trends in Fe–S structures, which tend to show decreases in Fe–S distances upon oxidation.⁵⁵ The average Fe–S distance in both [1-PhCCH]⁺ and [1-COE]⁺ is 2.279(2) Å, which is between that of [1-OEt₂]⁺ and [1-Cl]⁺, and, therefore, longer than would be predicted for an [Fe₄S₄]³⁺ cluster.

A similar picture emerges from Mössbauer studies of [1-PhCCH]⁺ and [1-COE]⁺. Mössbauer spectra of [1-PhCCH]⁺, [1-COE]⁺, and [1-OEt₂]⁺ were collected at 150 K due to magnetic broadening at lower temperatures (Fig S12–S14). Additionally, in these spectra, the quadrupole doublet for the unique Fe site cannot be resolved, and as such many simulations of the data are possible. For this reason, we limit our discussion to the average Mössbauer isomer shift (δ_avg). For reference, δ_avg for [1-OEt₂]⁺ is 0.49 mm/s at 150 K; if swapping the π-neutral Et₂O ligand for a π-acceptor ligand such as PhCCH or cyclooctene were to result in a two-electron oxidation of the cluster core to the [Fe₄S₄]³⁺ state, we would expect δ_avg to decrease by ~0.17 mm s⁻¹ to ~0.32 mm s⁻¹.⁵⁶ Instead, the observed δ_avg values for [1-PhCCH]⁺ and [1-COE]⁺ are only modestly lower than for [1-OEt₂]⁺, at 0.41 and 0.45 mm s⁻¹ respectively. Decreases in δ_avg of 0.08 and 0.04 mm s⁻¹, respectively, are consistent with increased Fe–L covalency via backbonding to the π-acceptor ligands and minor contributions from resonance structures involving electron transfer to the unsaturated ligands (i.e., an [Fe₄S₄]³⁺ description and/or resonance structures involving one-electron transfer to generate an alkyne or alkene radical anion and an [Fe₄S₄]²⁺ cluster; analogous resonance structures involving single-electron transfer have been used to describe the bonding in high-spin Fe¹⁺-alkyne complexes⁴⁸). Thus, the Mössbauer, structural, and vibrational analysis (vide supra), as well as the compounds’ cyclic voltammograms (which show that the redox couples for [1-PhCCH]⁺ and [1-COE]⁺ are shifted positively compared with those of [1-OEt₂]⁺; Fig. S24 and S25 and Table S4), all indicate modest backbonding from the cluster to the π-acidic ligands.

We additionally considered the possibility that, despite alkyne and alkene binding not leading to complete, two-electron oxidation of the cluster, these π-acidic ligands could induce localization of ferrous character at the unique Fe site, and that such localization could be experimentally probed using ¹H NMR spectroscopy. As is well-established,⁵⁷ [Fe₄S₄]⁺ clusters can be described as a pair of spin-aligned Fe^2.5+ centers (S = 9/2, majority spin) anti-ferromagnetically coupled to a pair of spin-aligned Fe²⁺ centers (S = 4, minority spin) to give an overall S = 1/2 spin state. For homoleptic, synthetic clusters such as [Fe₄S₄(IⁱPr^Me)₄]⁺ (IⁱPr^Me = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), all four Fe valences within the cluster rapidly exchange on the NMR timescale, such that each Fe site has an effective valence of Fe^2.25+ and net majority spin. As a result, only one set of ligand resonances is observed in the ¹H NMR spectrum.³⁵ Valence averaging is likewise observed for the three IMes-bound sites in [1-PhCCH]⁺ and [1-COE]⁺. However, exchange of valences between chemically inequivalent sites, such as between the IMes- and alkyne/alkene-bound sites in [1-PhCCH]⁺ and [1-COE]⁺, may or may not occur. That is, the difference in the Fe coordination spheres could lead to preferential localization of different valences at different sites^40,58,59—here, localization of Fe²⁺ at the Fe sites coordinated by the more π-accepting alkyne and alkene ligands. If, in the spectrum of thermally populated states, the unique Fe sites adopts primarily an Fe²⁺ valence, it will reside preferentially in the 2×Fe²⁺ pair and it will have net minority spin density. On the other hand, if no valence localization occurs at a given temperature and on a given timescale, then all four sites will adopt Fe²⁺ and Fe^2.5+ valences with equal probability and thus all four Fe centers will appear to have majority spin density. Therefore, if it is determined that the alkyne- or alkene-bound Fe sites have minority spin density, it follows that these sites adopt localized Fe²⁺ valences.

The sign of the spin density at the unique Fe site can be predicted from the ¹H NMR chemical shifts of the ligands bound to the unique site if the mechanism of spin transfer from the metal center to the H-atoms on the ligand is understood,⁶⁰ because the sign of the spin density on an H atom can be directly read out from the variable temperature behavior of the ¹H NMR resonance; at reasonably high temperatures (i.e., near room temperature), positive spin density on a ¹H center is manifested as the resonance shifting downfield with decreasing temperature, and negative spin density on a ¹H center is manifested as the resonance shifting upfield with decreasing temperature.^59,61–63

To determine how the sign of the spin density at the unique Fe site is reflected in the paramagnetic NMR shifts of the alkyne/alkene ligands, we computationally analyzed the molecular orbitals and spin densities of the Fe²⁺ model complexes [FeCl₃–C₂H₂]⁻ and [FeCl₃–C₂H₄]⁻. In both complexes, the spin on the alkyne or alkene ligand is dominated by the β-spin electron involved in Fe–alkyne/alkene π-bonding; the β-spin π-backbonding orbital is more delocalized over the alkyne/alkene than the α-spin π-backbonding orbital due to spin polarization induced by the unpaired α-spin electrons localized on the Fe center. For [FeCl₃–C₂H₂]⁻, the Fe d orbital involved in backbonding is mixed with both the C–C π-symmetry orbital and the C–H σ-symmetry orbitals, resulting in delocalization of β-spin from Fe to the alkynyl H atoms (Fig. 5A, left). We would therefore expect the Fe and alkynyl H atoms to have opposite spin because the Fe spin is dominated by the five unpaired d electrons in the α manifold and the H spin is dominated by the π-backbonding electron in the β manifold.

