Environmentally responsive threading, dethreading, and fixation of anion-induced pseudorotaxanes

Hydrogen-Bonded Organic Frameworks Chelated Manganese for Precise Magnetic Resonance Imaging Diagnosis of Cancers

Magnetic resonance imaging (MRI) is an important tool in the diagnosis of many cancers. However, clinical gadolinium (Gd)-based MRI contrast agents have limitations, such as large doses and potential side effects. To address these issues, we developed a hydrogen-bonded organic framework-based MRI contrast agent (PFC-73-Mn). Due to the hydrogen-bonded interaction of water molecules and the restricted rotation of manganese ions, PFC-73-Mn exhibits high longitudinal relaxation r1 (5.03 mM-1 s-1) under a 3.0 T clinical MRI scanner. A smaller intravenous dose (8 μmol of Mn/kg) of PFC-73-Mn can provide strong contrast and accurate diagnosis in multiple kinds of cancers, including breast tumor and ultrasmall orthotopic glioma. PFC-73-Mn represents a prospective new approach in tumor imaging, especially in early-stage cancer.

Hybrid Silver(I)-Doped Soybean Oil and Potato Starch Biopolymer Films to Combat Bacterial Biofilms

This study describes the preparation, characterization, and antimicrobial properties of novel hybrid biopolymer materials doped with bioactive silver(I) coordination polymers (bioCPs). Two new bioCPs, [Ag₂(μ₆-hfa)]_n (1) and [Ag₂(μ₄-nda)(H₂O)₂]_n (2), were assembled from Ag₂O and homophthalic (H₂hfa) or 2,6-naphthalenedicarboxylic (H₂nda) acids as unexplored building blocks. Their structures feature 2D metal-organic and supramolecular networks with 3,6L64 or sql topology. Both compounds act as active antimicrobial agents for producing bioCP-doped biopolymer films based on epoxidized soybean oil acrylate (SBO) or potato starch (PS) as model biopolymer materials with a different rate of degradability and silver release. BioCPs and their hybrid biopolymer films (1@[SBO]_n, 2@[SBO]_n, 1@[PS]_n, and 2@[PS]_n) with a very low loading of coordination polymer (0.05-0.5 wt %) show remarkable antimicrobial activity against Staphylococcus aureus and Staphylococcus epidermidis (Gram-positive) and Escherichia coli and Pseudomonas aeruginosa (Gram-negative) bacteria. Biopolymer films also effectively impair the formation of bacterial biofilms, allowing total biofilm inhibition in several cases. By reporting on new bioCPs and biopolymer films obtained from renewable biofeedstocks (soybean oil and PS), this study blends highly important research directions and widens a limited antimicrobial application of bioCPs and derived functional materials. This research thus opens up the perspectives for designing hybrid biopolymer films with outstanding bioactivity against bacterial biofilms.

Rigidity and Flexibility in Rotaxanes and Their Relatives; On Being Stubborn and Easy-Going

Rotaxanes are an emerging class of molecules composed of two building blocks: macrocycles and threads. Rotaxanes, and their pseudorotaxane and polyrotaxane relatives, serve as prototypes for molecular-level switches and machines and as components in materials like elastic polymers and 3D printing inks. The rigidity and flexibility of these molecules is a characteristic feature of their design. However, the mechanical properties of the assembled rotaxane and its components are rarely examined directly, and the translation of these properties from molecules to bulk materials is understudied. In this Review, we consider the mechanical properties of rotaxanes by making use of concepts borrowed from physical organic chemistry. Rigid molecules have fewer accessible conformations with higher energy barriers while flexible molecules have more accessible conformations and lower energy barriers. The macrocycles and threads become rigidified when threaded together as rotaxanes in which the formation of intermolecular interactions and increased steric contacts collectively reduce the conformational space and raise barriers. Conversely, rotational and translational isomerism in rotaxanes adds novel modes of flexibility. We find that rigidification in rotaxanes is almost universal, but novel degrees of flexibility can be introduced. Both have roles to play in the function of rotaxanes.

