Neutral receptors for anion recognition: synthesis, selectivity studies and transport properties

This thesis reports the synthesis and studies of a new series of compounds containing urea groups that can function as selective anion receptors and efficient anion transmembrane transporters. Two new asymmetric ureidic receptors L1 (1-(1H-indol-7-yl)-3- (quinolin-2-yl)urea) and L2 (1-(quinolin-2-yl)-3-(quinolin-8-yl)urea), shown in Figure 1, have been synthesised and their affinity towards different anions tested in DMSO-d6. L1 adopts both in solution and in the solid state an (E,Z) conformation. A moderate affinity for acetate has been observed with L1 while no interaction has been observed with L2. The different behaviour has been ascribed to the presence/absence of the indole group. In the case of L1 the indole group causes the formation of a peculiar supramolecular architecture with two molecules of the receptor binding the anions in (E,Z) conformation via H-bonds. L2 also adopts an (E,Z) conformation in the solid state. However, the absence of the indole in L2 hampers the formation of the supramolecular assembly with the participation of anionic species. The results are discussed in Chapter 2. In Chapter 3 a new family of bis-ureidic receptors (L3-L8) is reported (Figure 2). The binding properties of L3-L8 towards different anions (acetate, benzoate, glutarate, malonate, dihydrogen phosphate, hydrogen pyrophosphate, triphosphate, AMP and ADP) have been studied by means of 1H-NMR, UV-Vis and fluorescence spectroscopies and a remarkable affinity for HPpi3- has been observed in the case L5 (in DMSO- d6 and DMSO-d6/5% H2O) which also acts as fluorimetric chemosensor, even at naked eye, for this anion. Theoretical calculations helped us to explain the binding properties observed. A series of bis-ureidic receptor L9-L14 (Figure 3) was prepared and the studies of their transmembrane anion transport activity and anion binding properties are described in Chapter 4. Vesicle anion transport assays using ion selective electrodes demonstrate that these compounds function as efficient mobile carriers that are able to transport chloride through lipid bilayer predominantly via anion exchange mechanism including Cl-/NO3- and Cl-/HCO3- antiport. Compound L14 was found to be the most active transporter of the series. As anion receptors, we observed that compounds L10 and L12 demonstrate strong 1:1 binding with TBA fluoride (Ka >104 M-1) and moderate 1:1 binding with TBA dihydrogenphosphate. In Chapter 5, the anion transport efficiency of a series of trisureas L15-L20 (Figure 4) was investigated. The compounds showed a potent transmembrane anion transport activity. The mechanism of transport was extensively investigated using a combination ion selective electrode and fluorescence techniques. It was found that compounds L15-L20 mediate chloride transport via Cl-/NO3- and Cl-/HCO3- antiport and NaCl symport. The tris-urea L15 was found to be a remarkably potent anion carrier at low concentration.