Evidence of the Formation of Isoporphyrin in the Electrooxidation Processes of Meso-Tetraphenylporphyrin
Authors: Ruiz-Montoya Jose Gabriel , Macías-Ruvalcaba Norma Angélica 
Published: February 2020

In this research, the electrochemical oxidation of meso-tetraphenylporphyrin (H2TPP) has been studied in two solvents, benzonitrile and dichloromethane, with Bu4NClO4 as supporting electrolyte. Three oxidation waves are observed during the anodic sweep; the first two of nature quasi-reversible, and the third irreversible (Fig.1 a,b). The processes occurring at the first and second waves have been assigned to the formation of cation radical and dication, respectively1. Cyclic voltammograms show that when the switching potential occurred after peak IIa, a reduction peak is observed around -0.17 V vs Fc+/Fc (IIIc); this peak, along with the third oxidation wave (IIIa), might be associated to the formation of isoporphyrin. Our hypothesis is that traces of water in the solvent can act as nucleophiles at the meso position of porphyrin dication, giving rise to the isoporphyrin species2. Absorption bands at 445, 755, 794 and 883 nm observed during thin layer spectroelectrochemical UV-visible experiments (Fig.1 c,d,e) are also consistent with the formation of isoporphyrins following the formation of the dication3,4. Long-term experiments such as controlled potential electrolysis show that isoporphyrins can also be formed from the cation radical, however this reaction seems to be too slow to affect the voltammograms. Exhaustive electrolysis at the potential of the first oxidation peak Ia, consumed about one-electron per molecule, and at the end of the electrolysis an anodic sweep revealed the disappearance of peak Ia; in a cathodic sweep, peak IIIc was observed. When the electrolyzed solution containing the isoporphyrin was reduced to the starting porphyrin at a potential slightly cathodic from of wave IIIc, the original anodic signals of H2TPP were restored, showing the reversibility of the process. Similar results were obtained in both solvents. This demonstrates the existence of at least two reaction pathways, one for the formation of isoporphyrins from the radical cation H2TPP•+ at longer time scales, and other from H2TPP+2 at shorter times. This results contrast with previous studies that have shown that in benzonitrile the radical cation of the free-base porphyrins undergoes hydrogen atom abstractions leading to diprotonated species5, and with experiments using Bu4NPF6 in CH2Cl2 that do not lead to isoporphyrin species.

References:

Kadish, K. M. & Van Caemelbecke, E. Electrochemistry of porphyrins and related macrocycles. J. Solid State Electrochem.
7, 254–258 (2003).

Hinman, A. S., Pavelich, B. J., Kondo, A. E. & Pons, S. Oxidative voltammetry of some tetraphenylporphyrins in the presence of nucleophiles leading to isoporphyrins. J. Electroanal. Chem. Interfacial Electrochem.
234, 145–162 (1987).

Dolphin, D., Felton, R. H., Borg, D. C. & Fajer, J. Isoporphyrins. J. Am. Chem. Soc.
92, 743–745 (1970).

Bhuyan, J. Metalloisoporphyrins: from synthesis to applications. Dalton Trans.
44, 15742–56 (2015).

Mest, Y. Electrooxidation of porphyrin free bases: fate of the π-cation radical. New Journal of Chemistry,
22
, 823-830 (1998).

Figure 1