Synthesis of Pyridine Containing Crowned Fused Expanded Porphyrins

New examples of nonaromatic fused expanded porphyrins containing both pyridine and crown ether moiety as a part of macrocyclic framework were synthesized by condensing pyridine based pentapyrrane with polyether based diol in CH 2 Cl 2 in the presence of one equivalent of BF 3 ·OEt 2 under inert conditions followed by oxidation with DDQ in open air. The condensation was expected to form pyridine containing crowned expanded porphyrins but resulted in the formation of fused crowned expanded porphyrins due to intramolecular fusion of two pyrrole "N"s with two adjacent inverted thiophene "C"s as revealed by X-ray crystallography obtained for one of the macrocycle. The HRMS and NMR study supported the formation of fused crowned pyridine containing expanded porphyrins, and the macrocycles showed simple well-resolved NMR spectra where all resonances were identified easily by 2D NMR spectroscopy. The macrocycles exhibited typical nonaromatic absorption features and showed one broad band with peak maxima at 535 nm and one or two shoulder bands in the higher energy region. The protonation studies resulted in clear colour change from purple to blue and absorption bands experienced bathochromic shifts with a broad band at 662 nm which was extended up to 800 nm. The electrochemical studies revealed that the macrocycles were easier to oxidize but difficult to reduce. DFT studies indicated that the macrocycle attains a very puckered and distorted 'U' shaped structure owing to the flexibility of the crown ether chain and TD-DFT studies corroborated experimental results. The preliminary studies indicated that the macrocycles could be used as colorimetric optical sensor for detection of Cu 2+ ion.

Visualizing molecular weights differences in supramolecular polymers

Molecular weight determinations play a vital role in the characterization of supramolecular polymers. They are essential to assessing the degree of polymerization, which in turn can have a significant impact on the properties of the polymer. While numerous characterization methods have been developed to estimate the number-average molecular weight (M_n) of supramolecular polymers, a simple visual method could provide advantages in terms of ease of use. We have now developed a system wherein differences in the fluorescent signature, including changes in color, allow variations in the M_n of an anion-responsive supramolecular polymer [M1·Zn(OTf)₂]_n to be readily monitored. The present visual differentiation strategy provides a tool that may be used to characterize supramolecular polymers.

Issues of molecular weight determination have been central to the development of supramolecular polymer chemistry. Whereas relationships between concentration and optical features are established for well-behaved absorptive and emissive species, for most supramolecular polymeric systems no simple correlation exists between optical performance and number-average molecular weight (M_n). As such, the M_n of supramolecular polymers have to be inferred from various measurements. Herein, we report an anion-responsive supramolecular polymer [M1·Zn(OTf)₂]_n that exhibits monotonic changes in the fluorescence color as a function of M_n. Based on theoretical estimates, the calculated average degree of polymerization (DP_cal) increases from 16.9 to 84.5 as the monomer concentration increases from 0.08 mM to 2.00 mM. Meanwhile, the fluorescent colors of M1 + Zn(OTf)₂ solutions were found to pass from green to yellow and to orange, corresponding to a red shift in the maximum emission band (λ_max). Therefore, a relationship between DP_cal and λ_max could be established. Additionally, the anion-responsive nature of the present system meant that the extent of supramolecular polymerization could be regulated by introducing anions, with the resulting change in M_n being readily monitored via changes in the fluorescent emission features.

Coordination-Adaptive Polydentate Pseudorotaxane Ligand for Capturing Multiple Uranyl Species

The propensity of uranyl for hydrolysis in aqueous environments prevents precise control of uranyl species in the scenarios of on-demand separation and tailored synthesis. Herein, using cucurbit[7]uril (CB[7]) as the macrocyclic molecule and 4,4'-bipyridine-N,N'-dioxide (DPO) as the string molecule, we propose a new kind of multidentate pseudorotaxane ligand, DPO@CB[7] for capturing uranyl species at different pH's. With the aprotic nature of DPO for metal coordination, the coordination ability of the DPO@CB[7] ligand is less affected by pH and can work in a wide range of pH's. Furthermore, by adaptive uranyl coordination, this aprotic pseudorotaxane ligand achieves effective recognition for different uranyl species ranging from monomeric to tetrameric originating from hydrolysis at varying pH's, and four novel uranyl-rotaxane compounds (URC1-4) are successfully obtained. Single-crystal X-ray diffraction analysis reveals that the DPO@CB[7] ligand coordinates with uranyl centers from monomeric to tetrameric in four different modes, as a result of structural flexibility of the DPO@CB[7] pseudorotaxane ligand. A detailed discussion for conformation flexibility of the DPO@CB[7] ligand has been conducted on the position changes of the DPO ligand trapped in the CB[7], which thus reveals good adaptivity of DPO@CB[7] that is noncovalently bonded as a supramolecular motif. In addition, characterization of the physicochemical properties of URC1 and URC2 with high phase purity, including powder X-ray diffraction (PXRD), infrared spectroscopy (IR), thermogravimetric analysis (TGA), and luminescence properties, are also provided. This work provides a good case of an adaptive pseudorotaxane ligand for the recognition and capture of different uranyl species and will bring valuable hints to the design of multifunctional supramolecular ligands for actinide separation in the future.

