Characterization of an Fe≡N-NH2 Intermediate Relevant to Catalytic N2 Reduction to NH3.

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Characterization of an FeN−NH2 Intermediate Relevant to Catalytic N2 Reduction to NH3 John S. Anderson,†,§ George E. Cutsail III,‡,§ Jonathan Rittle,†,§ Bridget A. Connor,† William A. Gunderson,‡,# Limei Zhang,*,†,∥ Brian M. Hoﬀman,*,‡ and Jonas C. Peters*,† †

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States

‡

S Supporting Information *

ABSTRACT: The ability of certain transition metals to mediate the reduction of N2 to NH3 has attracted broad interest in the biological and inorganic chemistry communities. Early transition metals such as Mo and W readily bind N2 and mediate its protonation at one or more N atoms to furnish M(NxHy) species that can be characterized and, in turn, extrude NH3. By contrast, the direct protonation of Fe−N2 species to Fe(NxHy) products that can be characterized has been elusive. Herein, we show that addition of acid at low temperature to [(TPB)Fe(N2)][Na(12-crown-4)] results in a new S = 1/2 Fe species. EPR, ENDOR, Mössbauer, and EXAFS analysis, coupled with a DFT study, unequivocally assign this new species as [(TPB)FeN−NH2]+, a doubly protonated hydrazido(2−) complex featuring an Fe-to-N triple bond. This unstable species oﬀers strong evidence that the ﬁrst steps in Fe-mediated nitrogen reduction by [(TPB)Fe(N2)][Na(12crown-4)] can proceed along a distal or “Chatt-type” pathway. A brief discussion of whether subsequent catalytic steps may involve early or late stage cleavage of the N−N bond, as would be found in limiting distal or alternating mechanisms, respectively, is also provided.

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biological N2 ﬁxation.10 We have been interested in the postulate that a single Fe center can mediate N2 reduction via a ﬂexible ligand environment that enables N2 coordination at Fe and facilitates the stabilization of intermediates exhibiting Fe− N multiple bonding en route to NH3 formation.11 Isolated synthetic Fe complexes of a tris(phosphine)borane (TPB) scaﬀold (Scheme 1) with bound N2 or imido (NR2−) ligands

INTRODUCTION Since the discovery1 and crystallographic characterization2 of the FeMo-cofactor as the active site of FeMo-nitrogenase, there has been substantial interest in elucidating the mechanism of biological nitrogen reduction.3 While synthetic model chemistry cannot provide direct mechanistic information regarding enzymatic N2 reduction, it can play a crucial role in exploring the chemical viability of proposed pathways and in exposing new reactivity patterns that help stimulate, frame, and constrain various hypotheses. Early synthetic model work by Chatt, Hidai, and others,4 as well as recent examples of molecular Mo catalysts for N2 reduction,5 lent credibility to the initial proposal that the Mo center in the FeMo-cofactor could in principle serve as the site of N2 binding and reduction via a distal or Chatt-type mechanism.4a,6 More recent spectroscopic, biochemical, and structural studies of FeMo-nitrogenase have suggested that iron is the more likely site for N2 binding and reduction in the FeMo-cofactor.7 This hypothesis also reﬂects that, to date, iron is the only transition metal known to be essential to nitrogenase activity, underscored by the characterization of Fe-only nitrogenases.3b,8 This state of aﬀairs has motivated studies toward the synthesis of Fe complexes that mimic steps in proposed N2 reduction schemes and/or stabilize candidate iron intermediates of biological nitrogen ﬁxation.9 Indeed, numerous synthetic studies underscore the idea that an Fe−N2 binding site(s) may be mechanistically relevant to © XXXX American Chemical Society

