ABSTRACT
A novel squaramide cage (2) binds H2PO4- and HP2O73- with high selectivity and affinity in a highly polar protic solvent system. Receptor 2 is also able to extract these hydrophilic anions into a chloroform phase from water. The X-ray crystal structure demonstrated that compound 2 forms a complex with H2PO4- with 1 : 1 stoichiometry in the solid state.
ABSTRACT
A cyclo[2]carbazole[2]pyrrole (2) consisting of two carbazoles and two pyrroles has been synthesized by directly linking the carbazole 1- and 8-carbon atoms to the pyrrole α-carbon atoms. Macrocycle 2 is an extensively conjugated 16-membered macrocyclic ring that is fixed in a pseudo-1,3-alternate conformation. This provides a preorganized anion binding site consisting of two pyrrole subunits. 1H NMR spectroscopic analysis revealed that only the two diagonally opposed pyrrole NH protons, as opposed to the carbazole protons, take part in anion binding. Nevertheless, cyclo[2]carbazole[2]pyrrole 2 binds representative anions with higher affinity in CD2Cl2 than calix[4]pyrrole (1), a well-studied non-conjugated tetrapyrrole macrocycle that binds anions via four pyrrolic NH hydrogen bond interactions. On the basis of computational studies, the higher chloride anion affinity of receptor 2 relative to 1 is rationalized in terms of a larger binding energy and a lower host strain energy associated with anion complexation. In the presence of excess fluoride or bicarbonate anions, compound 2 loses two pyrrolic NH protons to produce a stable dianionic macrocycle [2-2H]2- displaying a quenched fluorescence.
ABSTRACT
Two-phenoxy walled calix[4]pyrroles 1 and 2 strapped with small rigid linkers containing pyridine and benzene, respectively, have been synthesized. 1H NMR spectroscopic analyses carried out in CDCl3 revealed that both of receptors 1 and 2 recognize only F- and HCO3 - among various test anions with high preference for HCO3 - (as the tetraethylammonium, TEA+ salt) relative to F- (as the TBA+ salt). The bound HCO3 - anion was completely released out of the receptors upon the addition of F- (as the tetrabutylammonium, TBA+ salt) as a result of significantly enhanced affinities and selectivities of the receptors for F- once converted to the TEAHCO3 complexes. Consequently, relatively stable TEAF complexes of receptors 1 and 2 were formed via anion metathesis occurring within the receptor cavities. By contrast, the direct addition of TEAF to receptors 1 and 2 produces different complexation products initially, although eventually the same TEAF complexes are produced as via sequential TEAHCO3 and TBAF addition. These findings are rationalized in terms of the formation of different ion pair complexes involving interactions both inside and outside of the core receptor framework.
ABSTRACT
The binding properties of the pyrrole-strapped calix[4]pyrrole 2 for cesium halide ion pairs were studied via 1H NMR spectroscopic and single crystal X-ray diffraction analyses. Receptor 2 was found to bind CsF, CsCl, and CsBr in the solid state and in chloroform/methanol (4/1, v/v) solution with relatively high affinity as compared with the parent calix[4]pyrrole 1. It was also revealed by solid-liquid extraction experiments that receptor 2 was capable of solubilizing CsF in CDCl3, a medium in which this salt is otherwise insoluble. Single crystal X-ray diffraction analyses and 1H NMR spectroscopic data recorded in 20% CD3OD in CDCl3 provide support for the suggestion that the strap pyrrolic NH proton of 2, as well as those of the calix[4]pyrrole framework, contribute to anion recognition, thus increasing affinity for cesium halide salts relative to the parent system 1. In the solid state, receptor 2 interacts with CsF to form a two dimensional coordination polymer in the presence of methanol. A linear coordination polymer is observed in the case of CsCl and CsBr. Receptor 2 was also found to form a complex with CsF in chloroform/methanol (4/1, v/v) solution, albeit with a different binding mode than is seen in the solid state.