Hydrophilic Tetraphenylethene-Based Tetracationic Cyclophanes: NADPH Recognition and Cell Imaging With Fluorescent Switch

A hydrophilic TPE-based tetracationic cyclophane TPE-cyc was synthesized, which could capture intracellular Nicotinamide adenine dinucleotide phosphate and fuel the antioxidative ability of tumor cells to detoxify reactive oxygen species (ROS). Meanwhile, upon the reduction by cellular GSH, TPE-cyc could light up tumor cells, acting as a GSH-responsive fluorescent switch to image cells with high resolution.

Synthesis and aqueous anion recognition of an imidazolium-based nonacationic cup

An imidazolium-based nonacationic cup (1·9X; X = PF₆^- or Cl^-) was synthesized via step-by-step S_N2 reactions without using any template. The water-soluble 1·9Cl^- as a molecular container can encapsulate anionic nucleoside triphosphate and dinucleotide molecules (e.g., ATP and NADH) inside its cavity through hydrogen bonds and electrostatic interactions in aqueous solution.

Synergistic regulation of nonbinary molecular switches by protonation and light

We report a molecular switching ensemble whose states may be regulated in synergistic fashion by both protonation and photoirradiation. This allows hierarchical control in both a kinetic and thermodynamic sense. These pseudorotaxane-based molecular devices exploit the so-called Texas-sized molecular box (cyclo[2]-(2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene); 1⁴⁺, studied as its tetrakis-PF₆ ^- salt) as the wheel component. Anions of azobenzene-4,4'-dicarboxylic acid (2H⁺•2) or 4,4'-stilbenedicarboxylic acid (2H⁺•3) serve as the threading rod elements. The various forms of 2 and 3 (neutral, monoprotonated, and diprotonated) interact differently with 1⁴⁺, as do the photoinduced cis or trans forms of these classic photoactive guests. The net result is a multimodal molecular switch that can be regulated in synergistic fashion through protonation/deprotonation and photoirradiation. The degree of guest protonation is the dominating control factor, with light acting as a secondary regulatory stimulus. The present dual input strategy provides a complement to more traditional orthogonal stimulus-based approaches to molecular switching and allows for the creation of nonbinary stimulus-responsive functional materials.

Pb2+-Containing Metal-Organic Rotaxane Frameworks (MORFs)

The metal-organic rotaxane framework (MORF) structures with the advantage of mechanically interlocking molecules (MIMs) have attracted intense interest from the chemical community. In this study, a set of MORFs (i.e., MORF-Pb-1 and MORF-Pb-2) are constructed using Pb2+, a tetraimidazolium macrocycle (Texas-sized molecular box; 14+), and aromatic dicarboxylate (p-phthalate dianions (PTADAs; 2) or 2,6-naphthalene dicarboxylate dianions (3)) via a one-pot three-layer diffusion protocol. In particular, an unusual Pb…Pb weak interaction was shown in MORF-Pb-1 (charactered with distance of 3.656 Å).

"Texas-Sized" Molecular Boxes: From Chemistry to Applications

The design and synthesis of novel macrocyclic host molecules continues to attract attention because such species play important roles in supramolecular chemistry. However, the discovery of new classes of macrocycles presents a considerable challenge due to the need to embody by design effective molecular recognition features, as well as ideally the development of synthetic routes that permit further functionalization. In 2010, we reported a new class of macrocyclic hosts: a set of tetracationic imidazolium macrocycles, which we termed “Texas-sized” molecular boxes (TxSBs) in homage to Stoddart’s classic “blue box” (CBPQT⁴⁺). Compared with the rigid blue box, the first generation TxSB displayed considerably greater conformational flexibility and a relatively large central cavity, making it a good host for a variety of electron-rich guests. In this review, we provide a comprehensive summary of TxSB chemistry, detailing our recent progress in the area of anion-responsive supramolecular self-assembly and applications of the underlying chemistry to water purification, information storage, and controlled drug release. Our objective is to provide not only a review of the fundamental findings, but also to outline future research directions where TxSBs and their constructs may have a role to play.

