Anion Recognition in Water, Including Sulfate, by a Bicyclam Bimetallic Receptor: A Process Governed by the Enthalpy/Entropy Compensatory Relationship

Novel Fluorescence Guanidine Molecules for Selective Sulfate Anion Detection in Water Complex Samples over a Wide pH Range

Quantitative analysis of sulfate anions in water still remains an important challenge for the society. Among all the methodologies, the most successful one is based on optical supramolecular receptors because the presence of small concentrations of sulfate anion modifies the photophysical properties of the receptor. In this case, fluorescence anion sensors have been designed by the incorporation of guanidine motifs into fluorenyl cores. The photophysical behaviors of the new mono- (M) and bis-guanidine (B) derivatives were studied through pH dependence, solvent effects, and ion sensing on steady-state spectra and time-resolved fluorescence spectroscopy. In more detail, the results demonstrate that M is a highly selective and sensitive sulfate ion receptor in real water samples and, even more importantly, its function remains unchanged at different ranges of pH. The reason behind this resides on the fluorescence quenching produced by an internal charge-transfer process when the sulfate anion is complexed with M. It is worth noting that the global and partial affinity constants (10¹⁰ M^-2 and 10⁵ M^-1, respectively) of complex formation are far above from the current sulfate sensors in water (10⁴ M^-1) which give an LOD of 0.10 μM in water with an analytical range of 2.5-10 μM. On the other hand, although it would seem, at first sight, that the B derivate will be the most promising one, the possibility of having two simultaneous protonation states reduces the complex formation and, therefore, its sensitivity to sulfate anions. The results presented here offer the possibility of using a new molecule in water environments, which opens the door to infinite applications such as the detection of trace amounts of sulfate ions in food or water.

Self-Assembled Anion-Binding Cryptand for the Selective Liquid-Liquid Extraction of Phosphate Anions

The ligands L1 and L2 form trinuclear self-assembled complexes with Cu2+ (i.e. [(L1 )2 Cu3 ]6+ or [(L2 )2 Cu3 ]6+ ) both of which act as a host to a variety of anions. Inclusion of long aliphatic chains on these ligands allows the assemblies to extract anions from aqueous media into organic solvents. Phosphate can be removed from water efficiently and highly selectively, even in the presence of other anions.

Towards Building Blocks for Supramolecular Architectures Based on Azacryptates

In this work, we report the synthesis of a new bis(tris(2-aminoethyl)amine) azacryptand L with triphenyl spacers. The binding properties of its dicopper complex for aromatic dicarboxylate anions (as TBA salts) were investigated, with the aim to obtain potential building blocks for supramolecular structures like rotaxanes and pseudo-rotaxanes. As expected, UV-Vis and emission studies of [Cu₂L]⁴⁺ in water/acetonitrile mixture (pH = 7) showed a high affinity for biphenyl-4,4′-dicarboxylate (dfc²⁻), with a binding constant of 5.46 log units, due to the best match of the anion bite with the Cu(II)-Cu(II) distance in the cage’s cavity. Compared to other similar bistren cages, the difference of the affinity of [Cu₂L]⁴⁺ for the tested anions was not so pronounced: conformational changes of L seem to promote a good interaction with both long (e.g., dfc²⁻) and short anions (e.g., terephthalate). The good affinity of [Cu₂L]⁴⁺ for these dicarboxylates, together with hydrophobic interactions within the cage’s cavity, may promote the self-assembly of a stable 1:1 complex in water mixture. These results represent a good starting point for the application of these molecular systems as building units for the design of new supramolecular architectures based on non-covalent interactions, which could be of interest in all fields related to supramolecular devices.

Bimacrocyclic Effect in Anion Recognition by a Copper(II) Bicyclam Complex

The dicopper(II) complex of the bimacrocyclic ligand α,α'-bis(5,7-dimethyl-1,4,8,11-tetraazacyclotetradecan-6-yl)-o-xylene, 2, interacts with selected anions in dimethyl sulfoxide solution according to two different modes: (i) halides (Cl^-, Br^-, and I^-) and N₃ ^- coordinate the two metal centers at the same time between the two macrocyclic subunits that face each other and (ii) anionic species that do not fit the bridging coordination mode (e.g., NCO^-, SCN^-, CH₃COO^-, NO₃ ^-, and H₂PO₄ ^-) interact with copper(II) ions only at the "external" positions or their interaction is too weak to be detected. Occurrence of the bridging interaction is demonstrated by X-ray crystallographic studies performed on the adduct formed by [Cu₂(2)]⁴⁺ with azide and by electron paramagnetic resonance investigation, as the anion coordination between the two copper(II) centers induces spin-spin coupling. Isothermal titration calorimetry experiments performed on [Cu₂(2)]⁴⁺ and, for comparison, on [(5,7-dimethyl-6-benzyl-1,4,8,11-tetraazacyclotetradecane)copper(II)], representing the mononuclear analogue, allowed determination of thermodynamic parameters (log K, ΔH, and TΔS) associated with the considered complex/anion equilibria. Thermodynamic data showed that adducts formed by [Cu₂(2)]⁴⁺ with halides and azide benefit from an extra stability that can be explained on the basis of the anion advantage of simultaneously binding the two metal centers, i.e., in terms of the bimacrocyclic effect.

Self-Assembly of an Anion-Binding Cryptand for the Selective Encapsulation, Sequestration, and Precipitation of Phosphate from Aqueous Systems

The self-assembled trimetallic species [L₂ Cu₃ ]⁶⁺ contains a cavity that acts as a host to many different anions. By using X-ray crystallography, ESI-MS, and UV/Vis spectroscopy we show that these anions are encapsulated both in the solid state and aqueous systems. Upon encapsulation, the anions Br^- , I^- , CO₃^2- , SiF₆^2- , IO₆^3- , VO₄^3- , WO₄^2- , CrO₄^2- , SO₄^2- , AsO₄^3- , and PO₄^3- are all precipitated from aqueous solution and can be removed by filtration. Furthermore, the cavity can be tuned to be selective to either phosphate or sulfate anions by variation of the pH. Phosphate anions can be removed from water, even in the presence of other common anions, reducing the concentration from 1000 to <0.1 ppm and recovering approximately 99 % of the phosphate anions.