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

Mechanical bonding of rigid MORFs using a tetratopic rotaxane

The preparation of highly rigid cobalt(II)- and copper(II)-organic frameworks incorporating a tetralactam [2]rotaxane as a ligand is described. The interlocked ligand is functionalized with two pairs of carboxylate groups placed at each counterpart, thus limiting its dynamics within the crystal. The solid structure of the metal-organic rotaxane frameworks showed different, unprecedented polycatenation modes of grids, depending on the employed metal, providing great rigidity to the structures. This rigidity has been evaluated by using single crystal X-ray diffraction analyses of the cobalt(II)-organic frameworks embedded in different solvents, observing that the lattices remain unchanged. Thus, this research demonstrates that rigid and robust materials with permanent porosity can be achieved using dynamic ligands.

Synthesis of 'Impossible' Rotaxanes

'Impossible' rotaxanes, which are constituted by interlocked components without obvious binding motifs, have attracted the interest of the mechanically interlocked molecules (MIMs) community. Within the synthetic efforts reported in the last decades towards the preparation of MIMs, some innovative protocols for accessing 'impossible' rotaxanes have been developed. This short review highlights different selected synthetic examples of 'impossible' rotaxanes, as well as suggests some future directions of this research area.

Guest-Shuttling in a Nanosized Metallobox

An iridium-conjoined long and narrow metallorectangle was obtained by combining a quinoxalinophenanthrophenazine-connected Janus-di-imidazolylidene ligand and pyrazine. The size and shape of this assembly together with the fused polyaromatic nature of its panels provides it with properties that are uncommon for other metallosupramolecular assemblies. For example, this nanosized 'slit-like' metallobox is able show very large binding affinities with planar organic molecules in such a way, that the cavity is asymmetrically occupied by the guest molecule. This unsymmetrical conformation leads to the existence of a large amplitude motion of these guests, which slide between the two sides of the cavity of the host, thus constituting rare examples of molecular shuttles.

Correlated translational motions in pseudo-rotaxane complexes controlled by a single chemical stimulus

Coordinated motions are essential in the operation of molecular machines. This feature can be achieved by landscaping the energy surface along the movement coordinates. Herein, we present an approach of using a single stimulus to modify the free energy curve describing the threading and shuttling of a ring along a linear molecule. This approach has been realized by locating two identical ring-binding sites near the axle termini.

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.

Preprogrammed assembly of supramolecular polymer networks via the controlled disassembly of a metastable rotaxane

In our daily life, some of the most valuable commodities are preprogrammed or preassembled by a manufacturer; the end-user puts together the final product and gathers properties or function as desired. Here, we present a chemical approach to preassembled materials, namely supramolecular polymer networks (SPNs), which wait for an operator's command to organize autonomously. In this prototypical system, the controlled disassembly of a metastable interlocked molecule (rotaxane) liberates an active species to the medium. This species crosslinks a ring-containing polymer and assembles with a reporting macrocycle to produce colorful SPNs. We demonstrate that by using identical preprogrammed systems, one can access multiple supramolecular polymer networks with different degrees of fluidity (μ^* = 2.5 to 624 Pa s^-1) and color, all as desired by the end-user.

Synthesis of multifunctional crown ether covalent organic nanospheres as stationary phase for capillary electrochromatography

Crown ethers are macrocyclic polyether compounds containing multiple -oxo-methylene-structural units, which are often used for recognition of metal ions and ammonium ions. Inspired by the molecular design of rotaxanes, a novel covalent organic nanospheres material (CON ADBC-Tp) constructed by 4,4'-diaminodibenzo-18-crown-6 (ADBC) and 2,4,6-triformylphloroglucinol (Tp) was rationally designed as stationary phase for the separation of compounds containing imidazole structure. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) were carried out to confirm the morphology and composition of ADBC-Tp and ADBC-Tp modified capillary column. Thanks to the introduction of crown ether building ligands, the prepared capillary column exhibited excellent separation selectivity towards protonated imidazole structure, with benzimidazole and its 2-substituted derivatives as modal analytes. Moreover, separation of fungicides, nucleobases, short peptides and sulfanilamides were achieved on ADBC-Tp@capillary. The multifunctional ADBC-Tp@capillary with satisfactory reproducibility and stability gives great promise for separation science.

