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We report the synthesis of a series of amphiphilic p‐sulfonatocalix[4]arenes with varying alkyl chain lengths (CX4‐Cn) and their application as efficient counterion activators for membrane transport of cell‐penetrating peptides (CPPs). The enhanced membrane activity is confirmed with the carboxyfluorescein (CF) assay in vesicles and by the direct cytosolic delivery of CPPs into CHO−K1, HCT 116, and KTC‐1 cells enabling excellent cellular uptake of the CPPs into two cancer cell lines. Intracellular delivery was confirmed by fluorescence microscopy after CPP entry into live cells mediated by CX4‐Cn, which was also quantified after cell lysis by fluorescence spectroscopy. The results present the first systematic exploration of structure‐activity relationships for calixarene‐based counterion activators and show that CX4‐Cn are exceptionally effective in cellular delivery of CPPs. The dodecyl derivative, CX4‐C12, serves as best activator. A first mechanistic insight is provided by efficient CPP uptake at 4 °C and in the presence of the endocytosis inhibitor dynasore, which indicates a direct translocation of the CPP‐counterion complexes into the cytosol and highlights the potential benefits of CX4‐Cn for efficient and direct translocation of CPPs and CPP‐conjugated cargo molecules into the cytosol of live cells. [ABSTRACT FROM AUTHOR]

The calix[4]arene scaffold, blocked in the cone conformation through alkylation with long alkyl chains, and decorated at the upper rim with four guanidine or arginine units, effectively catalyzes the cleavage of the phosphodiester bond of DNA and RNA model compounds in water. An exhaustive kinetic investigation unequivocally points to the existence of spontaneous aggregation phenomena, driven by hydrophobic effect, occurring at different critical concentrations that depend on the identity of the compound. A pronounced superiority of the assembled structures compared with the monomers in solution was observed. Moreover, the catalytically active units, clustered on the macrocyclic tetrafunctional scaffold, were proved to efficiently cooperate in the catalytic mechanism and result in improved reaction rates compared to those of the monofunctional model compounds. The kinetic analysis is also integrated and corroborated with further experiments based on fluorescence spectroscopy and light scattering. The advantage of the supramolecular assemblies based on tetrafunctional calixarenes leads to believe that the active units can cooperate not only intramolecularly but also intermolecularly. The molecules in the aggregates can probably mold, flex and rearrange but, at the same time, keep an ordered structure that favors phosphodiester bond cleavage. This dynamic preorganization can allow the catalytic units to reach a better fitting with the substrates and perform a superior catalytic activity. [ABSTRACT FROM AUTHOR]

Core‐fluorescent cavitands based on 2‐(2′‐resorcinol)benzimidazole fluorophores (RBIs) merged with the resorcin[4]arene skeleton were designed and synthesized. The cavitands, due to the presence of intramolecular hydrogen bonds and increased acidity, show excited state intramolecular proton transfer (ESIPT) and readily undergo deprotonation to form dianionic cavitands, capable of strong binding to organic cations. The changes in fluorescence are induced by deprotonation and binding events and involve huge Stokes shifts (due to emission from anionic double keto tautomers) and cation‐selective enhancement of emission originating from the restriction of intramolecular motion (RIR) upon recognition in the cavity. Ab initio calculations indicate that the macrocyclic scaffold stabilizes the ground state tautomeric forms of the fluorophores that are not observed for non‐macrocyclic analogs. In the excited state, the emitting forms for both macrocyclic scaffolds and non‐macrocyclic analogs are anionic double keto tautomers, which are the result of excited state intramolecular proton transfer (ESIPT) or excited state double proton transfer (ESDPT). [ABSTRACT FROM AUTHOR]

The recent discovery of calix[3]pyrrole, a porphyrinogen‐like tripyrrolic macrocycle, has provided an unprecedented strain‐induced ring expansion reaction into calix[6]pyrrole. Here, we synthesized calix[n]furan[3‐n]pyrrole (n=1∼3) macrocycles to investigate the reaction scope and mechanism of the ring expansion. Single crystal X‐ray analysis and theoretical calculations revealed that macrocyclic ring strain increases as the number of inner NH sites increases. While calix[1]furan[2]pyrrole exhibited almost quantitative conversion into calix[2]furan[4]pyrrole within 5 minutes, less‐strained calix[2]furan[1]pyrrole and calix[3]furan were inert. However, N‐methylation of calix[2]furan[1]pyrrole induced a ring‐expansion reaction that enabled the isolation of a linear reaction intermediate. The mechanism analysis revealed that the ring expansion consists of regioselective ring cleavage and subsequent cyclodimerization. This reaction was further utilized for synthesis of calix[6]‐type macrocycles. [ABSTRACT FROM AUTHOR]

Carbonic anhydrases (CAs) continue to represent a relevant pharmaceutical target. The need of selective inhibitors and the involvement of these metalloenzymes in many multifaceted diseases boost the search for new ligands able to distinguish among the different CA isoforms, and for multifunctional systems simultaneously able to inhibit CAs and to interfere with other pathological events by interacting with additional targets. In this work, we successfully explored the possibility of preparing new CAs ligands by combining calixarenes with benzensulfonamide units. Inhibition tests towards three human CA isoforms evidenced, for some of the ligands, Ki values in the nanomolar range and promising selectivity. X‐ray and molecular modeling studies provided information on the mode of binding of these calixarene derivatives. Thanks to the encouraging results and the structural features typical of the calixarene scaffold, it is then possible to plan for the future the design of multifunctional inhibitors for this class of widely spread enzymes. [ABSTRACT FROM AUTHOR]

