Pyrrole-Based Anion Recognition

Pyrrole contains a single NH H-bonding donor group and constitutes the basis for a wide variety of charged and neutral anion receptors. Pyrrole differs from urea discussed above as it does not contain an H-bond acceptor group. 

In 1990, Sessler and co-workers developed an efficient synthesis of sapphyrin 56, a pentapyrrolic macrocycle initially discovered by Woodward et al. [108]. X-ray structure and NMR analyses of its bis HPFe salt revealed the presence of a fluoride anion bound within the diprotonated pentapyrrcUc macrocycle (Fig. 5.44) [109]. A PFs counter anion was also found in the lattice but not proximate to the nearly planar sapphyrin skeleton. The fluoride anion itself was found to be within hydrogen bonding distance of all five proton-bearing nitrogen atoms. 

The simplest pyrrole-based anion receptor is mesooctamethyl calix[4]pyrrole 57 which can be synthesized in one step by the acid-catalyzed condensation of pyrrole with acetone. In 1996, Sessler and co-workers demonstrated that 57 binds fluoride anion selectively in CD2CI2 solution [110]. Crystal stmcmre of 57 (Fig. 5.45a) showed that two of the NHs point up, and the other two point down. When coordinated with anion, the four NHs aU points at the anion. The selectivity of calix [4]pyrrole for anions depends on the nature of the solvent [111, 112], and fluoride selectivity could be lost under certain conditions. A detailed study by Sessler et al. with several chloride salts in solution by ITC and H NMR titrations and in the solid state by X-ray analysis showed that the stability was highly dependent on the solvent (Fig. 5.45b) [113], with Xa ranging from 10 to 10 In dichloro-methane, a strong dependence on the counter cation was also observed, with of the complexes with chloride varying within 10 -10 M .  

Moyer et al. demonstrated that anion complexes of 57 could include large charge-diffuse cations, such as caesium and imidazolium, in the anion-induced calix [4]pyrrole cup [114]. Shielding of the imidazolium CH atoms in CD2CI2 in the presence of an anion-binding calixpyrrole was observed and considered as evidence for ion-pair complexation in solution. Similar behavior was also observed for N-ethylpyridinium cation [115]. 

Gale and co-workers also reported the ability of 57 to form bromide complexes with dibromide salt in the solid state and in solution [116]. Crystallization of 57 in the presence of dibromide salts resulted in assemblies where the bromide ion was coordinated by four NH Br H-bonds (N Br distances between 3.410 and 3.503 A) of the calixpyrrole (Fig. 5.45c). The for the association of dibromide salt was determined to be 2,220 M  

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