No abstract

The binding and selectivity of halides (spherical) and oxyanions (tetrahedral) toward a recently reported pentafluorophenyl-substituted tripodal urea-based receptor L(1) are examined thoroughly in the solid state by single-crystal X-ray crystallography as well as in solution by multinuclear NMR techniques. Crystallographic results show proof of a fluoride encapsulation in the cavity of L(1) in complex [L(1)(F)][Bu(4)N], . Fluoride encapsulation inside the C(3v) symmetric cleft is observed via six hydrogen bonds to all six urea protons of the receptor. In case of complex crystallographic results show encapsulation of sulfate ion inside a supramolecular cage formed upon 1 : 2 (guest-host) complex formation between sulfate and L(1). Sulfate encapsulation is observed via fourteen hydrogen bonding interactions from all six urea moieties of two L(1) units. Our effort to isolate single crystal of halides/oxyanions complexes of L(2) always yield single crystals of free L(2) though literature shows anion binding with this receptor in solution. Solution state binding studies of L(1) are carried out by (1)H-NMR titration to calculate binding constants, which show the following anion binding sequence H(2)PO(4)(-) > SO(4)(2-)> CH(3)COO(-) > F(-) > Cl(-) >> Br(-) whereas there is no binding with I(-), NO(3)(-) and ClO(4)(-) guests. Comparison of phosphate and sulfate binding in L(1) and L(2), show higher binding with the pentafluorophenyl substituted receptor, L(1). Further (19)F and (31)P-NMR experiments in solution are also carried out to probe the binding of F(-) and H(2)PO(4)(-) with L(1), respectively. Extensive (1)H-NMR experiments in solution and crystallization in the presence of multiple anions are also undertaken to evaluate the selectivity of H(2)PO(4)(-) over other anions.

A bowl-shaped tripodal receptor with an appropriately positioned amide functionality on the benzene platform and electron-withdrawing p-nitrophenyl terminals (L(1)) has been designed, synthesized, and studied for the anion binding properties. The single-crystal X-ray crystallographic analysis on crystals of L(1) with tetrabutylammonium salts of nitrate (1), acetate (2), fluoride (3), and chloride (4) obtained in moist dioxane medium showed encapsulation of two NO(3)(-), [(AcO)(2)(H(2)O)(4)](2-), [F(2)(H(2)O)(6)](2-), and [Cl(2)(H(2)O)(4)](2-) respectively as the anionic guests inside the staggered dimeric capsular assembly of L(1). The p-nitro substitution in the aryl terminals assisted the formation of dimeric capsular assembly of L(1) exclusively upon binding/encapsulating above different guests. Though L(1) demonstrates capsule formation upon anion or hydrated anion complexation for all of the anions studied here, its positional isomer with the o-nitro-substituted tripodal triamide receptor L(2) selectively formed the dimeric capsular assembly upon encapsulation of [F(2)(H(2)O)(6)](2-) and noncapsular aggregates in the cases of other anions such as Cl(-), NO(3)(-), and AcO(-). Interestingly, structural investigations upon anion exchange of the complexes revealed that both isomers have selectivity toward the formation of a [F(2)(H(2)O)(6)](2-) encapsulated dimeric capsule. In contrast, solution-state (1)H NMR titration studies of L(1) and L(2) in DMSO-d(6) with AcO(-) indicated 1:3 (host:guest) binding.

Researchers world-wide have employed diverse strategies to achieve various anion binding hosts and anion induced supramolecular architectures due to the increasing appreciation of anion receptor chemistry. Intellectual discovery of molecular capsules for the recognition of different guest species has opened up a new field of research in the area of supramolecular chemistry. This feature article aims to provide an overview of the current status and recent achievements made by us and others in the last decade in the area of anion induced construction of supramolecular capsules and anion binding in molecular capsules.

A new C3-symmetric drum-shaped homoditopic haxaamino bicyclic cyclophane and its hexachloride and hexaiodide complexes have been synthesized and characterized and dual recognition of guests has been demonstrated. Single-crystal X-ray analysis illustrates that bicyclic cyclophane has a cavity and side pockets for acetone molecules. The hexaprotonated state of this bicycle shows encapsulation of an iodide inside its cavity, and in hexachloride complex, chloride is recognized as Cl(-)...H2O in each of the three side pockets which are in extensive hydrogen bonding interactions with the water and chlorides. (1)H NMR experiments have also been carried out on hexatosylated cyclophane with the halides to study solution state binding.

An arene based tripodal amide receptor shows selective dimeric capsule formation templated by fluoride-water cluster, [F2(H2O)(6)]2-, and non-capsular aggregation for chloride, nitrate and acetate anions.

An unidirectional orientation of all six arms of an arene based hexapodal amide receptor shows dimeric capsular assembly templated by a [F(4)(H(2)O)(10)](4-) cluster.

A new benzene-based hexasubstituted bistripodal receptor is synthesized and explored as a new generation receptor for multiple anion binding. The solid state crystal structure showed the encapsulation of four nitrate anions in a bistripodand fashion with "ababab" conformation of the receptor, and upon complexation with acetate anions, the receptor adopted less favorable "aaabbb" conformation with two encapsulated acetate ions.