Copper based anion receptors

Owing to the popularity and vast modularity of the copper(l)-catalyzed azide-alkyne cycloaddition (CuAAC) [4,5], so-called click chemistry [24] (Fig. 1), many more 1,2,3-triazole-based anion receptors have been reported during the past 3 years (2008-2011). Reviews [25, 26] have been published to cover this new moiety in anion receptor chemistry. Therefore, this chapter will focus on triazole-based anion receptors that have not been reviewed to date. In addition, applications including sensors, ion-selective electrodes, catalysis, anion transport, and anion regulation, as well as their use in interlocked molecules, will be discussed. 

A competitive method for carbonate sensing is based on dicopper complex 17 and coumarin 343 as anion receptor and external fluorophore, respectively. The fluorophore 18 coordinates to the copper atoms through its carboxylic group, and energy transfer between coumarin ring and copper atoms quenches the fluorescence emission. Carbonate showed strong binding to the metal complex and coupled the displacement of the bound fluorophore with concomitant recovery of the coumarin emission. This ensemble showed selective response to carbonate over phosphate or carboxylate. 

The macrocyclic Schiff base L37 contains an hexyl linker between the two amine groups. In anion transport experiments, the corresponding metal complexes of L37 are found to successfully extract sulfate anion from aqueous solution to organic solvents. In the case of the copper complex of receptor L37, it was found that more than... 

The macrobicyclic cryptand BISTREN (3), as discussed earlier, binds anions when hexaprotonated. This compound is also capable of binding pairs of transition metal ions [e.g. the bis-copper(Il) Complex (55)] when unprotonated (39, 138). The bis-copper(II) based receptor (55) was observed to bind a chloride ion cascaded between the two metal ions. In fact, receptor 55 bound chloride more strongly (log K = 3.55) than the hexaprotonated form of 3 (log K = 2.36), and this was attributed to the formation of strong coordinate bonds. Hydroxide was also cascade bound forming a thermodynamically very stable complex (log K = 11.56), the resulting complex being of a different type from that formed by non-cryptate complexes of the Cu(II) ion. 

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