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Structural and Magnetic Effects of meso-Substitution in Alkyl-Substituted Metalloporphyrinate π-Cation Radicals: Characterization of [Fe(TalkylP)(Cl)]SbCl6 (alkyl = ethyl and n-propyl)

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posted on 06.09.2010, 00:00 by Ming Li, Teresa J. Neal, Graeme R. A. Wyllie, Charles E. Schulz, W. Robert Scheidt
We report the preparation and characterization of two meso-alkyl substituted porphyrin π-cation radical derivatives, [Fe(TalkylP)(Cl)]SbCl6 (alkyl = ethyl or propyl). Both complexes have been characterized by UV/vis/near-IR, IR, and Mössbauer spectroscopy, temperature-dependent solid-state magnetic susceptibility measurements, and X-ray structure determinations. All data for both oxidized species are consistent with the formulation of the complexes as ring-oxidized iron(III) porphyrin species. The molecular structures of the two five-coordinate species have the typical square-pyramidal coordination group of high-spin iron(III) derivatives. The crystal structures also reveal that the species form cofacial π−π dimers with lateral shifts of 1.44 Å and 3.22 Å, respectively, for the propyl and ethyl radical derivatives. Both radicals exhibit porphyrin cores with alternating bond distance patterns in the inner 16-membered ring. In addition, [Fe(TEtP)(Cl)]SbCl6 and [Fe(TPrP)(Cl)]SbCl6 have been characterized by temperature-dependent (6−300 K) magnetic susceptibility studies, the best fitting of the temperature-dependent moments reveal strong coupling between iron spins and porphyrin radical, and a smaller magnitude of antiferromagnetic coupling between ring radicals, which are opposite to those found in the five-coordinate iron(III) OEP radicals. The differences in structure and properties of the cation radical meso-alkyl and β-alkyl derivatives possibly reflect differences in properties of a1u- and a2u-forming radicals.