Scheme 1

Received: April 1, 2015

DOI: 10.1021/jacs.5b03432 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society demonstrate important aspects of this single-site hypothesis.11e,f Moreover, through the use of chlorosilanes, this scaﬀold permits the conversion of Fe−N2 to FeN−NR2, modeling the ﬁrst two steps in a Chatt-type mechanism. The generation and characterization of a parent FeN−NH2 unit, either directly from Fe−N2 or otherwise, proved more challenging. Recently, three Fe-based catalysts for N2 reduction to NH3 were reported by our laboratory.12 These catalysts appear to be single-site12a,13 and provide functional Fe−N2 reduction systems for which systematic mechanistic studies can be conducted. In our original report,12a a new S = 1/2 species was generated by the low temperature addition of excess acid to [(TPB)Fe(N2)][Na(12-crown-4)2] (1) in the absence of exogenous reductant. This doublet species was tentatively postulated to be the hydrazido(2−) complex [(TPB)FeN− NH2][BArF4] (BArF4 = (3,5-(CF3)2C6H3)4B−) (2),12a but several alternative structures such as those candidates depicted in Scheme 1 could not be excluded. Because 2 is a likely intermediate of catalytic nitrogen reduction by 1 (vide infra), its characterization is of primary interest. Herein, we report combined EPR, ENDOR, and EXAFS spectroscopic studies that assign 2 as the parent hydrazido(2−) complex [(TPB)FeN−NH2][BArF4]. 57Fe Mössbauer studies indicate that this species constitutes a signiﬁcant portion of the Fe-containing material upon exposure of 1 to acid. Finally, the geometric constraints provided by the spectroscopic data are corroborated by the optimized geometry of 2 obtained from Density Functional Theory (DFT) methods.

Figure 1. (A) The 77 K X-band (9.388 GHz) EPR of [(TPB)Fe(N2)][Na(12-crown-4)2] (1). (B) The 77 K X-band (9.409 GHz) EPR after addition of 10 equiv of HBArF4·2Et2O to 1 to generate [(TPB)FeN−NH2][BArF4] (2). The parameters for the simulation of the spectrum are g1 = 2.222, g2 = 2.091, g3 = 2.006, two 31P nuclei coupling A3(31Pα) = 64 MHz, A3(31Pβ) = 40 MHz determined from ENDOR spectroscopy and third phosphorus of A3(31Pλ) ≤ 20 MHz; line broadening = 256, 113, and 41 MHz for g1, g2, and g3, respectively. (C) The 77 K X-band (9.399 GHz) EPR of [(TPB)FeN− Ad][BArF4] (4).

metal imido complexes.5b,14 The cationic imido complex [(TPB)FeN−Ad][BArF4] (4) is isoelectronic to [(TPB)FeN−NH2][BArF4], but was found to be thermally stable. This imido complex has been crystallographically characterized (Supporting Information) and also displays a rhombic EPR spectrum (Figure 1C). The rhombic EPR spectrum of 4 reﬂects a pseudotetrahedral iron center resulting from Fe−B elongation (Fe−B = 2.770 Å) as the iron center slips above the P3 plane. This geometry should place an unpaired spin in a relatively nonbonding orbital arising from a 2Edx2−y2,xy state. This electronic structure is reminiscent of the ferrocenium cation and is also similar to various low spin L3FeIIINR imides that have been previously described.15,9d Fe−N triply bonded species supported by the TPB scaﬀold include [(TPB)FeN−Ad][BArF4] (this work) and (TPB)FeN(4-OMe-Ph).11e These species have characteristically short Fe−N distances of 1.660 and 1.668 Å, respectively. Structural data was sought for 2 to probe for an anticipated short FeN−NH2 triple bond. Iron K-edge extended X-ray absorption ﬁne structure (EXAFS) data were collected and allowed for the observation of Fe-ligand distances in solution samples. These data were collected on frozen preparations (Supporting Information) of 2 in 2-MeTHF and are shown in Figure 2. Three pronounced peaks are observed in the Fourier transform spectrum, including one peak much shorter than expected for a singly bonded Fe−N pair. Indeed the EXAFS and Fourier transform spectra can be best ﬁt with a short Fe−N distance of 1.64 Å and two Fe−P distances of 2.28 and 2.42 Å in a 1:2 ratio. Additionally, an Fe−B scattering path may be included in the ﬁt at a distance of 2.67 Å, although convolution from the Fe−P scatterers makes this assignment tentative. The observation of two diﬀerent Fe−P scatterers for a complex with three phosphorus ligands can arise from various scenarios. If one assumes the EXAFS sample contains a single species, the two distances observed could correspond to a distorted structure with two long Fe−P distances and one short distance. A similar, though substantially lessened, distortion is

