Novel calix[4]arene-based receptors with bis-squaramide moieties for colorimetric sensing of anions via two different interaction modes

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.

Squaramide-Based Heteroditopic [2]Rotaxanes for Sodium Halide Ion-Pair Recognition

A series of squaramide-based heteroditopic [2]rotaxanes consisting of isophthalamide macrocycle and squaramide axle components are synthesized using an alkali metal cation template-directed stoppering methodology. This work highlights the unprecedented sodium cation template coordination of the Lewis basic squaramide carbonyls for interlocked structure synthesis. Extensive quantitative 1 H NMR spectroscopic anion and ion-pair recognition studies reveal the [2]rotaxane hosts are capable of cooperative sodium halide ion-pair mechanical bond axle-macrocycle component recognition, eliciting up to 20-fold enhancements in binding strengths for bromide and iodide, wherein the Lewis basic carbonyls and Lewis acidic NH hydrogen bond donors of the squaramide axle motif operate as cation and anion receptive sites simultaneously in an ambidentate fashion. Notably, varying the length and nature of the polyether cation binding unit of the macrocycle component dramatically influences the ion-pair binding affinities of the [2]rotaxanes, even overcoming direct contact NaCl ion-pair binding modes in polar organic solvents. Furthermore, the cooperative ion-pair binding properties of the squaramide-based heteroditopic [2]rotaxanes are exploited to successfully extract solid sodium halide salts into organic media.

Potentiometric Sensing of Nonsteroidal Painkillers by Acyclic Squaramide Ionophores

We report here a small library of a new type of acyclic squaramide receptors (L1-L5) as selective ionophores for the detection of ketoprofen and naproxen anions (KF- and NS-, respectively) in aqueous media. 1H NMR binding studies show a high affinity of these squaramide receptors toward KF- and NS-, suggesting the formation of H-bonds between the two guests and the receptors through indole and -NH groups. Compounds L1-L5 have been tested as ionophores for the detection of KF- and NS- inside solvent PVC-based polymeric membranes. The optimal membrane compositions were established through the careful variation of the ligand/tridodecylmethylammonium chloride (TDMACl) anion-exchanger ratio. All of the tested acyclic squaramide receptors L1-L5 have high affinity toward KF- and NS- and anti-Hofmeister selectivity, with L4 and L5 showing the highest sensitivity and selectivity to NS-. The utility of the developed sensors for a high precision detection of KF- in pharmaceutical compositions with low relative errors of analysis (RSD, 0.99-1.4%) and recoveries, R%, in the range 95.1-111.8% has been demonstrated. Additionally, the chemometric approach has been involved to effectively discriminate between the structurally very similar KF- and NS-, and the possibility of detecting these analytes at concentrations as low as 0.07 μM with R2 of 0.947 and at 0.15 μM with R2 of 0.919 for NS- and KF-, respectively, was shown.

A comparative performance evaluation of cephalosporin's drugs for fluoride recognition

In this manuscript, readily available cephalosporin's drugs cefuroxime axetil (L1) cefpdoxime proxetil (L2), and cefditoren pivoxil (L3) possess dihydrothiazine ring as signaling unit, and -NH groups as the binding site were used for the sensing of fluoride (F^-) ions. In the presence of F^-, the drug selectively portrayed a naked-eye detectable color change from colorless. The binding constant of 1:1 stoichiometric complex of L1, L2, and L3 with F^- was found to be 2.36 × 10⁴ M^-1, 2.44 × 10³ M^-1 and 1.02 × 10⁴ M^-1 respectively. The lowest detection limit (LOD) of F^- was found to be 11 µM (209 ppb) with drug L1 and L2. The binding mechanism of the drug with F^- was studied by ¹H and ¹⁹F nuclear magnetic resonance (NMR) spectral titration, electrospray ionization mass spectra (ESI-MS) analysis, and density functional theory (DFT) studies. The presence of F^- was monitored in various spiked water and Colgate toothpaste samples. Overall, cephalosporin's drug demonstrates a promising potential for the detection of F^- ions in the semi-aqueous phase.

