Characterization Of An Fe≡N-NH2 Intermediate Relevant ... - PubMed

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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(N(x)H(y)) species that can be characterized and, in turn, extrude NH3. By contrast, the direct protonation of Fe-N2 species to Fe(N(x)H(y)) 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(12-crown-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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Figures

FIGURE 1

FIGURE 1

(A) 77 K X-Band (9.388…

FIGURE 1

(A) 77 K X-Band (9.388 GHz) EPR of [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ]…

FIGURE 1 (A) 77 K X-Band (9.388 GHz) EPR of [(TPB)Fe(N2)][Na(12-crown-4)2] (1). (B) 77 K X-Band (9.409 GHz) EPR after addition of 10 equivalents of HBArF4 · 2 Et2O 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) 77 K X-Band (9.399 GHz) EPR of [(TPB)Fe≡N-Ad][BArF4] (4).
FIGURE 2

FIGURE 2

Fe K-edge Fourier transform of…

FIGURE 2

Fe K-edge Fourier transform of the EXAFS spectrum collected on a sample of …

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 SI.
FIGURE 3

FIGURE 3

57 Fe Mössbauer spectrum obtained…

FIGURE 3

57 Fe Mössbauer spectrum obtained by the reaction of 57 Fe-enriched [(TPB) 57 …
FIGURE 3 57Fe Mössbauer spectrum obtained by the reaction of 57Fe-enriched [(TPB)57Fe(N2)][Na(Et2O)x] with 5 equiv HBArF4 · 2 Et2O in Et2O at −110 °C. Data is shown as black dots and the combined simulation is shown in red. The individual sub-spectra 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 SI for simulation parameters and more detailed discussion.
FIGURE 4

FIGURE 4

2D field-frequency 35.049 GHz Davies…

FIGURE 4

2D field-frequency 35.049 GHz Davies ENDOR pattern of 2 ( black solid )…

FIGURE 4 2D field-frequency 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, 1Hβ, 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.
FIGURE 5

FIGURE 5

Davies pulsed ENDOR spectra of …

FIGURE 5

Davies pulsed ENDOR spectra of 2 collected at 2 K for each 14 …

FIGURE 5 Davies pulsed ENDOR spectra of 2 collected at 2 K for each 14N and 15N isotopologues in red and blue, respectively. Observed 14/15N 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, is the ν of aiso(2-31Pβ) = 60 MHz. 11B/3: peaks at νB/3, the third harmonic of 11B nuclear Larmor frequency. Parameters: microwave frequency = 34.88 GHz; π/2 = 100 ns; τ = 600 ns; TRF = 60 μs; repetition time = 20 ms.
FIGURE 6

FIGURE 6

(A) Computed geometry for [(TPB)Fe≡N-NH …

FIGURE 6

(A) Computed geometry for [(TPB)Fe≡N-NH 2 ] + and (B) spin density plot…

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 1

SCHEME 1

SCHEME 1

SCHEME 1
SCHEME 2

SCHEME 2

Plausible scenarios that would transform…

SCHEME 2

Plausible scenarios that would transform [Fe-N 2 ] − catalyst 1 to [Fe-NH …

SCHEME 2 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 are provided for bookkeeping purposes only.
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