Redox and Spectroscopic Properties of Iron Porphyrin Nitroxyl in the Presence of Weak Acids

Md Hafizur Rahman; Michael D Ryan

doi:10.1021/acs.inorgchem.6b02665

Redox and Spectroscopic Properties of Iron Porphyrin Nitroxyl in the Presence of Weak Acids

Inorg Chem. 2017 Mar 20;56(6):3302-3309. doi: 10.1021/acs.inorgchem.6b02665. Epub 2017 Mar 2.

Authors

Md Hafizur Rahman¹, Michael D Ryan¹

Affiliation

¹ Marquette University Chemistry Department, PO Box 1881, Milwaukee, Wisconsin 53201, United States.

PMID: 28252947
DOI: 10.1021/acs.inorgchem.6b02665

Abstract

The spectroelectrochemistry and voltammetry of Fe(OEP) (NO) in the presence of substituted phenols was studied. Cyclic voltammetry showed that two closely spaced waves were observed for the reduction of Fe(OEP) (NO) in the presence of substituted phenols. The first wave was a single electron reduction under voltammetric conditions. The second wave was kinetically controlled, multielectron process. Visible spectroelectrochemistry of Fe(OEP) (NO) in the presence of substituted phenols showed that three species were present during the electrolysis. Additional spectroscopic studies indicated that the two reduction species were Fe(OEP) (HNO) and Fe(OEP)(H₂NOH). The Fe(OEP) (HNO) species, which can be generated chemically, was stable over a period of hours. Additional acid did not lead to further protonation. Proton NMR spectroscopy confirmed the Fe(OEP) (HNO) species could be deprotonated under basic conditions. The third species, Fe(OEP)(H₂NOH), was generated by the further reduction of the chemically generated Fe(OEP) (HNO) complex. Both the Fe(OEP) (HNO) and Fe(OEP)(H₂NOH) complexes could be slowly oxidized back to Fe(OEP) (NO). At millimolar concentrations of Fe(OEP) (HNO), there was no evidence for the disproportionation of Fe(OEP) (HNO) to Fe(OEP) (NO) and H₂ in the presence of substituted phenols. Nor was there evidence for the generation of N₂O. The FTIR spectroelectrochemistry showed changes in the infrared spectra in the presence of substituted phenols, but no isotopic sensitive bands were observed for the reduced species between 1450 and 1200 cm^-1. This may be because the ν_NO band shifted into a region (1500-1450 cm^-1) where it would be difficult to observe.