Synthesis and metal ions Complexation of Dibenzo-18-crown-6

Dibenzo-18-crown-6 is a macrocyclic crown ether compound that appears as a white to slightly beige solid powder under normal ambient conditions. This compound exhibits notable fluorescence properties and possesses a significant ability to complex with metal ions. Although it is insoluble in water, Dibenzo-18-crown-6 shows good solubility in a variety of organic solvents, including ethyl acetate and dichloromethane, which facilitates its handling and application in non-aqueous systems. Historically, Dibenzo-18-crown-6 was among the very first crown ethers ever reported, having been initially described by Pedersen, and it has since become recognized as a prototypical example of substituted crown ethers.

General Introduction

In solution, the complexation behavior of Dibenzo-18-crown-6 is generally analogous to that of the parent compound 18-crown-6 (18C6), exhibiting similar affinities and selectivities toward various cationic species. However, the presence of its two aromatic substituents significantly differentiates it from 18C6 in terms of solvent compatibility: Dibenzo-18-crown-6 is considerably less hydrophilic, showing reduced solubility in aqueous media, while simultaneously displaying enhanced solubility in nonpolar organic solvents compared to 18C6. This distinctive solubility profile renders Dibenzo-18-crown-6 particularly advantageous in situations where the partitioning of the ligand into an aqueous phase would be undesirable, making it a preferred choice in liquid-liquid extraction processes and other applications where maintaining the crown ether in the organic phase is critically important.

Synthesis

Figure1: Synthesis of Dibenzo-18-crown-6

Catechol (2.20 g, 20.0 mmol), NaOH (2.24 g, 40.0 mmol), and bis(2-chloroethyl) ether (2.86 g, 20.0 mmol) were dissolved in anhydrous DMSO (15 mL) within a microwave container, and the resulting mixture was stirred at 70°C for 4 hours. The reaction mixture was then poured into a combination of ice water (300 mL) and HCl (37 wt%, 10 mL), and the resulting solution was stirred at 25°C for an additional 2 hours. The aqueous mixture was subsequently extracted three times with CH₂Cl₂, and the combined organic phases were dried over anhydrous MgSO₄ and filtered. The solvent was removed under reduced pressure using a rotary evaporator, and the obtained solid powder was washed with acetone (100 mL). Finally, the solid was collected by filtration and dried under vacuum at 50°C for 24 hours to afford the desired product Dibenzo-18-crown-6.[1]

Microhydration Effects on the Encapsulation of Potassium Ion

Dibenzo-18-crown-6 (DB18C6) was employed to form hydrated complexes with potassium ion, namely K⁺·DB18C6·(H₂O)ₙ (n = 1–5), and the electronic and conformer-specific vibrational spectra of these complexes were measured in a cold 22‑pole ion trap. For comparison, Dibenzo-18-crown-6 was also used to prepare the corresponding Rb⁺ and Cs⁺ complexes with three water molecules, and their spectra were recorded under identical conditions. By analyzing the spectral data with the aid of quantum chemical calculations, Dibenzo-18-crown-6 complexes were found to exhibit distinct conformational behaviors depending on the metal ion and the number of water molecules. [2]

Complexation of metal ions

Specifically, the K⁺·DB18C6·(H₂O)₁ complex exists as a single conformer under our experimental conditions. For n = 2 and 3, Dibenzo-18-crown-6 complexes with K⁺ show at least two conformers even at low temperatures, whereas the corresponding Rb⁺ and Cs⁺ analogues with three water molecules each display only one isomer. This difference arises because the K⁺ ion fits optimally into the cavity of Dibenzo-18-crown-6; as K⁺ becomes deeply encapsulated by the crown ether, its interaction with water molecules weakens, allowing multiple hydration geometries to coexist for the K⁺·DB18C6 complex. In contrast, for n = 4 and 5, Dibenzo-18-crown-6 complexes with K⁺ revert to a single conformer, which is attributed to a cooperative hydration effect: the water molecules form a ring structure bound above the crown‑ether complex. According to the stable structures determined in this study, the K⁺ ion in Dibenzo-18-crown-6 complexes tends to be progressively pulled out of the crown cavity by the increasing number of water molecules as n increases. The multiple conformations observed for the K⁺ complexes of Dibenzo-18-crown-6 may provide an entropic advantage for the selective and efficient capture of K⁺ over other alkali metal ions during the formation of hydrated complexes.[2]

Reference

[1] Mohamed, Mohamed Gamal ; et al, Robust Nitrogen-Doped Microporous Carbon via Crown Ether-Functionalized Benzoxazine-Linked Porous Organic Polymers for Enhanced CO2 Adsorption and Supercapacitor Applications, ACS Applied Materials & Interfaces 2024, 16, 40858-40872.

[2] Inokuchi Y, Ebata T, Rizzo T R, et al. Microhydration effects on the encapsulation of potassium ion by dibenzo-18-crown-6[J]. Journal of the American Chemical Society, 2014, 136(5): 1815-1824.