Kimberly Riordan

Chemistry and Biochemistry

Faculty Advisor: Emily Tsui

Annual 2020 Project: Synthesis and Electrochemical Studies of T-shaped Sulfur Radicals

This year’s Nobel Prize in Chemistry was awarded to research discoveries on the lithium-ion battery. These batteries are lightweight and long-lasting, relying on the constant flow of lithium ions (1). The formation of soluble polysulfides is a degradation pathway for Li-S batteries, so therefore understanding its radical chemistry is important to longer battery lifetimes. Last year’s project targeted the synthesis and characterization of trigonal geometry sulfur radical. This approach was met with synthetic challenges, which are common. Consequently, this year’s project seeks to leverage an existing system to more readily access the relevant electrochemistry and spectroscopy. This research project focuses on synthesizing a different sulfur-based radical species. Imada, et al. have published a sulfur radical with T-shape geometry at the sulfur atom (2). By adding more electron-withdrawing groups, the radical should be more stable. These effects of changing substituents will be studied with the addition of sulfur in solution to measure the redox waves using cyclic voltammetry. This concept would have connections for the lithium-sulfur battery technology. We will also measure the isolated radical using electron paramagnetic resonance spectroscopy. Finally, we will study the solution-phase interactions with elemental sulfur as a method of stabilizing the soluble polysulfide species that are the reason why lithium-sulfur batteries degrade. In the synthesis of a T-shaped sulfur radical species, the starting compound will be 1-bromo-2,6-diorthoester. Treatment with C2F5Li and an excess of iodine is used to prepare an iodinane. The iodinane is trimetallated with n-BuLi and sulfur to yield the radical species. This route will be repeated with different substituents on the phenyl rings. We will consider groups with different electronics such as electron withdrawing and electron donating groups. Additionally, varying the positions and sterics of these substituents from the meta to the para positions will be studied. These compounds will be analyzed with electron paramagnetic resonance (EPR) spectroscopy. It can also be characterized by cyclic voltammetry and UV-vis spectroscopy.

(1) Press release: The Nobel Prize in Chemistry 2019. Nobel Media AB 2019. Mon. 28 Oct 2019. (2) J. Am. Chem. Soc. 2016, 138, 479−482