Fe(III) complexes of 1,4,8,11-tetraaza[14]annulenes as catalase mimics

The development of enzyme mimics of catalase which decompose hydrogen peroxide to water and molecular oxygen according to the 2:1 stoichiometry of native catalase and in aqueous solution at pH 7 and at micromolar concentrations of the enzyme model and hydrogen peroxide is reported. For this purpose, iron(III) complexes of 1,4,8,11-tetraaza[14]annulenes are prepared by various procedures. Efficacious preparations utilize reaction of the [N4] macrocyles with FeII salts in the presence of triphenylamine, followed by gentle oxidation of the FeII complexes by molecular oxygen or by tris(4-bromophenyl)aminium hexachloroantimonate. The complexes are characterized by SQUID magnetometry and by Mössbauer, EPR, and UV/vis spectrometry. In the solid state, the iron(III) center of the catalytically active complexes exists in the intermediate (quartet, S = 3/2) spin state. Several of these complexes decompose hydrogen peroxide in aqueous buffer solution at pH 7.2 at room temperature with turnover numbers between 40 and 80. The apparent second-order rate constant for hydrogen peroxide decomposition is in the range of 1400-2400 M(-1) s(-1), about 3 orders of magnitude lower than the value for native catalase. Besides oxygen production, a non-oxygen releasing pathway of hydrogen peroxide decomposition is unveiled.