Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical properties and catalytic activity for reduction of molecular oxygen in acid media

Abstract

Cobalt(III) triarylcorroles containing 0–3 nitro groups on the para-position of the three meso-phenyl rings of the macrocycle were synthesized and characterized by electrochemistry, mass spectrometry, (UV–vis) and 1H NMR spectroscopy. The examined compounds are represented as (NO2Ph)nPh3−nCorCo(PPh3), where n varies from 0 to 3 and Cor represents the core of the corrole. Each compound can undergo two metal-centered one-electron reductions leading to formation of Co(II) and Co(I) derivatives in CH2Cl2 or pyridine containing 0.1M tetra-n-butylammonium perchlorate (TBAP). A stepwise two electron reduction of each NO2Ph group of the compound is also observed. The first is reversible and occurs in a single overlapping step at the same potential which involves an overall one-, two- or three-electron transfer process for compounds 2–4, respectively. This indicates the lack of an interaction between these redox active sites on the corroles. The second reduction of the NO2Ph groups is irreversible and located at a potential which overlaps the Co(II)/Co(I) process of the compounds. Thin-layer UV–visible spectroelectrochemical measurements in CH2Cl2, 0.1M TBAP demonstrate the occurrence of an equilibrium between a Co(III) π-anion radical and a Co(II) derivative with an uncharged macrocycle after the first controlled potential reduction of the nitro-substituted corroles. All four cobalt corroles were also examined as catalysts for the electroreduction of O2 when coated on an edge-plane pyrrolytic graphite electrode in 1.0M HClO4. This study indicates that the larger the number of nitro-substituents on the cobalt corrole, the better the compound acts as a catalyst.