Cyclo[2]carbazole[2]pyrrole: a preorganized calix[4]pyrrole analogue. Issue 5 (12th January 2023)

• Record Type: Journal Article
• Authors: Lee, Areum; Yang, Ju Ho; Oh, Ju Hyun; Hay, Benjamin P.; Lee, Kyounghoon; Lynch, Vincent M.; Sessler, Jonathan L.; Kim, Sung Kuk
• Is Part Of: Chemical science. Volume 14:Issue 5(2023)
• Page Start: 1218
• Page End: 1226
• Publication Date: 2023-01-12
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/d2sc06376j
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : Less is more: two NH hydrogen bond donors in a preorganized receptor provide greater anion affinity than the four NH moieties present in the classic anion receptor, calix[4]pyrrole. 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. 1 H 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 CD2 Cl2 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.