Synthesis, Molecular Structures, and Properties of Six-Coordinate [Fe(OEP)(L)(NO)]⁺ Derivatives: Elusive Nitrosyl Ferric Porphyrins

The preparation and characterization of several {FeNO}6 porphyrinates ([Fe(OEP)(L)(NO)]+), where OEP is octaethylporphyrin and L is one of the neutral nitrogen-donating axial ligands 1-methylimidazole (1-MeIm), pyrazole (Pz), indazole (Iz), or pyrazine (Prz), are described. The synthesis and crystallization of these derivatives requires an NO atmosphere and careful attention to conditions. The characterization of these [Fe(OEP)(L)(NO)]+ complexes by IR and UV−vis spectroscopies provides for general distinctions between five- and six-coordination and the two common oxidation states of iron nitrosyl porphyrins. The UV−vis spectra of the [Fe(OEP)(L)(NO)]+ derivatives are all characterized by very distinct α and β bands. The solid-state NO stretching frequencies are found in the narrow range of 1890 to 1921 cm-1. Six X-ray structure determinations have been completed. The molecular structures of [Fe(OEP)(1-MeIm)(NO)]ClO4, [Fe(OEP)(Pz)(NO)]ClO4(1), [Fe(OEP)(Iz)(NO)]ClO4, and {[Fe(OEP)(NO)]2Prz}(ClO4)2 are described in detail. All structural features are consistent with the assignment of a low-spin state to the central iron. The Fe−Np bond lengths range from 1.995(8) to 2.004(5) Å. The axial Fe−NNO bond lengths are quite short (range 1.627(2)−1.6465(17) Å). The Fe−N−O groups are essentially linear (Fe−N−O angle range is 176.6(3)−177.6(3)°) which is that expected for these {FeNO}6 species. The Fe−L bond lengths trans to the nitrosyl are normal (range 1.988(2)−2.039(2) Å); there is no nitrosyl structural trans effect. The iron is displaced only slightly out of the plane of the 24-atom core toward the nitrosyl, and with the short Fe−NNO bond lengths steric interactions cause modest porphyrin core distortions.