Experimental and computational studies of anion recognition by pyridine-functionalised calixarenes

Terpyridine-Functionalized Calixarenes: Synthesis, Characterization and Anion Sensing Applications

Lanthanide complexes have been developed and are reported herein. These complexes were derived from a terpyridine-functionalized calix[4]arene ligand, chelated with Tb3+ and Eu3+. Synthesis of these complexes was achieved in two steps from a calix[4]arene derivative: (1) amide coupling of a calix[4]arene bearing carboxylic acid functionalities and (2) metallation with a lanthanide triflate salt. The ligand and its complexes were characterized by NMR (1H and 13C), fluorescence and UV-vis spectroscopy as well as MS. The photophysical properties of these complexes were studied; high molar absorptivity values, modest quantum yields and luminescence lifetimes on the ms timescale were obtained. Anion binding results in a change in the photophysical properties of the complexes. The anion sensing ability of the Tb(III) complex was evaluated via visual detection, UV-vis and fluorescence studies. The sensor was found to be responsive towards a variety of anions, and large binding constants were obtained for the coordination of anions to the sensor.

Winged-Cone Conformation in Hexa-p-tert-butylcalix[6]arene Driven by the Unusually Strong Guest Encapsulation

Hexa-p-tert-butylcalix[6]arene (1) is believed to adopt a winged conformation in a solution, featured by four phenyl rings perpendicular to the calix basis and two others at 1,4-positions lying down. However, there is some controversy on the occurrence of this conformation because it has never been found in the solid state of calix[6]arenes, regardless of the substitution pattern at lower and upper rims. Here, we have observed the winged-cone conformation for the first time in a solvate form of 1 with dimethyl sulfoxide (DMSO), dimethylformamide, and pyridine. The DMSO molecule is strongly encapsulated into 1 through two OH···O hydrogen bonds with both flattened phenolic moieties, one lp_(S)···π and four CH···π interactions with the four perpendicular phenyl rings. This host-guest complex has energy lower by 23.4 kcal mol^-1 than the isolated species. In addition, another DMSO solvate form with 1,2,3-alternate conformation was also obtained in this study, and its structure is compared with that of the precedent one. A detailed density functional theory study has also been carried out to understand the energetic relationships among cone conformers, intramolecular hydrogen-bonding patterns, and DMSO encapsulation.

A 1,3-Capped Calix[4] Conjugate Possessing an Amine Moiety as an Anion Receptor: Reversible Anion Sensing Detected by Spectroscopy and Characterization of the Supramolecular Features by Microscopy

A phenylenediamine-capped conjugate of calix[4]arene (Lamino ) was synthesized by reducing its precursor, Limino , with sodium borohydride in methanol. The Lamino sample binds to anions due to the more flexible and bent conformation of the capped aminophenolic binding core, compared to the precursor Limino . The Lamino sample showed selectivity towards H2 PO4 (-) by exhibiting a ratiometric increase in emission by about 11-fold with a detection limit of (1.2±0.2) μm ((116±20) ppb) over 15 anions studied, including other phosphates, such as P2 O7 (4-) , adenosine monophosphate (AMP(2-) ), adenosine diphosphate (ADP(2-) ), and adenosine triphosphate (ATP(2-) ). The Lamino sample shows an increase in the absorbance at λ=315 nm in the presence of H2 PO4 (-) , CO3 (2-) , HCO3 (-) , CH3 CO2 (-) , and F(-) . The (1) H NMR spectroscopic titration of Lamino with H2 PO4 (-) , F(-) , and CH3 CO2 (-) showed major changes in the phenylene-capped and salicyl moieties, and thereby, confirming the aminophenolic region as the binding core. However, the binding strength of these anions followed the trend H2 PO4 (-) >F(-) ≫CH3 CO2 (-) >HSO4 (-) . The heat changes observed by isothermal titration calorimetry support this trend. The Lamino sample showed reversible sensing towards H2 PO4 (-) and F(-) in the presence of Mg(2+) and Ca(2+) , respectively. NOESY studies of Lamino , in comparison with its anionic complexes, revealed that major conformational changes occurred in the capping region to facilitate the binding of anion. ESI-MS and the Job's method revealed 1:1 stoichiometry between Lamino and H2 PO4 (-) or F(-) . In the SEM micrographs of Lamino , the spherical particles are converted into spherical aggregates and further form large agglomerates and even branched sheets in the presence of anions, depending upon their binding strength.