The halogen bond has recently risen in prominence as a non-covalent interaction for use in supramolecular chemistry, allowing for the rational design of materials, pharmaceuticals, and functional molecules. The occurrence of the σ-hole opposite to the C-X covalent bond (X = F, Cl, Br, I) renders the halogen bond a highly directional and tuneable interaction, offering desirable features to crystal engineers. The halogen bond can be divided into its two components: the halogen bond donor bearing the halogen atom, and the electron-rich halogen bond acceptor. In this thesis, we investigate the nature of the halogen bond, its role in supramolecular assembly and impact on the local dynamics, along with developing synthetic methods to prepare this class of materials. We begin by fully characterizing the halogen bond donor by using 35Cl ultra-wideline solid-state nuclear magnetic resonance (NMR) spectroscopy on a series of single-component chloronitriles exhibiting the C-Cl···N halogen bond. We then perform the first modern nuclear quadrupole resonance (NQR) investigations of the halogen bond, observing the 79/81Br and 127I nuclei in a series of cocrystals exhibiting the C-Br···N and C-I···N halogen bond, respectively. Computational results attribute the observed increases in the quadrupolar coupling constants (CQ) to a reduction in the carbon-halogen σ-bonding contribution to V33 and an increase in the lone-pair and core orbital contributions, providing the first model of the electronic changes occurring on the halogen bond donor upon the formation of the halogen bond. Attention is then turned on characterizing the halogen bond acceptor and its surrounding environment, beginning by investigating a solid-state NMR approach relying on the 19F nucleus to characterize perfluorinated cocrystals. This strategy has reduced analysis times from hours to minutes while providing higher sensitivity and resolution, with the resulting chemical shifts permitting the unambiguous identification of the halogen bond and allowing for the refinement of X-ray crystal structures. The halogen bond acceptor is then investigated in a series of isomorphous dimers exhibiting both the halogen bond and hydrogen bond in the C≡C-I···X-···H-N+ motif, revealing the halogen bond’s relative contribution to the electric field gradient increasing in the order of Cl- > Br- > I-, contrasting the contributions of the hydrogen bond. We then explore the impact of the halogen bond on the surrounding environment, using the rotating methyl groups of 2,3,5,6-tetramethylpyrazine as a model. Upon the introduction of a halogen bond, we observe a reduction in the rotational energy barrier of 56% on average, overshadowing the 36% reduction observed in the hydrogen bonded cocrystals. This is the first instance of the halogen bond directly catalyzing the local dynamics, coining the term “dynamics catalyst”. These results provide an effective strategy of enhancing the dynamics in molecular systems, such as molecular machines, supramolecular catalyst, as well as correcting the faulty dynamics encountered in diseased proteins. The role of halogen bonding in crystal engineering is then explored, reporting the first supramolecular triangle, a series of discrete charged dimers, and supramolecular architectures built from 1,3,5-tri(iodoethynyl)-2,4,6-trifluorobenzene, with the potential of creating fully organic porous structures for gas absorption. Mechanochemistry is then investigated as a synthetic method, allowing for the preparation of cocrystals featuring 3-iodoethynylbenzoic acid as the donor, with the resulting structures exhibiting concurrent halogen and hydrogen bonding. Mechanochemical ball milling is shown to reduce preparation times of powdered cocrystals from days to a single hour, while using a fraction of the organic solvent. Lastly, we pioneer cosublimation as a solvent-free synthetic technique for rapidly preparing halogen bonded cocrystals, yielding quality single crystals within a few hours, and a microcrystalline product within 15 minutes. Among its advantages, cosublimation offers a significant acceleration of discovery, while eliminating the environmental footprint associated with conventional synthetic methods.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/39233 |
| Date | 24 May 2019 |
| Creators | Szell, Patrick |
| Contributors | Bryce, David |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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