Synthesis, crystal structures and competitive complexation property of a family of calix-crown hybrid molecules and their application in extraction of potassium from bittern

<div><p>A family of calix-crown hybrid molecules containing calix[4]arene and crown-5/6, either at lower rim or at both upper and lower rims, have been synthesised, characterised and their competitive complexation property towards alkali and alkaline earth metal ions in aqueous media have been investigated. The competitive metal ion extraction study, carried out with equimolar mixture of Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺ and Sr²⁺ in aqueous media, revealed that the amount of K⁺ extracted is remarkably high compared to other metal ions. Complexation with K⁺ has been investigated by ¹H NMR, association constants and thermodynamic parameters have been determined by isothermal calorimetric study. The molecular structures of one of the receptors and two of the K⁺ complexes have been established by single crystal X-ray study. One of the receptors formed bimetallic complex and it exhibited interesting polymeric network structure with bridged picrate anion. These receptors have been applied for extraction of metal ions from bittern.</p></div

Recognition of Hg²⁺ Ion through Restricted Imine Isomerization: Crystallographic Evidence and Imaging in Live Cells

A newly synthesized imine-based receptor (<b>L</b>) showed remarkable specificity toward the Hg²⁺ ion in aqueous media over other metal ions. Coordination of <b>L</b> to Hg²⁺ induces a <i>turn-on</i> fluorescence response. This was explained based on the restricted imine isomerization along with PET on coordination to Hg²⁺. X-ray structural evidence tends to favor a C–C bond rotation rather than CN isomerization for adopting a favorable conformation in <b>L</b> for coordination to Hg²⁺. This reagent could be used for imaging the accumulation of Hg²⁺ ions in HeLa cells

Heteroannulation of 3‑Nitroindoles and 3‑Nitrobenzo[<i>b</i>]thiophenes: A Multicomponent Approach toward Pyrrole-Fused Heterocycles

A simple, efficient, and general multicomponent reaction involving an enolizable ketone, a primary amine, and an N-protected 3-nitroindole was developed for the synthesis of a range of functionalized pyrrolo­[3,2-<i>b</i>]­indoles. The methodology was efficaciously utilized for the “pyrroloindoliztion” of natural products, the pyrrolization of 3-nitrobenzo­[<i>b</i>]­thiophene, and the gram-scale synthesis of pyrroloindole. Furthermore, a “one-pot” approach for accessing indolo­[3,2-<i>b</i>]­indoles was realized

Recognition of Hg²⁺ Ion through Restricted Imine Isomerization: Crystallographic Evidence and Imaging in Live Cells

A newly synthesized imine-based receptor (<b>L</b>) showed remarkable specificity toward the Hg²⁺ ion in aqueous media over other metal ions. Coordination of <b>L</b> to Hg²⁺ induces a <i>turn-on</i> fluorescence response. This was explained based on the restricted imine isomerization along with PET on coordination to Hg²⁺. X-ray structural evidence tends to favor a C–C bond rotation rather than CN isomerization for adopting a favorable conformation in <b>L</b> for coordination to Hg²⁺. This reagent could be used for imaging the accumulation of Hg²⁺ ions in HeLa cells

A copper(II) complex of benzimidazole-based ligand: synthesis, structure, redox aspects and fluorescence properties

<p>Employing 1-(2-methoxybenzyl)-2-(2-methoxyphenyl)-1<i>H</i>-benzimidazole (<b>bpb</b>) as a monodentate ligand, a new greenish-blue copper(II) complex, [Cu(<b>bpb</b>)₂(NO₃)₂] (<b>1a</b>), has been synthesized. <b>1a</b> has been characterized analytically and spectroscopically. The X-ray crystal structure of <b>1a</b> reveals that it adopts a <i>cis</i> disposition with respect to the ligands. The solid state structure of <b>1a</b> is stabilized by intramolecular offset face-to-face <i>π</i>–<i>π</i> stacking. Non-covalent supramolecular edge-to-face C–H⋯<i>π</i> interactions with neighboring molecules give 1-D supramolecular chains that further lead to the formation of an assembled 3-D supramolecular metal-organic framework via hydrogen bonding interactions. <b>1a</b> shows blue fluorescence most likely due to intramolecular offset face-to-face <i>π</i>–<i>π</i> stacking. At room temperature, <b>1a</b> is one-electron paramagnetic. It shows a rhombic EPR spectrum with <i>g</i>₁ = 2.12, <i>g</i>₂ = 2.42, and <i>g</i>₃ = 2.52 in the solid state at liquid nitrogen temperature. In cyclic voltammetry, <b>1a</b> displays a one-electron oxidative Cu(II)/Cu(III) couple. Our DFT calculations, corroborate the observed experimental results of <b>1a</b>.</p

