This tutorial review focuses on some recent aspects in the development of synthetic receptors for selective sulfate anion recognition and separation, with a special emphasis to: (i) receptors for selective recognition of sulfate in organic and aqueous media and (ii) receptors for separation of sulfate from water via liquid-liquid extraction and crystallization.

Three heteroditopic cryptands with different cavity dimensions have been synthesized in high yields at 278 K without employing any templating metal ion. The three secondary amino nitrogens in each cryptand could be derivatized with anthryl groups to have a fluorophore−spacer−receptor configuration. The fluorophores in these systems do not show any fluorescence due to an efficient photoinduced intramolecular electron transfer (PET) from nitrogen lone pairs. However, the fluorescence can be recovered to different extents in the presence of different metal ions and protons as well. On complexation by a transition metal ion or on protonation in a solvent like dry THF, each exhibits large fluorescence enhancement as the nitrogen lone pairs responsible for PET are engaged in bonding. Inner-transition-metal ions like Eu(III) or Tb(III) show remarkable discrimination and give high fluorescence enhancement only in one case where the cavity size is smaller than that of other two. Each system exhibits large fluorescence enhancement with Pb(II) among the heavy metal ions studied. The present study shows that transition metal ions and Pb(II), which are known for quenching, can indeed cause fluorescence enhancement in cryptand-based systems. It is also reported for the first time that inner-transition-metal ions can also cause fluorescence. The enhancement in each case is interpreted in terms of a communication gap between the metal ion and fluorophore. Such cryptand-based fluorophores can be useful as potential molecular photonic devices and metal ion sensors as well.

Hydrophobic n-alkanethiolate bottoms and walls consisting of hydrophilic three-layer poly(acrylic acid)/poly(ethylene glycol) nanocomposite polymers are used for making patterned corrals having lateral dimensions of 63 μm. Macrophage cells are confined within these corrals and are unable to grow over the corral walls (see picture).

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.

Efficient fixation of aerial carbon dioxide as carbonate by a simple tripodal urea receptor is demonstrated by crystallizing the carbonate encapsulated molecular capsule in almost quantitative yield, followed by regeneration of free receptor from the capsule under mild conditions.

Single crystal X-ray crystallographic signature of the pentafluorophenyl substituted tripodal urea-based receptor shows formation of a pseudo dimeric cage which also encapsulates a phosphate dimer via numerous hydrogen bonding and anion[dot dot dot]pi interactions.

The selective phosphate-sensing property of a bis-heteroleptic Ru complex, 1[PF ] , which has a halogen-bonding iodotriazole unit, is demonstrated and is shown to be superior to its hydrogen-bonding analogue, 2[PF ] . Complex 1[PF ] , exploiting halogen-bonding interactions, shows enhanced phosphate recognition in both acetonitrile and aqueous acetonitrile compared with its hydrogen-bonding analogue, owing to considerable amplification of the Ru -center-based metal-to-ligand charge transfer (MLCT) emission response and luminescence lifetime. Detailed solution-state studies reveal a higher association constant, lower limit of detection, and greater change in lifetime for complex 1 in the presence of phosphates compared with its hydrogen-bonding analogue, complex 2. The H NMR titration study with H PO ascertains that the binding of H PO occurs exclusively through halogen-bonding or hydrogen-bonding interactions in complexes 1[PF ] and 2[PF ] , respectively. Importantly, the single-crystal X-ray structure confirms the first ever report on metal-assisted second-sphere recognition of H PO and H P O with 1 through a solitary C-I⋅⋅⋅anion halogen-bonding interaction.

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