N₂reduction and hydrogenation to ammonia by a molecular iron-potassium complex

Abstract

The most common catalyst in the Haber-Bosch process for the hydrogenation of dinitrogen (N(2)) to ammonia (NH(3)) is an iron surface promoted with potassium cations (K(+)), but soluble iron complexes have neither reduced the N-N bond of N(2) to nitride (N(3-)) nor produced large amounts of NH(3) from N(2). We report a molecular iron complex that reacts with N(2) and a potassium reductant to give a complex with two nitrides, which are bound to iron and potassium cations. The product has a Fe(3)N(2) core, implying that three iron atoms cooperate to break the N-N triple bond through a six-electron reduction. The nitride complex reacts with acid and with H(2) to give substantial yields of N(2)-derived ammonia. These reactions, although not yet catalytic, give structural and spectroscopic insight into N(2) cleavage and N-H bond-forming reactions of iron.

Fig. 1

Simplified scheme of the ammonia formation pathway in the Haber-Bosch process.

Fig. 2

Iron complexes with varying steric bulk give different N₂ products upon reduction by potassium (introduced as KC₈, potassium graphite). A) Bulky ligand L¹ (14); B) Bulky ligand L² (15); C) Less bulky ligand L³ (this work). ^tBu = tert-butyl.

Fig. 3

Molecular structure of 2 using 50% probability ellipsoids. Hydrogen atoms and cocrystallized solvent molecules have been omitted for clarity. Selected bond lengths (Å) and angles (°): Fe1-N1, 1.812(2); Fe1-N2, 1.906(2); Fe2-N1, 1.809(2); Fe2-N2, 1.918(2); Fe3-N2, 1.832(2); Fe1-Fe2, 2.4531(4); N1-N2, 2.799(2); Fe1-N1-Fe2, 85.27(7); Fe1-N2-Fe2, 79.79(7); Fe3-N2-Fe1, 135.9(1); Fe3-N2-Fe2, 143.6(1).

Fig. 4

A) Zero-field Mössbauer spectrum of solid 2 at 80 K. The red line is a superposition of the three Lorentzian doublets (I), (II), and (III) with intensity ratio 2:1:1. B) Temperature dependence of the effective magnetic moment of 2 measured at B = 1 T. The solid line represents a spin Hamiltonian simulation with four spins and two exchange coupling constants as indicated in the inset.