Anionic Guests Research Articles

Spatially Designed Supramolecular Anion Receptors Based on Pillar[5]arene Scaffolds: Synthesis and Halide Anion Binding Properties.

Urea-functionalized anion receptors based on brominated functionalized pillar[5]arenes were prepared. The binding affinity toward halide anions was investigated and probed using 1H NMR titration and isothermal titration calorimetry (ITC). The complexation behavior was affected by the structure of the receptor and the nature of the anionic guest. The synthesized receptors are highly selective toward fluoride resulting in the formation of a 1:2 host-to-guest complex. The anion receptor based on the 1,3-alternate pillar[5]arene regioisomer shows the highest affinity toward fluorine anions. No significant interactions were observed with larger bromine anions. The formation of a self-assembled supramolecular polymer driven by hydrogen bonds in solution was demonstrated by diffusion-ordered spectroscopy (DOSY), ITC, and dynamic light scattering (DLS) measurements. From ITC dilution experiments, we found that the supramolecular polymer self-assembly at higher concentrations is a spontaneous process as indicated by the positive value of Gibbs free energy (ΔG = 12.04 kJ mol-1).

Front Cover: Enhanced Host‐Guest Association and Fluorescence in Copolymers from Copper Salphen Complexes by Supramolecular Internalization of Anions (ChemPlusChem 3/2023)

The cover picture shows a fluorescent Cu-Salphen host crystal structure interacting with an anion guest pink sphere that emits in the yellow-green region represented by fireflies, as well as the counterparts linked to a polymer chain represented as a blue ribbon, which yields blue-shifted hyperchromic cyan luminescence due to the protection from its coil conformation. They emerge from the UNESCO's World Heritage Site Central University City Campus of the National Autonomous University of Mexico. More information can be found in the Research Article by G. Zelada-Guillén, M. V. Escárcega-Bobadilla, and co-workers.

Binding of ion pairs and neutral guests by aryl-extended meso‑p-hydroxyphenyl calix[4]pyrrole: The interplay between three binding sites

An aryl-extended calix[4]pyrrole with four meso‑p-hydroxyphenyl substituents was investigated as a host for chloride, acetate, and benzoate anions. Crystal structures of pyridinium and imidazolium chloride complexes were obtained in which chloride ions are hydrogen bonded exo-cavity to the upper rim hydroxyl groups, and the aromatic cations are bound to the shallow cavity of the host. Furthermore, the calix[4]pyrrole formed a hydrogen bonded dimeric capsule templated by inclusion of adiponitrile guest in the endo-cavity binding site. NMR titrations revealed the preference of the OH groups of the host to bind anionic guests in solution. Benzoate anion had the highest binding constant (4 700 M−1) in acetonitrile. Density functional theory (DFT) calculations indicated that the exo-cavity complex with chloride anions was favoured by 23.2 kJ/mol over the endo-cavity complex, whereas the energies of endo- and exo-cavity benzoate complexes were of similar magnitude due to dispersion interactions between the host and the guest.

Enhanced Host-Guest Association and Fluorescence in Copolymers from Copper Salphen Complexes by Supramolecular Internalization of Anions.

We describe the synthesis, crystallographic characterization of a new Cu-Salphen compound and its use as a host Lewis-acid against guest anions in two versions: a) free molecule, b) copolymerized with methyl methacrylate:n-butyl acrylate (1 : 4-wt.) as protective co-monomers. Higher contents in Cu-Salphen yielded larger and more homogeneous polymer sizes. Polymer size together with glass transitions, heat capacity, thermal degradation, guest-saturation degrees and host-guest species distribution profiles from spectrophotometric titrations explained growths of up to 630-fold in K11 and 180000-fold in K12 for the host's binding site attributable to a solvophobic protection from the macromolecular structure. Spectrofluorimetry revealed blue-shifted×13-16 larger luminescence for Cu-Salphen in the polymers (λem =488-498 nm) than that of the non-polymerized counterpart (λem =510-543 nm) and "turn-on" blue-shifted enhanced fluorescence upon guest association. We propose a cooperative incorporation of the guests occurring from the outer medium toward internally protected binding site pockets in the random coil polymer conformations.

Binding propensity and selectivity of cationic, anionic, and neutral guests with model hydrophobic hosts: A first principles study.

