Stefan Will

Electrochemical and Spectral Characterization of Iron Corroles in High and Low Oxidation States: First Structural Characterization of an Iron(IV) Tetrapyrrole π Cation Radical

The electrochemistry and spectroscopic properties of three iron corroles were examined in benzonitrile, dichloromethane, and pyridine containing 0.1 M tetra-n-butylammonium perchlorate or tetra-n-ethylammonium hexafluorophosphate as supporting electrolyte. The investigated compounds are represented as (OEC)FeIV(C6H5), (OEC)FeIVCl, and (OEC)FeIII(py), where OEC is the trianion of 2,3,7,8,12,13,17,18-octaethylcorrole. Each iron(IV) corrole undergoes two one-electron reductions and two or three one-electron oxidations depending upon the solvent. Under the same solution conditions, the iron(III) corrole undergoes a single one-electron reduction and one or two one-electron oxidations. Each singly oxidized and singly reduced product was characterized by UV-vis and/or EPR spectroscopy. The data indicate a conversion of (OEC)FeIV(C6H5) and (OEC)FeIVCl to their iron(III) forms upon a one-electron reduction and to iron(IV) corrole π cation radicals upon a one-electron oxidation. The metal center in [(OEC)FeIII(C6H5)]- is low spin (S = 1/2) as compared to electrogenerated [(OEC)FeIIICl]-, which contains an intermediate-spin (S = 3/2) iron(III). (OEC)FeIII(py) also contains an intermediate-spin-state iron(III) and, unlike previously characterized (OEC)FeIII(NO), is converted to an iron(IV) corrole upon oxidation rather than to an iron(III) π cation radical. Singly oxidized [(OEC)FeIV(C6H5)]•+ is the first iron(IV) tetrapyrrole π cation radical to be isolated and was structurally characterized as a perchlorate salt. It crystallizes in the triclinic space group P1̄ with a = 10.783(3) Å, b = 13.826(3) Å, c = 14.151(3) Å, α = 78.95(2)°, β = 89.59(2)°, and γ = 72.98(2)° at 293 K with Z = 2. Refinement of 8400 reflections and 670 parameters against F o2 yields R1 = 0.0864 and wR2 = 0.2293. The complex contains a five-coordinated iron with average Fe-N bond lengths of 1.871(3) Å. The formulation of the electron distribution in this compound was confirmed by Mössbauer, X-ray crystallographic, and magnetic susceptibility data as well as by EPR spectroscopy, which gives evidence for strong antiferromagnetic coupling between the iron(IV) center and the singly oxidized corrole macrocycle.

Manganese(III) and manganese(IV) corroles: synthesis, spectroscopic, electrochemical and X-ray structural characterization

The synthesis, spectroscopic characterization and electrochemistry of four Mn(III) and Mn(IV) octaethylcorroles are reported and the potentials of the Mn(III)/Mn(IV) and Mn(IV)/Mn(III) processes examined as a function of the axial ligand. The investigated compounds are represented as (OEC)Mn, (OEC)MnCl, (OEC)Mn(py) and (OEC)Mn(C6H5) where OEC is the trianion of octaethylcorrole. The first one-electron oxidation of (OEC)MnIII and (OEC)MnIII(py) in PhCN or pyridine containing 0.1 M TBAP leads to the facile formation of a Mn(IV) species while the first one-electron reduction of (OEC)MnIVCl and (OEC)MnIV(C6H5) in the same two solvents leads to the Mn(III) corrole. All other redox reactions occur at the corrole macrocycle to give π-cation radicals or π-anion radicals and there is no evidence for electrogeneration of a compound with a Mn(II) oxidation state as is the case for manganese(III) porphyrins which are all easily reduced to the Mn(II) state in nonaqueous media. The products of each Mn(III)/Mn(IV) redox reaction were characterized by UV-visible and/or ESR spectroscopy and the structures of (OEC)MnCl, (OEC)Mn(py) and (OEC)Mn(C6H5) were determined by single-crystal X-ray diffraction.