Controllable Donor–Acceptor Neutral [2]Rotaxanes

Abstract

In pursuit of a neutral bistable [2]rotaxane made up of two tetraarylmethane stoppers--both carrying one isopropyl and two tert-butyl groups located at the para positions on each of three of the four aryl rings--known to permit the slippage of the pi-electron-donating 1,5-dinaphtho[38]crown-10 (1/5DNP38C10) at the thermodynamic instigation of pi-electron-accepting recognition sites, in this case, pyromellitic diimide (PmI) and 1,4,5,8-naphthalenetetracarboxylate diimide (NpI) units separated from each other along the rod section of the rotaxane's dumbbell component, and from the para positions of the fourth aryl group of the two stoppers by pentamethylene chains, a modular approach was employed in the synthesis of the dumbbell-shaped compound NpPmD, as well as of its two degenerate counterparts, one (PmPmD) which contains two PmI units and the other (NpNpD) which contains two NpI units. The bistable [2]rotaxane NpPmR, as well as its two degenerate analogues PmPmR and NpNpR, were obtained from the corresponding dumbbell-shaped compounds NpPmD, PmPmD, and NpNpD and 1/5DNP38C10 by slippage. Dynamic 1H NMR spectroscopy in CD2Cl2 revealed that shuttling of the 1/5DNP38C10 ring occurs in NpNpR and PmPmR, with activation barriers of 277 K of 14.0 and 10.9 kcal mol(-1), respectively, reflecting a much more pronounced donor-acceptor stabilizing interaction involving the NpI units over the PmI ones. The photophysical and electrochemical properties of the three neutral [2]rotaxanes and their dumbbell-shaped precursors have also been investigated in CH2Cl2. Interactions between 1/5DNP38C10 and PmI and NpI units located within the rod section of the dumbbell components of the [2]rotaxane give rise to the appearance of charge-transfer bands, the energies of which correlate with the electron-accepting properties of the two diimide moieties. Comparison between the positions of the visible absorption bands in the three [2]rotaxanes shows that, in NpPmR, the major translational isomer is the one in which 1/5DNP38C10 encircles the NpI unit. Correlations of the reduction potentials for all the compounds studied confirm that, in this non-degenerate [2]rotaxane, one of the translational isomers predominates. Furthermore, after deactivation of the NpI unit by one-electron reduction, the 1/5DNP38C10 macrocycle moves to the PmI unit. Li+ ions have been found to strengthen the interaction between the electron-donating crown ether and the electron-accepting diimide units, particularly the PmI one. Titration experiments show that two Li+ ions are involved in the strengthening of the donor-acceptor interaction. Addition of Li+ ions to NpPmR induces the 1/5DNP38C10 macrocycle to move from the NpI to the PmI unit. The Li+-ion-promoted switching of NpPmR in a 4:1 mixture of CD2Cl2 and CD3COCD3 has also been shown by 1H NMR spectroscopy to involve the mechanical movement of the 1/5DNP38C10 macrocycle from the NpI to the PmI unit, a process that can be reversed by adding an excess of [12]crown-4 to sequester the Li+ ions.

Efficient production of [ n ]rotaxanes by using template-directed clipping reactions

“…These observations can be explained by the constitutionally heterotopic environments of the macrocycles surrounding the dumbbells, that is, in the [4]rotaxane, the two homotopic macrocycles adjacent to the stopper (R A , signals a-f) are heterotopic with respect to the central macrocycle (R B , signals aЈ-fЈ). In the [5]rotaxane, rings R A are different from rings R B , and in [7]rotaxane, rings R B and R C share very similar chemical environments that differ from rings R A . The slight difference between rings R B and R C is even expressed in the separation of the peaks for the imine protons (HЈOCAN; Fig.…”

“…However, the efficiency of the fixing process has its limitations with the increasing numbers of macrocycles. The pure fixed [3]-, [4]-, and [5]rotaxanes were isolated in 77%, 74%, and 40% yields, respectively. All of these [n]rotaxanes were purified by preparative TLC to remove impurities remaining after reduction.…”

“…The efficiency of the fixing of dynamic [n]rotaxanes by reduction of the imine bonds shows some dependence on n, that is, it occurs with decreased efficiency as n becomes larger. The fixed [3]-, [4]-, and [5]rotaxanes as pure, well characterized compounds have been successfully prepared.…”

“…The organic layer was washed with H 2 O and dried (Na 2 SO 4 ), and the solvent was removed under vacuum. The residue was then purified by preparative TLC on silica gel plates by using different eluents (CHCl 3 /MeOH ϭ 4:1 for the [3]rotaxane; CHCl 3 /Et 3 N ϭ 3:2 for the [4]-and [5]rotaxanes) to give the neutral rotaxanes, which were dissolved in CH 2 Cl 2 before a few drops of TFA were added. The solvents were then removed under vacuum, and the residue was redissolved in a minimum amount of MeOH.…”

“…M echanically interlocked and knotted compounds, such as rotaxanes (1)(2)(3)(4)(5), catenanes (6-10), suitanes (11,12), trefoil knots (13)(14)(15)(16)(17), Borromean rings (18)(19)(20), and Solomon knots (21), represent challenging synthetic goals that have nevertheless been realized. These molecular compounds are usually synthesized by a template-directed approach (22) that depends on molecular recognition and self-assembly processes.…”