Anion-Tuning of Supramolecular Gel Properties

Abstract

Low molecular weight gelators (LMWGs) are a class of compounds that has garnered
great interest from material, synthetic, supramolecular and biological chemists. Anion
tuning of these supramolecular gels is a burgeoning field of study. Two classes of
compounds, chiral bisureas and urea derivatised pyrazoles are studied.
The synthesized chiral bisurea compounds act as gelators in a variety of solvents.
Addition of anions in the form of tetrabutyl ammonium salts was found to afford break
up of the gels. Studies reveal that the rheological characteristics of these materials can be
tuned using the simple addition of sub-stoichiometric amounts of anion. Variation in the
length of the alkyl chain of the spacer separating the chiral and bisurea moieties affects
the gel formation of a series of related compounds. Compounds with even numbered
spaced alkyl chains were found to gel, whereas the odd numbered spaced alkyl chain
compounds did not. Crystal growth within the gel matrix influences the crystallization
processes and the use of anion induced break down of the LMWGs allows for easy
recovery of the grown crystals. Drug compounds, bound upon gelation of a solution
containing the drugs, were found to be released in a controlled manner from the gel
matrix.
Urea functionalised pyrazoles were synthesized. Crystallographic determination of the
hydrogen bonding of the compounds as well as the coordination chemistry of these
compounds was obtained. Anion binding studies, in addition to the crystallographic
results, reveal that the urea or thiourea groups form an intramolecular hydrogen bond
with the pyrazole group resulting in an anti conformation. This prevents formation of gels
except in one case. The gelation of 1-(3-methyl-1H-pyrazol-5-yl)-3-(3-nitrophenyl)urea
in acidic water overcomes this problem by protonation of the pyrazole group resulting in
a syn conformation of the urea group. Anion tuning of the gel properties is revealed
where nitrate and chloride result in precipitation of gels and the rheological
characteristics can be tuned by changing the anion’s identity.