New Routes to Low-Coordinate Iron Hydride Complexes: The Binuclear Oxidative Addition of H₂

Abstract

The oxidative addition and reductive elimination reactions of H₂ on unsaturated transition-metal complexes are crucial in utilizing this important molecule. Both biological and man-made iron catalysts use iron to perform H₂ transformations, and highly unsaturated iron complexes in unusual geometries (tetrahedral and trigonal planar) are anticipated to give unusual or novel reactions. In this paper, two new synthetic routes to the low-coordinate iron hydride complex [L^tBuFe(μ-H)]₂ are reported. Et₃SiH was used as the hydride source in one route by taking advantage of the silaphilicity of the fluoride ligand in three-coordinate L^tBuFeF. The other synthetic method proceeded through the binuclear oxidative addition of H₂ or D₂ to a putative Fe(I) intermediate. Deuteration was verified through reduction of an alkyne and release of the deuterated alkene product. Mössbauer spectra of [L^tBuFe(μ-H)]₂ indicate that the samples are pure, and that the iron(II) centers are high-spin.

Introduction

The oxidative addition of H₂ to transition metal complexes to form hydride complexes is now a well known reaction.[1] It has been studied in detail in part due to chemists’ desire to understand the mechanism of homogeneous catalytic transformations of H₂ such as hydrogenation.[2] The interaction of H₂ with transition metal complexes during oxidative addition is usually thought to proceed through a dihydrogen complex, followed by scission of the H-H bond to give a dihydride complex.[3-7]

The standard organometallic description of the first part of this reaction is that the σ orbital of H₂ interacts with an empty d orbital (Figure 1a). [1,8] This model has been supported by the spectroscopic and crystallographic characterization of hundreds of dihydrogen complexes, which typically have diamagnetic transition metal sites from strong-field ligands. [6] The subsequent H₂ cleavage formally oxidizes the metal by two electrons (Figure 1b).

Figure 1.

Interaction of dihydrogen orbitals with metal d orbitals. In the standard model, an empty d orbital lobe forms a σ interaction with the H-H bonding orbital, and two lobes of a filled d orbital donate into the empty σ* orbital of H₂.

The Holland research group has focused its organometallic chemistry efforts on complexes that have a high-spin electronic configuration.[9] We use bulky β-diketiminate ligands that are weak-field π-donors to enforce a low coordination number. High-spin complexes like these with 5 or more d electrons have no empty d orbitals, so the model above does not apply unless a spin-state change or other electronic reorganization can occur.

Recent computational and experimental work has been instrumental in demonstrating the importance of spin-state changes in organometallic chemistry. [10] [11] [12] [13] [14] [15] [16] One of our goals with β-diketiminate complexes is to discover new reactions that do not fit the usual organometallic mold and that may involve spin-state changes or use singly-occupied orbitals as acceptors (2-center/3-electron interactions). For example, alkyl complexes L^RFeCH₂CH₂R (L^R = L^Me or L^tBu, Figure 2) readily undergo β-hydride elimination upon mild heating.[17] As shown in Scheme 1, the transient hydride complex is trapped by the addition of another alkene. The β-hydride elimination mechanism was confirmed by the use of isotope labeling, activation parameters, and an H/D kinetic isotope effect (KIE) of 2.2.[17] Therefore, a pathway exists for high-spin complexes to undergo β-hydride elimination in the absence of completely empty d orbitals. Based on these ideas, we have also been interested to find examples of oxidative addition and reductive elimination, especially involving the fundamentally interesting molecule H₂.

Figure 2.

β-diketimate ligands L^R, where R indicates the substituent on the 2 and 4 positions of the C₃N₂ backbone.

Scheme 1.

In previous work, we isolated a dimeric hydride complex, [L^tBuFe(μ-H)]₂, from the reaction of L^tBuFeCl with KBEt₃H (Scheme 2a). [18] This was the first iron hydride complex with a coordination number less than five, and the only other one known is its close analogue [L^MeFe(μ-H)]₂.[19] [L^tBuFe(μ-H)]₂ is curious because the hydride bridges hold the two iron(II) ions exceptionally close to one another (2.624(2) Å). [18] Despite the dimeric structure in the solid state, in solution the great steric interference between the diketiminate ligands on the two iron atoms leads the molecule to partially dissociate into monomers L^tBuFeH, which were observed by ¹H NMR spectroscopy. [18] Both the monomer and dimer show ¹H NMR spectra with broadened resonances over a large chemical shift range, which are characteristic of paramagnetic molecules. Therefore, the available data suggested a high-spin electronic configuration for the iron(II) centers in [L^tBuFe(μ-H)]₂, but were not definitive because an excited electronic state might be accessed at room temperature.

