Catalytic N2 Reduction to Silylamines and Thermodynamics of N2

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Catalytic N2 Reduction to Silylamines and Thermodynamics of N2 Binding at Square Planar Fe Demyan E. Prokopchuk,† Eric S. Wiedner,† Eric D. Walter,‡ Codrina V. Popescu,§ Nicholas A. Piro,∥,⊥ W. Scott Kassel,∥ R. Morris Bullock,† and Michael T. Mock*,† †

Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States ‡ Environmental Molecular Sciences Laboratory, Richland, Washington 99352, United States § Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346, United States ∥ Department of Chemistry, Villanova University, 800 E. Lancaster Ave., Villanova, Pennsylvania 19085, United States S Supporting Information *

ABSTRACT: The geometric constraints imposed by a tetradentate P4N2 ligand play an essential role in stabilizing square planar Fe complexes with changes in metal oxidation state. The square pyramidal Fe0(N2)(P4N2) complex catalyzes the conversion of N2 to N(SiR3)3 (R = Me, Et) at room temperature, representing the highest turnover number of any Fe-based N2 silylation catalyst to date (up to 65 equiv N(SiMe3)3 per Fe center). Elevated N2 pressures (>1 atm) have a dramatic effect on catalysis, increasing N2 solubility and the thermodynamic N2 binding affinity at Fe0(N2)(P4N2). A combination of high-pressure electrochemistry and variable-temperature UV−vis spectroscopy were used to obtain thermodynamic measurements of N2 binding. In addition, X-ray crystallography, 57Fe Mössbauer spectroscopy, and EPR spectroscopy were used to fully characterize these new compounds. Analysis of Fe0, FeI, and FeII complexes reveals that the free energy of N2 binding across three oxidation states spans more than 37 kcal mol−1.

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INTRODUCTION Since the discovery of transition-metal dinitrogen complexes over 50 years ago,1 designing such compounds for homogeneous N2 reduction catalysis remains a vibrant area of research. Synthetic Fe-N2 complexes have been widely studied to mediate the conversion of dinitrogen to ammonia, given their biological relevance to iron-containing nitrogenase enzymes2 and heterogeneous Haber−Bosch catalysts.3 Although welldefined Mo-N2 compounds are known to catalyze the conversion of N2 to NH3,4 Fe0-N2 complexes with multidentate phosphine-containing ligand scaffolds have recently emerged as nitrogen fixation catalysts in the presence of excess acid and electrons.5 In particular, trigonal bipyramidal Fe0-N2 precatalysts with tetradentate P3E ligands (E = B, C, Si) were developed by Peters and co-workers,5a−d and Nishibayashi and co-workers demonstrated catalysis with a square planar FeI-N2 pincer complex.5f Notably, Ashley and co-workers recently reported that the trigonal bipyramidal Fe0(N2)(depe)26 (depe = Et2PCH2CH2PEt2) can selectively catalyze the formation of N2H4 from N2.5g An alternative approach to functionalizing dinitrogen is the conversion of N2 into silylamines N(SiR3)3, which can be hydrolyzed to NH3.4c,d,7 In contrast to using proton and electron equivalents, this reaction commonly employs reagents such as Me3SiCl and a strong reductant (e.g., Li, Na, KC8) to © 2017 American Chemical Society

generate silyl radicals in situ that react with N2 coordinated to a transition metal.8 Examples of well-defined Mo-N2,7a,8a,b,9 VN2,10 and Co-N28c complexes have been reported to catalyze this reaction. Recently, iron complexes for the conversion of N2 to silylamines have been developed that vary in structural complexity (Figure 1). These include simple organometallic complexes such as Fe(CO)5 and ferrocene (Figure 1A/B),11 a novel carbene-supported Fe0 complex (Figure 1C),5e and multiiron [Fe4]/[Fe6] hydride clusters (Figure 1D; only [Fe4] cluster shown).12

Figure 1. Iron complexes shown to catalyze the reduction of N2 to N(SiMe3)3. Received: May 3, 2017 Published: June 14, 2017 9291

DOI: 10.1021/jacs.7b04552 J. Am. Chem. Soc. 2017, 139, 9291−9301

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Journal of the American Chemical Society Despite the diverse coordination environments of the catalysts used for NH3 or N(SiR3)3 formation, a discrete FeN2 complex is believed to play an essential role in the catalytic cycle. Surprisingly, none of the Fe silylation precatalysts mentioned above contain N2 bound to Fe at room temperature. The thermochemical requirements for binding N2 and release of fixed nitrogen products are fundamental parameters that can influence catalytic activity. While N2 binding affinity has been studied for Co13 and group 6 metals,14 a recent study by Peters and co-workers on the enhancement of N2 binding in a hydride-bridged square pyramidal diiron complex is the only study comparing N2 binding affinity at FeI and FeII.15 Little is known about N2 binding free energies to transition metals in solution, especially when varying the metal oxidation state while keeping the ligand scaffold in a fixed coordination geometry. In our efforts to design catalysts for N2 reduction,16 we sought to understand the electronic structure and thermodynamics of N2 binding as an axial ligand at Fe in a rigid square planar P4 coordination environment (Figure 1E), particularly in oxidation states 0, +I and +II. While square planar FeII is well established in porphyrin frameworks,17 to the best of our knowledge this geometry is unprecedented for a P4 ligand set coordinated to Fe0. Bis(diphosphine)Fe0-N2 complexes typically adopt a trigonal bipyramidal geometry,6,18 while linear tetradentate phosphine ligands are flexible, producing cis-α and cis-β isomers upon metal coordination.19 Herein, we present the N2 binding thermochemistry of Fe complexes bearing a rigid P4N2 ligand20 that enforces a square planar geometry at the metal due to the constraints imposed by the rigid eightmembered ring of the P4N2 backbone. The structural rigidity and stability of the P4N2 ligand is critical, since the P4 coordination geometry remains unchanged in all three metal oxidation states. We quantify the thermodynamics of N2 binding as the fifth ligand at Fe0, FeI, and FeII by constructing thermochemical cycles using variable-pressure electrochemistry and variable-temperature UV−vis spectroscopy. In addition, we present 57Fe Mössbauer spectroscopic data to provide additional insight into the electronic structure of these Fe complexes. In particular, the square pyramidal complexes [FeI(N2)(P4N2)]+ and Fe0(N2)(P4N2) are rare examples of well-defined Fe-N2 complexes that catalyze the reduction of N2 into N(SiR3)3 at room temperature. Fe0(N2)(P4N2) produces up to 65 ± 10 equiv of N(SiMe3)3 per Fe center under elevated N2 pressures, which is the highest turnover number of any Febased N2 silylation catalyst to date.

Scheme 1. Synthesis of Fe(P4N2) Complexes

Figure 2. Molecular structures of FeIBr, Fe0(N2), and [FeI]+[B(C6F5)4]− with 50% probability ellipsoids. Most H atoms and the B(C6F5)4− anion of [FeI]+ are omitted for clarity. Selected bond distances (deg) and angles (Å) are provided in the Supporting Information.

