Calix[4]arene‐Based Rotaxane Host Systems for Anion Recognition

The synthesis, structure and anion binding properties of the first calix[4]arene‐based [2]rotaxane anion host systems are described. Rotaxanes 9? Cl and 12? Cl, consisting of a calix[4]arene functionalised macrocycle wheel and different pyridinium axle components, are prepared via adaption of an anion templated synthetic strategy to investigate the effect of preorganisation of the interlocked host’s binding cavity on anion binding. Rotaxane 12? Cl contains a conformationally flexible pyridinium axle, whereas rotaxane 9? Cl incorporates a more preorganised pyridinium axle component. The X‐ray crystal structure of 9? Cl and solution phase ¹H NMR spectroscopy demonstrate the successful interlocking of the calix[4]arene macrocycle and pyridinium axle components in the rotaxane structures. Following removal of the chloride anion template, anion binding studies on the resulting rotaxanes 9? PF₆ and 12? PF₆ reveal the importance of preorganisation of the host binding cavity on anion binding. The more preorganised rotaxane 9? PF₆ is the superior anion host system. The interlocked host cavity is selective for chloride in 1:1 CDCl₃/CD₃OD and remains selective for chloride and bromide in 10 % aqueous media over the more basic oxoanions. Rotaxane 12? PF₆ with a relatively conformationally flexible binding cavity is a less effective and discriminating anion host system although the rotaxane still binds halide anions in preference to oxoanions.     

Carboxylated Pillar[5]arene‐Coated Gold Nanoparticles with Chemical Stability and Enzyme‐like Activity

A facile synthesis of gold nanoparticles (AuNPs) covered with a multidentate macrocycle, carboxylated pillar[5]arene ( CP ), via a one‐pot hydrothermal process is reported. The resulting AuNPs are highly stable against salts and pH variations, while their traditional counterparts are not stable at the same conditions. For the stabilization, multiple carboxylate groups of CP might contribute to electrostatic or steric stabilization. In addition, we found that CP ‐coated AuNPs exhibit greater peroxidase‐like activity than citrate‐stabilized AuNPs in the presence of silver cations. The system presented herein would provide a new scheme to fabricate unique sensory systems in combination with enzymes, which can bind to the pocket of CP .     

The Effect of Ligand Denticity in Size‐Selective Synthesis of Calix[n]arene‐Stabilized Gold Nanoparticles: A Multitechnique Approach

A series of gold nanoparticles (AuNPs) stabilized by monodentate, bidentate, and tridentate thiolate calix[n]arene ligands 1 – 3 was prepared by using the Brust–Schiffrin two‐phase direct synthesis and characterized with NMR spectroscopy, elemental analysis, transmission electron microscopy (TEM), and X‐ray photoelectron spectroscopy (XPS). The experimental data show that the particular multidentate structure of calix[n]arene derivatives 2 and 3 introduces a control element in the preparation of the gold nanoparticles that allows, in the particular experimental conditions here reported, to obtain very small (≈1 nm) AuNPs. These are the first experimental findings that identify a role of ligand “denticity” in the determination of the nuclearity of nanoparticles.     

Calix[4]arene‐Phosphine Dimers: Precursors of Flexible Metallo‐Capsules and Self‐Compacting Molecules

The first diphosphines based on a double calixarene, namely 1,4 (or 1,3)‐bis‐(5‐diphenylphosphino‐25,26,27,28‐tetrapropoxycalix[4]aren‐17‐yl)benzene ( L² , L³ ) were each prepared in four steps starting from 5,17‐dibromo‐25,26,27,28‐tetrapropoxycalix[4]arene. Upon reaction of L² with [Au(tht)(thf)]BF₄, (tht=C₄H₈S) a rigid metallo‐capsule was quantitatively formed, which adopts an oblique form owing to the distinct nature of the spacers linking the two calixarene half‐spheres. In the solid state, the 1,4‐substituted phenylene linker is turned towards the gold ion, suggesting the existence of weak bonding interactions between two aromatic CH protons of this ring and the metal centre (Au???H=2.67 Å). In contrast to this gold complex, the related silver complex shows dynamic behaviour in solution, the exchange between two enantiomeric oblique forms being facilitated by the greater stereochemical flexibility of Ag^I vs. Au^I. A heteronuclear ¹⁰⁹Ag{¹H} HMQC experiment established strong correlations between the CH protons of the phenylene linker and the ¹⁰⁹Ag ion. Dynamic behaviour similar to that observed for the silver complex was further observed in trans‐[PtCl₂? L² ], a chelate complex that could be obtained quantitatively from L² and [PtCl₂(PhCN)₂]. The intended formation of a chelate complex leading to a capsule with an endo‐oriented metal centre was achieved by reacting L³ with [Pd(allyl)(thf)₂]BF₄. The complex thus formed constitutes the first organometallic transition metal complex embedded in a cavity with large portals. Binding of [RuCl₂(p‐cymene)] to L² and L³ resulted in self‐compacting bimetallic complexes in which each calixarene basket entraps a Ru(p‐cymene) unit, thereby forming molecules occupying a minimal volume.     