Scheme 2.

The reactivity of the dimeric hydride complex, [L^tBuFe(μ-H)]₂, has been examined with a range of substrates in solution.[18-21] The most relevant reaction to this work is the addition of strong field ligands such as CO to the hydride complex, which resulted in reductive elimination of H₂ to form Fe(I) products (Scheme 3). [19] This observation shows that the reductive elimination of H₂ from [L^tBuFe(μ-H)]₂ is facile in the presence of strong field ligands. However, photolysis was required to eliminate H₂ in the presence of weaker ligands. For example, photolysis of the hydride complex under N₂ atmosphere resulted in loss of H₂ and formation of the dinitrogen complex, L^tBuFeNNFeL^tBu. [19] The intermediate species in this process are unknown, but it is conceivable that a highly unsaturated “L^tBuFe” species is formed; then this iron(I) intermediate would be trapped by N₂. The prospect of trapping this species with H₂ inspired us to evaluate the oxidative addition of H₂ to highly unsaturated iron(I) species in the absence of N₂.

Scheme 3.

The hydride complex also reacts with boranes R₃B to give L^tBuFe(μ-H)₂BR₂ and L^tBuFeR (Scheme 2b). [21] While the mechanistic study of this reaction was interesting, [21] the reactivity of [L^tBuFe(μ-H)]₂ towards boranes posed a practical problem. The synthesis of [L^tBuFe(μ-H)]₂ utilized KBEt₃H as the hydride source, giving BEt₃ as a byproduct. The desired hydride complex was the kinetic product of the reaction (formed within 15 minutes), but [L^tBuFe(μ-H)]₂ immediately began to react with the BEt₃ byproduct to give the thermodynamic product L^tBuFe(μ-H)₂BR₂ over several hours. Therefore, our samples of crude [L^tBuFe(μ-H)]₂ were invariably contaminated with the dihydridoborate complex, and the separation of the two complexes by crystallization was laborious. [19] Therefore, another motivation for the studies below was the development of a new synthetic route to [L^tBuFe(μ-H)]₂. Since [L^tBuFe(μ-H)]₂ demonstrated the ability to reductively eliminate H₂, we decided to explore the microscopic reverse, oxidative addition of H₂ to Fe(I), as a possible synthetic route. Here we report two new independent syntheses of [L^tBuFe(μ-H)]₂ and provide further characterization of the complex using Mössbauer spectroscopy.

Results and Discussion

Synthesis of [L^tBuFe(μ-H)]₂ from a Silane

We have previously shown that L^tBuFeF undergoes reactions with silylated substrates, eliminating Et₃SiF and leaving the formerly silicon-bound group on iron.[22] This methodology was used to generate [L^tBuFe(μ-H)]₂ from L^tBuFeF using Et₃SiH as a hydride source (Scheme 4). A slurry of pink L^tBuFeF in toluene was treated with 10 molar equivalents of Et₃SiH and was heated overnight at 100 °C to produce a red-brown solution. Removal of the volatile components and crystallization from a saturated Et₂O solution at -45 °C yielded [L^tBuFe(μ-H)]₂ in 83% yield. The identity of the product as the desired hydride complex was established through ¹H NMR spectroscopy in C₆D₆, which was compared to the literature spectrum.[18]

Scheme 4.

The synthesis of [L^tBuFe(μ-H)]₂ via Et₃SiH as a hydride source utilizes the formation of the strong Si-F bond as a thermodynamic driving force for the reaction. This method was inspired by Roesky, who used Et₃SiH as hydride source to generate a β–diketiminate zinc-hydride dimer from the corresponding zinc fluoride complex.[23] However, a stoichiometric amount of Et₃SiH was used in the zinc system while an excess was needed for the Fe system. Repeated attempts to generate [L^tBuFe(μ-H)]₂ using only 2 equivalents of Et₃SiH resulted in incomplete conversion, as evidenced by a small amount of L^tBuFeF remaining in the ¹H NMR spectrum. The use of excess Et₃SiH is not a problem as it is easily removed during the workup.

