Neutral guest binding by meso‑3,5-bis(trifluoromethyl)phenyl two-wall calix[4]pyrrole: N-oxide recognition features and unique acetonitrile binding mode

Abstract

Listen

Translate article

Two-wall calix[4]pyrroles bearing two aryl substituents at diametrically opposed meso-positions have been well explored toward the recognition and sensing of anionic substrates. However, their ability to form stable complexes with neutral guests remains underexplored. Herein, we report the heterocyclic N-oxides (such as pyridine N-oxide (PNO), 4-methylpyridine N-oxide (MPNO), and 4-phenylpyridine N-oxide (PPNO)) binding features of meso-3,5-bis(trifluoromethyl)phenyl two-wall calix[4]pyrrole 1. Single crystal X-ray diffraction analysis of the PNO complex of 1 revealed the formation of two types of closely resembled 1/1 complexes within the same crystal, wherein the calix[4]pyrrole core adopted a cone-like conformation and the PNO was bound to the cleft delineated by two aromatic walls adopting pseudo-edge-to-face (PETF) geometry. MPNO formed only one type of 1/1 complex with receptor 1 having PETF geometry in the solid state. Solution phase interaction of 1 with PNO, MPNO, and PPNO was investigated by 1H NMR spectroscopic titrations carried out in CD3CN. On the other hand, X-ray diffraction analysis of single crystals of 1, grown from CH3CN solution, unveiled the formation of a 1/2 host-acetonitrile solvate containing an unstable arrangement of CH3CN dimer displaying a unique CH3CN coordination mode in the solid state. Furthermore, a unique 1D supramolecular chain of CH3CN was stabilized within the narrow channel in the crystal lattice of 1•(CH3CN)2.