Molecular recognition and sensing of dicarboxylates and dicarboxylic acids

The recognition and detection of dicarboxylic acids and dicarboxylates is of significance for a wide variety of applications, including medical diagnosis, monitoring of health and of environmental contaminants, and in industry. Hence small molecule receptors and sensors for dicarboxylic acids and dicarboxylates have great potential for applications in these fields. This review outlines the challenges faced in the recognition and detection of these species, strategies that have been used to obtain effective and observable interactions with dicarboxylic acids and dicarboxylates, and progress made in this field in the period from 2014 to 2020.

Soft Materials Constructed Using Calix[4]pyrrole- and "Texas-Sized" Box-Based Anion Receptors

Soft materials have received considerable attention from supramolecular chemists and material scientists alike. This interest reflects the advantages provided by their soft, flexible nature and the convenience of the molecular self-assembly that underlies their preparation. Common soft supramolecular materials include polymeric gels, supramolecular polymers, nanoaggregates, and membranes. Polymeric gels are solidlike networks of cross-linked polymer chains. Supramolecular polymers contain repeat units connected through reversible non-covalent bonds. Nanoaggregates are formed as a result of hydrophobic interactions involving amphiphilic building blocks. Because of the presence of non-covalent interactions, supramolecular soft materials typically display stimuli-responsive or adaptive features. Various macrocyclic hosts, such as cyclodextrins, crown ethers, calixarenes, cucurbiturils, and pillararenes, and many classic non-covalent interactions have been harnessed to construct supramolecular soft materials. Only recently has anion binding been used as the underlying recognition motif. Anions are ubiquitous in the natural world. Their importance has inspired efforts to achieve good anion binding and to exploit anion recognition in a number of fields, including extraction, transport, sensing, and catalysis. Most of this effort has involved the use of stand-alone anion receptors. On the other hand, soft materials with anion recognition features could lead to new macromolecular systems of interest in the context of many application areas. In this Account, we summarize the latest efforts from our laboratory to prepare supramolecular soft materials, including polymeric gels, supramolecular polymers, and nanoaggregates, with bona fide anion recognition features. Two anion receptor systems, namely, calix[4]pyrroles (C4Ps) and a tetraimidazolium macrocycle known as the "Texas-sized" molecular box (TxSB), have been used for this purpose. To date, TxSB-based hydrogels have been utilized to capture anions from water and for coded information applications; C4P-based organic polymeric gels have been used to extract dianions from aqueous source phases and for the on-site detection of chloride anions. Polymers containing C4P and TxSB anion recognition subunits typically display responsive features and can be modified through application of appropriately chosen external stimuli. For instance, nanoaggregates may be formed as a result of the hydrophobic interactions of C4P- and TxSB-based amphiphiles. The resulting aggregates were found to mimic the structural evolution of organelles and could be used as effective anion and ion pair extractants. This Account summarizes progress to date while underscoring potential opportunities associated with combining anion recognition and soft materials chemistry. The hope is to stimulate further advances in broad areas, including polymer science, supramolecular chemistry, biology, materials research, and information storage.

Azobenzene-Bridged Expanded "Texas-sized" Box: A Dual-Responsive Receptor for Aryl Dianion Encapsulation

We report an expanded "Texas-sized" molecular box (AzoTxSB) that incorporates photoresponsive azobenzene bridging subunits and anion recognition motifs. The shape of this box can be switched through light induced E ↔ Z photoisomerization of the constituent azobenzenes. This allows various anionic substrates to be bound and released by using different forms of the box. Control can also be achieved using other environmental stimuli, such as pH and anion competition.

Convergent Ditopic Receptors Enhance Anion Binding upon Alkali Metal Complexation for Catalyzing the Ritter Reaction

A supramolecular approach to catalyzing the Ritter reaction by utilizing enhanced anion-binding affinity in the presence of alkali metal cations was developed with ditopic hydrogen-bonded amide macrocycles. With prebound cations in the macrocycle, particularly Li+ ion, their metal complexes exhibit greatly enhanced catalytic activities. The catalysis is switchable by removal or addition of the bound cation. The method described in this work may be generalized for use in other anion-triggered organic reactions involving heteroditopic receptors capable of ion pairing.

