Formation and Isolation of a Four-Electron-Reduced Porphyrin

Porphyrins are well known as 18π conjugated aromatic macrocycles with high stability and also as redox-active compounds exhibiting reversible multi-redox processes. In this study, we demonstrated that the two-electron reduction of a diprotonated dodecaphenylporphyrin derivative by Na2S2O4 in DMSO afforded the corresponding isophlorin (Iph) selectively. Formation of Iph was confirmed by spectroscopic measurements and the isolation of tetra-methylated Iph. Further two-electron reduction of Iph was examined using excess Na2S2O4 in DMSO to obtain an unprecedented four-electron reduced porphyrin (IphH2 ), which was successfully characterized by spectroscopic analysis and X-ray crystallography. IphH2 showed a unique conformation and could be oxidized to reproduce the starting porphyrin by the chemical oxidation, allowing us to establish a proton-coupled four-electron reversible redox system.