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Access Type

WSU Access

Date of Award

January 2023

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Claudio N. Verani

Abstract

Surfactants are a highly versatile group of molecules that find widespread application in various fields, including detergents, emulsifiers, and cosmetics. They are characterized by having a hydrophilic region capable of interacting with polar solutions, as well as a hydrophobic tail that can interact with organic solutions. Metal-containing surfactants, known as metallosurfactants, maintain the amphiphilic nature of traditional surfactants while incorporating a metal center into their ligand framework. The inclusion of a coordinated metal ion introduces novel properties to the surfactant, such as redox reactions, electrochemistry, magnetism, and photochemistry. This research focuses on exploring two specific applications of metallosurfactants. The first application involves utilizing metallosurfactants to facilitate directional electron transport through assemblies composed of gold electrodes sandwiching the metallosurfactant layer. By carefully controlling the arrangement and properties of the metallosurfactant molecules, it is possible to achieve controlled electron flow in specific directions, which has potential implications in electronic devices and molecular-scale circuits. The second application revolves around the use of surfactants for metal

recovery from dilute aqueous solutions through a process called ion flotation. By studying the fundamental, coordination chemistry of surfactant and metal interactions, it is possible to finesse a system to selectively recover one metal ion over another by forming metallosurfactant aggregates. The phenomenon of facilitating the directional flow of electrons in a molecule is referred to as rectification. Molecular rectification has attracted considerable attention from researchers ever since it was proposed by Aviram and Ratner in the 1970s. The appeal of molecular-based electronics lies in their potential for scalability, as electronic devices constructed using molecular components could offer reduced size and enhanced processing capabilities. The concept of molecular rectification involves designing molecules that exhibit an asymmetric electronic behavior, allowing them to preferentially conduct electrons in one direction while impeding or blocking their flow in the opposite direction. To this end, in Chapter 3 we developed three new metallosurfactants containing 3d4 manganese(III) ions, namely [MnIII(LNO2-N2O2)Cl(MeOH)](1), [MnIII(LN2O2)(H2O)(MeOH)]Cl(2), [MnIII(LN2O3)](3), bound to three distinct phenylenediamine-bridged phenolate-rich ligands. These were designed and deposited as Langmuir-Blodgett monolayer films on gold electrodes, and probed for directional electron transfer in Au|LB|Au junctions. All three MnIII metallosurfactants promote current rectification, and we compare the behavior of these metallosurfactants with that of our previously studied 3d1 V=OIV, 3d3 CrIII, and 3d5 FeIII species in similar environments through a rigorous effort. Based on the analysis electrochemical, spectroscopic, microscopic and DFT results, we propose distinct mechanisms by which electronic configurations and ligand frameworks influence and modulate the energy gap between the electrode Fermi levels and the molecular orbitals responsible for electron transport. Our previous work characterized distinct mechanisms, i.e., electron transport through below-Fermi for VIVO-based SOMOs, above-Fermi for FeIII -based SOMOs, and ligand-based HOMOs for the CrIII species. These MnIII species show electron transport similar to FeIII, however, low current capacities suggest that film quality is an important factor to be considered. Metal recovery through ion flotation has garnered significant interest since it was proposed by Sebba in the 1950s and has been used to remove toxic metals from water supplies as well as recover economically advantageous metals. This is accomplished by dissolving an oppositely charged collector surfactant in an aqueous solution rich with metal ions. The stereo-interaction between the two charged species will allow them to interact. Air is bubbled through this solution and the surfactant|metal complex is spontaneously concentrated on the surface of the bubble which then moves out of solution as a foam mixture. To achieve more selective metal ion recovery, a neutral chelating ligand is introduced to this mixture, allowing interactions based on geometry, denticity, and pH variations that will facilitate selectivity by forming chelate|metal|surfactant complexes. Thus, in Chapter 4 we investigate the role of two variables, namely pH and ligand field control based on fundamental transition metal principles to complex metal ions and selectively recover via ion flotation. Ion flotation experiments were conducted with sodium dodecyl sulfate (SDS) as the collector surfactant and triethylenetetramine (trien or 222), 1,2-Bis(3- aminopropylamino)ethane (323), and N,N′-Bis(3-aminopropyl)-1,3-propanediamine (333) as the chelating sequestrants. The results show that metal selectivity is achieved through chelate interactions, allowing NiII and GdIII to be recovered separately from each other at different pH values. The coordination geometry is not the determining factor for selectivity, as coordination studies suggest that the chelating ligand is the responsible factor in imposing either a square planar or octahedral geometry on the metal due to steric strain and the larger cavity size of the chelating sequestrant promotes a 1:1 binding ratio. Additionally, in Chapter 5 we investigate the use of ethylenediamine and cyclen as chelating sequestrants for the recovery of NiII, CoIII, and GdIII from dilute aqueous solutions using ion flotation. Our previous work described that larger chelate cavity size accommodates a 1:1 metal:ligand binding ratio and that small cavity sizes lead to steric issues between the metal and the chelate. In this study, we use a more flexible chelator ethylenediamine (en) and 1,4,7,10- tetraazacyclododecane (cyclen) compared to the previously studied triethylenetetramine to determine the effects of chelate rigidity on recovery of metal ions and on the coordination chemistry. Ethylenediamine is found to have low recovery rates at pH = 2.5 and 5, but increases to 99% at pH 8, 9.5, and 11 for NiII. Cyclen displays a 13% recovery rate at pH 2.5 and a recovery amount of 99% at pH 11 for NiII. Cyclen also exhibits high recovery rates for CoIII and GdIII. The use of cyclen results in a 1:1 binding ratio of the metal ion-chelate complex, while ethylenediamine coordinates in a 1:2 or 1:3 metal:chelate ratio, however, both CoIII and NiII appear to coordinate in an octahedral geometry, and the tunability of the ion flotation is controlled by pH effects rather than by preferential binding geometry. This research work showcases the diverse applications of metallosurfactants and highlights the significance of metallosurfactant chemistry. The use of MnIII metal ions in redox-active frameworks facilitates directional electron transport and demonstrates the feasibility of molecular electronic components. Our findings have shed light on the crucial role of chelate cavity size and ligand rigidity in influencing the binding interactions. Understanding these factors is pivotal for designing environmentally friendly experiments that utilize minimal amounts of product. Notably, our research has underscored the pH as a key determinant of metal ion selectivity, emphasizing its importance in controlling and directing metal recovery processes. Overall, this work contributes to expanding the scope of metallosurfactant chemistry and lays the foundation for future advancements in this exciting field.

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