Iron Complexes Bearing Diphosphine Ligands with Positioned

Article pubs.acs.org/Organometallics

Iron Complexes Bearing Diphosphine Ligands with Positioned Pendant Amines as Electrocatalysts for the Oxidation of H2 Tianbiao Liu, Qian Liao,†,‡ Molly O’Hagan,† Elliott B. Hulley,§ Daniel L. DuBois,† and R. Morris Bullock*,† †

Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-12, Richland, Washington 99352 United States ‡ Department of Chemistry, Tsinghua University, Beijing 100084, China § Department of Chemistry, University of Wyoming, Dept. 3838, Laramie, Wyoming 82071 United States S Supporting Information *

ABSTRACT: The synthesis and spectroscopic characterization of CpC5F4NFe(PtBu2NBn2)Cl, [3-Cl] (where C5F4N is a tetrafluoropyridyl substituent and PtBu2NBn2 = 1,5-dibenzyl-3,7di(tert-butyl)-1,5-diaza-3,7-diphosphacyclooctane), are reported. Complex 3-Cl and [CpC5F4NFe(PtBu2NtBu2)Cl], 4-Cl, are precursors to intermediates in the catalytic oxidation of H2, including Cp C 5 F 4 N Fe(P tBu 2 N Bn 2 )H (3-H), Cp C 5 F 4 N Fe(PtBu2NtBu2)H (4-H), [CpC5F4NFe(PtBu2NBn2)]BArF4 ([3](BAr F 4 ), [Cp C 5 F 4 N Fe(P tBu 2 N tBu 2 )]BAr F 4 ([4](BAr F 4 ), [Cp C 5 F 4 N Fe(P tBu 2 N Bn 2 )(H 2 )]BAr F 4 ([3-H 2 ]BAr F 4 ), and [CpC5F4NFe(PtBu2NtBu2H)H]BArF4 ([4-FeH(NH)]BArF4). All of these complexes were characterized by spectroscopic and electrochemical studies; 3-Cl, 3-H, and 4-Cl were also characterized by single crystal diffraction studies. 3-H and 4-H are electrocatalysts for H2 (1.0 atm) oxidation in the presence of an excess of the amine bases N-methylpyrrolidine, Et3N or iPr2EtN. Turnover frequencies at 22 °C for 3-H and 4-H with N-methylpyrrolidine as the base are 2.5 and 0.5 s−1, and overpotentials at Ecat/2 are 235 and 95 mV, respectively. Studies of individual chemical and electrochemical reactions of the various intermediates provide important insights into the factors governing the overall catalytic activity for H2 oxidation.

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INTRODUCTION Two classes of enzymes have bimetallic active sites containing Fe atoms, the [FeFe]-hydrogenase, and the [NiFe]-hydrogenase.1,2 These enzymes catalyze the reversible reduction of protons to form H2, and their high catalytic activity has inspired chemists to attempt to develop simpler synthetic catalysts using earth-abundant metals.3−10 Of particular interest is the development of iron-based catalysts because of the abundance of this metal and its lack of toxicity to the environment.11,12 Catalysts based on earth-abundant metals are of particular interest because most low-temperature fuel cells for oxidation of hydrogen are based on platinum. The study of model complexes has resulted in remarkable advances in our understanding of how the [FeFe]-hydrogenase functions,13−16 and iron catalysts are now known for both hydrogen production17−22 and oxidation.23,24 Similarly, iron glyoximate complexes25 have recently been reported for H2 production, and these complexes represent important extensions of previously reported cobalt catalysts26−30 using this ligand platform. Our group has reported that [Ni(PR2NR′2)2]2+ complexes are highly active catalysts for H2 production31−34 and oxidation,35−38 and bidirectional catalysts active for both H2 production and oxidation,39,40 where PR2NR′2 is a cyclic 1,5© XXXX American Chemical Society

diaza-3,7-diphosphacyclooctane ligand. A key feature of these catalysts is a positioned pendant amine that functions as a proton relay in close proximity to a vacant coordination site of a redox active metal center, which permits bifunctional activation of H2 during the heterolytic cleavage or formation of the H−H bond. Efficient bifunctional activation requires control of the hydride acceptor ability of the metal center and the proton acceptor ability of the pendant base, and it determines the internal bias of these catalysts to favor H2 oxidation, H2 production, or bidirectional catalysis. Efforts to extend these concepts to other metals resulted41,42 in [Co(PR2NR′2)(CH3CN)3]2+ catalysts for H2 production and related Co complexes with a tetraphosphine ligand.43 Similar iron complexes do not exhibit catalytic activity under the conditions studied, although they can facilitate the rapid formation and cleavage of the H−H bond. For example, trans[HFe(PNHP)(dmpm)(CH3CN)](BF4)244 (where PNHP is the N-protonated form of Et2PCH2N(CH3)CH2PEt2 and dmpm is Me2PCH2PMe2) undergoes rapid intramolecular exchange (k ≈ 104 s−1 at room temperature, ΔG⧧ ≈ 12 kcal/ mol) of the hydride ligand with the proton of the protonated Received: December 16, 2014

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DOI: 10.1021/om501289f Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1

Scheme 2

duced on the Cp ligand to lower the pKa of the corresponding Fe dihydrogen complex, [CpC6F5Fe(PtBu2NBn2)(H2)](BArF4) ([2-H2]BArF4),51 and facilitate intra- and intermolecular proton transfer during the catalytic cycle. The cationic species, [CpC6F5Fe(PtBu2NBn2)](BArF4) ([2]BArF4) does not bind sterically demanding bases such as Et3N, iPr2EtN and Nmethylpyrrolidine, and CpC6F5Fe(PtBu2NBn2)H (2-H) is a molecular electrocatalyst for H2 oxidation, with a turnover frequency (TOF) of 2.0 s−1 under 1.0 atm of H2 and modest overpotentials of 160−220 mV using various amine bases as proton acceptors (Scheme 1).51 We recently communicated52 the isolation and single crystal neutron diffraction study of the important catalytic intermediate [CpC5F4NFe(PtBu2NtBu2H) H]BArF4 [4-FeH(NH)](BArF4) (Scheme 1) in which the H2 ligand has been cleaved to give a hydride residing on Fe and a proton on the positioned pendant amine of the PR2NR′2 ligand, with a very short dihydrogen bond, Fe−H···H−N, 1.489(10) Å [PtBu2NtBu2 = 1,5-di(tert-butyl)-3,7-di(tert-butyl)-1,5-diaza-3,7diphosphacyclooctane]. These studies have also led to a series of Fe and Ru catalysts with similar molecular design for H2 oxidation, CpC5F4NFe(PRNR′PR)H (PRNR′PR = R2PCH2N(R′)CH2PR2, e.g., PEtNMePEt)53 and Cp*Ru(PR2NtBu2)H (R = tBu and Ph; Cp* = η5-C5Me5).54 In this paper, we report our detailed synthetic, spectroscopic, and electrochemical studies of two new Fe catalysts for oxidation of H2, CpC5F4NFe(PtBu2NBn2)H (3-H) and [CpC5F4NFe-

pendant amine. This exchange is thought to occur by reversible formation of a dihydrogen intermediate through formation and heterolytic cleavage of the H−H bond. These results and previous studies in other laboratories 45−49 of [CpFe(diphosphine)(H2)]+ complexes led us to study [CpFe(PPh2NBn2)(H2)]BArF4 ([1-H2]BArF4) and its derivatives (where Bn is benzyl and ArF is 3,5-bis(trifluoromethyl)phenyl).50 This complex catalyzes the conversion of H2 and D2 to form HD (Scheme 1). This reaction is facilitated by the pendant amines of the PPh2NBn2 ligand via reversible intramolecular heterolytic cleavage of H2 and D2 followed by intermolecular H+/D+ exchange. In this respect, the Fe dihydrogen complex can be regarded as a functional mononuclear model of the active site of the [FeFe]hydrogenase (Scheme 1). Studies of this system revealed that [CpFe(PPh2NBn2)](BArF4), the cationic intermediate required for H2 uptake during catalytic H2 oxidation, competitively binds the base required for proton removal during the catalytic cycle. As a result, CpFe(PPh2NBn2)H is not an electrocatalyst for H2 oxidation. To overcome the limitations of CpFe(PPh2NBn2)H, the phenyl substituents on both P atoms of the PPh2NBn2 ligand were replaced with bulky t-butyl groups in an effort to favor coordination of the small H2 molecule over the bulkier bases required for proton removal. In addition, the electronwithdrawing pentafluorophenyl substituent (C6F5) was introB


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Organometallics (PtBu2NtBu2)H (4-H) and their corresponding catalytic intermediates. For these complexes, the Cp ring has an electronwithdrawing tetrafluoropyridyl substituent,55,56 C5F4N, which is more-electron withdrawing than the C6F5 substituent of 2-H. As described below, 3-H and 4-H are electrocatalysts for H2 oxidation with TOFs of 2.5 and 0.5 s−1, respectively (1.0 atm of H2), and they exhibit modest overpotentials of approximately 100 mV. Detailed studies of proton/hydride exchange of [4FeH(NH)](BArF4) and other individual chemical and electrochemical reactions of 3-H and 4-H detailed herein offer insightful mechanistic understanding of electrochemical oxidation of H2 by this class of Fe catalysts.

diffraction studies and are reported in this study. Fe(PtBu2NtBu2)Cl2 is a monomeric complex that exhibits a pseudotetrahedral geometry similar to the previously studied Fe(PPh2NBn2)Cl2 complex.51 The Fe−Cl distances of Fe(PtBu2NtBu2)Cl2 are approximately 0.07 Å shorter than those of 4-Cl, while the Fe−P distances are approximately 0.20 Å longer. Both 3-Cl and 4-Cl display three-legged piano-stool geometries (Figure 1), with the six-membered rings of the PtBu2NR2 ligands adopting one boat and one chair conformation. The C5F4N substituent is directed away from the PtBu2NR2 ligands and toward the chloride ligand, minimizing steric repulsions between the C5F4N substituent and the PtBu2NR2 ligand. As observed for previous complexes of this class, the sixmembered ring adjacent to the Cl ligand adopts a chair conformation, while the ring adjacent to the substituted Cp ligand adopts a boat conformation.50,51 This geometry is consistent with a repulsive electrostatic interaction between the pendant amine and the chloride ligand. The complexes 3-Cl and 4-Cl possess structural parameters similar to those of the previously reported 2-Cl51 (see Table 1).