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RESULTS AND DISCUSSION Synthesis and Characterization of Fe(P4N2) Complexes. Following a modified preparation of the free P4N2 ligand (see the Supporting Information, SI), mixing 1 equiv of ligand with FeBr2 affords the yellow octahedral complex FeIIBr2 (Scheme 1), whose molecular structure is provided in the SI. Reduction of FeIIBr2 with KC8 generates FeIBr, a paramagnetic orange solid with a solution magnetic moment of 1.6 μB (Evans method), consistent with an S = 1/2 ground state. The molecular structure of FeIBr indicates a square pyramidal geometry with a τ5 value of 0.06, (τ5 = 0 for square pyramid, τ5 = 1 for trigonal bipyramid;21 Figure 2). Furthermore, the EPR spectrum of FeIBr in a toluene glass at 125 K is rhombic (g = 1.9909, 2.1499, 2.1834) with resolved hyperfine coupling to 31P (A31P = 70 MHz)22 and Br (ABr = 45 MHz; see SI).

Reduction of FeIIBr2 by 2 equiv NaC10H8 under N2 affords an orange-brown square pyramidal dinitrogen complex Fe0(N2) (τ5 = 0.07,21 Figure 2). In contrast to known square pyramidal Fe-N2 complexes with nitrogen and carbon-based ligands,23 Fe0(N2) is the first example of a zerovalent Fe-N2 complex in a P4 ligand environment with a square pyramidal geometry. The 31 P NMR spectrum of Fe0(N2) has an AA′BB′ splitting pattern that was simulated (see Experimental Section and SI). The IR spectrum of Fe0(N2) shows a ν(14N2) stretch at 2003 cm−1, shifting to 1936 cm−1 for Fe0(15N2). The presence of two alkylphosphine and two diarylphosphine moieties places its 14 N2 stretching frequency in between Fe0(N2)(dppe)2 (dppe = Ph2PCH2CH2PPh2), ν(14N2) = 2068 cm−1),24 and Fe0(N2)(depe)2, ν(14N2) = 1955 cm−1).6 Under 15N2, the 15N{1H} NMR spectrum of Fe0(15N2) contains multiplets at −54.79 and −58.80 ppm, assigned to the Nα and Nβ atoms, respectively. 9292


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Journal of the American Chemical Society The NN bond length (N1−N2) for the terminally bound N2 ligand is 1.101(4) Å. Fe0(N2) is unstable without an N2 atmosphere. A 1 mM solution of Fe0(N2) stored in toluene under argon at 298 K slowly converts into Fe nanoparticles over the course of 16 h, as indicated by a diagnostic broad EPR signal (Figure 3).25 At 298

Figure 3. Experimental X-band EPR spectra (toluene) of Fe0(N2) after 16 h under argon. Figure 4. Top: Experimental (black) and simulated (red) X-band EPR spectra (105 K, toluene glass) of [FeI]+ under Ar and [FeI(N2)]+ under N2. Bottom: Stacked variable-temperature EPR spectra of [FeI]+/[FeI(N2)]+ (105−373 K, toluene, N2).

K, the very broad feature has a line width of ca. 500 G, which shifts upfield and broadens to 900 G at 125 K. The sharp central feature at 3200 G contains some hyperfine features, however the observed species is not consistent with any other EPR-active compounds in this study (vide infra). As discussed in the next section, a redox wave is observable via cyclic voltammetry for the putative four-coordinate [Fe0(P4N2)] complex, [Fe0], under an argon atmosphere. Treatment of Fe I Br with NaBAr F 4 (Ar F = 3,5-bis (trifluoromethyl)phenyl) or KB(C6F5)4 in fluorobenzene immediately changes the solution color from orange to deep purple, affording [FeI]+ (S = 1/2, μeff = 1.5 μB, Scheme 1). The molecular structure of [FeI]+[B(C 6F 5)4]− confirms the distorted square planar geometry, formally a 15-electron species with a τ4 value of 0.23 (τ4 = 0 for square planar, τ4 = 1 for tetrahedral;26 Figure 2). Aside from four Fe−P bonds, the next closest atomic contact to Fe in the solid-state is a phenylphosphine ortho-CH interaction, (Fe···H20 = 2.786 Å, Fe···C20 = 3.389 Å), which is best described as a weak σ-agostic interaction.27 [FeI]+[B(C6F5)4]− is the second structurally characterized example of a square planar, cationic FeI(P4) complex.28 IR measurements of [FeI]+ in toluene under N2 reveal a weak 14 ν( N2) stretch at 2090 cm−1, which shifts to 2021 cm−1 using 15 N2. Furthermore, the FeI(P4N2)+ cation yields distinct X-band EPR spectra under either Ar or N2 gas, assigned to [FeI]+ and [FeI(N2)]+, respectively (Figure 4). In a frozen toluene glass, S = 1/2 [FeI]+ displays a rhombic spectrum29 (g = 1.986, 2.206, 2.303) with resolved 31P hyperfine coupling (A31P = 70 MHz), while [FeI(N2)]+ is nearly axial (g = 1.994, 2.092, 2.111) with moderately resolved 31P hyperfine features (A31P = 59 MHz). Under N2, EPR spectra contain temperature-dependent ratios of [FeI]+ and [FeI(N2)]+, with exclusively [FeI(N2)]+ (pale yellow) from 105 to 243 K and [FeI]+ at 373 K (deep purple). Remarkably, there are no signs of decomposition by EPR, even at 373 K. Computational analysis of [FeI]+ shows the unpaired Mulliken spin density at the metal center (1.24), while [FeI(N2)]+ contains electron spin density at the metal center (1.12) and distal Nβ atom (−0.08)30 (see SI). Recently, the square pyramidal FeI complex [Fe(N2)(depe)2][OTf] has been structurally characterized and has an Fe(14N2) stretch at 2052 cm−1 in THF.5g

Oxidation of [FeI]+ using Cp2FeX (X− = BArF4− or B(C6F5)4−) in fluorobenzene furnishes the green paramagnetic complex [FeII]2+ (Scheme 1, bottom), which was isolated and fully characterized in solution. The formally 14-electron complex exhibits a solution magnetic moment of μeff = 2.8 μB, consistent with an S = 1 intermediate-spin ground state. No νN2 stretches were observed in the solid-state or solution IR spectra, consistent with our electrochemical investigations (vide infra). In lieu of X-ray structural determination, DFT calculations evaluated the free energies of possible [FeII]2+ conformers, where the pendant nitrogen moiety can be oriented either toward (boat) or away (chair) from the metal center (Figure 5). The computed free energies of S = 1 [FeII]2+ are lowest, consistent with the solution magnetic moment data and indicating that the “boat−chair” ligand conformation is preferred.31 Low-spin (S = 0) and high-spin (S = 2) configurations are significantly higher in energy (24.8 and 14.1 kcal mol−1, respectively). Notably, the calculated S = 0

Figure 5. DFT optimized conformers and electronic isomers of [FeII]2+. For clarity, H atoms have been removed, and only the ipso phenyl carbons are shown. Free energies are relative to “boat−chair”[FeII]2+ and presented in kcal mol−1. 9293


