Abstract
The high activity and selectivity of enzymes have inspired many scientists to study the structure and working mechanism of bio-molecular complexes. Also in the catalysis community this subject is of topical interest, as it may provide inspiration for the development of a new generation of bio-inspired catalyst materials. Functionalization of
... read more
inorganic substrates, such as zeolites, with transition metal ion (TMI) complexes offers the possibility to design new materials with a high potential to serve as working enzyme mimics. An essential step for the further development of these kinds of materials is obtaining fundamental knowledge of the chemistry involved in the making of these mimics and an understanding of the factors that influence the molecular structure of the resulting materials.
These challenging objectives have been the main goal of the research described in this thesis. To this end complexes of Histidine (His) coordinated to Cu2+ have been selected as model TMI-complexes for introduction into the pore system of zeolite Y and to serve as a mimic for the active site of the enzyme galactose oxidase. In order to understand and describe the host-guest chemistry of these complexes in the pores of zeolite Y and to make the analogy with enzymatic systems a detailed study of the His ligand was conducted first. As a logical next step the coordination and geometrical structure of the Cu2+/His complexes in aqueous environment was studied, followed by the characterization of the complexes after immobilization in the inorganic zeolite matrix.
A large set of accurate chemical and physical data is inevitable to (1) establish the coordination geometry of the TMI-complexes before and after immobilization and (2) to understand the chemistry of the guest complexes within the pores of the zeolite host material. For the collection of these data a diversity of characterization methods has been used, either as stand-alone techniques or in an integrated set-up. IR, Raman, UV/Vis, ESR and XAFS spectroscopy have been successfully applied to obtain the desired information.
Summarizing the results of this thesis, the following conclusions can be drawn:
-The multiple spectroscopic techniques approach is highly beneficial for unambiguously elucidating the molecular structure of TMI-complexes in great detail, in solution as well as inside a zeolite support.
-Inorganic microporous substrates, such as zeolite Y, can be functionalized successfully by encaging complexes of transition metal ions with organic ligands via the method of ion exchange. The immobilization process is not only driven by the chemical properties of the guest complex, but the zeolite host material also actively participates in the coordination chemistry of the complex.
-Zeolite supports can be used to stabilize Cu2+/His complexes in two different coordination geometries, neither one of them identical to the complexes present in the initial exchange solution. The first one is very similar to the complex in aqueous environment at pH = 5. In the second one the zeolite support is involved in the coordination, i.e. the zeolite material acts as a monodentate ligand. This complex can be envisaged as a structural mimic of Galactose Oxidase, since it has a very similar first coordination environment around the Cu2+ cation.
-Synchrotron radiation effects can be used to evaluate the redox potential of metal complexes in solutions and solids if water is present.
show less
