The Electrochemistry and Spectroelectrochemistry of Sulfate Complexes of Iron Porphyrins
Format of Original
9 p.; 29 cm
Inorganica Chimica Acta
The spectroscopy and electrochemistry of [Fe(TPP)]2SO4 and [Fe(OEP)]2SO4 in methylene chloride, dimethylformamide and dimethyl sulfoxide was examined. In methylene chloride, the reduction of iron(III) to iron(II) porphyrins occurs in two well-separated reduction waves. The first wave corresponds to the reduction of the sulfate-bridged dimer to Fe(P) and Fe(P)(SO4)−, where P=TPP or OEP. The second wave corresponds to the reduction of Fe(P)(SO4)− to Fe(P). The reduction of Fe(P) occurs at the normal potential for the iron(II)/iron(I) porphyrin reduction. These results were confirmed by visible spectroelectrochemistry, proton NMR and EPR spectroscopy. In coordinating solvents such as DMF or DMSO, the sulfate-bridged dimer dissociated and a single iron(III)/iron(II) wave was observed. The addition of sulfate to the sulfate-bridged complex in methylene chloride or chloroform lead to the dissociation of the complex into the sulfate monomer complexes. The NMR spectrum of Fe(TPP)(SO4)− was typical of a high-spin ferric porphyrin complex, and was almost indistinguishable from Fe(TPP)Cl. In the presence of excess sulfate, only one iron(III)/iron(II) wave was observed, and this wave occurred at the potential of the second wave for the sulfate-bridged dimer. As with the dimer itself, this wave was quite quasi-reversible, and the reduction wave occurred substantially negative of the iron(III)/iron(II) wave for Fe(TPP)Cl. The spectroelectrochemistry of the reduced product was consistent with a Fe(II)(TPP)(SO4)2− complex. The strong complex between sulfate and iron(II) is probably due to the poor solvation of sulfate in these organic solvents. In DMSO, the results were similar to methylene chloride, except that there was no evidence for complexation of sulfate with the ferrous species. In addition to the sulfate complex, the bisulfate complex of ferric OEP was also examined, as well as the reaction of bisulfate with Fe(TPP)(ClO4). The infrared, visible and NMR spectra for Fe(OEP)(HSO4) were obtained.