Preparation and Electrochemical Behaviors of Phenoxide-coordinated Iron(III) Porphyrins

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Abstract
A new series of iron(III) porphyrins having various phenoxide anions as the axial ligand, [FeIII(porphyrin)(4-X–C6H4O−)] (porphyrin=octaethylporphyrin and tetraphenylporphyrin; X=OCH3, CH3, H, C6H5, Cl, Br, COCH3, CN, CHO, and NO2) were synthesized. All these complexes are five-coordinated high-spin complexes (S=5⁄2). Their electrochemical behaviors were investigated by the cyclic voltammetry and the controlled potential coulometry. The complexes showed significantly negative potentials for the FeII/FeIII redox couple (−0.4–−0.8 V). A good linear relationship was found between the half-wave potentials of the FeII/FeIII redox couple and the Hammett σ-values of the substituent X. A new characteristic feature is the appearance of the redox peaks of the coordinated phenoxide anions near the redox couple of the porphyrin ring oxidoreduction. The role of the tyrosine-67 in the enzymatic mechanism for cytochrome c is discussed.
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