Sapphyrin protonated

There are a limited number of fluorescent sensors for anion recognition. An outstanding example is the diprotonated form of hexadecyltetramethylsapphyrin (A-7) that contains a pentaaza macrocydic core (Figure 10.31) the selectivity for fluoride ion was indeed found to be very high in methanol (stability constant of the complex 105) with respect to chloride and bromide (stability constants < 102). Such selectivity can be explained by the fact that F (ionic radius 1.19 A) can be accommodated within the sapphyrin cavity to form a 1 1 complex with the anion in the plane of the sapphyrin, whereas Cl and Br are too big (ionic radii 1.67 and 1.82 A, respectively) and form out-of-plane ion-paired complexes. A two-fold enhancement of the fluorescent intensity is observed upon addition of fluoride. Such enhancement can be explained by the fact that the presence of F reduces the quenching due to coupling of the inner protons with the solvent. 

Expanded porphyrins (sapphyrins) in their protonated forms can bind anions such as phosphate and nucleotides, but no electrochemical investigations have been reported to date (Sessler et al., 1991,1992). 

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 ). 

Subsequent to the work with halide anions, efforts were made to quantify the extent to which phosphate anions are bound by protonated sapphyrin. " As for chloride and fluoride, the rationale for this work was both chemical and biological. 

To test further the self-assembly process, experiments were carried out in the presence of fluoride anion. As noted above, protonated sapphyrins have a significantly higher affinity for fluoride anion as compared to other anions. It was thus expected that adding fluoride anion to a solution of 6 would serve to inhibit the... 

Electrophilic proton exchange occurs readily for sapphyrin (39) and meso deuteration is completed in trifluoroacetic acid and at room temperature overnight.274 The substitution proceeds at a moderate rate for two of the meso protons but very slowly for the other two in the dioxa analogue... 

Another interesting class of anion receptors based upon protonated nitrogen atoms are the expanded porphyrin macrocycles such as 4.17 (diprotonated sapphyrin) and compound 4.18. The tetrapyrrole porphyrin macrocycles are excellent hosts for metal cations such as Fe2+ and Mg2+ (e.g. haemoglobin and chlorophylls, Sections 2.3-2.5) however, their cavity dimensions are too small to accommodate anions. Conversely, expanded porphyrins such as 4.17 comprising five or more pyrrole residues present a rigid macrocyclic cavity about 5.5A in diameter, in which (particularly when protonated) the Nff... 

In the case of sapphyrin-based carboxylate receptors, the relative binding affinities were found to depend on the nature of the substrate. In particular, aromatic carboxylates were found to be bound with higher affinity to protonated sapphyrins than aliphatic ones. Indeed, in the case of the decaalkyl sapphyrin 3, affinity constants measured in dichloromethane, were found to be ca. 103 M-1 for aromatic substrates but less than 102 M-1 for aliphatic ones. 

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