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RESULTS Syntheses and Characterization of Fe−Cl Complexes. The iron chloride complex CpC5F4NFe(PtBu2NBn2)Cl (3-Cl) was synthesized from Fe(PtBu2NBn2)Cl2 and NaCpC5F4N (C5F4N = tetrafluoropyridyl) in THF at 22 °C (Scheme 2). Its structure was established by a single-crystal X-ray diffraction study (Figure 1). The syntheses of Fe(PtBu2NtBu2)Cl2 and CpC5F4NFe(PtBu2NtBu2)Cl (4-Cl) were reported previously,51,52 and the structures of Fe(PtBu2NtBu2)Cl2 (Supporting Information Figure S1) and 4-Cl (see Figure 1) were determined by X-ray

Table 1. Calculated RH−H (Å) for [Fe(H2)]+ Complexes from JHD (Hz) Values Using Equations Developed by Other Groups Fe(H2) complex [CpFe(PPh2NPh2)(H2)]+ [CpFe(PPh2NBn2)(H2)]+ ([1-H2]+) [CpC6F5Fe(PtBu2NBn2)(H2)]+ ([2-H2]+) [CpC5F4NFe(PtBu2NBn2)(H2)]+ ([3-H2]+)

JHD (Hz)

eq 158

27.5 30

0.98 0.94

0.96 0.92

0.92 0.88

0.95 0.91

30

0.94

0.92

0.88

0.91

25.5

1.01

0.99

0.94

0.99

eq 259 eq 360 eq 461

Syntheses and Characterization of Fe Hydride and Dihydrogen Complexes. The Fe hydrides, 3-H and 4-H, were synthesized by the reaction of 3-Cl and 4-Cl with NaBH4 (Scheme 3). Characteristic triplets for these two hydride complexes are observed in the 1H NMR spectra of 3-H at −17.02 ppm (JPH = 67.0 Hz) and 4-H at −17.05 ppm (JPH = 70.0 Hz), as expected for hydride ligands coupling to two Scheme 3

Figure 1. Molecular structures of CpC5F4NFe(PtBu2NBn2)Cl (3-Cl, top), CpC5F4NFe(PtBu2NtBu2)Cl (4-Cl, middle), and CpC5F4NFe(PtBu2NBn2)H (3-H, bottom) with 30% probability ellipsoids. C


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complexes.58−61 Previously, we used the relatively simple equations reported by Heinekey (eq 1)58 and by Morris (eq 2)59 to calculate H−H bond distances using JHD coupling constants. Subsequently, Limbach, Chaudret, and co-workers (eq 3)60 and Heinekey and co-workers (eq 4)61 developed equations giving better fits with experimentally determined values. On the basis of the experimentally determined JHD, we calculated the H−H bond distance for Fe(H2)+ complexes we reported previously and those reported here, using all four equations; these results are summarized in Table 1. While the values in Table 1 suggest that [3-H2]+ has a longer H−H bond distance than other three Fe−dihydrogen complexes, the RH−H values calculated by eqs 1−4 indicate that there is not a very large range of differences (∼0.08 Å) in the H−H distances for these Fe complexes. Figure 2. 1H NMR spectra of (a) the H2 resonance of [CpC5F4NFe(PtBu2NBn2)(H2)]BArF4 ([3-H2]BArF4) (in PhCl-d5), (b) the HD resonance (1:1:1 triplet) of [CpC5F4NFe(PtBu2NBn2)(HD)]BArF4 ([3HD]BArF4) (in PhCl-d5), and (c) the hydride resonance (1:2:1 triplet) of CpC5F4NFe(PtBu2NBn2)(H) (3-H) (in THF-d8). All spectra were recorded at 22 °C.

RH − H = 1.44 − 0.0168 × JHD

(1)

RH − H = 1.42 − 0.0167 × JHD

(2)

RH − H = 0.74 − 0.404 × ln{0.5 × (17.3571 − [301.27 − 4 × (1 + 0.357143 × JHD )]0.5 }

equivalent phosphorus atoms (see Figure 2c for the 1H NMR spectrum of hydride region of 3-H). The molecular structure of 3-H was determined by a single-crystal X-ray diffraction study (Figure 1). The overall geometry of 3-H is similar to that of the precursor 3-Cl and the related complex studied previously, CpC6F5Fe(PtBu2NBn2)(H) (2-H),51 with the PtBu2NBn2 ligand adopting a boat−chair conformation. The Fe−H1A distance is 1.48(3) Å, similar to the values of CpFe(PPh2NBn2)(H) (1.47(2) Å)50 and 2-H (1.48(4) Å),51 although this comparison is subject to the uncertainties associated with determining H atom locations near heavy atoms by X-ray crystallography.57 The major differences are observed for the P−Fe−L, CpC5F4N− Fe−P, and CpC5F4N−Fe−L angles of 3-Cl and 3-H. The P−Fe− H1A angles, 82.6(11) and 79.4(11) of 3-H, are significantly smaller than the P−Fe−Cl1 angles, 93.38(2) and 93.54(2), by as much as 10°. As a result, the CpC5F4N−Fe−P and CpC5F4N− Fe−L angles of 3-H are greater than those of 3-Cl. These structural differences are attributed to repulsive ligand−ligand interactions between the PtBu2NBn2 ligand and the monodentate ligand, L, as observed previously for CpFe(PPh2NBn2)(L) (L = Cl and H)50 and CpC6F5Fe(PtBu2NBn2)(L) (L = Cl (2-Cl) and H (2-H)).51 The cationic species, [CpC5F4NFe(PtBu2NR2)]BArF4 ([3](BArF4) R = Bn and ([4](BArF4) R = tBu), were generated in situ from 3-Cl and 4-Cl by chloride abstraction using NaBArF4 in fluorobenzene. Addition of H2 to [3](BArF4) led to the rapid formation of the Fe dihydrogen complex, [3H2](BArF4), with an accompanying color change from darkbrown to yellow. This behavior is similar to that of [2](BArF4),51 which also contains the sterically bulky PtBu2NBn2 ligand. Both [2-H2](BArF4)51 and [3-H2](BArF4) undergo rapid H2 dissociation upon exposure to Ar, as established by UV−vis spectroscopy (see Supporting Information Figure S2 for reactions of [3-H2]BArF4). The dihydrogen complex [3H2]BArF4 exhibits a characteristic H2 resonance at −12.75 ppm (see Figure 2a) in the 1H NMR spectrum. The corresponding Fe−HD species ([3-HD]BArF4) displays a 1:1:1 triplet at −13.36 ppm (JHD = 25.5 Hz, Figure 2b). In the 2H NMR spectrum, [3-HD]BArF4 shows a doublet at −13.36 ppm (JHD = 25.3 Hz). Several groups have analyzed the relationship between JHD values and the H−H distance of dihydrogen

(3)

RH − H = 0.74 − 0.494 × ln{0.5 × (16.1447 − [260.65 − 4 × (1 + 0.33 × JHD)]0.5 }

(4)

II

We designed our Fe complexes to possess a vacant coordination site, analogous to the structurally characterized cationic FeII complexes of the type [Cp*Fe(L2)]+ (L2 = dppe (1,2-bis(diphenylphosphino)ethane), 46 dppp (1,3-bis(diphenylphosphino)propane), 47 and dippe (1,2-bis(diisopropylphosphino)ethane))48).47,48 Although κ3-coordination is uncommon with the P2N2 ligand family, the positioned amine could potentially bind to Fe; crystal structures of CrCl 3 (κ 3 -P Ph 2 N Bn 2 ), 62 [Mn(CO)(κ 3 -P Ph 2 N Bn 2 )(bppm)] + (bppm = (PArF2)2CH2),63,64 and [Mn(CO)(κ3-PPh2NBn2)(dppm)]+63 show that P2N2 ligands can adopt a κ3-bonding mode when the metal center is sufficiently electrophilic. Moreover, Rauchfuss and co-workers also reported a diiron complex with the amine of its azadithiolate ligand exhibiting κ3bonding to one Fe site.65 We established that intermolecular amine binding can occur, having previously found that binding of an amine ligand (DBU) to [CpFe(PPh2NBn2)]+ inhibited catalysis,66 and the crystal structure of the n-butylamine adduct [(CpC5F4N)Fe(PEtNMePEt)(NH2nBu)]+ has been recently reported.53 An intramolecular binding of the pendent amine to Fe in the κ3-bonding mode will be more favored entropically than binding a ligand such as NH2nBu, but the enthalpic and entropic considerations from ring strain of two Fe−N−C−P four-membered rings make it unclear which structure will be most stable. Although we have not been able to obtain a crystal structure of [3]+ or [4]+, the aforementioned studies suggest an equilibrium between the unsaturated Fe complex and its κ3isomer may be operative (Scheme 4). If that is the case, the facile reaction of H2 with [3]+ and [4]+ indicates that, if present, the Fe−N κ3-interaction is quite labile, as it must be rapidly displaced by H2 (1.0 atm) at 22 °C. As reported previously, addition of H2 (1.0 atm) to a fluorobenzene solution of [4]BArF4 at 22 °C leads to heterolytic cleavage of H2, producing [4-FeH(NH)]BArF4.52 This complex has a hydride residing on Fe and a proton on the positioned N of the PtBu2NtBu2 ligand, which was structurally D


