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Title: Functionally rigid and degenerate molecular shuttles
Authors: Stoddart, J. Fraser
Miljanić, Ognjen Š.
Leung, Ken C. F.
Khan, Saeed I.
Goddard III, William A.
Yoon, Il.
Benítez, Diego
Zhao, Yanli
Kim, Soo Young
Tkatchouk, Ekaterina
Keywords: DRNTU::Science::Chemistry::Organic chemistry
Issue Date: 2009
Source: Yoon, II, Benítez, D., Zhao, Y. L., Miljanić, O. Š., Kim, S. Y., Tkatchouk, E., et al. (2009). Functionally Rigid and Degenerate Molecular Shuttles. Chemistry - a European Journal, 15, 1115-1122.
Series/Report no.: Chemistry - a European journal
Abstract: The preparation and dynamic behavior of two functionally rigid and degenerate [2]rotaxanes (1⋅4 PF6 and 2⋅4 PF6) in which a π-electron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT4+) ring, shuttles back and forth between two π-electron-rich naphthalene (NP) stations by making the passage along an ethynyl-phenylene-(PH)-ethynyl or butadiyne rod, are described. The [2]rotaxanes were synthesized by using the clipping approach to template-directed synthesis, and were characterized by NMR spectroscopic and mass spectrometric analyses. 1H NMR spectra of both [2]rotaxanes show evidence for the formation of mechanically interlocked structures, resulting in the upfield shifts of the resonances for key protons on the dumbbell-shaped components. In particular, the signals for the peri protons on the NP units in the dumbbell-shaped components experienced significant upfield shifts at low temperatures, just as has been observed in the flexible [2]rotaxanes. Interestingly, the resonances for the same protons did not exhibit a significant upfield shift at 298 K, but rather only a modest shift. This phenomenon arises from the much reduced binding of the ethynyl-NP unit compared to the 1,5-dioxy-NP unit. This effect, in turn, increases the shuttling rate when compared to the 1,5-dioxy-NP-based rotaxane systems investigated previously. The kinetic and thermodynamic data of the shuttling behavior of the CBPQT4+ ring between the NP units were obtained by variable-temperature NMR spectroscopy and using the coalescence method to calculate the free energies of activation (ΔGc≠) of 9.6 and 10.3 kcal mol−1 for 1⋅4 PF6 and 2⋅4 PF6, respectively, probed by using the rotaxane's α-bipyridinium protons.
ISSN: 1521-3765
DOI: 10.1002/chem.200802096
Rights: © 2009 Wiley-VCH Verlag.
Fulltext Permission: open
Fulltext Availability: With Fulltext
Appears in Collections:SPMS Journal Articles

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