The stability of low-valent iron porphyrins in electrochemical solutions

Chathra Shirani De Silva, Marquette University

Abstract

The substituent effect on Fe(P)NO complex was studied using Fe(X-TPP)NO, where X was a substituent at 3- or 4-position on the phenyl ring. The slope of the E$\sb{1/2}$ value for the first wave was 0.045V indicative of metal centered reduction. A value of 0.072V was observed for the second wave suggesting a macrocyclic reduction. Two and three-electron reduced complexes of Fe(TPP) were electrochemically generated and characterized by electrochemical, visible and resonance Raman spectroscopic methods. The resonance Raman shifts that were observed upon the reduction of Fe(TPP)$\sp{-}$ to Fe(TPP)$\sp{2-}$ were quite similar to the shifts that were observed for other $\pi$-anion radicles such as Zn(TPP)$\sp{-}$. In particular the $\nu\sb2$ and $\nu\sb{10}$ bands were quite sensitive to reduction with downshifts of 18 and 22 cm$\sp{-1}$, respectively. The $\nu\sb4$ band was relatively insensitive to the reduction of the complex. Several modes maintained the normal depolarization values upon reduction whereas significantly higher depolarization ratios were observed for the $\nu\sb2$ and $\nu\sb3$ bands. $\sigma$-alkyl iron porphyrin complexes were found to be formed from the reaction of Fe(TPP)$\sp{2-}$ with tetraalkylammonium ions. At room temperature, this reaction was too slow to be observed with cyclic voltammetry. However, when electrolyzed at a potential that generate Fe(TPP)$\sp{2-}$ and initiate the scan in the forward direction, waves for alkylated products were observed. Visible spectroelectrochemical characterization of electrolysis products at Fe(TPP)$\sp{2-}$ potential with alkylated supporting electrolytes, showed the Fe(TPP)R$\sp{-}$ visible spectrum. Further characterization by resonance Raman spectroscopy of chemically generated Fe(TPP)(CH$\sb3)\sp{0/-}$ and Fe(TPP)(butyl)$\sp{0/-}$ complexes, and compared the results with coulometrically reduced complexes. The ferrous alkyl porphyrins gave spectra that were consistent with low-spin five coordinate complexes, with $\nu\sb2$ and $\nu\sb4$ bands being observed at 1566 and 1363 cm$\sp{-1}$, respectively. The $\nu\sb2$ and $\nu\sb4$ bands for the electrochemically generated product were at 1562 and 1363 cm$\sp{-1}$, respectively. The source of alkylation in the electrochemically reduced samples was investigated using a deuterated supporting electrolyte. Deuterium NMR data are consistent with the Fe(II)alkyl complexes. These results ensured that the stability of the highly reduced iron porphyrin complexes is affected by the alkylated electrolyte present in the electrolytic solutions. The activation energies for the reaction of Fe(TPP)$\sp{2-}$ with tetrabutyl ammonium perchlorate were more consistent with ET behavior than with S$\sb{\rm N}$2 behavior.

Recommended Citation

De Silva, Chathra Shirani, "The stability of low-valent iron porphyrins in electrochemical solutions" (1996). Dissertations (1962 - 2010) Access via Proquest Digital Dissertations. AAI9719076.
https://epublications.marquette.edu/dissertations/AAI9719076

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