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. 2015 Jan 7;54(2):532-5.
doi: 10.1002/anie.201409454. Epub 2014 Nov 13.

Low-temperature N2 binding to two-coordinate L2Fe(0) enables reductive trapping of L2FeN2(-) and NH3 generation

Gaël Ung  1 , Jonas C Peters
Affiliations

Low-temperature N2 binding to two-coordinate L2Fe(0) enables reductive trapping of L2FeN2(-) and NH3 generation

Gaël Ung et al. Angew Chem Int Ed Engl. .

Abstract

The two-coordinate [(CAAC)2Fe] complex [CAAC = cyclic (alkyl)(amino)carbene] binds dinitrogen at low temperature (T<-80 °C). The resulting putative three-coordinate N2 complex, [(CAAC)2Fe(N2)], was trapped by one-electron reduction to its corresponding anion [(CAAC)2FeN2](-) at low temperature. This complex was structurally characterized and features an activated dinitrogen unit which can be silylated at the β-nitrogen atom. The redox-linked complexes [(CAAC)2Fe(I)][BAr(F)4], [(CAAC)2Fe(0)], and [(CAAC)2Fe(-I)N2](-) were all found to be active for the reduction of dinitrogen to ammonia upon treatment with KC8 and HBAr(F)4⋅2 Et2O at -95 °C [up to (3.4±1.0) equivalents of ammonia per Fe center]. The N2 reduction activity is highly temperature dependent, with significant N2 reduction to NH3 only occurring below -78 °C. This reactivity profile tracks with the low temperatures needed for N2 binding and an otherwise unavailable electron-transfer step to generate reactive [(CAAC)2FeN2](-) .

Keywords: carbenes; coordination modes; iron; nitrogen fixation; structure elucidation.

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Figures

Figure 1
Figure 1
Representative Mo and Fe complexes studied for catalytic N2 reduction to NH3. R = tBu; R’ = H, OMe; HIPT = hexaisopropylterphenyl; E = B, C; M = K, Na.
Figure 2
Figure 2
A. Variable temperature UV-vis spectra of (CAAC)2Fe in a pentane solution. a) under N2 atmosphere; b) under vacuum. B. X-Band EPR spectra at 10 K: a) [(CAAC)2Fe(N2)][K(18-crown-6)] in a frozen Et2O glass; b) (CAAC)2Fe(N2SiEt3) in a frozen methylcyclohexane glass. C.57Fe Mössbauer spectra for microcrystalline samples obtained at 80 K in boron nitride pellets. a) [(CAAC)2Fe(N2)][K(18-crown-6)]; b) (CAAC)2Fe(N2SiEt3). Solid red lines correspond to the simulated data. Solid blue line in a) correspond to a minor species (~ 9% of Fe) parameters of δ = 0.74 mm s−1 and ΔEQ = 3.79 mm s−1. D. Structure of [(CAAC)2Fe(N2)][K(18-crown-6)] in the solid state. Ellipsoids are drawn at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): C1–Fe1 = 1.924(2), C2–Fe1 = 1.919(2), C1–N1 = 1.386(3), C2–N2 = 1.411(3), Fe1–N3 = 1.778(3), N3–N4 = 1.035(4), N4–K1 = 2.726(4); C1–Fe1–C2 = 140.81(10), C1–Fe1–N3 = 101.91(11), C2–Fe1–N3 = 117.08(11), Fe1–N3–N4 = 175.5(3).
Scheme 1
Scheme 1
Reduction of (CAAC)2Fe and subsequent trapping with silylating reagents. Dipp = 2,6-diisopropylphenyl; R = Me, Et.
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