Analyzing reaction pathways for imine hydrogenation reactions catalyzed by alkaline earth metal catalysts using DFT

Student thesis: Master's Thesis

Abstract

In this thesis, the alkaline earth metal catalysed imine hydrogenation was investigated by means of density functional theory (DFT) calculations. This work was inspired by experimental observations on the use of group II metals and, in particular, Ca in catalytic reactions and ab initio simulations on the exact mechanism of such processes. The role of the metal was analysed by performing static DFT calculations to create the energy profile of the reaction pathways for Mg, Ca, Sr and Ba catalysts for which limited experimental data is available. In a first part, a suitable computational method was established for expanding the scope of this work towards the heavier elements, Sr and Ba. The computed trends in reaction energetics and transition state energies are in excellent agreement with the improved catalytic activity upon descending group II of the periodic table observed from experiments. Moreover, the turnover frequency computed through the energetic span model qualitatively confirmed these observations. Structural differences in the catalytic complexes were found to be the main driving force behind the enhanced reactivity of heavier alkaline earth metals. In particular, varying bite angles and atomic radii for the different alkaline earth metal catalyst complexes were shown to drastically influence the catalytic activity. In this regard, it was found that Mg distinguished itself from the heavier elements, being Ca, Sr and Ba, by its high activation barriers. Activation strain analyses were carried out on the Mg and Ca pathways in order to reveal the origin of the increased activation barriers for Mg compared to the heavier elements. These investigations quantitatively confirmed that the higher transition state barriers for the Mg pathway originate from unfavourable bite angles in the Mg-complexes and, especially, the hydride species. The larger deformation of bite angles translated in more destabilizing strain for the formation of the Mg transition state structures. On top of the activation strain analysis, energy decomposition analyses on the transition state structures of all four metals revealed that each transition state is determined by either more stabilizing electrostatic and orbital interactions or lower repulsive Pauli interactions. Interestingly, stronger repulsive interactions destabilize the Mg species, indicating that the steric congestion due to the small Mg centre impedes reaction kinetics. Finally, static DFT calculations on the calcium-mediated hydrogenation of a small selection of imines were performed to evaluate the role of the substrate in the whole catalytic process. The computed catalytic efficiency, however, did not consistently reflect trends in experimental reaction times, especially for non-conjugated imines, indicating that an in-depth and systematic investigation of the substrate and its substituents is needed in order to fully understand the selectivity of certain alkaline earth catalysts for reducing specific imines.
Date of Award3 Jul 2019
Original languageEnglish
Awarding Institution
  • Vrije Universiteit Brussel
SupervisorMercedes Alonso Giner (Promotor)

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