Synthesis and Characterization of a MnIII Porphyrin Cation Radical and Its Conversion to MnIV by Ligand Metathesis

Article abstract of DOI:10.1021/ja00268a037

The complex (5,10,15,20-tetraphenylporphinato)manganese chloro hexachloroantimonate, 1, MnTPP(Cl)(SbCl6), was synthesized by the oxidation of MnTPP(Cl) with phenoxathiin hexachloroantimonte in dichloromethane.The assignment of 1 as a manganese(III) porphyrin ?-cation radical species was based on its X-ray crystal structure, visible and infrared spectra, and magnetic properties.The crystal structure was determined as a tetrachloroethane solvate.The manganese in the compound is five-coordinated with the chloride anion axially bound to the Mn with 50percent occupancy at sites above and below the porphyrin ring.The second anion, SbCl6(1-), is displaced from the Mn with a Mn-Cl-Sb angle of 132 deg.The porphyrin core carbon and nitrogen atoms as well as Mn are coplanar.Variable-temperature magnetic susceptibility measurements on the solid gave μ_eff = 4.9 μ_B which is close to that expected for a Mn^III cation (d⁴) S = 2, ?-cation radical S = 1/2, with independent spin states.Magnetic susceptibility measurements on solutions of 1 gave a range of values for μ_eff between 5.7 and 6.7 μ_B.This suggests that the spin independent state of 1 in the solid changes into a ferromagnetically coupled state, resultant spin of five-halves, in the solution phase.The IR spectrum of 1, with four solvent molecules (C2H2Cl4) present, shows a peak at 1280 cm^-1.A band in this region has been suggested as diagnostic of metallotetraphenylporphyrin ?-cation radical species.The 1280-cm^-1 band observed for 1 disappeared when all the solvent molecules from 1 were removed under vacuum.The crystalline nature of the solid was lost by the removal of the solvent molecules; however, magnetic moment (4.7 μ_B) and the visible spectrum remained virtually unchanged.We believe that the 1280-cm^-1 band in 1 is associated with the solvent molecules; neat C2H2Cl4 also shows strong absorption in the 1280-cm^-1 region.Treatment of 1 with basic methanol or oxidation of MnTPP(Cl) with phenoxathiin hexachloroantimonate in the presence of basic methanol gave another species whose visible and EPR spectra are identical with that reported for Mn^IVTPP(OCH3)2.These results suggest that the site of oxidation in MnTPP(Cl) is shifted from the ligand to the metal in the presence of the strong ?-donating ligand, CH3O(1-).