Squaramide-based receptors in anion supramolecular chemistry: insights into anion binding, sensing, transport and extraction

Over the last 15 years, squaramide-based receptors have attracted the attention of supramolecular chemists working in the field of anion recognition. Herein, we highlight examples of squaramide-based receptors that are able to bind, sense, extract and transport anions.

Contemporary Developments in Ferrocene Chemistry: Physical, Chemical, Biological and Industrial Aspects

Ferrocenyl-based compounds have many applications in diverse scientific disciplines, including in polymer chemistry as redox dynamic polymers and dendrimers, in materials science as bioreceptors, and in pharmacology, biochemistry, electrochemistry, and nonlinear optics. Considering the horizon of ferrocene chemistry, we attempted to condense the neoteric advancements in the synthesis and applications of ferrocene derivatives reported in the literature from 2016 to date. This paper presents data on the progression of the synthesis of diverse classes of organic compounds having ferrocene scaffolds and recent developments in applications of ferrocene-based organometallic compounds, with a special focus on their biological, medicinal, bio-sensing, chemosensing, asymmetric catalysis, material, and industrial applications.

Selectivity Rule of Cryptands for Anions: Molecular Rigidity and Bonding Site

Cryptands utilize inside CH or NH groups as hydrogen bond (H-bond) donors to capture anions such as halides. In this work, the nature and selectivity of confined hydrogen bonds inside cryptands were computationally analyzed with the energy decomposition scheme based on the block-localized wavefunction method (BLW-ED), aiming at an elucidation of governing factors in the binding between cryptands and anions. It was revealed that the intrinsic strengths of inward hydrogen bonds are dominated by the electrostatic attraction, while the anion preferences (selectivity) of inner CH and NH hydrogen bonds are governed by the Pauli exchange repulsion and electrostatic interaction, respectively. Typical conformers of cages are classified into two groups, including the C_3(h) -symmetrical conformers, in which all halide anions are located near the centroids of cages, and the "semi-open" conformers, which exhibit shifted bonding sites for different halide anions. Accordingly, the difference in governing factors of selectivity is attributed to either the rigidity of cages or the binding site of anions for these two groups. In details, the C₃ conformers of NH cryptands can be enlarged more remarkably than the C_3(h) -symmetrical conformers of CH cryptands as the size of anion (ionic radius) increases, resulting in the relaxation of the Pauli repulsion and a dramatic reduction in electrostatic attraction, which eventually rules the selectivity of NH cryptands for halide anions. By contrary, the CH cryptands are more rigid and cannot effectively reduce the Pauli repulsion, which subsequently governs the anion preference. Unlike C₃ conformers whose rigidity determines the selectivity, semi-open conformers exhibit different binding sites for different anions. From F^- to I^- , the bonding site shifts toward the outside end of the pocket inside the semi-open NH cryptand, leading to the significant reduction of the electrostatic interaction that dominates the anion preference. Differently, binding sites are much less affected by the size of anion inside the semi-open CH cryptand, in which the Pauli exchange repulsion remains the key factor for the selectivity of inner hydrogen bonds.

Recent Advancements in Ion-Pair Receptors

Over the past two decades, non-covalent chemistry has introduced various promising artificial receptors and revolutionized the host-guest chemistry. These versatile receptors have particularly been entertained in sensing and recognizing of diverse neutral molecules and/or ionic entities (e. g. anions, cations and ion-pair) of particular interest. Notably, supramolecular chemistry had given birth to a plethora of important molecules, explored in the chemical, biological, environmental, and pharmacological world to resolve the critical issues related to the human health while keeping environmental concerns in mind. Amongst the various types of supramolecular monotopic receptors (anions, cations, and neutral molecules), heteroditopic receptors (ion-pair receptors) consisting of distinct binding sites in one system for both cation and anion, have gained much interest from the scientific community in recent past because of their unique binding abilities. Interestingly, these promising artificial receptors have shown potential applications in sensing, recognition, transport and extraction processes besides their uses in salt/waste purification. Bearing the importance of these systems in mind, we intended to report the recent developments in ion-pair chemistry. Herein, we divided the whole document into three main sections; first one describes the introduction and history of the ion-pairs receptors. The second portion highlights the synthesis and applications of ion-pair receptors in sensing, recognition, molecular machines, photoswitching behaviour, extraction and transport properties, whereas the last part of this manuscript provides concluding remarks as well as future prospects of ion-pair receptors. We hope that this manuscript will be helpful to stimulating researchers around the globe to find out the hidden opportunities in this and related areas.

