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[Fe(OEP)(X)]+ π-Cation Radicals:  Characterization and Spin−Spin Interactions

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posted on 29.01.1997 by Charles E. Schulz, Hungsun Song, Anil Mislankar, Robert D. Orosz, Christopher A. Reed, Peter G. Debrunner, W. Robert Scheidt
The preparation and characterization of the π-cation radical derivatives of [Fe(OEP)(X)] (X = Cl-, Br-) is reported. Three different derivatives have been prepared:  [Fe(OEP)(Cl)]ClO4, [Fe(OEP)(Cl)][SbCl6], and [Fe(OEP)(Br)][SbCl6]. All derivatives have been characterized by UV−vis, IR, and Mössbauer spectroscopy. In addition, [Fe(OEP)(Cl)]ClO4 has been characterized by a single-crystal structure determination, and [Fe(OEP)(Cl)][SbCl6] and [Fe(OEP)(Br)][SbCl6] have been studied by temperature-dependent magnetic susceptibility measurements and Mössbauer measurements in an applied magnetic field. The X-ray structure of [Fe(OEP)(Cl)]ClO4 reveals a five-coordinate porphyrinate species that forms tight cofacial π−π dimers in which the two porphyrin rings are almost exactly overlapped with an inter-ring separation of 3.24 Å, a lateral shift of 0.2 Å, and a twist angle between the two rings of 31.2°; the two iron atoms are 4.112 Å apart. Crystal data:  C37H46FeCl4O4N4, a = 27.454(7) Å, b = 15.322(3) Å, c = 19.802(3) Å, β = 116.14 (2)°, monoclinic, C2/c, Z = 8. Iron(III) is found to be in the high-spin state in all derivatives. The magnetic data (susceptibility and Mössbauer) have been interpreted in terms of two spin coupling models. Both models give a picture of strong coupling between the various spins in the dimeric species. In the model judged to best fit all data with a physically meaningful zero-field splitting, there are three terms in the total Hamiltonian:  an axial zero-field splitting parameter D for the high-spin iron, an intramolecular antiferromagnetic coupling −2JFe-r(S⃗·s⃗) between the iron spin S = 5/2 and the π-cation radical s = 1/2 spin, and an intermolecular antiferromagnetic coupling −2JR-R(S⃗ + s⃗)·(S⃗‘ + s⃗‘) between the total spins on each half of the dimer.