Head vs. Tail Squaramide–Naphthalimide Conjugates: Self-Assembly and Anion Binding Behaviour

The syntheses of two squaramide–naphthalimide conjugates (SN1 and SN2) are reported; the structures of SN1 and SN2 differ by the attachment of a squaramide—either at the ‘head’ or the ‘tail’ of the naphthalimide fluorophore. Both compounds displayed weak fluorescence due to the inclusion of a nitro-aromatic squaramide which efficiently quenches the emission of the naphthalimide. Both compounds were also shown to undergo self-aggregation as studied by 1H NMR and scanning electron microscopy (SEM). Furthermore, SN1 and SN2 gave rise to stark colourimetric changes in response to basic anions such as AcO−, SO42− HPO42−, and F−. The observed colour changes are thought to be due to deprotonation of a squaramide NH. The same basic anions also result in a further quenching of the naphthalimide emission. No colour change or emission modulations were observed in the presence of Cl−; however, 1H NMR studies suggest that moderate H-bonding occurs between this anion and both SN1 and SN2.

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.

Allosteric binding properties of a 1,3-alternate thiacalix[4]arene-based receptor having phenylthiourea and 2-pyridylmethyl moieties on opposite faces

Thiacalix[4]arene receptors with two thiourea moieties and two 2-pyridyl moieties exhibited a heteroditopic dinuclear receptor with F− and Ag+ ions by a positive allosteric effect.

Multi-charged bis(p-calixarene)/pillararene functionalized gold nanoparticles for ultra-sensitive sensing of butyrylcholinesterase

A series of supramolecular assemblies based on multi-charged calixarene (SC4A), bis(p-calixarene) (BSC4A) and pillararene (CP5A) modified gold nanoparticles (AuNP) was constructed to realize colorimetric sensing of both succinylcholine (SuCh) and butyrylcholinesterase (BChE). With the high binding affinity of BSC4A and CP5A towards SuCh, BSC4A-AuNPs and CP5A-AuNPs could assemble with micromolar level SuCh as SuCh-BSC4A/CP5A-AuNPs. More interestingly, the enzymatic hydrolysis of SuCh by BChE could lead to the disassembly of SuCh-BSC4A/CP5A-AuNPs and provide a sensitive time-dependent color change from blue to red which could be observed by the naked eye and used to monitor BChE activity. As BChE activity is an important biomarker for diseases and poor health conditions, this novel supramolecular tandem colorimetric sensing strategy may have potential use for early diagnosis of diseases.

Phosphate and polyphosphate anion recognition by a dinuclear copper(ii) complex of an unsymmetrical squaramide

At pH = 7.4, ATP⁴⁻ is selectively removed from water containing PhPO₄²⁻, Haep⁻, AMP²⁻ and ADP³⁻ by the studied squaramide-based dicopper(ii) complex.

Recognition and Extraction of Sodium Chloride by a Squaramide-Based Ion Pair Receptor

We synthesized an ion pair receptor 1 consisting of a crown ether cation binding site and a squaramide anion binding domain and compared its binding properties to those of its analogous urea counterpart 2. We studied their salt binding properties using spectrophotometric and spectroscopic measurements in an acetonitrile solution and in acetonitrile/water mixtures. Apart from carboxylate anions, all of the anions tested were found to associate with receptor 1 and 2 more strongly in the presence of sodium cations. A homotopic anion receptor 3, lacking a crown ether unit, was unable to bind sodium salt more strongly than tetrabutylammonium salts. Solution and solid-state X-ray measurements revealed strong sodium chloride coordination to receptor 1, which is able to bind this salt even in the presence of 10% water. In contrast to the urea-based ion pair receptor 2 or anion receptor 3, ditopic receptor 1 is capable of extracting sodium chloride from aqueous media to the organic phase, as was evidenced unambiguously by ¹H nuclear magnetic resonance, mass spectrometry, and atomic absorption spectroscopy analyses.

