Electrochemistry and Spectroscopy of Sulfate and Thiosulfate Complexes of Iron Porphyrins

Authors

Philip Walter Crawford, Marquette UniversityFollow
Michael D. Ryan, Marquette UniversityFollow

Document Type

Article

Publication Date

1-30-2002

Source Publication

Inorganica Chimica Acta

Source ISSN

0020-1693

Abstract

The electrochemical and spectroscopic properties of the complex formed by the addition of thiosulfate to ferric porphyrins were examined. The NMR spectrum of the thiosulfate–ferric porphyrin complex was consistent with a high-spin ferric complex, while the EPR spectrum at liquid nitrogen temperatures indicated that the complex under these conditions was low-spin. Such behavior has been previously observed for other ferric porphyrin complexes. The visible spectra were characterized by a shift in the Soret band to higher energies, with smaller changes in the longer wavelength region. The complex was reasonably stable in DMF, but slowly reduced over several hours to Fe^II(TPP) and S₄O₆ ²⁻. The voltammetric behavior of the thiosulfate complex in DMF consists of two waves, the first of which was irreversible. The ferric/ferrous reduction in the presence of thiosulfate was shifted negatively about 400 mV, compared to the Fe(TPP)(Cl) reduction. The visible, NMR and EPR spectra were most consistent with a Fe–S bonded ferric porphyrin–thiosulfate complex, Fe(P)(SSO₃)⁻. The kinetics of the reduction of ferric porphyrin by thiosulfate in DMSO indicated an autocatalytic mechanism, where the first step is the formation of the catalyst. The identity of the catalyst could not be determined because it must be present at low concentrations, but it is formed from the reaction of the ferric complex with thiosulfate. Coordination of thiosulfate to the porphyrin was not necessary for the reduction to occur, and the reduction of Fe(TPP)(Cl) by thiosulfate was accelerated by the addition of sulfate. Under these conditions, sulfate had replaced thiosulfate as the axial ligand for the ferric porphyrin. In the presence of sulfate, the reduction occurred in a single kinetic pseudo-first order step.

The voltammetry, spectroelectrochemistry and kinetics for the reaction of thiosulfate with ferric porphyrins were examined. A rapid reaction between ferric porphyrins and thiosulfate was observed in DMF. The reaction was complex, involving the formation of a catalytic intermediate. Window factor analysis and multivariate curve resolution were used to deconvolute the kinetic data.

Comments

Accepted version. Inorganica Chimica Acta, Vol. 328, No. 1, (January 30, 2002): 13-22. DOI. © 2002 Elsevier. Used with permission.

Recommended Citation

Crawford, Philip Walter and Ryan, Michael D., "Electrochemistry and Spectroscopy of Sulfate and Thiosulfate Complexes of Iron Porphyrins" (2002). Chemistry Faculty Research and Publications. 151.
https://epublications.marquette.edu/chem_fac/151