Adamantane bisurea derivatives: anion binding in the solution and in the solid state

Overcoming barriers with non-covalent interactions: supramolecular recognition of adamantyl cucurbit[n]uril assemblies for medical applications

Adamantane, a staple in medicinal chemistry, recently became a cornerstone of a supramolecular host-guest drug delivery system, ADA/CB[n]. Owing to a good fit between the adamantane cage and the host cavity of the cucurbit[n]uril macrocycle, formed strong inclusion complexes find applications in drug delivery and controlled drug release. Note that the cucurbit[n]uril host is not solely a delivery vehicle of the ADA/CB[n] system but rather influences the bioactivity and bioavailability of drug molecules and can tune drug properties. Namely, as host-guest interactions are capable of changing the intrinsic properties of the guest molecule, inclusion complexes can become more soluble, bioavailable and more resistant to metabolic conditions compared to individual non-complexed molecules. Such synergistic effects have implications for practical bioapplicability of this complex system and provide a new viewpoint to therapy, beyond the traditional single drug molecule approach. By achieving a balance between guest encapsulation and release, the ADA/CB[n] system has also found use beyond just drug delivery, in fields like bioanalytics, sensing assays, bioimaging, etc. Thus, chemosensing in physiological conditions, indicator displacement assays, in vivo diagnostics and hybrid nanostructures are just some recent examples of the ADA/CB[n] applicability, be it for displacements purposes or as cargo vehicles.

Protonation and anion-binding properties of aromatic sulfonylurea derivatives

In this work the anion-binding properties of three aromatic sulfonylurea derivatives in acetonitrile and dimethyl sulfoxide were explored by means of NMR titrations. It was found that the studied receptors effectively bind anions of low basicity (Cl^-, Br^-, I^-, NO₃ ^- and HSO₄ ^-). The stoichiometry of the complexes with receptors containing one binding site was 1 : 1 exclusively, whereas in the case of the receptor containing two sulfonylurea groups 1 : 2 (receptor : anion) complexes were also detected in some cases. The presence of strongly basic anions (acetate and dihydrogen phosphate) led to the deprotonation of the sulfonylurea moiety. This completely hindered its anion-binding properties in DMSO and only proton transfer occurred upon the addition of basic anions to the studied receptors. In MeCN, a complex system of equilibria including both ligand deprotonation and anion binding was established. Since ionisation of receptors was proven to be a decisive factor defining the behaviour of the sulfonylurea receptors, their pK _a values were determined using several deprotonation agents in both solvents. The results were interpreted in the context of receptor structures and solvent properties and applied for the identification of the interactions with basic anions.

Formation of self-assembled supramolecular polymers by anti-electrostatic anion-anion and halogen bonding interactions

We report here the formation of self-assembled supramolecular polymers in which the cooperative action of anti-electrostatic anion-anion and halogen-bonding interactions serve as a powerful driving force for the formation of large supramolecular polymers. DOSY-NMR, DLS, TEM, SEM and X-ray experiments provide evidence of the formation of supramolecular structures in solution and solid state.

Supramolecular Chemistry of Anionic Dimers, Trimers, Tetramers and Clusters

Two or more anions constrained in close proximity within a single pocket are found in a number of natural systems but a less common motif in artificial systems. This review summarizes work on anion receptors capable of stabilizing anionic dimers, trimers, tetramers and clusters in a well-defined fashion. These systems may provide insights into the fundamental chemistry of anion-anion interactions and provide a guide for understanding in greater detail a number of biological and environmental processes, as well as key tenants of relevance to supramolecular chemistry, extraction, transport, crystal engineering, and the like. The primary goal of this review is to provide a general introduction into multi-anion recognition chemistry for the benefit of supramolecular and non-supramolecular chemists alike.