Tuning the shape anisotropy of loosely bound colloid-like ionic clusters in solution

We characterize the influence of the ionic ratio on the dynamic self-assembly process involving a macrocyclic tetraimidazolium molecular box and small dianionic salts into highly defined, colloid-like ionic clusters in solution, called ionoids.

Dynamic self-assembly of ions with variable size and charge in solution

Recently it was found that at ambient temperatures and in specific ternary solvents a cationic macrocyclic tetraimidazolium molecular box and small dianionic salts can self-assemble into highly defined, colloid-like ionic clusters, called ionoids. Here, we present evidence that the solution-based ionic self-assembly process leading to ionoids is a general phenomenon by characterizing new ionic building blocks which are capable of generating loosely bound globular and anisotropic structures similar to those in the established system. Using new cationic and anionic molecules, we show that variations in the size ratio between cationic and anionic component mainly affect size, shape and durability of the ionic clusters. Utilizing dynamic light scattering (DLS), continuously monitored phase-analysis light scattering (cmPALS) and continuous wave electron paramagnetic resonance (CW EPR) spectroscopy, we can thus define generalized ionic ratios, in which specific combinations of ionic compounds with certain size and charge densities are able to form these soft yet durable and long-lived ionic clusters. Furthermore, we characterize the temporal development of our dynamically self-assembled structures in solution from the level of the individual ionic building blocks to stable clusters with minimum lifetimes of months through previously established ionoid evolution diagrams (IEDs). The direct comparison of various cluster systems with respect to their shape, size and charges allows correlations of structural changes of the individual building blocks with the fate of self-assembled entities inside the crafted IEDs. This work generalizes the concept of ionoid formation to ions of specific sizes and charge densities, which may broaden the scope of this new type of highly dynamic and soft yet remarkably durable structures in the field of supramolecular chemistry.

Recently it was found that at ambient temperatures and in specific ternary solvents a cationic macrocyclic tetraimidazolium molecular box and small dianionic salts can self-assemble into highly defined, colloid-like ionic clusters, called ionoids.

Tetraphenylethene-based tetracationic cyclophanes and their selective recognition for amino acids and adenosine derivatives in water

Tetracationic cyclophane 1 with a trapezoid-like cavity exhibited highly-selective recognition for tryptophan and ATP through electrostatic and π–π interactions in water.

Supramolecular Properties of a Monocarboxylic Acid-Functionalized "Texas-Sized" Molecular Box

A new carboxylic acid-functionalized "Texas-sized" molecular box TxSB-CO₂H has been prepared by combining two separate building blocks via an iodide-catalyzed macrocyclization reaction. A single-crystal X-ray diffraction analysis revealed a paired "clip-like" dimer in the solid state. Concentration-dependent behavior is seen for samples of TxSB-CO₂H as prepared, as inferred from ¹H NMR spectroscopic studies carried out in DMSO- d₆. However, in the presence of excess acid (1% by weight of deuterated trifluoracetic acid; TFA- d₁), little evidence of aggregation is seen in DMSO- d₆ except at the highest accessible concentrations. In contrast, the conjugate base form, TxSB-CO₂^-, produced in situ via the addition of excess triethylamine to DMSO- d₆ solutions of TxSB-CO₂H acts as a self-complementary monomer that undergoes self-assembly to stabilize a formal oligomer ([TxSB-CO₂^-] _n) with a degree of polymerization of approximately 5-6 at a concentration of 70 mM. Evidence in support of the proposed oligomerization of TxSB-CO₂^- in solution and in the solid state came from one- and two-dimensional ¹H NMR spectroscopy, X-ray crystallography, dynamic light scattering (DLS), and scanning electron microscopy (SEM). A series of solution-based analyses carried out in DMSO and DMSO- d₆ provide support for the notion that the self-assembled constructs produced from TxSB-CO₂^- are responsive to environmental stimuli, including exposure to the acetate anion (as its tetrabutylammonium, TBA⁺, salt), and changes in overall concentration, temperature, and protonation state. The resulting transformations are thought to reflect the reversible nature of the underlying noncovalent interactions. They also permit the stepwise interconversion between TxSB-CO₂H and [TxSB-CO₂^-] _n via the sequential addition of triethylamine and TFA- d₁. The present work thus serves to illustrate how appropriately functionalized molecular box-type macrocycles may be used to develop versatile stimuli-responsive materials. It also highlights how aggregated forms seen in the solid state are not necessarily retained under competitive solution-phase conditions.

