Novel synthesis and evaluation of anion-p interactions in one-walled meso-cyanophenyl substituted calix[4]pyrrole (C4P): Experimental and computational insights

A novel one-walled meso-cyanophenyl functionalized calix[4]pyrrole (4) was synthesized from cyanophenyl appended dipyrromethane (3) through conventional strategy and also by utilizing deep eutectic solvent of N,N'dimethyl urea (DMU) & L-(+)-tartaric acid (TA) in a ratio of 7:3. The structural confirmation of 4 was done through H-1-NMR, C-13-NMR, FT-IR, and HRMS spectral data. Anion binding studies of the receptor 4 with several anions (halides, trigonal oxoanions, tetragonal oxoanions, etc.) used as tetrabutylammonium (TBA) salts, were successfully investigated by virtue of UV-Vis spectroscopy and time dependent density functional theory (TDDFT). Notably, the UV-Vis absorbance titrations performed in CH3CN solvent, revealed significantly higher anion binding of receptor 4 with F?, Cl?, Br?, and SCN? in comparison to the simple C4P. This can be ascribed to the fact that the cyanophenyl moiety is p-acidic in nature which participates in anion complexation through weak anion-p interactions (observed through H-1-NMR spectroscopy) and hence modulates anion binding through cooperative effect of preliminary NH-hydrogen bonding due to C4P scaffold and anion-p contacts due to cyanophenyl moiety.

Automated summary

A new one-walled meso-cyanophenyl functionalized calix[4]pyrrole (4) was synthesized using both conventional strategy and deep eutectic solvent technique. The binding ability of receptor 4 with various anions was successfully studied using UV-Vis spectroscopy and TDDFT. The significant enhancement in anion binding of receptor 4 compared to C4P was attributed to the p-acidity of the cyanophenyl moiety and the cooperative effect of the NH-hydrogen bonding and anion-p interactions.