Addressing Association Entropy by Reconstructing Guanidinium Anchor Groups for Anion Binding: Design, Synthesis, and Host–Guest Binding Studies in Polar and Protic Solutions

Fluorescent Guanidinium-Azacarbazole for Oxoanion Binding in Water

Oxoanions such as carboxylates, phosphates, and sulfates play important roles in both chemistry and biology and are abundant on the cell surface. We report on the synthesis and properties of a rationally designed guanidinium-containing oxoanion binder, 1-guanidino-8-amino-2,7-diazacarbazole (GADAC). GADAC binds to a carboxylate, phosphate, and sulfate in pure water with affinities of 3.6 × 104, 1.1 × 103, and 4.2 × 103 M-1, respectively. Like 2-azacarbazole, which is a natural product that enables scorpions to fluoresce, GADAC is fluorescent in water (λabs = 356 nm, λem = 403 nm, ε = 13,400 M-1 cm-1). The quantum yield of GADAC is pH-sensitive, increasing from Φ = 0.12 at pH 7.4 to Φ = 0.53 at pH 4.0 as a result of the protonation of the aminopyridine moiety. The uptake of GADAC into live human melanoma cells is detectable in the DAPI channel at low micromolar concentrations. Its properties make GADAC a promising candidate for applications in oxoanion binding and fluorescence labeling in biological (e.g., the delivery of cargo into cells) and other contexts.

Near Quantitative Removal of Selenate and Sulfate Anions from Wastewaters by Cocrystallization with Chelating Hydrogen-Bonding Guanidinium Ligands

Selenium (Se) has become an environmental contaminant of aquatic ecosystems as a result of human activities, particularly mining, fossil fuel combustion, and agricultural activities. By leveraging the high sulfate concentrations relative to Se oxyanions (i.e., SeO _n ^2-, n = 3, 4) present in some wastewaters, we have developed an efficient approach to Se-oxyanion removal by cocrystallization with bisiminoguanidinium (BIG) ligands that form crystalline sulfate/selenate solid solutions. The crystallization of the sulfate, selenate and selenite, oxyanions and of sulfate/selenate mixtures with five candidate BIG ligands are reported along with the thermodynamics of crystallization and aqueous solubilities. Oxyanion removal experiments with the top two performing candidate ligands show a near quantitative removal (>99%) of sulfate or selenate from solution. When both sulfate and selenate are present, there is near quantitative removal (>99%) of selenate, down to sub-ppb Se levels, with no discrimination between the two oxyanions during cocrystallization. Reducing the selenate concentrations by 3 orders of magnitude or more relative to sulfate, as found in many wastewaters, led to no measurable loss in Se removal efficiencies. This work offers a simple and effective alternative to selective separation of trace amounts of highly toxic selenate oxyanions from wastewaters, to meet stringent regulatory discharge limits.

A guanidino-γ-cyclodextrin superdimer generates a twin receptor for phosphate dimers assembled by anti-electrostatic hydrogen bonds

Octakis-6-guadinidino-γ-cyclodextrin (gguan) hydrochloride in the presence of phosphates crystallises from aqueous solution in the unprecedented form of a superdimer (dimer-within-a-dimer). The self-assembly exposes four circular octa-guanidinium regions that bind and stabilise discrete H-bonded phosphate anion dimers. The small (∼2 nm) gguan-phosphate assembly is preorganised and stable in aqueous solution, as demonstrated by DLS and NMR experiments.

Hydrophilic and hydrophobic carboxamide pincers as anion hosts

Hydrophobic and hydrophilic, monotopic and ditopic carboxamide pincer hosts containing ethyl, hexyl, 2-hydroxyethyl and 2-hydroxyethyl ethyl ether pendant arms were synthesized. Solubility trends indicated that solubilities in water or hydrocarbon solvents varied depending on the nature of the pendant arms. Binding constants for hydrophilic pincers were larger in general than their hydrophobic analogs. Significant synergistic binding effects for the ditopic hosts were not observed.

Catalytic hydration of cyanamides with phosphinous acid-based ruthenium(ii) and osmium(ii) complexes: scope and mechanistic insights

The catalytic hydration of cyanamides to ureas has been accomplished employing, for the first time, homogeneous catalysts, i.e. the phosphinous acid complexes [MCl₂(η⁶-p-cymene)(PMe₂OH)] (M = Ru, Os).

