Synthesis and electrochemistry of β-pyrrole nitro-substituted cobalt(II) porphyrins. The effect of the NO2 group on redox potentials, the electron transfer mechanism and catalytic reduction of molecular oxygen in acidic media

Four cobalt(II) porphyrins, two of which contain a beta-pyrrole nitro substituent, were synthesized and characterized by electrochemistry and spectroelectrochemistry. The investigated compounds are represented as (TRPP)Co and (NO2TRPP)Co, where TRPP is the dianion of a substituted tetraphenylporphyrin and R is a CH3 or OCH3 substituent on the four phenyl rings of the macrocycle. Two reductions and three oxidations are observed for each compound in CH2Cl2 containing 0.10 M tetra-n-butylammonium perchlorate. The first reduction of the compounds without a nitro substituent is metal-centered and leads to formation of a Co(I) porphyrin which then reacts with the CH2Cl2 solvent to generate a carbon sigma-bonded Co-III-R complex. A further reduction then occurs at more negative potentials to generate an unstable Co(II) sigma-bonded compound. In contrast to these reactions, the first reduction of the nitro-substituted porphyrins is macrocycle-centered under the same solution conditions and gives a Co(II) porphyrin pi-anion radical product. This reversible electron transfer is then followed at more negative potentials by a second reversible one-electron addition to give a Co(II) dianion. Three reversible one-electron oxidations are also seen for each compound. The first is metal-centered and the next two involve the conjugated pi-system of the macrocycle. Each neutral Co(II) porphyrin was also examined as to its catalytic activity for electro-reduction of molecular oxygen when coated on an edge-plane pyrolytic graphite electrode in 1.0 M HClO4. The beta-pyrrole nitro-substituted derivatives were shown to be better catalysts than the non-nitro substituted compounds under the utilized experimental conditions.