Efficient stabilisation of a dihydrogenphosphate tetramer and a dihydrogenpyrophosphate dimer by a cyclic pseudopeptide containing 1,4-disubstituted 1,2,3-triazole moieties

A cyclic pseudooctapeptide 2 is described containing 1,4-disubstituted 1,2,3-triazole moieties. This compound features eight converging hydrogen bond donors along the ring, namely four amide NH and four triazole CH groups, which enable 2 to engage in interactions with anions. While fully deprotonated sulfate anions exhibit only moderate affinity for 2, protonated anions such as dihydrogenpyrophosphate and dihydrogenphosphate anions are strongly bound. Complexation of the phosphate-derived anions involves sandwiching of a dihydrogenpyrophosphate dimer or a dihydrogenphosphate tetramer between two pseudopeptide rings. X-ray crystallography provided structural information, while 1H NMR spectroscopy, mass spectrometry, and isothermal titration calorimetry demonstrated that these complexes are stable in solution (2.5 vol% water/DMSO) and can even be transferred without decomposition into the gas phase. The observed high thermodynamic stabilities are attributed to the mutual reinforcement of the interactions between the individual complex components, namely, hydrogen-bonding between the anions, multiple hydrogen bonding interactions between the anion aggregates and the triazole CH and NH hydrogen bond donors of 2, and potential dispersive interactions between the closely arranged pseudopeptide rings. Pseudopeptide 2 thus represents a promising lead for the construction of phosphate receptors, whose binding selectivity makes use of the unique ability of certain anions to assemble into higher aggregates.

Dehydroacetic Acid Derivatives Bearing Amide or Urea Moieties as Effective Anion Receptors

Derivatives of dehydroacetic acid comprising amide or urea subunits have been synthesized and their anion-binding properties investigated. Among a series of halides and oxyanions, the studied compounds selectively bind acetate and dihydrogen phosphate in acetonitrile and dimethyl sulfoxide. The corresponding complexation processes were characterized by means of ¹ H NMR titrations, which revealed a 1:1 complex stoichiometry in most cases, with the exception of dihydrogen phosphate, which formed 2:1 (anion/ligand) complexes in acetonitrile. The complex stability constants were determined and are discussed with respect to the structural properties of the receptors, the hydrogen-bond-forming potential of the anions, and the characteristics of the solvents used. Based on the spectroscopic data and results of Monte Carlo simulations, the amide or urea groups were affirmed as the primary binding sites in all cases. The results of the computational methods indicate that an array of both inter- and intramolecular hydrogen bonds can form in the studied systems, and these were shown to play an important role in defining the overall stability of the complexes. Solubility measurements were carried out in both solvents and the thermodynamics of transfer from acetonitrile to dimethyl sulfoxide were characterized on a quantitative level. This has afforded a detailed insight into the impact of the medium on the complexation reactions.

Oxoanion binding to a cyclic pseudopeptide containing 1,4-disubstituted 1,2,3-triazole moieties

The converging arrangement of hydrogen bond donors from amide NH and 1,2,3-triazole CH groups enables the described cyclic pseudopeptide to interact with oxoanions in water/DMSO mixtures.

Fluorescent asymmetric bis-ureas for pyrophosphate recognition in pure water

Three fluorescent asymmetric bis-urea receptors (L1-L3) have been synthesised. The binding properties of L1-L3 towards different anions (fluoride, acetate, hydrogencarbonate, dihydrogen phosphate, and hydrogen pyrophosphate HPpi(3-)) have been studied by means of (1)H-NMR, UV-Vis and fluorescence spectroscopy, single crystal X-ray diffraction, and theoretical calculations. In particular, a remarkable affinity for HPpi(3-) has been observed in the case L1 (DMSO-d6/0.5% H2O) which also acts as a fluorimetric chemosensor for this anion. Interestingly, when L1 is included in cetyltrimethylammonium (CTAB) micelles, hydrogen pyrophosphate recognition can also be achieved in pure water.

New insights into dihydrogenphosphate recognition with dirhenium(i) tricarbonyl complexes bridged by a thiourea moiety

Three thiourea ligands and their dirhenium(i) complexes are reported that show an unusual cooperative binding to two dihydrogenphosphate anions; this could explain the high selectivity for phosphate in many reported anion receptors.