Catalytic Reduction of N2 to NH3 by an Fe–N2 Complex Featuring a C-Atom Anchor

Malnutrition is the leading cause of death of children, the elderly, and adults. The ability to produce fertilizer, and by extension food, is therefore directly correlated with global human health. Nitrogen fixation is the process by which atmospheric nitrogen is converted to a bio-available form; it is essential to sustaining all life on this planet. Global population growth drove the widespread deployment of industrial fertilizer production (Haber-Bosch Process), a remarkable technological achievement that enables the global agriculture industry to feed billions of people each day. The scale of this industrial process is daunting: approximately 140 million metric tons of ammonia are produced annually, accounting for about half of all nitrogen fixed globally and significant global energy (~ 2%) and natural gas (~ 4%) consumption. But environmental consequences from fertilizer production and use are severe, including heavy CO₂ emissions, surface and groundwater pollution from runoff, eutrophication of freshwater systems, and massive killing of aquatic organisms in coastal regions that comprise so-called dead zones due to depleted oxygen. Given the importance of nitrogen fixation to global health, developing environmentally sustainable ways to make fertilizer and deploy it is a worthy goal. While it is widely appreciated that iron-rich cofactors within nitrogenase enzymes facilitate biological nitrogen fixation under ambient conditions, how they do so remains poorly understood.

This has led to two important scientific questions.

(1) Can we understand biological nitrogen fixation at the atomic level? (2) Can we use a fundamental understanding of biological nitrogen fixation to design more efficient synthetic catalysts and sustainable fertilizer production technologies?

Our laboratory addresses both of these questions through the development of synthetic model complexes. In this paper, which followed a prior recent study from our lab, we disclosed the first molecular iron complexes to serve as a functional model catalysts for the reduction of N₂ to NH₃ under remarkably mild conditions (-78 °C and 1 atm N₂). Our results indicate that a single iron site is capable of stabilizing the various N_xH_y intermediates generated during catalytic NH₃ formation. Based on these studies we hypothesize that geometric tunability at iron, imparted by a flexible Fe-B or Fe-C interaction in our model systems, is important for catalysis. We have proposed that the interstitial light carbon atom recently located in the center of nitrogenase cofactors may serve a similar role. We are now trying to use this insight to develop more increasingly efficient synthetic nitrogen-fixing catalysts.