Alkaline earth organometallic compounds in homogeneous catalysis: Synthesis, characterization and catalytic activity of calcium and magnesium complexes
Homogeneous catalysis has been developed mainly through the use of transition metal complexes. However, transition metal catalysts, often highly toxic, are becoming increasingly rare and consequently more expensive. Therefore, the search for sustainable alternatives is nowadays of great importance. Among early main group metals, alkaline earth metals, especially calcium, seem particularly beneficial from an ecological and economical point of view. Calcium is essentially free of toxicity, very cheap and the fifth most abundant element in the earth's crust. This thesis describes the syntheses and structures of novel calcium, magnesium and zinc complexes with the objective of studying their application in homogeneous catalysis. Although alkaline earth metal complexes have been applied in asymmetric catalysis, their applications are limited and multiple possibilities are still to be explored. In that context, chiral phosphoramidites and secondary phosphine oxides are investigated for group 2 based asymmetric catalysis. Another potential application of alkaline earth metal complexes is as catalysts in the transformation of CO2. Hence, the reactivity of phosphorus/magnesium compounds as "frustrated" Lewis acid Lewis base pairs in CO2 activation is studied. In alkaline earth metal based catalysis, the influence of the metal is not clearly established. In order to evaluate its role, group 2 catalysts are replaced by comparable metal free catalysts and tested in intramolecular and intermolecular hydroamination reactions. Based on the results, two group 2 catalytic systems for intramolecular hydroamination are presented. They question the traditional though of group 2 catalytic systems and overcome the great challenge in homogeneous group 2 catalysis, i.e the Schlenk equilibria.