In this study, the ligand exchange mechanism at a biomimetic ZnII centre, embedded in a pocket mimicking the possible constrains induced by a proteic structure, is explored. The residence time of different guest ligands (dimethylformamide, acetonitrile and ethanol) inside the cavity of a calix[6]arene‐based tris(imidazole) tetrahedral zinc complex was probed using 1D EXchange SpectroscopY NMR experiments. A strong dependence of residence time on water content was observed with no exchange occurring under anhydrous conditions, even in the presence of a large excess of guest ligand. These results advocate for an associative exchange mechanism involving the transient exo‐coordination of a water molecule, giving rise to 5‐coordinate ZnII intermediates, and inversion of the pyramid at the ZnII centre. Theoretical modelling by DFT confirmed that the associative mechanism is at stake. These results are particularly relevant in the context of the understanding of kinetic stability/lability in Zn proteins and highlight the key role that a single water molecule can play in catalysing ligand exchange and controlling the lability of ZnII in proteins. [ABSTRACT FROM AUTHOR]

The hierarchical assembly, in aqueous solution, of a new multi‐metalloporphyrin/calixarene aggregate has been accomplished. In this supramolecular system transfer of chirality, from the outermost components to the central porphyrin reporter, takes place as a result of favorable and fully noncovalent long‐range electronic communication. [ABSTRACT FROM AUTHOR]

This work describes a calix[6]arene‐based wheel that binds, in non‐polar media, a stilbazolium salt to yield a mixture of pseudorotaxane orientational isomers. The isomer's abundance ratio evolves with time and can be reversibly tuned by adjusting the temperature. The spectroscopic properties, and notably the emission spectrum, of the bound guest depend on its orientation inside the non‐palindromic wheel, suggesting such a system as a switch with spectroscopic readout. [ABSTRACT FROM AUTHOR]

A calix[4]arene ligand, in which two of the phenol functions are replaced by pyrazole units has been employed to mimic the His2–Tyr2 (His: histidine, Tyr: tyrosine) ligand sphere within the active site of the galactose oxidase (GO). The calixarene backbone forces the corresponding copper(II) complex into a see‐saw‐type structure, which is hitherto unprecedented in GO modelling chemistry. It undergoes a one‐electron oxidation that is centered at the phenolate donor leading to a copper‐coordinated phenoxyl radical like in the GO. Accordingly, the complex was tested as a functional model and indeed proved capable of oxidizing benzyl alcohol to the respective aldehyde using two phenoxyl‐radical equivalents as oxidants. Finally, the results show that the calixarene platform can be utilized to arrange donor functions to biomimetic binding pockets that allow for the creation of novel types of model compounds. [ABSTRACT FROM AUTHOR]

Macrocyclic compounds (calix[4]‐ and calix[6]arene derivatives) with aryl rings interconnected by spirocyclopropyl groups have been synthesized and structurally characterized. The compounds were prepared by the reaction of dichlorocarbene with calixarenes possessing exocyclic double bonds at the bridges, followed by reductive perdechlorination of the spirocyclopropyl groups. In all systems, pairs of geminal rings connected to the quaternary spiro carbon atoms are oriented anti, and the methylene groups of the cyclopropyl rings are located in isoclinal positions. Calix[6]rotane adopts in the crystal and in solution a 1,3,5‐alternate conformation. The presence of the spirocyclopropyl groups increases the rigidity of the macrocyclic ring. [ABSTRACT FROM AUTHOR]

Covalently linked multiple calixarenes are valued in supramolecular chemistry. This work reports an easy and versatile synthetic route to covalently linked double and triple calix[4]arene and calix[6]arenes by a novel DMF‐controlled selective alkylation of a convenient and readily available upper‐rim dimethylaminomethyl‐substituted tetrahydroxy and hexahydroxy calix[4]arene and ‐[6]arenes. Synthetic routes to upper‐rim functionalised redox active disulfide‐linked double‐, tetra‐ and peptidohybrid‐calixarenes employing either redox chemistry (CH2SH) or thiolates (CH2S−) are also opened up from the same key starting material. Double and triple calix[4]‐ and calix[6]arenes are easily accessible by a novel selective alkylation reaction employing n‐propyl mesylate in DMF. Disulfide‐linked double‐ and peptidohybrid‐calixarenes and a tetra‐calixarene, formed either redox chemistry (CH2SH) or thiolate (CH2S−) addition, are also described. [ABSTRACT FROM AUTHOR]

Calixarenes have found widespread application as building blocks for the design and synthesis of functional materials in host–guest chemistry. The ongoing desire to develop a detailed understanding of the nature of NO bonding to multichromophoric π‐stacked assemblies led us to develop an electron‐rich methoxy derivative of calix[4]arene (3), which we show exists as a single conformer in solution at ambient temperature. Here, we examine the redox properties of this derivative, generate its cation radical (3+.) using robust chemical oxidants, and determine the relative efficacy of its NO binding in comparison with model calixarenes. We find that 3/3+. is a remarkable receptor for NO+/NO, with unprecedented binding efficacy. The availability of precise experimental structures of this calixarene derivative and its NO complex, obtained by X‐ray crystallography, is critically important both for developing novel functional NO biosensors, and understanding the role of stacked aromatic donors in efficient NO binding, which may have relevance to biological NO transport. A methoxy‐substituted calixarene derivative is an effective receptor for NO with unprecedented binding efficiency, the largest binding constant among all known NO complexes. [ABSTRACT FROM AUTHOR]

