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

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.