H-Bond Acceptor Parameters for Anions

Palladium-catalyzed cross-coupling of alcohols with olefins by positional tuning of a counteranion

Transition metal-catalyzed cross-couplings have great potential to furnish complex ethers; however, challenges in the C(sp3)-O functionalization step have precluded general methods. Here, we describe computationally guided transition metal-ligand design that positions a hydrogen-bond acceptor anion at the reactive site to promote functionalization. A general cross-coupling of primary, secondary, and tertiary aliphatic alcohols with terminal olefins to furnish >130 ethers is achieved. The mild conditions tolerate functionality that is prone to substitution, elimination, and epimerization and achieve site selectivity in polyol settings. Mechanistic studies support the hypothesis that the ligand's geometry and electronics direct positioning of the phosphate anion at the π-allyl-palladium terminus, facilitating the phosphate's hydrogen-bond acceptor role toward the alcohol. Ligand-directed counteranion positioning in cationic transition metal catalysis has the potential to be a general strategy for promoting challenging bimolecular reactivity.

Precise counter anion modulation of the self-assembly behavior-endowed ultrasensitive and specific dual-mode visualization of nitrate

Pt(II) polypyridine complex-based probe exhibits promising performance in anion detection by the change of the absorption and emission properties based on supramolecular self-assembly. However, whether one can develop a modulation strategy of the counter anion to boost the detection sensitivity and anti-interference capability of the Pt(II) complex-based probe remains a big challenge. Here, an effective modulation strategy was proposed by precisely regulating the interaction energy through adjusting the type of the counter anions, and a series of probes have been synthesized by counter anion (X = Cl-, ClO4-, PF6-) exchange in [Pt(tpy)Cl]·X (tpy=2,2':6',2''-terpyridine), and thus the colorimetric-luminescence dual-mode detection toward nitrate was achieved. The optimal [Pt(tpy)Cl]·Cl probe shows superior nitrate detection performance including a limit of detection (LOD) (8.68 nM), rapid response (<0.5 s), an excellent selectivity and anti-interference capability even facing 14 common anions. Moreover, a polyvinyl alcohol (PVA) sponge-based sensing chip loaded with the probe enables the ultra-sensitive detection of nitrate particles with an ultralow detection limit of 7.6 pg, and it was further integrated into a detection pen for the accurate recognition of nitrate particles in real scenarios. The proposed counter-anion modulation strategy is expected to start a new frontier for the exploration of novel Pt(II) complex-based probes.

Interplay between anion-receptor and anion-solvent interactions in halide receptor complexes characterized with ultrafast infrared spectroscopies

The competition between host-guest binding and solvent interactions is a crucial factor in determining the binding affinities and selectivity of molecular receptor species. The interplay between these competing interactions, however, have been difficult to disentangle. In particular, the development of molecular-level descriptions of solute-solvent interactions remains a grand experimental challenge. Herein, we investigate the prototypical halide receptor meso-octamethylcalix[4]pyrrole (OMCP) complexed with either chloride or bromide anions in both dichloromethane (DCM) and chloroform (trichloromethane, TCM) solvent using ultrafast infrared transient absorption and 2D IR spectroscopies. OMCP·Br- complexes in both solvents display slower vibrational relaxation dynamics of the OMCP pyrrole NH stretches, consistent with weaker H-bonding interactions with OMCP compared to chloride and less efficient intermolecular relaxation to the solvent. Further, OMCP·Br- complexes show nearly static spectral diffusion dynamics compared to OMCP·Cl-, indicating larger structural fluctuations occur within chloride complexes. Importantly, distinct differences in the vibrational spectra and dynamics are observed between DCM and TCM solutions. The data are consistent with stronger and more perturbative solvent effects in TCM compared to DCM, despite DCM's larger dielectric constant and smaller reported OMCP·X- binding affinities. These differences are attributed to the presence of weak H-bond interactions between halides and TCM, in addition to competing interactions from the bulky tetrabutylammonium countercation. The data provide important experimental benchmarks for quantifying the role of solvent and countercation interactions in anion host-guest complexes.

Modulation of Tannic Acid on the Cholesteric Structure of Cellulose Nanocrystals

The chiral nematic phase structure, formed by the self-assembly of cellulose nanocrystals (CNCs) in an aqueous suspension and maintained in a solid film, shows great potential for optical applications. To achieve complex structures in optical devices, it is crucial to subject CNCs to specific shearing processes, such as spinning and printing. Understanding the structural and property changes of the CNC liquid crystal phase in these processes is of utmost importance. In this study, we investigated the effect of adding tannic acid (TA) on the rheological properties and cholesteric phase structures of CNCs/TA mixed suspensions. By calculating the surface site interaction points, we observed that TA can adsorb onto the surface of CNC rods in suspensions through hydrogen bonding. Through characterization techniques, such as polarized optical microscopy, rheology, and synchrotron SAXS, we examined the effects of TA addition on the microstructure and rheological properties of the CNC liquid crystal phase and clarified the change relating to the system composition. Under the same CNC concentration, the volume fraction of the anisotropic phase, the pitch, and the rod spacing of the cholesteric phase were not significantly affected by the addition of TA. However, the system viscosity was significantly reduced with the appropriate amount of TA (2 wt %), in a wide range of CNC concentrations (up to 15 wt % CNCs). The flow indexes (n) in Region I and Region III of steady-state shear curves of CNCs/TA systems (11-15 wt % CNCs) were compared. Moreover, we introduced the well-established theoretical models for liquid crystal polymers to tentatively interpret Region I of the CNCs/TA cholesteric phase and realized that increased numbers of smaller cholesteric-phase domains in the CNCs/TA system and interfacial modification by TA may contribute to the fluidity change. The feature of the domain texture of CNCs/TA systems is verified by polarized optical microscopy observations.