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Organometallics

ppm is similar to that expected for an η2-HD complex, broadband 31P decoupling revealed a singlet without any significant 2H coupling (JHD ∼ 30 Hz for Fe(η2-HD), Table 1). The 2H NMR spectrum shows a broad singlet at −0.07 ppm (Figure 3c); the average of the 1H and 2H chemical shifts of [4FeD(NH)]+ ⇄ [4-FeH(ND)]+ (−6.24 ppm in PhCl-d5) is similar to the chemical shift of the 2H NMR signal in CD2Cl2 (−5.36 ppm). As we determined in a previous study of [HMn(CO)(PPh2NBn2H)(bppm)]+63 (bppm = (PArF2)2CH2), these data suggest the intramolecular reversible heterolytic cleavage of the H−H or H−D bonds to form [4-FeH(NH)]+ or [4-FeD(NH)]+ ⇄ [4-FeH(ND)]+, respectively (Scheme 5). For [4-FeD(NH)]+ ⇄ [4-FeH(ND)]+, the disparate 1H and 2 H chemical shifts result from an equilibrium isotope effect (EIE) that leads to the ND/FeH isotopomer being energetically favored over the NH/FeD isotopomer,68−70 as a result of the larger zero-point energy differences of the N−H/N−D bonds compared to the Fe−D/Fe−H bonds, as illustrated in Figure 4. The temperature dependence of the 1H and 2H NMR chemical shifts indicate that their values reflect the weighted averages of the FeH/ND and FeD/NH isotopomers, and thus the system must still be undergoing rapid exchange at the lowest temperatures measured experimentally. We believe that the HD experiment is critically important in exploring bifunctional H−H cleavage because it qualitatively establishes that rapid exchange is occurring and because quantifying the EIE provides a fundamental benchmark for future theoretical studies. Information about the EIE for [4-FeD(NH)]+ ⇄ [4FeH(ND)]+ can be obtained from NMR spectroscopic studies; it requires knowledge of the “frozen out” chemical shifts of the [4-FeD(NH)]+ ⇄ [4-FeH(ND)]+ isotopomers and the chemical shift of the averaged [4-FeD(NH)]+ ⇄ [4FeH(ND)]+ resonances (Figure 5; details of the calculation are provided in the Supporting Information). This Fe system has a sufficiently low exchange barrier that this information cannot be directly obtained experimentally, as we were unable to observe decoalescence for [4-FeD(NH)]+ ⇄ [4-FeH(ND)]+. However, we may approximate the required values from the experimentally measured averaged proton/ hydride resonance in [4-FeH(NH)]+, together with a suitable estimate for the hydride resonance of [4-FeD(NH)]+ ⇄ [4FeH(ND)]+ (calculation details are given in the Supporting Information). On the basis of closely related FeH complexes reported previously,51,52,66 we estimate that δ(FeH) is between −16 and −18 ppm, resulting in an estimated EIE ([4FeD(NH)]+ ⇄ [4-FeH(ND)]+) of 0.17−0.24 at −20 °C. This value is similar to the EIE of 0.11−0.23 estimated at −20 °C for HD isotopomers of the Mn complex [MnH(CO)(PPh2NBn2H)(bppm)]+.63 Chirik and co-workers reported an intramolecular EIE of Keq = 0.41 for the equilibrium between (C5Me4H)2Zr(H)(NDtBu) and (C5Me4H)2Zr(D)(NHtBu) at 56 °C; as in the reactions reported here, the MH/ND isotopomer is favored over MD/NH.71 In CD2Cl2, the averaged resonance for [4-FeH(NH)]+ appears at −5.36 ppm (slightly shifted from its value of −7.06 ppm in PhCl-d5) and remains almost unchanged upon cooling to −80 °C. Upon cooling a solution of the HD adduct, the 2H NMR resonance gradually shifted downf ield from 1.38 to 4.61 ppm as the temperature was lowered from 0 to −80 °C (Figure 6). Conversely, the 1H NMR resonance at −12.60 ppm gradually shifted upf ield to −15.10 ppm as the temperature was lowered from 0 to −80 °C (Figure 6). The proton resonances

Scheme 4

confirmed by a single-crystal neutron diffraction study. In the solid-state structure of [4-FeH(NH)]+, the PtBu2NtBu2 ligand adopts a chair−boat conformation that orients the N−H close to the Fe−H, in contrast to the boat−chair conformation of the precursor, 4-Cl. The opposite conformations of the sixmembered rings of [4-FeH(NH)]+ compared to 4-Cl are attributed to an attractive Fe−H···H−N dihydrogen bonding interaction67 between the protonated pendant amine and the hydride ligand of [4-FeH(NH)]+ and a repulsive interaction between the chloride ligand and the adjacent pendant amine in 4-Cl.50 Intramolecular exchange of the hydride and proton of [4FeH(NH)]+ and its HD isotopomers [4-FeH(ND)]+ ⇄ [4FeD(NH)]+ was briefly described earlier.52 Here we report a detailed description of 1H and 2H NMR studies of [4FeH(NH)]+ and [4-FeH(ND)]+ ⇄ [4-FeD(NH)]+ that provide further information on the NH/FeH exchange. The 1 H NMR spectrum of [4-FeH(NH)]+ at 22 °C in PhCl-d5 exhibits a resonance at −7.06 ppm, integrating to two protons (Figure 3a) that is more than 5 ppm downfield from the H2

Figure 3. (a) 1H NMR spectrum of ([4-FeH(NH)]BArF4) showing the averaged chemical shift of NH and FeH resonances. (b) 1H NMR spectrum of ([4-FeD(NH)]+ ⇄ [4-FeH(ND)]+). (c) 2H NMR spectrum of ([4-FeD(NH)]+ ⇄ [4-FeH(ND)]+). All spectra were recorded at 22 °C in PhCl-d5.

resonance (−12.75 ppm) of [3-H2]+. For [4-FeH(ND)]+ ⇄ [4-FeD(NH)]+, a 1:2:1 triplet is observed in the 1H NMR spectrum at −12.42 ppm arising from phosphorus coupling (JPH = 30.3 Hz) (Figure 3b). The coupling constant is about half that observed for the hydride resonance (e.g., JPH = 67 Hz for [3-H]) and thus cannot result from a “frozen-out” structure where the proton and deuteron are localized. Although the chemical shift of [4-FeH(ND)]+ ⇄ [4-FeD(NH)]+ at −12.42 E


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Figure 4. Qualitative diagram showing the energies of the FeH/NH vs FeD/NH and FeH/ND isotopomers, illustrating the origin of the equilibrium isotope effect (EIE).

= 2−1/2π(Δν) (where Δν is the separation of the Fe−H and N−H resonances, ∼10000 Hz on our 500 MHz 1H NMR spectrometer); for the present system, this value is 2.2 × 104 s−1 (ΔG⧧ < 7.3 kcal mol−1). However, being unable to reach decoalescence, we know that the exchange must be proceeding faster than that, and 2.2 × 104 s−1 represents a lower limit at our lowest temperature, −80 °C. If we assume the same ΔG⧧ at 22 °C, the estimated minimum exchange rate is greater than 2.2 × 107 s−1 at 22 °C. In related studies, the Mn complex, [HMn(CO)(P P h 2 N B n 2 H)(bppm)][BAr F 4 ] (bppm = (PArF2)2CH2), undergoes proton−hydride exchange at rates greater than 1.5 × 104 s−1 at −95 °C in CD2Cl2.63,64 H/D Exchange Reactions of Fe Dihydrogen Complexes. Chlorobenzene-d5 solutions of [3-H2]+ in the presence of mixtures of H2 and D2 catalyze H/D exchange, as indicated by the formation of [3-HD]+ (see Supporting Information

assigned to the P2N2 and CpC5F4N ligands in the 1H NMR spectra do not shift, implying that there is no significant structural change over this temperature range. This marked temperature dependence of 1H and 2H resonances arises directly from the aforementioned equilibrium isotope effect; the relative populations of [4-FeH(ND)]+ and [4-FeD(NH)]+ change, increasingly favoring the more thermodynamically stable [4-FeH(ND)]+ isotopomer at low temperature. Although we were unable to directly observe decoalescence of the Fe−H and N−H resonances, we can estimate the rate of reversible heterolytic cleavage of H2. We can estimate the peak separation of the Fe−H and N−H resonances of ([4FeH(NH)]+ from the knowledge of the average position (−5.36 ppm in CD2Cl2) and a reasonable estimate for the chemical shift of the static Fe−H (−17 ppm; see above).51,52,66 If the system were at coalescence, the exchange rate would be k F


