A translationally active ligand based on a [2]rotaxane molecular shuttle with a 2,2'-bipyridyl core.

A rigid H-shaped, [2]rotaxane molecular shuttle comprised of an axle containing two benzimidazole recognition sites and a central 2,2'-bipyridyl (bipy) group interlocked with a 24-crown-8 (24C8) wheel was synthesized using a threading followed by stoppering protocol. The central bipy chelating unit was shown to act as a speed bump that raised the barrier to shuttling for the [2]rotaxane. Coordination of a PtCl2 moiety to the bipy unit in a square planar geometry created an insurmountable steric barrier to shuttling. Addition of one equivalent of NaB(3,5-(CF3)2C6H3)4 removed one of the chloride ligands allowing for translation of the crown ether along the axle into the coordination sphere of the Pt(ii) centre but full shuttling of the crown ether could not be activated. In contrast, addition of Zn(ii) ions in a coordinating solvent (DMF) allowed shuttling to occur using a ligand exchange mechanism. DFT calculations showed this likely occurs via coordination of the 24C8 macrocycle to the Zn(ii) centre bound to the bipy chelate. This interplay of the rotaxane axle and wheel components is an example of a translationally active ligand that utilises the large amplitude displacement of a macrocycle along an axle in a molecular shuttle to access ligand coordination modes not possible with conventional ligand designs.

Structural Dynamics by NMR in the Solid State: II. The MOMD Perspective of the Dynamic Structure of Metal-Organic Frameworks Comprising Several Mobile Components.

We describe the application of the microscopic-order-macroscopic-disorder (MOMD) approach, developed for the analysis of dynamic 2H NMR lineshapes in the solid state, to unravel interactions among the constituents of metal-organic frameworks (MOFs) that comprise mobile components. MOMD was applied recently to University of Windsor Dynamic Material (UWDM) MOFs with one mobile crown ether per cavity. In this work, we study UWDM-9-d4, which comprises a mobile 2H-labeled phenyl-ring residue along with an isotopically unlabeled 24C8 crown ether. We also study UiO-68-d4, which is structurally similar to UWDM-9-d4 but lacks the crown ether. The physical picture consists of the NMR probe─the C-D bonds of the phenyl-d4 rotor─diffusing locally (diffusion tensor R) in the presence of a local ordering potential, u. For UiO-68-d4, we find it sufficient to expand u in terms of four real Wigner functions, D0|K|L, overall 2-3 kT in magnitude, with R∥ relatively fast, and R⊥ in the (2.8-5.0) × 102 s-1 range. For UWDM-9-d4, u requires only two terms 2-3 kT in magnitude and slower rate constants R∥ and R⊥. In the more crowded macrocycle-containing UWDM-9-d4 cavity, phenyl-d4 dynamics is more isotropic and is described by a simpler ordering potential. This is ascribed to cooperative phenyl-ring/macrocycle motion, which yields a dynamic structure more uniform in character. The experimental 2H spectra used here were analyzed previously with a multi-simple-mode (MSM) approach where several independent simple motional modes are combined. Where possible, similar features have been identified and used to compare the two approaches.

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.

Translational dynamics of a non-degenerate molecular shuttle imbedded in a zirconium metal-organic framework.

A new [2]rotaxane molecular shuttle linker based on the binding of a 24-crown-8 ether macrocycle at a benzimidazole recognition site was synthesised. The shuttling dynamics of the linker were studied in solution and the structure confirmed by X-ray crystallography. A multivariate Zr(iv) MOF, UWDM-11, containing the new MIM linker and primary linker tetramethylterphenyldicarboxylate was synthesised and the translational motion of the molecular shuttle studied in the solid state. The use of a 13C enriched MIM linker allowed the dynamics of both activated and mesitylene-solvated UWDM-11 to be elucidated by VT 13C CPMAS SSNMR. The incorporation of mesitylene into the pores of UWDM-11 resulted in a significant increase in the barrier for thermally driven translation of the macrocycle.

Exploring the dynamics of Zr-based metal-organic frameworks containing mechanically interlocked molecular shuttles.

Zr(iv) metal-organic frameworks (MOFs) UiO-68 and PCN-57, containing triphenylene dicarboxylate (TPDC) and tetramethyl-triphenylene dicarboxylate (TTDC) linkers, respectively, were doped with an H-shaped tetracarboxylate linker that contains a [2]rotaxane molecular shuttle. The new MOFs, UWDM-8 and UWDM-9, contain a [2]rotaxane crossbar spanning the tetrahedral cavities of the fcu topology while the octahedral cavities remain empty. 13C solid-state NMR (SSNMR) spectra and solution 1H NMR spectra verified that the [2]rotaxanes were included as designed. Variable-temperature (VT) cross polarization (CP) magic-angle spinning (MAS) 13C SSNMR was used to explore the translational motion of the macrocyclic ring in both MOFs. The SSNMR results clearly show that the structure of the linker (TPDCvs.TTDC) affects the shuttling rate of the macrocyclic ring, although questions remain as to how rotation of the central phenylene unit of the strut might also affect the motion of the macrocycle.

One-pot synthesis of porphyrin-based [5]rotaxanes.

