Electrochemical and catalytic assessment of peripheral bromoaryl-substituted manganese and iron porphyrazines

Abstract

The cyclotetramerization reaction following the Linstead approach led to novel symmetrical magnesium(II) aminoporphyrazines with methyl(6-bromo-3-pyridylmethyl) and methyl(4-bromobenzyl) substituents. These products were applied in the demetallation reaction procedure in trifluoroacetic acid in the dark, which resulted in free-base porphyrazine derivatives. Then these macrocycles were remetallated with iron(II) bromide in DMF to give the desired iron(III) porphyrazine macrocycles. Simultaneously, the cyclotetramerization reaction of maleonitrile derivatives with manganese(II) chloride in n-pentanol and DBU led to symmetrical manganese(III) porphyrazines. New porphyrazine complexes with methyl(6-bromo-3-pyridylmethyl) and methyl(4-bromobenzyl) substituents were thoroughly characterized by the use of various analytical techniques, including electronic spectra, mass spectrometry, and FTIR. Macrocycles were subjected to electrochemical and spectroelectrochemical characterization, accompanied by preliminary catalytic study. The electrochemical properties of all obtained macrocycles were assessed with the use of cyclic and differential pulse voltammetry. The electrochemical activity of new macrocycles and their susceptibility to oxidation/reduction processes depended on the presence of nitrogen substituents in their macrocyclic periphery. The electrochemical measurements provided information on the positions of energy levels (HOMO-LUMO) of each porphyrazine, allowing estimation of the electrochemical energy gaps. The calculated electrochemical energy gaps were higher for porphyrazines with methyl(6-bromo-3-pyridylmethyl)amino peripheral substituents than methyl(4-bromobenzyl)amino ones due to the more electronegative nature of nitrogen amine atom. The calculated electrochemical HOMO-LUMO energy level gap values and the optical band gaps were found to agree within approx. 0.2 eV. Iron(III) and manganese(III) porphyrazines were assessed in terms of their catalytic properties. In these studies, DPBF was employed as a substrate of oxidation processes due to its easy UV–vis assessment. The results revealed the catalytic activity of porphyrazines with peripheral methyl(4-bromobenzyl)amino substituents following first-order kinetics.