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Organometallics

followed by intermolecular H/D exchange in solution between protonated and unprotonated amine groups of these complexes. A similar exchange process has been observed previously for the closely related [CpC6F5Fe(PtBu2NBn2)(H2)]+, [CpFe(PPh2NBn2)(H2)]+, and [CpFe(PPh2NPh2)(H2)]+ complexes.50 In our study of [CpFe(PPh2NBn2)(H2)]+ and [CpFe(PPh2NPh2)(H2)]+, an analogous complex, [CpFe(PPh2C5)(H2)]+ (where PPh2C522 is 1,4-diphenyl-1,4-diphosphacycloheptane), lacking a pendant amine, did not exhibit H/D exchange,50 providing further evidence of the key role of the pendant amine as a proton relay. Deprotonation of [3-H2](BArF4) and [4-FeH(NH)](BArF4). Deprotonation of [3-H2](BArF) under 1.0 atm of H2 using 1 equiv of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene, pKa = 24.34 for [DBU-H]+ in CH3CN)72 leads to the clean formation of the corresponding purple Fe hydride, 3-H. Reprotonation of 3-H with 1 equiv HBF4·Et2O resulted in the regeneration of [3-H2]+. To obtain a more quantitative estimate of the acidity of [3-H2]+, the equilibrium reaction of [3-H2]+ with N-methylpyrrolidine was studied in fluorobenzene by 31P{1H} and 1H NMR spectroscopy (see Experimental Section for details). In the presence of N-methylpyrrolidine, rapid exchange was observed on the NMR time scale, as indicated by an averaging of the resonances for [3-H2]+ and 3H. A lower limit on the pseudo-first-order rate of proton exchange between [3-H2]+ and N-methylpyrrolidine under these conditions has been estimated to be ca. 3 × 103 s−1. Although the pKa value of protonated N-methylpyrrolidine is not known in fluorobenzene, its value in acetonitrile has been determined to be 18.42.51,73 Using this pKa value and the equilibrium constant (K) for the reaction of [3-H2]+ (10 mM) with N-methylpyrrolidine, a pKa value of 18.3 (±0.1) (on the acetonitrile scale) was estimated for [3-H2]+ (see Experimental Section for details). Consistent with the relative electronwithdrawing effect of C5F4N, C6F5, and H, the pKa value of [3H2]+ is 0.6 units lower than that of [CpC6F5Fe(PPh2NBn2)(H2)]+ (18.9) and at least 1.7 pKa units lower than that of [CpFe(PPh2NBn2)(H2)]+, which was previously bracketed between 20 and 24.50 The pKa value of [4-FeH(NH)](BArF4) was determined in a similar manner using NEt3 as the base, and it was found to be 20.7 (±0.1), about 2.4 pKa units more basic than [3-H2]+. This is similar to the difference in pKa values (3.1) observed for [Ni(PCy2Nt‑Bu2H)2]2+ (pKa = 16.5)37 and [Ni(PCy2NBn2H)2]2+ (pKa = 13.4).36 The increased electron density of the pendant amine arising from the more electron-donating t-butyl group compared to the benzyl group results in the observed decrease in acidity of [4-FeH(NH)]+ compared to [3-H2]+. 31P{1H} NMR spectroscopic studies indicate that the rate of exchange of the proton between triethylamine and [4-FeH(NH)]+ to form 4-H is slower on the NMR time scale than for [3-H2]+ and Nmethylpyrrolidine because individual phosphorus resonances were observed at 22 °C for [4-FeH(NH)]+ and 4-H. The slower rate for 4-H and NEt3 is likely the result of increased steric interactions between the t-butyl group of the pendant amine and NEt3 compared to a benzyl group of 3-H and Nmethylpyrrolidine. Electrochemical Studies. Cyclic voltammograms of 3-Cl exhibit a reversible one-electron oxidation wave corresponding to the FeIII/FeII couple at −0.47 V vs the Cp2Fe+/0 couple. Peak-to peak separations (ΔEp) of 70−75 mV (see Figure 7a for 3-Cl) were observed. The more negative potential (−0.54 V, see Supporting Information Figure S12a) of 4-Cl52

Figure 5. Equilibrium isotope effect (EIE) for H/D isotopomers as a function of the 1H NMR chemical shift of the M(H/D) complex and the limiting (“frozen out”) chemical shifts of the MH and NH positions.

Figure 6. (a) Variable temperature 2H and 1H NMR spectra of [4FeD(NH)]+ ⇄ [4-FeH(ND)]+ from 0 to −70 °C in CH2Cl2. (b) Plots of the 2H and 1H chemical shifts versus temperature.

Figure S3). This scrambling is proposed to result from the rapid intramolecular heterolytic cleavage of H2 and D2 to form the corresponding [3-FeH(NH)]+ and [3-FeD(ND)]+ complexes, G


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Organometallics

The cyclic voltammogram of [3-H2]+ (Figure 7c) shows a quasireversible oxidation wave at 0.33 V (FeIII/II couple) and a quasireversible reduction wave with Ep at −1.42 V (FeII/I couple). The small potential shifts for these oxidation and reduction waves compared to those of cationic species [3]+ suggest a weak binding of H2 to the FeII complex, consistent with the reversible loss of H2 under an argon atmosphere. For comparison, [4-FeH(NH)]+ exhibits two quasireversible redox waves at 0.29 V (FeIII/II couple) and −1.53 V (FeII/I couple).52 Again, the small differences in the potentials of the observed FeIII/II and FeII/I couples for [4]+ and [4-FeH(NH)]+ suggest weak H2 binding. The electrochemical characteristics observed for [4]+ and [4-FeH(NH)]+ are also similar to those of previously reported complexes [2]+ and [2-H2]+.51 The cyclic voltammogram of 3-H shows an irreversible oxidation at −0.70 V vs Cp2Fe+/0 at a 100 mV/s scan rate (Figure 7d). By comparison of the peak current of 3-H to that of the reversible one-electron oxidation wave of 3-Cl under the same conditions, the oxidation at −0.70 V of 3-H is assigned to a one-electron process. The oxidation of 3-H becomes more reversible at higher scan rates (see Figure S6b in the Supporting Information), consistent with a reversible electron transfer followed by a chemical reaction. Upon scanning to more positive potentials, a reversible oxidation is observed at 0.24 V that is assigned to [3]BArF4. Upon reversing the scan direction, the reversible reduction of [3]+ at −1.23 is also observed. The formation of [3]+ was further confirmed by recording cyclic voltammograms of 3-H under H2 (1.0 atm). Under these conditions, redox waves are observed that are consistent with the formation of [3-H2]+ (see Supporting Information Figure S6c) from the reaction of [3]+ with H2. In contrast to the behavior of [CpFe(PPh2NBn2)(H)],50 which forms a 1:1 mixture of [CpFe(PPh2NBn2)(H2)]+ and [CpFe(PPh2NBn2)]+ after oneelectron oxidation, [3-H2](BArF4) is not observed during the oxidation of 3-H under Ar. As shown in Supporting Information Figure S2 and noted in the Experimental Section, [3-H2]+ loses H2 in the absence of an atmosphere of H2. Formation of [3-H2]+ upon oxidation of 3-H will be followed by rapid loss of H2 to form [3](BArF4), consistent with weak H2 binding by [3]+. These experimental observations are consistent with the reaction sequence shown in Scheme 6 (where E indicates an electron transfer reaction and C indicates a chemical reaction). One-half equivalent of 3-H is oxidized to [3-H]+ (E1 in Scheme 6), and subsequently, the second half equivalent of 3-H deprotonates [3-H]+ to give one-half equivalent of 17 electron species, [CpC5F4NFeI(PtBu2NBn2)], and one-half equivalent of [3-H2]+ (C2 in Scheme 6). The half equivalent of [CpC5F4NFeI(PtBu2NBn2)], which has an oxidation potential of −1.23 V, undergoes oxidation at the potential of

Figure 7. (a) Cyclic voltammograms of 3-Cl (1.0 mM, under 1.0 atm Ar), (b) [3](BArF4) (1.0 mM, under 1.0 atm Ar), (c) [3-H2](BArF4) (1.0 mM, under 1.0 atm H2), and (d) 3-H (1.0 mM, under 1.0 atm Ar. For the two traces in (d), the initial scan direction is positive for the pink trace and negative for the blue dotted trace. Conditions: scan rate 100 mV/s; 0.1 M [nBu4N]B(C6F5)4 in fluorobenzene.

compared to that of 3-Cl is attributed to the greater electron donor ability of the PtBu2NtBu2 ligand compared to the PtBu2NBn2 ligand. The potential of 3-Cl is ca. 140 mV more positive than the oxidation potential of CpFe(PPh2NBn2)Cl (1-Cl, −0.61 V)50 and 90 mV more positive than that of CpC6F5Fe(PtBu2NPh2)(Cl) (2-Cl, −0.56 V),51 as expected for an electron withdrawing C5F4N substituent on the Cp ring. For comparison, electrochemical data of 1-Cl, 2-Cl, 3-Cl, and 4-Cl, as well as the data for related species, are summarized in Table 2. Cyclic voltammograms of ([3](BArF4), generated from 3-Cl by Cl− abstraction using NaBArF4 in fluorobenzene, exhibit a reversible oxidation wave at 0.24 V assigned to the FeIII/II couple and a reversible reduction wave at −1.23 V assigned to the FeII/I couple (Figure 7b). Removal of the chloride ligand from 3-Cl thus results in a positive shift in the oxidation potential by 0.71 V. Similarly, 4-Cl undergoes chloride abstraction to form the cationic species [Cp C 5 F 4 N Fe(PtBu2NtBu2)]BArF4, [4](BArF4), which exhibits a reversible oxidation wave at 0.15 V for the FeIII/II couple and a reversible reduction wave at −1.41 V for the FeII/I couple (see Supporting Information Figure S12b). These electrochemical properties are similar to those of previously reported [CpFe(diphosphine)]+ and [Cp*Fe(dppe)]+ complexes.50,74

Table 2. Electrochemical Data for Fe Complexes Recorded in 0.1 M [nBu4N]B(C6F5)4 in Fluorobenzenea [CpFe(PPh2NBn2)(L)]0/+ [CpC6F5Fe(PtBu2NBn2)(L)]0/+ [CpC5F4NFe(PtBu2NBn2)(L)]0/+ [CpC5F4NFe(PtBu2NtBu2)(L)]0/+

a

L = Cl, E1/2 (FeIII/II)

L = vacant site or κ3, E1/2 (FeIII/II), E1/2 (FeII/I)

L = H2, E1/2 (FeIII/II), E1/2 (FeII/I)

L = H E1/2 (FeIII/II)