Anion-Dependent Strength Scale of Interactions in Ionic Liquids from X-ray Photoelectron Spectroscopy, Ab Initio Molecular Dynamics, and Density Functional Theory

Using a combination of experiments and calculations, we have gained new insights into the nature of anion-cation interactions in ionic liquids (ILs). An X-ray photoelectron spectroscopy (XPS)-derived anion-dependent electrostatic interaction strength scale, determined using XPS core-level binding energies for IL cations, is presented here for 39 different anions, with at least 18 new anions included. Linear correlations of experimental XPS core-level binding energies for IL cations with (a) calculated core binding energies (ab initio molecular dynamics (AIMD) simulations were used to generate high-quality model IL structures followed by single-point density functional theory (DFT) to obtain calculated core binding energies), (b) experimental XPS core-level binding energies for IL anions, and (c) other anion-dependent interaction strength scales led to three main conclusions. First, the effect of different anions on the cation can be related to ground-state interactions. Second, the variations of anion-dependent interactions with the identity of the anion are best rationalized in terms of electrostatic interactions and not occupied valence state/unoccupied valence state interactions or polarizability-driven interactions. Therefore, the XPS-derived anion-dependent interaction strength scale can be explained using a simple electrostatic model based on electrostatic site potentials. Third, anion-probe interactions, irrespective of the identity of the probe, are primarily electrostatic, meaning that our electrostatic interaction strength scale captures some inherent, intrinsic property of anions independent of the probe used to measure the interaction strength scale.

Solvent effects in anion recognition

Anion recognition is pertinent to a range of environmental, medicinal and industrial applications. Recent progress in the field has relied on advances in synthetic host design to afford a broad range of potent recognition motifs and novel supramolecular structures capable of effective binding both in solution and at derived molecular films. However, performance in aqueous media remains a critical challenge. Understanding the effects of bulk and local solvent on anion recognition by host scaffolds is imperative if effective and selective detection in real-world media is to be viable. This Review seeks to provide a framework within which these effects can be considered both experimentally and theoretically. We highlight proposed models for solvation effects on anion binding and discuss approaches to retain strong anion binding in highly competitive (polar) solvents. The synthetic design principles for exploiting the aforementioned solvent effects are explored.

Anion-Bridged Dual Hydrogen Bond Enabled Concerted Addition of Phenol to Glycal

A noncovalent organocatalytic concerted addition of phenol to glycal is developed for the stereoselective and regioselective construction of biologically important phenolic 2-deoxyglycosides, featuring wide substrate tolerance. The method relies on an anion-bridged dual hydrogen bond interaction which is experimentally proved by Nuclear Magnetic Resonance (NMR), Ultraviolet and visible (UV-vis), and fluorescence analysis. Experimental evidence including kinetic analysis, Kinetic Isotope Effect (KIE) studies, linear free energy relationship, Hammett plot, and density functional theory (DFT) calculations is provided for a concerted mechanism where a high-energy oxocarbenium ion is not formed. In addition, the potential utility of this method is further demonstrated by the synthesis of biologically active glycosylated flavones. The benchmarking studies demonstrate significant advances in this newly developed method compared to previous approaches.

Anion-templated synthesis of a switchable fluorescent [2]catenane with sulfate sensing capability

Anion templation strategies have facilitated the synthesis of various catenane and rotaxane hosts capable of strong and selective binding of anions in competitive solvents. However, this approach has primarily relied on positively charged precursors, limiting the structural diversity and the range of potential applications of the anion-templated mechanically interlocked molecules. Here we demonstrate the synthesis of a rare electroneutral [2]catenane using a powerful, doubly charged sulfate template and a complementary diamidocarbazole-based hydrogen bonding precursor. Owing to the unique three-dimensional hydrogen bonding cavity and the embedded carbazole fluorophores, the resulting catenane receptor functions as a sensitive fluorescent turn-ON sensor for the highly hydrophilic sulfate, even in the presence of a large excess of water. Importantly, the [2]catenane exhibits enhanced binding affinity and selectivity for sulfate over its parent macrocycle and other acyclic diamidocarbazole-based receptors. We demonstrate also, for the first time, that the co-conformation of the catenane may be controlled by reversible acid/base induced protonation and deprotonation of the anionic template, SO42-. This approach pioneers a new strategy to induce molecular motion of interlocked components using switchable anionic templates.