Abstract: Operating molecular machines are based on switchable systems whose components can be set in motion in a controllable fashion. The presence of nonsymmetrical elements is a mandatory requirement to obtain and demonstrate the unidirectionality of motion. Calixarene‐based macrocycles have proved to be very efficient hosts in the design of oriented rotaxanes and of pseudorotaxanes with strict control over the direction of complexation. A series of two‐station rotaxanes based on bipyridinium–ammonium axles was synthesized and characterized. A recently reported supramolecularly assisted strategy for the synthesis of different orientational isomers was exploited, and the ammonium unit was identified as a proper secondary station for the calixarene. Displacement of the macrocycle was triggered by electrochemical reduction of the bipyridinium primary station, and it was shown that the shuttling is influenced both by the length of the chain of the axle component and by the position of the secondary station with respect to the calixarene rims. [ABSTRACT FROM AUTHOR]

Abstract: The triple‐stranded metallohelicates 1 a,/1 b–3 a/3 b possessing internal guest binding cavities surrounded by calix[4]arene units were synthesized through coordination‐driven self‐assembly. UV/Vis titration experiments verified that the metallohelicates encapsulated N‐methyl pyridinium cations bearing amino acid groups to form host–guest complexes. The guest chirality was transferred to the helicity of the helicates through the steric contact between the stereogenic center of the amino acid group and the metal cores. The (M) helicity was induced when guests the (R)‐4‐‐(R)‐6 were accommodated within the cavities. The multiple guest complexation within the self‐assembled helicates 2 a and 3 a displayed large positive cooperative effects, indicating that the first guest complexation preorganizes the rest of the cavities to facilitate a subsequent guest binding. This cooperativity results in the majority‐rules effect in the chiral guest binding for 2 a and 3 a. [ABSTRACT FROM AUTHOR]

Abstract: Environmental gas sensing needs stringent sensor requirements in terms of sensitivity, selectivity and ruggedness. One of the major issues to be addressed is combining in a single device the conflicting requirements of molecular‐level selectivity and low‐ppb sensitivity. The exploitation of synthetic molecular receptors as sensing materials is particularly attractive to address the selectivity issue, to single out the desired analytes in the presence of overwhelming amounts of interferents. This minireview summarizes the strategies in environmental gas and vapor sensing using molecular receptors as selective hosts for specific analytes, with the main focus on cavitands. In particular, we highlight the use of these macrocycles as selective preconcentrator units to be integrated into portable devices for environmental monitoring. Depending on the class of analytes to be detected, the molecular recognition properties of cavitands can be manipulated through the proper choice of the bridging groups at the upper rim, and their transducer integration can be implemented through the manifold functionalization options at the lower rim. [ABSTRACT FROM AUTHOR]

Abstract: The self‐assembly of internally ion‐paired, neutral AA/BB‐type supramolecular polymers composed of complementary di‐ionizable homoditopic pairs of monomers is reported. Host‐to‐guest double‐proton transfer mediates the recognition between bis‐calix[5]arenedicarboxylic acids and α,ω‐diaminoalkanes to yield cyclic, doughnut‐shaped assemblies with morphologies (i.e., cyclic vs. linear) that can be controlled by means of external chemical stimuli. The behavior of these intriguing aggregates, both in solution and on surfaces, has been investigated by a combination of 1H and DOSY NMR spectroscopy, light‐scattering, and atomic force microscopy. [ABSTRACT FROM AUTHOR]

The interactions of two mono-functionalized sulfonatocalix[ 4]arenes with cytochromec were investigated by structural and thermodynamic methods. The replacement of a single sulfonate with either a bromo or a phenyl substituent resulted in altered recognition of cytochrome c as evidenced by X-ray crystallography. The bromo-substituted ligand yielded a new binding mode in which a self-encapsulated calixarene dimer contributed to crystal packing. This ligand also formed a weak halogen bond with the protein. The phenyl-substituted ligand was bound to Lys4 of cytochrome c, in a 1.7 a resolution crystal structure. A dimeric packing arrangement mediated by ligand-ligand contacts in the crystal suggested a possible assembly mechanism. The different protein recognition properties of these calixarenes are discussed. [ABSTRACT FROM AUTHOR]

Mimicking the antibacterial activity of polyphenols in synthetic systems is an attractive approach for the development of new active pharmaceutical ingredients. Resorcinarenes represent a class of polyphenols, which have been exploited for decades for their attractive chemical scaffold suitable for forming host–guest complexes with hydrophobic guest molecules. However, the polyphenolic character of resorcinarenes, which could be a potential asset to the pharmaceutical industry, have been least exploited. The present work represents an unprecedented interplay of antimicrobial activity of resorcinarene together with its ability to interact chemically with an antibacterial drug gatifloxacin, improving the overall antibacterial activity. The chemistry and the clinical activities involved in this study were investigated simultaneously by spectroscopic techniques, as well as by in vitro measurement of antibacterial activity toward two human bacterial pathogens, a Gram-positive pathogen Staphylococcus aureus and a Gram-negative lung pathogen Legionella pneumophila. The initial positive result obtained from this study could revolutionize the use of synthetically modifiable resorcinarenes and their analogues in fine tuning the clinical behavior of drugs. [ABSTRACT FROM AUTHOR]

