Transmembrane anion transport mediated by adamantyl-functionalised imidazolium salts

We present the design, synthesis and transmembrane anion transport properties of a new class of mobile organic transporters, possessing a central imidazolium cation and two external adamantyl units. We demonstrate herein that the imidazolium cation can be incorporated in the structure of active mobile anion transporters. Depending on the nature of the counter-anion of the salt, as well as the extravesicular anions, different anion selectivities were obtained. We show the importance of the H2 proton of the imidazolium cation in order to obtain a higher binding constant of the chloride anion. Furthermore, we demonstrate the importance of the flexibility of the spacers between the adamantyl groups and the imidazolium cation in the transport process.     

Enhanced voltammetric anion sensing at halogen and hydrogen bonding ferrocenyl SAMs

Halogen bonding mediated electrochemical anion sensing has very recently been established as a potent platform for the selective and sensitive detection of anions, although the principles that govern binding and subsequent signal transduction remain poorly understood. Herein we address this challenge by providing a comprehensive study of novel redox-active halogen bonding (XB) and hydrogen bonding (HB) ferrocene-isophthalamide-(iodo)triazole receptors in solution and at self-assembled monolayers (SAMs). Under diffusive conditions the sensory performance of the XB sensor was significantly superior. In molecular films the XB and HB binding motifs both display a notably enhanced, but similar, response to specific anions. Importantly, the enhanced response of these films is rationalised by a consideration of the (interfacial) dielectric microenvironment. These effects, and the resolved relationship between anion binding and signal transduction, underpin an improved fundamental understanding of anion sensing at redox-active interfaces which will benefit not just the development of more potent, real-life relevant, sensors but also new tools to study host–guest interactions at interfaces.

Surface enhancement effects in the sensing of anions at redox-active molecular films are investigated in detail and rationalised based on a consideration of the dielectric binding microenvironment.     

Transmembrane anion transport by synthetic systems

This highlight examines recent advances in the development of synthetic membrane transporters for anions, highlighting the underlying principles of transporter design and the promising applications of such transporters to biological systems including potential future treatments for diseases like cystic fibrosis which is caused by dysregulation of chloride transport across epithelial cell membranes.     

Perfluorocarbon-hydrocarbons self-assembly: halogen bonding mediated intermolecular recognition

Halogen bonding is discussed as an interaction that is capable of overcoming the reluctance shown by fluorinated compounds towards interactions with hydrocarbons. This non-covalent interaction is sufficiently strong to enact formation of supramolecular architectures where perfluorocarbons, hydrocarbons, and inorganic salts are self-assembled. Examples are discussed which exemplify the consistency and versatility of this interaction in affording numerous and structurally different hybrid materials. The importance of fluorine in determining the overall packing of the formed architectures is also highlighted.     

Preorganization-enhanced halogen bonding via intramolecular hydrogen bonding: a theoretical study

Structural Chemistry - Cationic and neutral halogen bonding (XB) donors use two types (I and II) of intramolecular hydrogen bonding (HB) to preorganize structures and increase the efficiency of...     

Nonspherical anion sequestration by C–H hydrogen bonding

Macrocyclic arenes laid the foundations of supramolecular chemistry and their study established the fundamentals of noncovalent interactions. Advancing their frontier, here we designed rigidified resorcin[4]arenes that serve as hosts for large nonspherical anions. In one synthetic step, we vary the host''s anion affinity properties by more than seven orders of magnitude. This is possible by engineering electropositive aromatic C–H bond donors in an idealized square planar geometry embedded within the host''s inner cavity. The hydrogen atom''s electropositivity is tuned by introducing fluorine atoms as electron withdrawing groups. These novel macrocycles, termed fluorocages, are engineered to sequester large anions. Indeed, experimental data shows an increase in the anion association constant (K_a) as the number of F atoms increase. The observed trend is rationalized by DFT calculations of Hirshfeld Charges (HCs). Most importantly, fluorocages in solution showed weak-to-medium binding affinity for large anions like [PF₆]⁻ (10²< K_a <10⁴ M⁻¹), and high affinity for [MeSO₃]⁻ (K_a >10⁶).

