Aryl-Substituted Metalloporphyrins and Metallocorroles: Electrochemistry and Molecular Oxygen Electroreduction
Authors: Berezina Nadezhda M. , Maksimova Aleksandra A. , Berezin Dmitry B. , Bazanov Mikhail I. 
Published: February 2020

Among of macrocyclic complexes of transition metals N4-macrocyclic complexes are of particular interest on account of their high stability, possibilities of varying their structures in a wide range by targeted synthesis and their lower cost compare to noble metals as an electrocatalysts. One of the most important properties of porphyrins and their analogs are redox ones. This is due their biological functions and application in the catalysis of various redox reactions. It was shown that metalloporphyrins and metallophtalocyanines generally exhibit a good activity in the oxygen reduction reaction (ORR) [1-3]. Electrocatalytic properties are different as differ individual structural features for porphyrins of various classes.
Pyridyl-substituted porphyrins and their complexes with d-metals attract a special researchers’ attention. Thanks to the lone pair of electron of the nitrogen atom in pyridyl fragments, they can form donor-acceptor bonds with other molecules, cations and etc.
The main feature of metallocorrole is a difference in a size and composition of coordination centers which implies an ability to stabilize metals in different oxidation state {Co (III); Cu(II) or (III); Fe (III) or (IV)} which is normally higher in relation to complexes of MP. This property of metallocorrole to change the oxidation state of the metal gradually plays a key role in electrocatalysis.
The aim of this work is to compare electrochemical behavior and electrocatalytic abilities of ORR of MCor with MP of closely related structures. ms-Tetraphenylporphyrin {H2(Ph)4P}, pyridylporhyrins with different numbers of ms-pyridyl groups (one, two, three or four ones), their isomerism (4-, 3-, 2- ), β-alkyl substituents {H2(Py)4P} and corroles {H3Cor} as a porphyrin analogs bearing ms-aryl substituents are considered.
The electrochemical behavior of pyridyl-substituted porphyrins in aqueous alkaline medium has been characterized in the course of electroreduction (electrooxidation) processes not only on the metal (Co(II), Cu(II), Fe(III)) or the π-conjugated system of the macrocycle, but also on the electron-withdrawing (pyridyl) substituents, which are reduced in the potential range -0.7÷-0.8 V (vs. Ag/AgCl electrode), compared to the complexes of ms-tetraphenylporphyrin [4,5].
Moreover, it was found that the introduction of a pyridyl substituent in porphyrins leads to an increase in the rate of ORR. The ms-pyridylporphyrins Co(Py-4)4P and Co(Py-3)4P exhibit higher activity in the ORR, compared to the Co(Ph)4P complex [4].
It was established that the isomerism of the nitrogen atom in the pyridyl groups, though not strongly, but affects the electrochemical and electrocatalytic properties of studied porphyrins. The present of the cobalt ion in molecule decreases the reductive ability of the π-conjugated system and increases the electrocatalytic effect on the oxygen electroreduction.
The electrochemical behavior of corroles in aqueous alkaline medium has been characterized in the course of electroreduction (electrooxidation) processes on the metal (Cu(II and III), Co(III), Fe(III), Mn(III and IV)) and one on the π-conjugated system of the macrocycle. The potential of π-electronic system electroreduction of metallocorroles is lower (220-285 mV) than those of similar structure MP [6].
Electrocatalytic function of d-metal complexes of porphyrin analogs in an indicator of ORR is described by the following regularity of a half-wave potential E1/2(O2) from the structure of the aromatic macrocycle, for instance: Co(Ph)4P < CoCor.
In these studies we have demonstrated the possibility of evaluating the electrocatalytic properties in ORR on the fixed electrodes with active layers containing porphyrins or corroles in active mass. In our view, these investigations increasingly model of actual use of hydrophobized cathodes in power sources with oxygen (air) depolarization.
Thus, the electrocatalytic activity of tetrapyrrole compounds increases in the series CuCor < CoCor < MnCor < FeCor for corroles and FeP < CuP < CoP for pyridylporphyrins. It was noted that a number of metals activity in the ORR can vary significantly depending on the nature of the ligand, which is observed in the experiment.
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3. Tarasevich MR, Radyushkina KA. Catalysis and electrocatalysis by metalloporphyrins. Moscow: Nauka. 1982. 168 p.
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6. Bazanov MI, Berezina NM, Karimov DR, Berezin DB. Russian Journal of Electrochemistry. 2012; 48: 905–910.

The research was carried out in
Research Institute of Macroheterocyclic Compounds,
supported by the President grant of the Russian Federation (number MK-249.2017.3).

Figure 1