Rapid detection of HSO4- in water: Novel immobilized azo-azomethine colorimetric anion receptors on solid supports

The immobilized azo-azomethine receptors on amorphous SiO₂, S-B, SiO₂ nanoparticles, S-NPs, and NaY zeolite, S-ZY, have been prepared and applied as solid phase sensors for detection of HSO₄^-, over other interfering anions, in 100% aqueous media. Remarkably, S-B and S-ZY show unique and rapid sensitivity towards HSO₄^-, which could it easily visualized through naked eye detection even at 5×10^-4molL^-1 and 4×10^-4molL^-1, respectively. The fabricated solid phase sensors were characterized using powder XRD diffraction, TGA-DTA, FE-SEM and also FT-IR techniques. Moreover, the related molecular anion receptor, HL, has been prepared and used for naked eye detection of F^- and AcO^-, in dry DMSO. The anions recognition ability of HL was also evaluated using UV-Vis and ¹H NMR spectroscopic methods.

Recent applications of chiral calixarenes in asymmetric catalysis

The use of calixarenes in asymmetric catalysis is receiving increasing attention due to their tunable three-dimensional molecular platforms along with their easy syntheses and versatile modification at the upper and lower rims. This review summarizes the recent progress of synthesis and use of chiral calixarenes in asymmetric syntheses which emerged later than 2010.

A novel water soluble pillar[5]arene and phenazine derivative self-assembled pseudorotaxane sensor for the selective detection of Hg2+ and Ag+ with high selectivity and sensitivity

A bifunctional supramolecular pseudorotaxane chemosensor (G-WAP) based on pillar[5]arene and phenazine imidazole was constructed, which could detect Hg²⁺ and Ag⁺ in water.

Miranda M, Busschaert N, Howe ENW, Clarke HJ, Haynes CJE, Kirby IL, Rodilla AM, Pérez-Tomás R, Gale PA, Félix V. Full elucidation of the transmembrane anion transport mechanism of squaramides using in silico investigations

The anion carrier mechanism promoted by squaramide-based molecules has been elucidated by molecular dynamics and chloride efflux studies.

Kinetic Analysis and Mechanism of the Hydrolytic Degradation of Squaramides and Squaramic Acids

The hydrolytic degradation of squaramides and squaramic acids, the product of partial hydrolysis of squaramides, has been evaluated by UV spectroscopy at 37 °C in the pH range 3-10. Under these conditions, the compounds are kinetically stable over long time periods (>100 days). At pH >10, the hydrolysis of the squaramate anions shows first-order dependence on both squaramate and OH^-. At the same temperature and [OH^-], the hydrolysis of squaramides usually displays biphasic spectral changes (A → B → C kinetic model) with formation of squaramates as detectable reaction intermediates. The measured rates for the first step (k₁ ≈ 10^-4 M^-1 s^-1) are 2-3 orders of magnitude faster than those for the second step (k₂ ≈ 10^-6 M^-1 s^-1). Experiments at different temperatures provide activation parameters with values of ΔH^⧧ ≈ 9-18 kcal mol^-1 and ΔS^⧧ ≈ -5 to -30 cal K^-1 mol^-1. DFT calculations show that the mechanism for the alkaline hydrolysis of squaramic acids is quite similar to that of amides.

A dual-channel chemosensor could successively detect CN− and HSO4− in an aqueous solution and act as a keypad lock

A dual-channel sensor (T) based on a phenazine derivative has been designed and synthesized. The sensor T could successively detect CN⁻ and HSO₄⁻ in aqueous solution with high selectivity and sensitivity.

A study of anion binding behaviour of 1,3-alternate thiacalix[4]arene-based receptors bearing urea moieties

Thiacalix[4]arene receptors with two urea moieties exhibited a high affinity towards all of the selected anions.

