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

Two flexible Troger'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 H-1 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 center dot center dot center dot O or N-H center dot center dot center dot N hydrogen-bond distances observed in the range of 1.71-1.98 angstrom 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 H-1 NMR titration is traced to the conformational folding of the guest.