Protonated Macrobicyclic Hosts Containing Pyridine Head Units for Anion Recognition

Abstract

In this paper, we report two macrobicyclic receptors containing pyridine head units derived from 1,10-diaza-15-crown[5] (L1) or 4,13-diaza-18-crown[6] (L2) that can be protonated in MeCN and used for anion recognition. The interaction of these protonated lateral macrobicycles with different anions has been investigated by means of spectrophotometric titrations in MeCN. The association constants for the complexes of halide anions with the protonated macrobicycles follow the sequences Cl(-)>Br(-)>I(-)>F(-) (L1) and Cl(-)>F(-)>I(-)>Br(-) (L2), whereby an increase of more than two logarithmic units is observed from F(-) to Cl(-) for the binding constants of the receptor derived from L1. The association constants also indicate an important degree of selectivity of these macrobicyclic receptors for Cl(-) over Br(-) or I(-). The X-ray crystal structure analyses of the chloride and bromide complexes confirms the formation of the envisaged supramolecular complexes. Moreover, the binding constants indicate that these receptors present a high sulfate-to-nitrate binding selectivity. The stability trend observed for the recognition of halide anions by the macrobicycles presented herein as well as the sulfate-to-nitrate binding selectivity have been rationalised by means of DFT calculations at the B3LYP/LanL2DZ level. These studies indicate that the especially high binding selectivity for Cl(-) is the result of the optimum fit between the protonated macrobicyclic cavity and the size of the anion, whereas the sulfate-to-nitrate selectivity results from shape complementarity between the hydrogen-binding acceptor sites on sulfate and the hydrogen-bond donors of the macrobicycle.

A merged experimental and theoretical conformational study on alkaline-earth complexes with lariat ethers derived from 4,13-diaza-18-crown-6

“…We have previously reported that reaction of L 1 and L 2 with dialdehydes leads to the formation of lateral macrobicyclic structures that are useful for the coordination of both cations and anions [27][28][29]. We have found that these macrobicycles cannot be prepared by direct reaction between the organic precursors due to the anti arrangement adopted by the diamines L 1 and L 2 , but Ba(II) can act as an effective template, thereby facilitating the formation of the desired macrobicycles in high yields [30].…”

Stereochemistry and thermodynamics of the inclusion of aliphatic and aromatic anionic guests in a tetracationic calix[4]arene in acidic and neutral aqueous solutions

“…Calixarenes fit the purpose as they are available in a wide variety of geometries and may be turned into water soluble entities with relative ease. Amide, 9 polyamine or polyammonium 10 calixarene-based receptors have been employed for anion coordination. Since their strength and selectivity of complexation depends both on electrostatic interactions (charge) and structural effects (topology and/or dimensions), 11 they may be optimized by conveniently arranging the binding groups on a calixarene-based scaffold of suitable size and shape.…”

Fluorescent Sensing of Fluoride in Cellular System

“…Despite their high sensitivity and selectivity, hydrogen-bond probes have certain limitations for biosensing because of the high solvability of fluoride in water. The hydration free energy of fluoride is around ca .100~110 kcal /mol, close to the energy in a C-H covalent bond 55 . Most reported hydrogen-bonding fluoride probes work only in organic solvents, and their biological applications are inhibited in water.…”