Structural and electronic characterization of nitrosyl(octaethylporphinato)iron(III) perchlorate derivatives

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)degrees) w ith a small off-axis tilt of the Fe-N(NO) vector from the heme normal (angl e = 4.6 degrees); the Fe-N(NO) distance is 1.6528(13) Angstrom and the iron is displaced 0.32 Angstrom out-of-plane. The complex forms a tight cofacia l pi-pi dimer in the solid state. Mossbauer spectra for this derivative as well as for a related crystalline form are measured both in zero applied ma gnetic field and in a 7 T applied field. Fits to the measurements made in a pplied magnetic field demonstrate that both crystalline forms of [Fe(OEP)(N O)]ClO4 have a diamagnetic ground slate at 4.2 K. The observed isomer shift s (delta = 0.22-0.24 mm/s) are smaller than those typically observed for lo w-spin iron(III) porphyrinates. Analogous Mossbauer measurements are also o btained for a six-coordinate derivative, [Fe-(OEP)(Iz)(NO)]ClO4 (Iz = indaz ole). The observed isomer shift for this species is smaller still (delta = 0.02 mm/s). All derivatives show a strong temperature dependence of the iso mer shift. The data emphasize the strongly covalent nature of the FeNO grou p. The Mossbauer isomer shifts suggest formal oxidation states greater than +3 for iron, but the NO stretching frequencies are not consistent with suc h a large charge transfer to NO. Differences in the observed nitrosyl stret ching frequencies of the two crystalline forms of [Fe(OEP)(NO)]ClO4 are dis cussed.