Investigation of anion-π interactions involving thiophene walls incorporated calix[4]pyrroles.

Thiophene containing "two-wall" aryl extended calix[4]pyrroles were synthesized for the first time, through acid catalyzed condensation of 2-acetylthiophenes with pyrrole. Isomeric "two-walled" calix[4]pyrroles (8a-10a and 8b-10b) were obtained in satisfactory yields and their halide anion binding strengths were investigated in the solution phase by (1)H NMR and in the gas phase by computational methods and mass spectrometry. Change in the chemical shifts of thiophene -CH-protons during the course of NMR titrations entailed participation of the thiophene rings in anion binding; this fact was further substantiated by computational methods. The α,α-(cis)-isomers (8a, 9a, and 10a) showed strong binding toward F(-) and Cl(-) anions when compared to their isomeric α,β-(trans)-isomer (8b, 9b, and 10b). In both isomers, binding with F(-) anion was found to be stronger than that with Cl(-) anion. Both the solution-phase and gas-phase results revealed that the thiophene rings stabilize the anions through anion-π interactions.