Gelation is crucially dependent on functional group orientation and may be tuned by anion binding

Abstract

The gelation ability of a series of chiral bis(urea) gels alternates between even and odd chain length and for the even numbered spacers the rheological characteristics can be tuned by the addition of anions according to the anion binding constant.

Anion-switchable supramolecular gels for controlling pharmaceutical crystal growth

“…Results are summarised in Table 2 and selected photographs are given in the supplementary information ( Figure S3). Gel characterisation data for 2 has been reported previously 24 SEM data of a xerogel of 3 is shown in Fig. 1b and rheometry and additional SEM data is given in the supplementary information ( Figures S4 and S5).…”

“…This effect is in direct contrast to anion binding by bis(urea) gels in which the competition between anion binding and gelator selfassociation results in marked anion-dependent decreases in gel strength and yield stress. 24 The CBZ does, however, lower the gel-sol transition temperature (T gel ; see supplementary material Figure S1). NMR titration experiments showed only very weak apparent binding of CBZ to 1 with log K 11 = 0.84 (8) Because of its gelation ability the host itself is also likely to undergo significant selfassociation and hence the competition between both gelator and CBZ aggregation and gelator-CBZ binding results in a low overall apparent binding constant.…”

“…Acetate was chosen since it is strongly bound by ureas and previous work on 2 has shown that acetate induces the most significant reduction in gel strength. 24 Crystals of CBZ form III are produced 1-2 days after gelation. Crystallisation is faster in the non-gel reference sample and occurs after ca.…”

“…Compounds 2, and 4 have been reported previously. [23][24] Novel compounds 1 and 3 were prepared by standard methods as detailed in the supplementary information. Gelators 1 -3 were chosen because they reliably form gels in a number of solvents ranging from aqueous systems to toluene.…”

Halogen-bonding-triggered supramolecular gel formation

“…In a series of bis(urea) gels we have shown how a combination of metal-and anion-binding can allow comprehensive control over the system. Competitive anion binding reduces gel strength by inhibiting urea -tape hydrogen-bond formation [28][29][30][31] .Conversely, metal coordination in pyridyl-urea compounds results in metal binding to the pyridyl group which suppresses the alternative, gel-inhibiting urea-pyridyl hydrogenbonding interaction, freeing the urea groups to form fibrils, and hence gels, as a result of the urea tape hydrogen-bonding motif [32][33][34] . These competitive interaction modes are summarised in Figure 1. 3…”