Inducing asymmetry in free-base, MnIII, NiII and CuII (ethylsulfanyl)porphyrazines: synthetic aspects and spectro-electrochemical implications

Abstract

Treatment of 2,3,7,8,12,13,17,18-octakis(ethylsulfanyl)-5,10,15,20-porphyrazine (H₂OESPz) with CrCl₂ in a 1,2,4-trichlorobenzene–n-BuOH mixture resulted in one ethylsulfanyl branch at the pyrrolic β positions of the macrocycle being replaced by an hydrogen atom with >40% yield. The structure of the asymmetric pyrrolic subunit and of the nearest pyrrolic subunits have been determined by NMR spectroscopy. The reaction path leading to the asymmetric porphyrazine implies the formation of a two-electron reduced diprotonated porphyrazine, [H₂OESPz(−4)(2H⁺)], that, as a consequence of charge and structural intramolecular rearrangements at high temperature, undergoes nucleophilic substitution of an ethyl sulfide group by H⁻. UV-Vis and near-IR results suggest that the asymmetric substitution modifies the (sulfanyl)porphyrazine π and n_Sulfur levels. On the contrary, ESR spectroscopy shows that the electronic properties of the coordinated Cu²⁺ ion are affected only marginally. The half-waves of the ligand first reductive processes sensibly shift towards cathodic potentials in the asymmetric metal porphyrazines, but not in the asymmetric free-base porphyrazine, probably due to conflicting electronic and structural effects induced by removal of an ethylsulfanyl tail.