Computations play a critical role in deciphering the nature of host-guest interactions both at qualitative and quantitative levels. Reliable quantum chemical computations were employed to assess the nature, binding strength, and selectivity of ionic, and neutral guests with benzenoid hosts. Optimized complex structures reveal that alkali and ammonium ions are found to be in the hydrophobic cavity, while halide ions are outside, while both complexes elicit substantial binding energy. The origin of the selectivity of host toward the guest has been traced to the interaction and deformation energies, and the nature of associated interactions is quantified using energy decomposition and the Quantum Theory of Atoms in Molecules analyses. While the larger hosts lead to loosely bound complexes, as assessed by the longer intermolecular distances, the binding strengths are proportional to the size of the host systems. The binding of cationic complexes is electrostatic or polarization driven while exchange term dominates the anionic complexes. In contrast, dispersion contribution is a key in neutral complexes and plays a pivotal role in stabilizing the polyatomic complexes.

Intermolecular Interaction of Tetrabutylammonium and Tetrabutylphosphonium Salt Hydrates by Low-Frequency Raman Observation.

Semi-clathrate hydrates are attractive heat storage materials because the equilibrium temperatures, located above 0 °C in most cases, can be changed by selecting guest cations and anions. The equilibrium temperatures are influenced by the size and hydrophilicity of guest ions, hydration number, crystal structure, and so on. This indicates that intermolecular and/or interionic interaction in the semi-clathrate hydrates may be related to the variation of the equilibrium temperatures. Therefore, intermolecular and/or interionic interaction in semi-clathrate hydrates with quaternary onium salts was directly observed using low-frequency Raman spectroscopy, a type of terahertz spectroscopy. The results show that Raman peak positions were mostly correlated with the equilibrium temperatures: in the semi-clathrate hydrates with higher equilibrium temperatures, Raman peaks around 65 cm−1 appeared at a higher wavenumber and the other Raman peaks at around 200 cm−1 appeared at a lower wavenumber. Low-frequency Raman observation is a valuable tool with which to study the equilibrium temperatures in semi-clathrate hydrates.

A Synthetic Protocell-Based Heparin Scavenger.

Heparin is a commonly applied blood anticoagulant agent in clinical use. After treatment, excess heparin needs to be removed to circumvent side effects and recover the blood-clotting cascade. Most existing heparin antidotes rely on direct heparin binding and complexation, yet selective compartmentalization and sequestration of heparin would be beneficial for safety and efficiency. However, such systems have remained elusive. Herein, a semipermeable protein-based microcompartment (proteinosome) is loaded with a highly positively charged chitosan derivative, which can induce electrostatics-driven internalization of anionic guest molecules inside the compartment. Chitosan-loaded proteinosomes are subsequently employed to capture heparin, and an excellent heparin-scavenging performance is demonstrated under physiologically relevant conditions. Both the highly positive scavenger and the polyelectrolyte complex are confined and shielded by the protein compartment in a time-dependent manner. Moreover, selective heparin-scavenging behavior over serum albumin is realized through adjusting the localized scavenger or surrounding salt concentrations at application-relevant circumstances. In vitro studies reveal that the cytotoxicity of the cationic scavenger and the produced polyelectrolyte complex is reduced by protocell shielding. Therefore, the proteinosome-based systems may present a novel polyelectrolyte-scavenging method for biomedical applications.

Oligopyrrolic Cages: From Classic Molecular Constructs to Chemically Responsive Polytopic Receptors.

