THEORETICAL EVALUATION OF PYRAZOLYL-BASED IRON, COBALT, NICKEL AND PALLADIUM COMPLEXES AS ETHYLENE OLIGOMERIZATION CATALYSTS

Abstract

encouragement. iv ABSTRACT Transition metal complexes in catalysis have gained a lot of interest for a decade now. Because of the widespread importance and applications of transition metal complexes in catalytic reactions of industrial importance, it is desirable to theoretically design and evaluate their catalytic activities. This dissertation involves a series of modeling investigations that have been designed to probe the influence of the electronic structure of the metal cation, the nature of the ligand, as well as the effect of chelation and steric interactions on the activity of the catalysts for ethylene oligomerization reactions. A series of transition metal cations, Fe2+, Co2+, Ni2+, and Pd2+ chelated by 2-(3, 5-dimethyl-pyrazol-1-yl)-ethanol and 1-(2-chloro-ethyl)-3, 5-dimethyl-1H-pyrazole ligands have been investigated as ethylene oligomerization catalysts. Six chelates in total were studied: Fe1, Co1, Ni1, Pd1, Co2, and Ni2. Electronic structure calculations were employed to determine stable low-energy structures, energetics, and chemical reactivity’s of the transition metal complexes. Density Functional Theory (DFT) was employed to gain a conceptual understanding of the structures of the initial metal complexes in solution that are likely to be pre-catalysts in the ethylene oligomerization reaction. A further goal was to analyze the electronic factors and ligand effects that impact on the chemical reactivity order of the six metal complexes within the context of DFT simulations. Theoretical studies used B3LYP and B3PW91 DFT methods and LanL2DZ basis set for metal atoms and 6-311+G (2d, p) basis set for all the remaining atoms. Theoretical studies of the Ni1 and Ni2 metal complexes were compared to the previous experimental studies of analogous complexes. The ground-state structures of all the six metal complexes show that the ligands bind in a fashion consistent with the simple valence shell electron pair repulsion model, where minor distortions from the idealized geometries are correlated with the structure of the ligands and more significant distortions with the valence electron configuration of the metal cation. The nature of the metal cation and ligand moiety had a discernible impact on the global chemical reactivity’s parameters of the six metal complexes. The chemical potential, the electrophilicity indices, the charge, and the electronegativity values suggest that Ni1 is the best catalyst in the series. Comparison between theory and experiment further confirm that Ni1 is indeed the best catalyst of the six catalysts studied for ethylene oligomerization reaction. Both electronic and steric factors correlate with the metal-ligand distances, and appear to be significant factors underpinning the reactivity of the six metal complexes. Thus, the results showcase the importance of using theoretical simulations to design catalysts rapidly and the ability to develop potentially active catalysts for ethylene oligomerization reaction through a thorough analysis of the global chemical reactivity parameters.