N₂ Generation from Nitric Oxide Coordinated to Iron(III) Porphyrin in Acidic Glycine Buffer

Nitric oxide (NO) was efficiently converted to molecular nitrogen (N₂) in an acidic aqueous solution containing iron(III) porphyrin encapsulated in the cyclodextrin (CD) nanocavity. The supramolecular iron(III) porphyrin/CD dimer complexes (hemoCD-P and hemoCD-I), where the iron(III) is axially coordinated by a nitrogenous ligand (pyridine or imidazole) in the linker of the CD dimer, form stable 6-coordinated ferric nitrosyls {FeNO}⁶ in acidic aqueous solution (pH ∼3). When the solution contained glycine as the buffer component, N₂ bubbles were significantly generated within several minutes at room temperature. In this system, a new N-N bond is formed on the iron-porphyrin due to the nucleophilic attack of glycine on the {FeNO}⁶ complex. The resulting diazo compound, O═N-NH-CH₂-COOH ⇄ HO-N═N-CH₂-COOH, was readily hydrolyzed to generate N₂ along with a formation of α-hydroxyacid (HO-CH₂-COOH). The reaction mechanism was evidenced by isotope-labeling experiments using ¹⁵NO and ¹⁵N-glycine, quantitative NMR detection of α-hydroxyacid, and theoretical calculation by DFT. The present study will provide the possibility of N-N bond formation promoted by the nucleophilic attack of amines to {FeNO}⁶ on the native heme iron.