Porphyrin self-exchange electron transfer

The electron self-exchange rate constants for several Fe(II)/Fe(III) porphyrin couples have been measured by H NMR line-broadening techniques in 5 1 acetone/water at -20 The relative rate constants for the [Fe(P)(l-MeIm)2] couples, P = octaethylporphyrin chlorin < isobacteriochlorin, have been attributed to differences in outer-sphere reorganization, related to the steric bulk. The rate-determining step in the metallopophyrin-catalyzed reductions of dioxygen by substituted ferrocenes is the electron transfer between the ferrocene and the metalloporphyrin (M = Fe, Co, and The Marcus relationship provides a... 

Self-exchange rates of the three tetrakis-(4-pyridyl)porphine complexes [Fe(P)(imidazole)a] +/ +, [Fe(P)(OH)(OH2)] +/ +, and [Fe(P)(OHa)]6+/ + have been estimated as > 10 , > 10 , and 1.2 x 10 respectively. Of the iron(iii) complexes in this series, the first is low-spin, the second is believed to be an equilibrium mixture of high-spin and low-spin forms, and the third is high-spin. These results support a previous suggestion that electron transfer to or from a low-spin iron porphyrin system has a negligible Franck-Condon barrier. ... 

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