H-Bond-Assisted Cleavage of N–O Bond in the Electrochemical Reduction of N2O Catalyzed by Iron Tetraphenylporphyrin

Abstract

Reductive deoxygenation reactions play a crucial role in electrocatalytic processes relevant to energy and environmental challenges, including the reduction of CO₂, O₂, and N₂O. In particular, N₂O reduction is essential for closing the nitrogen cycle and preventing its accumulation in the atmosphere. These reactions are also significant in chemical processes, such as the reduction of phosphine and sulfur oxides. In this context, we demonstrate that the molecular catalysis of N₂O to N₂ reduction by iron tetraphenylporphyrin occurs predominantly via an innersphere mechanism. We then reveal that acids of varying strengths, including water, ethanol, trifluoroethanol, phenol, and acetic acid, can accelerate N–O bond cleavage. Surprisingly, the effect of acid pK_a is minimal, suggesting that the acceleration arises from hydrogen-bond-assisted N–O bond cleavage rather than direct conventional protonation. The process begins with coordination of N₂O to the low-valent iron tetraphenylporphyrin, activating the bond. At high acid concentrations, this binding step becomes rate-determining. Density functional theory calculations support the proposed mechanism, highlighting the importance of a dual activation strategy: bond activation by a low-valent transition metal and hydrogen-bond assistance from an acid acting as an H-bond donor. These findings provide valuable insights for designing more effective catalysts for N–O bond activation and, more broadly, for advancing reductive deoxygenation reactions.