Sapphyrin diprotonated

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. 

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... 

The fifth sapphyrin-phosphate structure to be solved is of the mixed chlo-ride/monobasic cyclic AMP salt of diprotonated sapphyrin 4 (Figure 9). While the general features of binding are similar to the other phosphate ester structures... 

In addition to phosphate and halide anion binding, carboxylate chelation by sapphyrin macrocycles has been the subject of recent investigation. To date, two crystal structures have been solved. A 2 1 complex formed between diprotonated sapphyrin 3 and trifluoroacetic acid shows that the oxyanions are chelated above and below the sapphyrin plane (Figure 10). ° Greater complexity of organi-... 

Nitrogen-bearing cyclophanes like 351 [16] and 352 [17] bind larger organic anions in water due to superposition of the hydrophobic effect and electrostatic attraction. The phenanthridinium hosts like 351 have been found to form the most stable nucleotide complexes known so far. On the other hand, free tetrapyrrolic porphyrins do not bind anions since their cavity is too small to take advantage of the convergent N-H dipoles for the complex stabilization [18]. However, expanded diprotonated porphyrins like sapphyrin 353 were shown to form stable complexes with phosphate [19a] and halide [19b] anions. 

Like porphyrins, sapphyrins are easily diprotonated in acidic media the basicity of sapphyrins... 

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... 

Figure 4.15 (a) X-ray crystal structure of the diprotonated expanded porphyrin host sapphyrin... 

The first diprotonated sapphyrin-derived anion complex, a fluoride-containing chelate [15], was obtained in quite an unusual fashion. During an attempt to obtain an X-ray diffraction quality crystal of the diprotonated sapphyrin 216,... 

Taken together, these two structures, the only ones so far obtained for mono-protonated sapphyrin derivatives, are important. They demonstrate clearly that, at least in the solid state, sapphyrins need not be doubly protonated to bind anions. These results thus provide critical structural support for the notion that sapphyrins could prove useful as anion receptors in solution. This is because in aqueous media it is the mono-, not the diprotonated, form of sapphyrin that dominates at or near neutral pH vide infra) ... 

In order to support the contention that the protonated forms of sapphyrin have a role to play as anion receptors, particularly for physiologically important species such as phosphates and carboxylates, efforts were made to crystallize complexes containing these anions. In the case of phosphate, a number of such structures have in fact been obtained. Two are considered particularly informative. The first involves the 1 1 complex formed between monobasic phosphoric acid and diprotonated sapphyrin 10.6 (Figure 10.5.15) and the second, the mixed salt of diprotonated sapphyrin 10.29, chloride anion, and cyclic-AMP (Figure 10.5.16). ... 

In both cases, the phosphate oxyanion is clearly chelated to the diprotonated sapphyrin core by anisotropic hydrogen-bonding interactions as well as by, presumably, charge effects. Taken together, these effects appear to result in strong binding, at least as inferred from bond distances the phosphate oxyanion is found 1.22 A and 1.38 A above the mean N5 macrocyclic plane in the case of these two complexes, respectively. ... 

By such methods, it proved possible to determine that fluoride anion is bound to the diprotonated form of sapphyrin with highest affinity (A a > lO"... 

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