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Spectral, Electrochemical, and ESR Characterization of Manganese Tetraarylporphyrins Containing Four β,β′-Pyrrole Fused Butano and Benzo Groups in Nonaqueous Media

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journal contribution
posted on 05.02.2019 by Yuanyuan Fang, Liping Wang, Weijie Xu, Zhongping Ou, Mingyuan Chen, Lei Cong, Wenqian Shan, Xiangyi Ke, Karl M. Kadish
Two series of β,β′-pyrrole butano- and benzo-substituted mangenese­(III) tetraarylporphyrins were synthesized and characterized with regard to their spectral and electrochemical properties. The investigated compounds have the general formula butano­(Ar)4PorMnCl and benzo­(Ar)4PorMnCl, where Por is the dianion of the porphyrin and Ar is a p-CH3Ph, Ph or p-ClPh group on each of the four meso-positions of the macrocycle. Each manganese­(III) butano- or benzoporphyrin was examined in CH2Cl2 and/or pyridine containing 0.1 M tetra-n-butylammonium perchlorate and the data then were compared to that of the parent tetraarylporphyrins having the same meso-substituents. Up to four reductions are observed for each compound, the first being metal-centered to generate a Mn­(II) porphyrin, and the second and third being porphyrin ring-centered to give a Mn­(II) porphyrin π-anion radical and dianion, respectively. The one-electron reduced manganese porphyrins have an ESR spectrum with signals at g= 5.6–5.8 and g// = 2.0, indicating a mixture of the four- and five-coordinated Mn­(II) complexes in a high-spin state (3d5, S = 5/2, I = 5/2). Data from cyclic voltammetry and spectroelectrochemistry both suggest that formation of the porphyrin dianion is followed by a chemical reaction at the electrode surface to give an electroactive phlorin anion. The effects of solvent and porphyrin substituents on ultraviolet–visible light (UV-vis) spectra, redox potentials, and electron transfer mechanisms are discussed.