Effect of conformation, flexibility and intramolecular interaction on ion selectivity of calix[4]arene-based anion sensors: experimental and computational studies

<p>A number of calix[4]arene-based molecules were designed incorporating amide moiety with variation in conformation, rigidity at the binding sites and steric crowding at the upper rim to investigate the anion sensing property of this series of ionophores. These compounds were synthesised and characterised, molecular structures of two of the compounds were established by single-crystal X-ray study. Anion binding property of these ionophores, investigated with the aid of ¹H NMR and UV–vis spectroscopy, revealed that three (<b>1–3</b>) out of four ionophores strongly interact with F⁻, in addition, ionophore <b>2</b> interacts with CN⁻ and , ionophore <b>3</b> interacts with CH₃COO⁻ and and ionophore <b>4</b> does not interact with any anions. NMR titration was carried out to determine binding constant with strongly interacting anions. Crystal structure analysis revealed that strong intramolecular interaction in <b>4</b> prevented the anions to interact with the N–H protons of the amide moiety. Interestingly, <b>2</b> with F⁻ and CN⁻ exhibits sharp colour change in acetonitrile–chloroform. Apparently, conformation of the calix moiety, flexibility of the binding sites and intramolecular H-bonding played critical role towards determination of selectivity. Computational study was performed to investigate the interaction site(s) and also to corroborate some of the experimental results.</p> <p>Anion binding study of functionalised calix[4]arenes revealed that conformation, flexibility and intramolecular interaction in calix moiety play critical role to determine ion selectivity. One of the receptors performs as sensitive colorimetric sensor for F⁻ and CN⁻, computational study also corroborates most of the experimental results.</p

Recognition of Hg²⁺ and Cr³⁺ in Physiological Conditions by a Rhodamine Derivative and Its Application as a Reagent for Cell-Imaging Studies

A new rhodamine-based receptor, derivatized with an additional fluorophore (quinoline), was synthesized for selective recognition of Hg²⁺ and Cr³⁺ in an acetonitrile/HEPES buffer medium of pH 7.3. This reagent could be used as a dual probe and allowed detection of these two ions by monitoring changes in absorption and the fluorescence spectral pattern. In both instances, the extent of the changes was significant enough to allow visual detection. More importantly, the receptor molecule could be used as an imaging reagent for detection of Hg²⁺ and Cr³⁺ uptake in live human cancer cells (MCF7) using laser confocal microscopic studies. Unlike Hg­(ClO₄)₂ or Hg­(NO₃)₂ salts, HgCl₂ or HgI₂ failed to induce any visually detectable change in color or fluorescence upon interaction with <b>L</b>_<b>1</b> under identical experimental conditions. Presumably, the higher covalent nature of Hg^II in HgCl₂ or HgI₂ accounts for its lower acidity and its inability to open up the spirolactam ring of the reagent <b>L</b>_<b>1</b>. The issue has been addressed on the basis of the single-crystal X-ray structures of <b>L</b>_<b>1</b>·HgX₂ (X^– = Cl^– or I^–) and results from other spectral studies

Recognition of Hg²⁺ and Cr³⁺ in Physiological Conditions by a Rhodamine Derivative and Its Application as a Reagent for Cell-Imaging Studies