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RESULTS AND DISCUSSION Mixing of precatalyst 1 and 10 equiv of HBArF4·2Et2O in thawing 2-MeTHF solutions at −136 °C (Scheme 1) resulted in the disappearance of the dark red color characteristic of 1 and the formation of a brown-yellow solution. Analysis by CW X-band EPR indicates complete consumption of 1 and appearance of a new, more rhombic, S = 1/2 signal at 77 K (Figure 1B). Spin integration of the signal of 2 suggests a yield of 87(8)%. If stoichiometric acid is added to 1 at low temperature, rapid oxidation to neutral S = 1 (TPB)Fe(N2) with loss of 0.5 equiv H2 occurs instead (eq 1) and no EPR signal is observed (Supporting Information).12a 1 + HBAr F 4·2Et 2O → (TPB)Fe(N2) + 0.5H 2

(1)

(TPB)Fe(N2) + HBAr F 4·2Et 2O → [(TPB)Fe]+ + 0.5H 2 (2)

The signal for 2 can be generated, albeit at lower intensity, with as little as 2 equiv of acid. EPR spectra collected at 10 K reveal weak, complicated signals at low ﬁeld that are consistent with one or more S = 3/2 species (Supporting Information), in addition to those stemming from 2. The EPR spectrum of 2 is distinct from the more axial signature of 1 (Figure 1A) and it is also distinct from the previously reported EPR spectrum for (TPB)Fe(N2SiMe3) (3).11g The fact that structurally characterized 3, a model complex of the as yet uncharacterized complex ‘(TPB)FeN2H’, displays a more axial EPR spectrum than that of 2 suggests that 2 is electronically distinct from 3, and by extension is unlikely to be the parent (TPB)FeN2H diazenido species (candidate 2′-E in Scheme 1). Moreover, the requirement of ≥2 equiv of acid strongly suggests an alternative structure to 2′-E. Metal hydrazido(2−) complexes typically display substantial M−N multiple bonding, and are thus electronically similar to B

DOI: 10.1021/jacs.5b03432 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 3. 57Fe Mössbauer spectrum obtained by the reaction of 57Feenriched [(TPB)57Fe(N2)][Na(Et2O)x] with 5 equiv HBArF4·2Et2O in Et2O at −110 °C. Data is shown as black dots, and the combined simulation is shown in red. The individual subspectra representing (TPB)Fe(N2), [(TPB)Fe][BArF4], and [(TPB)FeN−NH2][BArF4] (2) are shown in blue, orange, and green, respectively, and are oﬀset for clarity. The spectrum was collected at 80 K in the presence of a 50 mT applied magnetic ﬁeld to minimize line broadening (fast relaxation regime). Refer to the Supporting Information for simulation parameters and more detailed discussion.

Figure 2. Fe K-edge Fourier transform of the EXAFS spectrum collected on a sample of 2. Inset shows the EXAFS oscillations. Scatterer distances (Å) are Fe−P = 2.28, Fe−P = 2.42, Fe−N = 1.64, and Fe−B = 2.67 with the phosphines present in a 1:2 ratio. Full ﬁt parameters may be found in the Supporting Information.

