The solid-state of ionic liquids: Impacts of non-covalent interactions

Arsalan Mirjafari, Matthias Zeller, Patrick C Hillesheim

Arsalan Mirjafari - SUNY Oswego
Matthias Zeller - Purdue University
Patrick C Hillesheim - Ave Maria University

Ionic liquids have emerged as focal points of both academic research and industrial applications, garnering considerable attention over the past three decades. Despite the extensive exploration of modern air and moisture-stable ionic liquids, ongoing fundamental investigations into their structure and properties persistently unveil surprising discoveries, propelling advancements in the field.

A prominent challenge, known as the "anion crisis," underscores the prevalent reliance on a limited repertoire of anions in current ionic liquid synthesis. Concurrently, a parallel issue, termed the "cation crisis," highlights the predominant utilization of a narrow spectrum of cations (e.g., ammonium, imidazolium, pyridinium, etc.). Our research endeavors are devoted to addressing this challenge through the exploration and fundamental analysis of 'non-traditional' cations. Our objective is to construct predictive models aimed at expediting the discovery of novel ionic liquids hitherto unreported.

In recent investigations, our group has delved into the realm of triarylphosphonium-based salts. Remarkably, these compounds exhibit unexpectedly low melting points despite their high molecular weights and highly symmetric cations, characteristics traditionally considered unfavorable for the formation of low melting ionic liquids. By employing a combination of X-ray crystallography and computational studies, we aim to unravel the underlying physicochemical properties responsible for the observed phenomena.

Our findings underscore the pivotal role of precisely tailoring both the sterics and electronics of the aryl groups in dictating the formation of π interactions, thereby exerting control over the melting points of the compounds. Through this multifaceted approach, we aim to contribute to the development of a deeper understanding of ionic liquid behavior and pave the way for the rational design of innovative materials with tailored properties.