1-Cl −0.61 2-Cl −0.56 3-Cl −0.47 4-Cl −0.54

[1]+ 0.07, NA [2]+ 0.16, −1.38 [3]+ 0.24, −1.23 [4]+ 0.15, −1.41

[1-H2]+ 0.43, −1.83 [2-H2]+ 0.24, −1.62 [3-H2]+ 0.33, −1.42 [4-FeH(NH)]+ 0.29, −1.53

1-H −0.72 2-H −0.77 3-H −0.70 4-H −0.79

All potentials (V) are referenced to the Cp2Fe+/0 couple. H


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the electrode (−0.70 V) to give half an equivalent of [3]+ (E2 in Scheme 6), while [3-H2]+ loses H2 to give the second half equivalent of [3]+ (C3 in Scheme 6). The net stoichiometry of the oxidation of 3-H is a one-electron process, resulting in the formation of one equivalent of [3]+ and a half equivalent of H2. In the presence of a base such as N-methylpyrrolidine, the oxidation of 3-H is a two-electron process (see Supporting Information Figure S6d), similar to that observed previously for [CpFe(PPh2NBn2)H] in the presence of DBU. Under these conditions, the oxidation of 3-H is followed by deprotonation by N-methylpyrrolidine, which replaces the neutral 3-H shown by step C2 of Scheme 6. As a result, 3-H undergoes the two one-electron oxidations shown in steps E1 and E2 of Scheme 6 to form one equivalent of [3] + and protonated Nmethylpyrrolidine without sacrificing one-half equivalent of 3H as a proton acceptor. In this case, the net reaction is the oxidation of 3-H by two electrons to form H+ and [3]+. Thus, in the absence of an exogenous base, the hydride ligand is oxidized to form H2. In the presence of an exogenous base, the hydride ligand is oxidized to H+. Complex 4-H displays an irreversible oxidation wave at −0.79 V, and this oxidation is accompanied by the formation of [4]+ (see Supporting Information Figures S12d and S13). The chemical processes following the oxidation of 3-H and 4-H in the presence and absence of base are thought to be similar to those observed for 2-H.51 Electrocatalytic Oxidation of H2 by 3-H and 4-H. Both 3-H and 4-H were investigated as electrocatalysts for H2 oxidation using N-methylpyrrolidine, Et3N, and iPr2EtN as bases under 1.0 atm H2. The results for 4-H are similar to those found for 3-H and will be discussed only briefly. As can be seen from Figure 7 and the red trace shown in Figure 8, 3-H exhibits an irreversible one-electron oxidation wave with a peak at −0.70 V in the absence or presence of H2. In the presence of 1.0 atm H2, the oxidation current at this wave initially increases as the concentration of N-methylpyrrolidine is increased until the base concentration reaches approximately 0.060 M. Above this concentration, no significant increase in current is observed as the base concentration is increased to 0.080 M. In addition to the current enhancement observed upon addition of base, the waves become plateau-shaped at higher base concentrations

Figure 8. Cyclic voltammograms of a fluorobenzene solution of 3-H (1.0 mM) under 1.0 atm H2 with increasing concentrations of Nmethylpyrrolidine as indicated in the legend. Conditions: scan rate, 20 mV/s; 0.1 M nBu4NB(C6F5)4 as supporting electrolyte; glassy carbon working electrode. Potentials are referenced to Cp2Fe+/0, and [Cp2Co]PF6 (the wave at −1.33 V) was used as a secondary internal reference.

rather than exhibiting a distinct peak. This behavior is expected for a catalytic wave that is not limited by substrate diffusion. The observation that the catalytic wave coincides with the oxidation potential of 3-H indicates that oxidation of 3-H initiates the catalytic oxidation of H2. To determine the rate of catalysis, the ratio of icat/ip is plotted versus the concentration of N-methylpyrrolidine (where icat is the catalytic current measured in the presence of base and H2 and ip is the peak current for the oxidation current of 3-H; see Figure 8). It can be seen that this ratio increases initially with the base concentration and then becomes independent of base concentration. In the base-independent region, the maximum icat/ip value achieved is 7.7 (see Figure 9). This value can be used in equations75,76 5 and 6 to calculate an observed catalytic rate constant (or turnover frequency) for H2 oxidation of 2.5 s−1 under 1.0 atm H2 for base concentrations exceeding 0.05 M. I


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Organometallics

To provide further evidence for the catalytic nature of the electrochemical oxidation of H2, chemical oxidation of D2 (1.0 atm) was carried out using 3-H in the presence of a large excess of base (N-methylpyrrolidine or Et3N), using [Cp2Fe]B(C6F5)4 as the oxidant (eq 7). The product, D+ in the form of [BaseD]B(C6F5)4, was characterized by 2H NMR spectroscopy after a 5 min reaction time. Control experiments under the same conditions without catalyst did not show the formation of [Base-D]B(C6F5)4. The yield of [Base-D]B(C6F5)4 indicated that D2 oxidation occurs with ca. 100% yield with respect to Nmethylpyrrolidine (or Et3N) with 2 mol % loading of 3-H. This corresponds to a turnover number of 25, confirming the catalytic process. The second product, Cp2Fe (showing an absorption at 420 nm), was identified by UV−vis spectroscopy and confirmed that formation of Cp2Fe was also essentially quantitative.

Figure 9. Plot of icat/ip versus base concentration, [N-methylpyrrolidine], for a 1.0 mM solution of 3-H in fluorobenzene (0.1 M n Bu4NB(C6F5)4) under 1.0 atm H2 at 20 mV/s scan rate.

In eq 5, n is 2, corresponding to the number of electrons involved in H2 oxidation, R is the gas constant, F is the Faraday constant, T is the temperature, and υ is the scan rate. At low base concentrations, the ratio of icat/ip shows a linear relationship with the square root of base concentration, indicating the catalytic rate is first-order with respect to the base concentration (see Supporting Information Figure S7). When the electrochemical H2 oxidation was performed using the in situ generated catalyst precursor, [3-H2]+ (see Supporting Information Figure S8), a slightly higher turnover frequency (3.0 s−1) was obtained. icat n = ip 0.4463 kobs

RTkobs Fυ

3‐H,2%

2CpFe+ + 2B + D2 (1.0atm) ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2B‐D+ + 2Cp2Fe PhF,22 ° C

B = N ‐methylpyrrolidine, Et3N (7)

Complex 4-H also functions as an electrocatalyst for H2 oxidation (see Supporting Information Figures S15, S16, and S17). A turnover frequency of 0.5 s−1 was estimated using eq 6 for this complex using N-methylpyrrolidine as exogeneous base, with a 95 mV overpotential at Ecat/2 determined using the same procedures as described for 3-H. When using Et3N or iPr2EtN as exogeneous base, the turnover frequencies for 4-H were estimated to be 0.3 and 0.1 s−1 with overpotentials of 150 and 155 mV, respectively. Turnover frequencies and overpotentials for the electrocatalytic oxidation of H2 by 2-H, 3-H, and 4-H using different bases are summarized in Table 3. The catalytic rate of 4-H also decreases as the steric bulk of the exogenous base increases (see Table 3), as observed for the other catalysts. The slower catalytic rates of 4-H compared to 3-H could arise from either the additional steric hindrance of t-butyl groups or an increase in the basicity of the pendant amine. As discussed above, slow proton exchange of [4-FeH(NH)]+ and NEt3 observed in the NMR studies supports this hypothesis.

(5)

2 0.1992Fυ ⎛⎜ icat ⎞⎟ = TOF = ⎜ ⎟ n2RT ⎝ i p ⎠

(6)

3-H also shows a dependence of the catalytic rate on the identity of the base as well as the concentration of base under 1.0 atm H2. When Et3N or iPr2EtN was used as the exogeneous base, the observed catalytic rates under 1.0 atm H2 were determined to be 1.7 and 1.4 s−1, respectively, based on the maximum icat/ip (see Supporting Information Figures S9 and S10). As with N-methylpyrrolidine, the ratio of icat/ip shows a linear relationship with the square root of base concentration at low concentrations of Et3N or iPr2EtN (below 18 mM), consistent with a first-order dependence of the catalytic rate on the base concentration. This behavior is similar to that observed previously for 2-H. The overpotentials77 at Ecat/2 for H2 oxidation using 3-H as the catalyst were determined to be 235, 290, and 295 mV for N-methylpyrrolidine, Et3N, and i Pr2EtN, respectively, based on open circuit potential measurements to determine the potential of the H2/H+ couple, as described previously.51 These overpotentials are about 80 mV larger than those reported for 2-H (Table 3).

■

DISCUSSION Our previous studies50 indicated that [CpFe(PPh2NBn2)(H2)]+ catalyzes the formation of HD from H2 and D2 by a mechanism that requires cleavage of H2/D2 and intermolecular proton transfers. This complex did not catalyze the oxidation of H2, as catalysis was inhibited by binding of the base that is required for removal of the protons generated during the catalytic cycle. To overcome this problem, t-butyl groups were introduced on phosphorus in the PtBu2NBn2 ligand with the intent that their greater steric bulk would disfavor binding of the base to Fe. However, it was anticipated that the replacement of the phenyl substituent of the P atom of the PPh2NBn2 ligand by tert-butyl would make the corresponding dihydrogen complex, [CpFe(PtBu2NBn2)(H2)]+, too low in acidity to be deprotonated by bases such as Et3N. The pKa of [CpFe(PPh2NBn2)(H2)]+ in fluorobenzene was previously estimated to be between 20 and 24 on the acetonitrile scale, and the pKa of [CpFe(PtBu2NBn2)(H2)](BArF4) was anticipated to be even greater. This led us to use the less electron-donating CpC6F5 ligand and synthesize [CpC6F5Fe(PtBu2NBn2)(H2)](BArF4) ([2-H2]+), which has a pKa value of 18.9 in fluorobenzene by reference to the pKa (18.42)51 of protonated N-methylpyrrolidine in CH3CN. The

Table 3. Turnover Frequencies and Overpotentials (at Ecat/2) of Electrocatalytic H2 Oxidation by 2-H, 3-H, and 4-H Using Different Bases Fe catalyst

