Urea- and Thiourea-Based Receptors for Anion Binding

Abstract

Conspectus Over the past five decades, significant progress has been made in the field of anion recognition with a diverse variety of synthetic receptors because of the fundamental importance of anions in chemical, environmental, and biological processes. In particular, urea- and thiourea-based molecules offering directional binding sites are attractive receptors for anions due to their ability to bind anions employing primarily hydrogen-bonding interactions under neutral conditions and have gained a recent paramount attention in the area of supramolecular chemistry. The presence of two imine (−NH) groups on each urea/thiourea functionality in these receptors gives them potential for excellent binding of an anion, mimicking the natural binding process in living cells. The increased acidity offered by thiocarbonyl groups (CS) in a thiourea-functionalized receptor could enhance its anion binding ability as compared to its analogous urea-based receptor containing a carbonyl (CO) group. During the last several years, our group has been involved in exploring a wide variety of synthetic receptors, and we have studied them with anions experimentally and computationally. In this Account, we will highlight the overall summary of our group’s efforts focusing on anion coordination chemistry of urea- and thiourea-based receptors with varying linkers (rigid and flexible), dimensions (dipodal and tripodal), and functionalities (bifunctional, trifunctional, and hexafunctional). Depending on the linkers and attached groups, bifunctional-based dipodal receptors can bind anions forming 1:1 or 1:2 complexes. A dipodal receptor with flexible aliphatic or rigid m-xylyl linkers forms a cleft to bind a single anionic species in the pocket. However, a dipodal receptor with p-xylyl linkers binds anions in both 1:1 and 1:2 binding modes. As compared to a dipodal receptor, a tripodal receptor provides a more organized cavity for an anion, forming predominantly a 1:1 complex, while the binding strength and selectivity are influenced by linking chains and terminal groups. A hexafunctional-based tripodal receptor bridged with o-phenylene groups provides two clefts that can host two small anions or one large anion. However, a hexafunctional receptor with p-phenylene groups as linkers binds two anions, one at an inner pocket and the other at an outer pocket. It was shown that the presence of suitable chromophores at the terminal groups makes the receptor useful for the naked-eye detection for certain anions (e.g., fluoride, acetate) in solution. The field of anion binding chemistry is rapidly growing, and this Account aims to provide fundamental aspects influencing the binding strength and selectivity of anionic species with abiotic receptors which might eventually be useful for the development of new devices for binding, sensing, and separating biologically and environmentally important anions.

Dynamic mechanostereochemical switching of a co-conformationally flexible [2]catenane controlled by specific ionic guests

“…Noteworthily, literature examples of sulfate-selective receptors frequently invoke a hydrophobic effect from host encapsulation of sulfate anions, which shields the highly hydrophilic anion and assists its desolvation in aqueous environment. 54,[69][70][71][72][73][74][75] In contrast, sulfate anion induces a large-amplitude co-conformational change in the interlocked backbone of C, bringing the urea donors in the two macrocycles in proximity for convergent sulfate binding. The high flexibility and ability to undergo large-amplitude co-conformational change thus enable the catenane host to adapt to ionic guests of different size, charge, geometry and binding stoichiometry (i.e., 1:1 binding of monocationic, spherical Cu + and 1:2 binding of dianionic, tetrahedral SO4 2-).…”

Disrupting the Hofmeister bias in salt liquid–liquid extraction with an arylethynyl bisurea anion receptor

“…Advancing this eld, hostmediated liquid-liquid extraction (LLE) has emerged as a powerful approach to impart enhanced selectivity and high binding affinities for desired guest ions. [9][10][11][12] Research into hostmediated LLE has explored the inuence of receptors on cation, 13 anion, 14 and ion-pair 15 extraction. A signicant body of this work has focused on the development of cation receptors, whereas complementary studies on anion receptors have lagged in comparison.…”

Gold nanoshells with magnetic cores and a urea-based receptor for SERS sensing of fluoride anions: experimental and computational study

“…The urea/thiourea moiety forms hydrogen bonds with certain anions, typically oxoanions like H 2 PO 4 − or carboxylates, but also with halide anions (see the complexation of fluoride anion in Scheme 1). 22 Many studies have focused on increasing the binding constant ( cf. Scheme 1, eqn (1)), on increasing selectivity for certain anions, and on improving the sensitivity by coupling urea/thiourea with efficient chromophores and fluorophores, which are active in UV-vis spectrophotometry/colorimetric analysis or spectrofluorimetry (see the excellent reviews by Kundu et al , 22 Li et al 23 or Bregović et al 24 ).…”

“…22 Many studies have focused on increasing the binding constant ( cf. Scheme 1, eqn (1)), on increasing selectivity for certain anions, and on improving the sensitivity by coupling urea/thiourea with efficient chromophores and fluorophores, which are active in UV-vis spectrophotometry/colorimetric analysis or spectrofluorimetry (see the excellent reviews by Kundu et al , 22 Li et al 23 or Bregović et al 24 ). An important observation was made by Boiocchi et al in 2004, who studied interactions of N , N ′-bis(4-nitrophenyl)urea with a series of various anions (including strongly interacting carboxylates, H 2 PO 4 − , F − and others) in acetonitrile and DMSO and showed that the interaction of the sensor with the fluoride anion was different.…”