Solvent Acts as the Referee in a Match-Up Between Charged and Preorganized Receptors

The prevalence of anion-cation contacts in biomolecular recognition under aqueous conditions suggests that ionic interactions should dominate the binding of anions in solvents across both high and low polarities. Investigations of this idea using titrations in low polarity solvents are impaired by interferences from ion pairing that prevent a clear picture of binding. To address this limitation and test the impact of ion-ion interactions across multiple solvents, we quantified chloride binding to a cationic receptor after accounting for ion pairing. In these studies, we created a chelate receptor using aryl-triazole CH donors and a quinolinium unit that directs its cationic methyl inside the binding pocket. In low-polarity dichloromethane, the 1 : 1 complex (log K1 : 1 ~ 7.3) is more stable than neutral chelates, but fortuitously comparable to a preorganized macrocycle (log K1 : 1 ~ 6.9). Polar acetonitrile and DMSO diminish stabilities of the charged receptor (log K1 : 1 ~ 3.7 and 1.9) but surprisingly 100-fold more than the macrocycle. While both receptors lose stability by dielectric screening of electrostatic stability, the cationic receptor also pays additional costs of organization. Thus even though the charged receptor has stronger binding in apolar solvents, the uncharged receptor has more anion affinity in polar solvents.

Towards molecularly imprinted polymers that respond to and capture phosphorylated tyrosine epitopes using fluorescent bis-urea and bis-imidazolium receptors

Early detection of cancer is essential for successful treatment and improvement in patient prognosis. Deregulation of post-translational modifications (PTMs) of proteins, especially phosphorylation, is present in many types of cancer. Therefore, the development of materials for the rapid sensing of low abundant phosphorylated peptides in biological samples can be of great therapeutic value. In this work, we have synthesised fluorescent molecularly imprinted polymers (fMIPs) for the detection of the phosphorylated tyrosine epitope of ZAP70, a cancer biomarker. The polymers were grafted as nanometer-thin shells from functionalised submicron-sized silica particles using a reversible addition-fragmentation chain-transfer (RAFT) polymerisation. Employing the combination of fluorescent urea and intrinsically cationic bis-imidazolium receptor cross-linkers, we have developed fluorescent sensory particles, showing an imprinting factor (IF) of 5.0. The imprinted polymer can successfully distinguish between phosphorylated and non-phosphorylated tripeptides, reaching lower micromolar sensitivity in organic solvents and specifically capture unprotected peptide complements in a neutral buffer. Additionally, we have shown the importance of assessing the influence of counterions present in the MIP system on the imprinting process and final material performance. The potential drawbacks of using epitopes with protective groups, which can co-imprint with targeted functionality, are also discussed.

BILFF: All-Atom Force Field for Modeling Triazolium- and Benzoate-Based Ionic Liquids

We present an extension of our previously developed all-atom force field BILFF (Bio-polymers in Ionic Liquids Force Field) to three different ionic liquids: 1-ethyl-3-methyl-1,2,3-triazolium acetate ([EMTr][OAc]), 1-ethyl-3-methyl-1,2,3-triazolium benzoate ([EMTr][OBz]), and 1-ethyl-3-methylimidazolium benzoate ([EMIm][OBz]). These ionic liquids are of practical importance as they have the ability to dissolve significant amounts of cellulose even at room temperature. Our force field is optimized to accurately reproduce the strong hydrogen bonding in the system with nearly quantum chemical accuracy. A very good agreement between the microstructure of the quantum chemical simulations over a wide temperature range and experimental density data with the results of BILFF were observed. Non-trivial effects, such as the solvation shell structure and π-π stacking of the cations, are also accurately reproduced. Our force field enables accurate simulations of larger systems, such as solvated cellulose in different (aqueous) ionic liquids, and is the first to present the optimized parameters for mixtures of these solvents and water.

Neutral MOF Anion Receptor: Radical-Promoted Precise Anion Recognition

Precise ion recognition plays a key role in the anionic decontamination in water. However, the established anionic recognition based on neutral or cationic anion receptor is still restricted by the inherent limitation, such as narrow application scope in organic solvent rather than water for neutral anion receptor and poor selectivity due to non-directional electrostatic interaction for cationic anion receptor. Herein, for the first time, a neutral metal-organic framework (MOF) anion receptor is shown, enabling precise anion recognition, for example, the presence of a variety of 1000-fold competitive anions does not affect the selective adsorption of the target anion at all. A radical-dominating anion-recognition mechanism is proposed for rationalizing the efficacy of the neutral MOF.

Aromatic pentaamide macrocycles bind anions with high affinity for transport across biomembranes

The convergent positioning of functional groups in biomacromolecules leads to good binding, catalytic and transport capabilities. Synthetic frameworks capable of convergently locking functional groups with minimized conformational uncertainty-leading to similar properties-are highly desirable but rare. Here we report C5-symmetric aromatic pentaamide macrocycles synthesized in one pot from the corresponding monomers. Their crystal structures reveal a star-shaped, fully constrained backbone that causes ten alternating NH/CH hydrogen-bond donors and five large amide dipoles to orient towards the centre of the macrocycle. With a highly electropositive cavity in a high-energy unbound state, the macrocycles bind anions in a 1:1 stoichiometry in solution, with high affinity for halides and very high affinity for oxoanions. We demonstrate that such macrocycles are able to transport anions across lipid bilayers with a high chloride selectivity and restore the depleted airway surface liquid of cystic fibrosis airway cell cultures.