"Functional molecular systems", discrete and self-assembled constructs where control over molecular recognition, structure, bonding, transport, release, catalytic activity, etc., is readily achieved, are a topic of current interest. Within this broad paradigm, oligopyrrolic cages have garnered attention due to their responsive recognition features. Due to the presence of slightly polar pyrrole subunits which can also behave as hydrogen-bonding donors, these oligopyrrolic cages are potential receptors for various polarized species. In this Account, we summarize recent advances involving the syntheses and study of (1) covalent oligopyrrolic macrobicyclic cages, (2) oligopyrrolic metallacages, and (3) oligopyrrolic noncovalently linked cages. Considered in concert, these molecular constructs have allowed advances in applied supramolecular chemistry; to date, they have been exploited for selective guest encapsulation studies, anion binding and ion-channel formation, and gas absorption, among other applications. While key findings from others will be noted, in this Account will focus on our own contributions to the chemistry of discrete oligopyrrolic macrocycles and their use in supramolecular host-guest chemistry and sensing applications. In terms of specifics, we will detail how oligopyrrole cages with well-defined molecular geometries permit reversible guest binding under ambient conditions and how the incorporation of pyrrole subunits within larger superstructures allows effective control over anion/conjugate acid binding activity under ambient conditions. We will also provide examples that show how derivatization of these rudimentary macrocyclic cores with various sterically congested β-substituted oligopyrroles can provide entry into more complex supramolecular architectures. In addition, we will detail how hybrid systems that include heterocycles other than pyrrole, such as pyridine and naphthyridine, can be used to create self-assembled materials that show promise as gas-absorbing materials and colorimetric reversible sensors. Studies involving oligopyrrolic polymetallic cages and oligopyrrolic supramolecular cages will also be reviewed. First, we will discuss all-carbon-linked oligopyrrolic bicycles and continue on to present systems linked via amines and imines linkages. Finally, we will summarize recent work on pyrrolic cages created through the use of metal centers or various noncovalent interactions. We hope that this Account will provide researchers with an initial foundation for understanding oligopyrrolic cage chemistry, thereby allowing for further advances in the area. It is expected that the fundamental design and recognition principles made in the area of oligopyrrole cages, as exemplified by our contributions, will be of general use to researchers targeting the design of functional molecular systems. As such, we have structured this Account so as to summarize the past while setting the stage for the future.

Hydroxy Groups Enhance [2]Rotaxane Anion Binding Selectivity.

We report the synthesis of two [2]rotaxanes containing an interlocked three dimensional binding cavity formed from a pyridinium bis(amide) axle component containing two phenol donors, and an isophthalamide based macrocycle. In the competitive solvent mixture 1 : 1 CDCl3 : CD3OD, one of the receptors exhibits a much higher selectivity preference for chloride than an analogous rotaxane without the hydroxy groups. X‐ray crystal structures reveal the chloride anion guest encapsulated within the interlocked binding cavity, though not all of the hydrogen bond donors are utilised. Computational semi‐empirical simulations indicate that secondary intermolecular interactions occur between the axle hydroxy hydrogen bond donors and the [2]rotaxane macrocycle components, contributing to a more preorganised binding pocket, which may be responsible for the observed enhanced selectivity.

Design Principles and Applications of Selective Lanthanide-Based Receptors for Inorganic Phosphate.

Phosphate is an anion of both environmental and medical significance. The increase in phosphate levels in surface waters due primarily to run-offs from fertilized agricultural fields causes widespread eutrophication and increasingly large dead-zones. Hyperphosphatemia, a condition in which blood phosphate levels are elevated, is a primary cause of increased mortality and morbidity in chronic or advanced kidney disease. Resolving both of these issues require, in part, new technology that could selectively sequester phosphate in water at neutral pH. The high hydration energy of phosphate, which prevents organic receptors from functioning in water with sufficient affinity, can be overcome via coordination to a hard metal ion. The hardness, oxophilicity and lability of lanthanide ions make them excellent candidates for the design of high affinity phosphate receptors. In this perspective, we discuss how the principles of lanthanide coordination chemistry can be exploited to design sensitive and selective receptors for phosphate. Unlike many supramolecular systems, these hosts do not recognize their anionic guests via directed electrostatic and hydrogen bonding interactions. Instead, the selectivity of our fluxional receptors is governed entirely by acid-base chemistry and electrostatic forces. Parameters that affect the affinity and selectivity of the receptors include the basicities of the coordinating ligand and of the targeted anion, the acidity of the lanthanide ion, and the geometry of the ligand. Uniquely, their affinity for phosphate can be readily tuned by orders of magnitude either by peripheral interactions or by the lanthanide ion itself without affecting their exquisite selectivity over competing anions such as bicarbonate and chloride.

