Catalysts that efficiently interconvert between electrical energy and chemical bonds (fuels) are needed for sustainable, secure energy. Ammonia has been proposed as a carbon-free chemical fuel. To this goal, oxidation of ammonia by molecular complexes is a burgeoning area of research, but poses critical scientific challenges that must be addressed. Understanding the thermodynamics of the elementary steps in which the N-H bond is cleaved can contribute to improved reactivity. We have focused on the design of molecular catalysts for oxidation of NH₃ to give N₂, using both aryloxy radicals to abstract H atoms from M-NH₃ complexes and direct electrochemical approaches. A key challenge is balancing the need for cleavage of N-H bonds and the formation of an N-N bond. Computational studies closely coupled with experimental studies have been pivotal to progress due to their ability to provide an atomistic picture and give energetics for experimentally unobservable species. We use density functional theory (DFT) to calculate the bond dissociation free energies (BDFEs) for HAT to show that improvement in the BDFE of over 20 kcal/mol can be made by moving from late to early transition metals in both polypyridyl and porphyrin complexes. Particular attention is paid to N-N coupling, a step previously found to be difficult in early metals, to show that pentadentate ligands disrupt nitride binding in order to enable N-N coupling. The mechanism for ammonia oxidation by both HAT and direct electrochemical approaches in metal porphyrins will be discussed, with a particular focus on the N–N coupling step. We show that Ru porphyrin complexes can also react with the H-atom acceptor to make C–N bonds, this can be avoided by choosing a bulky H-atom acceptor, thus achieving turnover. Thus, the mechanism by which this complex is able to generate N₂ with turnover will be explored. We also focus on the kinetics for H atom transfer, showing that the barrier can be broken into approach, transfer, and decoupling steps, giving guidance for catalyst design in the future. We show also how Cr and Os porphyrins overstabilize intermediates on the potential surface, preventing the formation of N₂. These results highlight how using both experimental and theoretical methods have advanced our understanding of ammonia oxidation and lead to the design of faster and more effective catalysts.

Alexander Phearman

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