In this work, model complexes for iron-containing enzymes like methane monooxygenase were investigated. The most important part is the analysis of the formation of so called oxygen-adducts . In the first part of this thesis synthetic and reaction kinetics investigations on iron complexes of the ligand N-benzyl-N,N´,N´-tris(2-pyridylmethyl)ethylendiamin (bztpen) were carried out.Before the beginning of this work it had been known, that iron(II) and iron(III) complexes of this ligand react with hydrogen peroxide to form hydroperoxido- and peroxido complexes. In the past it was pointed out that the iron oxido complexes could oxidize relatively inert alkanes like cyclohexane. Due to this fact, kinetic research on these complexes seemed to be particularly interesting in order to gain a more detailed knowledge of the reaction mechanism. In the past, there had already been investigations of the temperature dependency of the iron(III) bztpen complex in reaction with hydrogen peroxide in the Schindler group. They had the result that reaction proceeds with an associative character. This means that the reaction has a seven-fold coordinated transition state. This was justified with the determination of a negative activation entropy. (DeltaS = -72 ± 8 Jmol-1K-1). These results were reproduced successfully in this thesis.However, it is known that temperature dependent kinetic investigations often have a large error in the allocation of the activation entropy. Due to this fact, high pressure experiments for determination of activation volumes were performed at the van Eldik group (University of Erlangen). Activation volumes can be determined more exactly compared to activation entropies, yet they provide the same details of reaction mechanisms. Using high-pressure stopped-flow -experiments it was observed that the reaction is not dependent on the pressure. The activation volume is close to zero. This means that the reaction proceeds via a mere interchange mechanism in with the ligands substitute simultaneously. For the reaction with hydrogen peroxide including the additional investigations mentioned the mechanism can be depicted as follows:Another goal of this thesis was the structural characterization of the formed hydroperoxido and peroxido complex. Unfortunately, many experiments on crystallisation of these compounds were unsuccessful. This was not unexpected, as such complexes are relatively unstable, and other working groups had not been successful in doing so either. Additional experiments to isolate an iron superoxido complex from iron(II) bztpen and dioxygen had no results. In this context, an iron-nitric oxide complex could be isolated and characterized by X-ray crystallography, a compound which is also not easy to isolate. Surprisingly, the formation of this complex could be monitored by stopped-flow -measurements spectroscopically. Due to its electronic character, nitric oxide is a good model compound for oxygen adducts and supplies useful structural informations.It is well known that cobalt complexes form peroxido complexes very easily. Based on this fact, in this work it was tried to investigate the cobalt-bztpen complexes additional to the analogous iron-bztpen complexes. Unfortunately, neither cobalt hydroperoxido nor cobalt peroxido-complex could be isolated and crystallograpically characterized. Even UV/ Vis spectroscopy and mass spectrometric investigations did not have any unambiguous results. Still several new cobalt(II) complexes with the ligands bztpen and metpen could be structurally characterized.In this work, the new bridging ligand 1,3-bis(N,N,N´-tris(2-pyridylmethyl-))diaminoethyl-benzol (bz-b-tpen) could be synthesized. The lifetime of the iron(III) hydroperoxido and peroxido complexes could not been extended by this ligand as expected by secondary interactions (chapter 5). The formation of these complexes could be monitored by UV/ Vis-spectroscopy. However, no stabilization of the intermediates was observed.The ligand bztpen can be derivated and synthesized from the tripodal ligand (2-amino-ethyl)-bis(2-pyridylmethyl)amine (uns-penp). This ligand and its derivatives themselves are also very interesting ligands for the coordination chemistry of iron.Based on former investigations of the Schindler group, who had already worked with of these compounds as model complexes for catechol dioxygenases, several new iron(II)/ iron(III) and the analogous cobalt(II)/ cobalt(III) complexes could be synthesized and characterized in this work. These compounds are described in detail in chapter 6 and 7. Particularly interesting is the crystal structure of the complex [Fe2(acetyl-uns-penp)2(OH)](ClO4)2. Concluding the facts this work contributes important knowledge for the understanding of the reactions of iron complexes with hydrogen peroxide as well as with dioxygen. It demonstrates that such compounds can be used in oxidation catalysis in the future.
Verknüpfung zu Publikationen oder weiteren Datensätzen
