Exploring Hydrogen Evolution Accompanying Nitrogen Reduction on Biomimetic Nitrogenase Analogs: Can Fe-N xH yIntermediates Be Active Under Turnover Conditions?

Zsolt Benedek,Julianna Oláh,Tibor Szilvási,Marcell Papp

doi:10.1021/acs.inorgchem.9b00719

Abstract

Nitrogen reduction reaction (N2RR) carried out on biomimetic catalytic systems is considered to be a promising alternative for the traditional Haber-Bosch ammonia synthesis. Unfortunately, the selectivity of the currently known biomimetic catalysts is poor, as they also catalyze the unproductive hydrogen evolution reaction (HER). In the present computational study, we examine the HER activity of early N2RR intermediates in EP3 (E = B, Si) ligated single-site biomimetic iron complexes by calculating and comparing the activation Gibbs free energies of HER and N2RR elementary steps. We find that, in contrast to previous suggestions, early N2RR intermediates are not likely sources of HER under turnover conditions, as the barriers of the competing N2RR steps are significantly lower. Consequently, future research should focus on preventing other potential HER mechanisms, e.g., hydride formation, rather than accelerating the consumption of early N2RR intermediates as proposed earlier to design more efficient biomimetic catalysts.

Highlights

Even though more than a century has elapsed since the invention of the Haber−Bosch ammonia synthesis, this process is still vital for modern agriculture and industry
The N2 reduction reaction (N2RR) and potential bimolecular hydrogen evolution reaction (HER) reactivity of the selected intermediates are examined in detail by comparing the activation Gibbs free energies of the competing elementary steps
It has not escaped our attention that our data point to different conclusions than the results presented in ref 17, where the general stabilizing effect of the boron atom and surprisingly large differences between BDFEN−Hs (13 kcal/mol in average; consult with Table S9 of the Supporting Information for details) were reported.[17]

Summary

Introduction

Even though more than a century has elapsed since the invention of the Haber−Bosch ammonia synthesis, this process is still vital for modern agriculture and industry. In the past few years, the annual global production of Haber−Bosch plants exceeded 140 million tons of NH3 gas,[1] which accounted for roughly 1% of the world’s energy consumption.[2] In spite of the extensive efforts to develop more effective and environmental friendly ammonia synthesis methods, the industrial application of alternative synthesis procedures is at present insignificant. Numerous “artificial nitrogenase” families were developed,[5] among which the single-site Fe B, C, Si)[6−8] (Scheme 1, left) complexes with EP3 ligand (E = resemble the natural enzymatic structure most closely.[9] These complexes are apt to bind dinitrogen and catalyze the N2 reduction reaction (N2RR) in the presence of proton and electron sources (Scheme 1, top right).[10]

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Results

Conclusion