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.