Fluorocages: new class of rigidified host utilizing nontraditional C–H hydrogen bonds to capture the nonspherical anions.     

Intramolecular hydrogen bonding and anion binding of N-benzamido-N'-benzoylthioureas

N-(p-Dimethylamino)benzoyl-N'-phenylthiourea as an N-acylthiourea is known to be unable to bind anions due to a strong intramolecular hydrogen bond (IHB). We show here that by inserting an amido group in the N'-phenyl side the newly designed N-benzamido-N'-benzoylthioureas, despite this IHB too, bind strongly to anions with binding constants on the order of 10(6)-10(7) mol(-1) L. Results suggest that potential anion receptors or organocatalysts could be developed on the basis of this framework with a wide structural diversity.     

Early-late transition metal ferromagnetic coupling mediated by hydrogen bonding

Hydrogen bonding between early and late transition metal complexes is proposed as a strategy to obtain ferromagnetic interactions based on a theoretical study using density functional calculations.     

Participation of benzene hydrogen bonding upon anion binding

A m-xylene-bridged imidazolium receptor, 1, has been designed and synthesized. The receptor 1 utilizes two imidazole (C-H)(+)- - -anion hydrogen bonds and one benzene hydrogen- - -anion hydrogen bond. The major driving force of complexation between the receptor 1 and anions comes from two imidazole (C-H)(+)- - -anion hydrogen bonds. However, both NMR experiments and ab initio calculations show that the benzene hydrogen attracts the anion guests, clearly indicating benzene (C-H)- - -anion hydrogen bonding. [reaction: see text]     

Engineering functional materials by halogen bonding

Engineering functional materials endowed with unprecedented properties require the exploitation of new intermolecular interactions, which can determine the characteristics of the bulk materials. The great potential of Halogen Bonding (XB), namely any noncovalent interaction involving halogens as electron acceptors, in the design of new and high‐value functional materials is now emerging clearly. This Highlight will give a detailed overview on the energetic and geometric features of XB, showing how some of them are quite constant in most of the formed supramolecular complexes (e.g., the angle formed by the covalent and the noncovalent bonds around the halogen atom), while some others depend strictly on the nature of the interacting partners. Then, several specific examples of halogen‐bonded supramolecular architectures, whose structural aspects as well as applications in fields as diverse as enantiomers' separation, crystal engineering, liquid crystals, natural, and synthetic receptors, will be fully described. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: PolymChem 45: 1–15, 2007     

Solid-state NMR studies of hydrogen bonding networks and proton transport pathways based on anion and cation dynamics

Proton dynamics in polymer electrolyte membranes are multifaceted processes, and the relative contributions of various mechanisms can be difficult to distinguish. Judicious choices of model systems can aid in understanding the critical steps. In this study, we characterize anion dynamics in a series of benzimidazole-alkyl phosphonate salts, and compare those dynamics to a membrane prototype, built on a decane backbone. The series of salts are characterized, using high resolution (1)H solid-state magic angle spinning (MAS) NMR, DQ MAS NMR, and (31)P centreband-only detection of exchange (CODEX) NMR spectroscopy, to determine the influence of the nature of the alkyl group on the rates and geometries of anion dynamics, and overall proton exchange processes. The alkyl group is shown to slow the correlation times for anion reorientation, when compared at ambient temperature. However, it is also apparent that the lowered lattice energy of the salt lowers the activation energy and allows good dynamics at intermediate temperatures in both the benzimidazolium ethylphosphonate and in the HBr adduct of 1,10-(1-H-imidazol-5-yl)decanephosphonic acid (Imi-d-Pa).     

