Evidence of the Participation of Phenolic Groups on Hydrogen-Atom Transfer Reactions during the Electrochemical Oxidation of Meso-(p-hydroxyphenyl)Porphyrin
Authors: Hernández-Muñoz Lindsay Sidney , Aguilar Martínez Martha , Macías Ruvalcaba Norma Angélica 
Published: February 2020

The electron transfer reactions of porphyrins have been widely studied due to their relevance in biological systems [1-2]. In general, the oxidation of free-base porphyrins proceeds by two ring-centered one-electron transfers, leading to the the radical cation and dication respectively [3-4]. Depending on the electrolyte-solvent system and the type and number of substituents on the porphyrin macrocycle, the radical cation can follow different pathways. While in THF the radical cation of free-base tetraphenylporphyrin leads to diprotonated porphyrin species [5], in benzonitrile, isoporphyrin species follow the formation of the radical cation.
It is known that the electrochemical behavior of many organic compounds is modified when the molecule contains in its structure functional groups capable to participate in hydrogen bonding interactions or hydrogen exchange reactions. However, there are not reports in the literature focused to understand how this kind of substituents can affect the fate of the radical cation of porphyrins. To deep on the understanding of the electrochemical oxidation mechanisms of porphyrins, in this work we carried out an electrochemical study of a porphyrin substituted with phenolic groups; the 5,10,15,20-tetrakis-(4-hydroxyphenyl)porphyrin (1). In benzonitrile solution, the oxidation of 1 occurs in two consecutive quasi-reversible processes at E1/2 (Ia/Ic) = 0.32 V and EpIIa = 0.364 V vs Fc+/Fc at 0.1 Vs-1. The first electron transfer presents a linear variation of the peak potential with the scan rate of 27 mV dec-1, consistent with the presence of a coupled chemical reaction. The number of electron transferred was determined by coulometry. The electrolysis was carried-out at a constant potential of 0.388 V vs Fc+/Fc; after the experiment, the two original oxidation processes Ia/Ic and IIa/IIc disappeared, and a new reversible process I'a/I'c was observed at higher oxidation potential, E1/2 (I'a / I'c) = 0.588 V vs Fc+/Fc, Figure 1A. Also, on a cathodic sweep, two new reversible reduction processes at less negative potentials than those of 1 were observed (Figure 1B). This behavior suggests that the final product of the electrooxidation is the diprotonated form of porphyrin 1. This assumption was supported by the fact that the voltammetric behavior of benzonitrile solutions of porphyrin 1 in presence of two equivalents of perchloric acid parallels the behavior of the electrolyzed solution. The UV-visible spectrum of the electrolyzed solution also confirms the formation of diprotonated porphyrin 1. From these results it can be stated that the phenolic –OH groups on the porphyrin 1 act as hydrogen atom donors to the electrogenerated radical cation provoking its reduction, similar to the mechanism of phenolic antioxidants which tend to transfer hydrogen atoms to free radicals.

Figure 1: A) Cyclic voltammograms of 0.25 mM of 1 before (Ia-Ic) and (IIa-IIc) and after electrolysis (I´a-I´c) at Eapp = 0.388 V vs Fc+/Fc. B) 0.25 mM of 1 with increasing concentrations of HClO4 in benzonitrile + 0.1 M n-Bu4ClO4 at 0.1 Vs-1 on a glassy carbon electrode (3 mm φ).

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Figure 1