Synthesis from Dihydrogen

In another method, [L^tBuFe(μ-H)]₂ can be synthesized directly from L^tBuFeCl using H₂ as the hydride source (Scheme 5). Reduction of L^tBuFeCl in Et₂O with potassium graphite (KC₈) under argon gave a dark green mixture. The mixture was degassed and placed under an atmosphere of purified H₂, and the mixture turned red-brown. (The reaction is complete within 5 min of addition, as shown by a small-scale experiment in a J. Young NMR tube in C₆D₆.) After 18 h, H₂ was removed, and the mixture was filtered through Celite to yield a red-brown solution. The red-brown solution was identified as [L^tBuFe(μ-H)]₂ by ¹H NMR spectroscopy in C₆D₆.[18] Crystalline [L^tBuFe(μ-H)]₂ was obtained in 58% yield by cooling a saturated Et₂O solution to -45 °C. [L^tBuFe(μ-D)]₂ was obtained in 62% yield through the same reaction conditions by substituting D₂ for H₂. Characterization of [L^tBuFe(μ-D)]₂ by ¹H NMR spectroscopy in C₆D₆ revealed that the deuteride and the hydride complexes have identical ¹H NMR spectra. The hydrides are not visible by ¹H NMR spectroscopy, probably due to the proximity to the paramagnetic Fe atoms. Therefore, a different method was required to quantify the amount of deuteration in [L^tBuFe(μ-D)]₂.

Scheme 5.

We took advantage of the high-yield reaction of [L^tBuFe(μ-H)]₂ with 3-hexyne to form the three-coordinate Fe vinyl complex, L^tBuFeC(Et)=CHEt.[18] Subsequently, a sample of L^tBuFeC(Et)=CHEt was quenched with H₂O, and the volatile components were separated and examined by GC-MS. As expected, the parent ion of 3-hexene (m/z 84) was observed. The experiment was repeated with [L^tBuFe(μ-D)]₂ to give L^tBuFeC(Et)=CDEt, leading to 3-hexene-d₁. The same analysis as above gave a mass spectrum with the parent ion at m/z 85 and a small peak at m/z 84. The relative intensities of the peaks showed that the sample of [L^tBuFe(μ-D)]₂ was 89% deuterated.

The absence of N₂ is crucial for the success of the reaction of L^tBuFeCl with KC₈ and H₂ to give [L^tBuFe(μ-H)]₂. If any N₂ was present, the previously characterized dinitrogen complex, L^tBuFeNNFeL^tBu, was isolated. [24] Morris and co-workers have documented the parallels between H₂ and N₂ binding to unsaturated metal complexes. [25-27] They found that the fragments that bind N₂ to give a M-N₂ complex with an N-N stretching frequency below 2060 cm^-1 did not form a stable H₂ complex; rather, they oxidatively add H₂ to give a dihydride species. A few complexes that deviate from the trend have been reported.[28-32] In the system examined here, the parallel is that two “L^tBuFe” fragments can either cooperatively bind N₂ in L^tBuFeNNFeL^tBu or cooperatively add H₂ to give [L^tBuFe(μ-H)]₂. The N-N stretching frequency in L^tBuFeNNFeL^tBu is 1778 cm^-1,[24] so the observation of H₂ oxidative addition fits the established trend. We note that Mössbauer spectroscopy and DFT computations on L^tBuFeNNFeL^tBu show that the N₂ ligand is reduced by two electrons (N₂²⁻).[33] Therefore, the iron(I) L^tBuFe fragment gives exceptionally strong backbonding, and it is reasonable that the same bimetallic system oxidatively adds H₂ and binds N₂.