Template Synthesis of Three-Dimensional Hexakisimidazolium Cages

A procedure for the synthesis of three-dimensional hexakisimidazolium cage compounds has been developed. The reaction of the trigonal trisimidazolium salts H₃ L(PF₆ )₃ , decorated with three N-olefinic pendants, and silver oxide yielded trinuclear trisilver(I) hexacarbene molecular cylinders of the type [Ag₃ L₂ ]³⁺ with the olefinic pendants from the two different tricarbene ligands arranged in three pairs. Subsequent UV irradiation gave three cyclobutane links between the two tris-NHC ligands in three [2+2] cycloaddition reactions, thereby generating a three-dimensional hexakis-NHC ligand. Removal of the metal ions resulted in the formation of three-dimensional hexakisimidazolium cages with a large internal cavity.

Flexible imidazolium macrocycles: building blocks for anion-induced self-assembly

This feature article summarises recent contributions of the authors in the area of anion-induced supramolecular self-assembly. It is based on the chemistry of a set of tetracationic imidazolium macrocycles, specifically the so-called 'Texas-sized' molecular box, cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene) (1⁴⁺), and its congeners, cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,2-dimethylenebenzene) (2⁴⁺), cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,3-dimethylenebenzene) (3⁴⁺), and cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](2,6-dimethylenepyridine) (4⁴⁺). These systems collectively have been demonstrated as being versatile building blocks that interact with organic carboxylate or sulfonate anions, as well as substrates (e.g., neutral molecules or metal cations). Most work to date has been carried out with 1⁴⁺, a system that has been found to support the construction of a number of stimuli responsive self-assembled ensembles. This macrocycle and others of the 'Texas-sized' box family also show the potential to react as carbene precursors and to undergo post-synthetic modification (PSM) to produce new functional macrocycles, such as trans- and cis-cyclo[2]((Z)-N-(2-((6-(1H-imidazol-1-yl)pyridin-2-yl)amino)vinyl)formamide)[2](1,4-bismethylbenzene) (5²⁺ and 6²⁺, respectively). On the basis of the work reviewed in this Feature article, we propose that the imidazolium macrocycles 1⁴⁺-4⁴⁺ can be considered as useful tools for the construction of ensembles with environmentally responsive features, including control over self-assembly and an ability to undergo precursor-specific PSM.

Phosphate-phosphate oligomerization drives higher order co-assemblies with stacks of cyanostar macrocycles

The importance of phosphate in biology and chemistry has long motivated investigation of its recognition. Despite this interest, phosphate's facile oligomerization is only now being examined following the discovery of complexes of anion-anion dimers of hydroxyanions. Here we address how oligomerization dictates phosphate's recognition properties when engaged with planar cyanostar macrocycles that can also oligomerize by stacking. The crystal structure of cyanostar with phosphate shows an unprecedented tetrameric stack of cyanostar macrocycles threaded by a phosphate trimer, [H₂PO₄···H₂PO₄···H₂PO₄]^3-. The solution behaviour, studied as a function of solvent quality, highlights how dimers and trimers of phosphate drive formation of higher order stacks of cyanostar into dimer, trimer and tetramer co-assemblies. Solution behaviors differ significantly from simpler complexes of bisulfate hydroxyanion dimers. Phosphate oligomerization is: (1) preferred over ion pairing with tetrabutylammonium cations, (2) inhibits disassembly of the complexes upon dilution, and (3) resists interference from competitive anion solvation. The phosphate oligomers also appear critical for stability; complexation of just one phosphate with cyanostars is unfavored. The cyanostar's ability to self-assemble is found to create a tubular, highly electropositive cavity that complements the size and shape of the phosphate oligomers as well as their higher charge. When given the opportunity, phosphate will cooperate with the receptor to form co-assembled architectures.

