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Functionally Rigid and Degenerate Molecular Shuttles
Author(s) -
Yoon Il,
Benítez Diego,
Zhao YanLi,
Miljanić Ognjen Š.,
Kim SooYoung,
Tkatchouk Ekaterina,
Leung Ken C.F.,
Khan Saeed I.,
Goddard William A.,
Stoddart J. Fraser
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200802096
Subject(s) - chemistry , dumbbell , crystallography , proton nmr , degenerate energy levels , ring (chemistry) , cyclophane , rotaxane , molecular switch , catenane , electron paramagnetic resonance , stereochemistry , molecule , nuclear magnetic resonance , supramolecular chemistry , organic chemistry , physics , medicine , crystal structure , quantum mechanics , physical therapy
The preparation and dynamic behavior of two functionally rigid and degenerate [2]rotaxanes ( 1⋅ 4 PF 6 and 2⋅ 4 PF 6 ) in which a π‐electron deficient tetracationic cyclophane, cyclobis(paraquat‐ p ‐phenylene) (CBPQT 4+ ) 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. 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 (Δ G c ≠ ) of 9.6 and 10.3 kcal mol −1 for 1⋅ 4 PF 6 and 2⋅ 4 PF 6 , respectively, probed by using the rotaxane's α‐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 Å) twice as long as that (3.8 Å) 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 (Δ G ≠ =10.4 kcal mol −1 for 1⋅ 4 PF 6 and 2⋅ 4 PF 6 ) that are in good agreement with the experimental values (Δ G c ≠ =9.6 and 10.3 kcal mol −1 for 1⋅ 4 PF 6 and 2⋅ 4 PF 6 , respectively, probed by using their α‐bipyridinium protons).