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Synthesis and Study of Long Pyridyl-Endcapped Oligoynes on the Way to Carbyne
- Author / Creator
- Gao, Yueze
The versatility of elemental carbon is revealed not only in the formation of diverse structures of organic compounds but also in its all-carbon forms, carbon allotropes, which feature unique properties. During the last 35 years, the discovery and synthesis of new carbon allotropes have made it a reality to uncover the unique properties of these new forms of carbon. The sp-carbon allotrope, commonly referred to as carbyne, however, is effectively absent from this list, and intense discussion continues over its very existence. Throughout the past 60 years, oligoynes and polyynes have been targeted as model systems toward carbyne.
This thesis deals with the synthesis, characterization, and properties of pyridyl-endcapped oligo-/polyynes. Chapter 1 introduces recent developments in carbon-rich compounds. Then, this chapter reviews the state-of-the-art synthetic methods for assembling oligo-/polyynes, focusing on the formation of “long” derivatives that are at least the length of an octayne. The synthesis of shorter oligoynes is also briefly discussed with the introduction of commonly used methods, such as the Hay, Eglinton–Galbraith, and Cadiot–Chodkiewicz coupling reactions. Finally, the properties of oligo-/polyynes are summarized and discussed, with the purpose of providing insights into understanding and predicting the characteristics of carbyne.
Chapter 2 focuses on the development of pyridyl-endcapped oligoynes (PEOs), and the fundamental goal has been the use of PEOs as a model to study carbyne. The chemical stabilization of oligoynes by sterically encumbered endgroups, particularly the 3,5-bis(3,5-di-tert-butylphenyl)pyridyl group, is key to assemble an extended series of stable oligoynes. Versatile synthetic strategies toward elongation of the sp-carbon chains have been developed, resulting in a series of stable PEOs that consists of as many as 48 contiguous sp-carbons. Spectroscopic and X-ray crystallographic analyses show that endgroups influence the properties of oligoyne derivatives, but this effect diminishes as length increases toward the polyyne/carbyne limit. For instance, with UV-vis spectroscopy, molecular symmetry clearly documents the evolution of characteristics from oligoynes to polyynes. The combined experimental data are used to refine predictions for the D∞h structure of carbyne.
When oligo- and polyynes are formed under Hay conditions, unusual byproducts resulting from the loss of alkyne units from the desired oligo-/polyyne are sometimes observed, particularly in the synthesis of longer derivatives. Chapter 3 deals with a mechanistic study of alkyne loss under the typical oxidative coupling conditions. A pentayne precursor with 13C labelling has been designed and synthesized, and its subsequent coupling reactions are examined. The mass spectrometric and NMR spectroscopic analyses support that the terminal alkyne unit is being removed from the carbon chain during the course of the oxidative coupling reaction. Two plausible mechanisms are provided suggesting pathways that might be responsible for the C–C bond cleavage. Finally, suggestions for methods to avoid/suppress the loss of alkyne unit are discussed.
Chapter 4 deals with the synthesis and characterization of a series of platinum complexes coordinated to the terminal pyridyl groups of PEOs. This chapter provides preliminary results in exploring the relationship between axial chirality and the helical frontier molecular orbitals of oligoynes.
Chapter 5 presents the hierarchical synthesis of three porphyrin and four bisporphyrin derivatives. This strategy relies on the incorporation linkers based on azo moieties appended with pyridyl and/or acetylenic groups that facilitate axial coordination to Ga- and Ru-metalloporphyrins. These porphyrinic systems allow for a quantitative analysis of the effects of diamagnetic anisotropy (DA) on the ligand, arising from the porphyrin, using 1H NMR spectroscopic and X-ray crystallographic analyses. A simple power-law relationship between the proton chemical shift of ligand protons and distance from the porphyrin core is experimentally outlined, which confirms previous theoretical predictions and shows that the limit of DA is about 2 nm. Photophysical properties of the azo-linked porphyrins are analyzed by UV-vis spectroscopy, showing that appreciable cis-trans isomerization is not observed for azo ligands bound only to Ga-porphyrins. Incorporation of Ru-porphyrins to an azo ligand facilitates photoswitching behavior, but the process faces competition from decarbonylation of the Ru-porphyrin, and appreciable switching is only documented for GaL1Ru.
Chapter 6 gives a summary and outlook for the topics discussed in this thesis, including a discussion of some future directions. Finally, Chapter 7 provides the experimental details and supporting spectra discussed in this thesis.
- Graduation date
- Spring 2021
- Type of Item
- Doctor of Philosophy
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.