observed in [(TPB)FeN−Ad][BArF4]. Alternatively, these disparate distances could arise from the presence of more than one species in the sample, each with a narrow distribution of Fe−P distances, a scenario that seems more likely based on the observation that oxidation of 1 by acid is a competing side reaction (eq 1). The apparently larger than expected Debye− Waller factor in the ﬁtting for the Fe−N scatterer would thereby result from the presence of multiple species; the short and long Fe−P distances correspond closely to those observed in low-spin and high-spin iron complexes of the (TPB) ligand scaﬀold, which display variable Fe−N distances.11f−h To further probe the possible presence of multiple iron species generated on addition of acid to 1 at low temperature, Mössbauer analysis of in situ generated solutions of 2 was undertaken. In brief, an ethereal solution of 57Fe-enriched [(TPB)57Fe(N2)][Na(Et2O)x] was frozen and then layered with excess HBArF4·2Et2O in Et2O that was then also frozen. This sample was allowed to thaw to approximately −110 °C and mechanically stirred. The sample was then transferred to a Mössbauer cup chilled at 77 K for analysis. The Mössbauer spectrum of a representative sample is shown in Figure 3 and suggests the presence of three primary iron-containing species. Similarly prepared samples showed the same features but in variable ratios and hence caution must be exercised with respect to extrapolating the population of 2 in this speciﬁc Mössbauer sample relative to the population of 2 in independently prepared EXAFS and EPR samples. Nevertheless, the Mössbauer data conﬁrm the presence of three (TPB)Fe-species generated under relatively comparable conditions. One of the three species present in the representative Mössbauer spectrum shown (∼20% of total Fe present; Supporting Information) can be deﬁnitively assigned as the neutral S = 1 Fe−N2 adduct (TPB)Fe(N2) by comparison with an authentic sample (δ = 0.56 mm/s; ΔEQ = 3.34 mm/s). The two other species are present in approximately equal amounts (40% each of total Fe). These two components can be simulated in multiple ways (see Supporting Information for a detailed discussion), but comparison of the simulation parameters with those of independently characterized, pure (TPB)Fe complexes suggests the simulation of Figure 3 to be the most reasonable. The simulation parameters suggest one component is the previously characterized S = 3/2 cation [(TPB)Fe]+ (δ = 0.75 mm/s; ΔEQ

= 2.55 mm/s);16 the other by default is therefore the iron hydrazido(2−) cation 2 (δ = 0.35 mm/s; ΔEQ = 1.02 mm/s). The presence of (TPB)Fe(N2) and [(TPB)Fe]+ in the sample is consistent with our previous ﬁnding that the addition of HBArF4·2Et2O to 1 leads to net oxidation of the complex to generate (TPB)Fe(N2) via loss of H2.12a Likewise, (TPB)Fe(N2) can be further oxidized to [(TPB)Fe]+ via addition of HBArF4·2Et2O (eq 2, Supporting Information). In sum, the available Mössbauer data predicts that variously prepared samples of 2 may contain (TPB)Fe(N2) and [(TPB)Fe]+, and this fact helps explain the additional P scatterer in the EXAFS data, as well as the larger than expected Fe−N Debye− Waller factor. As (TPB)Fe(N2) is EPR-silent, and [(TPB)Fe]+ does not display EPR signals at 77 K, this Fe speciation is fully consistent with the clean spectrum observed by CW X-band EPR (Figure 1B). While the aforementioned characterization data are most consistent with [(TPB)FeN−NH2][BArF4], the techniques used are not sensitive to the presence or location of the protons. As a direct probe of these protons, as well as other nuclei in 2, we turned to electron nuclear double resonance (ENDOR) spectroscopy. This technique is particularly attractive because it selectively monitors S = 1/2 2 regardless of the presence of the other species that complicate the EXAFS and Mössbauer analysis. Figure 4 presents a 2D ﬁeld-frequency pattern of 1H Davies pulsed Q-band ENDOR spectra collected across the EPR envelope of 2. In addition to ancillary ligand signals from weakly coupled protons with A < 4 MHz, the spectra show strongly coupled signals that arise from protonation of the N2 ligand (Figure 4). The spectra collected at the “crystal-like” ﬁelds, g1 and g3, show two distinct 1H doublets, Ag1(1Hα) ∼ 16 MHz, and Ag1(1Hβ) ∼ 10 MHz (Figure 4, blue and red, respectively), indicating the presence of two well-deﬁned, inequivalent protons, consistent with assignment of 2 as exhibiting an unsymmetrically bound diprotonated (−N−NH2) species, inconsistent with the proposed structures 2′-B and 2′-E (Scheme 1). Structure 2′-B has two protons which are anticipated to be approximately equivalent with couplings C

DOI: 10.1021/jacs.5b03432 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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parameters for the two protons are inconsistent with every other model visualized in Scheme 1. Q-band 14/15N ENDOR data was also acquired to further characterize the hydrazido ligand of 2 (Figure 5). A 15N