Squaramide-Tethered Sulfonamides and Coumarins: Synthesis, Inhibition of Tumor-Associated CAs IX and XII and Docking Simulations

(1) Background: carbonic anhydrases (CAs) are attractive targets for the development of new anticancer therapies; in particular, CAs IX and XII isoforms are overexpressed in numerous tumors. (2) Methods: following the tail approach, we have appended a hydrophobic aromatic tail to a pharmacophore responsible for the CA inhibition (aryl sulfonamide, coumarin). As a linker, we have used squaramides, featured with strong hydrogen bond acceptor and donor capacities. (3) Results: Starting from easily accessible dimethyl squarate, the title compounds were successfully obtained as crystalline solids, avoiding the use of chromatographic purifications. Interesting and valuable SARs could be obtained upon modification of the length of the hydrocarbon chain, position of the sulfonamido moiety, distance of the aryl sulfonamide scaffold to the squaramide, stereoelectronic effects on the aromatic ring, as well as the number and type of substituents on C-3 and C-4 positions of the coumarin. (4) Conclusions: For sulfonamides, the best profile was achieved for the m-substituted derivative 11 (Ki = 29.4, 9.15 nM, CA IX and XII, respectively), with improved selectivity compared to acetazolamide, a standard drug. Coumarin derivatives afforded an outstanding selectivity (Ki > 10,000 nM for CA I, II); the lead compound (16c) was a strong CA IX and XII inhibitor (Ki = 19.2, 7.23 nM, respectively). Docking simulations revealed the key ligand-enzyme interactions.

Bis[squaramido]ferrocenes as electrochemical sulfate receptors

The bis[squaramido]ferrocene scaffold is introduced and shown to selectively bind and electrochemically report sulfate in competitive water/DMSO mixtures.

Fluorescence Recognition of Anions Using a Heteroditopic Receptor: Homogenous and Two-Phase Sensing

In contrast to monotopic receptor 3, the anthracene functionalized squaramide dual-host receptor 1 is capable of selectively extracting sulfate salts, as was evidenced unambiguously by DOSY, mass spectrometry, fluorescent and ion chromatography measurements. The receptors were investigated in terms of anion and ion pair binding using the UV-vis and 1H NMR titrations method in acetonitrile. The reference anion receptor 3, lacking a crown ether unit, was found to lose the enhancement in anion binding induced by the presence of cations. Besides the ability to bind anions in an enhanced manner exhibited by ion pair receptors 2 and 4, changing the 1-aminoanthracene substituent resulted in their exhibiting a lower anion affinity than receptor 1. By using receptor 1 and adjusting the water content in organic phase it was possible to selectively detect sulfates both by "turn-off" and "turn-on" fluorescence, and to do so homogenously and under interfacial conditions. Such properties of receptor 1 have allowed the development of a new type of sensor capable of recognizing and extracting potassium sulfate from the aqueous medium across a phase boundary, resulting in an appropriate fluorescent response in the organic solution.

Synthesis of Novel Crown Ether-Squaramides and Their Application as Phase-Transfer Catalysts

This work presents the synthesis of six new phase-transfer organocatalysts in which the squaramide unit is directly linked to the nitrogen atom of an aza-crown ether. Four chiral skeletons, namely hydroquinine, quinine, cinchonine (cinchonas), and α-d-glucopyranoside were responsible for the asymmetric construction of an all-carbon quaternary stereogenic center in α-alkylation and Michael addition reactions of malonic esters. We investigated the effects of these different chiral units and that of crown ethers with different sizes on catalytic activity and enantioselectivity. During extensive parameter investigations, both conventional and emerging green solvents were screened, providing valuable α,α-disubstituted malonic ester derivatives with excellent yields (up to 98%).

