Facile and Reversible Electrogeneration of Porphyrin Trianions and Tetraanions in Nonaqueous Media

The first examples for the facile, reversible, and stepwise electrogeneration of triply ring-reduced porphyrin macrocycles are presented. The investigated compounds are represented as MTPP(NO2)(PE)(6), MTTP-(PE)(8), NiTPP(NO2)(Ph)(4), and MTPP(CN)(4), where TTP and TPP are the dianions of tetratolylporphyrin and tetraphenylporphyrin, respectively, NO2 phenylethynyl (PE), and CN are substituents at the beta-pyrrole positions of the macrocyde, and M = Cu-II, Ni-II, Zn-II, Co-II, or 2H. Each porphyrin undergoes three or four reductions within the negative potential limit of the electrochemical solvent. The UV-visible spectra of the first three reduction products were characterized by means of, thin-layer UV-vis spectroelectrochemistry, and the generation of multianionic porphyrins is interpreted in terms of extensive stabilization of the LUMOs due to the electron-withdrawing and/or extended pi-conjugation of the beta-substituents.