Fe^III Complexes of 1,4,8,11-Tetraaza[14]annulenes as Catalase Mimics

Abstract

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.

Hydrogen Peroxide Decomposition by a Non‐Heme Iron(III) Catalase Mimic: A DFT Study

“…These can be compared with the experimental Gibbs free energies of activation of DG°e xp = 13.2-12.9 kcal mol À1 as calculated from the experimental rate constants for H 2 O 2 decomposition by complexes 1, 2 and their derivatives (k exp = (1.3-2.3) 10 3 m À1 s À1 at 20 8C). [30] The strikingly close correspondence of the experimental and computed activation free energies thus further supports a homolytic route for the cleavage of the first associated molecule of H 2 O 2 in the catalytic process.…”

“…Relative energies were calculated with respect to the energy of intermediate-spin complex 4 2 (plus the energies of H 2 O 2 and/or H 2 O), because the S = 3 = 2 spin state was found experimentally for the catalytically active complex in the solid state. [30] Relative energies are collected in Table 1; relative, zero-point-corrected electronic energies at 0 K are also displayed in Figure 2; graphical schemes of the relative enthalpies and Gibbs free energies, respectively, are provided in Figures S1 and S2 in the Supporting Information.…”

“…[30] Mçssbauer spectroscopy and susceptibil- In addition to ongoing experimental studies on the mechanism by which the above catalase mimics decompose hydrogen peroxide, we studied the mechanism also by quantum-chemical calculations utilizing density functional theory (DFT). Here, we present our first results.…”

“…This is justified because it was found that the Fe III complex of ligand L2 (as the chloride salt) exhibits catalase-like activity in aqueous buffer at pH 7 similar to that of 1. [30] We therefore selected Fe III complex 2 as a model in which a hydroxide anion occupies the proximal coordination site and thereby renders the complex uncharged. Besides mimicking the tyrosinate of native catalase, [32] coordination of a hydroxide ion seems to be realistic with respect to the observed catalytic action of 1.…”

OO Bond Formation Mediated by a Hexanuclear Iron Complex Supported on a Stannoxane Core

“…A major doublet with an isomer shift d = 1.10 mm s À1 and quadrupole splitting DE Q = 2.80 mm s À1 in the zero-field Mçssbauer spectrum of 2 demonstrates equivalent high-spin Fe II centers in a five coordinate geometry. [10,11] Based on the DFT calculation [12] the most stable coordination environment of the Fe II centers in 2 corresponds to a distorted trigonal bipyramidal geometry (Scheme 2, Figure S5 and Table S1) with two equatorial triflate ligands. The ligand L1 acts as a tridentate ligand, with one of the nitrogens of the 5-membered imidazolidinyl ring remaining out of the coordination sphere.…”