Figure 5. Q-band (∼35.05 GHz) Davies pulsed ENDOR spectra of 2 collected at 2 K for each 14N and 15N isotopologues in red and blue, respectively. Observed 14/15 N coupling are Larmor centered (triangles) split by A and P for 14N (goalposts). A stronger nitrogen coupling is observed in 2 following protonation of 1. At g2: A2(2-15N) = +9.0 MHz; A2(2-14N) = −6.4 MHz; 3P(2-14N) = 2.7 MHz. At g3: A3(2-15N) = +8.0 MHz, but the corresponding A3(2-14N) = −5.7 MHz is possibly further quadrupole split and not observed. The ν− feature of aiso(2-11B) = −8.7 MHz at ∼11 to 12 MHz. The features observed at 10.0 and 10.5 MHz at g2 and g3, respectively, are the ν− of aiso(2-31Pβ) = 60 MHz. 11B/3: peaks at νB/3, the third harmonic of 11B nuclear Larmor frequency. Parameters: π/2 = 100 ns; τ = 600 ns; TRF = 60 μs; repetition time = 20 ms.

Figure 4. 2D ﬁeld-frequency Q-band (35.049 GHz) Davies ENDOR pattern of 2 (black solid) collected at 2 K. 1Hα (blue), A = [18.0, 10.5, 8.0] MHz, β = 30°; 1Hβ (red), A = [11.5, 6.0, 4.5] MHz, β = 0°; 31P (green), A = [43, 42, 40] MHz. Summed ENDOR simulation of 1Hα, 1 Hβ, and 31P is in black dashed line. Conditions: π = 60 ns; τ = 600 ns; TRF = 30 μs; repetition time = 20 ms; RF frequency randomly hopped.

substantially larger than observed (see below), and structure 2′E should only have one proton signal. As the magnetic ﬁeld is increased, the 1Hα feature splits, with the outer peak reaching a maximum 1H coupling between g1 and g2 (11661 G), Amax = 18 MHz for 1Hα; with further increase in ﬁeld, the pattern coalesces into the single doublet at g3. The maximum coupling for a metal-bound hydride (Amax ∼ 40 MHz) is much larger,17 inconsistent with structures 2′-C and 2′-D, Scheme 1. The 1Hα pattern resembles that of a heme hydroperoxy Fe−O−OH proton,18 an appealing analogy to the Fe−N−NH2 unit in 2, and those analyses provide a good starting point for analysis of the 1Hα hyperﬁne tensor; simulation of the 1Hα pattern (Figure 4, blue) yields a slightly rhombic tensor of A = +[18.0, 10.5, 8.0] MHz (see Figure 4 caption), whose anisotropic contribution corresponds to an Fe−Hα distance of d > 3.1 Å, with the Fe−Hα vector rotated away from g1, which coincides approximately with the Fe−N bond and the molecular pseudo-C3 axis.19 In short, the analysis requires a nonlinear Fe−N−N geometry (Supporting Information Figure S8). The smaller 1Hβ coupling (Figure 4, red) exhibits a 2D pattern that can be described by a hyperﬁne tensor whose anisotropic contribution corresponds to an Fe−Hβ distance of d > 3.5 Å, with the Fe−Hβ vector along g1 (the Fe−N bond). The absolute signs of the 1Hα, 1Hβ, couplings are implied by the assumption that the dominant anisotropic component is determined by through-space dipolar coupling, and were conﬁrmed by the Variable Mixing Time (VMT) Pulsed ENDOR Saturation and Recovery (PESTRE) protocol (Supporting Information). The estimated Fe−1Hα/1Hβ distances, along with the orientations of the hyperﬁne coupling tensors relative to g, imply that both protons have been added to the β-N of a bound N2, to generate a bent FeN−NH2 moiety, with an Fe−N−N bond angle δ ∼ 150° (Supporting Information). Notably, as indicated above, the metrical

