The Influence of Steric Effects on the Oxidation Kinetics of [FeII(aminopolycarboxylato)] Complexes with Dioxygen

Abstract The oxidation kinetics for the reaction of [FeII(1,2- pdta)] (1,2-pdta = 1,2-N,N′-1,2-propylenediaminetetraacetate) and [FeII(1,3-pdta)] (1,3-pdta = 1,2-N,Ν′- 1,3-propylenediaminetetraacetate) with molecular oxygen were studied as a function of [FeII], [O2], pH, temperature and pressure. It was observed that the reaction with [FeII(1,2-pdta)] is ca. 200 times faster than with the isomeric [FeII(1,3-pdta)] complex under comparable conditions. In the presence of an excess of [FeII(1,2-pdta)] three reaction steps could be identified for which the rate constants are = 3385 ± 607M-1 s-1, k2 = 226±9M-1s-1 and k3 = 111 ± 4M-1s-1 at pH = 6 and T = 25°C. Since [FeII(1,3-pdta)] is unstable during the reaction, only approximate rate constants for two of the electron-transfer steps could be determined, which are two orders of magnitude smaller than in the case of the [FeII(1,2-pdta)] complex. Rate and activation parameters for the 1,2-pdta system are reported and discussed in reference to our earlier work on related systems.