N-methylpyrrolidine TOF/overpotential

Et3N TOF/overpotential

i Pr2EtN TOF/overpotential

2-H 3-H 4-H

2.0 s−1/155 mV 2.5 s−1/235 mV 0.5 s−1/95 mV

1.35 s−1/210 mV 1.7 s−1/290 mV 0.3 s−1/150 mV

0.66 s−1/215 mV 1.4 s−1/295 mV 0.1 s−1/155 mV J


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(PPh2NBn2H)(bppm)(CO)]BArF4 (bppm = (PArF2)2CH2),63 which has a similar Mn−H···H−N dihydrogen bond, and for [CpFe(PPh2NBn2)(CO)]Cl50 and Mn(PPh2NBn2)(dppm)(CO)H,78 which have attractive electrostatic interactions between the partial negative charge on N atoms of pendant amines and the partial positive charge on carbon atoms of CO ligand. In summary, the spectroscopic and structural data support retention of the H−H bond for [3-H2]+ and the heterolytic cleavage of the H−H bond for [4-FeH(NH)]+. This is attributed to the increased basicity of the t-butyl N atom of the pendant amine of [4-FeH(NH)]+ compared to the benzyl N atom of [3-H2]+. This indicates that the dihydrogen complexes and their heterolytic cleavage products are very close in energy for these two complexes, although the H−H bond distances differ by about 0.50 Å. Reversible Heterolytic Cleavage of H2. Proton/hydride exchange was observed previously for the complex trans[HFe(PNHP)(dmpm)(CH3CN)]2+,44 with an estimated exchange rate of approximately 104 s−1 at room temperature (ΔG⧧ = 12 kcal/mol). This rate is at least 3 orders of magnitude slower than that of [4-FeH(NH)]+. This difference in exchange rates is thought to arise from the fact that the most stable conformation of the six-membered of the PNHP ligand of [HFe(PNHP)(dmpm)(CH3CN)](BF4)2 is the chair conformation. The boat conformation, which is appropriately positioned for proton/hydride exchange, is expected to be 5−6 kcal/mol less stable than the chair conformation. Thus, the proper positioning of the pendant amine in [4-FeH(NH)]+ is thought to be critical for the extremely high exchange rates observed. Consistent with this argument, the Mn complex [(PPh2NBn2H)Mn(H)(CO)(bppm)][BArF4], with a pendant amine positioned in the boat conformation, undergoes proton/hydride exchange with ΔG⧧ < 6.8 kcal/mol and a rate greater than 107 s−1 at 25 °C. In contrast, separate Mn−H and N−H resonances of [(PPhNMePPhH)MnH(CO)(bppm)]+ (where P Ph N Me P Ph is Ph 2 PCH 2 N(Me)CH 2 PPh 2 and PPhNMePPhH is its N-protonated form) can be frozen out in low-temperature NMR experiments.64 A chair−boat ring flip is required to achieve the geometry required for proton−hydride exchange, and the rate of exchange is 9.7 × 103 s−1 at 20 °C, which is at least 3 orders of magnitude slower than for the related complex containing positioned pendant amines.64 The structural and spectroscopic characterization of [1-H2]+ and [4-FeH(NH)]+ as well as spectroscopic studies of [3-H2]+ provide a unique opportunity to assess the spectral properties of both the reactants and products of H−H bond cleavage. [3-

corresponding Fe-hydride, 2-H, is an electrocatalyst for H2 oxidation using Et3N, N-methypyrrolidine, or iPr2EtN as bases. The bifunctional activation of H2 requires matching of the hydride acceptor ability of the iron center of the catalyst with the proton acceptor ability of the pendant base, and we were interested in preparing [CpRFe(PR2NR′2)]+ complexes in which subtle structural and electronic modifications would result in either dihydrogen complexes, [CpRFe(PR2NR′2)(H2)]+, or the heterolytic cleavage product, [CpRFe(H)(PR2NR′2H)]+. In the results described above, the synthesis of such energetically biased tautomers, [3-H2]+ and [4-FeH(NH)]+, is achieved through electronic modulation of the P2N2 ligands. The dihydrogen complex [3-H2]+, with a benzyl substituent on N, contrasts with [4-FeH(NH)]+, which exists as the tautomer resulting from the heterolytic cleavage of the H−H bond. Thus, the increase in the basicity of the N atom from the benzyl derivative to the t-butyl derivative changes the energetics, leading to heterolytic cleavage of the H−H bond (Scheme 7). DFT calculations on the previously reported [CpFe(PPh2NBn2)(H2)]+, [1-H2]+, indicate that the tautomer resulting from heterolytic H−H bond cleavage, [CpFe(PPh2NBn2H)(H)]BArF4, [1-FeH(NH)]+, features a similar chair−boat conformation with an Fe−H···H−N dihydrogen bond.50 Although [1-FeH(NH)]+ is not experimentally observable, DFT calculations indicate this tautomer is ca. 4.7 kcal/mol higher in energy than the dihydrogen complex. The dihydrogen complexes [1-H2]+, [2-H2]+, and [3-H2]+ are also observable by NMR spectroscopy, indicating that they are more energetically favorable than their corresponding H−H heterolytic cleavage tautomers. In contrast, the neutron diffraction study of [4-FeH(NH)]+ indicated that the H−H bond has been cleaved to form a hydride ligand bound to Fe and a proton bound to one of the pendant amines of the of PtBu2NtBu2 ligand.52 In the structure of [4-FeH(NH)]+, the six-membered ring containing the protonated amine adopts a boat conformation. Consequently, the proton bound to N is located in an endo position with respect to the hydride ligand, and an energetically significant dihydrogen bond was found in this complex. The structural differences between [3-H2]+ and [4-FeH(NH)]+ indicate that the hydride acceptor abilities of the Fe centers and the proton acceptor abilities of the pendant amine in these complexes are closely matched so that the free energy for the heterolytic cleavage of H2 is low for these complexes·. The conformations of the six-membered rings of [4FeH(NH)]+ are similar to those observed for [Mn(H)K


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H2]+ has spectroscopic properties typical of H2 complexes of iron, with the exception that the H−D coupling constant of [3HD]+ is slightly smaller than those observed for the [CpFe(PPh2NBn2)(H2)]+ complexes studied previously, consistent with an H−H bond that is slightly longer (by 0.07 Å). Of particular interest are the spectroscopic properties of the HD adduct of [4]+, which indicates a strong preference for the hydride ligand to reside on the Fe atom and for the deuteron to reside on the pendant amine because of a large equilibrium isotope effect.68−70 The equilibrium isotope effect is primarily the result of the N−H bond having a significantly higher stretching frequency than the Fe−H bond. Variable temperature NMR studies of [4FeH(NH)]+ and [4-FeD(NH)]+ ⇄ [4-FeH(ND)]+ indicate that the proton bound to N and the hydride bound to Fe are undergoing rapid exchange, with rates exceeding 2 × 107 s−1 at 22 °C. This exchange rate is many orders of magnitude faster than the maximum observed catalytic rates of H2 oxidation by 3-H (2.5 s−1) and 4-H (0.5 s−1), clearly indicating that the intramolecular heterolytic H−H bond cleavage is not the ratedetermining step in the catalytic process. The extremely fast intramolecular exchange of the Fe−H and the N−H in solution suggests a very small activation barrier of the exchange process, even though the driving force for this reaction is expected to be less than the difference in the pKa values of [4-FeH(NH)]+ (20.7) and [3-H2]+) (18.3) or 3.3 kcal/mol. In other words, the intrinsic barrier for intramolecular heterolytic H−H bond cleavage in these complexes is extremely low. Comparisons to [FeFe]-Hydrogenase and Synthetic Hydrogenase Mimics. The structural and NMR spectroscopic studies of the [CpFe(PPh2NBn2)(H2)]+ and [4FeH(NH)]+ complexes provide strong experimental support for the previously proposed binding of H2 to the distal Fe of the [FeFe]-hydrogenase and the participation of a pendant amine in the heterolytic cleavage of the bound dihydrogen molecule, as shown in Scheme 8. The catalytic intermediate resulting from the addition of H2 to the [FeFe]-hydrogenase enzyme may be biased toward an H2 complex analogous to [2-H2]+, [3H2]+, and [CpFe(PPh2NBn2)(H2)]+, or toward a heterolytic cleavage product such as [4-FeH(NH)]+. On the basis of DFT calculations,79 Hall and co-workers have proposed that the product resulting from the addition of H2 to the [FeFe]hydrogenase active site is similar to that observed for [4FeH(NH)]+, with a hydride ligand on the distal Fe and the proton bound to the pendant amine. As noted above, Rauchfuss

and co-workers recently reported an [FeFe]-hydrogenase model complex that possesses a similar Fe−H···H−N interaction, formed by protonation of the azadithiolate ligand of the precursor.22 Interestingly, although their complex has a positioned protonated pendant amine adjacent to a hydride ligand, rapid intramolecular proton/hydride exchange is not observed. It is likely that the pKa of the protonated pendant amine of their diiron complex is too large (i.e., it is not sufficiently acidic) to protonate the hydride ligand to form a dihydrogen complex. Both positioning of the protonated pendant amine and energy matching are important for rapid, reversible H−H bond cleavage and formation. All of these data taken together provide strong evidence for a functional role of a pendant amine in the intramolecular heterolytic cleavage and formation of the H−H bond.22 The results described for these Fe complexes contrast with previous studies of [Ni(PR2NR′2)2]2+ complexes for which the first observable intermediate by spectroscopic and crystallographic studies of H2 addition products are doubly protonated Ni0 complexes, [Ni(PR2NR′2H)2]2+ (Scheme 8).80−82 For these Ni complexes, two of the pendant amines are protonated, and the two electrons from the H−H bond have reduced NiII to Ni0. However, detailed theoretical and NMR studies83,84 suggest that [NiH(NH)]2+ species (shown in Scheme 8, analogous to [4-FeH(NH)]+) form as unstable intermediates upon reaction of [Ni(PR2NR′2)]2+ complexes with H2, prior to formation of the doubly protonated Ni0 species. Thus, the observation of [3-H2]+ and [4-FeH(NH)]+ provide additional support for the formation of similar [NiH(NH)]2+ intermediates during the addition of H2 to [Ni(PR2NR′2)2]2+ complexes that have not been experimentally observable but that are implicated by theoretical and NMR spectroscopic studies. Proposed Mechanism of the Electrocatalytic Oxidation of H2. On the basis of the reactions reported here and the electrochemical studies of [3]+, [3-H2]+, 3-H, and 4-H, the mechanism for catalytic H2 oxidation shown in Scheme 9 is proposed. A similar mechanism has been proposed for electrocatalysis by 2-H.51 The studies of 3-H and 4-H and their derivatives provide important additional insights into this proposed mechanism. As discussed above, in the absence of base, 3-H undergoes a reversible one-electron oxidation at high scan rates, as shown in step a of Scheme 9 (see Supporting Information Figure S6b). This observation indicates that this electron transfer step is not rate-determining for the overall L