ENDOR response is observed as a peak at ν+ = +9.8 MHz (Figure 5), corresponding to A2(2-15N) = +9.0 MHz, A2(2-14N) = −6.4 MHz for the coordinated nitrogen (upon scaling by the gyromagnetic ratios of 14N and 15N (γ ≡ gn(14N)/gn(15N) ≡ A(14N)/A(15N) = −0.71)). The 2-14N ENDOR response has a quadrupole splitting of 3P = 2.7 MHz at g2. Furthermore, the isotopically labeled (2-15N) sample has a resolved hyperﬁne coupling of A3(15N) = +8.0 MHz near g3. The corresponding 14 N ENDOR response of 2-(14N) is expected to have a coupling of A3(14N) = −5.7 MHz, but is not observed. The additional quadrupole splitting of the 14N at g3 may orient A and P in such a manner that the quadrupole ENDOR response is too broad at g3 and sharpest at g2. No distinguishable nitrogen ENDOR features were observed at ﬁelds below g2 for either 2-(14N) or 2-(15N), but the single-ﬁeld three-pulse ESEEM spectrum of 2-(14N) (Supporting Information) is well simulated with a single 14N tensor of A(2-14N) = −[4.34, 7.18, 6.22] MHz and quadrupole parameters of e2qQ/h = 1.76 (P = 0.88 MHz) and rhombicity factor, η = 0.64, in excellent agreement with the observed P2 = 0.90 MHz from ENDOR spectroscopy. The presence of quadrupole coupling with high rhombicity supports the idea of a nonlinear Fe−N−N linkage. 11 B ENDOR spectra of 2 (Supporting Information, Figures S10 and S11) show a coupling of aiso ∼ −8.8 MHz, completing the characterization of the axial ligands to Fe. As shown in Supporting Information Figure S11, the 11B nuclei exhibits a VMT-PESTRE response, typical of a nuclei with negative spin density and a negative A. Using the 11B as a ‘reference’, the observed 14N nuclei possesses negative spin density and 1Hα and 1Hβ both exhibit positive spin density. The negative spin D

DOI: 10.1021/jacs.5b03432 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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carried out at −78 °C in Et2O by addition of acid followed by addition of reductant.12a As such, we sought to determine whether 2 could be detected in mixtures more relevant to catalysis, prior to the addition of the reductant. X-band EPR spectra of solutions prepared from the addition of acid to 1 at either −136 °C in 2-MeTHF or at −78 °C in Et2O indicate that 2 is present in both preparations (Supporting Information, Figure S15). Furthermore, upon warming to −40 °C or to room temperature, both preparations show the growth of S = 3/2 signals concomitant with the decay of the signals of 2. The identity of these S = 3/2 species is hard to determine due to convolution of the quartet signals. One such S = 3/2 species, [(TPB)Fe(NH3)]+, has been deﬁnitively assigned by inspection of the 1H NMR spectrum of a similarly prepared mixture following warming to room temperature.12a Therefore, 2 decays at least in part to [(TPB)Fe(NH3)]+ upon warming even without exogenous reductant.

density observed on the 11B and 14N nuclei is a result of the spin-polarization of the Fe−B bond by the positive-spin density of the Fe center. Finally, a nearly isotropic 31P ENDOR signal from the phosphine ligand is simulated with A(31Pα) = [43, 42, 40] MHz (Figure 4, green). Features of a second phosphine are observed in Figure 5, around 10.0−10.5 MHz and assigned as the ν− transitions of 31P doublet with A ∼ 60 MHz; the ν+ partner of this 31P response unfortunately falls underneath the intense ENDOR response of weakly coupled solvent protons (1H) (∼55 MHz) (Supporting Information Figure S12). This observation of two 31P couplings, one with A3(31Pβ) ∼ 64 MHz the other with A3(31Pα) = 40 MHz by ENDOR spectroscopy correlates well with the simulation of the Xband EPR spectrum (Figure 1). Computational studies were undertaken to compare the parameters of the theoretically predicted structure with those obtained spectroscopically. An optimized geometry (Supporting Information) of [(TPB)FeN−NH2]+ was found, utilizing the core of [(TPB)FeNAd]+ for an initial guess. This geometry shows similar bonding metrics to those observed by EXAFS and ENDOR analysis. Thus, ENDOR requires diprotonation of the remote Nβ, while EXAFS requires a short Fe−Nα distance consistent with the calculated value 1.70 Å, and calculated Fe−P distances of 2.29, 2.31, and 2.36 Å (Figure 6A). The characteristically short Fe−N distance (i.e. ≤

