REACTION OF LOW-VALENT IRON PORPHYRINS WITH ALKYL CONTAINING SUPPORTING ELECTROLYTES

A sigma-alkyl iron porphyrin complex was found to be formed from the r eaction of Fe(TPP)(2-) with tetraalkylammonium ions (TPP=tetraphenylpo rphyrin). At room temperature, this reaction was too slow to be observ ed with cyclic voltammetry. But, if one electrolytes at a potential th at generated Fe(TPP)(2-), and then initiates the scan in the forward d irection, waves for Fe(TPP)(R)(-)/Fe(TPP)(R) and Fe(TPP)(R)/Fe(TPP)(R) (+), where R=alkyl group, were observed. Both tetramethyl- and tetrabu tylammonium ions were found to react with Fe(TPP)(2-). No alkylated pr oducts were observed if alkali salts were used as the supporting elect rolyte. Visible spectroelectrochemistry with a thin layer cell showed that electrolysis at the first and second waves of Fe(TPP)(CI) 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)(CI) because of t he poor stability of Fe(TPP)(R)(+). Further characterization was carri ed out by obtaining the resonance Raman spectra of chemically generate d Fe(TPP)(CH3)(0/-) and Fe(TPP)(butyl)(0/-) complexes, and comparing t he results with the electrochemically generated complexes. The sigma-a lkyl iron porphyrins were not very stable under laser irradiation, and the best spectra were obtained in THF with frozen samples. The sigma- alkyl ferric porphyrins were considerably less stable than the corresp onding ferrous complexes, and the photoproduct, Fe(TPP), could be obse rved in all spectra. The ferrous alkyl porphyrins gave spectra that we re consistent with low-spin five-coordinate complexes, with nu(2) and nu(4) bands being observed at 1566 and 1363 cm(-1), respectively. The nu(2) and nu(4) bands for the ferric alkyl porphyrin were at 1565 and 1359 cm(-1), respectively. The product, obtained by electrolyzing Fe(T PP)(CI) at the potential for the generation of Fe(TPP)(2-), gave a spe ctrum in THF that was consistent with an iron alkyl complex. The photo product appeared to be Fe(TPP)(OH)(-) rather than Fe(TPP), as was obse rved 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 n o 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: nuc leophilic attack or electron transfer between iron('0') on the tetrabu tylammonium ion. Steric effects would favor an electron transfer mecha nism over the nucleophilic process, but the data at this time are not conclusive.