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.

A Carbazole-1,8-Disulfonamide-Derived Cryptand Receptor for Anion Recognition

A novel cryptand-like anion receptor 1 was synthesized in reasonable yield by a one-step condensation reaction. The UV-vis spectroscopic titrations indicated that cryptand 1 bound AcO- in preference to other monovalent anions (including its competing F- and H2PO4-) in CH3CN, generating a 1:1 binding complex with Ka = 51,000 M-1. Moreover, the crystal structures revealed that the acetate ion was encapsulated inside the cryptand's cavity in a 1:1 manner, through multiple N-H···O hydrogen bonds (although having two different crystal forms).

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.

Organocatalytic carbon dioxide fixation to epoxides by perfluorinated 1,3,5-triols catalysts

In order to improve epoxides conversion to carbonates by fixation of CO2 a new type of perfluorinated triol catalysts was developed. These simple acyclic scaffolds of enhanced acidity are efficient for catalysis through selective H-bonding activation of the epoxide. In combination with TBAI as co-catalyst, this useful transformation is performed under only 1 atmosphere of CO2 and between 30 to 80 °C. Both the 1,3,5-triol motif and the perfluorinated side chains are crucial in order to observe this epoxide opening under such mild conditions. In addition, the stereochemistry of the starting epoxide can efficiently be conserved during the carbonate formation.

Anion coordination chemistry using O-H groups

This review covers significant advances in the use of O-H groups in anion coordination chemistry. The review focuses on the use of these groups in synthetic anion receptors, as well as more recent developments in transport, self-assembly and catalysis.

Control of Molecular Recognition via Modulation of the Nanoenvironment

Many biological processes are driven by the interaction of a host with a guest molecule. We show such interactions can be modulated by carefully defining the local molecular environment to give a specific chemical outcome. Particularly, the selectivity of a host toward two different ions (Ca²⁺ and Al³⁺) is defined by it being in solution or the physisorbed state. In solution, the host displays greater selectivity toward Ca²⁺. When physisorbed, the selectivity profile of the host is reversed with enhanced binding of Al³⁺. This demonstrates a single host molecule can be tailored to selectively bind multiple guests by altering its nanoenvironment.

Triazole-bearing calixpyrroles: strong halide binding affinities through multiple N–H and C–H hydrogen bonds

Triazole-bearing calixpyrroles (TCPs) were synthesized as artificial anion binding receptors.

Anion binding of a neutral bis(cyclopeptide) in water-methanol mixtures containing up to 95% water

Anion receptor 2b was designed and synthesized, which was structurally derived from a previously described bis(cyclopeptide) 2a comprising two covalently linked cyclic hexapeptide rings with alternating L-proline and 6-aminopicolinic acid subunits. Solubilizing groups attached to the aromatic cyclopeptide subunits of 2b cause a substantial improvement of water solubility with respect to 2a, but have negligible effects on anion binding properties. Thus, anion affinity of 2b could be evaluated in aqueous solvent mixtures in which 2a is not sufficiently soluble, namely in water–methanol with a water content of up to 95 vol%. The solvent-dependent characterization of anion binding showed that the logKa values of the iodide and sulfate complexes of 2b decrease linearly with increasing water content while the individual contributions of complexation enthalpy and entropy correlate with the solvent composition in a more complex manner. The obtained results provide insight into the factors that control anion affinity and selectivity of these neutral receptors in aqueous media. In addition, they show that substantial anion affinity can be expected even in 100% water.