Porphyrin-diones and porphyrin-tetraones: Reversible redox units being localized within the porphyrin macrocycle and their effect on tautomerism

Porphyrin-2,3-diones and porphyrin-2,3,7,8- and porphyrin-2,3,12,13- tetraones were shown to have a redox-active unit that can function independently of the macrocycle at large. Electroreduction of 5,10,15,20-tetrakis(3,5-di- tert-butylphenyl)porphyrin-2,3-diones [(P-dione)M] and the corresponding -2,3,12,13-tetraones [L-(P-tetraone)M] and -2,3,7,8-tetraones [C-(P-tetraone)M], where M = 2H, Cu^II, Zn^II, Ni^II, and Pd ^II was investigated and the products were characterized by ESR and thin-layer UV-visible spectroelectrochemistry. Electrochemical and spectroelectrochemical data show that the first two reductions of the porphyrin-diones and the first three reductions of the porphyrin-tetraones occur at the dione units. This was confirmed by ESR spectra of first reduction products which show that the electron spin is totally localized on a semidione unit, independent of the central metal ion and of the number and location of dione units. ESR spectra of the radical anions derived from free-base porphyrin-2,3-dione [(P-dione)2H] and porphyrin-2,3,12,13-tetraone [L-(P-tetraone)2H] confirm the trans-arrangement of the two inner protons and their location on nonsubstituted pyrrolic rings, thereby maintaining an 18-atom 18-π electron bacteriochlorin-like aromatic delocalization pathway. The redox unit is not similarly isolated in the corner free-base porphyrin-2,3,7,8- tetraone [C-(P-tetraone)2H]. A one-electron reduction of C-(P-tetraone)2H leads to the formation of a tautomer with trans inner hydrogens with one residing on the N of the ring with the reduced unit as the only detectable product. This process is favorable because it creates a more delocalized 18-atom 18-π electron aromatic pathway. This result is consistent with the measured redox potentials which show the first reduction of C-(P-tetraone)2H to be substantially easier than (P-dione)2H or L-(P-tetraone)2H.