[2]pseudorotaxanes derived from 27-and 29-membered oxaazacyclophanes and 1,2-bis(benzimidazolium)ethane salts

Four novel 27- and 29-membered oxaazacyclophanes were synthetized via the high dilution method in moderate to good yields, employing aromatic dialdehydes and diamines as precursors to generate first the corresponding cyclic Schiff bases, which were then reduced in almost quantitatively yields to the corresponding macrocyclic diamines. All compounds were characterized by conventional techniques such as IR and NMR (H-1, C-13) spectroscopy, elemental analysis, and mass spectrometry. Besides, two macrocycles were studied by single-crystal X-ray diffraction analysis, which revealed that their cavities are appropriate to include small molecules. Using two representative oxaazacyclophanes (OX-3 and OX-4) as wheels and 1,2-bis(benzimidazolium)ethane salts as axles, [2]pseudorotaxanes were formed in 1:1 (v/v) solvent mixtures of CHCl3 and MeCN and studied by H-1 NMR and ultraviolet/visible (UV/Vis) spectroscopy, and electrospray ionization-time-of-flight mass spectrometry, finding in all cases the formation of stable 1:1 complexes. The binding constants of the complexes were determined by UV/Vis and H-1 NMR titration experiments, giving values ranging from 0.52 x 10(3) to 4.6 x 10(3) M-1. Likely, complex structures were investigated in the gas-phase using density functional theory calculations at the B3LYP/6-31G(d) level of theory. The complexation experiments together with the results from the molecular modeling studies indicate that dicationic axles can penetrate the cavity of the oxaazacyclophanes synthesized herein to give a new family of [2]pseudorotaxanes. These supramolecular structures are held together by pi-pi stacking interactions (charge-transfer), charge-assisted hydrogen bonds, and ion dipole forces. Copyright (C) 2012 John Wiley & Sons, Ltd.