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

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Abstract
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 Fo2 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.

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