Further Understanding the Roles of Solvent, Brønsted Acids, and Hydrogen Bonding in Iron Porphyrin-Mediated Carbon Dioxide Reduction

Improving our understanding of how molecules and materials mediate the electrochemical reduction of carbon dioxide (CO2) to upgraded products is of great interest as a means to address climate change. A leading class of molecules that can facilitate the electrochemical conversion of CO2 to carbon monoxide (CO) is iron porphyrins. These molecules can have high rate constants for CO2-to-CO conversion; they are robust, and they rely on abundant and inexpensive synthetic building blocks. Important foundational work has been conducted using chloroiron 5,10,15,20-tetraphenylporphyrin (FeTPPCl) in N,N-dimethylformamide (DMF) solvent. A related and recent report points out that the corresponding perchlorate complex, FeTPPClO4, can have superior function due to its solubility in other organic solvents. However, the importance of hydrogen bonding and solvent effects was not discussed. Herein, we present a detailed kinetic study of the triflate (CF3SO3-) complex of FeTPP in DMF and in MeCN using a range of phenol Brønsted acid additives. We also detected the formation of Fe(III)TPP-phenolate complexes using cyclic voltammetry experiments. Importantly, our new analysis of apparent rate constants with different added phenols allows for a modification to the established mechanistic model for CO2-to-CO conversion. Critically, our improved model accounts for hydrogen bonding and solvent effects by using simple hydrogen bond acidity and basicity descriptors. We use this augmented model to rationalize function in other reported porphyrin systems and to make predictions about operational conditions that can enhance the CO2 reduction chemistry.

Acetamido-TEMPO mediated electrochemical oxidation of alcohols to aldehydes and ketones

A protocol for the oxidation of alcohols to aldehydes and ketones employing an electrochemical aminoxyl-mediated reaction is presented. The approach employs a catalytic amount of the radical and the use of a base is not required. It is performed using readily available electrodes in a commercially available electrochemistry apparatus and does not require a reference electrode. The methodology is applicable to a range of structurally and electronically diverse substrates, including the oxidation of primary alcohols to aldehydes rather than the more commonly formed carboxylic acids.

Stereoselective Synthesis of Z-Styryl Sulfides from Nucleophilic Addition of Arylacetylenes and Benzyl Thiols

The stereoselective synthesis of Z-anti-Markovnikov styryl sulfides via an anionic thiolate-alkyne addition reaction was achieved when the terminal alkynes and benzyl mercaptans were reacted using tBuOLi (0.5 equiv) in EtOH under ambient conditions. Exclusive stereoselectivity (ca. 100%) was achieved by stereoelectronic control via anti-periplanar and anti-Markovnikov addition of benzylthiolates to phenylacetylenes. Solvolysis of lithium thiolate ion pairs in ethanol significantly suppresses the competing formation of the E-isomer. A remarkable enhancement of the Z-selectivity under a longer reaction time was observed.

Noncovalent tailoring of coordination complexes by resorcin[4]arene-based supramolecular hosts

Molecular recognition of guest molecules in a confined cavity is one of the important phenomena in biological and artificial molecular systems. When the guest is trapped within an artificial nano-space, its conformation is fixed in an unusual fashion by noncovalent interactions with host frameworks, and also the guest is kept away from the bulk solvent by the steric effect of the host. Therefore, host-guest formations lead to the effective modulation of the chemical and physical properties of guests via noncovalent interactions. In contrast to the many examples of organic guests, the examples of host-guest formation using coordination complex guests have been less explored. This is simply due to the size and shape complementarity problem between small hosts and large coordination complex guests. Resorcin[4]arene-based supramolecular hosts have been shown to provide internal cavities that are large enough to fully accommodate coordination complexes within the internal spaces via effective molecular interactions. In this article, we focus on supramolecular strategies to control the chemical and physical properties of the coordination complex guests within resorcin[4]arene-based supramolecular hosts. By the careful selection of the host and guest complexes, these combinations can produce a new supramolecular system, showing unusual structures, redox, catalytic, and photophysical properties derived from the entrapped coordination complexes.

Selectivity Rule of Cryptands for Anions: Molecular Rigidity and Bonding Site

Cryptands utilize inside CH or NH groups as hydrogen bond (H-bond) donors to capture anions such as halides. In this work, the nature and selectivity of confined hydrogen bonds inside cryptands were computationally analyzed with the energy decomposition scheme based on the block-localized wavefunction method (BLW-ED), aiming at an elucidation of governing factors in the binding between cryptands and anions. It was revealed that the intrinsic strengths of inward hydrogen bonds are dominated by the electrostatic attraction, while the anion preferences (selectivity) of inner CH and NH hydrogen bonds are governed by the Pauli exchange repulsion and electrostatic interaction, respectively. Typical conformers of cages are classified into two groups, including the C_3(h) -symmetrical conformers, in which all halide anions are located near the centroids of cages, and the "semi-open" conformers, which exhibit shifted bonding sites for different halide anions. Accordingly, the difference in governing factors of selectivity is attributed to either the rigidity of cages or the binding site of anions for these two groups. In details, the C₃ conformers of NH cryptands can be enlarged more remarkably than the C_3(h) -symmetrical conformers of CH cryptands as the size of anion (ionic radius) increases, resulting in the relaxation of the Pauli repulsion and a dramatic reduction in electrostatic attraction, which eventually rules the selectivity of NH cryptands for halide anions. By contrary, the CH cryptands are more rigid and cannot effectively reduce the Pauli repulsion, which subsequently governs the anion preference. Unlike C₃ conformers whose rigidity determines the selectivity, semi-open conformers exhibit different binding sites for different anions. From F^- to I^- , the bonding site shifts toward the outside end of the pocket inside the semi-open NH cryptand, leading to the significant reduction of the electrostatic interaction that dominates the anion preference. Differently, binding sites are much less affected by the size of anion inside the semi-open CH cryptand, in which the Pauli exchange repulsion remains the key factor for the selectivity of inner hydrogen bonds.