2‐(4′‐Pyridyl‐N‐oxide)‐Substituted Hemithioindigos as Photoresponsive Guests for a Super Aryl‐Extended Calix[4]pyrrole Receptor

We report the synthesis of two 2‐(4′‐pyridyl‐N‐oxide)‐substituted hemithioindigos (HTIs). We probed their photoisomerization by using UV/Vis and ¹H NMR spectroscopy techniques. Light irradiation at λ=450 nm provoked the isomerization of the HTI Z isomer to the E counterpart to a large extent (≈80 % at the photostationary state). ¹H NMR titration experiments revealed the formation of thermodynamically and kinetically stable 1:1 inclusion complexes of the (Z)‐HTI isomers with a super aryl‐extended host (association constant>10⁴ m ^?1). Photoirradiation at λ=450 nm of the inclusion complexes induced the isomerization of the bound HTI N‐oxide to afford the (E)‐HTI?calix[4]pyrrole complex. We determined accurate association constant values for the 1:1 inclusion complexes of the (Z)‐ and (E)‐HTI isomers by using isothermal titration calorimetry experiments. The results showed that the stability constants of the (E)‐HTI complexes were 2.2–2.8‐fold lower than those of the (Z)‐HTI counterparts, which explains the lack of light‐induced release of the former to the bulk solution.     

Calix[4]arene‐Based Bis(Nitric Oxide) Complexes: Synthesis,Physical Properties,and Structural Characterization

Calix[4]arene‐based molecules hold great promise as candidate sensors and storage materials for nitric oxide (NO), owing to their unprecedented binding affinity for NO. However, the structure of calix[4]arene is complicated by the availability of four possible conformers: 1,3‐alternate, 1,2‐alternate, cone, and partial cone (paco). Whilst complexes of NO with several of these conformers have previously been established, the 1,2‐alternate conformer complex, that is, [1,2‐alter ? NO]⁺, has not been previously reported. Herein, we determine the crystal structure of the NO complex with the 1,2‐alternate conformer for the first time. In addition, we have also found that the 1,2‐alternate and 1,3‐alternate conformers can combine with two NO molecules to form stable bis(nitric oxide) complexes. These new complexes, which exhibit remarkable binding capacity for the construction of NO‐storage molecules, were characterized by using X‐ray crystallography and NMR, IR, and UV/Vis spectroscopy. These findings will extend our understanding of the interactions between nitric oxide and cofacially and non‐cofacially arrayed aromatic rings, and we expect them to aid in the design and development of new supramolecular sensors and storage materials for NO with high capacity and efficacy.     

Pillar[5]arene‐Mediated Synthesis of Gold Nanoparticles: Size Control and Sensing Capabilities

We present a simple procedure for the synthesis of quasi‐spherical Au nanoparticles in a wide size range mediated by macrocyclic host molecules, ammonium pillar[5]arene (AP[5]A). The strategy is based on a seeded growth process in which the water‐soluble pillar[5]arene undergoes complexation of the Au salt through the ammonium groups, thereby avoiding Au nucleation, while acting as a stabilizer. The presence of the pillar[5]arene onto the Au nanoparticle particle surface is demonstrated by surface‐enhanced Raman scattering (SERS) spectroscopy, and the most probable conformation of the molecule when adsorbed on the Au nanoparticles surface is suggested on the basis of theoretical calculations. In addition, we analyze the host–guest interactions of the AP[5]A with 2‐naphthoic acid (2NA) by using ¹H NMR spectroscopy and the results are compared with theoretical calculations. Finally, the promising synergetic effects of combining supramolecular chemistry and metal nanoparticles are demonstrated through SERS detection in water of 2NA and a polycyclic aromatic hydrocarbon, pyrene (PYR).     

