Electrochemistry and spectra of six coordinated high-spin bis (tetrahydrofurane) protoporphyrinato IX-iron encapsulated in aqueous surfactant micelles

Abstract

The optical spectra of the bis (tetrahydrofurane) adduct of iron(III) and iron(II) protoporphyrinato IX in aqueous solutions of anionic (SDS), cationic (CTAB), and neutral (Triton X-100) surfactant micelles are typical of six-coordinated high-spin (6 cHS) ferrous and ferric hemes. The characteristic spectral bands in the ferric form are observed at. 400, 575 nm and 598 nm while these in the ferrous form are found at 420, 540 nm and 567 nm. The 6 cHS ferrous heme is very rare and the complex reported here is the first example in aqueous solutions. The mid-point potential of the bis(thf) adducts of heme are -0.35 V, -0.36 V and -3.0V vs. Ag-AgCl in SDS, TX- 100 and CTAB micelles, respectively. Binding of thf to iron porphyrin in SDS micelles gives a 67 mV anodic shift of the potential. As the high-spin iron is brought into the porphyrin plane in the six-coordinated bis (thf) complexes, the ferrous porphyrin is preferentially stabilised compared to the ferric hemin. The displcement of the iron atom to the porphyrin plane influence the charge transfer bands and the mid-point potential to a significant extent.