Ion-specific binding of cations to the carboxylate and of anions to the amide of alanylalanine

Studies of ion-specific effects on oligopeptides have aided our understanding of Hofmeister effects on proteins, yet the use of different model peptides and different experimental sensitivities have led to conflicting conclusions. To resolve these controversies, we study a small model peptide, L-Alanyl-L-alanine (2Ala), carrying all fundamental chemical protein motifs: C-terminus, amide bond, and N-terminus. We elucidate the effect of GdmCl, LiCl, KCl, KI, and KSCN by combining dielectric relaxation, nuclear magnetic resonance (¹H-NMR), and (two-dimensional) infrared spectroscopy. Our dielectric results show that all ions reduce the rotational mobility of 2Ala, yet the magnitude of the reduction is larger for denaturing cations than for anions. The NMR chemical shifts of the amide group are particularly sensitive to denaturing anions, indicative of anion-amide interactions. Infrared experiments reveal that LiCl alters the spectral homogeneity and dynamics of the carboxylate, but not the amide group. Interaction of LiCl with the negatively charged pole of 2Ala, the COO^- group, can explain the marked cationic effect on dipolar rotation, while interaction of anions between the poles, at the amide, only weakly perturbs dipolar dynamics. As such, our results provide a unifying view on ions' preferential interaction sites at 2Ala and help rationalize Hofmeister effects on proteins.

The surface site interaction point approach to non-covalent interactions

The functional properties of molecular systems are generally determined by the sum of many weak non-covalent interactions, and therefore methods for predicting the relative magnitudes of these interactions is fundamental to understanding the relationship between function and structure in chemistry, biology and materials science. This review focuses on the Surface Site Interaction Point (SSIP) approach which describes molecules as a set of points that capture the properties of all possible non-covalent interactions that the molecule might make with another molecule. The first half of the review focuses on the empirical non-covalent interaction parameters, α and β, and provides simple rules of thumb to estimate free energy changes for interactions between different types of functional group. These parameters have been used to have been used to establish a quantitative understanding of the role of solvent in solution phase equilibria, and to describe non-covalent interactions at the interface between macroscopic surfaces as well as in the solid state. The second half of the review focuses on a computational approach for obtaining SSIPs and applications in multi-component systems where many different interactions compete. Ab initio calculation of the Molecular Electrostatic Potential (MEP) surface is used to derive an SSIP description of a molecule, where each SSIP is assigned a value equivalent to the corresponding empirical parameter, α or β. By considering the free energies of all possible pairing interactions between all SSIPs in a molecular ensemble, it is possible to calculate the speciation of all intermolecular interactions and hence predict thermodynamic properties using the SSIMPLE algorithm. SSIPs have been used to describe both the solution phase and the solid state and provide accurate predictions of partition coefficients, solvent effects on association constants for formation of intermolecular complexes, and the probability of cocrystal formation. SSIPs represent a simple and intuitive tool for describing the relationship between chemical structure and non-covalent interactions with sufficient accuracy to understand and predict the properties of complex molecular ensembles without the need for computationally expensive simulations.

Measuring anion binding at biomembrane interfaces

The quantification of anion binding by molecular receptors within lipid bilayers remains challenging. Here we measure anion binding in lipid bilayers by creating a fluorescent macrocycle featuring a strong sulfate affinity. We find the determinants of anion binding in lipid bilayers to be different from those expected that govern anion binding in solution. Charge-dense anions H2PO4- and Cl- that prevail in dimethyl sulfoxide fail to bind to the macrocycle in lipids. In stark contrast, ClO4- and I- that hardly bind in dimethyl sulfoxide show surprisingly significant affinities for the macrocycle in lipids. We reveal a lipid bilayer anion binding principle that depends on anion polarisability and bilayer penetration depth of complexes leading to unexpected advantages of charge-diffuse anions. These insights enhance our understanding of how biological systems select anions and guide the design of functional molecular systems operating at biomembrane interfaces.

Non-Globular Organic Ionic Plastic Crystal Containing a Crown-Ether Moiety - Tuning Its Behaviour Using Sodium Salts

Organic ionic plastic crystals (OIPCs) are a class of soft materials showing positional order while still allowing orientational freedom. Due to their motional freedom in the solid state, they possess plasticity, non-flammability and high ionic conductivity. OIPC behavior is typically exhibited by 'simple' globular molecules allowing molecular rotation, whereas the interactions that govern the formation of OIPC phases in complex non-globular molecules are less understood. To better understand these interactions, a new family of non-globular OIPCs containing a 15-crown-5 ether moiety was synthetized and characterized. The 15C5BA molecule prepared does not exhibit the sought-after behavior because of its non-globular nature and strong intermolecular H-bonds that restrict orientational motion. However, the OIPC behavior was successfully obtained through complexation of the crown-ether moiety with sodium salts containing chaotropic anions. Those anions weaken the interactions between the molecules, allowing rotational freedom and tuning of the thermal and morphological properties of the OIPC.

