Enhanced Ammonia Oxidation Catalysis by a Low-Spin Iron Complex Featuring Cis Coordination Sites

The goal of using ammonia as a solar fuel motivates the development of selective ammonia oxidation (AO) catalysts for fuel cell applications. Herein, we describe Fe-mediated AO electrocatalysis with [(bpyPy(2)Me)Fe(MeCN)(2)](2+), exhibiting the highest turnover number (TON) reported to date for a molecular system. To improve on our recent report of a related iron AO electrocatalyst, [(TPA)Fe(MeCN)(2)](2+) (TON of 16), the present [(bpyPy(2)Me)Fe(MeCN)(2)](2+) system (TON of 149) features a stronger-field, more rigid auxiliary ligand that maintains cis-labile sites and a dominant low-spin population at the Fe(II) state. The latter is posited to mitigate demetalation and hence catalyst degradation by the presence of a large excess of ammonia under the catalytic conditions. Additionally, the [(bpyPy(2)Me)Fe(MeCN)(2)](2+) system exhibits a substantially faster AO rate (ca. 50x) at significantly lower (similar to 250 mV) applied bias compared to [(TPA)Fe(MeCN)(2)](2+). Electrochemical data are consistent with an initial E-1 net H-atom abstraction step that furnishes the cis amide/ammine complex [(bpyPy(2)Me)Fe(NH2)(NH3](2+), followed by the onset of catalysis at E-2. Theoretical calculations suggest the possibility of N-N bond formation via multiple thermodynamically plausible pathways, including both reductive elimination and ammonia nucleophilic attack. In sum, this study underscores that Fe, an eartha-bundant metal, is a promising metal for further development in metal-mediated AO catalysis by molecular systems.

Automated summary

This study details Fe-mediated AO electrocatalysis with a high TON and fast AO rate, highlighting the promising potential of Fe as a metal catalyst for further development in metal-mediated AO catalysis.