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CONCLUSIONS The combined spectroscopic data presented herein conﬁrm the assignment of 2 as a [(TPB)FeN−NH2]+ hydrazido(2-) complex. Alternative formulations for 2 can be excluded by the data presented. The observation of a short Fe−N distance by EXAFS spectroscopy conclusively excludes the two diazene formulations, 2′-A,B, as well as 2′-E. In addition, an η1 diazene adduct (2′-A) should have two 1H hyperﬁne couplings, but with one much stronger than the two observed. An η2 diazene adduct (2′-B) is expected to have two 1H couplings larger than observed. Hydride formulations (2′-C, 2′-D) are ruled out by the 1H couplings and metric parameters already discussed. Finally, 2′-E would have only one coupled proton. To summarize, double protonation at the distal nitrogen, as invoked in a distal or Chatt type mechanism, is viable for this iron catalyst and likely occurs in the Fe-mediated N2 ﬁxation cycle catalyzed by 1 (Scheme 2). One or more downstream species arising from the decomposition of 2 involve higher spin (S = 3/2) species based on X-band EPR data (see Supporting Information). Plausible candidates for these S = 3/2 species include [(TPB)Fe(NH3)]+, (TPB)Fe(NH2), and [(TPB)-

Figure 6. (A) Computed geometry for [(TPB)FeN−NH2]+ and (B) spin density plot for [(TPB)FeN−NH2]+. Both plots generated with the M06L functional with TZVP, SVP, and 6-31G(d) basis sets on Fe; P, B, N, C; and H, respectively.

Scheme 2. Distal, Alternating, and Hybrid Pathwaysa

1.7 Å) is consistently found when exploring a variety of functionals, while the Fe−P distances show slightly more variation (Supporting Information). In addition, the bending at Nα required for the interpretation of the ENDOR data is observed in the calculated gas-phase structure. Figure 6B depicts the predicted spin distribution in [(TPB)FeN− NH2]+ suggesting that the majority of spin is carried by Fe with spin polarization observed on the NNH2 and borane ligands, consistent with the ENDOR results. Other candidate structures for 2 were similarly optimized and found to be higher energy isomers. For example, a comparison of the energies of the hydrazido(2−) species versus a diazene isomer (Scheme 1, 2′A) shows that the assigned [(TPB)FeN−NH2]+ isomer is lower in energy by 17.0 (BP86) or 8.2 (M06L) kcal/mol (Supporting Information). Taken together, the computational work is also consistent with the assignment of 2 as [(TPB)Fe N−NH2]+. While 2 has been generated herein with excess acid at −136 °C in 2-MeTHF, N2 reduction catalysis by 1 was originally

Plausible scenarios that would transform [Fe−N2]− catalyst 1 to [Fe−NH3]+ where [FeN−NH2]+, 2, is a key intermediate. Species shown in blue (along with spin states) have been experimentally characterized. Top path illustrates a limiting distal mechanism, bottom path an alternating mechanism, and the dotted arrows illustrate hybrid paths that could shuttle distal intermediate 2 to an alternating pathway that features late stage N−N cleavage. The order of protons, electrons, and/or H-atoms is provided for bookkeeping purposes only. a

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Fe]+;11h these species, each of which is shown in blue in Scheme 2, are compatible with both distal and alternating scenarios. Another candidate S = 3/2 species is the hydrazine complex that would form on the alternating pathway, [(TPB)Fe(N2H4)]+, which was previously characterized and shown to decompose to [(TPB)Fe(NH3)]+.11h A branching scenario whereby the distal intermediate 2 isomerizes to an alternate diazene structure, such as either 2′-A or 2′-B in Scheme 1, that then proceeds to (TPB)Fe−N2H4+ as a laterstage intermediate of the alternating pathway, remains a mechanistic possibility.7 Likewise, formal H atom addition to 2 at the Nα atom (Scheme 1) provides an alternative possible branching point from the distal to the alternating pathway. The key point we wish to underscore is that the observation of 2 is not incompatible with late stage N−N cleavage via a hydrazine intermediate. Sorting out further mechanistic details for the present system warrants additional studies. But one conclusion is clear: an imide-like [(TPB)FeN−NH2]+ species can be directly generated at low temperature by protonation of the Fe−N2 catalyst precursor, and solutions of this species are known to decay to [(TPB)Fe(NH3)]+. This is a satisfying conclusion as it links the chemistry of iron-to-nitrogen multiple bonding to an N2 ﬁxation cycle. In this context, a conceptual parallel can be drawn to catalytic O2 reduction wherein iron-to-oxygen multiply bonded species (e.g., FeO) are generated as intermediates.20

