Anion coordination chemistry - synthesis and anion binding features of cyclophane type macrobicyclic anion receptor molecules

Stimulus-Controlled Anion Binding and Transport by Synthetic Receptors

Anionic species are omnipresent and involved in many important biological processes. A large number of artificial anion receptors has therefore been developed. Some of these are capable of mediating transmembrane transport. However, where transport proteins can respond to stimuli in their surroundings, creation of synthetic receptors with stimuli-responsive functions poses a major challenge. Herein, we give a full overview of the stimulus-controlled anion receptors that have been developed thus far, including their application in membrane transport. In addition to their potential operation as membrane carriers, the use of anion recognition motifs in forming responsive membrane-spanning channels is discussed. With this review article, we intend to increase interest in transmembrane transport among scientists working on host-guest complexes and dynamic functional systems in order to stimulate further developments.

Helical Anion Foldamers in Solution

Using anions to induce molecular structure is a rapidly growing area of dynamic and switchable supramolecular chemistry. The emphasis of this review is on helical anion foldamers in solution, and many of the beautiful complexes described herein are accentuated by their crystal structures. Anion foldamers are defined as single- or multistrand complexes-often helical-that incorporate one or more anions. The review begins by discussing foldamer structure and nomenclature and follows with discourse on the anions which are employed. Recent advances in functional foldamers that bind a single anion are examined, including: induced chirality, stimuli-responsive dynamics, fluorescence changes, organocatalysis, anion transport, and halogen bonding. The review then inspects multianion foldamers, and this section is organized by the number of strands within the foldamer-from single- to triple-strand foldamers. Finally, the review is punctuated by recent hydrogen- and halogen-bonding triple-strand anion foldamers.

The usefulness of energy decomposition schemes to rationalize host–guest interactions

The findings reported here reveal the robustness and practical application of EDA-NOCV in rationalizing molecular recognition situations in host–guest systems.

Tracking the absence of anion–π interactions in modified [23](1,3,5)cyclophanes: insights from computation

[2₃](1,3,5)Cyclophanes containing 1,3,5-triazine and 1,3,5-triphosphinine planar rings present different degrees of aromaticity as well as σ-donor and π-acceptor abilities.

"Bis-Click" Ligation of DNA: Template-Controlled Assembly, Circularisation and Functionalisation with Bifunctional and Trifunctional Azides

Ligation and circularisation of oligonucleotides containing terminal triple bonds was performed with bifunctional or trifunctional azides. Both reactions are high yielding. Template-assisted bis-click ligation of two individual non-complementary oligonucleotide strands was accomplished to yield heterodimers exclusively. In this context, the template fulfils two functions: it accelerates the ligation reaction and controls product assembly (heterodimer vs. homodimer formation). Intermolecular bis-click circularisation of one oligonucleotide strand took place without template assistance. For construction of oligonucleotides with terminal triple bonds in the nucleobase side chain, 7- or 5-functionalised 7-deaza-dA and dU residues were used. These oligonucleotides are directly accessible by solid-phase synthesis. When trifunctional azides were employed instead of bifunctional linkers, functionalisation of the remaining azido group was performed with small molecules such as 1-ethynyl pyrene, biotin propargyl amide or with ethynylated oligonucleotides. By this means, branched DNA was constructed.

Computational Design of Thiourea-based Cyclophane Sensors for Small Anions

The host–guest binding properties of a tri-thiourea cyclophane receptor (1) with several common anions have been investigated using density functional theory (DFT) and molecular dynamics calculations. Receptor 1 is predicted to be an effective receptor for binding small halogen and Y-shaped (NO3– and AcO–) anions in the gas phase, cyclohexane and chloroform. The calculated order of anion binding affinity for the receptor 1 in chloroform is F– > Cl– > AcO– > NO3– >Br– > H2PO4– > HSO4–. The binding free energies are strongly influenced by a dielectric solvent medium. The structures of the receptor–anion complexes are characterized by multiple (typically 6) hydrogen bonds in all cases. The overall binding affinity of various anions is determined by the basicity of anion, size and shape of the binding site, and solvent medium. Explicit chloroform solvent molecular dynamics simulations of selected receptor–anion complexes reveal that the anions are strongly bound within the binding pocket via hydrogen-bonding interactions to all the receptor protons throughout the simulation. A sulfur analogue of receptor 1 (2), with a larger central cavity, is shown to be a more effective sensor than 1 for small anions. Two different approaches to develop the thiourea-based cyclophane receptor into a chromogenic sensor were examined.

Anion binding in covalent and self-assembled molecular capsules

This critical review describes selected examples extracted from the extensive literature generated during the past 42 years on the topic of anion binding in molecular capsules. The goal of including anions in molecular capsules emerges from the idea of incorporating the traits exhibited by biological receptors into synthetic ones. At the outset of this research area the capsules were unimolecular. The scaffold of the receptor was designed to covalently link a series of functional groups that could converge into a cavity and to avoid its collapse. The initial examples involved the encapsulation of one monoatomic spherical anion. With time, the cavity size of the receptor was increased and encapsulation of polyatomic anions and co-encapsulation became a reality. Synthetic economy fueled the use of aggregates of self-complementary molecules rather than one large molecule as capsules. The main purpose of this review is to give a general overview of the topic which might be of interest to supramolecular or non supramolecular chemists alike (149 references).

