Structural modifications and their effects on the catalytic behavior of metal surfaces and organometallic complexes: Theoretical studies of oxygen adsorption on silver surfaces and aziridination catalysis by iron-tetracarbene complexes
Date of Award
Doctor of Philosophy
Robert Hinde, Tessa Calhoun, David Keffer
Great interest exists in efficient synthesis of epoxides and aziridines, molecules containing three-membered rings with oxygen and nitrogen atoms, respectively, due to their use in the chemical and pharmaceutical industries. Epoxides, such as ethylene oxide, can be partially oxidized to form ethylene glycol, a feedstock in many consumer products. Typically, ethylene oxide is made over a silver surface; however, there remain many questions as to the structure of the active catalyst. In this work, the interaction of atomic oxygen with two facets of silver has been investigated with molecular dynamics simulations and density functional theory (DFT) calculations. These studies revealed that oxygen interacts with Ag(111) and Ag(110) very differently as a function of coverage. The occupation of nearest-neighbor sites on the Ag(111) surface significantly decreases adsorption energies which is not observed on Ag(110). This result has been used to construct a simple adsorption model, which can predict the adsorption energy of a given arrangement of oxygen atoms if the number and types of neighbor interactions are known. Additionally, aziridines have been shown to possess antibiotic and anticancer properties. Due to enhanced stereochemical control, many aziridines are synthesized using organometallic catalysts. Our work investigates the effect of structural modifications on the reaction mechanism of two generations of iron-tetracarbene catalysts using DFT calculations. This study reveals both catalysts proceed through a radical intermediate, making them susceptible to intramolecular rotation, potentially reducing their stereochemical retention. We also show that the undesired metallotetrazene product can be destabilized by use of bulkier azides, further improving the atom-economy of the reaction. We then investigated further modifications of the second-generation catalyst, adding chiral wings in an effort to improve selectivity of a given enantiomer. This work reveals that though initially promising, these macrocycles do not catalyze aziridine through the same mechanism as the achiral catalysts, if at all. In conclusion, our work reveals the complex role that structure plays in both homogeneous and heterogeneous catalytic systems.
Isbill, Sara Beth, "Structural modifications and their effects on the catalytic behavior of metal surfaces and organometallic complexes: Theoretical studies of oxygen adsorption on silver surfaces and aziridination catalysis by iron-tetracarbene complexes. " PhD diss., University of Tennessee, 2019.