Enhanced Ammonia Oxidation Catalysis by a Low-Spin Iron Complex Featuring 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 [(bpyPyMe)Fe(MeCN)], 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)] (TON of 16), the present [(bpyPyMe)Fe(MeCN)] system (TON of 149) features a stronger-field, more rigid auxiliary ligand that maintains -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 [(bpyPyMe)Fe(MeCN)] system exhibits a substantially faster AO rate (ca. 50×) at significantly lower (∼250 mV) applied bias compared to [(TPA)Fe(MeCN)]. Electrochemical data are consistent with an initial net H-atom abstraction step that furnishes the amide/ammine complex [(bpyPyMe)Fe(NH)(NH)], followed by the onset of catalysis at . 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 earth-abundant metal, is a promising metal for further development in metal-mediated AO catalysis by molecular systems.