Dynamics of a [2]rotaxane wheel in a crystalline molecular solid.

An H-shaped [2]rotaxane comprising a bis(benzimidazole) axle and a 24-membered crown ether wheel appended with four trityl groups forms a highly crystalline material with enough free volume to allow large amplitude motion of the interlocked macrocycle. Variable-temperature (VT) 2H solid-state nuclear magnetic resonance (SSNMR) was used to characterize the dynamics of the [2]rotaxane wheel in this material.

Threading-gated photochromism in [2]pseudorotaxanes.

Rigid, Y-shaped imidazole compounds containing the bis(thienyl)ethene moiety were designed and synthesized. The 4,5-bis(benzothienyl)-2-phenylimidazolium cations were then used as axles for [2]pseudorotaxane formation with 24-membered crown ether wheels. It was demonstrated using 1H NMR spectroscopy, UV-Vis absorption and emission spectroscopies that this host-guest interaction results in significant changes in the photochromic properties of the imidazolium axles. This is a rare example of gated photochromism, which exploits the recognition event of an interpenetrated molecular system to tune the photochromic properties in one of the components.

Ring-through-ring molecular shuttling in a saturated [3]rotaxane.

Mechanically interlocked molecules such as rotaxanes and catenanes comprise two or more components whose motion relative to each other can be controlled. A [2]rotaxane molecular shuttle, for example, consists of an axle bearing two recognition sites and a single macrocyclic wheel that can undergo a to-and-fro motion along the axle-shuttling between the recognition sites. The ability of mechanically interlocked molecules to undergo this type of large-amplitude change is the core mechanism behind almost every interlocked molecular switch or machine, including sophisticated mechanical systems such as a molecular elevator and a peptide synthesizer. Here, as a way to expand the scope of dynamics possible at the molecular level, we have developed a molecular shuttling mechanism involving the exchange of rings between two recognition sites in a saturated [3]rotaxane (one with no empty recognition sites). This was accomplished by passing a smaller ring through a larger one, thus achieving ring-through-ring molecular shuttling.

Rotationally Active Ligands: Dialing-Up Multiple Interlocked Co-Conformations for Silver(I) Coordination.

A novel [2]rotaxane ligand has been prepared that contains a bidentate N,N'-chelate as part of a rigid, H-shaped axle and a 24-membered crown ether macrocycle containing six ether O-atoms and a trans olefinic group as the wheel. This unique interlocked connectivity allows access to a number of different donor sets, which are shown to be capable of binding AgI metal ions.

Reversible mechanical protection: building a 3D "suit" around a T-shaped benzimidazole axle.

The T-shaped benzimidazolium/crown ether recognition motif was used to prepare suit[1]anes. These novel mechanically interlocked molecules (MIMs) were fully characterized by 1H and 13C NMR spectroscopy, single-crystal X-ray diffraction, UV-vis absorption and fluorescence spectroscopy. By conversion to a suit[1]ane, a simple benzimidazole was shown to be protected from deprotonation by strong base. Moreover, it was demonstrated that this unique three-dimensional encapsulation can be made reversible, thus introducing the concept of "reversible mechanical protection"; a protecting methodology that may have potential applications in synthetic organic chemistry and the design of molecular machinery.

Optical Distinction between "Slow" and "Fast" Translational Motion in Degenerate Molecular Shuttles.

A series of six [2]rotaxane molecular shuttles was designed which contain an axle with a benzo-bis(imidazole) core (in either a neutral or dicationic form) and a single 24-membered, crown ether wheel (24C6, B24C6, or DMB24C6), and the shuttling rates of the ring along the axle were determined. The charged versions showed much slower shuttling rates as a result of the increase in noncovalent interactions between the axle and wheel. The [2]rotaxane with a B24C6 wheel shows a difference in fluorescence between the charged and neutral species, while the [2]rotaxane with a DMB24C6 wheel exhibits a difference in color between the charged and neutral compounds. These changes in optical properties can be attributed to the structural differences in the co-conformations of the [2]rotaxane as they adapt to the changes in acid/base chemistry. This allowed the relative rate of the translational motion of a molecular shuttle to be determined by observation of a simple optical probe.

Metal-organic frameworks utilising an interlocked, hexadentate linker containing a tetra-carboxylate axle and a bis(pyridine) wheel.

A [2]rotaxane linker was synthesized which combines an H-shaped axle containing four 3-carboxyphenyl groups and a macrocyclic wheel with two 4-pyridyl groups. Metal-organic framework materials were prepared with CuII and ZnII ions X-ray structures show that the materials contain unique frameworks that are threaded solely due to the interpenetrated nature of the linker.

