Reaction of Low-Valent Iron Porphyrins with Alkyl Containing Supporting Electrolytes

A σ-alkyl iron porphyrin complex was found to be formed from the reaction of Fe(TPP)2− with tetraalkylammonium ions (TPP=tetraphenylporphyrin). At room temperature, this reaction was too slow to be observed with cyclic voltammetry. But, if one electrolyzes at a potential that generated Fe(TPP)2−, and then initiates the scan in the forward direction, waves for Fe(TPP)(R)−/Fe(TPP)(R) and Fe(TPP)(R)/Fe(TPP)(R)+, where R=alkyl group, were observed. Both tetramethyl- and tetrabutylammonium ions were found to react with Fe(TPP)2−. No alkylated products were observed if alkali salts were used as the supporting electrolyte. Visible spectroelectrochemistry with a thin layer cell showed that electrolysis at the first and second waves of Fe(TpP)(Cl) in DMF yielded the expected Fe(TPP) and Fe(TPP)− complexes, but Fe(TPP)2− was not observed at the third wave. Instead, the spectrum of Fe(TPP)(R)− was obtained. Oxidation of this species led to the formation of Fe(TPP)(R), and further oxidation regenerated Fe(TPP)(Cl) because of the poor stability of Fe(TPP)(R)+. Further characterization was carried out by obtaining the resonance Raman spectra of chemically generated Fe(TPP)(CH3)O/− and Fe(TPP)(butyl)O/− complexes, and comparing the results with the electrochemically generated complexes. The σ-alkyl iron porphyrins were not very stable under laser irradiation, and the best spectra were obtained in THF with frozen samples. The σ-alkyl ferric porphyrins were considerably less stable than the corresponding ferrous complexes, and the photoproduct, Fe(TPP), could be observed in all spectra. The ferrous alkyl porphyrins gave spectra that were consistent with low-spin five-coordinate complexes, with v2 and v4 bands being observed at 1566 and 1363 cm−1, respectively. The v2 and v4 bands for the ferric alkyl porphyrin were at 1565 and 1359 cm−1, respectively. The product, obtained by electrolyzing Fe(TPP)(Cl) at the potential for the generation of Fe(TPP)2−, gave a spectrum in THF that was consistent with an iron alkyl complex. The photoproduct appeared to be Fe(TPP)(OH)− rather than Fe(TPP), as was observed in the chemically generated product due to trace amounts of water in the supporting electrolyte. The visible spectrum of the resonance Raman was identical to the authentic Fe(TPP)(R)− sample. There was no evidence for Fe(TPP)(OH)−, which would have been quite apparent in the visible spectrum. The generation of Fe(TPP)(R)− from Fe(TPP)2− and tetrabutylammonium ion could occur by at least two pathways: nucleophilic attack or electron transfer between iron(`O\u27) on the tetrabutylammonium ion. Steric effects would favor an electron transfer mechanism over the nucleophilic process, but the data at this time are not conclusive