Calix[6]arene‐Based Cascade Complexes of Organic Ion Triplets Stable in a Protic Solvent

Herein we report a D_3h‐symmetric tail‐to‐tail bis‐calix[6]arene 3 featuring two divergent cavities triply connected by ureido linkages. This calix[6]tube was synthesized by a domino Staudinger/aza‐Wittig reaction followed by a macrocyclization reaction. This process also afforded a C_2h‐symmetric isomer that represents a rare example of a self‐threaded rotaxane based on calix[6]arene subunits. The binding properties of 3 have been evaluated by NMR studies. Thus, bis‐calix[6]arene 3 is able to bind simultaneously two neutral ureido guests through an induced‐fit process. The guests are located in the cavities and are recognized through multiple hydrogen‐bonding interactions with the ureido bridges. Host 3 can also simultaneously bind multiple ions and is especially efficient for the complexation of organic ion triplets. The anion is recognized through hydrogen‐bonding interactions at the ureido binding site and is thus located between the two ammonium ions accommodated in the cavities. The resulting [1+1+2] quaternary complexes represent rare examples of cascade complexes with organic cations. These complexes are unique: 1) They are stable even in a markedly protic solvent, 2) the recognition of the ion triplets proceeds in a cooperative way through an induced‐fit process and with a high selectivity, linear cations and doubly charged anions being particularly well recognized, 3) the ions are bound as contact ion triplets thanks to the closeness of the three binding sites, 4) the cationic guests can be exchanged and thus mixed [1+1+1+1] complexes can be obtained, 5) the ureido linkers wrapped around the anion adopt a helical shape and the resulting chirality is sensed by the cations. In other words, bis‐calix[6]arene 3 presents a selective inner tunnel in which multiple guests such as organic ion triplets can be aligned in a cooperative way through induced‐fit processes.     

Self‐Assembly of Calix[6]arene–Diazapyrenium Pseudorotaxanes: Interplay of Molecular Recognition and Ion‐Pairing Effects

The calix[6]arene wheel CX forms pseudorotaxane species with the diazapyrenium‐based axle 1? 2PF₆ in CH₂Cl₂ solution. The macrocyclic component is a heteroditopic receptor, which can complex the electron‐acceptor moiety of the axle inside its cavity and the counterions with the ureidic groups on the upper rim. The self‐assembled supramolecular species is a complex structure, which involves three components—the wheel, the axle and its counterions—that can mutually interact and affect. The stoichiometry of the resulting supramolecular complex depends on the nature and concentration of the counterions. Namely, it is observed that in dilute solution and with low‐coordinating anions the axle takes two wheels, whereas with highly coordinating anions or in concentrated solutions the complex has a 1:1 stoichiometry.     

A Self‐Sorting Scheme Based on Tetra‐Urea Calix[4]arenes

Size and shape do matter : When dimerized in nonpolar solvents, an equimolar mixture of eleven tetra‐urea calix[4]arenes with different wide‐rim substituents self‐sorts into only six out of 35 different homo‐ and heterodimers (see picture). Since the calixarene scaffold and the four urea units are the same in all cases, the self‐sorting process is driven only by the cooperative action of steric requirements and stoichiometry.

     

Cavity‐Shaped Ligands: Calix[4]arene‐Based Monophosphanes for Fast Suzuki–Miyaura Cross‐Coupling

Five conical calix[4]arenes that have a PPh₂ group as the sole functional group anchored at their upper rim were assessed in palladium‐catalysed cross‐coupling reactions of phenylboronic acid with aryl halides (dioxane, 100 °C, NaH). With arylbromides, remarkably high activities were obtained with the catalytic systems remaining stable for several days. The performance of the ligands is comparable to a Buchwald‐type triarylphosphane, namely, (2′‐methyl[1,1′‐biphenyl]‐2‐yl)diphenylphosphane, which in contrast to the calixarenyl phosphanes tested may display chelating behaviour in solution. With the fastest ligand, 5‐diphenylphosphanyl‐25,26,27,28‐tetra(p‐methoxy)benzyloxy‐calix[4]arene ( 8 ), the reaction turnover frequency for the arylation of 4‐bromotoluene was 321 000 versus 214 000 mol(ArBr).mol(Pd)^?1. h^?1 for the reference ligand. The calixarene ligands were also efficient in Suzuki cross‐coupling reactions with aryl chlorides. Thus, by using 1 mol % of [Pd(OAc)₂] associated with one of the phosphanes, full conversion of the deactivated arenes 4‐chloroanisole and 4‐chlorotoluene was observed after 16 h. The high performance of the calixarenyl–phosphanes in Suzuki–Miyaura coupling of aryl bromides possibly relies on their ability to stabilise a monoligand [Pd⁰L(ArBr)] species through supramolecular binding of the Pd‐bound arene inside the calixarene cavity.