title: Functionally rigid and degenerate molecular shuttles.
abstract: The preparation and dynamic behavior of two functionally rigid and degenerate [2]rotaxanes (14 PF(6) and 24 PF(6)) in which a pi-electron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT(4+)) ring, shuttles back and forth between two pi-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. (1)H 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 CBPQT(4+) ring between the NP units were obtained by variable-temperature NMR spectroscopy and using the coalescence method to calculate the free energies of activation (DeltaG(c) ( not equal)) of 9.6 and 10.3 kcal mol(-1) for 14 PF(6) and 24 PF(6), respectively, probed by using the rotaxane's alpha-bipyridinium protons. The solid-state structure of the free dumbbell-shaped compound (3) shows the fully rigid ethynyl-PH-ethynyl linker with a length (8.1 A) twice as long as that (3.8 A) of the butadiyne linker. Full-atomistic simulations were carried out with the DREIDING force field (FF) to probe the degenerate molecular shuttling processes, and afforded shuttling energy barriers (DeltaG( not equal)=10.4 kcal mol(-1) for 14 PF(6) and 24 PF(6)) that are in good agreement with the experimental values (DeltaG(c) ( not equal)=9.6 and 10.3 kcal mol(-1) for 14 PF(6) and 24 PF(6), respectively, probed by using their alpha-bipyridinium protons).
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095, USA.
authors.forename: Il
authors.initials: I
authors.lastname: Yoon
authors.forename: Diego
authors.initials: D
authors.lastname: Benítez
authors.forename: Yan-Li
authors.initials: YL
authors.lastname: Zhao
authors.forename: Ognjen S
authors.initials: OS
authors.lastname: Miljanić
authors.forename: Soo-Young
authors.initials: SY
authors.lastname: Kim
authors.forename: Ekaterina
authors.initials: E
authors.lastname: Tkatchouk
authors.forename: Ken C-F
authors.initials: KC
authors.lastname: Leung
authors.forename: Saeed I
authors.initials: SI
authors.lastname: Khan
authors.forename: William A
authors.initials: WA
authors.lastname: Goddard
authors.forename: J Fraser
authors.initials: JF
authors.lastname: Stoddart