Quantification of secondary electrostatic interactions in H-bonded complexes

The H-bonding properties of compounds that contain multiple functional groups are difficult to predict, because there are through-bond polarisation effects and long-range secondary electrostatic interactions that have significant effects on the interactions with solvents and other molecules. Here we use experimental measurements of association constants for formation of 1 : 1 H-bonded complexes that contain a single well-defined H-bond and a single well-defined secondary electrostatic interaction to quantify the magnitude of this effect. The results were used to develop a computational method for calculating functional group H-bond parameters that accurately reproduce the magnitudes of both primary H-bonding interaction and secondary electrostatic interactions. The effects of secondary electrostatic interactions are observed in calculations of ab initio Molecular Electrostatic Potential (MEP) values, but at the van der Waals surface, the magnitude of the effect is highly overestimated. MEP values calculated on electron density isosurfaces that lie closer to the nuclei provide a more accurate description of the experimental observations. H-bond parameters calculated using this approach successfully account for the properties of arrays of multiple H-bond donor and acceptor groups in different configurations. The results provide insight into the factors that govern the interaction properties of molecules that contain multiple functional groups and provide an accurate method for prediction of solution phase complexation free energies based on gas phase calculations of individual molecules.

Calix[6]arenes with halogen bond donor groups as selective and efficient anion transporters

Here we present the anion binding and anion transport properties of a series of calix[6]arenes decorated on their small rim with either halogen bond or hydrogen bond donating groups. We show that the halogen bond donating iodotriazole groups enable highly selective transport of chloride and nitrate anions, without transport of protons or hydroxide, at rates similar to those observed with thiourea or squaramide groups.

A calix[6]arene with three preorganised halogen bond donating groups gives >100-fold selectivity for Cl⁻ uniport over HCl symport, in contrast to analogous compounds with strong hydrogen bond donating groups.

The pKBHX Hydrogen-Bond Basicity Scale: From Molecules to Anions

The pK_BHX (logarithm of complexation constant K of 4-fluorophenol with bases) hydrogen-bond basicity scale of neutral hydrogen-bond acceptors (HBAs) is extended to anionic HBAs. The scale is constructed for 26 anions through (i) the infrared measurement of K on NBu₄⁺X^- ion pairs in CCl₄, (ii) the estimation of K from linear free energy relationships between measured K values and literature K values for various phenols in polar solvents, and (iii) the computation of K at the density functional theory level in CCl₄. The scale extends on a 9.4 pK unit range from fluoride to tetraphenylborate. Considering a number of anions as organic functions substituted with unipolar substituents, their pK_BHX values can be related to the Hammett-Taft substituent constants σ. Unipolar substituents (O^- and S^-) obey the same pK_BHX versus σ relationships as dipolar ionic (N^-N⁺R₃) and dipolar (OH, CF₃, NR₂, or OR) ones for the nitrile, carbonyl, nitroso, nitro, sulfonyl, and phosphoryl functions. Like dipolar substituents, unipolar substituents at carbon and nitrogen operate by field-inductive and resonance effects, whereas substituents at sulfur and phosphorus operate only by the field-inductive effect.

Reversible Capture and Release of a Ligand Mediated by a Long-Range Relayed Polarity Switch in a Urea Oligomer

Ethylene-bridged oligoureas characterized by a continuous, switchable chain of hydrogen bonds and carrying a binding site (an N,N′-disubstituted urea) for a hydrogen-bond-accepting ligand (a phosphine oxide) were synthesized. These oligomers show stronger ligand binding when the binding site is located at the hydrogen-bond-donating terminus than when the same binding site is at the hydrogen-bond-accepting terminus. An acidic group at the terminus remote from the binding site allows hydrogen bond polarity, and hence ligand binding ability, to be controlled remotely by a deprotonation/reprotonation cycle. Addition of base induces a remote conformational change that is relayed through up to five urea linkages, reducing the ability of the binding site to retain an intermolecular association to its ligand, which is consequently released into solution. Reprotonation returns the polarity of the oligomer to its original directionality, restoring the function of the remote binding site, which consequently recaptures the ligand. This is the first example of a synthetic molecular structure that relays intermolecular binding information, and these “dynamic foldamer” structures are prototypes of components for chemical systems capable of controlling chemical function from a distance.

The effect of solvent on the binding of anions and ion-pairs with a neutral [2]rotaxane

