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 hexafluorophosphate as supporting electrolyte. The investigated compounds are represented as (OEC)Fe(IV)(C(6)H(5)), (OEC)Fe(IV)Cl, 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)(C(6)H(5)) and (OEC)Fe(IV)Cl 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)(C(6)H(5))](-) is low spin (S = (1)/(2)) as compared to electrogenerated [(OEC)Fe(III)Cl](-), 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)(C(6)H(5))](*)(+) 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&onemacr; with a = 10.783(3) Å, b = 13.826(3) Å, c = 14.151(3) Å, alpha = 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(o)(2) 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.
Caemelbecke
Eric Van
EV
Department of Chemistry, University of Houston, Houston, Texas 77204-5641, Institut für Organische Chemie, Universität zu Köln, Greinstrasse 4, D-50939 Köln, Germany, and Département de Chimie, URA CNRS 405, Université Louis Pasteur, 67000 Strasbourg, France.
Will
Stefan
S
Autret
Marie
M
Adamian
Victor A.
VA
Lex
Johann
J
Gisselbrecht
Jean-Paul
JP
Gross
Maurice
M
Vogel
Emanuel
E
Kadish
Karl M.
KM