Fluoride anion binding

Calix[4]pyrrole and derivatives involved in this chapter are those receptors which are able to recognise selectively the fluoride anion in solution. It is shown that the fluoride anion binding ability of these receptors can be tuned. 

Good fluoride anion binding affinities were observed in the case of the two hexapyrrolic calix[4]pyrrole receptors 9 reported in 2014 by Anzenbacher and coworkers [24]. The incorporation of two additional carbonyl ester pyrrole units on either side of the macrocycle produced the cis- and traws-configurational isomers, 9a and 9b, respectively (cf. Fig. 12.7). In acetonitrile solution, a value of 1.0 X 10 for fluoride anion binding was estimated for both 9a and 9b on the... 

Following the discovery of the above solid-state fluoride anion complex, the authors succeeded in crystallizing the bishydrochloride salt of this same system, 3. The X-ray structure of this derivative revealed the presence of two chloride counterions bound via hydrogen bonds to the diprotonated macrocycle (Figure 2). In contrast to what was seen in the case of the fluoride anion structure, in this instance neither of the counteranions is located in the plane of the sapphyrin macrocycle. Rather, they are found in near symmetric fashion ca. 1.8 A above and below the mean Nj plane. This out-of-plane binding reflects, presumably, the fact... 

After completing initial mass spectrometric studies, it was quickly found, much to the authors delight, that sapphyrin 3, when protonated, acts as an efficient receptor and carrier for chloride and fluoride anions (as judged from the U-tube model membrane studies) vide infra Further, using visible absorption and fluorescence spectroscopic analyses, it proved possible to derive a binding constant (K ) of ca. 9.6 X lO for fluoride anion in methanol. For comparison, chloride anion was found to be bound with much lower affinity in this same solvent (K 10 M ), whereas bromide anion hardly formed a complex at all < 10 M ). 

Other groups have subsequently reported anion receptors that work on the same principle. For instance, an Eu(III) complex of the bis-bipyridinephen-ylphosphine oxide ligand 86 made by Ziessel and co-workers is able to sense anions by luminescence enhancement in acetonitrile, with stability constants which follow the trend fluoride>acetate>chloride>nitrate [61]. Tsukube and co-workers have investigated the properties of the Eu(III) and Tb(III) complexes of the chiral ligand 87 [62]. Anion binding was assessed by profiling luminescence enhancement in acetonitrile, and it was found that the different metal centres provided different selectivities. The emission at 548 nm of the Tb(III) complex was increased by 5.5 times in the presence of 3 equivalents of chloride compared to 2.2 for nitrate and 1.1 for acetate. Conversely the emission at 618 nm of the Eu(III) complex was increased 8.3 times by 3 equivalents of nitrate, 2.5 times for chloride and 1.0 times for acetate. Stability constants were not reported. 

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