Synthesis and X‑ray Structures of Bis-Functional Resorcinarene Crown Ethers

Abstract

Two resorcin[4]-arene-crowns were synthesized with crown ether substituents as the lower rim functionalities, with either the Crown-6 (TRC6) or Crown-7 (TRC7) moieties being incorporated. X-ray crystallographic data show that both molecules prefer the boat conformation in the solid state. The crown ethers were observed in an askew anti-arrangement in TRC6 and a syn-arrangement in TRC7. TRC6 crystallized with four DMF and four H₂O molecules. Two of the DMF solvent molecules are hydrogen bonded to the resorcinolic OHs, and the other two are entrapped as a dimer between two ether moieties from adjacent TRC6 molecules. In TRC7, four intramolecular hydrogen bonds from the same resorcinol OH group to the crown ether oxygens are observed, with O···O contact distances of 2.742(2) Å and 2.872(2) Å and O-H···O angles of 169.8° and 175.4°. Host-guest binding of potassium and rubidium cations as their chloride and tetrafluoroborate salts in highly competitive DMSO shows 1:1 and 1:2 host-guest complexes for TRC6 and TRC7, respectively. More intense NMR signal changes were observed in less competitive acetonitrile solvent with the tetrafluoroborate salts.

1

Resorcinarene crown ether hosts (TRC6 and TRC7) and alkali metal (K⁺ and Rb⁺) salt guests.

2

Synthesis of resorcinarene tetracrowns TRC6 and TRC7.

3

Ball-and-stick (left) and space-filling (right) figures of TRC6 (top) and TRC7 (bottom).

4

(a) The hydrogen-bonded DMF (green), the non-hydrogen-bonded DMF (yellow), the two hydrogen bonded water molecules (pink), and the water molecules encapsulated by the crown ether (light blue). (b) The DMF dimer (yellow) entrapped between two adjacent crown ethers of TRC6 (other solvent molecules omitted for clarity; hydrogen bonds shown as turquoise dashed lines).

5

Crown ether hydrogen bonded (turquoise dashed lines) water molecules (a, light blue) and formation of a 1-D chain formed by hydrogen bonding of the encapsulated water to the crown ether (b) in TRC6 (other solvent molecules omitted for clarity).

6

Two water molecules (pink) hydrogen bonded (turquoise dashed lines) to four molecules of TRC6.

7

Four intramolecular hydrogen bonds (turquoise dashed lines) in TRC7.

8

Channels (highlighted with orange boundaries) running through the lattice of TRC7 along the crystallographic a-axis.

9

Conformational analysis of resorcinarene macrocycles TRC6 and TRC7. (a) Minimum energy conformation of TRC6 displaying the characteristic “three-down, one-up” arrangement of crown ether rings. (b) Overlay of all generated conformers for TRC6 showing flexibility in the crown ether regions while maintaining core structural integrity. (c) Minimum energy conformation of TRC7 exhibiting similar “three-down, one-up” orientation. (d) Conformational ensemble of TRC7 showing comparable flexibility patterns in the crown ether moieties while preserving the central cavity structure. In all panels, gray represents carbon atoms, red represents oxygen atoms, and white represents hydrogen atoms.

10

Stacked ¹H NMR spectra (DMSO-D₆, 298 K) showing (a) pure TRC6 and up to four equivalents of KCl, (b) pure TRC7 and up to four equivalents of RbCl. The dashed lines indicate the signal changes in ppm.

11

Stacked ¹H NMR spectra (CD₃CN, 298 K) show up to four equivalents of (a) KBF₄ into TRC6, (b) KBF₄ into TRC7, (c) RbBF₄ into TRC6, and (d) RbBF₄ into TRC7. The dashed lines indicate the signal changes in ppm.