Investigation of the redox interaction between iron(III) 5,10,15,20-tetrakis(p-sulfonatophenyl)porphyrinate and aminoiminomethanesulfinic acid in aqueous solution

Abstract

Stability studies have been made on aminoiminomethanesulfinic acid [aimsa, thiourea dioxide, (NH₂)₂CSO₂] and the reactions of aimsa and its decomposition product (dithionite, S₂O₄^2–) with iron(III) 5,10,15,20-tetrakis(p-sulfonatophenyl)porphyrinate [Fe^III(tpps)] in the presence of oxygen have been investigated. Application of NMR and stopped-flow spectrophotometry revealed direct evidence for the existence of two different forms of aimsa in aqueous solution. The slow formation of one of these forms, NH₂NHCSO₂H, is shown to determine the strong dependence of the reactivity of aimsa on the age of the stock solutions. Both aimsa and dithionite react in a similar way with Fe^III(tpps) in alkaline solutions. The SO₂^˙– radical plays a key role in the redox reactions. The ratio of the oxygen and radical concentration determines which kind of reaction (oxidation, reduction or decomposition) will dominate. In general a high oxygen concentration and a low radical concentration favour the oxidation and decomposition of the metalloporphyrin, whereas a high radical concentration and a low oxygen concentration favour the reduction. In strongly basic solutions ([NaOH] = 0.5 M) a redox cycle between Fe^III(tpps) and Fe^II(tpps) is observed at low aimsa and dithionite concentration. Possible mechanisms for the decomposition of aimsa in alkaline solutions, as well as for the reactions between Fe^III(tpps) and aimsa or dithionite, are proposed.