Halide Functionality Dependent Formation of Molecular Receptors and Their Ion Recognition Properties

Abstract

The halide functionality on N-bridged tripodal receptors has shown a distinct behavior on their self-assembly structures and binding ability toward HgCl(2) and ClO(4)(-) anions. The receptors containing fluoro, chloro, and iodo groups crystallized to form hemicarcerands in the solid state, whereas the receptor with a bromo group forms a molecular capsule via C-H...Br and C-H...pi interactions. The cavity of the molecular capsule is tunable and is capable of reversible encapsulating-releasing the guest molecules by pH modulation.

Structurally Flexible C3‐Symmetric Receptors for Molecular Recognition and Their Self‐Assembly Properties

“…We started our work on N‐bridged systems with simple tripodal receptors 1 – 4 (Figure ) bearing a halide functionality at the para position of the phenyl rings . We envisioned that incorporating various halides into the tripodal framework would modulate the binding affinity for hazardous pollutants like HgCl 2 (Lewis acid) via the inductive effect exerted by halides with different degrees of electronegativity.…”

“…Inspired by the formation of dimeric capsule 3●3 , where the driving force for binding of perchlorate anion by receptor 3 was mainly ionic interactions, we envisioned that incorporation of multiple hydrogen‐bond motifs into the receptor structure would result in a receptor capable of selectively encapsulating a planar triangular anion like the nitrate anion. Furthermore, a protonated amine would bring the flexible tripodal branches into a convergent cone‐shaped conformation where three amide N–H groups would perfectly accommodate a planar triangular‐shaped anion through intermolecular hydrogen‐bonding interactions …”

“…Hydrogen bonds can be easily modulated depending upon external stimuli in artificial systems and, motivated by our earlier results, our goal was to achieve control over nitrate anion encapsulation through dynamic self‐assembly. We envisioned that two convergent tripodal receptors could be assembled into a capsule form by a subtle balance of hydrogen bond donor (D)–acceptor (A) interactions on each of the tripodal branches (Figure ) [26b,e]…”

“…The bridging N atom was designed to serve two purposes: first, to trap a Lewis acidic center, and second, to allow the resulting receptor to be easily modulated depending upon the acidity of the surrounding medium. With judicious design of a series of N‐bridged tripodal receptors, we have shown substitution‐dependent structural patterns in the solid state and selective recognition and sensing of nitrate anion in aqueous medium . Later, stimuli‐responsive structural tuning in solution was reported for reversible binding of nitrate anion through dynamic self‐assembly,[26b,e] and finally encapsulated nitrate anion was used as a promoter for a photochemical [2+2] cycloaddition reaction to make high‐energy product assisted by a tripodal receptor …”

“…Previously, various approaches for molecular recognition and sensing have been attempted by our group . We initiated a project a few years ago to explore ion recognition and directed self‐assembly processes by flexible tripodal systems (Figure ) . The bridging N atom was designed to serve two purposes: first, to trap a Lewis acidic center, and second, to allow the resulting receptor to be easily modulated depending upon the acidity of the surrounding medium.…”