M. Arunachalam

Anion complexation of a pentafluorophenyl-substituted tripodal urea receptor in solution and the solid state: selectivity toward phosphate

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 Versatile Tripodal Amide Receptor for the Encapsulation of Anions or Hydrated Anions via Formation of Dimeric Capsules

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.

Anion induced capsular self-assemblies

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 Hexaaza Bicyclic Cyclophane with Dual Binding Sites

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.

Mercaptothiazolinyl functionalized hexapodal and tripodal receptors on benzene platform: formation of silver ion assisted hexanuclear metallocage vs metal-organic polymer.

Mercaptothiazolinyl functionalized hexapodal (L(1)) and tripodal (L(2)) receptors on the benzene platform have been synthesized easily in good yields and structurally characterized by a single-crystal X-ray crystallographic study. In the solid state, L(1) shows an orientation of six arms in 1,3,5 vs 2,4,6 facial steric gearing fashion, whereas L(2) adopted C(2v) symmetry where two of its thiazolinyl arms are oriented in one direction and the third arm in the another direction. Two silver complexes of L(1), 1 ([2(L(1))·6(AgClO(4))·2(CHCl(3))·HClO(4)]) and 2 ([2(L(1))·6(AgClO(4))]), that are suitable for single-crystal X-ray studies are isolated upon the slow diffusion of a dimethylformamide solution of AgClO(4) to the solution of L(1) in chloroform and dichloromethane, respectively. Similarly, upon the slow diffusion of an acetonitrile solution of AgClO(4) to the chloroform solution of L(2), colorless crystals of the silver complex of L(2), 3, are successfully isolated. The structural analyses of 1 and 2 show the formation of a silver ion assisted hexanuclear metallocage Ag(6)(L(1))(2) via dimeric assembly of L(1) with multiple clefts and pockets toward guests binding. In 1, two chloroform molecules sit in top and bottom pockets, whereas six perchlorate counteranions are bound in six clefts between the silver ion pillared side arms of the metallocage. Though complex 2 shows the formation of a metallocage like 1, the single crystal structural analysis depicts perchlorate counteranions bonded to the silver atoms of the metallocage. On the contrary, the silver complex of tripodal receptor L(2), 3, shows the formation of a metallo-organic polymeric network of L(2) and Ag(+). To the best of our knowledge, this work represents the first report on the formation of an M(6)L(2) type metallosupramolecular cage topology with multiple clefts for guest binding by a semirigid hexapodal receptor.

Binding of ammonium hexafluorophosphate and cation-induced isolation of unusual conformers of a hexapodal receptor

The pyrazole-based hexa-host receptor showed the preference of an unusual conformation via 1:1 hydrogen bonding with ammonium hexafluorophosphate over steric gearing. Two different unusual conformers of the receptor are also isolated upon complexation with Cu(2+) and Cd(2+) ions.

Arene platform based hexa-amide receptors for anion recognition: single crystal X-ray structural and thermodynamic studies

Five newly synthesized hexa-amide receptors (L1–L5) and previously reported three such receptors (L6–L8) have been explored to investigate binding propensity of anions of various shapes and sizes. Single crystal X-ray structures of five new anion complexes of receptors (L1–L5), complexes (1–5) and previously reported seven such complexes of (L6–L8), complexes (6–12) fall under four categories of conformations such as aaabbb (A), ababab (B), aabaab (C) and aaaaaa (D) depending upon basicity of the anions as well as substituents on the receptor backbones. Moderately basic guest; chloride exhibits two different conformers with A and B patterns with L1 (R = –oNO2C6H4, complex 1) and L4 (R = –pFC6H4, complex 2) respectively in 1 : 2 (host : guest) stoichiometry. On the other hand, strongly basic acetate complexes of L2 (R = –pNO2C6H4), L3 (R = –mCF3C6H4), L5 (R = 4-pyridyl), L6 (R = –C6F5), L7 (R = –mNO2C6H4) and L8 (R –oCF3C6H4) i.e. complexes 3–8 respectively, exhibit a chair like conformer (A) with 1 : 2 (host : guest) stoichiometry. Our previous studies showed recognition of nitrate–water cluster (9) by L6 and nitrate (10) by L8 with (B) and (C) conformations respectively and an unusual conformation (D) was isolated in cases of L7 and L8 with hydrated fluoride clusters [(F)4(H2O)10]4− (11) and [(F)4(H2O)6]4− (12) respectively. Solution state ITC and NMR studies have also shown 1 : 2 stoichiometry of host : guest binding of acetate and fluoride with L1–L8.

Nitrate directed organized assemblies of protonated arene based tripodal receptors

Three tripodal and one dipodal receptors with benzene platform and heterocyclic terminals have been chosen to study nitrate and chloride dependent aggregation properties in their protonated states. A detailed single-crystal X-ray diffraction study showed the formation of nitrate (planar) ion assisted capsular/distorted capsular/macrocyclic structures in cases of tripodal receptors and 1D-zigzag polymeric chain in the case of the dipodal receptor. Single crystal structures of two of the tripodal receptors show that the spherical anion like chloride failed to form a capsular aggregate. Furthermore, the nitrate complex of the dipodal receptor reveals the solid state structural evidence of the uncommon hydrogen dinitrate anion where the central hydrogen atom is in a general position midway between two nitrate oxygen atoms.