Structural and Electronic Characterization of Nitrosyl(Octaethylporphinato)iron(III) Perchlorate Derivatives

• Published in: Inorganic chemistry 2000-10, Vol.39 (22), p.5102-5110
• Main Authors: Ellison, Mary K, Schulz, Charles E, Scheidt, W. Robert
• Format: Article
• Language: English
• Subjects: Crystallography, X-Ray; Ferric Compounds - chemical synthesis; Ferric Compounds - pharmacology; Indicators and Reagents; Metalloporphyrins - chemical synthesis; Metalloporphyrins - pharmacology; Nitric Oxide - chemistry; Spectrophotometry, Infrared; Spectroscopy, Mossbauer
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic000789e

The synthesis and crystallographic characterization of the five-coordinate iron(III) porphyrinate complex [Fe(OEP)(NO)]ClO4 are reported. This {FeNO}6 complex has a nearly linear Fe−N−O group (angle = 173.19(13)°) with a small off-axis tilt of the Fe−N(NO) vector from the heme normal (angle = 4.6°); the Fe−N(NO) distance is 1.6528(13) Å and the iron is displaced 0.32 Å out-of-plane. The complex forms a tight cofacial π−π dimer in the solid state. Mössbauer spectra for this derivative as well as for a related crystalline form are measured both in zero applied magnetic field and in a 7 T applied field. Fits to the measurements made in applied magnetic field demonstrate that both crystalline forms of [Fe(OEP)(NO)]ClO4 have a diamagnetic ground state at 4.2 K. The observed isomer shifts (δ = 0.22−0.24 mm/s) are smaller than those typically observed for low-spin iron(III) porphyrinates. Analogous Mössbauer measurements are also obtained for a six-coordinate derivative, [Fe(OEP)(Iz)(NO)]ClO4 (Iz = indazole). The observed isomer shift for this species is smaller still (δ = 0.02 mm/s). All derivatives show a strong temperature dependence of the isomer shift. The data emphasize the strongly covalent nature of the FeNO group. The Mössbauer isomer shifts suggest formal oxidation states greater than +3 for iron, but the NO stretching frequencies are not consistent with such a large charge transfer to NO. Differences in the observed nitrosyl stretching frequencies of the two crystalline forms of [Fe(OEP)(NO)]ClO4 are discussed.