N2 -to-NH3 Conversion by a triphos-Iron Catalyst and Enhanced Turnover under Photolysis

Abstract

Bridging iron hydrides are proposed to form at the active site of MoFe-nitrogenase during catalytic dinitrogen reduction to ammonia and may be key in the binding and activation of N₂ via reductive elimination of H₂ . This possibility inspires the investigation of well-defined molecular iron hydrides as precursors for catalytic N₂ -to-NH₃ conversion. Herein, we describe the synthesis and characterization of new P₂^P'Ph Fe(N₂ )(H)_x systems that are active for catalytic N₂ -to-NH₃ conversion. Most interestingly, we show that the yields of ammonia can be significantly increased if the catalysis is performed in the presence of mercury lamp irradiation. Evidence is provided to suggest that photo-elimination of H₂ is one means by which the enhanced activity may arise.

Keywords: ammonia; hydrides; iron complexes; nitrogen fixation; photolysis.

Figure 1

Top: The FeMoco active site of MoFe-nitrogenase.^[6] Bottom: Conversion of a proposed E₄(4 H) intermediate state of FeMoco into an activated E₄ state with N₂ bound.^[21]

Figure 2

The new diiron(I)–μ-N₂ catalyst (left) and previously reported P₃^BFe(N₂)⁻ (right) that provide higher yields of ammonia under Hg lamp photolysis.

Figure 3

a) X-ray structure of 3 with ellipsoids set at 50% probability (solvent and second dinuclear Fe molecule not shown; minor component of disordered isopropyl groups omitted for clarity).^[45] b) The 80 K, 50 mT, solid-state ⁵⁷Fe Mössbauer spectrum of 3. Data: black points, simulation: purple line. c) The 80 K, 50 mT, ⁵⁷Fe Mössbauer spectrum of a 2-MeTHF solution of 3. Major S = 0 component: blue, minor S = 1/2 component: yellow. d) X-band Continuous Wave (CW) EPR spectra (black) of 4 (top trace) and 4-D (bottom trace) in 2-Me-THF with simulations of each (red).^[46]

Figure 4

Hydride region of ¹H NMR spectrum of a [D₈]toluene solution of (P₃^B)(μ-H)Fe(N₂)(H) pre-photolysis (spectrum a) and after 10 minutes of Hg lamp photolysis at −78°C (spectrum b). The proton-(s) corresponding to the ¹H resonance are depicted in red and are underlined.

Scheme 1

Reductive elimination of H₂ from a polyphosphine iron complex in the presence of N₂ and sunlight, leading to an activated Fe(N₂) complex.^[–39]

Scheme 2

Synthesis of Fe complexes discussed herein.

Scheme 3

Possible roles for light in catalysis: photodissociation of dinuclear 3 to a monomer and/or reductive elimination of H₂ from an Fe(H)₂ complex 5.