There are several possible mechanisms through which [L^tBuFe(μ-H)]₂ might be formed (Scheme 6). In each mechanism, we assume that L^tBuFeCl is first reduced by KC₈ to yield KCl and “L^tBuFe” (see below). In Scheme 6a, the L^tBuFe fragment binds H₂ to form L^tBuFe(H₂), and subsequently attracts another L^tBuFe fragment to give [L^tBuFe(μ-H)]₂. The π–backbonding from two Fe(I) centers results in homolytic cleavage of the H-H bond. However, it is possible that H₂ is reduced by one or two electrons in the mononuclear intermediate. A one electron reduction of H₂ would form a Fe(II) intermediate, L^tBuFe(H₂), with an H₂⁻ ligand (Scheme 6b). Though the radical anion H₂⁻ is unprecedented as a ligand, previous studies on alkyne and N₂ binding to the L^tBuFe fragment gave evidence for transfer of a single electron from the metal to the π-acceptor ligand. [33] [34] Alternatively, oxidative addition of H₂ to the L^tBuFe fragment would give the iron(III) intermediate L^tBuFeH₂ (Scheme 6c). Either mononuclear complex could interact with an additional L^tBuFe fragment to give [L^tBuFe(μ-H)]₂. Finally, the reaction of two L^tBuFe species simultaneously with H₂ is possible (Scheme 6d). This reaction would be topologically similar to the homolytic cleavage of H₂ and alkanes by rhodium porphyrins, which has been demonstrated to be termolecular [35-37]. The monomeric hydride complexes would then dimerize to give [L^tBuFe(μ-H)]₂.

Scheme 6.

Possible mechanisms for addition of H₂ to the putative “L^tBuFe” fragment. The diketiminate ligand is simplified in these pictures for clarity.

It is most likely that reaction with H₂ with L^tBuFeCl does not precede reduction, as L^tBuFeCl showed no reactivity with H₂ in the absence of a reducing agent. Reduction of a red solution of L^tBuFeCl with KC₈ in the absence of N₂ or H₂ produced a dark green mixture, for which the ¹H NMR spectrum in C₆D₆ changed to have many peaks over the course of 3 h, accompanied by a pronounced color change from dark green to brown. These observations suggest the formation of a reactive intermediate, which unfortunately we have not been able to isolate or further characterize in Et₂O, toluene, or pentane.

In order to test the reactivity of H₂ toward a more stable high-spin Fe(I) complex with a very labile KCl ligand, we treated the iron(I) species L^tBuFe(μ-Cl)K(18-crown-6)[24] with H₂. Surprisingly, L^tBuFe(μ-Cl)K(18-crown-6) did not react with H₂ over 2 d at room temperature in C₆D₆, as judged by ¹H NMR spectroscopy. In addition, this isolable Fe(I) complex was not very reactive with N₂ as only partial conversion to L^tBuFeNNFeL^tBu was observed after 3 d. The low reactivity of L^tBuFe(μ-Cl)K(18-crown-6) provides insight into the pathway of hydride formation: we surmise that the incoming H₂ molecule must have access to the site occupied by the chloride ligand in L^tBuFe(μ-Cl)K(18-crown-6), and that coordination of the (18-crown-6)KCl ligand is strong enough to inhibit the formation of essential L^tBuFe(H₂) or L^tBuFe(N₂) intermediates.

To our knowledge, the reaction reported here is the first example of binuclear oxidative addition of H₂ to give a bridging diiron-hydride complex. There are a few other examples of binuclear oxidative addition of H₂ to give a four-coordinate metal with two bridging hydride ligands. Bach and coworkers synthesized [(dtbpe)Ni(μ-H)]₂ by the reduction of (dtbpe)NiCl₂ with excess Mg under a H₂ atmosphere.[38] [(dtbpe)Ni(μ-H)]₂ was also prepared independently from a Ni(0) complex by treating (dtbpe)Ni-benzene with H₂.[38] Schwartz and Andersen isolated a series of [P₂PtH]₂ complexes, where P₂ is a chelating phosphine ligand,[39]by treating P₂PtCl₂ complexes with sodium amalgam under an atmosphere of H₂. However, ¹H NMR experiments show that these [P₂PtH]₂ complexes have terminal hydride ligands unlike [(dtbpe)Ni(μ-H)]₂, which has bridging hydride ligands. These systems, like ours, utilize a reducing agent to open a coordination site for H₂ and to reduce the metal to a low oxidation state that is more amenable to oxidative addition. Note that there are other complexes that utilize H₂ as a hydride source for the synthesis of four-coordinate dinuclear bis(μ-hydride) complexes; however, these hydride complexes are generated via the hydrogenolysis of alkyl, aryl, or allyl metal complexes.[40-47]

