Structural and functional investigations on diiron complexes: Catalase-like activity and mechanistic studies on the formation of (mu-peroxo)diiron(III) adducts

The new diiron complex [Fe-2(tbpo){O2As(CH3)(2)}(CH3O)(CH3OH)](ClO4)(3) . 5 CH3OH . 2 H2O (1) containing a (mu-alkoxo)(mu-dimethylarsinato)diiron(III) core was synthesized using the heptadentate ligand N,N,N',N'-Tetrakis(2-be nzimidazolylmethyl)-1,3-diamino-2-propanol (Htbpo). The complex was charact erized structurally by X-ray crystallography. 1 reproduces the coordination mode and the stoichiometry of the proposed purple acid phosphatase-arsenat e inhibitor complex. More importantly. 1 is a good functional model for the activation of small molecules, since the solvent molecule in the coordinat ion sphere of each iron ion can be substituted very easily by a small subst rate molecule. This is confirmed by the comparatively high pH-dependent cat alase-like activity of 1. In order to study the influence of the cacodylate bridge on the formation of the metastable adduct with hydrogen peroxide, t he analogous hydroxo-bridged complex [Fe-2(tbpo)(OH)(NO3)(2)](NO3)(2) . CH3 OH . 2 H2O (2) was employed. The reactions of 1 and 2 with H2O2 were studie d as a function of [H2O2], pH, temperature, and pressure, and the kinetic r esults including the activation parameters are reported. In the case of com pound 2 the reaction proceeds in one step, and the observed first order rat e constant, k(obs), shows a linear dependence on the hydrogen peroxide conc entration with a zero intercept. For complex 1 the kinetic traces could be fitted to two exponential functions. One of the observed pseudo-first-order rate constants, k(obs1), exhibits a linear dependence on the hydrogen pero xide concentration with a zero intercept, whereas the other rate constant. k(obs2). was independent of the hydrogen peroxide concentration. A mechanis tic interpretation is presented.