Anomalous behaviour of the bis(benzimidazolium)butane [2]pseudorotaxanes of dibenzo-24-crown-8: A comparative study of methane, ethane, propane and butane bis(benzimidazolium)alkane]dibenzo-24-crown-8 systems with variable spacers

Bis(benzimidazolium)butane (BBIM-butane) system acts as a new guest molecule for dibenzo-24-crown-8 (DB24C8). This is the only [BBIM]DB24C8 pseudorotaxanes where two conformers coexist (detected by ¹H NMR and structure established by DFT studies); major conformer and minor conformer. Low temperature ¹H NMR (240, 253 K) was done for a representative [BBIM-butane] DB24C8 pseudorotaxane that could not be detected at room-temperature. DFT calculations for this [BBIM-butane] DB24C8 pseudorotaxane were done both with B3LYP and MPW1PW91 functionals. The MPW1PW91 optimized structures identified possible stable major and minor conformer with the strongest H-bonding. We have compared the characteristics of [BBIM]DB24C8 [2]pseudorotaxanes comprising the [BBIM-methane]DB24C8, [BBIM-ethane] DB24C8, [BBIM-propane]DB24C8 and [BBIM-butane] DB24C8 systems. The feasibility of formation of these pseudorotaxanes decreases considerably from [BBIMmethane] DB24C8 to the [BBIM-butane]DB24C8 system. The presence of these threaded complexes was initially screened based on ¹H NMR and substantiated further by 2D NMR spectroscopy (NOESY and COSY), X-ray crystallography, HR-ESI-MS and DFT calculations.