A novel calix[4]arene-based bifunctional squaramide organocatalyst for enantioselective Michael addition of acetylacetone to nitroolefins

A new chiral calix[4]arene-based organocatalyst 3 bearing bis-squaramide moieties was designed and synthesized from p-tert-butylcalix[4]arene. This bifunctional organocatalyst was used in the enantioselective conjugate addition of acetyl acetone to β-nitrostyrenes. The corresponding adducts were obtained in good to excellent yields with high enantioselectivities.

A turn-on fluorescent sensor for relay recognition of two ions: from a F−-selective sensor to highly Zn2+-selective sensor by tuning electronic effects

A simple ion sensor bearing quinoline and an amide group was designed and synthesized, which showed both colorimetric detection for F⁻ and a fluorescence turn-on response for Zn²⁺. Moreover, sensor L2 can distinguish F⁻ and Zn²⁺via different sensing mechanisms.

Colorimetric detection of anions in aqueous media using N-monosubstituted diaminomaleonitrile-based azo-azomethine receptors: real-life applications

New N-monosubstituted diaminomaleonitrile-based azo-azomethine dyes have been synthesized in order to develop colorimetric sensors for detection of biologically important anions in aqueous media. Importantly, the reported sensor decorated with strong electron-withdrawing group can detect inorganic fluoride in water even at 0.037 ppm level, which is lower than WHO permissible level (below 1 ppm). Successfully, the prepared dyes were used for qualitative and quantitative detection of inorganic fluoride in toothpaste and mouthwash. The anion recognition mechanism was also investigated by detailed UV-Vis and (1)H NMR experiments. The detailed (1)H NMR experiments corroborated that anion recognition is based on the deprotonation phenomenon.

Positive and negative allosteric effects of thiacalix[4]arene-based receptors having urea and crown-ether moieties

Allosteric binding behaviors of heteroditopic receptors bearing the 1,3-alternate conformation based thiacalix[4]arene have been evaluated by absorption and ¹H NMR titration experiments.

Two and three input molecular logic operations mediated by a novel azo-azomethine based chromogenic probe through intramolecular charge transfer processes

A novel azo-azomethine based chromogenic receptor has been successfully designed and synthesized for the construction of multifunctional molecular logic circuits.

Naked-eye detection of biologically important anions by a new chromogenic azo-azomethine sensor

A new chromogenic azo-azomethine sensor, containing active phenolic sites, has been designed and synthesized via condensation reaction of N,N,N',N'-tetrakis(2-aminoethyl)-2,2-dimethyl propane-1,3-diamine with 1-(3-formyl-4-hydroxyphenylazo)-4-nitrobenzene. The anion recognition ability of the synthesized receptor was evaluated using UV-Vis spectroscopy and (1)H NMR technique. The anion recognition studies exhibited that the receptor acts as a sensor for biologically important anions such as F(-), AcO(-) and H2PO4(-) over other anions. The binding stoichiometry between sensor and anions was found to be 1:2. (1)H NMR experiment revealed that sensor recognizes anions via H-bonds and subsequent deprotonation to elicit a vivid color change. Interestingly, the sensory system not only let for the naked eye detection without any spectroscopic instrumentation but also helped to discriminate between anions.

Anion-induced dimerization in p-squaramidocalix[4]arene derivatives

Spherical anions induce the dimerization of calix[4]arene derivatives 3 and 4 bearing squaramide moieties at the exo rim (p-squaramidocalixarenes). (1)H NMR titration experiments showed that unlike the distal isomer 3, proximal p-squaramidocalixarene 4 is also able to form dimeric complexes with trigonal-planar anions.

A catalyst-free approach to a novel imidazo [4,5-f][1,10] phenanthroline ligand and its corresponding ruthenium(ii) complex: insights into their applications in colorimetric anion sensing

Novel imidazo[4,5-f][1,10]phenanthroline ligand and its corresponding Ru(ii) complex are synthesized from green protocol for colorimetric detection of anions in water.