Neutral Acyclic Anion Receptor with Thiadiazole Spacer: Halide Binding Study and Halide-Directed Self-Assembly in the Solid State

A halide binding study of a newly synthesized neutral
acyclic receptor LH2 with a thiadiazole
spacer has
been methodically performed both in solution and in the solid state.
Crystal structure analysis of the halide complexes elucidate the fact
that fluoride forms an unusual 1:1 hyrogen-bonded complex with monodeprotonated
receptor, whereas in the case of other congeners, such as chloride
and bromide, the receptor binds two halide anions along with formation
of a halide-bridged 1D polymeric chain network by participation of
N–H···X– and aromatic C–H···X– hydrogen-bonding (where X = Cl and Br) interactions.
The presence of a rigid thiadiazole spacer presumably opens up enough
space for capturing two halide anions by a single receptor molecule,
where the coordinated −NH protons are pointed in the same direction
with respect to the spacer and eventually favor formation of halide
(Cl– and Br–) induced polymeric
architecture, although no obvious chloride- or bromide-directed polymeric
assembly is found in solution. A significant red shift of 243 nm in
the absorption spectra of LH2 was solely observed in the presence of excess fluoride anion, which
enables LH2 as an efficient colorimetric
sensor for optical detection of fluoride anion (yellow to blue). Furthermore,
spectroscopic titration experiments with increasing equivalents of
fluoride anion suggest formation of a H-bonded complex with subsequent
stepwise deprotonation of two N–H groups, which can be visually
monitored by a change in color from yellow to blue via pink.