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 pi radical cation complex

The molecular structure of the chloroiron(III) porphyrinate [(FeCl)-Cl-III-(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) Angstrom 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) Angstrom 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) Angstrom , respectively, Upon treatment of [Fe(III)Tf(tmtmp)] (2) with m-chloroperoxybenzoic acid, the green oxoferryl pi radical cation complex [Fe-IV = O(tmtmp(.))](+) (3) is obtained. It has been studied by EPR, Mossbauer, 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 v(2) in the resonance Raman spectra upon oxidation of 2 to 3 confirms that the electronic state of the porphyrin pi radical cation is predominantly (2)A(1u) this tmtmp compound I model.