Figure 5.

Spin density analysis of [Cl₃Fe–L]⁻ model complexes and [Fe₄S₄]⁺ cluster π complexes. A) Calculated spin density plots of (left) [Cl₃Fe–C₂H₂]⁻ (0.001 au) showing β-spin density on the alkynyl H-atom and (right) [Cl₃Fe–C₂H₄]⁻ (0.0005 au) showing α-spin density on the alkenyl H-atom. Blue is α-spin and red is β-spin. B) VT NMR spectra of (left) the alkynyl proton in [1-PhCCH]⁺, demonstrating that the resonance shifts downfield with decreasing temperature and (right) the alkenyl protons in [1-COE]⁺, demonstrating that the resonance shifts upfield with decreasing temperature. The change in lineshape around −60 °C is due to a decrease in molecular symmetry to C₃. C) Experimentally deduced spin densities of (left) [1-PhCCH]⁺ and (right) [1-COE]⁺.

In contrast, a similar analysis of the alkene adduct [FeCl₃–C₂H₄]⁻ shows that the dominant spin-transfer mechanism to the alkenyl H is via spin polarization⁶⁰ of the C–H bond by the β-spin density in the C–C π system (Fig. 5A, right). This results in the Fe center and alkenyl H atoms bearing spin density of the same sign in alkene adducts of high-spin, tetrahedral Fe²⁺. Thus, if we observe that the alkynyl proton in [1-PhCCH]⁺ shifts downfield with decreasing temperature and the alkenyl protons in [1-COE]⁺ shift upfield with decreasing temperature, we would conclude that the Fe center bears minority spin density and therefore adopts a localized Fe²⁺ valence in both clusters. [Note that in cases where there is no valence localization (all Fe centers are spin averaged to yield net majority spin) or there is localization of the majority spin Fe^2.5+ pair, we would expect the opposite trends in the ¹H NMR spectra.]

The chemical shift of the alkynyl proton in [1-PhCCH]⁺ follows approximately Curie behavior and shifts downfield from 203 ppm at 25 °C to 318 ppm at −60 °C (Fig. 5B, left and S19). Thus, the alkynyl proton bears positive spin density, and based on the above analysis, we conclude that the alkyne-bound Fe site bears minority spin density (Fig. 5C). The alkenyl protons in [1-COE]⁺ shift in the opposite direction, moving upfield with decreasing temperature from −36 ppm at 25 °C to −73 ppm at −60 °C (Fig. 5B, right and S20). Since the alkenyl protons are expected to bear spin of the same sign as the alkene-bound Fe, this finding is likewise consistent with minority spin at the unique Fe site (Fig. 5C). The experimental data therefore demonstrate valence localization of Fe²⁺ at the alkyne- and alkene-bound sites in both [1-PhCCH]⁺ and [1-COE]⁺. Broken-symmetry density functional theory analysis on [1-PhCCH]⁺ and [1-COE]⁺ reveals the same sign of the spin distribution onto the alkyne/alkene ligands as observed for the mononuclear model complexes (vide supra) and likewise supports the favorability of localizing Fe²⁺ at the alkyne-/alkene-bound site (Fig. S22 and Table S2).

Conclusions

We reported herein the first well-defined alkyne and alkene adducts of [Fe₄S₄]⁺ clusters. EPR spectroscopic analysis demonstrated that [1-PhCCH]⁺ and [1-COE]⁺, like the [Fe₄S₄]⁺–alkyne/alkene adducts proposed in IspG and IspH, have unusual g-tensors, with g_iso > 2. Examination of the clusters’ structural and spectroscopic properties revealed that [1-PhCCH]⁺ and [1-COE]⁺ are best described as [Fe₄S₄]⁺ clusters with covalent π-backbonding to alkyne/alkene ligands. The unusual g-tensors observed in both the synthetic and biogenic clusters likely arise from factors other than the presence of an [Fe₄S₄]³⁺ cluster, such as changes in the degree of spin canting and/or changes in the local site g-tensors. Variable-temperature ¹H NMR spectroscopy revealed that binding π-accepting alkynes and alkenes to the unique Fe site of 3:1 site differentiated [Fe₄S₄]⁺ clusters results in localization of ferrous character at the unique Fe site, and we propose this phenomenon also occurs in IspG and IspH intermediates and inhibitor-bound species, as well as in intermediates in reduction of organic substrates by nitrogenases. Lastly, we note that, in addition to being structurally faithful models for the proposed alkyne- and alkene-bound species in IspH and IspG, [1-PhCCH]⁺ and [1-COE]⁺ are rare examples of alkynes or alkenes bound to high-spin, formally Fe²⁺ sites.^64–66 This work therefore illustrates the broader utility of Fe–S clusters in studying high-spin, electron-rich Fe²⁺ sites with the ability to bind weakly donating ligands.