Towards sustainable materials: earth abundant elements for transformations of CO₂ and epoxides

Andrea, Kori A. (2021) Towards sustainable materials: earth abundant elements for transformations of CO₂ and epoxides. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The utilization of carbon dioxide (CO₂) as a renewable C-1 feedstock has received significant attention in recent years. In particular the coupling and/or polymerization of epoxides and CO₂ to yield cyclic and/or polycarbonates is a growing area of research leading to the production of sustainable materials. Due to the high thermodynamic stability of CO₂ these reactions typically proceed in the presence of a metal catalyst, while in the recent literature organocatalysts have found promising results. In this thesis, both iron and boron containing catalytic systems are reported that have shown excellent activity for these reactions. In Chapter 2, a series of iron(III) chloride and iron(III) μ-oxo compounds supported by tetradentate amino-bis(phenolate) ligands containing a homopiperazinyl backbone were prepared and characterized by electronic absorption spectroscopy, magnetic moment measurement, and MALDI-TOF mass spectrometry. We provide evidence that an epoxide deoxygenation step occurs when employing monometallic iron(III) chlorido species as catalysts. This affords the corresponding μ-oxo compounds which can then enter their own catalytic cycle. Deoxygenation of epoxides during their catalytic reactions with carbon dioxide is frequently overlooked and should be considered as an additional mechanistic pathway when investigating catalysts. Through extensive studies of the iron systems reported herein, we have shown a structure/geometry of the catalyst and product selectivity relationship. This study (reported in Chapter 3) demonstrated that the highly modifiable aminophenolate ligands can be tailored to yield iron complexes for both CO₂/epoxide coupling and ring-opening copolymerization activity. Metal-free catalysts have gained interest in the recent years as alternatives for these transformations but is still in its infancy compared to transition metal-based systems. In Chapter 4, we have shown that arylboranes can be used as catalysts in these transformations to produce either cyclic or polycarbonates. Kinetic studies revealed a process that was first-order in all reagents with the exception of CO₂, where an inverse dependence was shown. Building upon this report and taking into account the extensive research focused on Frustrated Lewis pairs and their ability to catalyze a range of transformations such as hydrosilylations I chose to combine these two ideas. Via assisted tandem catalysis BPh₃ could first catalyze the copolymerization of epoxides and CO₂ to give polycarbonates and then with the addition of a silane could catalyse the hydrosilylation of these materials. This work is reported in Chapter 5. Finally, Chapter 6 reports the same arylborane systems as a catalyst for the block copolymerization of epoxides, CO₂ and anhydrides in a controlled fashion. Further, by switching from BPh₃ to the more Lewis acidic BCF, the carbonate block in the obtained polymer could be selectivity degraded to the corresponding cyclic carbonate. This is the first report of an arylborane for both the polymerization of epoxides, anhydrides, and CO₂ as well as the first for the selective depolymerization of these materials.

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