Triggering N(2) uptake via redox-induced expulsion of coordinated NH(3) and N(2) silylation at trigonal bipyramidal iron

Abstract

The biological reduction of N(2) to give NH(3) may occur by one of two predominant pathways in which nitrogenous N(x)H(y) intermediates, including hydrazine (N(2)H(4)), diazene (N(2)H(2)), nitride (N(3-)) and imide (NH(2-)), may be involved. To test the validity of hypotheses on iron's direct role in the stepwise reduction of N(2), model systems for iron are needed. Such systems can test the chemical compatibility of iron with various proposed N(x)H(y) intermediates and the reactivity patterns of such species. Here we describe a trigonal bipyramidal Si(o-C(6)H(4)PR(2))(3)Fe-L scaffold (R = Ph or i-Pr) in which the apical site is occupied by nitrogenous ligands such as N(2), N(2)H(4), NH(3) and N(2)R. The system accommodates terminally bound N(2) in the three formal oxidation states (iron(0), +1 and +2). N(2) uptake is demonstrated by the displacement of its reduction partners NH(3) and N(2)H(4), and N(2) functionalizaton is illustrated by electrophilic silylation.

Figure 1. Synthetic scheme for the generation of Fe-N₂⁺, Fe-N₂, and Fe-N₂^- (3, 4b, 5 and 5’)

Exposure of ammonia to cationic Fe-THF⁺ (2a, R = Ph or 3, R = iPr) affords the ammonia complexes 7a and 7b. Upon addition of Cp*₂Cr nitrogen uptake generates Fe-N₂ (4a or 4b) with quantitative release of NH₃. Sodium naphthalide reduction of 4b generates Fe-N₂^- (5 and 5’).

Figure 2. Cyclic voltammetry behavior of (SiP^iPr₃)Fe(N₂) (4b)

(a) CV under an N₂ atmosphere; (b) after sparging sample with argon for 30 sec; (c) after partial removal of argon under vacuum and re-exposure to an N₂ atmosphere; (d) after another vacuum/N₂ exposure cycle. Data collected in tetrahydrofuran at 100 mV/s and 0.3 M {ⁿBu₄}{PF₆}.

Figure 3. Solid-state structures of 3, 5, and 5’

(a) {(SiP^iPr₃)Fe(N₂)}{B(ArF)₄} (3); (b) {(SiP^iPr₃)Fe(N₂)}{Na(THF)₃} (5); (c) {(SiP^iPr₃)Fe(N₂)}{Na(12-C-4)₂} (5’). All hydrogen atoms and molecules of co-crystallization have been omitted for clarity. See SI for complete details.

Figure 4. Solid-state structures of {6b}{OTf}, 7a, N₂H₄B(C₆F₅)₃, and 9b

(a) {(SiP^iPr₃)Fe^II(N₂H₄)}{OTf} ({6b}{OTf}); (b) {(SiP^Ph₃)Fe^II(NH₃)}{B(ArF)₄} (7a); (c) N₂H₄B(C₆F₅)₃; (d) (SiP^iPr₃)Fe^II(N₂H₃B(C₆F₅)₃) (9b). Selected hydrogen atoms and the {B(ArF)₄} anion of 7a have been omitted for clarity. See SI for details.

Figure 5. Synthesis and characterization of (SiP^Ph₃)Fe^II(N₂C₆H₅) (10) and {(SiP^Ph₃)Fe^II(N₂C₆H₅)}{B(C₆H₃(CF₃)₂)₄} (11)

(a) Synthetic scheme for the generation of 10 and 11; (b) Core atom 50% probability ellipsoid representations of the solid-state structures of 10 and 11.

Figure 6. Synthesis and characterization of (SiP^iPr₃)Fe^II(N₂SiMe₃) (12)

(a)Synthesis of 12 via silylation of 5 or via reductive silylation of 4b; (b) Solid-state structure of 12. Hydrogen atoms have been removed for clarity. Selected bond distances (Å) and angles (°) for 12: Fe1-N1 1.695(2), N1-N2 1.195(3), Si2-N2 1.720(3), Fe1-Si1 2.3104(9), Fe1-P1 2.2508(8), Fe1-P2 2.2577(8), Fe1-P3 2.2500(8); P1-Fe1-P2 119.80(3), P2-Fe1-P3 114.28(3), P3-Fe1-P1 116.94(3), N1-Fe1-Si1 175.78(9), N2-N1-Fe1 175.7(3), N1-N2-Si2 165.6(3); (c) DFT calculated HOMO and HOMO-1 of 12 (see SI for details).

Figure 7. Zero field Mössbauer spectra

Spectra are recorded at 77 K and offset from top to bottom in the following order: (SiP^iPr₃)Fe(Cl),^a {(SiP^iPr₃)Fe(N₂)}{B(ArF)₄} (3), (SiP^iPr₃)Fe(N₂) (4b), {(SiP^iPr₃)Fe(N₂)}{Na(12-C-4)₂} (5’), {(SiP^iPr₃)Fe(N₂)}{Na(THF)₃} (5), and (SiP^iPr₃)Fe(N₂SiMe₃) (12). The dotted lines are the raw data and the solid lines are fits using the parameters listed. ^aNo effect with an applied external magnetic field of 45 mT was observed.