Solvent Effects and Internal Functions Control Molecular Recognition of Neutral Substrates in Functionalized Self-Assembled Cages

A suite of internally functionalized Fe4L6 cage complexes has been synthesized with lipophilic
end groups to allow dissolution in varied solvent mixtures, and the
scope of their molecular recognition of a series of neutral, nonpolar
guests has been analyzed. The lipophilic end groups confer cage solubility
in solvents with a wide range of polarities, from hexafluoroisopropanol
(HFIP) to tetrahydrofuran, and the hosts show micromolar affinities
for neutral guests, despite having no flat panels enclosing the cavity.
These hosts allow interrogation of the effects of an internal functional
group on guest binding properties, as well as solvent-based driving
forces for recognition. Introducing polar effects to the interior
of the cavity enhances guest binding affinity in nonpolar solvents;
adding space-filling aliphatic groups reduces affinity in all cases.
While high dielectric solvents such as acetonitrile strongly favor
guest binding, “low dielectric, high polarity” solvents
such as HFIP strongly occupy the cavity and prevent guest recognition.
Analysis of the cage optical transitions shows that the guests interact
with the central ligand cores and reside in close proximity to the
internal functions. These results have implications for supramolecular
catalysis: balancing directed host:guest interactions (e.g., H-bonds)
with entropic effects from solvent displacement is essential for reactions
in these (and related) biomimetic hosts.