Abstract
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 [BBIM-methane]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.
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Notes
Pseudorotaxanes have been defined in Ashton et al. [1] as the noninterlocked counterparts of rotaxanes, in which one or more linear “rods” or “threads” are encircled by one or more “wheels” or “beads” to form inclusion complexes. As distinct from rotaxanes, these supramolecular complexes are free to dissociate into their separate components since they do not possess bulky stopper groups.
The term rotaxane derives from the Latin words rota and axis meaning wheel and axle, respectively, see Schill and Zollenkopf [2].
The distinction between rotaxanes and pseudorotaxanes is far from being a straightforward one. For a more detailed discussion of the fuzziness that exists between rotaxanes and pseudorataxanes, see Ashton et al. [5].
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Acknowledgments
One of the authors (SG) thanks the University Grants Commission, New Delhi, for her fellowship (SRF). We thank Professor Amarendranath Patra of Department of Chemistry, University of Calcutta. We thank the CAS Instrumentation Facility, University of Calcutta for spectral data. The authors acknowledge computer use of the Lion-X series of computational clusters of the Research Computing and Cyber infrastructure group, a unit of Information Technology Services at Penn State University. Author (TC) acknowledges the infrastructural use of IBM-HS21 server cluster/LAN running parallel version with Linda of University of Burdwan. We also acknowledge grant received from UGC funded Major project, F. No. 37-398/2009 (SR) dated 11-01-2010. We also wish to thank Chembiotek, a TCG Lifesciences Enterprise for low temperature NMR experiments.
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Ghosh, S., Schmiedekamp, A.M., Padmanaban, E. et al. 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. J Incl Phenom Macrocycl Chem 81, 321–340 (2015). https://doi.org/10.1007/s10847-014-0459-6
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DOI: https://doi.org/10.1007/s10847-014-0459-6