Post-synthetic modification of conjugated microporous polymer with imidazolium for highly efficient anionic dyes removal from water

• A cationic CMP-Im was synthesized by post-synthetic modification. • CMP-Im showed superior adsorption and separation capacity for anionic dyes. • The conjugation, porosity and cation benefit the adsorption results of CMP-Im . • CMP-Im can be used to prepare packed column and beads to remove anionic dyes. Water pollution caused by organic dyes is a severe environmental problem in developing society. Adsorption has been considered as a versatile and simple water-purification method. Therefore, it is desirable to develop adsorbents with high capacity and efficiency to remove dyes. In this work, through two-step post-synthetic modification to introduce imidazolium in a conjugated microporous polymer (CMP), an effective adsorbent ( CMP-Im ) was obtained to study the rational conjugated and cationic skeleton for anionic dyes removal. All these polymers exhibited inherent porous structures and high stability. CMP-Im showed Brunauer–Emmett–Teller (BET) surface area of 533 m 2 g −1 and possessed cationic framework of imidazolium unit favoring interaction with anionic guests. Due to the conjugated, cationic and porous framework, CMP-Im exhibited fast and efficient adsorption of anionic dyes such as methyl orange (MO) and Congo red (CR), and the adsorption capacity for MO and CR were 588 and 2500 mg g −1 , respectively. CMP-Im also exhibited excellent removal ability when it was used as stuffing to prepare packed column and adsorption beads. Hence, CMP-Im can be a promising candidate for dyes removal from water. This work opens up the exploration of post-synthetic CMPs and expands the application of cationic CMPs in dye adsorption.

Anion-Afforded Functions of Ionic Metal-Organic Frameworks: Ionochromism, Anion Conduction, and Catalysis.

The exchangeable counterions in ionic metal-organic frameworks (IMOFs) provide facile and versatile handles to manipulate functions associated with the ionic guests themselves and host-guest interactions. However, anion-exchangeable stable IMOFs combining multiple anion-related functions are still undeveloped. In this work, a novel porous IMOF featuring unique self-penetration was constructed from an electron-deficient tris(pyridinium)-tricarboxylate zwitterionic ligand. The water-stable IMOF undergoes reversible and single-crystal-to-single-crystal anion exchange and shows selective and discriminative ionochromic behaviors toward electron-rich anions owing to donor-acceptor interactions. The IMOFs with different anions are good ionic conductors with low activation energy, the highest conductivity being observed with chloride. Furthermore, integrating Lewis acidic sites and nucleophilic guest anions in solid state, the IMOFs act as heterogeneous and recyclable catalysts to efficiently catalyze the cycloaddition of CO2 to epoxides without needing the use of halide cocatalysts. The catalytic activity is strongly dependent upon the guest anions, and the iodide shows the highest activity. The results demonstrate the great potential of developing IMOFs with various functions related to the guest ions included in the porous matrices.

Dynamic [1]rotaxanes via a reversible covalent bond and host-guest anion recognition.

Bambus[6]uril-based [1]rotaxanes were formed quantitatively, utilizing a bis(acyloxy)iodate(I) reversible covalent bond and host-guest anion recognition. These novel [1]rotaxanes exhibited a dynamic nature facilitating carboxylate component exchange reactions in acetonitrile.

Flexible Vertex Engineers the Controlled Assembly of Distorted Supramolecular Tetrahedral and Octahedral Cages.

Designing and building unique cage assemblies attract increasing interest from supramolecular chemists but remain synthetically challenging. Herein, we propose the use of a flexible vertex with adjustable angles to selectively form highly distorted tetrahedral and octahedral cages, for the first time, in which the flexible vertex forms from the synergistic effect of coordination and covalent interactions. The inherent interligand angle of the vertex can be modulated by guest anions present, which allows for the fine-tuning of different cage geometries. Furthermore, the reversible structural transformation between tetrahedral and octahedral cages was achieved by anion exchange monitored by mass spectrometric technique, the smaller anions favoring tetrahedral cages, while the larger anions supporting octahedral cages. Additionally, the KBr-based cage thin films exhibited prominent enhancement of their third-order NLO responses in two or three orders of magnitude compared to those obtained for their corresponding solutions. This work not only provides a new methodology to build irregular polyhedral structures in a controlled and tunable way but also provides access to new kinds of promising functional optical materials.

Being Positive is Not Everything - Experimental and Computational Studies on the Selectivity of a Self-Assembled, Multiple Redox-State Receptor that Binds Anions with up to Picomolar Affinities.

