Lipid-Inspired Low Melting Ionic Liquids via Synergistic Cyclopropanation and Branching of Terpenoids

Abstract

Bacteria employ cyclopropane motifs as bioisosteres for unsaturations to modulate lipid bilayer fluidity and protect cellular membranes under environmental stress. Drawing inspiration from this biological strategy, we investigated how cyclopropanation impacts the thermophysical properties of lipid-inspired ionic liquids. We synthesized a series of imidazolium-based ionic liquids incorporating cyclopropanated derivatives of three renewable terpenoids: phytol, farnesol, and geraniol. Through an integrated approach combining property-driven design, thermophysical analysis, X-ray crystallography, and computational modeling, we systematically examined how these structural modifications influence quantitative structure–property relationships. Our findings demonstrate that ionic liquids with long alkyl appendages respond to side-chain modifications─particularly the synergistic combination of cyclopropanation and branching─in a manner that mimics homeoviscous adaptation in living organisms. The strategic incorporation of cyclopropyl moieties combined with chiral methyl branching produced dramatic melting point depressions, with phytol-derived ionic liquids achieving the lowest melting points reported to date for these bioinspired materials. This effectiveness results from positioning these structural elements within the symmetry-breaking region of alkyl chains, where they maximally disrupt molecular packing and enhance fluidity. X-ray crystallographic analysis of a cyclopropanated citronellyl-based ionic liquid revealed that the cyclopropyl ring induces significant conformational distortions that prevent efficient molecular organization. The use of terpenoids from the chiral pool as starting materials imparts inherent sustainability to these ILs. Enantiopure ILs can be synthesized from renewable feedstocks like phytol and citronellol while exploiting bioinspired structural design principles. This work provides new insights into IL structure–property relationships that both complement and extend previous discoveries, establishing a framework for the rational design of lipidic ionic liquid systems with enhanced fluidity and chemical stability from renewable resources.