Spin-coupling in ferric metalloporphyrin radical cation complexes: Mössbauer and susceptibility studies

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Abstract
The ferric metalloporphyrin π‐radical cation complexes Fe(III) (OClO3)2 (TPP.) and [Fe(III) Cl (TPP.)] [SbCl6] were examined in microcrystalline form by Mössbauer spectroscopy and magnetic susceptometry over a range of temperatures and applied fields. All measurements on the six‐coordinate Fe(OClO3)2 (TPP.) were consistent with isolated molecules having an S=5/2 iron site with zero field splitting (12 cm−1) S2z that is ferromagnetically coupled to the S=1/2 porphyrin radical by an energy term (−110 cm−1) S⋅s. Thus the ground state is overall spin‐3. In the five‐coordinate [FeCl (TPP.)] [SbCl6] the susceptibility is in reasonable agreement with the results of a calculation based on zero field splitting (12 cm−1) S2z for the S=5/2 iron and antiferromagnetic coupling (200 cm−1) S⋅s with the radical to give an overall spin‐2 ground state. However, the Mössbauer measurements require a more complicated model having the same large intramolecular iron–radical coupling, a smaller zero field splitting (3 cm−1) S2z, and weak intermolecular antiferromagnetic coupling between heme pairs given by (32 cm−1) s1⋅s2 or, equivalently, (0.65 cm−1) S1⋅S2. A slightly improved correspondence with the measured susceptibility results. The intermolecular antiferromagnetic coupling probably results from crystallization of the [FeCl (TPP.)]+ cations in face‐to‐face dimers as observed in other closely related five‐coordinate iron (III) porphyrins.