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reduction product of [3]+ (described above in the electrochemical section) has an oxidation potential of −1.23 V compared to an electrode potential of −0.70 V required for the oxidation of 3-H. Consequently [3]• is oxidized by one electron at −0.70 V to generate [3]+ (step d of Scheme 9). The reversibility of the [3]+/[3]• couple, and the large driving force dictated by the potential of the electrode, indicate that this electron transfer reaction will be much faster than the catalytic rate. [3]+ reacts with H2 to generate [3-H2]+ (step e in Scheme 9). On the basis of the H/D scrambling experiments of [3-H2]+ and the spectroscopic and structural characterization of [4FeH(NH)]+, [3-H2]+ undergoes an extremely fast intramolecular proton transfer process from the dihydrogen ligand to a pendant amine to form [3-FeH(NH)]+ (step f). For catalyst 4-H, the rate of this heterolytic cleavage step is at least 2.2 × 107 s−1, and this very high rate excludes this step as ratelimiting for any of the catalysts [2-H], [3-H], or [4-H]. During this intramolecular heterolytic cleavage of H2, the oxidation state of the Fe center does not change, remaining FeII. The final step in the catalytic cycle involves a fast intermolecular proton transfer from the protonated amine of [3-FeH(NH)]+ to a base in solution to regenerate 3-H and complete the catalytic cycle. That this proton transfer step is indeed fast can be inferred from the observation that a single average chemical shift is observed for [3-H2]+ and 3-H by 31P{1H} NMR spectroscopy

catalytic process. The lack of reversibility of this couple at lower scan rates is attributed to an intramolecular proton transfer from the FeIIIH, formed by oxidation of 3-H, to a pendant amine to form [3-NH]+ (step b in Scheme 9 and C1 of Scheme 6), with a net rate of proton transfer of approximately 3 s−1. In this step, the formal oxidation state of Fe changes from FeIII to FeI. In the presence of an exogenous base, i.e., under catalytic conditions, oxidation of 3-H to [3]+ is irreversible at scan rates between 0.1 and 50 V/s (see Supporting Information Figure S6e). The scan rate dependence studies of the cyclic voltammograms indicate that the rate of proton transfer, estimated to be more than 60 s−1, is much faster than the observed rate for H2 oxidation. This interpretation is also supported by the negative shift in the potential of the oxidation wave of 3-H when base is added (Supporting Information Figure S6d), consistent with an intramolecular proton transfer rate greater than 60 s−1. This suggests that this intramolecular proton transfer is near equilibrium in the absence of base but becomes irreversible in the presence of an exogenous base. Under these conditions, the intramolecular proton transfer step is followed by an intermolecular proton transfer from the pendant amine of [3-NH]+ to an exogenous base in solution to form the 17-electron species [3]• (step c of Scheme 9, or in the absence of base to 3-H, step C2 of Scheme 6). This intermolecular proton transfer step is likely the rate-limiting step at low base concentrations. [3]• is the one-electron M


Organometallics when N-methylpyrolidine was added to a fluorobenzene solution of [3-H2]+ in an NMR tube. Because the chemical shift differences of [3-H2]+ and 3-H differ by 1300 Hz, it can be concluded that the rate of exchange exceeds 3 × 103 s−1. Because intermolecular deprotonation of [3-H2]+ to 3-H takes place through [3-FeH(NH)]+, the rate of the intermolecular deprotonation also indicates a fast rate for intramolecular heterolytic cleavage of H2 from [3-H2]+ to [3-FeH(NH)]+. Taken together, these results suggest the H2 addition (step e in Scheme 9) is rate-limiting for 3-H at high base concentrations. Indeed, preliminary results show that the rate of catalytic H2 oxidation by 3-H is dependent on H2 pressure. For 4-H, in addition to H2 addition, either of the intermolecular proton transfer steps (step c or g in Scheme 9) could be rate-limiting for the overall catalytic oxidation of H2. It was previously found that the closely related Fe complex (CpC5F4N)Fe(PEtNMePEt)H [PEtNMePEt = Et2PCH2N(Me)CH 2 PEt 2 ] catalyzes the oxidation of H 2 (1 atm) in fluorobenzene with a faster turnover frequency (8.6 s−1) than the complexes reported here,53 although at a higher overpotential. Structural studies show that the P−Fe−P angle of (CpC5F4N)Fe(PEtNMePEt)H is 92.0°, which is significantly larger than the bite angle of 81−83° found in the Fe complexes that have P2N2 ligands. In contrast to the proposed rate-limiting addition of H2 in the catalysis by 3-H, experimental and computational studies indicate that the slow step in catalysis by (CpC5F4N)Fe(PEtNMePEt)H is likely the intramolecular proton transfer from the FeIII−H complex (cf. step b in Scheme 9). More recently, we found that incorporation of an additional pendant amine in the outer coordination sphere of a Fe complex facilitates even faster proton movement, leading to a turnover frequency of 290 s−1 at 22 °C (1 atm H2).85 Further studies of these and related catalytic systems are in progress to determine more precisely the factors limiting the catalytic rate. However, there are some important conclusions that can be drawn from the studies presented herein regarding the relationship of these catalytic systems to those of the active site of the [FeFe]-hydrogenase enzyme. First, the pendant amines clearly play an important role in the heterolytic cleavage of H2, and the matching of the hydride acceptor ability of the Fe center and the proton acceptor ability of the pendant amine are necessary to avoid high energy intermediates. Second, the pendant amine facilitates the transfer of a proton from the metal to a base in solution (steps c and g of Scheme 9). However, the rate of this proton transfer process may depend on the strength of hydrogen bonding interactions between the protonated pendant amine and a hydride ligand (step g of Scheme 9) or between the protonated pendant amine and the low-valent FeI center, step c of Scheme 9. Third, the steric bulk of the t-butyl substituents of the PtBu2NBn2 and PtBu2NtBu2 ligands favor the selective binding of H2 over the larger exogenous bases. In this case, the t-butyl groups play a role similar to that provided by the protective pocket of the [FeFe]hydrogenase enzyme, which excludes strongly coordinating ligands while allowing access of H2 to the Fe center and protons to the pendant amine. Finally, it is interesting to note that only a single iron is required for the catalysts described here, suggesting that a primary role of the second iron (the socalled distal iron) in the active site of the [FeFe]-hydrogenases is to serve as a terminus of the electron transfer channel.

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CONCLUSIONS

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EXPERIMENTAL SECTION

The iron complex 3-H is an electrocatalyst for oxidation of H2 with a turnover frequency up to 2.5 s−1 and an overpotential of approximately 235 mV under 1.0 atm H2. Among the Fe electrocatalysts for oxidation of H2, 4-H exhibits the lowest overpotential, approximately 95 mV, albeit with a slower turnover frequency of 0.5 s−1. The different catalytic rates for 3H and 4-H with different exogeneous bases indicate intermolecular proton transfer is very sensitive to the chemical nature of the pendant amine substituent of the P2N2 ligand and exogeneous base. Comprehensive studies of individual catalytic steps for 3-H suggest addition of H2 to [3]+ is likely rate-limiting for catalysis at high base concentrations. Comparative studies of catalytic intermediates [3-H2]+ and [4-FeH(NH)]+ demonstrate that small changes in the electron density at the pendant amine alter the thermodynamic preference for heterolytic H−H bond cleavage vs H−H formation; [3-H2]+, with a benzyl group on the amine, is a dihydrogen complex, whereas [4-FeH(NH)]+, with a tert-butyl group on the amine, leads to heterolytic cleavage, placing the hydride on iron and the proton on the amine. As a result of the more basic tert-butyl group, the pKa of [4-FeH(NH)]+ is greater than that of [3-H2]+ by about 2.4 pKa units. Rates and overpotentials for catalysis are affected by these changes; the overpotential for catalysis by 4-H is lower than that for 3-H because the redox potential for 4-H is more negative. Together with additional results reported by our group, the fast exchange of the hydride ligand and the NH proton for [4-FeH(NH)]+ indicates that the presence of a positioned pendant amine indeed facilitates the rapid intramolecular cleavage and formation of the H−H bond as proposed for the active site of the [FeFe]-hydrogenase. Through the examples of 3-H and 4-H as well as previously reported 1-H and 2-H, we have demonstrated that mononuclear Fe complexes possessing essential but minimal features of the active site of the enzyme can effectively reproduce the function of the [FeFe]-hydrogenase for catalytic oxidation of H2.

