Photochemical Sciences Ph.D. Dissertations

Iminium Salts and Their Derivatives as Models for Catalytic Water Oxidation

Date of Award

2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Ksenija Glusac

Second Advisor

Thomas Kinstle (Committee Member)

Third Advisor

Marshall Wilson (Committee Member)

Fourth Advisor

Michael Zamkov (Committee Member)

Abstract

The solar energy utilization is one of the most promising strategies for catering the ever-increasing energy demand in a renewable manner. For this reason, several approaches are pursued for solar energy storage, one of which involves the photocatalytic splitting of water. Over recent years, much research has been directed towards the design of transition-metal based water oxidation catalysts to obtain oxygen based on transition metal complexes. The major drawback of most of these catalysts is the cost of transition- metal complexes. For these reasons, the main focus of our research is based on the design of a fully organic catalyst suitable for water oxidation. Our group recently discovered that a flavinium ion performs electrode-mediated electrocatalytic water oxidation at large overpotentials. It was found that catalysis occurs only in the presence of the electrodes that produce active oxides on their surfaces. The mechanism of the catalysis by the flavinium ions was proposed to involve the coupling reaction two oxygen-centered radicals, one of which is derived from to the flavin moiety and the other one is formed at the electrode surface. The electrochemical oxidation of the formed peroxide species then proposed to release the oxygen molecule and recover the catalyst. However, it is important to note, that the detailed study of the mechanism is limited due the fact that electrode participates in the catalytic cycle. For these reasons, it is crucial to develop a fully homogeneous system to study the mechanism of the catalysis. One approach towards a fully molecular catalysis involves a system composed of two- iminium ion moieties joined covalently by a suitable linker. The mechanism of a catalysis is proposed to involve four individual steps: (i) pseudobase formation via a reaction of flavinium ions with water; (ii) proton-coupled oxidation of pseudobases to generate alkoxyl radicals; (iii) coupling of alkoxyl radicals to generate the peroxide intermediate; (iv) two-electron oxidation of the peroxide to release molecular oxygen and regenerate the catalyst. Therefore, we decided to study each individual step of the proposed mechanism above in great detail. A series of iminium salts and their pseudobases were synthesized. It was found that the efficiency of a pseudobase formation depends on the nature of heterocyclic ion and the nature of substituents bound to it. The proton-coupled electrocatalytic oxidation of pseudobases was studied using cyclic voltammetry. We found that the deprotonation of the amine radical cation formed after one-electron oxidation of pseudobase derivative occurs via two competing pathways: OH vs. C-H deprotonation. To elucidate the side responsible for C-H deprotonation event we synthesized the methoxy derivatives of iminium ions since these compounds do not contain an O-H proton. Additionally, to investigate the general chemistry of the alkoxyl radicals, we prepared 2- ethyl-4-nitroisoquinolinium hydroperoxide. Since hydroperoxides also tend to form alkoxyl radicals upon the decomposition, we decided to investigate the thermal behavior of 2-ethyl-4-nitroisoquinolinium hydroperoxide. The thermal decomposition was investigated using steady-state UV/Vis and NMR spectroscopy. Finally in order to study the two electron-oxidation processes of peroxide leading to the formation of oxygen we report the modified procedures for the synthesis of symmetric peroxide xanthrene based moiety.

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