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shift of 0.21 mm s−1 and a quadrupole splitting of 1.52 mm s−1.37 In addition, a spectrum in an applied magnetic field of 7.0 T (Figure 6B) shows splitting resulting only from the nuclear Zeeman interaction, consistent with an S = 0 electronic ground state. To the best of our knowledge, there are no precedents of Mössbauer spectra for square pyramidal P4FeN2 complexes of any oxidation state; however, trigonal bipyramidal FeIN2 and (Fe0N2)− complexes with a tetradentate P3Si− ligand have been characterized by Peters and co-workers, with an isomer shift of 0.226 mm s−1 reported for Fe0(N2)(P3Si)−.38 The Mössbauer spectrum of [FeII]2+ (Figure 7C) consists of two relatively broad quadrupole doublets. While the fit could be achieved with either staggered or nested doublets, given that this complex is mononuclear, a unique isomer shift was considered the proper choice. The best fit was obtained with two nested doublets of unequal contributions with an isomer shift of 0.43 mm/s and large quadrupole splittings of 4.0 and 3.5 mm s−1. The broader spectra and the observation of two quadrupole splittings may reflect the heterogeneity of this sample and the existence of a minority crystalline form with small variations in the local symmetry. An assignment of [FeII]2+ as an intermediate-spin configuration is plausible and is consistent with the solution magnetic moment data and DFT calculations; however, caution is stressed in absence of comparable Mö ssbauer data of P4Fe complexes in this geometry.39 Interestingly, the isomer shifts are nearly identical for FeIIBr2, [FeIBr], and [FeII]2+ (Figure 7A−C, respectively), suggesting that removing halide ligands does not drastically change the selectron density at the Fe nucleus.40 The change in coordination number from the six-coordinate FeIIBr2 to the four-coordinate [FeII]2+ is expected to give rise to lower isomer shifts, but a weakening of the ligand field should correlate with an increase in isomer shift.41 Therefore, the change in coordination number and weakening of the ligand field may have opposite effects, explaining the apparent lack of change in the isomer shift values. A change in the spin-state in from singlet (S = 0) FeIIBr2 to triplet (S = 1) [FeII]2+ may also contribute to this observation. An example related to FeIIBr2 is the low-spin complex [FeII(dppv)2Cl2] for which δ = 0.38 mm s−1 at 10 K (dppv = 1,2-bis(diphenylphosphino)ethylene).42 Moreover, relevant examples of S = 1 square planar iron(II) complexes include FeII(Mes)2(PMe2Ph)2, (δ = 0.31 mm s−1 and ΔEQ = 4.63 mm s−1; Mes = 2,4,6-Me3C6H2) and (dppe)FeII(Mes)2, (δ = 0.33 mm s−1 and ΔEQ = 4.53 mm s−1),43 although meaningful comparisons are limited in absence of complexes with similar ligands and geometry.44 Thermodynamics of N2 Binding. As shown above, [FeI]+ reversibly binds N2 under ambient conditions. Temperaturedependent UV−vis absorption maxima at 467 and 559 nm are observed under 1 atm N2 in fluorobenzene (Figure 8). The equilibrium constant Keq is 0.4(1) atm−1, indicating that [FeI(N 2)]+ slightly favors releasing N2 under standard conditions (ΔG298 = 0.41(5) kcal mol−1, PN2 = 1 atm). A van’t Hoff analysis (263−313 K) allows us to dissect the thermodynamic components of this equilibrium, and the negative entropic contribution (ΔS = −23.4(4) cal K−1 mol−1) is consistent with a more ordered state after N2 binding.13,14b,15 In the cyclic voltammogram (CV), the reversible FeI/0 couple for [FeI]+ appears at −1.88 V vs Cp2Fe+/0 under 1 atm Ar, indicating that the four-coordinate complex [Fe0] is stable on

[FeII]2+ also binds a pendant amine (Fe−N = 2.177 Å), which has been experimentally observed for Cr(P2N2),32 Mo(PNP),33 Mn(P2N2),34 and Mo(P2N2)35 complexes. The related FeII compounds FeIIX(depe)2+ (X = Cl, Br) have an S = 1 ground state and weakly bind N2 after a low-energy triplet−singlet spin crossover.36 Due to the electron-deficient nature of [FeII]2+, this complex readily binds solvents such as THF and acetonitrile, indicated by an instantaneous color change from deep green to yellow. The solid-state Mössbauer spectra of complexes Fe0(N2), FeIIBr2, [FeII]2+, and FeIBr were collected at variable temperatures and in several applied magnetic fields (Figures 6

Figure 6. Mössbauer spectra of Fe0(N2). (A) 4.2 K, 0.1 T; (B) 4.2 K, 7.0 T. The data are shown as hash marks, and solid blue lines are the overall least-squares fits for Fe0(N2) and two diamagnetic impurities (red and green lines with parameters shown in Table S4).

Figure 7. Low-field Mössbauer spectra of (A) FeIIBr2, 7.0 K, 0.07 T; (B) FeIBr, 7.0 K, 0.07 T; and (C) [FeII]2+, 4.2 K, 0.045 T. The data are shown as hash marks, the solid blue lines are the overall leastsquares fits, and red solid lines correspond to the principal species discussed in the text, with the parameters shown. The green and brown lines are spectral simulations of impurities.

and 7, Table S4). The spectrum of Fe0(N2) at 4.2 K in an applied field of 0.1 T (Figure 6A) consists of a single sharp quadrupole doublet (line width of 0.27 mm s−1) with an isomer 9294


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Figure 8. (A) [FeI]+/[FeI(N2)]+ equilibrium. (b) Variable-temperature UV−vis spectroscopy with equilibrium constant. (c) a van’t Hoff analysis with thermodynamic parameters.

the CV time scale. Under 1 atm N2, the redox couple shifts anodically to −1.60 V (Figure 9A). We attribute this shift in the FeI/0 couple under N2 to the reversible binding of N2 to [FeI]+ in the bulk solution and irreversible binding of N2 to Fe0(N2) formed in the diffusion layer at negative potentials. The FeI/0 couple can be shifted between these two potentials by sparging a solution of [FeI]+ with either N2 or Ar, indicating facile N2 exchange. The FeII/I couple appears at −0.33 V under either N2 or Ar, consistent with N2 binding to [FeII]2+ being very unfavorable. Similar electrochemical behavior was observed in studies on isolated [FeII]2+ under either N2 or Ar (see SI). Variable scan rate CV studies were performed to gain kinetic information on N2 binding at FeI and Fe0 (Figure 9B). At scan rates ≥5 V s−1 under 1 atm N2, CVs of [FeII]2+ and [FeI]+ show a second reduction wave around −1.9 V that corresponds to the FeI/0 couple with no N2 bound in either oxidation state. Simulation of these data suggests the pseudo-first-order rate constant for N2 binding to FeI is ∼103 s−1 at 1 atm N2 (see SI). The FeI/0 couple with no N2 bound was irreversible at all scan rates studied, thus no kinetic information could be gained on the rate of N2 binding to [Fe0]. High-pressure CV studies gave further insight into the impact of N2 binding on the FeII/I and FeI/0 couples. Increasing the N2 pressure above 1 atm causes the FeI/0 couple to shift anodically until reaching a constant potential of −1.56 V at PN2 ≥ 30 atm (Figure 9C). This pressure-independent potential corresponds to the FeI/0(N2) couple, with N2 bound in both the oxidized and reduced state. At N2 pressures of 40−100 atm, the intensity of the FeII/I couple decreases relative to the FeI/0 couple, and no new anodic features are observed up to +0.94 V (Figure 9D). These results provide a conservative lower limit of +0.94 V on the potential of the FeII/I(N2) couple, indicating that the putative [FeII(N2)]2+ is a very high-energy species that does not