Real-time Voltammetric Anion Sensing Under Flow*

The development of real‐life applicable ion sensors, in particular those capable of repeat use and long‐term monitoring, remains a formidable challenge. Herein, we demonstrate, in a proof‐of‐concept, the real‐time voltammetric sensing of anions under continuous flow in a 3D‐printed microfluidic system. Electro‐active anion receptive halogen bonding (XB) and hydrogen bonding (HB) ferrocene‐isophthalamide‐(iodo)triazole films were employed as exemplary sensory interfaces. Upon exposure to anions, the cathodic perturbations of the ferrocene redox‐transducer are monitored by repeat square‐wave voltammetry (SWV) cycling and peak fitting of the voltammograms by a custom‐written MATLAB script. This enables the facile and automated data processing of thousands of SW scans and is associated with an over one order‐of‐magnitude improvement in limits of detection. In addition, this improved analysis enables tuning of the measurement parameters such that high temporal resolution can be achieved. More generally, this new flow methodology is extendable to a variety of other analytes, including cations, and presents an important step towards translation of voltammetric ion sensors from laboratory to real‐world applications.

Macrocyclic squaramides as ion pair receptors and fluorescent sensors selective towards sulfates

Through the high dilution technique, we obtained macrocyclic ion pair receptors R1 and R2, an anion receptor R3, and a fluorescent sensor R4 using a combination of particular members of simple libraries consisting of synthesized diamines and methyl squarates, respectively. The receptors were investigated in terms of anion and ion pair binding using the ¹H NMR titration method in DMSO-d₆. We found that the major contribution to the anion binding comes from the interaction with the squaramide protons rather than with the amide functions of the receptors. The receptors demonstrated the highest affinity towards benzoates and sulfates over the anions tested, and in the case of sulfate binding more complex equilibria in solution were observed. Unlike the anion receptor R3, the ion pair receptor R1 was found to recognize anions in an enhanced manner with the assistance of sodium or potassium cations. Tethering of a simple fluorophore in close proximity to the amide function of receptor R4 resulted in an optical ion pair sensor selective towards sulfates. DFT calculations carried out for the 1 : 1 complexes of R3 with the anions helped clarify this selectivity, showing more effective participation of tetrahedral sulfate anions in binding with the amide function than in the case of benzoates or chlorides.

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.

Cooperative Transport and Selective Extraction of Sulfates by a Squaramide-Based Ion Pair Receptor: A Case of Adaptable Selectivity

The use of a squaramide-based ion pair receptor offers a solution to the very challenging problem of extraction and transport of extremely hydrated sulfate salt. Herein we demonstrate for the first time that a neutral receptor is able not only to selectively extract but also to transport sulfates in the form of an alkali metal salt across membranes and to do so in a cooperative manner while overcoming the Hofmeister bias. This was made possible by an enhancement in anion binding promoted by cation assistance and by diversifying the stoichiometry of receptor complexes with sulfates and other ions. The existence of a peculiar 4:1 complex of receptor 2 with sulfates in solution was confirmed by UV-vis and 1H NMR titration experiments, DOSY and DLS measurements, and supported by solid-state X-ray measurements. By varying the separation technique and experimental conditions, it was possible to switch the depletion of the aqueous layer into extremely hydrophilic or less lipophilic salts, thus obtaining the desired selectivity.

Unraveling the Origin of Differentiable 'Turn-On' Fluorescence Sensing of Zn2+ and Cd2+ Ions with Squaramides

A squaramide ring conjugated with Schiff-bases decorated with hydroxy and methoxy functional groups differentially senses zinc and cadmium ions, which turn on the fluorescence. The feebly emitting free ligands light up in the presence of zinc and cadmium acetates, with the acetate ion playing a pivotal role as a conjugate anion. The selective and differentiable emission responses for zinc and cadmium ions make these ligands efficient multi-analyte sensing agents. Furthermore, these ligands could be used to differentially sense zinc and cadmium ions even in aqueous environments. The NMR investigations reveal marginal differences in the binding of zinc and cadmium ions to the ligands, whereas density functional theory calculations suggest the different extent of ligand-to-metal charge transfer (LMCT) contributes to the differential behavior. Finally, comparison of the excited-state dynamics of free ligand and the metal complexes reveal the appearance of longer lifetime (about 500-700 ps) component with complexation, due to rigidified molecular skeleton, thereby impeding the non-radiative processes.