Characterization

The stretching frequency, ν_Fe-H, of the Fe-H bond in [L^tBuFe(μ-H)]₂ could not be assigned in previous reports due to the lack of a deuterated isotopomer. [18,19] The isolation of [L^tBuFe(μ-D)]₂ led to the examination of the IR spectra of [L^tBuFe(μ-H)]₂ and [L^tBuFe(μ-D)]₂ for a band that shifts between the two isotopomers. However, these complexes have identical IR spectra, and the spectra are featureless in the hydride stretch region between 1700 cm^-1 and 2300 cm^-1. The reason for the apparently low oscillator strength of the Fe-H stretching modes is not clear. We note that the hydride formulation is not in question, based on the X-ray crystal structure, [18] the insertion reactivity, [19] and the reaction with ligands to form H₂. [19]

The purity and electronic structure of solid [L^tBuFe(μ-H)]₂ was evaluated using Mössbauer spectroscopy. The zero-field Mössbauer spectrum of solid [L^tBuFe(μ-H)]₂ at 80 K is shown in Figure 3. The spectrum exhibits one quadrupole doublet with δ = 0.59 mm/s and ΔE_Q = 1.58 mm/s. The isomer shift is very close to the range (0.62-0.86 mm/s) observed in other high-spin Fe(II) diketiminate complexes.[33,34,48-52] High-spin, tetrahedral iron(II) sites in iron-sulfide clusters have similar parameters (δ = 0.6-0.7 mm/s and ΔE_Q = 2-3 mm/s).[53,54] Low-spin octahedral iron sites have very different isomer shifts and quadrupole splittings: δ = 0.3-0.45 mm/s and ΔE_Q < 1.5 mm/s.[54] The intermediate-spin (S = 1) iron(II) hydride complex [Fe(dppe)₂H]⁺ has been characterized by Mössbauer spectroscopy, giving δ = 0.23 mm/s and ΔE_Q = 1.53 mm/s.[55] The much higher isomer shift in [L^tBuFe(μ-H)]₂ (0.59 mm/s) strongly supports the assignment of the spin state at iron as high-spin (S = 2), and is consistent with the paramagnetically shifted ¹H NMR spectrum.

Figure 3.

Mössbauer spectrum of solid [L^tBuFe(μ-H)]₂ at 80 K. The black circles are the data, and the red line represents a simulated spectrum using the parameters given in the text.

Concluding Remarks

Two new synthetic routes to [L^tBuFe(μ-H)]₂ were developed. In one, [L^tBuFe(μ-H)]₂ was synthesized from L^tBuFeF using Et₃SiH as a hydride source, utilizing the formation of the strong Si-F bond as the driving force for the reaction. [L^tBuFe(μ-H)]₂ was also synthesized via the binuclear oxidative addition of H₂ to a low-coordinate Fe(I) intermediate. This reaction results in the homolytic binuclear cleavage of the H-H bond. These new syntheses were adapted to enable the isolation of the first low-coordinate iron deuteride complex, [L^tBuFe(μ-D)]₂.

These results show a new example of a high-spin complex that undergoes an organometallic reaction that is typically viewed through “2-electron” mechanisms. The oxidative addition of H₂ is facile, and complements a number of recently reported ligand-induced reductive elimination reactions. [19] Continued research will address the mechanisms of these reactions.

Experimental

General Considerations

All manipulations were performed under a nitrogen atmosphere (or argon atmosphere where specified) by Schlenk techniques or in an M. Braun glovebox maintained at or below 1 ppm of O₂ and H₂O. NMR data were recorded on a Bruker Avance 500 spectrometer (500 MHz). All peaks in the NMR spectra are referenced to residual protiated solvents (benzene δ 7.16 ppm; toluene δ 2.08 ppm; cyclohexane δ 1.38 ppm). Infrared spectra (450-4000 cm^-1) were recorded on KBr pellet samples in a Shimadzu FTIR spectrophotometer (FTIR-8400S) using 32 scans at 2 cm^-1 resolution. GC-MS was performed using a Shimadzu QP2010 system with electron impact ionization. Pentane, hexane, tetrahydrofuran (THF), diethyl ether, and toluene were purified by passage through activated alumina and “deoxygenizer” columns from Glass Contour Co. (Laguna Beach, CA). Deuterated solvents were first dried over CaH₂, then over Na/benzophenone, and then vacuum transferred into a storage container. Before use, an aliquot of each solvent was tested with a drop of sodium benzophenone ketyl in THF solution. Glassware was dried at 150 °C overnight, and Celite was dried overnight at 200 °C under vacuum. Ultra-high purity H₂ was purchased from Air Products and was dried by passage through a column of activated alumina. D₂ was purchased from Sigma-Aldrich and was dried by passage through a column of activated alumina. Et₃SiH was stored under N₂ over 4 Å molecular sieves. L^tBuFeCl,[56] L^tBuFeCH₃[48] and L^tBuFeF[22] were prepared by published procedures.

