From Coordination Complexes to Conductive Polymers: The Synthesis and Characterization of Anionic Molecules and Materials
The field of synthetic chemistry provides an unparalleled opportunity to study the relationship between molecular structure and the physical and chemical properties of a system. Toward this end, this dissertation describes efforts to develop new systems containing negatively charged components with an eye toward applying them to energy storage applications. Chapter One begins by explaining the importance of energy storage in harnessing renewable energy sources and how photosynthesis can serve as inspiration for converting solar energy into useful chemical fuels. It also outlines the motivation and core concepts for projects described in later chapters.
Chapter Two is presented in two parts. The first describes the synthesis of a series of ruthenium complexes bearing the pentadentate ligand 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine (PY5Me2) and the subsequent electrochemical evaluation of [(PY5Me2)Ru(H2O)]2+ as a water oxidation catalyst. The second investigates [(PY5Me2)Co(H2O)]2+ for the same application. While both systems provided initial electrochemical evidence for water oxidation, it was ultimately found that the ruthenium complex served only as a stoichiometric oxidant for water oxidation while the cobalt complex appeared to decompose to a catalytically active side product.
Based on lessons learned in Chapter Two, a fresh initiative was undertaken to synthesize new ligand scaffolds that might better support the high-valent metal species necessary to perform water oxidation. Consequently, pentadentate ligands possessing anionic donors were pursued. Chapter Three presents the synthesis and characterization of alkali metal salts of the tetraanionic ligand 2,2′-(pyridine-2,6-diyl)bis(2-methylmalonate) ([PY(CO2)4]4−) via deprotection of the neutral tetrapodal ligand tetraethyl 2,2′-(pyridine-2,6-diyl)bis(2-methylmalonate) (PY(CO2Et)4). The [PY(CO2)4]4− ligand, which features an axial pyridine and four equatorial carboxylate groups, cleanly reacts with a number of divalent first-row transition metals to form the series of complexes K2[(PY(CO2)4)M(H2O)] (M = Mn2+, Fe2+, Co2+, Ni2+, Zn2+). The metal complexes were comprehensively characterized via single-crystal X-ray diffraction, 1H NMR and UV-Vis absorption spectroscopy, and cyclic voltammetry. Additionally, Chapter Three recounts a barrage of synthetic routes that have been attempted in order to generate a new N4C− ligand possessing four equatorial pyridine donors and an axial, anionic carbon donor. While this ligand has not yet been successfully isolated in sufficient amounts, the most promising options moving forward are highlighted.
Although the final chapter continues to focus on the synthesis of negatively charged systems, the desired application switches to that of single-ion conducting electrolytes for Li-ion batteries. Hence, Chapter Four reports the synthesis of a series of poly(ethylene glycol) (PEG) based network polymers incorporating fluorinated tetraphenylborate nodes into the polymer backbone. The modular nature of the building units for this polymer allowed for a systematic study of the effect of linker length and composition on the conductivity of Li-ions through the material. Whereas long linkers produced flexible materials that were conductive at elevated temperatures, materials made with short linkers were brittle and exhibited no conductivity. However, when loaded with 68 wt% propylene carbonate, materials containing short linkers outperformed those with long linkers, exhibiting conductivity as high as 2.5 × 10–4 S/cm for the polymer made with ethylene glycol. It was also found that the conductivity could be further increased by exchanging the PEG linker for 1,5-pentanediol, which produced conductivity values of 3.5 × 10–4 S/cm.