The interaction of the self-assembled trinuclear ruthenium bowl 13+ , that displays three other accessible oxidation states, with oxo-anions is investigated. Using a combination of NMR and electrochemical experimental data, estimates of the binding affinities of 14+ , 15+ , and 16+ for both halide and oxo-anions were derived. This analysis revealed that, across the range of oxidation states of the host, both high anion binding affinities (>109 M-1 for specific guests bound to 16+ ) and high selectivities (a range of >107 M-1 ) were observed. As the crystal structure of binding of the hexafluorophosphate anion revealed that the host has two potential binding sites (named the α and β pockets), the host-guest properties of both putative binding sites of the bowl, in all of its four oxidation states, were investigated through detailed quantum-based computational studies. These studies revealed that, due to the interplay of ion-ion interactions, charge-assisted hydrogen-bonding and anion-π interactions, binding to the α pocket is generally preferred, except for the case of the relatively large and lipophilic hexafluorophosphate anionic guest and the host in the highest oxidation states, where the β pocket becomes relatively favourable. This analysis confirms that host-guest interactions involving structurally complex supramolecular architectures are driven by a combination of non-covalent interactions and, even in the case of charged binding pairs, simple ion-ion interactions alone cannot accurately define these recognition processes.

Tuning Dynamically Formed Active Phases and Catalytic Mechanisms of In Situ Electrochemically Activated Layered Double Hydroxide for Oxygen Evolution Reaction.

The active phase and catalytic mechanisms of Ni-based layered double hydroxide (LDH) materials for oxygen evolution reaction (OER) have no common consensus and remain controversial. Moreover, engineering the site activity and the number of active sites of LDHs is an efficient approach to advance the OER activity, as the thickness and stacking structure of the LDHs layer limit the catalytic activity. This work presents an interesting in situ approach of tuning the site activity and number of active sites of NiMn-LDHs, which exhibit the superior OER performance (onset overpotential of 0.17 V and overpotential of 0.24 V at 10 mA cm-2). The fundamental mechanistic insights and active phases during the OER process are characterized by in operando techniques along with the computational density functional theory calculations, revealing that the Ni site constitutes the OER activity and the dynamically generated NiOOH moiety is the active phase. We also prove that Ni sites undergo a reversible oxidation state under the working conditions to create active NiOOH species which catalyze the water to generate oxygen. These findings suggest that the Ni(III) phase in NiMn-LDHs is the OER active site and Mn promotes the electronic properties of Ni sites. Utilizing in situ/in operando techniques and theoretical calculation, we find that the in situ intercalation of guest anions allows the expansion of the LDH layers and keeps the active NiOOH species under the oxidation state of +3 via electron coupling, which ultimately tunes the site populations and site activity toward the superior OER activity, respectively. This work thus targets to provide insight into strategies to design the next generation of highly active catalysts for water electrolysis and fuel cell technologies.

Guest‐Induced Transformations in Metal‐Organic Cages

AbstractStimuli‐responsive structural transformations are common in artificial coordination assemblies and can help expand the engineering of programmable functional materials. In particular, chemical guests have proven to act as stimuli and signals inducing structural interconversions in Metal‐Organic Cages (MOCs). In addition to cages, other related classes of coordination assemblies can express structural reorganization upon chemical binding to produce new MOC constructs, many of which do not form otherwise, and exhibit properties and functions not available before the transformation. This review highlights recent examples of such a class of guest‐induced transformations. The connection between changes and host–guest interactions is examined for both neutral and anionic guests, pointing out that in‐situ formed molecular cargoes may also exert conversions. Cases of mixtures of MOCs that redistribute upon encapsulation of complementary species are also featured. This review thus aims to provide an update and new methodological insights into guest‐responsive MOCs for enhancing their design and applications.

Molecular Environment-Specific Atomic Charges Improve Binding Affinity Predictions of SAMPL5 Host-Guest Systems.

