Molecular structure of the chloroiron(III) derivative of the meso-unsubstituted 2,7,12,17-tetramethyl-3,8,13,18-tetramesitylporphyrin and weak ferromagnetic exchange interactions in the A_1u oxoiron(IV) porphyrin π radical cation complex

The molecular structure of the chloroiron(III) porphyrinate [Fe ^IIICl(tmtmp)] (1) is described. The doming of the bifacially encumbered tmtmp dianionic ligand present in 1 is characterized by a separation of 0.09(1) Å between the mean plane of the four pyrrole nitrogens (4N _p) and that of the porphyrin core. The coordination polyhedron of the five-coordinate iron atom is a square pyramid of C_4v symmetry. The metal is displaced by 0.44(1) Å from the 4N_p mean plane towards the axial chloro ligand. The Fe-N_p and Fe-Cl bond lengths are 2.057(6) and 2.223 (5) Å, respectively. Upon treatment of [Fe ^IIITf(tmtmp)] (2) with m-chloroperoxybenzoic acid, the green oxoferryl π radical cation complex [Fe^IV=O(tmtmp)]⁺ (3) is obtained. It has been studied by EPR, Mössbauer, and resonance Raman spectroscopy. It exhibits an EPR spectrum strikingly similar to those of compounds I of Micrococcus lysodeikticus catalase (MLC-I) and ascorbate peroxidase (APX-I). The exchange interactions between the spins of the radical cation and the ferryl iron are of the ferromagnetic type and are the weakest ever found for a synthetic compound I model. The shift of + 22 cm^-1 in the radical marker band ν₂ in the resonance Raman spectra upon oxidation of 2 to 3 confirms that the electronic state of the porphyrin π radical cation is predominantly ²A_1u in this tmtmp compound I model.