In this work we report the binding properties of rotaxane 1 towards a series of tetraalkylammonium salts of Cl^-, OCN^- and NO₃^- anions in acetone and a CHCl₃/MeOH solvent mixture. We used ¹H NMR titrations and Isothermal Titration Calorimetry (ITC) experiments to monitor and analyze the binding processes. We compared the obtained results with those previously described by us in chloroform solution. In acetone solution, the determined binding constants for the 1 : 1 complexes were 1 to 3 orders of magnitude larger than those measured in chloroform, a less competitive solvent for hydrogen-bonding. The thermodynamic signatures of the binding processes in acetone, determined by ITC experiments, revealed favorable enthalpic and entropic contributions having similar magnitudes. These results suggested that solvation/desolvation processes in acetone play a significant role in the binding processes. Conversely, the addition of just 5% of methanol to chloroform solutions of 1 significantly reduces the magnitude of the binding constants of all studied ion-pairs. In this solvent mixture, the entropy term is also favorable but it does not compensate the experienced loss of binding enthalpy. Moreover, in acetone solution, the addition of the Cl^- and OCN^- tetraalkylammonium salts in excess (more than 1 equiv.) led to the immediate appearance of 2 : 1 complexes. Related high-stoichiometry complexes are not observed in the solvent mixture (CHCl₃/MeOH 95/5). In chloroform, a large excess of the salt (> 6 equiv.) is required for its formation. From the analysis of the obtained binding data we infer that, in acetone and in CHCl₃/MeOH mixture, the formed complexes are mainly anionic.

Trisubstituted 1,3,5-Triazines: The First Ligands of the sY12-Binding Pocket on Chemokine CXCL12

CXCL12, a CXC-type chemokine, binds its receptor CXCR4, and the resulting signaling cascade is essential during development and subsequently in immune function. Pathologically, the CXCL12–CXCR4 signaling axis is involved in many cancers and inflammatory diseases and thus has sparked continued interest in the development of therapeutics. Small molecules targeting CXCR4 have had mixed results in clinical trials. Alternatively, small molecules targeting the chemokine instead of the receptor provide a largely unexplored space for therapeutic development. Here we report that trisubstituted 1,3,5-triazines are competent ligands for the sY12-binding pocket of CXCL12. The initial hit was optimized to be more synthetically tractable. Fifty unique triazines were synthesized, and the structure–activity relationship was probed. Using computational modeling, we suggest key structural interactions that are responsible for ligand–chemokine binding. The lipophilic ligand efficiency was improved, resulting in more soluble, drug-like molecules with chemical handles for future development and structural studies.

Orthogonal binding and displacement of different guest types using a coordination cage host with cavity-based and surface-based binding sites

The octanuclear Co(ii) cubic coordination cage system H (or HW if it bears external water-solubilising substituents) has two types of binding site for guests. These are (i) the partially-enclosed central cavity where neutral hydrophobic organic species can bind, and (ii) the six 'portals' in the centres of each of the faces of the cubic cage where anions bind via formation of a network of CH⋯X hydrogen bonds between the anion and CH units on the positively-charged cage surface, as demonstrated by a set of crystal structures. The near-orthogonality of these guest binding modes provides the basis for an unusual dual-probe fluorescence displacement assay in which either a cavity-bound fluorophore (4-methyl-7-amino-coumarin, MAC; λ_em = 440 nm), or a surface-bound anionic fluorophore (fluorescein, FLU; λ_em = 515 nm), is displaced and has its emission ‘switched on’ according to whether the analyte under investigation is cavity-binding, surface binding, or a combination of both. A completely orthogonal system is demonstrated based using a Hw/MAC/FLU combination: addition of the anionic analyte ascorbate displaced solely FLU from the cage surface, increasing the 515 nm (green) emission component, whereas addition of a neutral hydrophobic guest such as cyclooctanone displaced solely MAC from the cage central cavity, increasing the 440 nm (blue) emission component. Addition of chloride results in some release of both components, and an intermediate colour change, as chloride is a rare example of a guest that shows both surface-binding and cavity-binding behaviour. Thus we have a colourimetric response based on differing contributions from blue and green emission components in which the specific colour change signals the binding mode of the analyte. Addition of a fixed red emission component from the complex [Ru(bipy)₃]²⁺ (Ru) provides a baseline colour shift of the overall colour of the luminescence closer to neutral, meaning that different types of guest binding result in different colour changes which are easily distinguishable by eye.

Orthogonal binding of neutral or anionic fluorophores to the cavity or surface, respectively, of a coordination cage host allows a dual-probe displacement assay which gives a different fluorescence colorimetric response according to where analyte species bind.

Molecular level insight into the solvation of cellulose in deep eutectic solvents

Deep eutectic solvents as sustainable and new-generation solvents show potential in the field of cellulose dissolution. Although these novel materials are tested for numerous industrial, environmental, and medical applications, little is known about the structural features of cellulose interacting with deep eutectic solvents. In this work, the interplay of cellulose is studied in two deep eutectic solvents: choline acetate mixed with urea and choline chloride mixed with urea using classical molecular dynamics simulations. Dissolution of cellulose in the studied liquids was not observed to be in agreement with experimental work from the literature. However, a slight swelling in the chloride, as compared to the acetate-based solvent, is apparent. A possible rationale might be found in the stronger hydrogen bonding of the chloride anion compared to the acetate anion with the hydrogen atoms of the cellulose. Moreover, chloride approaches the outer glucose units comparatively more, which could be interpreted as the onset of entering and thus dissolving the cellulose as was previously observed. Specific hydrogen bonds between all units are analyzed and discussed in detail.