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental and spectroscopic data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.5b03432.

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Article

AUTHOR INFORMATION

Corresponding Authors

*[emailprotected] *[emailprotected] *[emailprotected] Present Addresses #

Department of Chemistry, Illinois College, 1101 West College Avenue, Jacksonville, IL, 62650, United States. ∥ Department of Biochemistry and Redox Biology Center, University of NebraskaLincoln, Lincoln, NE 68588, United States. Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGMENTS This work was supported by the NIH (GM 070757 to J.C.P., GM 111097 to B.M.H.), the NSF (MCB-1118613 to B.M.H., DGE-0824162 to G.E.C.), an NSERC fellowship (to L.Z.), a Caltech Center for Environmental Microbial Interactions fellowship (to L.Z.), and the Gordon and Betty Moore Foundation. We thank the staﬀ at Beamline 9−3, Stanford Synchrotron Radiation Lightsource (SSRL). SSRL is operated for the DOE and supported by OBER and by the NIH, NIGMS (P41GM103393) and the NCRR (P31RR001209). F

DOI: 10.1021/jacs.5b03432 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society (15) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 125, 322. (16) While we favor [(TPB)Fe]+, structurally and electronically similar S = 3/2 species that feature a weakly bound axial ligand, including [(TPB)Fe(NH3)][BArF4] and [(TPB)Fe(N2H4)][BArF4], cannot be strictly excluded as alternative candidates. (17) Kinney, R. A.; Hetterscheid, D. G. H.; Hanna, B. S.; Schrock, R. R.; Hoffman, B. M. Inorg. Chem. 2010, 49, 704. (18) (a) Davydov, R.; Kofman, V.; Fujii, H.; Yoshida, T.; Ikeda-Saito, M.; Hoffman, B. M. J. Am. Chem. Soc. 2002, 124, 1798. (b) Davydov, R.; Makris, T. M.; Kofman, V.; Werst, D. E.; Sligar, S. G.; Hoffman, B. M. J. Am. Chem. Soc. 2001, 123, 1403. (19) (a) McNaughton, R. L.; Chin, J. M.; Weare, W. W.; Schrock, R. R.; Hoffman, B. M. J. Am. Chem. Soc. 2007, 129, 3480. (b) McNaughton, R. L.; Roemelt, M.; Chin, J. M.; Schrock, R. R.; Neese, F.; Hoffman, B. M. J. Am. Chem. Soc. 2010, 132, 8645. (c) Kinney, R. A.; McNaughton, R. L.; Chin, J. M.; Schrock, R. R.; Hoffman, B. M. Inorg. Chem. 2011, 50, 418. (d) Lee, Y.; Kinney, R. A.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 16366. (e) Gunderson, W. A.; Suess, D. L. M.; Fong, H.; Wang, X.; Hoffmann, C. M.; Cutsail, G. E., III; Peters, J. C.; Hoffman, B. M. J. Am. Chem. Soc. 2014, 136, 14998. (f) Cutsail, G. E., III; Stein, B. W.; Subedi, D.; Smith, J. M.; Kirk, M. L.; Hoffman, B. M. J. Am. Chem. Soc. 2014, 136, 12323. (20) Nam, W. Acc. Chem. Res. 2007, 40, 522.

DOI: 10.1021/jacs.5b03432 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Characterization of an Fe≡N-NH2 Intermediate Relevant to Catalytic N2 Reduction to NH3. - PDF Download Free (2024)

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