Dinitrogen binding and cleavage by multinuclear iron complexes

Abstract

The iron-molybdenum cofactor of nitrogenase has unprecedented coordination chemistry, including a high-spin iron cluster called the iron-molybdenum cofactor (FeMoco). Thus, understanding the mechanism of nitrogenase challenges coordination chemists to understand the fundamental N2 chemistry of high-spin iron sites. This Account summarizes a series of studies in which we have synthesized a number of new compounds with multiple iron atoms, characterized them using crystallography and spectroscopy, and studied their reactions in detail. These studies show that formally iron(I) and iron(0) complexes with three- and four-coordinate metal atoms have the ability to weaken and break the triple bond of N2. These reactions occur at or below room temperature, indicating that they are kinetically facile. This in turn implies that iron sites in the FeMoco are chemically reasonable locations for N2 binding and reduction. The careful evaluation of these compounds and their reaction pathways has taught important lessons about what characteristics make iron more effective for N2 activation. Cooperation of two iron atoms can lengthen and weaken the N-N bond, while three working together enables iron atoms to completely cleave the N-N bond to nitrides. Alkali metals (typically introduced into the reaction as part of the reducing agent) are thermodynamically useful because the alkali metal cations stabilize highly reduced complexes, pull electron density into the N2 unit, and make reduced nitride products more stable. Alkali metals can also play a kinetic role, because cation-π interactions with the supporting ligands can hold iron atoms near enough to one another to facilitate the cooperation of multiple iron atoms. Many of these principles may also be relevant to the iron-catalyzed Haber-Bosch process, at which collections of iron atoms (often promoted by the addition of alkali metals) break the N-N bond of N2. The results of these studies teach more general lessons as well. They have demonstrated that N2 can be a redox-active ligand, accepting spin and electron density in complexes of N2(2-). They have shown the power of cooperation between multiple transition metals, and also between alkali metals and transition metals. Finally, alkali metal based cation-π interactions have the potential to be broadly useful for bringing metals close together with sufficient flexibility to allow multistep, multielectron reactions. At the same time, the positive charge on the alkali metal cation stabilizes charge buildup in intermediates.

Figure 1

Structure of iron–molybdenum cofactor (FeMoco) of nitrogenase in its resting state.

Figure 2

Two hypothetical activation modes in FeMoco: (a) Fe–C weakening or cleavage (indicated by a dashed line) and (b) Fe–S cleavage (indicated with one possible product after protonation).

Figure 3

β-Diketiminate ligands stabilize low-coordinate iron complexes (X = Cl, Br), which react with N₂ under reducing conditions to give products with weakened N₂.

Figure 4

Key “correlated pair” of orbitals from broken-symmetry calculations on L^Me,iPrFeNNFeL^Me,iPr, showing that there is β spin more localized on the π* orbital of N₂. (S_αβ is the value of the overlap integral between the α and β orbitals.) This is consistent with antiferromagnetic coupling of an N₂^2– bridge to two Fe²⁺ ions.

Figure 5

Reaction of L^Me,iPrFeNNFeL^Me,iPr with 2-hydroxypyridine leads not to protonation by the pendent acid, but gives N₂ displacement instead. Reduction of 2 H⁺ to form H₂ is assumed to account for the net oxidation from 2 Fe(I) to 2 Fe(II).

Figure 6

Smaller β-diketiminate, L^Me3, gives a four-iron product where the N–N triple bond has been cleaved to form two nitrides.

Scheme 1. Series of Half-Reactions That Lead to the Bis-Nitride Product

(a) Reduction of iron to the reactive iron(I) form and (b) cleavage of N₂. Two molar equivalents of KCl are also lost in the second reaction; these are represented as “2 K⁺” in the overall redox reaction.

Figure 7

Computation of the energy barrier for the conversion from end-on/end-on/side-on (EES) geometry to the end-on/side-on/side-on (ESS) geometry using truncated ligands. (a) Computed barrier in the absence of potassium. (b) Computed barrier in the presence of potassium. These results indicate that the addition of an electron and a potassium ion stabilizes the N–N cleaved isomer substantially, favoring the rearrangement of the core.

Figure 8

Product of reduction with 4 equiv of CsC₈ is Cs₂[L^Me3Fe(μ-N₂)]₃, which has intact N–N bonds. This shows that more reducing conditions do not necessarily lead to N–N cleavage, and implies a kinetic influence on N₂ activation.