Conformational switching between carboxylic acid and carboxylate anion states by NH-O hydrogen bonding

Biologically important Ca-proteins and Ca-biominerals as metal-polymer complexes are often regulated by the complexation and demetalation with the biopolymers. Metal-oxygen bond is supported by NH···O hydrogen bonds between coordinating oxyanion and neighboring amide NH and by the successive hydrogen bonding networks. Carboxylate anion under hydrophobic conditions has a high basicity that leads to a covalent Ca-O bond character that is significantly affected by the NH···O hydrogen bond. The hydrogen bonds in Asp-containing tripeptide fragments in Ca-proteins presumably control coordination/dissociation of metal-oxygen bonds. Our systematic studies of carboxylate, sulfonate and phosphate Ca(II) complexes demonstrate a relationship between the basicity of oxyanion in carboxylate and hydrogen bonds as cooperating with the oligopeptide conformation in Asp-containing Ca(II) complexes. Hydrogen bonds between carboxylate oxyanion and amide NH, controlled by a conformational switching of oligopeptide fragments, seem to be one of essential factors for the regulatory formation of Ca-proteins and nano-architectures in connection with the interface structure of inorganic and organic phases in biominerals.     

Macrocyclic anion receptors based on directed hydrogen bonding interactions

Natural anion receptors use charge-neutral dipoles to bind small anions with high affinities and selectivities. A convergent and rigid display of hydrogen bond donors such as amide, thiourea and urea functional groups in macrocyclic scaffolds would be one of the most efficient ways to create synthetic anion receptors that mimic natural ones. In this article, we present examples of natural anion receptors and discuss the synthesis of neutral macrocyclic receptors and their anion binding properties.     

A completely selective and strongly accelerated Diels-Alder reaction mediated by hydrogen bonding

A Diels-Alder cycloaddition between a furan and a maleimide is presented in which the presence of complementary hydrogen bonding sites dramatically accelerate the reaction and, additionally, ensure that only one of two possible diastereoisomers is formed.     

Fluorinated liquid crystals formed by halogen bonding

New, halogen-bonded fluorinated mesogens are reported; the expected microphase separation associated with perfluoroalkyl chains is surprisingly absent in the mesophase.     

Molecular assemblies from imidazolyl-containing haloalkenes and haloalkynes: competition between halogen and hydrogen bonding

The structural characterization of molecular assemblies constructed from imidazolyl-containing haloalkenes and haloalkynes is reported. 1-(3-Iodopropargyl)imidazole (2) and 1-(2,3,3-triiodoallyl)imidazole (5) were synthesized from 1-propargylimidazole (1). In the solid state, these wholly organic modules self-assemble through N...I halogen-bonding interactions, thus giving rise to polymeric chains. The N...I interaction observed in 2 (d(N...I)=2.717 A, angle-spherical C(sp)-I...N=175.8 degrees) is quite strong relative to previously reported data. The N...I interaction in 5 (d(N...I)=2.901 A, angle-spherical C(sp2)-I...N=173.6 degrees) is weaker, in accordance with the order C(sp)-X<--base>C(sp2)-X<--base. Compound 5 was found to give a 1:1 cocrystal 4 with morpholinium iodide (6). In the X-ray crystal studies of 4, N...I halogen-bonding interactions similar to those observed in 5 were shown not to be present, as the arrangement of the molecules is governed by two interwoven hydrogen-bonding networks. The first network involves N-H...O interactions between nearby morpholinium cations, and the second network is based on N-H...N hydrogen bonding between morpholinium cations and imidazolyl groups. Both hydrogen-bonding schemes are charge-assisted. Halogen bonding is not completely wiped out, however, as the triiodoalkene fragment forms a halogen bond with an iodide anion in its vicinity (d(I...I)=3.470 A, angle-spherical C(sp2)-I...I=170.7 degrees). X-ray crystal studies of 6 show a completely different arrangement from that observed in 4, namely, N-H...O interactions are not present. In crystalline 6, morpholinium cations are interconnected through C-H...O bridges (d(H...O)=2.521 and 2.676 A), and the NH2+ groups interact with nearby iodide anions (d(H...I)=2.633 and 2.698 A).     

A catenane host system containing integrated triazole C-H hydrogen bond donors for anion recognition

A 3,5-bis(triazole)-pyridinium motif is integrated into a catenane structural framework via chloride anion templation. The catenane host system displays a high degree of selectivity for halide anions over dihydrogen phosphate.