Oxoanion recognition by benzene-based tripodal pyrrolic receptors

Anchoring H-Bond Donating/Accepting Pyrrolic Derivatives on Preorganized Scaffolds: Conformationally Switchable Bipedal/Tripodal and Locked Molecular Cage Ionophores for Potentiometric Sensing of Phosphate and Fluoride

The ionophore properties of a myriad of conformationally switchable bipedal/tripodal receptors and locked molecular cages were evaluated here for the first time to fabricate potentiometric sensors for the determination of environmentally important phosphate and fluoride. Owing to the competent ionophore properties such as high binding selectivity and affinity, the developed ion-selective electrodes displayed response preference for phosphate and fluoride with a selectivity pattern that differs distinctly from traditional Hofmeister series. Binding constants of the ionophore-anion complexes are determined to underscore how modifications in the preorganization and H-bond donating/accepting ability of a given series of ionophores can be exploited to improve the performance for potentiometric sensing. While conformationally switchable bipedal/tripodal ionophores prefer tetrahedral oxyanions, locked molecular cages shift their preference to spherical halides gradually. Nernstian potential responses with good reversibility to target anions can be observed when shifting the optimized membrane electrodes in aqueous solutions within the concentration range of 10^-6.5-10^-2.0 M. Moreover, potentiometric determination of phosphate and fluoride in mineral water, soil, and tap water samples was achieved in a low μM concentration range with high accuracy, confirming their promising utility in real world applications.

Highly Efficient, Tripodal Ion-Pair Receptors for Switching Selectivity between Acetates and Sulfates Using Solid-Liquid and Liquid-Liquid Extractions

A tripodal, squaramide-based ion-pair receptor 1 was synthesized in a modular fashion, and ¹H NMR and UV-vis studies revealed its ability to interact more efficiently with anions with the assistance of cations. The reference tripodal anion receptor 2, lacking a crown ether unit, was found to lose the enhancement in anion binding induced by presence of cations. Besides the ability to bind anions in enhanced manner by the “single armed” ion-pair receptor 3, the lack of multiple and prearranged binding sites resulted in its much lower affinity towards anions than in the case of tripodal receptors. Unlike with receptors 2 or 3, the high affinity of 1 towards salts opens up the possibility of extracting extremely hydrophilic sulfate anions from aqueous to organic phase. The disparity in receptor 1 binding modes towards monovalent anions and divalent sulfates assures its selectivity towards sulfates over other lipophilic salts upon liquid–liquid extraction (LLE) and enables the Hofmeister bias to be overcome. By changing the extraction conditions from LLE to SLE (solid–liquid extraction), a switch of selectivity from sulfates to acetates was achieved. X-ray measurements support the ability of anion binding by cooperation of the arms of receptor 1 together with simultaneous binding of cations.

Small triiminopyrrolic molecular cage with high affinity and selectivity for fluoride

A small molecular cage (4) with high affinity and complete selectivity for fluoride to the limit of detection over other competing small anions was synthesized. Cage 4 was also found to retain the encapsulated fluoride anion within its cavity even after one or two pyrrolic NH protons were subject to deprotonation.

Synthesis and Anion Recognition Features of a Molecular Cage Containing Both Hydrogen Bond Donors and Acceptors

A molecular cage, macrobicycle 2, containing amide and pyrrole groups as hydrogen-bonding donors and imine groups as hydrogen-bonding acceptors has been synthesized. Compound 2 was found to recognize tetrahedral oxyanions with high affinities, such as H₂PO₄^-, HSO₄^-, SO₄^2-, and HP₂O₇^3-, as well as the spherical halide anions, in chloroform. A single-crystal X-ray diffraction analysis revealed that compound 2 formed a 1:1 complex with H₂PO₄^- in the solid state.

Supramolecular Chemistry of Anionic Dimers, Trimers, Tetramers and Clusters

Two or more anions constrained in close proximity within a single pocket are found in a number of natural systems but a less common motif in artificial systems. This review summarizes work on anion receptors capable of stabilizing anionic dimers, trimers, tetramers and clusters in a well-defined fashion. These systems may provide insights into the fundamental chemistry of anion-anion interactions and provide a guide for understanding in greater detail a number of biological and environmental processes, as well as key tenants of relevance to supramolecular chemistry, extraction, transport, crystal engineering, and the like. The primary goal of this review is to provide a general introduction into multi-anion recognition chemistry for the benefit of supramolecular and non-supramolecular chemists alike.

Pyrrole N-H Anion Complexes

Synthetic pyrrole-based anion receptors date back to the 1990s. They have been extensively developed in the context of macrocyclic systems as expanded porphyrins and calixpyrroles, and related systems. The chemistry of open-chain pyrrolic systems is, in many respects, no less venerable. It also has more direct analogy to naturally occurring pyrrole-based anion binding motifs. However, it has not been the subject of a comprehensive review. Presented herein is a summary of efforts devoted to the creation of de novo pyrrole-based receptors, as well as the anion recognition chemistry of naturally occurring pyrrolic systems as prodigiosins and their synthetic analogues.

Remarkably selective NH4+ binding and fluorescence sensing by tripodal tris(pyrazolyl) receptors derived from 1,3,5-triethylbenzene: structural and theoretical insights on the role of ion pairing

Differences in pyrazole substitution result in dramatic changes in NH₄⁺ binding and fluorescence sensing, due to ion pairing and solvation effects.

Non-cyclic formylated dipyrromethanes as phosphate anion receptors

Tetrakis- and hexakis(1H-pyrrole-2-carbaldehyde) anion receptors are described that undergo conformational reorganization in order to accommodate the dihydrogenphosphate and pyrophosphate anions.

New tetrakis- and hexakis(1H-pyrrole-2-carbaldehyde) anion receptors are described. The anion binding properties of these receptors were studied in organic media and in the solid state. The receptors displayed good affinity for the dihydrogenphosphate and pyrophosphate anions (as the tetrabutylammonium salts) in chloroform even in the presence of a polar protic solvent, methanol. Solution phase spectroscopic analyses proved consistent with the binding mode seen in single crystal structural studies of the dihydrogenphosphate and pyrophosphate complexes and provided support for the contention that these receptors undergo conformational reorganization in order to accommodate the bound oxoanions both in chloroform solution and in the solid state.

Anion directed conformational diversities of an arene based hexa-amide receptor and recognition of the [F4(H2O)6]4− cluster

The TOC shows difference in binding energies between different conformers after binding with anions of different dimensionalities and conformers A, B & C show structural diversities with anions in case of L.