Synthesis of [L^tBuFe(μ-H)]₂ from Et₃SiH

L^tBuFeF (400 mg, 0.694 mmol) was added to a flask with a Teflon pin closure. Toluene (40 mL) was added to the flask to produce a pink slurry. Et₃SiH (1.10 mL, 6.89 mmol, 9.9 equiv) was added to the flask via syringe. The flask was sealed, and the mixture was heated and stirred at 100 °C overnight. The solution changed color from pink to red-brown over the course of the reaction (~14 h). The solution was cooled to room temperature and the volatile components were removed under reduced pressure to yield a red-brown residue. The residue was dissolved in Et₂O (70 mL) and was filtered through Celite. This solution was concentrated to 35 mL and was cooled to -45 °C to yield 243 mg of red-brown crystalline product. The mother liquor was concentrated to 5 mL and was cooled to -45 °C to produce a second crop of crystals (79 mg). The total yield was 322 mg (83%). ¹H NMR (500 MHz, C₆D₆): 118, 70, 42, 24, 22, 20, 15, 12, 10, 6, 3, 1, -2, -3, -5, -8, -11, -12, -15, -16, -17, -23, -27, -28, -32, -38, -52, -58, -111, -115, -124 ppm. The complexity of the spectrum is attributed to hindered rotations and partial dissociation into monomeric L^tBuFeH, as discussed previously. [18] IR (KBr pellet): 3057 (w), 3020 (w), 2962 (s), 2939 (s), 2869 (s), 1579 (w), 1539(m), 1485(s), 1475 (s), 1433 (s), 1385 (s), 1362 (s), 1312 (s), 1273 (m), 1253(m), 1215 (m), 1201 (m), 1184 (m), 1155 (w), 1120 (m), 1099 (m), 1072 (w), 1055 (w), 1022 (w), 935 (w), 887 (w), 843 (w), 800 (w), 779 (s), 756 (m), 711 (m), 667 (w) cm^-1.

Synthesis of [L^tBuFe(μ-H)]₂ from H₂

In an N₂-filled glove box, L^tBuFeCl (334 mg, 0.563 mmol, 1 equiv) was added to a flask with a Teflon pin closure and was dissolved in Et₂O (40 mL) to produce a bright red solution. The flask was sealed, and the solution was degassed. In an argon-filled glove box, KC₈ (86.5 mg, 0.640 mmol, 1.08 equiv) was added to the solution which resulted in an immediate color change from red to dark green. The flask was sealed, and the mixture was degassed again. The dark green mixture was stirred for 1 h. The mixture was frozen, and purified H₂ (1 bar) was added to the frozen reaction mixture. H₂ was removed after stirring overnight, and the mixture was filtered through Celite to yield a red brown solution. This solution was concentrated to 12 mL and cooled to -45 °C to yield 149 mg of red-brown crystalline product. The mother liquor was concentrated to 2 mL and was cooled to -45 °C to produce a second crop of crystals (32 mg). The total yield was 181 mg (58%). Synthesis of [L^tBuFe(μ-D)]₂ from D₂ used the same method, and gave a yield of 62%.

Mössbauer spectroscopy

Mössbauer data were recorded on a spectrometer with alternating constant acceleration. The minimum experimental line width was 0.24 mm/s (full width at half-height). The sample temperature was maintained constant in an Oxford Instruments Variox cryostat. The γ-ray source was ca. 0.6 GBq ⁵⁷Co/Rh. Isomer shifts are quoted relative to iron metal at 300 K. The zero-field spectra were simulated by using Lorentzian doublets.