USING EXPERIMENTAL AND COMPUTATIONAL METHODS TO EVALUATE ANION COORDINATING ABILITY AND Z-SELECTIVE ISOMERIZATION OF TERMINAL ALKENES USING TUNGSTEN

Abstract

There are two main focuses to this thesis. The first is to explore interactions of weakly coordinating anions, specifically observing how they affect the cation they are paired with. Choosing the right anion for a reaction could be crucial to get high yields or good selectivity, so having an anion that could be tuned to different coordination strengths is useful. The previously synthesized imidazolyl phenyl (IMP) anions can be made with various functionalities that vary the coordination strength. The IMP anions, which have only been paired and studied with metal cations, were paired with triethylammonium to be able to investigate the interactions in an organic ion pair. IR and NMR spectroscopy as well as computational methods were used to explore the steric and electronic interactions between the ions. IR spectroscopy was used to attempt to see the triethylammonium N-H stretching frequency shift depending on which anion was paired with it. Similarly, 1H NMR spectroscopy saw a shift in the triethylammonium CH2 resonances when changing the coordinating anion. A scale was made using this data to see which anions are weakly coordinating anions and which are strongly coordinating anions. Computational approaches were used to supplement the experimental results we obtained. DFT calculations were done to calculate the energy of interaction between the anion and cation and compared to the experimental scale obtained using NMR spectroscopy. Using the web program SambVca 2.1, % buried volume calculations were performed to gain an understanding of the steric volume each anion had around the position it coordinated to the cation. Lastly, NBO calculations were performed to determine the charge on each atom of the ions before and after coordination to investigate how coordination affected the individual charges at each atom. The second focus is to find new catalysts for the Z-selective isomerization of terminal alkenes. Previous members in the Dobereiner lab have done extensive research on molybdenum catalyzed isomerization of terminal alkenes. It was proposed that the PCy3 ligand plays a large role in inducing Z-selectivity because of its large size. To test this, a smaller ligand, specifically 1,3-dimethyl-N-heterocyclic carbene, was used in place of PCy3. DFT experiments showed the energy barrier for the E mechanism is lower than the Z mechanism. Using the molybdenum catalyst as inspiration, DFT was used to test the same complex, but replacing Mo with W. The computational results suggest that the tungsten catalyst would also be Z-selective and would have better conversion because it has lower energy barriers compared to the Mo complex. These results are promising, but synthesizing the complex to test it experimentally would help be certain that the tungsten catalyst will perform better.