Host-guest systems are widely used in benchmarks as model systems to improve computational methods for absolute binding free energy predictions. Recent advances in sampling algorithms for alchemical free energy calculations and the increase in computational power have made their binding affinity prediction primarily dependent on the quality of the force field. Here, we propose a new methodology to derive the atomic charges of host-guest systems based on quantum mechanics/molecular mechanics calculations and minimal basis iterative stockholder (MBIS) partitioning of the polarized electron density. A newly developed interface between the OpenMM and ORCA software packages provides D-MBIS charges that represent the guest's average electrostatic interactions in the hosts or the solvent. The simulation workflow also calculates the average energy required to polarize the guest in the bound and unbound state. Alchemical free energy calculations using the general Amber force field parameters with D-MBIS charges improve the binding affinity prediction of six guests bound to two octa acid hosts compared to the AM1-BCC charge set after correction with the average energetic polarization cost. This correction originates from the difference in potential energy that is required to polarize the guest in the bound and unbound state and contributes significantly to the binding affinity of anionic guests.

Layered double hydroxide photocatalysts for solar fuel production

Splitting water or reducing CO2via semiconductor photocatalysis to produce H2 or hydrocarbon fuels through the direct utilization of solar energy is a promising approach to mitigating the current fossil fuel energy crisis and environmental challenges. It enables not only the realization of clean, renewable, and high-heating-value solar fuels, but also the reduction of CO2 emissions. Layered double hydroxides (LDHs) are a type of two-dimensional anionic clay with a brucite-like structure, and are characterized by a unique, delaminable, multidimensional, layered structure; tunable intralayer metal cations; and exchangeable interlayer guest anions. Therefore, it has been widely investigated in the fields of CO2 reduction, photoelectrocatalytic water oxidation, and water photolysis to produce H2. However, the low carrier mobility and poor quantum efficiency of pure LDH limit its application. An increasing number of scholars are exploring methods to obtain LDH-based photocatalysts with high energy conversion efficiency, such as assembling photoactive components into LDH laminates, designing multidimensional structures, or coupling different types of semiconductors to construct heterojunctions. This review first summarizes the main characteristics of LDH, i.e., metal-cation tunability, intercalated guest-anion substitutability, thermal decomposability, memory effect, multidimensionality, and delaminability. Second, LDHs, LDH-based composites (metal sulfide-LDH composites, metal oxide-LDH composites, graphite phase carbon nitride-LDH composites), ternary LDH-based composites, and mixed-metal oxides for splitting water to produce H2 are reviewed. Third, graphite phase carbon nitride-LDH composites, MgAl-LDH composites, CuZn-LDH composites, and other semiconductor-LDH composites for CO2 reduction are introduced. Although the field of LDH-based photocatalysts has advanced considerably, the photocatalytic mechanism of LDHs has not been thoroughly elucidated; moreover, the photocatalytic active sites, the synergy between different components, and the interfacial reaction mechanism of LDH-based photocatalysts require further investigation. Therefore, LDH composite materials for photocatalysis could be developed through structural regulation and function-oriented design to investigate the effects of different components and interface reactions, the influence of photogenerated carriers, and the impact of material composition on the physical and chemical properties of the LDH-based photocatalyst.

Computational Evaluation of Me2 TCCP as Lewis Acid.

Supramolecular adducts between dimethyl‐2,2,3,3‐tetracyanocyclopropane (Me2TCCP) with 21 small (polar) molecules and 10 anions were computed with DFT (B3LYP‐D3/def2‐TZVP). Their optimized geometries were used to obtain interaction energies, and perform energy decomposition and ‘atoms‐in‐molecules’ analyses. A set of 38 other adducts were also evaluated for comparison purposes. Selected examples were further scrutinized by inspection of the molecular electrostatic potential maps, Noncovalent Interaction index plots, the Laplacian, the orbital interactions, and by estimating the Gibbs free energy of complexation in hexane solution. These calculations divulge the thermodynamic feasibility of Me2TCCP adducts and show that complexation is typically driven by dispersion with less polarized partners, but by orbital interactions when more polarized or anionic guests are deployed. Most Me2TCCP adducts are more stable than simple hydrogen bonding with water, but less stable than traditional Lewis adducts involving Me3B, or a strong halogen bond such as with Br2. Several bonding analyses showed that the locus of interaction is found near the electron poor sp3‐hydridized (NC)2C−C(CN)2 carbon atoms. An empty hybrid σ*/π* orbital on Me2TCCP was identified that can be held responsible for the stability of the most stable adducts due to donor‐acceptor interactions.