Roles of Cation-Macrocycle Interactions in Modulating the Reactivity of Transition Metal Pincer-Crown Ether Complexes
Public Deposited- Last Modified
- March 21, 2019
- Creator
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Smith, Jacob
- Affiliation: College of Arts and Sciences, Department of Chemistry
- Abstract
- Pincer-crown ether ligands were designed to exhibit hemilability and also support cation- macrocycle interactions. These ligands feature strong phosphine and phenyl donors, and a pendent aza-crown ether group that provides hemilabile donors that can bind to the transition metal center and serve as a docking site for cations. Cation-macrocycle interactions can be exploited to directly control the primary coordination sphere of these complexes leading to control over ligand substitution reactions and catalysis. When the macrocycle acts simply as a docking site for cations, bound cation-substrate interactions can be harnessed to modulate reactivity. In chapter 2, nickel catalysts supported by diethylamine- or aza-crown ether-containing aminophosphinite (NCOP) pincer ligands catalyze the insertion of benzaldehyde into a C−H bond of acetonitrile. Neutral tert-butoxide precatalysts are active without any added base and give good yields of product after 24 h, while the cationic precatalysts require a base cocatalyst and operate much more slowly. In situ spectroscopic studies identified several intermediates and a common deactivated species. The deactivated cationic species is inactive under standard base-free conditions, but catalysis can be reinitiated by the addition of base. In Chapter 3, the thermochemistry of cation−macrocycle interactions in nickel pincer complexes bearing a hemilabile aza-15-crown-5 or aza-18-crown-6 macrocycle is investigated and applied to cation-controlled reversible ligand binding. Cation−crown interactions were examined in the solid-state and in solution (dichloromethane, and acetonitrile). Tridentate-bound pincer complexes bind cations more than 100,000 times more strongly than the tetradentate counterparts with a crown ether oxygen donating to nickel. The newfound thermodynamic insight guided the development of in situ switchable ligand binding and release at nickel using cations. In Chapter 4, the impact of post-macrocyclization modification of 1-aza-15-crown 5 through synthetic organic and organometallic tuning is discussed. Binding affinities for Li+ and Na+ salts are reduced substantially moving from the simple organic macrocycle to an organometallic pincer-crown ether. The selectivity for Li+ over Na+ increases dramatically, by as much as 30-fold. The impact of transition metal and ligand substitution at the metal center significantly impacts Li+ binding strength. Preliminary heteroditopic binding of LiCl by an iridium pincer-crown ether is also demonstrated.
- Date of publication
- August 2018
- Keyword
- DOI
- Resource type
- Rights statement
- In Copyright
- Advisor
- Leibfarth, Frank
- Miller, Alexander
- Schauer, Cynthia
- Johnson, Jeffrey
- Templeton, Joseph
- Degree
- Doctor of Philosophy
- Degree granting institution
- University of North Carolina at Chapel Hill Graduate School
- Graduation year
- 2018
- Language
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