Selective and tuneable recognition of anions using C3v-symmetrical tripodal urea-amide receptor platformsw

The design and synthesis of receptors for the selective recognition and sensing of anions, through the use of weak interactions such as hydrogen bonding, is an active area of research within supramolecular chemistry. Structures based on urea and thiourea recognition sites are of particular interest due to their strong, and tuneable hydrogen binding abilities, and their relatively easy syntheses, which facilitates their use in both simple and complex systems. Incorporation of such moieties into the tripodal molecular platform can give rise to the formation of preorganised structures, which can allow for cooperative hydrogen bonding interactions to take place from several binding sites. Hence, such systems have also been employed for studying transport of anions such as sulfate across lipid bilayers. To date, only a limited number of tripodal anion receptors have been reported in the literature, but these are highly attractive for the formation of anion receptors that possess high coordination requirements such as halides, sulfates and phosphates. We have recently demonstrated that simple amido-urea and thiourea receptors can be designed that enable positive cooperative binding of anions. Herein we present the synthesis and anion binding studies of six new tripodal receptors, 1–6, Fig. 1. Each receptor consists of three urea moieties, connected to a central C3v symmetrical phenyl platform via amide linkage. This centralN-arylbenzamide motif has previously been identified by Lewis et al., as a candidate for developing extended geometries for molecular recognition, but to the best of our knowledge, have not been used as part of an anion receptor platform of the kind presented here. We anticipated that for 1–6, this central platform would enable cooperative or synergetic binding of anions by all three urea sites. We also foresaw that this binding would affect the chemical shifts of the amide protons in the H NMR due to the conformational changes that these structures would undergo. While 1 and 2 possess electron withdrawing groups, 3–6 all have long alkyl chains which we hoped would facilitate the use of these structures as potential membrane transporters. The synthesis of receptors 1–6 (see Scheme S1 in ESIw) was achieved in a few steps from 1,3,5-benzenetricarbonyl trichloride. In general, these were first reacted with either meta or para nitroaniline, followed by reduction to the corresponding amines, using 10% Pd/C and hydrazine monohydrate. The final receptors 1–6 were then formed by suspending these amines into hot CH3CN followed by the addition of the desired isocyanates, and heating the resulting mixtures at reflux under an inert atmosphere overnight. This resulted in the formation of precipitates, which were filtered and washed with cold CH3CN giving 1–6 in 91%, 85%, 89%, 95%, 91% and 67% yields respectively (see ESIw). In order to evaluate the binding affinity of 1–6 for various anions, we initially carried out UV-Vis absorption studies using acetate (AcO ), sulfate (SO4 2 ), dihydrogen phosphate (H2PO4 ) and chloride (Cl ), respectively, as their TBA (tetrabutyl ammonium) salt solutions. However, to our surprise, the changes in the absorption spectra were minor, and it was difficult to assess the binding affinity of these receptors for the above anions accurately. Consequently, the anion recognition of 1–6 was instead examined by using H NMR titrations Fig. 1 Structures 1–6 employed in the current study.