General Experimental Procedures. 1H, 2H, 19F, and 31P{1H} NMR spectra were recorded on a Varian Inova spectrometer (500 MHz for 1H) at 20 °C. All 1H chemical shifts have been internally calibrated to the monoprotio impurity of the deuterated solvent. The 2 H NMR spectra were internally calibrated to the added internal reference or residule deuterated solvent. The 31P{1H} NMR spectra were proton decoupled and are referenced to external phosphoric acid. The 19F{1H} NMR spectra are referenced to CFCl3 (0.00 ppm) using external C6F6 (−163 ppm) as a secondary reference. UV−vis spectra were recorded on an Ocean Optics USB2000 UV−vis spectrometer. All electrochemical experiments were carried out under an atmosphere of argon or hydrogen as indicated in 0.1 M [nBu4N]B(C6F5)4 fluorobenzene electrolyte solutions. Cyclic voltammetry experiments were performed with a CH Instruments model 660C potentiostat. Cobaltocenium hexafluorophosphate (Cp2CoPF6) or bis(η5-pentamethylcyclopentadienyl)iron (Cp*2Fe) were used as secondary internal standards with all potentials referenced to the Cp2Fe+/Cp2Fe0 couple. The working electrode (1 mm PEEK-encased glassy carbon, Cypress Systems EE040) was polished using Al2O3 (BAS CF-1050, dried at 150 °C under vacuum) suspended in fluorobenzene and then rinsed with neat fluorobenzene. A glassy carbon rod (Structure Probe, Inc.) was used as the counter electrode. The reference electrode consisted of a silver wire coated with a layer of AgCl and suspended in a solution of 0.1 M [nBu4N]B(C6F5)4 in N


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Organometallics

[CpC5F4NFe(PtBu2NBn2)(H2)]BArF4, [3-H2](BArF4). 3-Cl (0.150 g, 0.200 mmol) and NaBArF4 (0.019 g, 0.210 mmol) were dissolved in 5 mL of fluorobenzene. Then H2 gas (1.0 atm) was purged through the solution, resulting in a rapid color change from dark to yellow, indicating formation of [3-H2](BArF4), which was confirmed by 1H NMR and 31P{1H} NMR spectroscopies. After filtration to remove insoluble solids, [3-H2](BArF4) was precipitated by adding 60 mL of pentane to the filtrate. The resulting solid was collected by filtration and dried under vacuum for 30 min. The isolated [3-H2](BArF4) (0.292 g, yield, 92%) was stored under H2 at −35 °C in a glovebox. Alternatively, [3-H2]BArF4 can be prepared from [3](BArF4) and H2. Anal. Calcd for [3-H2](BArF4): C69H58BN2F29FeP2: C, 51.76; H, 3.71; N, 2.66. Found: C, 50.99; H, 4.53; N, 2.44. 1H NMR (PhCl-d5): δ 8.00 (s, 8 H, B(C8F6H3)4), 7.37 (s, 4 H, B(C8F6H3)4), 7.01−6.61 (m, 10 H, CH2C6H5), 5.01 (s, 2 H, C6F5C5H4), 4.28 (s, 2 H, C6F5C5H4), 3.17 (s, 2 H, NCH2C6H5), 3.10 (s, 2 H, NCH2C6H5), 2.35 (d, JHH = 15.0 Hz, 2 H, NCH2P), 2.18 (d, JHH = 15.0 Hz, 2 H, NCH2P), 1.89 (d, JHH = 15.0 Hz, 2 H, NCH2P), 1.78 (d, JHH =15.0 Hz, 2 H, NCH2P), 0.64 (s, 18 H, PC(CH3)3), −12.75 (s, br, 2 H, FeH2). 31P{1H} NMR (PhCl-d5): δ 74.0 (s). 19F NMR (PhCl-d5): δ −86.02 (m, 2 F, C5F4N), −136.17 (m, 2 F, C5F4N). UV−vis (PhF): 407 nm (786 M−1 cm−1). H2 Dissociation from [CpC5F4NFe(PtBu2NBn2)(H2)]BArF4, [3-H2](BArF4). A standard solution of [3-H2](BArF4) (10 mM) was prepared from 3-Cl. This solution (0.15 mL) was added to a cuvette containing 2.75 mL of fluorobenzene saturated with 1.0 atm H2. A UV−vis spectrum of this solution containing (0.5 mM [3-H2]+) was recorded. For comparison, a solution of [3-H2]+ of the same concentration was prepared and injected into a cuvette under 1.0 atm Ar. UV−vis spectra of this solution were recorded as a function of time. The spectrum recorded at 10 s (the fastest time for data collection) confirmed formation of [3]BArF4 as indicated by the appearance of the peaks at 376, 492, and 604 nm. H2 dissociation was complete within 5 min (see Supporting Information Figure S2). Generation of [CpC5F4NFe(PtBu2NBn2)(HD)]BArF4, [3-HD](BArF4). A J. Young NMR tube was loaded with a mixture of NaBArF4 (0.018 g, 0.020 mmol) and 3-Cl (0.015 g, 0.020 mmol) and 0.5 mL of chlorobenzene or fluorobenzene were added. The NMR tube was immersed in a liquid N2 bath, evacuated and refilled with HD gas (1.0 atm). After warming to 22 °C, NMR spectra were recorded. 1H NMR (PhCl-d5): δ −13.36 (t, JHD = 25.5 Hz). 2H NMR: δ −13.36 (d, JHD = 25.3 Hz). Variable Temperature NMR Studies of [Cp C 5 F 4 N Fet tBu (P 2NtBuNt BuD)(H)]BArF4, [4-FeH(ND)](BArF4). A sample of [4FeH(ND)](BArF4) (0.02 mmol) in 0.5 mL CH2Cl2 was prepared in a J. Young NMR tube as described as above. 1H NMR spectra were recorded for [4-FeH(ND)](BArF4) from 0 to −80 °C. At each temperature, the sample was retained for 20 min before NMR data acquisition. The large solvent peak of CH2Cl2 (5.32 ppm) was used to reference the chemical shifts of the complex at 0 °C and was then suppressed for each spectrum. With decreasing temperature, the Fe−H resonance gradually upfield shifted from −12.50 to −15.10 ppm, while the proton resonances assigned to the P2N2 ligand and CpC5F4N ligand did not shift. The sample was warmed back to 0 °C, and 1H and 31P NMR spectra were recorded to ensure there was no decomposition. 2 H NMR spectra were recorded for [4-FeH(ND)](BArF4) from 0 to −80 °C following the same procedure. With decreasing temperature, the N−D resonance, which is referenced to the resonance of natural abundance CHDCl2 (5.32 ppm), gradually shifted downfield from 1.38 to 4.61 ppm. The 1H NMR and 2H NMR spectra showing the Fe−H resonance and the N−D resonance are shown in Figure 4. [CpC5F4NFe(PtBu2NBn2)(H)], (3-H). Ethanol (20 mL) was added to a mixture of 3-Cl (0.300 g, 0.40 mmol) and 10 equiv of NaBH4 (ca. 0.150 g) in a 50 mL flask. Stirring the reaction mixture resulted in the formation of a purple solution and gas evolution. The solution was stirred until no further gas evolution was observed, indicating complete consumption of NaBH4. The solvent was removed from the reaction mixture by applying a vacuum, and the purple powder that resulted was extracted with 10 mL of toluene. The purple solution was filtered through Celite, and the solvent was removed from the filtrate under

fluorobenzene. The solution of the reference electrode was separated from the analyte solution by a Vycor frit. X-ray Structure Determinations. For all studies, a 10× microscope was used to identify suitable crystals of the same habit. Each crystal was coated in Paratone, affixed to a Nylon loop, and placed under streaming nitrogen (110 K) in a Bruker KAPPA APEX II CCD diffractometer with 0.71073 Å Mo Kα radiation. The space groups were determined on the basis of systematic absences and intensity statistics. The structures were solved by direct methods and refined by full-matrix least-squares on F2. Anisotropic displacement parameters were determined for all nonhydrogen atoms. The hydride ligand of 3-H was located by electron density difference map. Other hydrogen atoms were placed at idealized positions and refined with fixed isotropic displacement parameters. The following is a list of programs used: data collection and cell refinement, BRUKER APEX2;86 data reductions, BRUKER SAINT;87 absorption correction, SADABS;88 structural solutions, SHELXS-97;89 structural refinement, SHELXL-97;89 graphics, Xceed for Windows.90 Synthesis and Materials. All reactions and manipulations were performed under an Ar or H2 atmosphere using standard Schlenk techniques or a glovebox. Solvents were dried using an activated alumina column and stored under Ar. N-methylpyrrolidine, Et3N, and i Pr2EtN were distilled from KOH and stored under Ar. All NMR solvents were purified according to standard methods. Sodium (tetrafluoropyridinyl)cyclopentadienide (NaCpC5F4N) was prepared using an improved procedure52 based on a published procedure,56 and 1,5-dibenzyl-3,7-di(tert-butyl)-1,5-diaza-3,7-diphosphacyclooctane (PtBu2NBn2),91 1,3,5,7-tetra(t-butyl)-1,5-diaza-3,7-diphosphacyclooctane (P tBu 2 N tBu 2 ), 52 Fe(P tBu 2 N Bn 2 )Cl 2 , 51 Fe(P tBu 2 N tBu 2 )Cl 2 , 52 CpC5F4NFe(PtBu2NtBu2)Cl (4-Cl),52 [CpC5F4NFe(PtBu2NtBu2)]BArF4 ([4](BArF4)),52 [CpC5F4NFe(P tBu2NtBu2H)(H)]BAr F4, [4-FeH(NH)](BArF4),52 and CpC5F4NFe(PtBu2NtBu2D)(H)]BArF4, [4-FeH(ND)](BArF4)52 were also prepared using published procedures. All other reagents were used as received. CpC5F4NFe(PtBu2NBn2)Cl, 3-Cl. NaCpC5F4N (0.213 g, 1.00 mmol) and [Fe(PtBu2NBn2)Cl2] (0.570 g, 1.00 mmol) were dissolved in 50 mL of THF and stirred for 2 h at 22 °C to yield a dark solution. 31P{1H} NMR and 19F NMR spectroscopic monitoring confirmed the reaction had proceeded to completion. 19F NMR spectroscopic monitoring indicated that a trace amount of CpC5F4N2Fe56 byproduct (

Iron Complexes Bearing Diphosphine Ligands with Positioned

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