Functionalizing ortho-carboranes for redox-switchable metal capture and release of critical minerals essential for clean energy.

The transition from fossil fuels towards renewable energy and energy storage is vital for reducing greenhouse gas emissions. Uranium, lithium, and cobalt are critical for nuclear power and battery storage. The demand for these minerals is increasing rapidly because of the shift towards clean energy. However, they are typically sourced from terrestrial mining, an environmentally taxing process. The mining industry accounts for 8% of global greenhouse gas emissions. Accordingly, it is crucial that we develop new strategies for sourcing critical minerals used in clean energy technology. Advancements in selective electrochemical metal capture chemistry could expand critical mineral extraction technologies to unconventional sources – spent nuclear fuel, seawater, end-of-life batteries. In this thesis, we present new derivatized o-carborane (C2B10H12) clusters functionalized with donor (phosphine oxide, crown-ether, bipyridyl) appendages to target selective, electrochemical metal capture and release. Derivatized o-carboranes are redox switchable between the neutral (closo) and dianionic (nido) states. Herein, we observe metal binding in nido form and metal release in closo form. In Chapter 2, we discuss how tuning phosphine oxide carborane basicity may alter the donicity by installing electron withdrawing or electron donating groups on the carborane cage or aryl substituents. We further investigate these effects on uranyl (UO22+) ligation and biphasic capture/release chemistry and discover that hydrophobicity may be more important than tuning electronics. In Chapter 3, we extend carborane chemistry to crown-ether substituted derivatives for size-selective capture. We report interesting experimental and simulated voltammetry of the redox switch with alkali metals (Li+, Na+, K+) and utilize the observed phenomena to derive binding constants. In Chapter 4, we target sterically encumbered carboranes with bipyridyl appendages for transition metal (Mn2+, Fe2+, Co2+, Ni2+) binding. We observe interesting electrochemistry which suggests that increasingly Lewis acidic metals may bind more strongly. We begin to probe the metal coordination environments and reveal that square pyramidal geometries may be preferred. In Chapter 5, we explore additional donor groups (carboxylic acids, amides, sulfides, sulfoxides) adhered to o-carborane and discover that the ligating groups have a profound impact on carborane formal reduction potentials.