Electrochemical and spectral characterization of iron corroles in high and low oxidation states: First structural characterization of an iron(IV) tetrapyrrole pi 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 hexafluorophospfiate as supporting electrolyte. The investigated compounds are represented as (OEC)Fe-IV(C6H5), (OEC)(FeCl)-Cl-IV, and (OEC)Fe-III(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)Fe-IV(C6H5) and (OEC)(FeCl)-Cl-IV to their iron(III) forms upon a one-electron reduction and to iron(IV) corrole pi cation radicals upon a one-electron oxidation. The metal center in [(OEC)Fe-III(C6H5)](-) is low spin (S = 1/2) as compared to electrogenerated [(OEC)(FeCl)-Cl-III]-, which contains an intermediate-spin (S = 3/2) iron(III). (OEC)Fe-III(py) also contains an intermediate-spin-state iron(III) and, unlike previously characterized (OEC)Fe-III(NO), is converted to an iron(IV) corrole upon oxidation rather than to an iron(III) pi cation radical. Singly oxidized [(OEC)Fe-IV(C6H5)](.+) is the first iron(IV) tetrapyrrole pi cation radical to be isolated and was structurally characterized as a perchlorate salt. It crystallizes in the triclinic space group P (1) over bar with a 10.783(3) Angstrom Angstrom, b = 13.826(3) Angstrom, c = 14.151(3) Angstrom, a = 78.95(2)degrees, beta = 89.59(2)degrees, and gamma = 72.98(2)degrees at 293 K with Z = 2. Refinement of 8400 reflections and 670 parameters against F-0(2) yields R1 = 0.0854 and wR2 = 0.2293. The complex contains a five-coordinated iron with average Fe-N bond lengths of 1.871(3) Angstrom. The formulation of the electron distribution in this compound was confirmed by Mossbauer, 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.