Crystal structures of 4-meth-oxy-benzoic acid-1,3-bis-(pyridin-4-yl)propane (2/1) and biphenyl-4,4'-di-carb-oxy-lic acid-4-meth-oxy-pyridine (1/2)

The crystal structures of two hydrogen-bonded compounds, namely 4-meth-oxy-benzoic acid-1,3-bis-(pyridin-4-yl)propane (2/1), C13H14.59N2·C8H7.67O3·C8H7.74O3, (I), and biphenyl-4,4'-di-carb-oxy-lic acid-4-meth-oxy-pyridine (1/2), C14H9.43O4·C6H7.32NO·C6H7.25NO, (II), have been determined at 93 K. In (I), the asymmetric unit consists of two crystallographically independent 4-meth-oxy-benzoic acid mol-ecules and one 1,3-bis-(pyridin-4-yl)propane mol-ecule. The asymmetric unit of (II) comprises one biphenyl-4,4'-di-carb-oxy-lic acid mol-ecule and two independent 4-meth-oxy-pyridine mol-ecules. In each crystal, the acid and base mol-ecules are linked by short O-H⋯N/N-H⋯O hydrogen bonds, in which H atoms are disordered over the acid O-atom and base N-atom sites, forming a linear hydrogen-bonded 2:1 or 1:2 unit of the acid and the base. The 2:1 units of (I) are linked via C-H⋯π, π-π and C-H⋯O inter-actions into a tape structure along [101], while the 1:2 units of (II) form a double-chain structure along [-101] through π-π and C-H⋯O inter-actions.

Supramolecular anion recognition in water: synthesis of hydrogen-bonded supramolecular frameworks

The interaction of tetratopic amidinium-containing receptors with terephthalate anions leads to porous framework materials assembled through charge-assisted hydrogen bonds. The frameworks form in good yield within minutes in water at room temperature, but no framework material is obtained if other anions (Cl^-, Br^-, NO₃^-, SO₄^2- or isophthalate^2-) are used in place of terephthalate. Two forms of the framework can be prepared: one with a connected pore network, and a more dense phase with discrete voids. We demonstrate that these are the kinetic and thermodynamic products, respectively. Either framework can be prepared independently and can be converted to the other form in response to stimuli. Furthermore, the frameworks can be controllably disassembled and reassembled in response to acid/base triggers suggesting that this new class of materials may have applications in the selective encapsulation and release of guests.

A ruthenium(iii) complex derived from N,N′-bis(salicylidene)ethylenediamine as a chemosensor for the selective recognition of acetate and its interaction with cells for bio-imaging: experimental and theoretical studies

A ruthenium(iii) complex ofN,N′-bis(salicylidene)ethylenediamine (L¹) was used as chemosensor for the recognition of acetate in cells for bio-imaging.

Imidazolium Based Probes for Recognition of Biologically and Medically Relevant Anions

The imidazolium derivatives due to their positive charge possess one of the most polarized and positively charged proton at C2-H to form strong ionic hydrogen bond (also termed as double ionic hydrogen bond) with anions and also provide opportunities for anion - π interactions with electron-deficient imidazolium ring. In the present review article, imidazolium based molecular probes for their ability to recognize inorganic anions like halides, cyanide, perchlorate, carboxylic acids, phosphate, sulfate etc. and their derived molecules viz. nucleotides, DNA, RNA, surfactants, proteins, etc have been discussed. The review covers the literature published after year 2009 and has > 130 references. The previous literature has already been discussed by Yoon et al. in two review articles published in Chem. Soc. Rev. 2006 and 2010.

An Unprecedented Two-Fold Nested Super-Polyrotaxane: Sulfate-Directed Hierarchical Polythreading Assembly of Uranyl Polyrotaxane Moieties

The hierarchical assembly of well-organized submoieties could lead to more complicated superstructures with intriguing properties. We describe herein an unprecedented polyrotaxane polythreading framework containing a two-fold nested super-polyrotaxane substructure, which was synthesized through a uranyl-directed hierarchical polythreading assembly of one-dimensional polyrotaxane chains and two-dimensional polyrotaxane networks. This special assembly mode actually affords a new way of supramolecular chemistry instead of covalently linked bulky stoppers to construct stable interlocked rotaxane moieties. An investigation of the synthesis condition shows that sulfate can assume a vital role in mediating the formation of different uranyl species, especially the unique trinuclear uranyl moiety [(UO2 )3 O(OH)2 ](2+) , involving a notable bent [O=U=O] bond with a bond angle of 172.0(9)°. Detailed analysis of the coordination features, the thermal stability as well as a fluorescence, and electrochemical characterization demonstrate that the uniqueness of this super-polyrotaxane structure is mainly closely related to the trinuclear uranyl moiety, which is confirmed by quantum chemical calculations.

