Studies of water-soluble paramagnetic iron(III) porphyrins

The effect of porphyrin ring protection on the electronic structure of the iron complex in aqueous solution has been investigated. Paramagnetic H-1 NMR studies have been carried out on iron(lll) unhindered sulfonato porphyrin, Fe-III(TPPS), and iron(lll) ''one-face''-hindered sulfonato porphyrin, Fe-III(e-HSP(C-12)). In this way, the longitudinal relaxation times of the hyperfine-shifted resonances, use of the integration method and COSY spectra have allowed us to assign the metalloporphyrin resonances. The pyrrolic protons present a pattern of resonances typical for high-spin iron(ill) porphyrins. Some H-1 NMR signals exhibit significant relaxation times, corresponding to a putative iron(ll) 5,10,15,20-tetra(4-sulfonatophenyl)porphyrin pi-anion radical. The electron spin resonance spectra of the FeTPPS complex were measured from 125 to 423 K. The formation of the pi-anion radical is confirmed as manifested by the drastic EPR spectral changes. Finally, it has been shown that the complexes undergo a reversible one-electron reduction (E(1/2)=-0.43 V for Fe-III(TPPS) and E(1/2)=0.47 V for Fe-III(e-HSP(C-12))versus SCE in H2O/KNO3 solution) forming Fe(II) complexes.