Calix[4]arenes are versatile ligands that, whilst also serving other purposes, can act as platforms for the synthesis of a wide range of 3 d, 4 f, and 3 d-4 f polymetallic clusters. The empirical metal ion binding rules established for calix[4]arene are closely mirrored by bis-calix[4]arene, a relatively new ligand in which two equivalents of the former are directly tethered at a methylene bridge position. The direct tethering within bis-calix[4]arene gives rise to some structural features that are related to calix[4]arene coordination chemistry, but the prevailing clusters have fascinating new topologies and coordination behaviors. Here, we present the synthesis of a family of new bis-calix[4]arene-supported 3 d-4 f clusters, as well as their structural characterization and magnetic properties. Comparison is drawn with calix[4]arene coordination chemistry, showing logical extension of common structural fragments and cluster capping behaviors upon moving to bis-calix[4]arene. This approach therefore holds great potential for tuning cluster formation and composition at a high level through subsequent ligand alteration. [ABSTRACT FROM AUTHOR]

Fully exploiting the electronic and mechanical properties of 2D laminar materials not only requires efficient and effective means of their exfoliation into low dimensional layers, but also necessitates a means of changing their morphology so as to explore any enhancement that this may offer. MXenes are a rapidly emerging new class of such laminar materials with unique properties. However, access to other morphologies of MXenes has not yet been fully realised. To this end we have developed the synthesis of MXenes (Ti2C) as plates, crumpled sheets, spheres and scrolls, which involves selective intercalation of p-phosphonic calix[ n]arenes, with control in morphology arising from the choice of the size of the macrocycle, n=4, 5, 6, or 8. This opens up wider avenues of discovery/design for new morphologies from the wider family of MXenes beyond Ti2C, along with opportunities to exploit any new physico-chemical properties proffered. [ABSTRACT FROM AUTHOR]

The simple tetraminocalix[4]arene 1, which contains weak H-bond-donor NH2 groups, is reported to be a highly efficient organocatalyst for the Vinylogous Mukaiyama Aldol Reaction (VMAR) of 2-(trimethylsilyloxy)furan 5 with α-ketoesters 6 a- l under 'on-water' conditions owing to the hydrophobic amplification of weak H-bond interactions. The catalytic efficiency of calixarene catalyst 1 was shown to be closely related to its recognition abilities towards the reactants 5 and 6 through a multipoint recognition model. The proposed model provided good explanations for the differences on the reaction rate acceleration and on the stereoselectivity observed with different substrates. [ABSTRACT FROM AUTHOR]

A heteroditopic ligand associated with a calix[6]arene scaffold bearing a tris(imidazole) coordinating site at its small rim and an amine/pyridine ligand at its large rim has been prepared, and its regioselective coordination to ZnII at the small rim and FeII in the amine/pyridine ligand has been achieved. The heterodinuclear complex obtained displays an overall cone conformation capped by the tris(imidazole)ZnII moiety and bears a non-heme FeII complex at its base. Each of the metal centers exhibits one labile position, allowing the coordination inside the cavity of a guest alkylamine at ZnII and the generation of reaction intermediates (FeIII(OOH) and FeIVO) at the large rim. A dependence between the chain length of the encapsulated alkylamine and the distribution of FeIII(OOH) intermediates and FeIII(OMe) is observed. In addition, it is shown that the generation of the FeIVO intermediate is enhanced by addition of the alkylamine guest. Hence, this supramolecular system gathers the three levels of reactivity control encountered in oxidoreductases: i) control of the FeII redox properties through its first coordination sphere, allowing us to generate high valent reactive species; ii) control of guest binding through a hydrophobic funnel that drives its alkyl chain next to the reactive iron complex, thus mimicking the binding pocket of natural systems; iii) guest-modulated reactivity of the FeII center towards oxidants. [ABSTRACT FROM AUTHOR]

The development of highly selective sensors for potassium is of great interest in biology. Two new hydrosoluble potassium sensors (Calix-COU-Alkyne and Calix-COU-Am) based on a calix[4]arene bis(crown-6) and an extended coumarin were synthesized and characterized. The photophysical properties and complexation studies of these compounds have been investigated and show high molar extinction coefficients and high fluorescence quantum yields. Upon complexation with potassium in the millimolar concentration range, an increase of one- and two-photon fluorescence emission is detected. A twofold fluorescence enhancement is observed upon excitation at λ=405 nm. The ligands present excellent selectivity for potassium in the presence of various competitive cations in water and in a physiological medium. The photophysical properties are not affected by the presence of a large amount of competing cations (Na+, Ca2+, Mg2+, etc.). Ex vivo measurements on mouse hippocampal slices show that Calix-COU-Alkyne accumulates extracellularly and does not alter the neuronal activity. Furthermore, the sensor can be utilized to monitor slow extracellular K+ increase induced by inhibition of K+ entry into the cells. [ABSTRACT FROM AUTHOR]