Influence of axle length on the rate and mechanism of shuttling in rigid H-shaped [2]rotaxanes.

A series of [2]rotaxane molecular shuttles was prepared containing a dibenzo[24]crown-8 (DB24C8) wheel and a rigid H-shaped axle with varying track lengths between recognition sites; from 7.4 to 20.3 Å as defined by 1-4 phenyl rings or a naphthyl group. The rate of shuttling was measured by variable temperature 1H NMR spectroscopy for neutral compounds and EXSY experiments for dicationic species. The rates were found to be independent of the length of the axle, except when the distance between the two recognition sites might be short enough (n = 1) to allow the crown ether to simultaneously interact with both recognition sites providing a short-cut mechanism which could lower the energy barrier. This notion is supported by DFT calculations and solid-state characterization of model compounds that mimic possible transition states.

Rotationally Active Ligands: Dialing-Up the Co-conformations of a [2]Rotaxane for Metal Ion Binding.

A novel [2]rotaxane was constructed that has a bidentate N,N'-chelate as part of a rigid, H-shaped axle and a 24-membered crown ether macrocycle containing six ether O-atoms and an olefinic group as the wheel. This unique topology produces a ligand with the ability to dial-up different donor sets for complexation to metal ions by simply rotating the wheel about the axle. The solution and solid-state structures of the free ligand and complexes with Li(+) and Cu(+) show how the ligand adopts different rotational co-conformations for each. The Li(+) ion uses the N,N'-chelate and O-donors while the Cu(+) center is coordinated to both O-donors and the olefinic group. This concept of rotationally active ligands should be possible with a wide variety of donor sets and could find broad application in areas of coordination chemistry, such as catalysis and metal sequestration.

Enhancing the anion affinity of urea-based receptors with a Ru(terpy)2(2+) chromophore.

Covalent linking of a Ru(terpy)2(2+) substituent improves recognition and sensing properties of the urea subunit toward anions. Urea's anion affinity is enhanced by the electrostatic attraction exerted by the Ru(II) cation and by the electron-withdrawing effect exerted by the entire polypyridine-metal complex. Such an enhancement of the anion affinity, which results from the combination of a through-space and a through-bond effect, is greater than that exerted by the classical neutral electron-withdrawing substituent nitrophenyl. Small yet significant modifications of π-π* and MLCT bands of the Ru(terpy)2(2+) chromophore, detected through UV-vis spectrophotometric titrations, allowed the determination of the constants for the formation of receptor-anion H-bond complexes in diluted MeCN solution. On (1)H NMR titration experiments, carried out under more concentrated conditions, the interaction of a second Cl(-) ion was observed, taking place through an outer-sphere mechanism. The Ru(terpy)2(2+) substituent favors the deprotonation of a urea N-H fragment on addition of a second equivalent of fluoride, with formation of HF2(-).

Moderate and advanced intramolecular proton transfer in urea-anion hydrogen-bonded complexes.

The study of the interactions of the three urea-based receptors AH, BH(+) and CH(2+) with a variety of anions, in MeCN, has made it possible to verify the current view that hydrogen bonding is frozen proton transfer from the donor (the urea N-H fragment in this case) to the acceptor (the anion X(-)). The poorly acidic, neutral receptor AH establishes two equivalent hydrogen bonds N-H···X(-), with all anions, including CH(3)COO(-) and F(-), in which moderate proton transfer from N-H to the anion takes place. The strongly acidic, dicationic receptor CH(2+) forms, with most anions, complexes in which two inequivalent hydrogen bonds are present: one involving moderate proton transfer (N-H···X(-)) and one in which advanced proton transfer has taken place, described as N(-)···H-X. The degree of proton advancement is directly related to the basic tendencies of the anion. The cationic receptor BH(+) of intermediate acidic properties only forms complexes with two inequivalent hydrogen bonds (moderate+advanced proton transfer) with CH(3)COO(-) and F(-), and complexes with two equivalent hydrogen bonds (moderate proton transfer) with all the other anions. Moreover, [B···HF] and [C···HF](+), on addition of a second F(-) ion, lose the bound HF molecule to give HF(2)(-). Release of CH(3)COOH, with the formation of [CH(3)COOH···CH(3)COO](-), also takes place with the [B···CH(3)COOH] complex in the presence of a large excess of anion.