MOLECULAR-STRUCTURE OF THE CHLOROIRON(III) DERIVATIVE OF THE MESO-UNSUBSTITUTED ,12,17-TETRAMETHYL-3,8,13,18-TETRAMESITYLPORPHYRIN AND WEAKFERROMAGNETIC 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 encumbere d tmtmp dianionic ligand present in 1 is characterized by a separation of 0.09 (1) Angstrom between the mean plane of the four pyrrole nitro gens (4N(p)) and that of the porphyrin core. The coordination polyhedr on of the five-coordinate iron atom is a square pyramid of C-4v symmet ry. The metal is displaced by 0.44 (1) Angstrom from the 4N(p) mean pl ane 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 o f [Fe(III)Tf(tmtmp)] (2) with m-chloroperoxybenzoic acid, the green ox oferryl pi radical cation complex [Fe-IV = O(tmtmp(.))](+) (3) is obta ined. It has been studied by EPR, Mossbauer, and resonance Raman spect roscopy. It exhibits an EPR spectrum strikingly similar to those of co mpounds 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 o f the porphyrin pi radical cation is predominantly (2)A(1u) this tmtmp compound I model.