A new rhodamine-based receptor, derivatized with an additional fluorophore (quinoline), was synthesized for selective recognition of Hg²⁺ and Cr³⁺ in an acetonitrile/HEPES buffer medium of pH 7.3. This reagent could be used as a dual probe and allowed detection of these two ions by monitoring changes in absorption and the fluorescence spectral pattern. In both instances, the extent of the changes was significant enough to allow visual detection. More importantly, the receptor molecule could be used as an imaging reagent for detection of Hg²⁺ and Cr³⁺ uptake in live human cancer cells (MCF7) using laser confocal microscopic studies. Unlike Hg­(ClO₄)₂ or Hg­(NO₃)₂ salts, HgCl₂ or HgI₂ failed to induce any visually detectable change in color or fluorescence upon interaction with <b>L</b>_<b>1</b> under identical experimental conditions. Presumably, the higher covalent nature of Hg^II in HgCl₂ or HgI₂ accounts for its lower acidity and its inability to open up the spirolactam ring of the reagent <b>L</b>_<b>1</b>. The issue has been addressed on the basis of the single-crystal X-ray structures of <b>L</b>_<b>1</b>·HgX₂ (X^– = Cl^– or I^–) and results from other spectral studies

Specific Reagent for Cr(III): Imaging Cellular Uptake of Cr(III) in Hct116 Cells and Theoretical Rationalization

A new rhodamine-based reagent (<b>L</b>_<b>1</b>), trapped inside the micellar structure of biologically benign Triton-X 100, could be used for specific recognition of Cr­(III) in aqueous buffer medium having physiological pH. This visible light excitable reagent on selective binding to Cr­(III) resulted in a strong fluorescence <i>turn-on</i> response with a maximum at ∼583 nm and tail of that luminescence band extended until 650 nm, an optical response that is desired for avoiding the cellular autofluorescence. Interference studies confirm that other metal ions do not interfere with the detection process of Cr­(III) in aqueous buffer medium having pH 7.2. To examine the nature of binding of Cr­(III) to <b>L</b>_<b>1</b>, various spectroscopic studies are performed with the model reagent <b>L</b>_<b>2</b>, which tend to support Cr­(III)-η²-olefin π-interactions involving two olefin bonds in molecular probe <b>L</b>_<b>1</b>. Computational studies are also performed with another model reagent <b>L</b>_<b>M</b> to examine the possibility of such Cr­(III)-η²-olefin π-interactions. Presumably, polar functional groups of the model reagent <b>L</b>_<b>M</b> upon coordination to the Cr­(III) center effectively reduce the formal charge on the metal ion and this is further substantiated by results of the theoretical studies. This assembly is found to be cell membrane permeable and shows insignificant toxicity toward live colon cancer cells (Hct116). Confocal laser scanning microscopic studies further revealed that the reagent <b>L</b>_<b>1</b> could be used as an imaging reagent for detection of cellular uptake of Cr­(III) in pure aqueous buffer medium by Hct116 cells. Examples of a specific reagent for paramagnetic Cr­(III) with luminescence <i>ON</i> response are scanty in the contemporary literature. This ligand design helped us in achieving the turn on response by utilizing the conversion from spirolactam to an acyclic xanthene form on coordination to Cr­(III)

Ratiometric Detection of Cr³⁺ and Hg²⁺ by a Naphthalimide-Rhodamine Based Fluorescent Probe

Newly synthesized rhodamine derivatives, <b>L</b>_<b>1</b> and <b>L</b>_<b>2</b>, are found to bind specifically to Hg²⁺ or Cr³⁺ in presence of large excess of other competing ions with associated changes in their optical and fluorescence spectral behavior. These spectral changes are significant enough in the visible region of the spectrum and thus, allow the visual detection. For <b>L</b>_<b>1</b>, the detection limit is even lower than the permissible [Cr³⁺] or [Hg²⁺] in drinking water as per standard U.S. EPA norms; while the receptor, <b>L2</b> could be used as a ratiometric sensor for detection of Cr³⁺ and Hg²⁺ based on the resonance energy transfer (RET) process involving the donor naphthalimide and the acceptor Cr³⁺/Hg²⁺-bound xanthene fragment. Studies reveal that these two reagents could be used for recognition and sensing of Hg²⁺/Cr³⁺. Further, confocal laser microscopic studies confirmed that the reagent <b>L</b>_<b>2</b> could also be used as an imaging probe for detection of uptake of these ions in A431 cells