Graduate Program

Chemistry

Degree Name

Master of Science (MS)

Semester of Degree Completion

Summer 2020

Thesis Director

Radu F. Semeniuc

Thesis Committee Member

Daniel J. Sheeran

Thesis Committee Member

Zhiqing Yan

Thesis Committee Member

Michael W. Beck

Abstract

Mechanically Interlocked Molecules (MIMs) are interlocked molecular architectures, characterized by the fact that they are linked through a mechanical bond. Catenanes, rotaxanes, molecular knots, and Borromean rings are typical examples for these molecules. Since MIMs are not connected by covalent bonds, hydrogen bonding, π- π stacking, and/or electrostatic interactions help bring the components together. Dialkyl-ammonium centers, bis(pyridinium)ethane dications, viologen species, and naphthalenediimide π-deficient groups are well known as recognition motifs for crown ethers. In the research described here, we have synthesized several pseudorotaxanes and their tricyclohexyltin metallic derivatives, based on benzobis(imidazolium) (BBI) salts as axles, in combination with the 15DN38C10 crown ether acting as a wheel. These species are characterized by the fact that the benzobis(imidazolium) core is trapped inside the cavity of the crown ether and the four arms of the axle are extending away directly from the cavity of the crown. This produces a straddled orientation of the wheel onto the axle. We named these species starburst [24]pseudorotaxanes, where the number 2 represents the number of components of the pseudorotaxane, and the number 4 indicates the number of the arms protruding from the cavity of the crown. 1H-NMR studies showed that the synthesized pseudorotaxanes exhibit a fast exchange on the NMR timescale. In order to calculate the association constant Ka, we performed NMR titrations in acetone-d6 at 293 K. The chemical shifts of the protons of crown ether were monitored and association constant values were determined using the Bindfit software. The pseudorotaxane based on the metal complex of the axle containing two donor arms showed the highest association constant, followed by the ditopic carboxylate species, while the tetratopic carboxylate species showed the smallest Ka value, in agreement with the hydrogen bonding possibilities of each species. The axle containing four tricyclohexyltin iv centers does not form a pseudorotaxane architecture, because the size of the metallic groups prevents the interaction between the axle and the crown. Solid-state studies performed on one of the prepared species showed that indeed these pseudorotaxanes have the predicted straddled structure. This architecture is held in place by hydrogen bonding, π-π stacking, and several [N – CH – N]+ ··· O ion – dipole interactions. In addition, all carboxylic groups on the BBI core are involved in multiple hydrogen bonding interactions, either among themselves or with solvent molecules, thus confirming our conclusions drawn from our studies on the association constants of the prepared species.

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