Figure 9. (A) CVs of [FeI]+/[FeI(N2)]+ (1 mM) under 1 atm N2 and 1 atm Ar, 0.05 V s−1 scan rate. (B) CVs of [FeI]+ at various scan rates (0.1 V s−1 − 10 V s−1). (C) Pressure dependence on E1/2(I/0) (V) for [FeI]+/[FeI(N2)]+ 1 mM) in fluorobenzene under N2 (1−100 atm), 0.1 V s−1 scan rate at each pressure increment (see SI for CV traces). (D) CVs for [FeI]+/[FeI(N2)]+ at N2 pressures of 1 and 100 atm, 0.1 V s−1 scan rate. All CVs are recorded in fluorobenzene with [nBu4N][B(C6F5)4] electrolyte.

form under experimental conditions. CVs simulated with this thermodynamic model reproduced the characteristic features of the experimental CVs (see SI). The electrochemical and UV−vis spectroscopic data can be used to construct thermochemical cycles for N2 binding at Fe0, FeI, and FeII (Scheme 2; also see SI).45 The experimentally derived free energy of N2 binding at [Fe0] is −7.0 kcal mol−1, leading to a ΔΔG°N2([FeI]+ − [Fe0]) of 7.4 kcal mol−1. N2 binding to FeII is thermodynamically unfavorable by at least 30 kcal mol−1, since the FeII/I(N2) couple is absent within the potential range employed in the CV studies. These free energies correspond to at least a 1022-fold increase in KN2 upon reduction from FeII to FeI and a 105-fold increase in KN2 upon 9295


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every order of magnitude increase in N2 pressure at 298 K, the binding free energy ΔG°′N2 changes by −1.36 kcal mol−1 (eq 1). Thus, the increased N2 solubility and thermodynamic driving force for N2 binding at Fe may inhibit the formation of off-cycle Fe compounds. To the best of our knowledge, N2 pressure dependence has not been reported for any homogeneous N2 reduction catalyst.

Scheme 2. Thermochemical Scheme Representing N2 Binding Affinities at Fe0, FeI, and FeII (1 atm N2, 298 K)a

ΔG N2 = ΔG°N2 − RT ln(PN2) a

[FeI]+ is also a competent N2 silylation precatalyst at 1 atm N2 (5.7 equiv, entry 6), but the comparatively low yield of N(SiMe3)3 at 100 atm (7.0 equiv, entry 7) suggests that zerovalent Fe is critical for attaining high catalytic activity. While Fe0(N2) is synthesized using NaC10H8 in THF (Scheme 1), this reduction reaction is slow. Under catalytic conditions in a low polarity solvent such as toluene, undesirable deactivation pathways may occur at [FeI]+ before formation of Fe0(N2). The choice of solvent and reductant is also important under high N2 pressure, as Fe0(N2) only generates slightly higher amounts of N(SiMe3)3 using THF and Na powder at 100 atm N2 (12 equiv, Table S2). Several control experiments conclusively indicate that Fe(P4N2) and an N2 atmosphere are required for the catalytic formation of N(SiMe3)3 (entries 8−11). For example, under 1 atm argon, Fe0(N2) generates 3.8 fixed N equivalents, presumably from silylating the coordinated N2 moiety and the N atoms in the P4N2 ligand during the decomposition of [Fe0] (vide infra; entry 8). In the absence of coordinated Fe, the N atoms of free P4N2 ligand are not converted to N(SiMe3)3 under catalytic conditions in an N2 atmosphere (entry 9). A proposed catalytic cycle is presented in Scheme 3. We propose that a pre-equilibrium between [Fe0] and Fe0(N2)

All experiments were conducted in fluorobenzene.

reduction from FeI to Fe0. For comparison, a 106-fold increase in KN2 was reported for an FeII2(μ-H)2 complex upon reduction by one electron to the mixed-valent FeII(μ-H)2FeI state.15 Catalysis. We found that Fe0(N2) catalyzes the silylation of N2 in the presence of equimolar amounts of trialkylchlorosilanes R3SiCl (R = Me, Et) and strong reductants M (M = Na, KC8).5e,11 In toluene, after 24 h, N2 reduction with Fe0(N2) yields a maximum of 11 ± 0.4 equiv (Me3Si)3N under 1 atm N2 (entries 1−2, Table 1). Fe0(N2) is a catalyst in THF with Na Table 1. N2 Silylation Catalysis Experimentsa

entry

cat.

mmol ClSiMe3, KC8

N2 press. (atm)

time (h)

N(SiMe3)3 equiv/Feb

1 2c 3 4c 5c 6 7 8d 9e 10 11

Fe0(N2) Fe0(N2) Fe0(N2) Fe0(N2) Fe0(N2) [FeI]+ [FeI]+ Fe0(N2) P4N2 − −

2 2 2 2 4 2 2 2 2 2 2

1 1 20 100 100 1 100 1 1 1 100

4 24 24 24 24 18 18 24 24 24 24

5.4 11 ± 0.4 22 38 ± 4 65 ± 10 5.7 7.0 3.8 0.2 0.0 0.0

(1)

Scheme 3. Proposed Catalytic Cycle for N2 Silylation Using Fe0(N2)(P4N2)

a

Experiments performed in 5 mL toluene using 0.004 mmol catalyst at 22 °C under N2 unless otherwise specified. bDetermined by acidification and NH4+ quantification using 1H NMR spectroscopy (see Experimental Section). cAverage of three runs. dUnder Ar. eUsing 0.016 mmol free ligand.

powder as the reductant, but catalytic activity is diminished (9.4 equiv, Table S2). Under 1 atm N2, using Et3SiCl also produces 11 fixed N equivalents after 24 h (Table S2). Increasing the N2 pressure has a dramatic effect on catalytic activity. At 20 atm, there is a 2-fold increase in N(SiMe3)3 (22 equiv) and nearly a 4-fold increase at 100 atm (38 ± 4 equiv; entries 3−4). Increasing the loading of silane and reductant at 100 atm N2 further enhances catalysis, with a maximum of 65 ± 10 equiv N(SiMe3)3 per Fe center (entry 5). The concentration of Fe0(N2) under the conditions outlined in Table 1 is 8.0 × 10−4 M, which is approximately six times lower than the concentration of dissolved N2 in toluene at standard state ([N2] = 5.1 × 10−3 M, PN2 = 1 atm, 298 K).46 Since we have determined that N2 loss from Fe0(N2) gradually leads to decomposition (vide supra), increasing the pressure increases N2 solubility, thus kinetically favoring N2 binding while also changing the N2 binding thermodynamics at [Fe0] (ΔG°N2 = −7.0 kcal mol−1 when PN2 = 1 atm, Scheme 2). For

occurs, with elevated N2 pressures biasing the formation of Fe0(N2) in solution and preventing catalyst deactivation. Next, it is expected that sequential silylation of the distal N atom forms Fe-silyldiazenido and Fe-disilylhydrazido intermediates, consistent with reported stoichiometric N2 silylation experiments.5e,8b,9,47 At this point, it is plausible for silylation to proceed through multiple mechanistic pathways, e.g., via silylation of the proximal8a,c or distal8b,9 N atom; efforts are ongoing in our laboratory to isolate molecular Fe intermediates and obtain further mechanistic information for this catalytic process. 9296


