Selective aqueous anion recognition in an anionic host

Abstract

Water-soluble Fe₄L₄ ^4- cages can be synthesized in a multicomponent self-assembly process exploiting functionalized trigonal ligands, Fe^II salts, and water-soluble sulfonated formylpyridine components. The cages are soluble in purely aqueous solution and display an overall 4- charge, but are capable of binding suitably sized non-coordinating anions in the host cavity despite their anionic nature. Anions such as PF₆ ^- or AsF₆ ^- occupy the internal cavity, whereas anions that are too small (BF₄ ^-) or too large (NTf₂ ^-) are not encapsulated. The external anionic charge and sterically blocked ligand cores limit the exchange rate of bound anions, as no exchange is seen over a period of weeks with the anion-filled cages, and internalization of added PF₆ ^- by an empty cage takes multiple weeks, despite the strong affinity of the cavity for PF₆ ^- ions. In the future, this recognition mechanism could be used to control release of anions for environmental applications.

Keywords: Chemistry; Supramolecular chemistry.

Graphical abstract

Figure 1

Self-assembled cage synthesis Self-assembly process for the formation of water-soluble cages 1 and 2.

Figure 2

Structure and characterization of anion-bound cage (A) Minimized structure of 1⋅AsF₆ (SPARTAN 20). (B) Observed and calculated isotope pattern for [Fe₄L₄⋅AsF₆]^5- ions in the ESI-MS spectrum of 1⋅AsF₆. (C) ¹H NMR spectrum of 1⋅AsF₆ (D₂O, 400 MHz, 298K; NOTE—peak H_f overlaps the D₂O peak, see Figure S10 for COSY spectrum). (D) ¹⁹F NMR spectrum of 1⋅AsF₆ (D₂O, 376 MHz, 298K).

Figure 3

Size-selective anion encapsulation (A) ¹⁹F NMR spectra of templated cage 1⋅PF₆, along with spectra for cage + added NaPF₆. (B) ¹⁹F NMR spectra of unoccupied cage 1 with residual BF₄⁻ along with spectra for cage + added NaBF₄ showing no encapsulation of anion (D₂O, 298K, 376 MHz).

Figure 4

Restricted anion exchange in the cage (A) ¹⁹F–¹⁹F EXSY spectrum of 1⋅PF₆+ PF₆⁻, indicating no exchange on the NMR timescale (2 mM, D₂O, 298K, 376 MHz, 300 ms mixing time). (B) ¹⁹F NMR spectra of 10 mM NaAsF₆ added to a solution of 1mM 1⋅PF₆+ PF₆⁻ over time, indicating no exchange over a period of weeks (D₂O, 298K, 376 MHz, see Figure S51 for full spectra).

Figure 5

Slow anion exchange into empty cage 1 ¹⁹F NMR spectra of 10 mM NaPF₆ added to a solution of 1mM 1⁴⁻ over time, indicating slow formation of 1⋅PF₆ over a period of weeks (D₂O, 298K, 376 MHz).

Figure 6

Cage disassembly and anion release ¹⁹F NMR spectra of 10 mM tris-(2-aminoethyl)amine (tren) added to a solution of 1mM [1⋅PF₆]^5- over time, indicating incomplete transimination of the cage and PF₆⁻ release after a period of weeks (D₂O, 298K, 376 MHz).

Figure 7

Anion binding mechanism Illustration of the molecular recognition process: (A) anions can template the formation of anionic cage 1, but (B) the external anionic slows guest entry and severely restricts guest egress.