A 10(6)-fold enhancement in N2-binding affinity of an Fe2(μ-H)2 core upon reduction to a mixed-valence Fe(II)Fe(I) state

Abstract

Transient hydride ligands bridging two or more iron centers purportedly accumulate on the iron-molybdenum cofactor (FeMoco) of nitrogenase, and their role in the reduction of N2 to NH3 is unknown. One role of these ligands may be to facilitate N2 coordination at an iron site of FeMoco. Herein, we consider this hypothesis and describe the preparation of a series of diiron complexes supported by two bridging hydride ligands. These compounds bind either one or two molecules of N2 depending on the redox state of the Fe2(μ-H)2 unit. An unusual example of a mixed-valent Fe(II)(μ-H)2Fe(I) is described that displays a 10(6)-fold enhancement of N2 binding affinity over its oxidized congener, quantified by spectroscopic and electrochemical techniques. Furthermore, these compounds show promise as functional models of nitrogenase as substantial amounts of NH3 are produced upon exposure to proton and electron equivalents. The Fe(μ-H)Fe(N2) sub-structure featured herein was previously unknown. This subunit may be relevant to consider in nitrogenases during turnover.

Scheme 1. Synthesis of [SiP₂O]H₂ and Supported Di-iron Complexes

Figure 1

X-ray diffraction crystal structures of 3-N₂ and 4-(N₂)₂ with thermal ellipsoids drawn at 50% probability. Hydrogen atoms (other than the iron hydrides), the Na(12-crown-4)₂ cation of 4-(N₂)₂, and cocrystallized solvent molecules have been removed for clarity. The isopropyl substituents have been rendered transparent to aid in visualization of the inner coordination sphere around the diiron unit.

Figure 2

Variable temperature 500 MHz ¹H NMR spectra of 3-N₂ in toluene-d₈. To exclude additional N₂, NMR samples were subjected to three freeze–pump–thaw cycles in a J Young tube before backfilling with Ar gas prior to data collection. Spectra were obtained following equilibration at the listed temperature for at least 10 min. The hydride resonances are not shown.

Figure 3

Spectroscopic observation of 3-(N₂)₂ at low temperatures. (A) Solution IR absorption spectra of 3-N₂ at 296 K (red) and 3-(N₂)₂ at 193 K (black) dissolved in N₂-saturated toluene. (B) UV–visible spectra of an N₂-saturated hexane solution of 3-N₂/3-(N₂)₂ at the listed temperatures. (C) Van’t Hoff plot derived from the UV–visible spectral changes recorded at 675 nm and the published N₂ solubility values in cryogenic hexane. (D) ⁵⁷Fe Mössbauer spectra (zero field, 80K) of polycrystalline 3-N₂ (bottom, green dots) and 3-(N₂)₂ in 2-MeTHF (top, red dots). The two subspectra of 3-N₂ shown are only one possible fit to the data (refer to Supporting Information for complete details). (E) Chemical equilibrium and thermodynamic parameters derived from the Van’t Hoff analysis of the equilbrium N₂-binding process.

Figure 4

Temperature dependence of the Fe-(μ-H)-Fe chemical shift in 3-N₂ and thermal population of an excited triplet state. (A) ¹H NMR spectra of 3-N₂ collected at the listed temperatures. (B) Fit of the ¹H chemical shift of 3-N₂ to the magnetization equation defined in the Supporting Information. (C) Orbital surfaces of the two singly occupied (occupancy = 1.00) natural orbitals. All other orbitals had occupancies of >1.90 or <0.10 electrons.

Figure 5

(A) X-band EPR spectra of 4-(N₂)₂-d₂ (red), 4-(N₂)₂ (blue) and the simulated EPR spectrum of 4-(N₂)₂. Simulation parameters are listed in Table 1. Spectra were collected a 77 K in a 2-MeTHF glass at υ = 9.395 GHz at 2 mW power and modulation amplitude of 2 G. (B) Spin density plot of 4-(N₂)₂ shown with an isovalue of 0.0015.

Figure 6

⁵⁷Fe Mössbauer spectra recorded at 5 K on frozen 2-MeTHF solutions of 5 mM 4-(N₂)₂ with a 50 mT magnetic field applied (A) perpendicular and (B) parallel to the propagation of the gamma beam. The difference spectrum are in (C). The experimental traces are in black and the simulated traces are shown in red. δ = 0.35 mm/s, ΔE_Q = 0.69 mm/s, and η = 0.86 for 4-(N₂)₂. Refer to the Supporting Information for further details.

Figure 7

(A) Cyclic voltammetry measurements on N₂-saturated THF electrolyte solutions of 3-(N₂)₂ at 198 K. (B) Cyclic voltammograms obtained on solutions of 3-N₂ at 298 K. (C) Simulation of the voltammograms obtained at 298 K. (D) Square scheme model considered in the simulation traces shown in (C). Equations 1 and 2 represent the electrochemical reduction equilibria of 3-N₂ and 3-(N₂)₂. Equations 3 and 4 represent the N₂-binding equilibria for 3-N₂ and 4-N₂. The charges shown are representative of the full molecule. (E) Table listing critical parameters defined in the model and the resulting values found upon simulation. An asterisk denotes values that were held constant during the simulation. Refer to the Supporting Information for additional fit parameters.