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than used in the experiment and should also include fail-safe pressure relief valves in case of accidental overpressurization. A commercially purchased pressure reactor (Parr Instrument Co. 4793Q) was equipped with two 4-lead signal feedthroughs, platinum wire pseudoreference electrode, graphite counterelectrode, three prepolished glassy carbon working electrodes, a glass reactor sleeve, and a Teflon coated stir bar. The calibrated pressure gauge (0−1500 psig) used was NIST certified with 10 psi pressure gradation marks and a readability error of ±5 psi. The system was loaded and sealed in a nitrogen-filled glovebox before each experiment, and the solution was stirred vigorously for at least 5 min in between each pressure interval to ensure thorough gas−liquid mixing. A very similar reaction vessel has been described in detail elsewhere.54 Electrochemical simulations were performed using the DigiElch 7.0 software package. NMR Spectroscopy. Experiments were conducted on a Varian INova or Varian NMR S spectrometer unless noted otherwise. 1H and 13 C spectra were referenced to their respective residual protio solvent,55 31P spectra were referenced to external 85% H3PO4 (0 ppm), and 15N spectra were referenced to external neat Me15NO2 (0 ppm). Solution magnetic moments were obtained using Evans’ method.56 The diffusion coefficient of Fe0(N2) was determined using 1H diffusion ordered spectroscopy (DOSY)57 using a 300 MHz 1 H frequency Agilent VNMRS system equipped with a direct detect dual band probe. The standard VNMRJ 4.0 1H DOSY pulse sequence was used with the following experimental parameters: 16 scans, 3.0 ms gradient pulse lengths, and a diffusion delay of 0.09 s. The gradient strengths were varied from 0−20 G cm−1. X-ray Crystallography. Single crystals were selected and mounted using NVH immersion oil onto a glass fiber or nylon fiber and cooled to the data collection temperature of 100(2) K with a stream of dry nitrogen gas, unless otherwise specified. Data were collected on a Bruker-AXS Kappa APEX II CCD diffractometer with 0.71073 Å Mo− Kα radiation. Unit cell parameters were obtained from 60 data frames, 0.5° ϕ, from three different sections of the Ewald sphere and complete data collection strategies were determined for each crystal using the APEX2 suite.58 Each data set was treated with SADABS (or TWINABS) absorption corrections based on redundant multiscan data.59 The structures were solved by direct methods (XS) or intrinsic phasing (XT) and refined by least-squares method on F2 using the XL program package interfaced through OLEX2.60 All non-hydrogen atoms were refined with anisotropic displacement parameters, and all hydrogen atoms were treated as idealized contributions. Details regarding specific solution refinement for each compound are provided in the SI. EPR Spectroscopy. Spectra were collected using a Bruker E580 spectrometer equipped with a SHQE resonator and a continuous flow cryostat. Spectra were recorded at 105 K suspended in a frozen toluene glass (1−5 mM) and contained in 4 mm OD quartz tubes. Microwave power was typically 2 milliwatts, and the frequency was 9.34 GHz. The field was swept by 800 G in 84 s and modulated at 100 kHz with 3 G amplitude, and a time constant of 20 ms was used. Spectra were simulated using EasySpin.61 57 Fe Mö ssbauer Spectroscopy. Spectra were recorded on constant acceleration Mö ssbauer spectrometers at low applied magnetic fields at temperatures between 4.2 and 295 K. Spectra of complexes FeIIBr2 and FeIBr were recorded at 0.07 T and variable temperature (5−200 K) on a closed-cycle refrigerator spectrometer (Colgate University), model CCR4K, equipped with a 0.07 T permanent magnet. Spectra for compound Fe0(N2) and [FeII]2+ were recorded at 0.045 T and 4.2 K on a spectrometer with a liquid helium cryostat at Carnegie Mellon University. Samples were prepared as polycrystalline powders dispersed in adamantane and placed in 1.0 mL Delrin cups. Mössbauer spectra were analyzed using the software WMOSS (SeeCo, Edina MN, www.SeeCo.us). Spectral simulations were elaborated in the framework of the quadrupole interaction Hamiltonian:

CONCLUSION In summary, we have synthesized a distinctive series of Fe complexes bearing a rigid tetraphosphine (P4N2) ligand. These include rare square planar FeII and FeI complexes and a Fe0-N2 complex, which is the first square pyramidal Fe0-N2 complex in a P4 ligand environment. Our results quantify the broad range of N2 binding affinities at Fe0, FeI, and FeII complexes in a square planar coordination environment, with comprehensive spectroscopic and thermochemical data showing the crucial dependence of oxidation state on the binding and activation of dinitrogen at iron. Under standard conditions, the N2 binding free energy at Fe0 is favorable (−7.0 kcal mol−1), reversible at FeI (0.4 kcal mol−1), and very unfavorable at FeII (>30 kcal mol−1). The reversible N2 binding observed at FeI in this work may serve as an example of a relevant oxidation state for N2 binding to Fe sites in the nitrogenase enzymes.2b,48 With a thorough understanding of the N2 thermochemistry and redox behavior for this system, we demonstrate that Fe0(N2) is the most active Fe-based N2 silylation catalyst to date. Elevated N2 pressures have a significant impact on the amount of N2-derived N(SiMe3)3 that is produced, generating up to 65 equiv N(SiMe3)3 per Fe center at 100 atm N2. Strategies to electrocatalytically reduce N2 to NH3 are currently being explored in our laboratory.

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EXPERIMENTAL SECTION

General Comments. All reactions were carried out under an atmosphere of nitrogen or argon using standard glovebox or Schlenk line techniques. Unless otherwise specified, commercially available chemicals were used as received. All reagents and solvents were stored in argon or nitrogen filled glove boxes prior to use. THF, toluene, dichloromethane, diethyl ether, acetonitrile, ethanol, and fluorobenzene were dried and degassed over activated alumina using a solvent purification system. Ethanol and THF were dried over 20% w/v activated 3 Å molecular sieves prior to use.49 Benzene was subjected to three freeze−pump thaw cycles and stored over 10% w/v activated 3 Å molecular sieves. Fluorobenzene was distilled prior to all electrochemical experiments. Celite and graphite were dried overnight at 200 °C under high vacuum, and glass wool was dried in a 180 °C oven overnight. CD2Cl2 was degassed and dried over CaH2, while THF-d8 was degassed and dried over Na/K alloy. C6D5Cl was subjected to three freeze−pump thaw cycles and stored over 10% w/v activated 3 Å molecular sieves. Sodium powder dispersion in mineral oil was rinsed with pentane and dried under high vacuum. PPh2CH2CH2PH2,20 [nBu4N][B(C6F5)4],50 [Fc][BArF4],51 and KC852 were prepared according to known literature procedures. [Fc][B(C6F5)4] was prepared according to a known procedure53 but using KB(C6F5)4 as the anion source. Aniline was dried over CaH2 and vacuum distilled prior to use. Gaseous 15N2 was added to J. Young NMR tubes using a stainless-steel gas delivery manifold attached to a Schlenk line. Electrochemistry. Experiments were conducted under N2 or Ar at 295 ± 3 K using a standard three-electrode setup, consisting of a 1 mm PEEK-encased glassy carbon working electrode, Pt wire pseudoreference electrode, and graphite counterelectrode. The working electrode was polished with 0.25 μm diamond polishing paste inside a glovebox and then rinsed with fluorobenzene. A CHI Instruments 620D potentiostat was used for data collection. Experiments under 1 atm gas were conducted in a glass vial containing 0.1 M [nBu4N][B(C6F5)4] electrolyte in distilled fluorobenzene (0.5 mL) with analyte concentrations of 1 mM and an internal Fc (0 V) or AcFc reference (309 mV vs Fc0/+). High-pressure electrochemical experiments were performed using ultra high purity N2 gas, 0.05 M [nBu4N][B(C6F5)4] dissolved in fluorobenzene (10 mL), analyte concentrations of 1 mM, and an internal AcFc reference (0.5 mM). Caution should be taken when performing experiments under high pressure! The system should be designed such that the components used are rated to a higher pressure