Cone calix[4]arenes and calix[6]arenes bearing two, three, and four short peptide units each having two chiral carbon atoms were prepared. The syntheses were performed by using an efficient modular approach that includes the Ugi preparation of the azido-peptide followed by its reactions with the propargylated calixarenes under CuAAC (CuI-catalyzed azide-alkyne cycloaddition) conditions. The three novel multitopic hosts were probed for their ability to bind metal ions by UV titration, and showed the highest complexation efficiency towards copper(II) and lead(II). These two cations possessed quite different complexation modes with copper(II) bound predominantly by multiple-triazole sites, in contrast to lead(II), which is stabilized mainly by multiple interactions with amide groups of the peptide units. Circular dichroism data for the free chiral hosts, their equimolar mixtures with copper(II) perchlorate and lead(II) perchlorate, and for tertiary mixtures of all three compounds showed the formation of mono- and binuclear complexes, or a switching behavior, depending on the structure of the host and the addition order of the cations. [ABSTRACT FROM AUTHOR]

Calix[4]arenes (C[4]s) are versatile platforms for the construction of polymetallic clusters containing paramagnetic metal ions. Synthetic modification at the C[4] methylene bridge allows for the design of bis-C[4]s that, depending on the linker employed, can be used to either dictate which clusters can be formed or direct the assembly of a new metal-organic polyhedron (MOP). The assembly resulting from the latter approach displays thermal stability and uptake of N2 or H2 gas, confirming that this is a viable route to the synthesis of new, functional supramolecular architectures. [ABSTRACT FROM AUTHOR]

A phenylenediamine-capped conjugate of calix[4]arene ( Lamino) was synthesized by reducing its precursor, Limino, with sodium borohydride in methanol. The Lamino sample binds to anions due to the more flexible and bent conformation of the capped aminophenolic binding core, compared to the precursor Limino. The Lamino sample showed selectivity towards H2PO4− by exhibiting a ratiometric increase in emission by about 11-fold with a detection limit of (1.2±0.2) μ m ((116±20) ppb) over 15 anions studied, including other phosphates, such as P2O74−, adenosine monophosphate (AMP2−), adenosine diphosphate (ADP2−), and adenosine triphosphate (ATP2−). The Lamino sample shows an increase in the absorbance at λ=315 nm in the presence of H2PO4−, CO32−, HCO3−, CH3CO2−, and F−. The 1H NMR spectroscopic titration of Lamino with H2PO4−, F−, and CH3CO2− showed major changes in the phenylene-capped and salicyl moieties, and thereby, confirming the aminophenolic region as the binding core. However, the binding strength of these anions followed the trend H2PO4−>F−≫CH3CO2−>HSO4−. The heat changes observed by isothermal titration calorimetry support this trend. The Lamino sample showed reversible sensing towards H2PO4− and F− in the presence of Mg2+ and Ca2+, respectively. NOESY studies of Lamino, in comparison with its anionic complexes, revealed that major conformational changes occurred in the capping region to facilitate the binding of anion. ESI-MS and the Job's method revealed 1:1 stoichiometry between Lamino and H2PO4− or F−. In the SEM micrographs of Lamino, the spherical particles are converted into spherical aggregates and further form large agglomerates and even branched sheets in the presence of anions, depending upon their binding strength. [ABSTRACT FROM AUTHOR]

Molecular recognition of small molecules and ions by artificial receptors in microheterogeneous media such as micelles and vesicles can, in principle, provide better models of biological systems in comparison with bulk solutions. In this work we have investigated the complexation of an organic fluorescent probe with amphiphilic calixarene receptor below and above the critical micelle concentration (CMC). For concentrations below the CMC, the probe forms a host-guest complex with the calixarene behaving like a traditional host-guest system operating in bulk solution. Above the CMC, multiple equilibrium processes are established and the probe can exchange between the recognition site of the calixarene in the monomeric state, micellized state and/or the micellar hydrophobic core. Careful analysis of the results obtained from NMR spectroscopy and fluorescence experiments allowed us to propose a quantitative model to describe the system. The increment of the local concentration of Na+ counterions at the Stern layer displace the dye to the micelle core through competitive binding of Na+ in the cavity of the receptor and is decisive for the observed self-sorting behavior. [ABSTRACT FROM AUTHOR]

Six tetraaza[1.1.1.1]cyclophane derivatives bearing peripheral amide groups were prepared according to two distinct synthetic strategies that depend on the connection pattern between the aryl units. NMR experiments combined with the X-ray structures of two tetraamide derivatives 4 b and 10 show that these cavitands adopt a 1,3-alternate conformation both in solution and in the solid state. Consequently, the four amide groups of the aza[1.1.1.1] -m, m, m, m-cyclophane isomer 10 can contribute to the same recognition process towards neutral water molecules or anion guests. NMR experiments, mass spectrometry analyses and single-crystal X-ray structures confirm the anion-binding ability of this receptor. Absorption spectrophotometric titrations in nonpolar solvents provided evidence for the selectivity of 10 to chloride anions in the halide series, with a corresponding association constant Ka reaching 2.5×106 m−1. [ABSTRACT FROM AUTHOR]

The Huisgen thermal reaction between an organic azide and an acetylene was employed for the selective monofunctionalization of a X6-azacryptand ligand bearing a tren coordinating unit [X6 stands for calix[6]arene and tren for tris(2-aminoethyl)amine]. Supramolecular assistance, originating from the formation of a host-guest inclusion complex between the reactants, greatly accelerates the reaction while self-inhibition affords a remarkable selectivity. The new ligand possesses a single amino-leg appended at the large rim of the calixarene core and the corresponding Zn2+ complex was characterized both in solution and in the solid state. The coordination of Zn2+ not only involves the tren cap but also the introverted amino-leg, which locks the metal ion in the cavity. Compared with the parent ligand deprived of the amino-leg, the affinity of the new monofunctionalized X6tren ligand 6 for Zn2+ is found to have a 10-fold increase in DMSO, which is a very competitive solvent, and with an enhancement of at least three orders of magnitude in CDCl3/CD3OD (1:1, v/v). In strong contrast with the fast binding kinetics, decoordination of Zn2+ as well as transmetallation appeared to be very slow processes. The monofunctionalized X6tren ligand 6 fully protects the metal ion from the external medium thanks to the combination of a cavity and a closed coordination sphere, leading to greater thermodynamic and kinetic stabilities. [ABSTRACT FROM AUTHOR]

