Reaction of nitric oxide with sulfur(IV) oxides in the presence of iron(II) complexes in aqueous solution

Abstract

Complexes of the type Fe^IIL (L = diethylenetriaminepentaacetate, ethylenediaminetetraacetate, N-2-hydroxyethylethylenediaminetriacetate or nitrilotriacetate) can effectively bind NO in aqueous solution to produce FeL(NO) species which can react with sulfur(IV) oxides to produce Fe^IIIL, N₂O, SO₄^2– and various nitrogensulfur oxides. The kinetics of these reactions was studied as a function of [S^IV]_T and pH using stopped-flow and UV/VIS spectroscopic techniques. Depending on the selected experimental conditions, two subsequent reactions were observed: the first involved the reaction of unco-ordinated NO with sulfur(IV) oxides to produce mainly ^–ON(NO)SO₃^– whereas the second involved attack of sulfoxides on the co-ordinated NO (presumably as nitrosyl) according to the Boedeker process to produce Fe^IIIL, N₂O, SO₄^2– and HON(SO₃)₂^2–. Reactions were also studied in a flow reactor in which a stream of NO gas was passed through a Fe^IIL–sulfite solution, and the reaction products were monitored on-line. The information obtained from both methods was used to suggest possible reaction mechanisms for the various steps. The results are discussed in reference to available literature information, and an overall catalytic cycle to account for the role of Fe^IIL is suggested.