Understanding Superatomic Cluster Tunability for Use as Building Blocks for Extended Structures

This dissertation summarizes my efforts and research in the Roy group to study the tunability of superatoms through ligand effects, create microporous structures from molecular cluster precursors to act as battery materials, and understand the electronic structure governing the interesting magnetic properties of Fe₆S₈(CN)₆, as well as efforts to design novel extended structures utilizing Fe₆S₈(CN)₆. Chapter 1 serves as an introduction to superatoms. It briefly discusses the quantum nature of small materials and how this gives rise to properties exhibited by superatoms. Properties which will prove important to this dissertation and methods of altering those properties through core composition and ligand choice are explored. Next, an overview of many methods to create extended structures is provided. Select examples of how superatomic clusters have already been used to increase our knowledge of fundamental concepts in science are then discussed. Finally, a brief summary and explanation of how these concepts will be explored in later chapters is given. This chapter is meant to serve as a targeted review with plenty of further reading cited for any incoming students with interest in continuing my projects. Chapter 2 discusses studies to understand the effects of either replacing PEt₃ ligands with CO ligands or the removal of PEt₃ ligands in the Co₆S₈(PEt₃)ₓ(CO)₆₋ₓ and Co₆S₈(PEt₃)ₓ systems, respectively. It presents a collaborative approach to synthesize a series of clusters for analysis by anion photoelectron spectroscopy and evaluation of results using computational chemistry. A drastic change in the donor/acceptor behavior of the cluster is observed, but surprisingly little change in the HOMO-LUMO gap is observed as the HOMO and LUMO experience similar energetic changes upon ligand removal or substitution. Chapter 3 presents a practical application for ligand removal of superatomic clusters. I present a synthesis of microspherical, highly porous materials derived from superatomic clusters. These microsphere materials display very different morphology from typical materials made using the same elemental ratio. This altered morphology results in a material which is favorable for use as a battery electrode. Its increased porosity improves its capacity retention upon cycling and at high power. The Co₆S₈(PEt₃)₆ derived material also shows promise as a Na+ ion battery material. In this chapter I also discuss unfinished studies on mixed chalcogenide materials. Chapter 4 explores the electronic basis for the high magnetic moment of the Fe₆S₈(CN)₆ cluster. Through collaboration with computational chemists, I present evidence of a phenomenon known as dual-subshell filling allowing for two spin channels holding different number of electrons resulting in many unpaired electrons. This cluster is also uniquely prepared for use as an extended material due to its cyanide ligands which may readily be used to form Prussian blue analogs. Chapter 5 describes efforts to design extended structures using the Fe₆S₈(CN)₆ cluster. Attempts towards Prussian blue analogs, covalently bound clusters using DCNQI, and EDT-TTF-CONH2 utilizing structures are discussed. Detailed notes on the synthesis of [NEt₄]₅[Fe₆S₈(CN)₆] are also provided. 2 structures which have successfully been synthesized, a 4 bridging ligand and a 2 bridging ligand “wire” are described in detail. In Chapter 6, collaborative efforts to increase our understanding of the cluster building blocks which can function as nanoscale atoms that assemble to form superatomic solids are described. We characterize a representative superatomic cluster, Co₆S₈(PEt₃)₆, in terms of structural, electronic, and magnetic properties using Solid State Nuclear Magnetic Resonance (SSNMR), Density Function Theory (DFT) calculations, and Superconducting Quantum Interference Device (SQUID) measurements. Evidence of delocalized HOMO orbitals and a delocalized spin in the oxidize cluster is shown. The findings presented in this chapter will assist the design of superatomic clusters and state-of-the-art applications, such as single-electron devices. Finally, Chapter 7 is much shorter than the other chapters as it is used to describe smaller projects which do not fit in the scope of the overall thesis. Magnetic measurements on a compound designed in the Norton lab are described.