A bis- tert-alcohol-functionalized crown-6-calix[4]arene (BACCA) was designed and prepared as a multifunctional organic promoter for nucleophilic fluorinations with CsF. By formation of a CsF/BACCA complex, BACCA could release a significantly active and selective fluoride source for SN2 fluorination reactions. The origin of the promoting effects of BACCA was studied by quantum chemical methods. The role of BACCA was revealed to be separation of the metal fluoride to a large distance (>8 Å), thereby producing an essentially 'free' F−. The synergistic actions of the crown-6-calix[4]arene subunit (whose O atoms coordinate the counter-cation Cs+) and the terminal tert-alcohol OH groups (forming controlled hydrogen bonds with F−) of BACCA led to tremendous efficiency in SN2 fluorination of base-sensitive substrates. [ABSTRACT FROM AUTHOR]

A series of mononuclear and dinuclear platinum(II) terpyridine complexes with amide-, sulfonamide-, and urea-containing ligands ( 1- 9) has been successfully designed and synthesized, and their photophysical and anion-binding properties have been studied. The anion-binding properties of the complexes were studied by electronic absorption, emission, and 1H NMR spectroscopy. Upon the binding with anions, the dinuclear complexes 1- 3 exhibit large electronic absorption and emission spectral changes, while for the mononuclear analogues 4- 6 much smaller spectral changes were observed. Interesting colorimetric responses towards different anions were observed in dinuclear complexes 1- 3. Dinuclear complexes 7- 9 are found to be insensitive to anion-binding, which could be attributed to the presence of intramolecular Pt⋅⋅⋅Pt interactions and/or π-π stacking in the complexes that led to the reduced anion-binding affinity. [ABSTRACT FROM AUTHOR]

Short segments of zigzag single-walled carbon nanotubes (SWCNTs) were obtained from a calixarene scaffold by using a completely new, simple and expedited strategy that allowed fine-tuning of their diameters. This new approach also allows for functionalised short segments of zigzag SWCNTs to be obtained; a prerequisite towards their lengthening. These new SWCNT short segments/calixarene composites show interesting behaviour in solution. DFT analysis of these new compounds also suggests interesting photophysical behaviour. Along with the synthesis of various SWCNTs segments, this approach also constitutes a powerful tool for the construction of new, radially oriented π systems. [ABSTRACT FROM AUTHOR]

Enzyme-responsive assembly represents one of the increasingly significant topics in biomaterials research and finds feasible applications to the controlled release of therapeutic agents at specific sites at which the target enzymes are located. In this work, based on the concept of host-guest chemistry, a trypsin-responsive supramolecular vesicle using p-sulfonatocalix[4]arene as the macrocyclic host and natural serine protease trypsin-cleavable cationic protein protamine as the guest molecule, is reported. The complexation of p-sulfonatocalix[4]arene with protamine directs the formation of a supramolecular binary vesicle, which is dissipated by trypsin with high selectivity. Therefore, the present system represents a principle-of-concept to build a controlled-release carrier at trypsin-overexpressed sites. [ABSTRACT FROM AUTHOR]

A synthetic route towards homodiselenacalix[4]arene macrocycles is presented, based on the dynamic covalent chemistry of diselenides. The calixarene inner rim is decorated with either alkoxy or tert-butyl ester groups. Single-crystal X-ray analysis of two THF solvates with methoxy and ethoxy substituents reveals the high similarity of their molecular structures and alterations on the supramolecular level. In both crystal structures, solvent channels are present and differ in both shape and capacity. Furthermore, the methoxy-substituted macrocycle undergoes a single-crystal-to-single-crystal transformation during which the molecular structure changes its conformation from 1,3-alternate (loaded with THF/water) to 1,2-alternate (apohost form). Molecular modelling techniques were applied to explore the conformational and energetic behaviour of the macrocycles. [ABSTRACT FROM AUTHOR]

A series of lipophilic gold nanoparticles (AuNPs) circa 5 nm in diameter and having a mixed organic layer consisting of 1-dodecanethiol and 1-(11-mercaptoundecyl) pyridinium bromide was synthesised by reacting tetraoctylammonium bromide stabilised AuNPs in toluene with different mixtures of the two thiolate ligands. A bidentate ω-alkylthiolate calix[4]arene derivative was instead used as a functional protecting layer on AgNPs of approximately 3 nm. The functionalised nanoparticles were characterised by transmission electron microscopy (TEM), and by UV/Vis and X-ray photoelectron spectroscopy (XPS). Recognition of the pyridinium moieties loaded on the AuNPs by the calix[4]arene units immobilised on the AgNPs was demonstrated in solution of weakly polar solvents by UV/Vis titrations and DLS measurements. The extent of Au-AgNPs aggregation, shown through the low-energy shift of their surface plasmon bands (SPB), was strongly dependent on the loading of the pyridinium moieties present in the organic layer of the AuNPs. Extensive aggregation between dodecanethiol-capped AuNPs and the Ag calix[4]arene-functionalised NPs was also promoted by the action of a simple N-octyl pyridinium difunctional supramolecular linker. This linker can interdigitate through its long fatty tail in the organic layer of the dodecanethiol-capped AuNPs, and simultaneously interact through its pyridinium moiety with the calix[4]arene units at the surface of the modified AgNPs. [ABSTRACT FROM AUTHOR]