Thermodynamics of Anion Binding by Chalcogen Bonding Receptors

The application of chalcogen bonding (ChB) to anion recognition is an underdeveloped area of host-guest supramolecular chemistry. The chemical instability of heavier chalcogen derivatives may in part be responsible for the lack of progress. Herein, the synthesis of a new structurally simple, tellurium-based ChB binding motif is reported, the robust stability of which has enabled the thermodynamic properties for ChB halide anion binding in polar aprotic and wet protic organic solvent media to be elucidated. The thermodynamic data reveals how the subtle interplay between ChB host, anion guest and solvent dictates halide binding selectivity and affinity trends. These findings help to provide a deeper insight into the nature of the ChB-anion interaction.

Capping the calix: how toluene completes cesium(i) coordination with calix[4]pyrrole

The role of solvent in molecular recognition systems is under-researched and often ignored, especially when the solvent is considered "non-interacting". This study concerns the role of toluene solvent in cesium(i) recognition by calix[4]pyrrole. We show that π-donor interactions bind toluene molecules onto the open face of the cation-receptor complex, thus "capping the calix." By characterizing this unusual aromatically-saturated complex, we show how "non-interacting" aromatic solvents can directly coordinate receptor-bound cations and thus influence recognition.

A conformationally persistent pseudo-bicyclic guanidinium for anion coordination as stabilized by dual intramolecular hydrogen bonds

The first example of a pseudo-bicyclic guanidinium ligand is reported, and its complexes are evaluated by crystallographic and computational approaches.

Supramolecular electron transfer-based switching involving pyrrolic macrocycles. A new approach to sensor development?

The potential utility of energy transfer in the design of pyrrolic macrocycle-based molecular switches and ability to serve as the readout motif for molecular sensors development is discussed.

Green synthesis, optical properties and catalytic activity of silver nanoparticles in the synthesis of N-monosubstituted ureas in water

We report the green synthesis of silver nanoparticles by using Euphorbia condylocarpa M. bieb root extract for the synthesis of N-monosubstituted ureas in water. UV-visible studies show the absorption band at 420 nm due to surface plasmon resonance (SPR) of the silver nanoparticles. This reveals the reduction of silver ions (Ag+) to silver (Ago) which indicates the formation of silver nanoparticles (Ag NPs). This method has the advantages of high yields, simple methodology and easy work up.

Aromatic and aliphatic CH hydrogen bonds fight for chloride while competing alongside ion pairing within triazolophanes

Triazolophanes are used as the venue to compete an aliphatic propylene CH hydrogen-bond donor against an aromatic phenylene one. Longer aliphatic C-H...Cl(-) hydrogen bonds were calculated from the location of the chloride within the propylene-based triazolophane. The gas-phase energetics of chloride binding (ΔG(bind) , ΔH(bind) , ΔS(bind) ) and the configurational entropy (ΔS(config) ) were computed by taking all low-energy conformations into account. Comparison between the phenylene- and propylene-based triazolophanes shows the computed gas-phase free energy of binding decreased from ΔG(bind) =-194 to -182 kJ mol(-1) , respectively, with a modest enthalpy-entropy compensation. These differences were investigated experimentally. An (1) H NMR spectroscopy study on the structure of the propylene triazolophane's 1:1 chloride complex is consistent with a weaker propylene CH hydrogen bond. To quantify the affinity differences between the two triazolophanes in dichloromethane, it was critical to obtain an accurate binding model. Four equilibria were identified. In addition to 1:1 complexation and 2:1 sandwich formation, ion pairing of the tetrabutylammonium chloride salt (TBA(+) ⋅Cl(-) ) and cation pairing of TBA(+) with the 1:1 triazolophane-chloride complex were observed and quantified. Each complex was independently verified by ESI-MS or diffusion NMR spectroscopy. With ion pairing deconvoluted from the chloride-receptor binding, equilibrium constants were determined by using (1) H NMR (500 μM) and UV/Vis (50 μM) spectroscopy titrations. The stabilities of the 1:1 complexes for the phenylene and propylene triazolophanes did not differ within experimental error, ΔG=(-38±2) and (-39±1) kJ mol(-1) , respectively, as verified by an NMR spectroscopy competition experiment. Thus, the aliphatic CH donor only revealed its weaker character when competing with aromatic CH donors within the propylene-based triazolophane.

Stopped-flow kinetic analysis of the interaction of cyclo[8]pyrrole with anions

The on and off rates corresponding to the binding of two test anions (acetate, AcO(-), and dihydrogen phosphate, H(2)PO(4)(-), studied as their tetrabutylammonium salts) to diprotonated cyclo[8]pyrrole have been determined in CH(3)CN using stopped-flow analyses carried out at various temperatures. For dihydrogen phosphate, this afforded the activation enthalpies and entropies associated with both off and on processes. The different dynamic behavior seen for these test anions underscores the utility of kinetic analyses as a possible new tool for the advanced characterization of anion receptors.