A Functional Model of Diiron Nitrite Reductases: Overcoming Catalyst Poisoning in the Diiron-Mediated Catalytic...

The biomimetic reduction of nitrite (NO₂^-) to nitric oxide (NO) is a fundamental transformation in the biogeochemical nitrogen cycle. However, developing synthetic catalysts that replicate the efficiency of diiron-containing nitrite reductases like YtfE remains a significant challenge. Herein, we report the design and synthesis of a novel diiron(II,II) complex, [Fe^II₂(cit)(CH₃CN)₄](PF₆)₄ (1), supported by a macrocyclic triazole/NHC hybrid ligand. This platform enabled the stabilization and crystallographic characterization of a rare {Fe₂(μ-(η¹-N:η¹-O)-NO₂)} intermediate (1-NO₂), providing unique structural insights into bimetallic nitrite activation. Mechanistic studies revealed that while thiols promote stoichiometric N-O bond cleavage to release NO, the formation of stable thiolate-bridged species eventually halts the catalytic cycle. By contrast, employing a proton-coupled electron transfer (PCET) strategy with cobaltocene and exogenous protons facilitates robust catalytic turnover. This system achieves highly selective nitrite-to-NO conversion with a record-breaking cumulative turnover number (TON) exceeding 1000. Crucially, the diiron framework exhibits an inherent resistance to NO coordination, effectively bypassing the "catalyst poisoning" that typically plagues mononuclear iron analogs. Although the PCET-driven mechanism departs from the mononuclear-like Fe-NO₂H pathway proposed for YtfE, this study establishes bimetallic synergy as a potent strategy for catalytic nitrite reduction and provides a robust structural platform for investigating nonheme diiron reactivity.