A one-pot reaction is used to make a series of [5]rotaxanes. The protocol involves simultaneous threading-followed-by-stoppering to trap a macrocycle (dibenzo[24]crown-8, DB24C8) on an axle to form a mechanically interlocked molecule (MIM) - in this case a rotaxane - and the condensation of an aldehyde with a pyrrole to form a porphyrin precursor. For each [5]rotaxane, a different combination of recognition site and stoppering group was used; the protonation state of the [5]rotaxane can be used to generate different co-conformational states for each [5]rotaxane making these systems potential multi-state switches for further study in solution or the solid-state.

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.

A pyridinium/anilinium [2]catenane that operates as an acid-base driven optical switch.

A two-station [2]catenane containing a large macrocycle with two different recognition sites, one bis(pyridinium)ethane and one benzylanilinium, as well as a smaller DB24C8 ring was synthesized and characterized. 1H NMR spectroscopy showed that the DB24C8 ring can shuttle between the two recognition sites depending on the protonation state of the larger macrocycle. When the aniline group is neutral, the DB24C8 ring resides solely at the bis(pyridinium)ethane site, while addition of acid forms a charged benzylanilinium site. The DB24C8 then shuttles between the two charged recognition sites with occupancy favoring the bis(pyridinium)ethane site by a ratio of 4:1. The unprotonated [2]catenane has a deep yellow/orange color when the DB24C8 ring resides solely at the bis(pyridinium)ethane site and changes to colorless when the crown ether is shuttling (i.e., circumrotating) back and forth between the two recognition sites thus optically signalling the onset of the shuttling dynamics.

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.

Metal ions in the synthesis of interlocked molecules and materials.

The use of metal ions to template the synthesis of catenanes by Sauvage and co-workers was a pivotal moment in the development of the field of interlocked molecules. In this Review Article we shall examine the different roles metal-ligand interactions play in modern syntheses of interlocked molecules and materials, with a particular focus on seminal contributions and the advantages and disadvantages of employing metal ligand interactions.

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.

Acid-Base Switchable [2]- and [3]Rotaxane Molecular Shuttles with Benzimidazolium and Bis(pyridinium) Recognition Sites.

For the purpose of developing higher level mechanically interlocked molecules (MIMs), such as molecular switches and machines, a new rotaxane system was designed in which both the 1,2-bis(pyridinium)ethane and benzimidazolium recognition templating motifs were combined. These two very different recognition sites were successfully incorporated into [2]rotaxane and [3]rotaxane molecular shuttles which were fully characterized by 1 H NMR, 2D EXSY, single-crystal X-ray diffraction and VT NMR analysis. By utilizing benzimidazolium as both a recognition site and stoppering group it was possible to create not only an acid/base switchable [2]rotaxane molecular shuttle (energy barrier 20.9 kcal⋅mol-1 ) but also a [3]rotaxane molecular shuttle that displays unique dynamic behavior involving the simultaneous motion of two macrocyclic wheels on a single dumbbell. This study provides new insights into the design of switchable molecular shuttles. Due to the unique properties of benzimidazoles, such as fluorescence and metal coordination, this new type of molecular shuttle may find further applications in developing functional molecular machines and materials.

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.

From Binuclear Complexes to Molecular Necklaces: Incorporating Flexible Ligands into Rotaxanes.

The conversion of binuclear complexes into larger molecular necklaces can be achieved through rigidifying flexible ligands by threading them through a crown ether to form either an interpenetrated [2]pseudorotaxane or a permanently interlocked [2]rotaxane. The resulting complexes and assemblies are characterized by (1) H and DOSY NMR in solution and single-crystal X-ray diffraction in the solid-state.

Rigid, Bistable Molecular Shuttles Combining T-shaped Benzimidazolium and Y-shaped Imidazolium Recognition Sites.

Two different recognition sites, one a T-shaped 2,4,7-triphenylbenzimidazolium and the other a Y-shaped 2,4,5-triphenylimidazolium are combined to construct a rigid bistable [2]rotaxane molecular shuttle on which the position of a 24-membered crown ether macrocycle can be controlled by acid-base chemistry. Molecular shuttling in both the neutral and dicationic versions were studied.

Thermally Driven Dynamics of a Rotaxane Wheel about an Imidazolium Axle inside a Metal-Organic Framework.

A new mechanically interlocked molecular linker was prepared by using ring-closing metathesis (Grubbs I) to clip a [24]crown-6 ether wheel around an axle containing both Y-shaped diphenylimidazole and isophthalic acid groups. A metal-organic framework (MOF) material was prepared using this linker and ZnII ions. Single-crystal X-ray diffraction experiments showed that the MOF contains an imidazolium-based rotaxane linked by dimeric [Zn2 (NO3 )(DEF)] secondary building units (SBUs). Variable-temperature (VT), 2 H solid-state NMR spectroscopy was used to characterize the motion of the "soft" wheel component around the rigid "hard" lattice of the framework. At higher temperatures (above 150 °C), it was demonstrated that the 24-membered, macrocyclic ring of the MOF undergoes rapid, thermally driven rotation about the axle inside the voids of the lattice.