Structural and electronic distortions of the iron nitrosyl group in iron porphyrinates

The focus of this work is the study of nitric oxide (NO) with iron(II) and iron(III) porphyrin systems. The (nitrosyl)iron porphyrinates were synthesized under carefully controlled oxygen- and water-free conditions. The newly synthesized species were characterized by UV-visible, infrared, and Mössbauer spectroscopies as well as by X-ray diffraction. High quality structures of two formally iron(II) nitrosyl porphyrinates, [Fe(OEP)(NO)], (OEP = octaethylporphyrin) reveal an unusual tilt of the FeNO group and a correlated asymmetry pattern in the iron-porphyrin bond lengths. This structural distortion of the iron coordination group geometry is supported only by bonding effects. A minor structural distortion, seen in the formally iron(III) nitrosyl porphyrinate, [Fe(OEP)(NO)]ClO₄, is used to explain the 30 wavenumber difference in the infrared nitrosyl stretching frequency for two chemically equivalent five-coordinate complexes. The addition of a nitrite ligand to give the six-coordinate, formally iron(III), nitro(nitrosyl) porphyrinates, [Fe(Porphyrin)-(NO₂)(NO)], strongly red-shifts the Soret band in the UV-visible spectrum and increases the infrared nitrosyl stretching frequency compared to the five-coordinate precursor. Structural data reveal only slight increases in iron-nitrogen bond lengths from coordination of nitrite. The characterization of several [Fe(OEP)(L)(NO)]+ species (L = 1-methylimidazole, indazole, pyrazole, pyrazine, 2-methylpyrazine, 4-cyanopyridine) shows that the effect of the added neutral nitrogen-donor ligand is to increase further the nitrosyl stretching frequency. Mössbauer data confirm a diamagnetic ground state for [Fe(OEP)(Iz)(NO)]ClO₄ (Iz = indazole). The very small isomer shift parameter and the large quadrupole splitting parameter indicate increased π-donation from iron to NO. Structural data for all of the [Fe(OEP)(L)-(NO)]+ complexes show that the Fe-N-O group is essentially linear and that neither the Fe-N(NO) nor the Fe-N(L) bond lengths are affected by coordinating a trans ligand. In contrast, the presence of the strongly σ-donating phenyl ligand trans to NO in [Fe(OEP)(C₆H₄-p-F)(NO)], significantly affects the FeNO coordination geometry. [Fe(OEP)(C₆H₄- p-F)(NO)] has a bent Fe-N-O group, a long Fe-N(NO) bond length, a reduced nitrosyl stretching frequency, and a very small quadrupole splitting constant.