Air-stable, heme-like water-soluble iron(II) porphyrin: in situ preparation and characterization

Abstract

Preparation of the water-soluble, kinetically labile, high-spin iron(II) tetrakis(4-sulfonatophenyl)porphyrin, Fe(II)TPPS(4-), has been realized in neutral or weakly acidic solutions containing acetate buffer. The buffer played a double role in these systems: it was used for both adjusting pH and, via formation of an acetato complex, trapping trace amounts of iron(III) ions, which would convert the iron(II) porphyrins to the corresponding iron(III) species. Fe(II)TPPS(4-) proved to be stable in these solutions even after saturation with air or oxygen. In the absence of acetate ions, however, iron(II) ions play a catalytic role in the formation of iron(III) porphyrins. While the kinetically inert iron(III) porphyrin, Fe(III)TPPS(3-), is a regular one with no emission and photoredox properties, the corresponding iron(II) porphyrin displays photoinduced features which are typical of sitting-atop complexes (redshifted Soret absorption and blueshifted emission and Q absorption bands, photoinduced porphyrin ligand-to-metal charge transfer, LMCT, reaction). In the photolysis of Fe(II)TPPS(4-) the LMCT process is followed by detachment of the reduced metal center and an irreversible ring-opening of the porphyrin ligand, resulting in the degradation of the complex. Possible oxygen-binding ability of Fe(II)TPPS(4-) (as a heme model) has been studied as well. Density functional theory calculations revealed that in solutions with high acetate concentration there is very little chance for iron(II) porpyrin to bind and release O(2), deviating from heme in a hydrophobic microenvironment in hemoglobin. In the presence of an iron(III)-trapping additive that is much less strongly coordinated to the iron(II) center than the acetate ion, Fe(II)TPPS(4-) may function as a heme model.