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, S2O42–) with iron(III) 5,10,15,20-tetrakis(p-sulfonatophenyl)porphyrinate [FeIII(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 FeIII(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 FeIII(tpps) and FeII(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 FeIII(tpps) and aimsa or dithionite, are proposed.