Ĥ Q = 9297

eQVzz , i ⎡ 2 15 2 2 ⎤ + η(Ix̂ , i − Iŷ , i)⎥ ⎢Iẑ , i − ⎦ 12 ⎣ 4 DOI: 10.1021/jacs.7b04552 J. Am. Chem. Soc. 2017, 139, 9291−9301

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2.95 (br, 4H; Ph2PCH2CH2). 13C NMR (CD2Cl2, 126 MHz, −25 °C): δ 151.0 (NCquat), 136.4 (PCquat), 134.1 (CHPPh), 129.0 (CHNPh), 129.0 (CHPPh), 127.3 (CHPPh), 118.7 (CHNPh), 114.5 (CHNPh), 47.7 (NCH2), 29.7 (Ph2PCH2), 25.7 (Ph2PCH2CH2). 31P{1H} NMR (CD2Cl2, 202 MHz, −25 °C): δ 94.01 (d, 2J(P,P)=85 Hz, 2P), 77.36 (d, 2J(P,P) = 85 Hz, 2P). Fe(P4N2)Br (FeIBr). A 20 mL vial was charged with a Teflon coated stir bar and FeIIBr2 (0.212 mmol, 200 mg) and then suspended in THF (4 mL). KC8 (0.255 mmol, 34 mg, 1.2 equiv) was weighed in a separate vial and added to the stirring solution. Residual KC8 was added to the reaction solution by rinsing the vial with THF (1 mL) and transferring to the stirring solution. The solution was vigorously stirred for 6 h, after which it became deep orange. The solids were removed by filtering the solution through a pad of Celite/glass wool and dried. The solid was stirred vigorously with diethyl ether (3 mL) until it became a fine suspension and then was dried under vacuum. The solid was again stirred vigorously with diethyl ether (5 mL), isolated on a medium pore glass frit, washed with diethyl ether (3 mL), and dried under vacuum to yield FeIBr as an orange solid (154 mg, 84%). Crystals suitable for X-ray diffraction were grown from dissolving FeIBr in THF and layering with pentane. Anal. calcd for C44H46N2P4BrFe: C, 61.27; H, 5.38; N, 3.25. Found: C, 61.00; H, 5.62; N, 3.40. μeff (C4D8O, Evans, 25 °C) = 1.6 μB (S = 1/2). Due to the paramagnetic nature of this compound, 1H NMR signals are broad and weak. 1H NMR (C4D8O, 500 MHz, 25 °C): δ 13.02, 10.78, 9.87, 7.55. 7.36, 7.18, 7.11, 6.91, 6.83, 6.59, 5.17. 4.92, 4.69, 4.42, 4.13, 3.26, 2.98, 2.50, 2.39, 1.29, −3.52. Fe(P4N2)(N2) (Fe0(N2)). A 25 mL Teflon-capped Schlenk tube was charged with a Teflon coated stir bar and FeIIBr2 (0.16 mmol, 150 mg) and set aside. NaC10H8 was prepared by dissolving naphthalene (0.43 mmol, 55 mg, 2.2 equiv) in THF (5 mL) with excess sodium powder (approximately 15 mg). After stirring vigorously for 2 h, deep green NaC10H8 was filtered through a pad of Celite/glass wool. The 25 mL Schlenk tube containing solid FeIIBr2 was cooled to −78 °C, and NaC10H8 was injected via syringe under a purge stream of N2 gas. The vessel was sealed under N2 (1 atm) and stirred at −78 °C for 2 h, during which the solution gradually became orange. The cooling bath was removed, causing the orange solution to turn deep orange, and was stirred vigorously at room temperature for 22 h. The deep orangebrown solution was dried under high vacuum, redissolved in toluene (3 mL), filtered through a pad of Celite/glass wool, and dried under high vacuum. The residue was suspended in pentane (10 mL) and vigorously stirred until it became a fine powder. The solid was isolated on a medium pore glass frit, washed with pentane (5 mL), and dried under vacuum to yield Fe0(N2) as an orange-brown powder (114 mg, 88%). Crystals suitable for X-ray diffraction were grown via slow vapor diffusion of pentane into Fe0(N2) dissolved in toluene. Anal. calcd for C44H46N4P4Fe: C, 65.20; H, 5.72; N, 6.91. Found: C, 65.26; H, 5.91; N, 6.46. 1H NMR (C4D8O, 500 MHz, 25 °C): δ 7.22−7.14 (m, 4H; CHNPh + CHPPh), 7.13−7.06 (m, 12H; CHPPh), 6.89 (m, 6H; CHNPh + CHPPh), 6.78 (m, 3H; CHNPh + CHPPh) 6.73 (t, 2J(H,H) =7.2 Hz, 1H; CHNPh), 6.57 (t, 2J(H,H) =7.2 Hz, 4H; CHNPh), 4.13 (m, 2H; NCH2), 3.50 (d, 2J(H,H) =12.7 Hz, 2H; NCH2), 3.31 (d, 2J(H,H) =12.7 Hz, 2H; NCH2) 3.26 (m, 2H; NCH2), 2.61 (m, 2H; Ph2PCH2), 2.40 (m, 2H; Ph2PCH2), 2.11 (m, 2H; Ph2PCH2CH2), 2.03 (m, 2H; Ph2PCH2CH2). 13C NMR ([D8]THF, 25 °C): δ 154.8 (NCquat), 153.6 (NCquat), 142.0 (PCquat), 141,3 (PCquat), 133.2 (CHPPh), 132.4 (CHPPh), 129.4 (CHNPh), 129.4 (CHNPh), 128.0 (CHPPh), 127.8 (CHPPh), 127.5 (CHPPh), 127.3 (CHPPh), 120.3 (CHNPh), 118.4 (CHNPh), 118.1 (CHNPh), 115.8 (CHNPh), 51.2 (NCH2), 50.4 (NCH2), 34.4 (Ph2PCH2), 26.3 (Ph2PCH2CH2). 31P{1H} NMR (C4D8O, 202 MHz, 25 °C): δ 92.89 (m, 2J(PA,PA′) = 173.5 Hz, 2 J(PA,PB) = 68.4 Hz, 2J(PA,PB′) = −75.9 Hz), 90.56 (m, 2J(PB,PB′) = 48.5 Hz, 2J(PB,PA) = 68.4 Hz, 2J(PB,PA′) = −75.9 Hz) (see SI for spin simulation). 15N{1H} NMR (C4D8O, 51 MHz, 25 °C): δ −54.8 (m, 1 15 15 J( N, N) = 5 Hz; Nα), −58.8 (d, 1J(15N,15N) = 5 Hz; Nβ). IR (THF): ν = 2003 cm−1 (FeN2), 1936 cm−1 (Fe15N2). [Fe(P4N2)][X] ([FeI][X]) (X = BArF4; X = B(C6F5)4). [FeI][BArF4]. A 20 mL vial was charged with a Teflon coated stir bar and FeIBr (0.093 mmol, 80 mg) and suspended in fluorobenzene (4 mL). NaBArF4

