A Non-Heme Iron Hyponitrite Complex

The reduction of nitric oxide (NO) is an important reaction in biology. For example, bacterial NO reductases (NORs) can reduce NO to N₂O. Flavodiiron NO reductases (FNORs) in pathogens can use this reaction to provide resistance to the human immune defense agent NO. These enzymes have non-heme diiron active sites. Identifying intermediates in the reduction of NO at these sites could provide useful insights. The results of computations indicate that iron-hyponitrite complexes could be key species in this process. However, there is little information about the coordination chemistry of non-heme iron centers with hyponitrite (N₂O₂²⁻).

Nicolai Lehnert, The University of Michigan, Ann Arbor, USA, and colleagues have synthesized a non-heme iron hyponitrite complex (simplified structure pictured). The team first reacted the complex [Fe^II(TPA)(MeCN)₂](OTf)₂ (TPA = tris(methylpyridyl)amine, OTf^– = triflate) with a sodium trans-hyponitrite hydrate (Na₂N₂O₂ • x H₂O) in methanol. They found that this led to the cleavage of the hyponitrite and the formation of the dimeric complex [Fe₂(TPA)₂(OMe)₂](OTf)₂. When they reacted a complex with a less Lewis-acidic iron center, i.e., [Fe₂(BMPA-PhO)₂(OTf)₂] (BMPA-PhOH = (2-((bis(pyridin-2-ylmethyl)amino)methyl)phenol), with Na₂N₂O₂ in acetonitrile as a polar aprotic solvent, they found that the tetranuclear iron-hyponitrite complex [{Fe₂(BMPA-PhO)₂}₂(μ-N₂O₂)](OTf)₂ was formed.

The product structure was confirmed using X-ray crystallography. The hyponitrite ion is bound to the four iron centers in the tetramer by both of its N and O atoms. According to the researchers, this represents a unique binding mode that had not been observed before. They also observed that upon the addition of acid to the tetrameric hyponitrite complex, quantitative N₂O formation took place. This indicates that protonation could be key for hyponitrite activation and N₂O formation. Oxidation of the iron centers also induced N₂O formation by modulating the Lewis acidity.