Post-synthetic modification of a macrocyclic receptor via regioselective imidazolium ring-opening

A facile post-synthetic modification of a tetracationic tetraimidazolium macrocycle, 14+ (i.e., the "Texas-sized" molecular box (cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene)), is described. Under mild basic conditions, ring-opening of the imidazolium moieties occurs. This results in two new isomeric dicationic macrocycles. This simple yet efficient modification serves to alter the size of the molecular cavity, the charge of the macromolecular receptor, and the manner whereby it interacts with dianionic guest molecules. The isomeric mixture of imidazolium ring opened macrocycles can be synthesized in relatively high overall yield (86-93%). The reaction shows regioselectivity and the ratio of major to minor (i.e., trans : cis ring-opened products) was determined to be ca. 3 : 1 via1H NMR spectroscopy. The major isomer, trans-cyclo[2]((Z)-N-(2-((6-(1H-imidazol-1-yl)pyridin-2-yl)amino)vinyl)formamide)[2](1,4-bismethylbenzene) hexafluorophosphate (22+·2PF6-), was isolated in its pure form in 42% yield via recrystallization. The molecular recognition properties of 22+ were investigated using a series of dianionic guests (i.e., 2,6-naphthalenedicarboxylate (4), 2,6-naphthalenedisulfonate (5), and 1,5-naphthalenedisulfonate (6)) whose binding interactions with 14+ have been previously reported. This allowed us to evaluate how imidazolium ring-opening affects the inherent host/guest interactions of 14+. On the basis of solution spectroscopic studies (e.g., 1H NMR, 1H-1H COSY NMR, DOSY NMR, and NOESY NMR), in tandem with mass spectrometric analyses (ESI-MS) and single-crystal X-ray diffraction studies, we conclude that opening up the macrocyclic structure (i.e., converting 14+ to 22+) leads to considerable changes in the recognition behavior, with so-called outside binding or weak ion pair interactions, rather than pseudorotaxane formation, being favored both in solution and the solid-state. We postulate that methodologies such as those described herein could provide a means to control the molecular interactions of both free-standing macrocycles and those used to construct mechanically-interlocked molecules. Indeed, the application of hydroxide anion under the present conditions not only serves to effect the ring-opening of 14+, but also pseudorotaxane structures, such as, e.g., [14+·4] or [14+·5] derived there from.

Syntheses, crystal structures and properties of metal–organic rotaxane frameworks with cucurbit[6]uril

Five new cucurbit[6]uril-based MORFs are hydrothermally synthesized by the mixed ligand strategy and their fluorescence properties have been investigated.

pH-dependent binding of guests in the cavity of a polyhedral coordination cage: reversible uptake and release of drug molecules

A range of organic molecules with acidic or basic groups exhibit strong pH-dependent binding inside the cavity of a polyhedral coordination cage. Guest binding in aqueous solution is dominated by a hydrophobic contribution which is compensated by stronger solvation when the guests become cationic (by protonation) or anionic (by deprotonation). The Parkinson's drug 1-amino-adamantane ('amantadine') binds with an association constant of 104 M-1 in the neutral form (pH greater than 11), but the stability of the complex is reduced by three orders of magnitude when the guest is protonated at lower pH. Monitoring the uptake of the guests into the cage cavity was facilitated by the large upfield shift for the 1H NMR signals of bound guests due to the paramagnetism of the host. Although the association constants are generally lower, guests of biological significance such as aspirin and nicotine show similar behaviour, with a substantial difference between neutral (strongly binding) and charged (weakly binding) forms, irrespective of the sign of the charged species. pH-dependent binding was observed for a range of guests with different functional groups (primary and tertiary amines, pyridine, imidazole and carboxylic acids), so that the pH-swing can be tuned anywhere in the range of 3.5-11. The structure of the adamantane-1-carboxylic acid complex was determined by X-ray crystallography: the oxygen atoms of the guest form CH···O hydrogen bonds with one of two equivalent pockets on the internal surface of the host. Reversible uptake and release of guests as a function of pH offers interesting possibilities in any application where controlled release of a molecule following an external stimulus is required.

Metal-free site-selective C–N bond-forming reaction of polyhalogenated pyridines and pyrimidines

A metal-free method for highly site-selective C–N bond-formation of polyhalogenated pyridines and pyrimidines is developed.