DFT Calculations. Calculations were performed using Gaussian 09.62 For benchmarking, various functionals (M06L,63 B3P86,64 ωB97XD 65 ) and basis sets (TZVP/TZVPfit, 66 6-31G(d) 67 (C,H,N,P), SDD68(Fe)) were tested on [FeI]+ and [FeI(N2)]+ and compared with the experimental N2 binding free energy (see SI). The combination M06L/6-31G(d)/SDD(Fe) was chosen for all further calculations. Either normal (opt) or tight (opt = tight) convergence criteria were met using a pruned (99,590) integration grid (grid = ultrafine). Except for N2(g), all optimizations were performed in a fluorobenzene solvent continuum (1 atm, 298 K) using the integral equation formalism polarizable continuum model (IEF-PCM)69 with radii and nonelectrostatic terms from the SMD solvation model (scrf = smd).70 Unrestricted open-shell calculations had negligible spin contamination. Full vibrational and thermochemical analyses were performed on optimized structures to obtain solvent-corrected free energies (G°) and enthalpies (H°). Optimized ground states had zero imaginary frequencies. Other Physical Measurements. MS analysis was performed using a 12T or 15T Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS) (Bruker SolariX, Billerica, MA) outfitted with a standard electrospray ionization (ESI) interface. The mass spectrometer was set to acquire data in positive mode, transient length was 1.1 s, and resolution was 170 K at m/z 539.19. Samples were directly infused at a flow rate of 3 μL/min using a 250 μL gastight syringe. The coated glass capillary temperature was set to 180 °C and the capillary set to −4.6 kV in positive mode. The data were collected from 100 m/z to 1500 m/z, ion accumulation time was 0.1 s for 32 scan averages co-added, and Q1 was set to 100 m/z. UV−vis experiments were carried out using a Shimadzu UV-2401PC spectrophotometer with a variable-temperature cuvette holder attachment. The sample and solvent blank temperatures were controlled with a PolyScience external cooling bath. A 40% aqueous ethylene glycol solution was constantly flowed through the temperature control system with 20 min of equilibration time between 5 K data collection intervals. Samples were placed into 1.00 cm quartz cuvettes fitted with a greased stopcock and sealed inside a glovebox. IR spectra were collected at room temperature on a Thermo Fisher Nicolet iS10 FT-IR spectrometer under a purge stream of N2 gas. Elemental analyses were performed at Atlantic Microlab, Inc. (Norcross, GA). Despite multiple attempts on independently prepared batches, samples [FeI][B(C6F5)4], [FeII][BArF4]2, and [FeII][B(C6F5)4]2 were consistently low in carbon by about 2% with consistent and accurate H/N percentages. We attribute this discrepancy to incomplete combustion associated with tetraarylborate anions.71 Nitrogen values were consistently low for Fe0(N2) by about 0.5%, probably due to partial loss of the coordinated N2 ligand. Fe(P4N2)Br2 (FeIIBr2). A 20 mL vial was charged with a Teflon coated stir bar, and P4N2 (0.688 mmol, 500 mg) was dissolved in THF (9 mL). FeBr2 (0.688 mmol, 148 mg) was weighed into a separate vial and added to the stirring solution. Residual FeBr2 was added to the reaction solution by rinsing the vial with THF (1 mL) and transferring to the stirring solution. A yellow precipitate gradually formed after 1 h of vigorous stirring. After the addition of pentane (10 mL) to the stirring suspension, the solid was isolated on a medium pore glass frit, rinsed with pentane (3 mL), and dried under high vacuum to yield analytically pure FeIIBr2 as a bright yellow solid (622 mg, 96%). Crystals suitable for X-ray diffraction were grown from dissolving FeIIBr2 in CH2Cl2 and layering with pentane. Anal. calcd for C44H46N2P4Br2Fe: C, 56.08; H, 4.92; N, 2.97. Found: C, 56.10; H, 5.01; N, 2.87. At room temperature, 1H and 31P NMR signals are broad. Cooling the sample to −25 °C significantly improves peak resolution. 1H NMR (CD2Cl2, 500 MHz, 25 °C): δ 7.54 (br, 8H; CHPPh), 7.30 (br m, 6H; CHPh), 7.23 (br m, 10H; CHPh), 6.91 (br m, 4H; CHPh), 6.76 (br m, 2H; CHPh), 4.33 (br, 4H; NCH2), 4.08 (br, 4H; NCH2), 3.27 (br, 4H; Ph2PCH2), 2.95 (br, 4H; Ph2PCH2CH2). 31 1 P{ H} NMR (CD2Cl2, 25 °C): δ 94.96 (br, 2P), 77.14 (br, 2P). 1H NMR (CD2Cl2, 500 MHz, −25 °C): δ 7.48 (br m, 8H; CHPPh), 7.28 (br m, 4H; CHPPh), 7.25−7.15 (br m, 12H; CHNPh + CHPPh), 6.91 (br m, 4H; CHNPh), 6.76 (br m, 2H; CHNPh), 4.36 (br m, 4H; NCH2), 4.05 (br d, 2J(H,H) =11 Hz, 4H; NCH2), 3.27 (br, 4H; Ph2PCH2), 9298


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The solids were dissolved in C6D5Cl, the tube was placed on a rotating stirrer overnight, and the solution became deep green. Based on the 1 H integral ratio between Fc (δ 6.5, br s) and 1,3,5-trimethoxybenzene (δ 3.50, s, OMe), the yield of [FeII][BArF4]2 is 81%. Representative Procedure for Fe0(N2)-Catalyzed Reduction of N2 to N(SiMe3)3. A 20 mL vial was charged with a Teflon coated stir bar, KC8 (2.0 mmol, 500 equiv, 270 mg), and toluene (5.0 mL). ClSiMe3 (2.0 mmol, 500 equiv, 0.25 mL) was added, followed by Fe0(N2) (0.0040 mmol, 1.0 equiv, 3.2 mg). The solution was vigorously stirred for 24 h, after which a 0.50 mL aliquot was transferred to a septum-capped vial charged with a Teflon coated stir bar. The 0.50 mL portion was acidified with 1 M HCl/Et2O (0.40 mL, 100 equiv/total Fe) and stirred for 10 min. The solution was dried under high vacuum for at least 30 min, dissolved in DMSO-d6, and NH4+ was quantified by 1H NMR spectroscopy5g relative to a known amount of 1,3,5-trimethoxybenzene (δ 6.10, s, 3H, CHAr; T1 = 2 s; conc. = 15−40 mM).72 To ensure complete relaxation of the internal standard, an acquisition delay of 10 s was used. Catalytic runs at N2 pressures >1 atm were prepared in a glovebox and transferred to a commercially available pressure reactor as described above. Approximately 10 min after combining all reagents, the vessel was pressurized and vigorously stirred for 24 h, followed by the same workup protocol in a glovebox. N(SiMe3)3 formation was confirmed by analyzing a 30 μL aliquot dissolved in CDCl3.8a The acidification protocol was verified using an authentic sample of commercially available N(SiMe3)3.