A new fluorescent 1,3-diaminonaphthalimide conjugate of calix[4]arene receptor ( R) was synthesized and characterized. The receptor displays good selectivity towards trinitrophenol (TNP) over other explosive aromatic- and aliphatic-nitro compounds by exhibiting changes in its fluorescence emission. Receptor-coated cellulose paper strips are equally effective in terms of their selective detection of TNP over other aromatic- and aliphatic-nitro compounds. When used in solution or on cellulose paper strips, R can detect up to submicromolar concentration of TNP by exhibiting changes in its fluorescence emission and in its supramolecular structure upon interaction. Interestingly, the microscopy features of R, TNP, and { R+TNP} are quite distinct, indicating the interactions present between R and TNP, as studied by using AFM and TEM. Computationally modeled complexes of receptor with TNP and TNT show enormous difference in their interaction energies in the favor of TNP by showing the host-guest interaction of cation ⋅⋅⋅anion type in the presence of TNP but not TNT. This is because the receptor adopts an 'arms-open'-type structure in the case of the TNP complex, whereas it adopts an 'arms-closed'-type structure in the presence of TNT. Both the experimental and the computational studies reveal that the receptor selectively binds to TNP over TNT. Thus, R-coated Whatman No.1 filter paper strips provide easy, rapid, and economical detection of trace amounts of TNP both by visual and spectral measurement. [ABSTRACT FROM AUTHOR]

The central MnII ions in a series of calix[4]arenestabilised butterflies can be sequentially replaced with LnIII ions, maintaining the structural integrity of the molecule but transforming its magnetic properties. The replacement of MnII for GdIII allows for the examination of the transferability of spin-Hamiltonian parameters within the family as well as permitting their reliable determination. The introduction of the 4f ions results in weaker intramolecular magnetic exchange, an increase in the number of low-lying excited states, and an increase in magnetisation relaxation, highlighting the importance of exchange over single-ion anisotropy for the observation of SMM behaviour in this family of complexes. The presence of the [TMII/III(TBC[4])(OH)(solvent)] metalloligand (TM=transition metal, TBC=p-tBu-calix[4]arene) suggests that magnetic calix[n]arene building blocks can be employed to encapsulate a range of different "guests" within structurally robust "hosts". [ABSTRACT FROM AUTHOR]

Tetraannulation of a resorcinarene-octaamino cavitand with ferrocenecarboxaldehyde allows the preparation of a tetrabenzimidazole-resorcinarene cavitand with four ferrocenyl moieties directly linked to the C2 atom of the imidazole units. Oxidation of the four ferrocenyl moieties produces important structural modifications of the molecule, as indicated by DFT calculations performed for the neutral and tetraoxidized forms of the cavitand. By means of ¹H NMR spectroscopic analysis, the encapsulating properties of the new tetraferrocenyl-resorcinarene cavitand toward a series of ammonium salts were evaluated, and a clear cutoff point in binding affinity with respect to size was observed. Cyclic voltammetric studies allowed us to estimate the relative association constants for the neutral and oxidized forms of the cavitand, thus indicating that the guest was bound to the neutral (reduced) state of the cavitand and was released from the oxidized form. These redox-addressable conformational and binding properties of the resorcinarene-tetraferrocenyl cavitand constitute all the necessary features of a redox-switchable molecular gripper. By means of massspectrometric analysis, we could unambiguously confirm the molar stoichiometry of the host-guest complex (1:1) and assess the strong guest encapsulation, as indicated by triggering the covalent coupling between host and guest in the gas phase. [ABSTRACT FROM AUTHOR]

New oval-shaped capsular and bilayer-type hydrogen-bonded arrangements of C-propyl-ol-pyrogallol[4]arene (PgC3-OH) with bipyridine-type spacer complexes are reported here. These complexes are engineered by virtue of derivatization of C-alkyl tails of pyrogallol[4]arene and the use of divergent spacer ligands. Complexes of PgC3-OH, PgC3-OH with bpy (4,4'-bipyridine) and PgC3-OH with bpa (1,2-bis(4-pyridyl)acetylene) have bilayer type arrangements; however, the use of hydrogen chloride causes protonation of bpy molecule, which is then entrapped flat within an offset ovalshaped dimeric hydrogen-bonded PgC3-OH nanocapsule. The presence of chloride anion in the crystal lattice controls the geometry of the resultant nanoassembly. [ABSTRACT FROM AUTHOR]

A procedure for studying "dynamic structural behavior" of large chiral macrocycles is presented. Ion mobility MS, diffusion-ordered NMR spectroscopy (DOSY NMR), and optical rotation (OR) measurements, supported by calculations, are used together as effective complementary methods to study dynamic formation of noncovalent aggregates. It is shown that the monomer-dimer equilibrium is driven by π-π or CH-π interactions and controlled largely by the substitution pattern of the calixsalen skeleton. [ABSTRACT FROM AUTHOR]

