A R T I C L E S
Chart 1
Webb et al.
Scheme 1. Synthesis of 1
gels that are responsive to pH changes,12a light,12b,c catalysis
(both chemical12d and enzymatic12e-g), and cation recognition.12h,i
The use of metal salts (e.g., AgBF4) has also been found to
template the formation of a gel from a simple bis(urea)
compound and the anion shown to influence the gel structure
by hydrogen-bonding interactions.13 Prior to this work, however,
there has only been one communication of using anions to
inhibit the formation of self-assembled molecular gels.14 This
is somewhat surprising, as many low-molecular mass organic
gelators (LMOG),8 that is, compounds that form self-assembled
gels, aggregate by the virtue of bis-urea groups or amide groups
often found in anion receptors. Among amide-based anion
receptors, the isophthalamides15 were the first acyclic anion
receptor to gain popularity in supramolecular chemistry and are
now one of the most heavily used in this area of research.7a,16
To date, however, there have been no reports of a ditopic version
of this receptor, or to the best of our knowledge, no reports on
self-assembled gels based on this class of molecules.
We report herein the synthesis of N,N′,N′′,N′′′-1,2,4,5-tetra-
(ethylhexanoate) pyromellitamide 1 (Chart 1). Remarkably,
although the parent pyromellitamide 2 has been known since
(8) For reviews, see: (a) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133.
(b) Abdallah, D. J.; Weiss, R. G. AdV. Mater. 2000, 12, 1237. (c) van Esch,
J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (d) Estroff, L.
A.; Hamilton, A. D. Chem. ReV. 2004, 104, 1201. (e) Hirst, A. R.; Smith,
D. K. Chem.-Eur. J. 2005, 11, 5496. (f) de Loos, M.; Feringa, B. L.; van
Esch, J. H. Eur. J. Org. Chem. 2005, 3615. (g) George, M.; Weiss, R. G.
Acc. Chem. Res. 2006, 39, 489.
1914,17 there are not many reports of pyromellitamides in the
literature18 and no reports regarding their supramolecular
properties. The pyromellitamides can be viewed structurally as
the ditopic version of the isophthalamides. The pyromellitamide
1 reported here displays aggregation behavior, including gel
formation, and binds to anions with a 1:2 host/anion stoichi-
ometry and negative cooperativity. The ability of pyromellita-
mide 1 to form self-assembled gels is not unexpected as it fits
well within the “rigid core/flexible tail” motif,8,19 commonly
found among other LMOG, being composed of a rigid aromatic
core with four amides linked to four flexible ethyl hexanoate
tails. The gels formed by 1 respond to the addition of anions,
by collapsing back to the solution state (anion-induced gel f
sol transition).
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Results and Discussion
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Synthesis and Solid-State Structure of Pyromellitamide
1. The pyromellitamide 1 was synthesized in two steps by the
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