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

TL;DR

A new method converts nitric oxide to nitrogen gas using iron porphyrin in an acidic glycine solution.

Contribution

A novel N–N bond formation mechanism via glycine's nucleophilic attack on iron(III) nitrosyl complexes is revealed.

Findings

N2 is efficiently generated from NO in acidic glycine buffer with iron(III) porphyrin complexes.

Glycine nucleophilic attack forms a diazo compound that hydrolyzes to release N2.

Isotope labeling and DFT calculations confirm the proposed reaction mechanism.

Abstract

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.

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