(0.093 mmol, 82 mg) was weighed in a separate vial and added to the stirring solution. Residual NaBArF4 was added to the reaction solution by rinsing the vial with fluorobenzene (1 mL) and transferring to the stirring solution. The solution immediately turned deep purple and was stirred vigorously for 1 h. The solids were removed by filtering the solution through a pad of Celite/glass wool, and then the solution was dried under high vacuum. The oily residue was washed with 3 mL pentane and dried again under high vacuum. The tacky residue was dissolved in a minimal amount of benzene (4−5 mL) and lyophilized to yield [FeI][BArF4] as a deep purple powder (148 mg, 97%). [FeI][B(C6F5)4−]: A 20 mL vial was charged with a Teflon coated stir bar and FeIBr (0.035 mmol, 30 mg) and suspended in fluorobenzene (3 mL). KB(C6F5)4 (0.035 mmol, 25 mg) was added directly to the stirring solution. The solution immediately turned deep purple and was stirred vigorously for 1 h. The solids were removed by filtering the solution through a pad of Celite/glass wool, and the filtrate was concentrated to about 1 mL. About 15 mL pentane was added to the stirring solution, which precipitated a maroon solid. The solid was isolated on a medium-pore glass frit and dried under vacuum to yield [FeI][B(C6F5)4−] as a maroon powder (48 mg, 94%). Crystals suitable for X-ray diffraction were grown via slow vapor diffusion of diethyl ether into [FeI][B(C6F5)4−] dissolved in fluorobenzene. Anal. calcd for C76H58N2BF24P4Fe ([FeI][BArF4]): C, 55.46; H, 3.55; N, 1.70. Found: C, 55.53; H, 3.72; N, 1.87. Anal. calcd for C68H46N2BF20P4Fe ([FeI][B(C6F5)4−]): C, 55.88; H, 3.17; N, 1.92. Found: C, 53.27; H, 3.17; N, 1.94. μeff (C6D5Cl, Evans, 25 °C) = 1.5 μB (S = 1/2). Due to the paramagnetic nature of this compound, 1H NMR signals are broad and weak. 1H NMR of [FeI][B(C6F5)4−] (C6D5Cl, 500 MHz, 25 °C): δ 8.92, 5.64, 4.54. IR (toluene, N2): ν = 2021 cm−1 (FeN2), 2090 cm−1 (Fe15N2). UV−vis (fluorobenzene, Ar): ε, M−1cm−1 (λmax, nm) = 3310 (467), 3390 (559). See above for variable-temperature UV−vis under N2. [Fe(P4N2)][X]2 ([FeII][X]2) (X = BArF4; X = B(C6F5)4). [FeII][BArF4]2. A 20 mL vial was charged with a Teflon coated stir bar and [FeI][BAr F4 ] (0.061 mmol, 101 mg) and dissolved in fluorobenzene (5 mL). [Fc][BArF4] (0.061 mmol, 65 mg) was added directly to the stirring solution in three portions. The solution gradually turned deep green and was stirred vigorously for 1 h. The solids were removed by filtering the solution through a pad of Celite/ glass wool, and the filtrate was concentrated to about 2 mL. About 10 mL pentane was added to the stirring solution, which precipitated a tacky, dark green solid. The orange solution was decanted off. The solid was triturated with pentane (3 × 5 mL) and dried under high vacuum to yield [FeII][BArF4]2 as a dark green powder (127 mg, 82%). [FeII][B(C6F5)4]2: A 20 mL vial was charged with a Teflon coated stir bar and [FeI][B(C6F5)4] (0.034 mmol, 50 mg) and dissolved in fluorobenzene (3 mL). [Fc][B(C6F5)4] (0.034 mmol, 30 mg) was added directly to the stirring solution in three portions. The solution gradually turned deep green and was stirred vigorously for 2 h. The solids were removed by filtering the solution through a pad of Celite/ glass wool, and the filtrate was concentrated to about 1 mL. About 10 mL pentane was added to the stirring solution, which precipitated a tacky, dark green solid. The orange solution was decanted off. The solid was stirred with pentane (10 mL) until it became a fine powder, which was isolated on a medium pore glass frit and dried under high vacuum to yield [FeII][B(C6F5)4]2 as a dark green powder (65 mg, 89%). FT-ICR-MS, fluorobenzene ([FeII][BArF4]2): Anal. calcd m/z for C44H46FeN2P42+: 391.0975. Found 391.1005 [M]2+. Anal. calcd for C108H70N2B2F48P4Fe ([FeII][BArF4]2): C, 51.79; H, 2.85; N, 1.11. Found: C, 49.73; H, 3.02; N, 1.13. Anal. calcd for C92H46N2B2F40P4Fe ([FeII][B(C6F5)4]2): C, 51.62; H, 2.17; N, 1.31. Found: C, 49.53; H, 2.59; N, 1.29. μeff (C6D5Cl, Evans, 25 °C) = 2.8 μB (S = 1). Due to the paramagnetic nature of this compound, 1H NMR signals are broad and weak. 1H NMR of [FeII][B(C6F5)4]2 (C6D5Cl, 500 MHz, 25 °C): δ 12.27, 10.34, 3.13, −7.20. Chemical Oxidation of Fe0(N2) to [FeII][BArF4]2. A J. Young NMR tube was charged with Fe0(N2) (0.011 mmol, 9.1 mg), [Fc][BArF4] (0.022 mmol, 24 mg, 2.0 equiv), and 1,3,5-trimethoxybenzene (0.0095 mmol, 1.6 mg, 0.86 equiv) as an internal standard.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data (CIF) The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b04552. Spectra (NMR, IR, EPR), cyclic voltammograms, X-ray crystallographic details, DFT structures and Cartesian coordinates (PDF) Crystallographic data Fe0(N2) (CIF) Crystallographic data FeIIBr2 (CIF) Crystallographic data FeIBr (CIF) Crystallographic data [FeI][B(C6F5)4] (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Demyan E. Prokopchuk: 0000-0002-6352-3509 Eric S. Wiedner: 0000-0002-7202-9676 Nicholas A. Piro: 0000-0003-4219-0909 W. Scott Kassel: 0000-0002-6764-9045 R. Morris Bullock: 0000-0001-6306-4851 Michael T. Mock: 0000-0002-7310-2791 Present Address ⊥

Department of Chemistry and Biochemistry, Albright College, 1621 N. 13th Street, Reading, PA 19604, United States

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. EPR experiments were performed using EMSL, a national scientific user facility sponsored by the DOE’s Office of Biological and Environ9299


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mental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the U.S. DOE. Computational resources were provided by the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory. The authors thank Prof. Yisong Alex Guo (Carnegie Mellon University), Emma Wellington (Colgate University), and Kaye Kuphal (Colgate University) for their assistance in recording Mössbauer data, Dr. Katarzyna Grubel for X-ray assistance, and Dr. Rosalie Chu for mass spectrometry assistance. The authors also thank Dr. Aaron Appel and Dr. Alex Kendall for helpful discussions.

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