CATALYSIS OF THE DECOMPOSITION OF HYDROGEN-PEROXIDE BY A COMPLEX OF IRON(III) WITH A SYNTHETIC MACROCYCLIC LIGAND

The complex dichloro-meso-2, 12-dimethyl-3, 7, l1, 17-tetraazabicyclo[ 11.3.1 ]heptadeca-l(17), 13, 15-trieneiron(III) tetrafluoroborate, abbreviated [Fe(CRH4)Cl2]BF4, is an effective catalyst for the decomposition of H2O2 to form H2O and O2 in aqueous solution. The solution behavior of the complex [Fe(CRH4)Cl2] BF4 was studied in order to facilitate the analysis of the rate data. The iron complex exists primarily as the aquo-hydroxo complex in the pH range of interest. This aquo-hydroxo complex slowly self-condenses in the presence of air and acetate buffers to give an antiferromagnetically coupled, μ-oxo dimer. The dimerization rate has been measured. The active species in the catalysis of H2O2 decomposition is the aquo-hydroxo complex Fe(CRH4)(OH)(H2O)2+. The dimer is catalytically inactive, as is the corresponding dimer of hemin. The kinetics of H2O2 decomposition in the pH range from 3.8 to 5.1 and in the temperature range from 8 to 32°C were studied thoroughly by oxygen evolution measurements using an apparatus which allows very rapid pressure changes to be monitored. The observed rate law is shown to be consistent with a mechanism formulated in terms of free-radical intermediates. Acetate inhibits the reaction, possibly by competing for hydroxyl radicals. © 1979, American Chemical Society. All rights reserved.