Reversible two-electron-one-proton systems in the ring-centered oxidation of metalloporphyrins bearing secondary amide-linked superstructures

Article abstract of DOI:10.1021/ja00005a022

Complexes of porphyrins derived from tetraphenylporphyrin by substitution of the ortho position of the phenyl rings by secondary amide groups in a basket-handle or picket configuration undergo a reversible two-electron oxidation whatever the nature of the central metallic ion, Cu²⁺, Zn²⁺, Ni²⁺, Fe³⁺, Co³⁺, in solvents such as 1,2-dichloroethane, methylene chloride, and benzonitrile. The reaction mechanism is investigated by cyclic voltammetry (as a function of the scan rate and the addition of a base or an acid) and UV-vis-near-IR and Fourier transform IR thin-layer spectroelectrochemistry. The reaction product is an endogeneous isoporphyrin resulting from the formation of an oxazine ring formed upon condensation of a meso carbon of the porphyrin dication with the oxygen of the amide group. The same reaction occurs with tertiary amide substituents, but in the case of secondary amide the concomitant loss of the amide proton facilitates the formation of the isoporphyrin. It thus drives the uphill disproportionation of two cation radicals to the right-hand side to such an extent that the uptake of the two electrons takes place at nearly the same potential. The reduction of the internal isoporphyrin is also a two-electron reaction at low scan rates with both the secondary and tertiary amide substituted compounds. The two-electron character of the isoporphyrin reduction is the result of an autocatalytic process in which the porphyrin cation radical serves as redox catalyst.