Copper(I)-catalyzed addition of limited amounts of azides to multiple alkynes, which led to statistical mixtures of triazole/acetylene derivatives or, in other cases, resulted in preferred formation of multiple triazoles, was studied at pre-organizable calixarene platforms bearing up to four propargyl groups. Depending on calixarene structures and reaction conditions, the unprecedented specific or selective formation of exhaustively triazolated calixarenes or a complete loss of the selectivity were observed. Both autocatalytic copper activation and a local copper(I) concentration increase due to copper-triazole complexation were thoroughly studied as the most expected reasons for the selectivity and both were disproved. Mixed triazolated/propargylated calixarenes and their copper(I) complexes proved not to be involved in the cascade-like process that was modeled to be driven by an intramolecular transfer of two copper(I) ions from a just-formed binuclear copper intermediate to the adjacent acetylene unit. [ABSTRACT FROM AUTHOR]

A calix[4]arene host equipped with two bis-[Zn(salphen)] complexes self-assembles into a capsular complex in the presence of a chiral diamine guest with an unexpected 2:1 ratio between the host and the guest. Effective chirality transfer from the diamine to the calix-salen hybrid host is observed by circular dichroism (CD) spectroscopy, and a high stability constant K2,1 of 1.59×1011 M−2 for the assembled host-guest ensemble has been determined with a substantial cooperativity factor α of 6.4. Density functional calculations are used to investigate the origin of the stability of the host-guest system and the experimental CD spectrum compared with those calculated for both possible diastereoisomers showing that the M, M isomer is the one that is preferentially formed. The current system holds promise for the chirality determination of diamines, as evidenced by the investigated substrate scope and the linear relationship between the ee of the diamine and the amplitude of the observed Cotton effects. [ABSTRACT FROM AUTHOR]

Solvothermal reactions of the calix[4]arene tetraacetic acid (H4CTA) with zinc nitrate in the presence of α,ω-diaminoalkanes afford two-dimensional metallopolycapsular networks of the formula {[Me2NH2]2[G@(Zn2(CTA)2)] ⋅(DMF)2 ⋅(H2O)4} n (G=+NH3-(CH2) n-NH3+, n=2, 3, 4; DMF= N, N-dimethylformamide). These metallopolycapsular networks are built up of metallocapsules that consist of two CTA and two ZnII ions. Short alkanediyldiammonium (+NH3-(CH2) n-NH3+, n=2, 3, 4) guest ions are accommodated in each capsule of the metallopolycapsular network through a variety of supramolecular interactions. The thermal behaviours and the solid-state photoluminescent properties of these complexes were also investigated. [ABSTRACT FROM AUTHOR]

Methylene-bridged calix[4]arenes have emerged as extremely versatile ligand supports in the formation of new polymetallic clusters possessing fascinating magnetic properties. Metal ion binding rules established for this building block allow one to partially rationalise the complex assembly process. The ability to covalently link calix[4]arenes at the methylene bridge provides significantly improved control over the introduction of different metal centres to resulting cluster motifs. Clusters assembled from bis-calix[4]arenes and transition metal ions or 3 d-4 f combinations display characteristic features of the analogous calix[4]arene supported clusters, thereby demonstrating an enhanced and rational approach towards the targeted synthesis of complex and challenging structures. [ABSTRACT FROM AUTHOR]

We have established that bipyridinium radicals can be reversibly π-dimerized under the combined effects of chemical (proton transfer) and electrochemical (electron transfer) stimuli. Our investigations also led to the discovery that a bis-pyridinyl appended calixarene intermediate is involved in a fully reversible redox-triggered σ-dimerization process. The structure of the most stable intramolecular σ-dimer was provided by computational chemistry and its complete conversion into a noncovalent π-dimer could be triggered chemically by addition of protons, leading to the formation of protonated cation radicals. Theoretical data collected with the N-methylated and N-protonated π-dimers also support the existence of multivariant orientations in π-bonded dimers of viologen cation-radicals. [ABSTRACT FROM AUTHOR]

A biscalix[5]arene-C60 supramolecular structure was utilized for the development of supramolecular fullerene polymers. Di- and tritopic hosts were developed to generate the linear and network supramolecular polymers through the complexation of a dumbbell-shaped fullerene. The molecular association between the hosts and the fullerene were carefully studied by using 1H NMR, UV/Vis absorption, and fluorescence spectroscopy. The formation of the supramolecular fullerene polymers and networks was confirmed by diffusion-ordered 1H NMR spectroscopy (DOSY) and solution viscometry. Upon concentrating the mixtures of di- or tritopic hosts and dumbbell-shaped fullerene in the range of 1.0-10 mmol L−1, the diffusion coefficients of the complexes decreased, and the solution viscosities increased, suggesting that large polymeric assemblies were formed in solution. Scanning electron microscopy (SEM) was used to image the supramolecular fullerene polymers and networks. Atomic force microscopy (AFM) provided insight into the morphology of the supramolecular polymers. A mixture of the homoditopic host and the fullerene resulted in fibers with a height of (1.4±0.1) nm and a width of (5.0±0.8) nm. Interdigitation of the alkyl side chains provided secondary interchain interactions that facilitated supramolecular organization. The homotritopic host generated the supramolecular networks with the dumbbell-shaped fullerene. Honeycomb sheet-like structures with many voids were found. The growth of the supramolecular polymers is evidently governed by the shape, dimension, and directionality of the monomers. [ABSTRACT FROM AUTHOR]