|
On the formation and reactivity of multinuclear silsesquioxane metal complexes/ by Robbie W.J.M. Hanssen. - Eindhoven : Technische Universiteit Eindhoven, 2003.Proefschrift. - ISBN 90-386-2924-9 NUR 913 Trefwoorden: anorganische chemie / coördinatiechemie / silsesquioxanen / overgangsmetaalcomplexen / dichtheidsfunctionaaltheorie ; DFT Subject headings: inorganic chemistry / coordination chemistry / silsesquioxanes / transition metal complexes / density functional theory ; DFT |
Full thesis available (PDF, 1.15 MB)
Silsesquioxane chemistry has taken a tremendous flight over the past decade. New synthetic approaches were developed; new complexes based on elements throughout the periodic table were synthesized, and were applied in catalytic processes such as alkene epoxidation, Oppenauer oxidation, alkene polymerization, Diels-Alders reactions and alkene metathesis. The use of silsesquioxane ligands as models for silica surface disclosed intimate knowledge of the reactions on the surface of this much-used support. New methods were developed to incorporate silsesquioxanes into new materials. Encapsulation in siloxane polymers provided precision supports for catalysts and controlled calcination produced materials with well-defined metal oxide particles.
A detailed knowledge of the synthetic part towards catalytically active complexes is needed in order to make the appropriate complexes. This thesis is concerned with the interaction of main group and transition metal precursors with silsesquioxane trisilanol ligands in order to study the coordination chemistry and to synthesize new catalysts based on silsesquioxane ligands. In the first chapter, concepts and literature of silsesquioxane ligands and their metal complexes are reviewed. The main conclusions from the literature to date (February 2003) are (i) that silsesquioxane are the best available models for silica surfaces, (ii) that the ligands can be used to bind to a large variety of metals, be it main group, early or late transition metals, and (iii) that the application of silsesquioxane metal complexes in catalysis is not yet widespread, but promising in some areas.
The second chapter describes the reactivity of silsesquioxane trisilanol ligands with Grignard and dialkyl magnesium reagents. Reaction of Grignard reagents with trisilanol ligands leads to the formation of a new tetranuclear di-silsesquioxane structure. The formation of this new material is investigated by the use of model reactions that represent phases in the synthesis steps of the complex. It was found that these model reactions fit well to the proposed three-step model. The apparent electron-withdrawing character of the silsesquioxide ligand was noticed in the very short length of the magnesium-chloride bond, and was confirmed by DFT computations. The strength of the magnesium chloride bond was also of influence in the study of the reactivity of the complexes towards protic and metallated substances like alcohols, alkoxides and amides. No reaction could be determined. Only the use of silver triflate resulted in the formation of silver chloride and a degradated silsesquioxane framework. Activation of the silicon-oxygen-magnesium bond proved to be easier, and could be used to prepare silsesquioxane complexes with other metals than magnesium.
Chapter three deals with the investigation of the interaction of silsesquioxane trisilanol ligands with alkyl zinc precursors. Reaction of these ligands with dialkyl zinc reagents in non-coordinating solvents yielded planar three-coordinate silsesquioxide zinc alkyl complexes with the same tetranuclear motif as found in the second chapter. The formation of this type of complexes was independent of the silsesquioxane side group, contrary to the magnesium type compounds. The polar character of the carbon-zinc moiety and the low coordination of zinc atoms made the complexes very reactive towards coordinating and polar substances. Addition of methyl zinc chloride, a zinc congener of a Grignard reagent, to a solution of trisilanol in THF formed the zinc alternative to the magnesium complexes in the second chapter. The zinc complexes were tested in the polymerization of lactide and were found to be active for a very short time before they precipitated from solution.
In the fourth chapter, the knowledge of formation mechanisms gained in the previous two chapters was applied to the synthesis of heterobimetallic compounds. The formation of heterobimetallic silsesquioxane complexes proved to be dependent on the oxophilicity of both metal compounds. Oxophilic ('hard') metals could easily replace softer metals to gain a more oxygen-rich environment. In case of zinc/magnesium, this led to exchange of the metal sites; in the case of aluminum and titanium, it led to complete expulsion of the softer metal. A mixed-metal magnesium-vanadium complex was tested in the catalytic polymerization of ethene and found to be reasonably active in producing high-molecular weight polyethene. However, it was found to be sensitive toward metal leaching depending on the temperature and aluminum activor.
Based on the successful application of vanadium silsesquioxane chemistry polymerization of ethene in the previous chapter, chapter five is involved with trivalent vanadium and titanium complexes of silyl-substituted silsesquioxane disilanol ligands. The coordination behavior of solvents like THF and pyridine is studied with UV-Vis and qualitatively confirmed by DFT computations. The position of the UV-Vis absorption bands and the relative stability of adducts match well with the computational results. Polymerization of ethene to high molecular weight polyethene was possible with both vanadium and titanium complexes, although the activity of the titanium catalyst was very low. Under the conditions studied, the vanadium catalysts were very active, but only for a few minutes A possible way of self-immobilizing catalysts by incorporating polymerizable groups in the ligand was proposed and tested, but the current systems could not be used with this approach. A tentative catalytic cycle was described and supported by DFT computations. The energetics of the reactions were found to be significantly lower than literature values, possibly explaining the low stability of the catalyst at reaction conditions.
For more information, ">contact the author