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. 2016;7(9):5736-5746.
doi: 10.1039/C6SC00423G. Epub 2016 Jun 14.

Stepwise N-H Bond Formation From N2-Derived Iron Nitride, Imide and Amide Intermediates to Ammonia

K Cory MacLeod  1 , Sean F McWilliams  1 , Brandon Q Mercado  1 , Patrick L Holland  1
Affiliations

Stepwise N-H Bond Formation From N2-Derived Iron Nitride, Imide and Amide Intermediates to Ammonia

K Cory MacLeod et al. Chem Sci. 2016.

Abstract

Reduction of N2 to ammonia in nature and in electrocatalysis takes place through 1-proton/1-electron steps, motivating efforts to experimentally study the steps during proton/electron transfer to well-characterized N2-derived species with bridging nitrides. We report here the protonation and reduction reactions of an N2-derived iron bis(nitride) complex (Rodriguez et al., Science, 2011, 334, 780). We isolate and definitively characterize triiron imido and amido intermediates that lie along the path to ammonia formation, and Mössbauer spectroscopy shows the oxidation level of iron atoms in these mixed-valence clusters. The first two H atoms add to one of the two nitrides of the bis(nitride) complex, and the proton-coupled electron transfer in the second step can be concerted or stepwise depending on the sources of protons and electrons. The characterization of partially protonated nitrides and their mechanisms of formation are expected to guide efforts to convert N2 to ammonia with mild acids.

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Figures

Fig. 1
Fig. 1. Thermal-ellipsoid plots of the molecular structures of LFe(OC6H2tBu3) (2, left) and [LFe(μ-OH)]2 (6, right) using 50% thermal ellipsoids.
Scheme 1
Scheme 1. Ammonia formation from bis(nitride) 1.
Fig. 2
Fig. 2. Reaction of [LFe]2(μ2-NH)(μ3-N)[FeL] (3, [black circle], [8.3 mM]) with tBu3C6H2OH [80.8 mM] in C6D6, which proceeds through intermediates [LFe]2(μ2-NH2)(μ3-N)[FeL] (4, ♦) and [LFe(μ-NH2)]2 (5, ■) to the final product LFe(OC6H2tBu3) (2, ▲). Reaction monitored by 1H NMR with concentrations determined from integration of resonances relative to an internal Cp2Co standard. It was not possible to integrate the peaks of 2 accurately at later times because they are broadened by ammonia (see ESI†). The reactions are faster in THF or 2,5-dimethyltetrahydrofuran (see text).
Fig. 3
Fig. 3. Thermal-ellipsoid plots of the molecular structures and Mössbauer spectra of solid [LFe]2(μ2-NH)(μ3-N)[FeL] (3, top), [LFe]2(μ2-NH2)(μ3-N)[FeL] (4, middle), and [LFe(μ-NH2)]2 (5, bottom) using 50% thermal ellipsoids. The methyl and 2,6-dimethylphenyl groups are omitted for clarity. In the Mössbauer spectra, the fits are indicated with colored lines and the black circles are the data.
Scheme 2
Scheme 2. Formation of the imide/nitride 3 (step 1).
Scheme 3
Scheme 3. Stepwise ET/PT using separate electron and proton sources (step 2).
Scheme 4
Scheme 4. Reactivity of the tetrairon bis(nitride) complex with H2: detected products are shown.
Scheme 5
Scheme 5. Proposed mechanism of Fe-mediated N2 reduction to NH3.
See this image and copyright information in PMC

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