Date of Award

Spring 2017

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

First Advisor

Gonghu Li

Second Advisor

Howard R. Mayne

Third Advisor

Christine A. Caputo

Abstract

Carbon dioxide (CO2) is a known greenhouse gas and in recent years has been identified as one of the primary contributors to changes in global climate. Nature utilizes CO2 through photosynthesis in which the plants harness energy from the sun to extract electrons from water to convert CO2 into sugars to fuel cellular activities. Using this as inspiration, our research aims to photochemically convert CO2 to higher value products by designing appropriate catalytic systems. Diimine-tricarbonyl rhenium compounds have demonstrated excellent activity in photo and electrochemical CO2 reduction. A low-energy pathway has been postulated in which Re(I)-based catalysts mediate CO2 reduction via binuclear interactions. However, limited experimental evidence has been reported. We aim to design innovative photocatalytic systems based on these Re(I) compounds and elucidate mechanistic pathways for CO2 reduction involving dimeric interactions of the transition metal catalysts.

We have designed several approaches, including homogenous molecular templating, physical adsorption, and surface immobilization to promote the binuclear pathway in photochemical CO2 reduction using the Re(I) compounds. In the homogenous approach, we attempted to template dimeric interactions by synthesizing a polymeric Re-amidohexyl compound as well as melamine-based compounds featuring multiple metal centers for photo and electrochemical CO2 reduction. Re(I) catalysts with hydroxyethyl pendant groups were also synthesized to investigate catalyst interactions with electrode surfaces. These materials provided insights regarding the effects of integrating Re(I) catalysts into both conjugated and non-conjugated polymeric systems.

Immobilization on solid-state surfaces could potentially improve stability and recyclability of molecular catalysts. We investigated the effects of forcing proximity of catalytic sites on surfaces by physically adsorbing both tricarbonyl Re(I) and Mn(I) compounds in mesoporous silica. Investigation with infrared spectroscopy revealed differences in binding strength of CO2-adducts to the metal centers in the presence of triethylamine. Covalent linkages were designed as another approach to promote close proximity of surface metal sites and induce dimeric interactions. A series of covalent linkages were investigated, including butyl, amide, and alkylamine. The electronic effects of the linkages involved were shown to have a significant impact on the catalytic activity of Re(I) on silica surfaces. Additionally, both monopodal and dipodal linkages were investigated to further explore the effects of surface Re(I) sites in forced proximity. Results obtained through this work will guide our efforts to develop robust and highly efficient systems for artificial photosynthesis.

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