Design and synthesis of Troger's base ditopic receptors: host–guest interactions, a combined theoretical and experimental study

Abstract

Two flexible Tröger's base ditopic receptors C4TB and C5TB incorporating monoaza crown ether were designed and synthesized for bisammonium ion complexation. A comprehensive study of host–guest interactions was established by ¹H NMR spectroscopy and DFT calculations. Bisammonium chloride (A1) with a shorter alkyl chain spacer showed the highest affinity for the receptors. M06-2X/cc-pVTZ calculations including the solvent effects on host–guest complexes were employed to explain and rationalize the experimental trends. The short N–H⋯O or N–H⋯N hydrogen-bond distances observed in the range of 1.71–1.98 Å indicate the existence of a strong charge assisted hydrogen bonding between the host and the guest. The unusual behaviour (higher binding constant) of A5 in ¹H NMR titration is traced to the conformational folding of the guest.