Tritopic Bis-Urea Receptors for Anion and Ion-Pair Recognition

Abstract

This work reports on the synthesis and characterization of three tritopic receptors and their binding properties toward various anions, as their tetrabutylammonium salts, and three alkali metal-acetate salts by UV-vis, fluorescence, ¹H, ⁷Li, ²³Na, and ³⁹K NMR in MeCN/dimethyl sulfoxide (DMSO) 9:1 (v/v). Molecular recognition studies showed that the receptors have good affinity for oxyanions. Furthermore, these compounds are capable of ion-pair recognition of the alkali metal-acetate salts studied through a cooperative mechanism. Additionally, molecular modeling at the density functional theory (DFT) level of some lithium and sodium acetate complexes illustrates the ion-pair binding capacity of receptors. The anion is recognized through strong hydrogen bonds of the NH- groups from the two urea sites, while the cation interacts with the oxygen atoms of the polyether spacer. This work demonstrates that these compounds are good receptors for anions and ion pairs.

Scheme 1. Chemical Structures of the Receptors Studied in this Work

Scheme 2. Synthesis of Bis-Urea Receptors R1–R3

Figure 1

Absorption spectra of R2 (3 × 10^–5 M) with increasing amounts of OAc^– ((0–7.94) × 10^–4 M) in MeCN/DMSO 9:1 (v/v) at 298 K. The upper right inset shows the abundance of species during titration, where H = receptor and G = anion guest. Dashed lines represent the theoretical profiles from data fit.

Figure 2

Fluorescence spectra of R2 (5.0 × 10^–6 M) with increasing amounts of OAc^– ((0–2.35) × 10^–3 M) in MeCN/DMSO 9:1 (v/v) at 298 K and λ_ex = 298 nm. The upper right inset shows the abundance of species during titration, where H = receptor and G = anion guest. Dashed lines represent the theoretical profiles from data fit.

Figure 3

Fluorescence spectra of a solution of R3 (5.0 × 10^–6 M) and 1 equiv of Na⁺ in MeCN/DMSO 9:1 (v/v), at 298 K and λ_ex = 307 nm, with increasing amounts of OAc^– ((0–3.31) × 10^–4 M). The upper right inset shows the abundance of species during titration, where H = receptor and G = anion guest. Dashed lines represent the theoretical profiles from the data fit.

Figure 4

¹H NMR spectra of R1 (2.5 × 10^–3 M) with increasing amounts of OAc^– (0–0.05 M) in CD₃CN/DMSO-d₆ 9:1 (v/v) at 298 K. The upper right inset shows the abundance of species during titration, where H = receptor and G = anion guest. Dashed lines represent the theoretical profiles from data fit.

Figure 5

¹H NMR spectra of R1 (2.5 × 10^–3 M) with increasing amounts of F^– (0–6.25 × 10^–3 M) in CD₃CN/DMSO-d₆ 9:1 (v/v) at 298 K.

Figure 6

⁷Li NMR spectra of solutions of free LiClO₄ (2.5 × 10^–3 M) and its mixtures with 1 equiv of R1, R2, and R3; in the latter case, both the concentrations of the salt and the receptor were 2.0 × 10^–3 M. All spectra were obtained in CD₃CN/DMSO-d₆ 9:1 (v/v) at 298 K.

Figure 7

Perspective view of the calculated molecular structures of (a) R2–sodium acetate and (b) R3–lithium acetate with the B3LYP/6-31G* level of theory.