C-Glycoside Clustering on Calix[4]arene, Adamantane, and Benzene Scaffolds through 1,2,3-Triazole Linkers

A route has been paved toward the preparation of triazole glycocluster libraries via the copper(I)-catalyzed modern version of the classical Huisgen 1,3-dipolar cycloaddition of azides to alkynes. Up to four 1,4-disubstituted 1,2,3-triazole rings bearing carbon-linked glycosyl fragments were constructed on various scaffolds via multiple cycloadditions of suitably polyfunctionalized calix[4]arene, adamantane, and benzene derivatives with ethynyl and azidomethyl C-glycosides. Each cycloaddition occurred with high regioselectivity to give exclusively the 1,4-disubstituted triazole ring in very high yield up to an average value of 98%. The high degree of efficiency of this approach and its wide scope constitute a simple and practical means for the attachment of various sugar units to polyfunctionalized substrates.

Binuclear Cu2+ complex mediated discrimination between l-glutamate and l-aspartate in water

L-glutamate and L-aspartate selectivity is achieved by the action of two Cu2+ metal ions rightly disposed in a cyclophane-type macrocyclic framework; electrochemical sensing of glutamate has been achieved by adsorption of the copper complexes on graphite electrodes.

A Highly Efficient, Preorganized Macrobicyclic Receptor for Halides Based on CH··· and NH···Anion Interactions

The preorganized, macrobicyclic azaphane (1) exhibits remarkable strong, selective fluoride binding comparable to the most effective bis(tren) cryptands despite binding anions via only three NH groups coupled with three CH hydrogen bond donors. The lower intrinsic affinity of CH donors is compensated by the high degree of preorganization exhibited by azacyclophane 1. Compound 1 is prepared via a tripod-tripod cyclization reaction between 1,3,5-tris-bromomethyl-benzene and an aliphatic tripodal hexatosylated polyamine, followed by the reduction of the resulting bicyclic tosylamine. The crystal structures of the bicyclic tosylamine 2 and four macrobicyclic polyammonium halide salts of 1 are reported. X-ray studies revealed the formation of inclusive 1:1 complexes of 1 with fluoride, chloride, bromide, and iodide. Potentiometric titrations showed very high binding constants for fluoride and chloride with a F(-)/Cl(-) selectivity of more than five logarithmic units. The final geometry of the anion cryptates is largely determined by optimization of NH and CH...anion interactions coupled with unfavorable anion-pi repulsion for the larger anions.

Synthesis of a Macrobicycle Incorporating the Tris(pyrazolyl)methane Ligand

Steric crowding of the 3-position of tris(pyrazolyl)borate and -methane ligands has produced tetrahedral metal complexes with controlled reactivity. As an alternative, we propose to incorporate the tris(pyrazolyl)methane chelate in a macrobicyclic structure in order to create a cavity with well-defined dimensions and shape. Acid-catalyzed equilibration of excess of the new pyrazole 3-(1H-pyrazol-3-yl)benzenemethanethiol acetate with HC(3,5-Me(2)pz)(3) followed by hydrolysis affords a functionalized tris(pyrazolyl)methane, which reacts with 1,3,5-tris(bromomethyl)benzene in K(2)CO(3)/DMF to give the title compound. [structure: see text]

New 1H-Pyrazole-Containing Polyamine Receptors Able To Complex l-Glutamate in Water at Physiological pH Values

The interaction of the pyrazole-containing macrocyclic receptors 3,6,9,12,13,16,19,22,25,26-decaazatricyclo-[22.2.1.1(11,14)]-octacosa-1(27),11,14(28),24-tetraene 1[L1], 13,26-dibenzyl-3,6,9,12,13,16,19,22,25,26-decaazatricyclo-[22.2.1.1(1)(1,14)]-octacosa-1(27),11,14(28),24-tetraene 2[L2], 3,9,12,13,16,22,25,26-octaazatricyclo-[22.2.1.1(11,14)]-octacosa-1(27),11,14(28),24-tetraene 3[L3], 6,19-dibenzyl-3,6,9,12,13,16,19,22,25,26-decaazatricyclo-[22.2.1.1(11,)(14)]-octacosa-1(27),11,14(28),24-tetraene 4[L4], 6,19-diphenethyl-3,6,9,12,13,16,19,22,25,26-decaazatricyclo-[22.2.1.1(11,14)]-octacosa-1(27),11,14(28),24-tetraene 5[L5], and 6,19-dioctyl-3,6,9,12,13,16,19,22,25,26-decaazatricyclo-[22.2.1.1(11,14)]-octacosa-1(27),11,14(28),24-tetraene 6[L6] with l-glutamate in aqueous solution has been studied by potentiometric techniques. The synthesis of receptors 3-6[L3-L6] is described for the first time. The potentiometric results show that 4[L4] containing benzyl groups in the central nitrogens of the polyamine side chains is the receptor displaying the larger interaction at pH 7.4 (Keff = 2.04 x 10(4)). The presence of phenethyl 5[L5] or octyl groups 6[L6] instead of benzyl groups 4[L4] in the central nitrogens of the chains produces a drastic decrease in the stability [Keff = 3.51 x 10(2) (5), Keff = 3.64 x 10(2) (6)]. The studies show the relevance of the central polyaminic nitrogen in the interaction with glutamate. 1[L1] and 2[L2] with secondary nitrogens in this position present significantly larger interactions than 3[L3], which lacks an amino group in the center of the chains. The NMR and modeling studies suggest the important contribution of hydrogen bonding and pi-cation interaction to adduct formation.