Sapphyrin complex with fluoride

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. 

Different compounds which complex the fluoride anion were recently developed. Sapphyrine (1) is a porphyrin with 5 pyrrole rings, and when it is doubly protonated it can exclusively complex F in its cavity68. Similarly, Seppelt prepared the thermally stable salt 2 with a bulky cation where the fluoride can be completely desolvated, thus increasing its efficiency as a nucleophile and a base69. 

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 ). 

At present some preliminary evidence has been obtained which suggests that sapphyrins can bind anions other than fluoride. In fact, an X-ray diffraction study shows that NJ binds to the monoprotonated form of sapphyrin [157]. Specifically, as shown in Fig. 29, the monoprotonated sapphyrin, H4Sap Nj, does not complex azide anion in an in-plane fashion but in an end-on manner, with the terminal azide nitrogen atom being 1.13 A above the sapphyrin plane [157]. Nonetheless, this atom is still vsdthin typical hydrogen bonding distance (2.8 to 3,0 A) of at least four of the five pyrrolic nitrogens [59, 173-176]. 

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