Structural, Electrochemical, and Spectroscopic Properties of a Class of Dodecasubstituted Iron Porphyrins Bearing Four Positive Charges Close to the Metal

Abstract

AbstractThe first iron complexes of the tetracationic 2,3,7,8,12,13,17,18‐octaethyl‐5,10,15,20‐tetra‐N‐pyridiniumylporphyrin, (OEPy4P)4+, in which four pyridine molecules are attached to the meso‐carbon atoms through their nitrogen atoms, were synthesized in three steps starting from Zn(OEP) (Zn‐β‐octaethylporphyrin) with an overall yield of ca. 30 %. The X‐ray structure of [FeII(OEPy4P)(pyridine)2](CF3SO3)3(Br) established that the molecule adopts a severely distorted nonplanar saddle conformation. 1H NMR spectroscopy showed that the [FeII(OEPy4P)(B)2]4+ ligated complexes, B = pyridine (Py) or imidazole (Im), are low‐spin, FeII, S = 0. These complexes exhibit redox potentials for the FeIII/FeII couple that range between 0.62 and 0.38 V (vs. Ag/AgCl) and are shifted by as much as +0.7 V relative to those of the corresponding planar iron complexes of OEP and of the nonplanar complexes of OETPP. Such highly positive redox potentials explain the stability of these iron(II) porphyrins in air. As previously found for dodecasubstituted iron porphyrins bearing electron‐withdrawing groups, the UV/Vis spectra of the[FeII(OEPy4P)(B)2]4+ complexes exhibit Soret and visible bands which are redshifted by ca. 40 nm relative to those of Fe(OEP)(Py)2. These metallo‐OEPy4P4+complexes offer promising synthetic avenues to novel classes of highly charged porphyrins and multiporphyrin arrays, which are soluble in aqueous and polar organic solvents.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)