The fluoride permeation pathway and anion recognition in Fluc family fluoride channels

Fluc family fluoride channels protect microbes against ambient environmental fluoride by undermining the cytoplasmic accumulation of this toxic halide. These proteins are structurally idiosyncratic, and thus the permeation pathway and mechanism have no analogy in other known ion channels. Although fluoride-binding sites were identified in previous structural studies, it was not evident how these ions access aqueous solution, and the molecular determinants of anion recognition and selectivity have not been elucidated. Using x-ray crystallography, planar bilayer electrophysiology, and liposome-based assays, we identified additional binding sites along the permeation pathway. We used this information to develop an oriented system for planar lipid bilayer electrophysiology and observed anion block at one of these sites, revealing insights into the mechanism of anion recognition. We propose a permeation mechanism involving alternating occupancy of anion-binding sites that are fully assembled only as the substrate approaches.

Classification and Design of HIV-1 Integrase Inhibitors Based on Machine Learning

A key enzyme in human immunodeficiency virus type 1 (HIV-1) life cycle, integrase (IN) aids the integration of viral DNA into the host DNA, which has become an ideal target for the development of anti-HIV drugs. A total of 1785 potential HIV-1 IN inhibitors were collected from the databases of ChEMBL, Binding Database, DrugBank, and PubMed, as well as from 40 references. The database was divided into the training set and test set by random sampling. By exploring the correlation between molecular descriptors and inhibitory activity, it is found that the classification and specific activity data of inhibitors can be more accurately predicted by the combination of molecular descriptors and molecular fingerprints. The calculation of molecular fingerprint descriptor provides the additional substructure information to improve the prediction ability. Based on the training set, two machine learning methods, the recursive partition (RP) and naive Bayes (NB) models, were used to build the classifiers of HIV-1 IN inhibitors. Through the test set verification, the RP technique accurately predicted 82.5% inhibitors and 86.3% noninhibitors. The NB model predicted 88.3% inhibitors and 87.2% noninhibitors with correlation coefficient of 85.2%. The results show that the prediction performance of NB model is slightly better than that of RP, and the key molecular segments are also obtained. Additionally, CoMFA and CoMSIA models with good activity prediction ability both were constructed by exploring the structure-activity relationship, which is helpful for the design and optimization of HIV-1 IN inhibitors.

Mechanistic insights into carbon dioxide utilization by superoxide ion generated electrochemically in ionic liquid electrolyte

Understanding the reaction mechanism that controls the one-electron electrochemical reduction of oxygen is essential for sustainable use of the superoxide ion (O2˙-) during CO2 conversion. Here, stable generation of O2˙- in butyltrimethylammonium bis(trifluoromethylsulfonyl)imide [BMAmm+][TFSI-] ionic liquid (IL) was first detected at -0.823 V vs. Ag/AgCl using cyclic voltammetry (CV). The charge transfer coefficient associated with the process was ∼0.503. It was determined that [BMAmm+][TFSI-] is a task-specific IL with a large negative isovalue surface density accrued from the [BMAmm+] cation with negatively charged C(sp2) and C(sp3). Consequently, [BMAmm+][TFSI-] is less susceptible to the nucleophilic effect of O2˙- because only 8.4% O2˙- decay was recorded from 3 h long-term stability analysis. The CV analysis also detected that O2˙- mediated CO2 conversion in [BMAmm+][TFSI-] at -0.806 V vs. Ag/AgCl as seen by the disappearance of the oxidative faradaic current of O2˙-. Electrochemical impedance spectroscopy (EIS) detected the mechanism of O2˙- generation and CO2 conversion in [BMAmm+][TFSI-] for the first time. The EIS parameters in O2 saturated [BMAmm+][TFSI-] were different from those detected in O2/CO2 saturated [BMAmm+][TFSI-] or CO2 saturated [BMAmm+][TFSI-]. This was rationalized to be due to the formation of a [BMAmm+][TFSI-] film on the GC electrode, creating a 2.031 × 10-9 μF cm-2 double-layer capacitance (CDL). Therefore, during the O2˙- generation and CO2 utilization in [BMAmm+][TFSI-], the CDL increased to 5.897 μF cm-2 and 7.763 μF cm-2, respectively. The CO2 in [BMAmm+][TFSI-] was found to be highly unlikely to be electrochemically converted due to the high charge transfer resistance of 6.86 × 1018 kΩ. Subsequently, O2˙- directly mediated the CO2 conversion through a nucleophilic addition reaction pathway. These results offer new and sustainable opportunities for utilizing CO2 by reactive oxygen species in ionic liquid media.

Emissive Supramolecular Systems Based on Reversible Bond Formation and Noncovalent Interactions

Noncovalent interactions and reversible bond formations are widely seen in natural systems for the construction of sophisticated molecular systems that perform various biological processes. Inspired by the natural systems, luminescent supramolecular systems constructed by coordination-driven self-assembly and homometallic metal-metal interations have been studied increasingly. These supramolecular systems show fascinating luminescent behaviors that are not observed from single components. This review summarizes our progress in the development of two types of unique luminescent supramolecular systems. The mononuclear Pt(II) complex units can sandwich coinage metal ions to form heteropolynuclear complexes involving heterometallic metal-metal interactions. A close proximity of the two or three different metal ions by the noncovalent forces lead to orbital overlapping among the coinage metal ions and the Pt(II) complex units, showing emission color change accompanied with structural transformation and reversible metal binding behaviors. Emissive host-guest systems consisting of mononuclear metal complexes and a hydrogen-bonded capsule are also developed, that show a unique encapsulation-induced emission enhancement (EIEE) behavior.