X‑ray Structure and Properties of the Ferrous Octaethylporphyrin Nitroxyl Complex

The preparation and characterization
of the iron octaethylporphyrin nitroxyl ion, [Fe­(OEP)­(NO)−], is reported. The complex was synthesized by the one-electron reduction
of Fe­(OEP)­(NO) using anthracenide as the reducing agent. The compound
was isolated as the potassium (2.2.2)­cryptand salt. The anion was
characterized using X-ray analysis with visible and infrared spectroscopy.
The spectral features of the iron nitroxyl complex were consistent
with previous literature reports. The important structural changes
upon reduction were a significant decrease in the Fe–N–O
bond angle from 142° to 127° and an increase in the N–O
bond length from that in the starting nitrosyl moiety. The porphyrin
ring became significantly less planar upon reduction, but the displacement
of the iron atom from the 24-atom plane was essentially unchanged.
In spite of the attempt to encapsulate the potassium ion with the
(2.2.2)­cryptand, significant interaction between K+ and
the oxygen of the nitroxyl were observed, indicating a contact ion
pair in the crystal structure. Comparison between the experimental
structure and the DFT-calculated parameters were reported. The results
are consistent with the Fe–N–O moiety being the site
of the reduction, with little evidence for the reduction of the iron
itself or the porphyrin ring. The proton NMR spectrum was also obtained,
and the chemical shifts were significantly different from other S = 0 metalloporphyrin complexes. These shifts, though,
were consistent with the DFT calculations.