Oxygen complexes : reduction kinetics and other model studies

Abstract

An investigation of the relationship between the reduction potentials of ferrous chelates and the rate of reduction of oxygen adducts of cobaltous-polyamines is reported. Potentiometric equilibrium studies of 1:1 molar ratios of Fe^2+ and ethylenediaminetetraacetic acid, (EDTA), demonstrate the existence of the [MHL] and [ML] species, but show no evidence of the formation of any [MOHL] complex. Similar studies of the ferrous ion and N-(2-hydroxyethyl)ethylenedinitrilo-N,N',N'-triacetic acid, (HEDTA), and the ferrous ion and N, N'-bis(2-hydroxy-5-sulfobenzyl) ethylenediaminediacetic acid, (SHBED), also failed to produce any evidence for the existence of hydrolyzed ferrous chelates. Using data from these potentiometric equilibrium studies and a program written by the author the reduction potentials of the ferrous complexes as a function of p[H+] have been calculated. Second-order rate constants for reduction by these ferrous complexes are reported for the oxygen adducts of the cobaltous-polyamines involving the following ligands: ethylenediamine, (EN); diethylenetriamine, (DIEN), triethylenetetraamine, (TRIEN); tetraethylenepentaamine, (TETREN) and 2,2',2'-tris(aminoethyl)amine, (TREN). A mechanism involving a rapid outer-sphere pre-equilibrium is proposed for reduction of several of these oxygen complexes. The possibility of an inner-sphere-mechanism for reduction of the oxygen adduct of cobaltous-TREN by the ferrous-HEDTA and the ferrous-EDTA systems is examined. Factors affecting rates, including reduction potentials, are discussed.