Measurement of the ground-state distributions in bistable mechanically interlocked molecules using slow scan rate cyclic voltammetry.

In donor-acceptor mechanically interlocked molecules that exhibit bistability, the relative populations of the translational isomers--present, for example, in a bistable [2]rotaxane, as well as in a couple of bistable [2]catenanes of the donor-acceptor vintage--can be elucidated by slow scan rate cyclic voltammetry. The practice of transitioning from a fast scan rate regime to a slow one permits the measurement of an intermediate redox couple that is a function of the equilibrium that exists between the two translational isomers in the case of all three mechanically interlocked molecules investigated. These intermediate redox potentials can be used to calculate the ground-state distribution constants, K. Whereas, (i) in the case of the bistable [2]rotaxane, composed of a dumbbell component containing π-electron-rich tetrathiafulvalene and dioxynaphthalene recognition sites for the ring component (namely, a tetracationic cyclophane, containing two π-electron-deficient bipyridinium units), a value for K of 10 ± 2 is calculated, (ii) in the case of the two bistable [2]catenanes--one containing a crown ether with tetrathiafulvalene and dioxynaphthalene recognition sites for the tetracationic cyclophane, and the other, tetrathiafulvalene and butadiyne recognition sites--the values for K are orders (one and three, respectively) of magnitude greater. This observation, which has also been probed by theoretical calculations, supports the hypothesis that the extra stability of one translational isomer over the other is because of the influence of the enforced side-on donor-acceptor interactions brought about by both π-electron-rich recognition sites being part of a macrocyclic polyether.

Syntheses and dynamics of donor-acceptor [2]catenanes in water.

A subset of mechanically interlocked molecules, namely, donor-acceptor [2]catenanes, have been produced in aqueous solutions in good yields from readily available precursors. The catenations are templated by strong hydrophobic and [π···π] stacking interactions, which serve to assemble the corresponding supramolecular precursors, prior to postassembly covalent modification. Dynamic (1)H NMR spectroscopic investigations performed on one of these [2]catenanes reveal that the pirouetting motion of the butadiyne-triethylene glycol chain occurs with a dramatically lower activation enthalpy, yet with a much higher negative activation entropy in water, compared to organic solvents. The preparations of mechanically interlocked molecules in water constitute the basis for the future development of complex functional molecular machinery in aqueous environments.

Rigid-strut-containing crown ethers and [2]catenanes for incorporation into metal-organic frameworks.

To introduce crown ethers into the struts of metal-organic frameworks (MOFs), general approaches have been developed for the syntheses of dicarboxylic acid dibenzo[30]crown-10 (DB30C10DA), dicarboxylic acid di-2,3-naphtho[30]crown-10 (DN30C10DA), dicarboxylic acid bisparaphenylene[34]crown-10 (BPP34C10DA), and dicarboxylic acid 1,5-naphthoparaphenylene[36]crown-10 (NPP36C10DA). These novel crown ethers not only retain the characteristics of their parent crown ethers since they can 1) bind cationic guests and 2) serve as templates for making mechanically interlocked molecules (MIMs), such as catenanes and rotaxanes, but they also present coordination sites to connect with secondary building units (SBUs) in MOFs. The binding behavior of BPP34C10DA with 1,1'-dimethyl-4,4'-bipyridinium bis(hexafluorophosphate) (DMBP2.PF(6)) has been investigated by means of UV/Vis, fluorescence, and NMR spectroscopic techniques. The crystal superstructure of the complex DMBP2.PF(6) subset BPP34C10DA was determined by X-ray crystallography. The NPP36C10DA-based [2]catenane (H(2)NPP36C10DC-CAT4.PF(6)) and the BPP34C10DA-based [2]catenane (H(2)BPP34C10DC-CAT4.PF(6)) were prepared in DMF at room temperature by the template-directed clipping reactions of the planarly chiral NPP36C10DA and BPP34C10DA with 1,1'-[1,4-phenylenebis(methylene)]di-4,4'-bipyridin-1-ium bis(hexafluorophosphate) and 1,4-bis(bromomethyl)benzene, respectively. The crystal structure of the dimethyl ester (BPP34C10DE-CAT4.PF(6)) of the [2]catenane H(2)BPP34C10DC-CAT4.PF(6) was investigated by X-ray crystallography, which revealed racemic R and S isomers with planar chirality present in the crystal in a 1:1 ratio. These crown ether based struts serve as excellent organic ligands to bind with transition metal ions in the construction of MOFs: the crown ethers BPP34C10DA and NPP36C10DA in the presence of Zn(NO(3))(2)4.H(2)O afforded the MOF-1001 and MOF-1002 frameworks, respectively. The crystal structures of MOF-1001 and MOF-1002 are both cubic and display Fm3m symmetry. The unit cell parameter of the metal-organic frameworks is a=52.9345 A. Since such MOFs, containing electron-donating crown ethers are capable of docking incoming electron-accepting substrates in a stereoelectronically controlled fashion, the present work opens a new access to the preparation and application of MOFs.

Rapid thermally assisted donor-acceptor catenation.

Charged donor-acceptor [2]catenanes containing cyclobis(paraquat-p-phenylene) as the ring component can be synthesised in yields of up to 88% in under one hour by heating two precursors in the presence of macrocyclic polyether templates in N,N-dimethylformamide at 80 °C.

Docking in metal-organic frameworks.

The use of metal-organic frameworks (MOFs) so far has largely relied on nonspecific binding interactions to host small molecular guests. We used long organic struts (approximately 2 nanometers) incorporating 34- and 36-membered macrocyclic polyethers as recognition modules in the construction of several crystalline primitive cubic frameworks that engage in specific binding in a way not observed in passive, open reticulated geometries. MOF-1001 is capable of docking paraquat dication (PQT2+) guests within the macrocycles in a stereoelectronically controlled fashion. This act of specific complexation yields quantitatively the corresponding MOF-1001 pseudorotaxanes, as confirmed by x-ray diffraction and by solid- and solution-state nuclear magnetic resonance spectroscopic studies performed on MOF-1001, its pseudorotaxanes, and their molecular strut precursors. A control experiment involving the attempted inclusion of PQT2+ inside a framework (MOF-177) devoid of polyether struts showed negligible uptake of PQT2+, indicating the importance of the macrocyclic polyether in PQT2+ docking.