Date of Award

Fall 2020

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

First Advisor

Christine A Caputo

Second Advisor

Gonghu Li

Third Advisor

Arthur Greenberg

Abstract

The linked problems of renewable energy scarcity and global climate change have grown increasingly dire as the world’s population and standard of living increases, with insufficient societal action to address them effectively. Scientific and technological progress is expected to address the gap between the wants and needs of our civilization, and the available physical and political resources. A truly sustainable solution must use an abundant energy source – sunlight – to simultaneously remove carbon dioxide from our atmosphere and produce carbon-neutral fuel such as hydrogen gas. Photocatalysts can achieve this process directly, while electrocatalysts can use electricity which can be generated by sunlight via photovoltaics if the energy is stored or if a complete circuit can be achieved. Compounds which use expensive metals to do this work are generally more effective than those which use more earth-abundant metals, but the latter are more sustainable. Therefore, it is important to develop new catalysts which use earth-abundant metals more effectively than those currently available. Effectiveness is dependent on the stability, efficiency, net energy requirements, and cost of the catalyst.

Two of the most direct catalytic transformations in this field – reduction of carbon dioxide to carbon monoxide and reduction of hydrogen ions to hydrogen gas – both require two electrons and two protons as part of an efficient catalytic cycle. Transition metals in the second and third rows of the d-block of the periodic table are more effective at two-electron transformations than those in the first row. This shortcoming can be overcome by the use of a redox non-innocent ligand, which can formally hold an electron for catalytic use. The protons can be more effectively brought to the active site of the catalyst by use of a pendant base which acts as a proton relay in the secondary coordination sphere, accepting protons in the bulk solution and transferring them to substrates bound to the metal center in an intramolecular fashion. By combining these two features, highly effective catalysts can be developed even with first-row transition metals.

This dissertation uses ligands with both redox non-innocent and pendant base functional groups. Both 2,2ʹ-bipyridine and bis(imino)acenaphthene are well-established moieties which can accept an electron during the catalytic cycle and which can chelate the metal center to improve stability. These groups were functionalized with pendant amines which were designed to improve catalysis by either acting as a pendant relay or by stabilizing reactive intermediates through the use of hydrogen bonding. These ligands were coordinated to a first-row transition metal, in particular manganese or cobalt, and investigated for electrocatalytic activity.

The electrochemical behavior of manganese complexes was simplified by first analyzing ligands without pendant amine groups. It was found that the position of other substituent groups had a significant impact on the properties of these complexes, in particular with regard to their propensity to form dimers via a metal-metal bond rather than forming the desired active species. Ligands with steric bulk adjacent to the metal center were found to destabilize the complex overall, leading to less dimer-forming behavior relative to ligands with steric bulk at other positions.

The addition of pendant amines to catalysts did not uniformly improve the effectiveness of those catalysts. In most cases, these catalysts did not outperform analogous catalysts which lacked the pendant amine groups, possibly due to the coordination of the nitrogen atom directly to the metal center, terminating the catalytic cycle. This behavior was avoided by introducing a pendant amine which would form an unstable ring upon coordination to the metal center, leading to catalytic enhancement. Catalytic activity was not demonstrated for catalysts employing the bis(imino)acenaphthene moiety despite the presence of pendant amines. Preliminary work was performed investigating a catalyst which appeared capable of catalytic hydrogen production without the use of a metal.

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