Dynamic network of intermolecular interactions in metal-organic frameworks functionalized by molecular machines

Molecular machines enable external control of structural and dynamic phenomena at the atomic level. To efficiently transfer their tunable properties into designated functionalities, a detailed understanding of the impact of molecular embedding is needed. In particular, a comprehensive insight is fundamental to design hierarchical multifunctional systems that are inspired by biological cells. Here, we applied an on-the-fly trained force field to perform atomistic simulations of a systematically modified rotaxane functionalized metal-organic framework. Our atomistic studies reveal a symmetric and asymmetric interplay of the mechanically bonded rings (MBRs) within the framework depending on the local environment. As a result, their translational motion is modulated ranging from fast oscillatory behavior to cooperative and potentially directed shuttling. The derived picture of competitive interactions, which influence the operation mechanism of the MBRs embedded in these soft porous materials, promotes the development of responsive functional materials, which is a key step toward intelligent matter.

Mechanically interlocked molecules in metal-organic frameworks

Mechanically interlocked molecules (MIMs) have great potential in the development of molecular machinery due to their intercomponent dynamics. The incorporation of these molecules in a condensed phase makes it possible to take advantage of the control of the motion of the components at the macroscopic level. Metal-organic frameworks (MOFs) are postulated as ideal supports for intertwined molecules. This review covers the chemistry of the mechanical bond incorporated into metal-organic frameworks from the seminal studies to the latest published advances. We first describe some fundamental concepts of MIMs and MOFs. Next, we summarize the advances in the incorporation of rotaxanes and catenanes inside MOF matrices. Finally, we conclude by showing the study of the rotaxane dynamics in MOFs and the operation of some stimuli-responsive MIMs within MOFs. In addition to emphasising some selected examples, we offer a critical opinion on the state of the art of this research field, remarking the key points on which the future of these systems should be focused.

Docking rings in a solid: reversible assembling of pseudorotaxanes inside a zirconium metal-organic framework

An unprecedented zirconium metal–organic framework featuring a T-shaped benzimidazole strut was constructed and employed as a sponge-like material for selective absorption of macrocyclic guests. The neutral benzimidazole domain of the as-synthesized framework can be readily protonated and fully converted to benzimidazolium. Mechanical threading of [24]crown-8 ether wheels onto recognition sites to form pseudorotaxanes was evidenced by solution nuclear magnetic resonance, solid-state fluorescence, and infrared spectroscopy. Selective absorption of [24]crown-8 ether rather than its dibenzo counterpart was also observed. Further study reveals that this binding process is reversible and acid–base switchable. The success of docking macrocyclic guests in crystals via host–guest interactions provides an alternative route to complex functional materials with interpenetrated structures.

A T-shaped ligand was designed as struts for building a zirconium metal–organic framework. Acid–base switchable docking and releasing a 24-membered crown ether inside crystals was successfully accomplished via post-synthetic modification.

2D and 3D metal-organic frameworks constructed with a mechanically rigidified [3]rotaxane ligand

A mechanically interlocked [3]rotaxane was newly designed, synthesized, and employed as a ligand for constructing metal-organic frameworks (MOFs). The nano-confinement by macrocycles forces the soft bis-isophthalate axle into a pseudo-rigid conformation and coordinates to zinc(II) ions, affording a two- or three-dimensional MOF under controlled conditions. The 2D MOF exhibits a neutral framework with a periodic puckering sheet structure, while an anionic framework with a pts topology was observed for the 3D MOF. The phase purity of both bulk materials was confirmed by powder X-ray diffraction. Thermogravimetric analysis reveals that both materials are stable up to 250 °C. The success of applying mechanical bonds to rigidify flexible ligands provides new insights for the design of metal-organic frameworks.

Controllable photomechanical bending of metal-organic rotaxane crystals facilitated by regioselective confined-space photodimerization

Molecular machines based on mechanically-interlocked molecules (MIMs) such as (pseudo) rotaxanes or catenates are known for their molecular-level dynamics, but promoting macro-mechanical response of these molecular machines or related materials is still challenging. Herein, by employing macrocyclic cucurbit[8]uril (CB[8])-based pseudorotaxane with a pair of styrene-derived photoactive guest molecules as linking structs of uranyl node, we describe a metal-organic rotaxane compound, U-CB[8]-MPyVB, that is capable of delivering controllable macroscopic mechanical responses. Under light irradiation, the ladder-shape structural unit of metal-organic rotaxane chain in U-CB[8]-MPyVB undergoes a regioselective solid-state [2 + 2] photodimerization, and facilitates a photo-triggered single-crystal-to-single-crystal (SCSC) transformation, which even induces macroscopic photomechanical bending of individual rod-like bulk crystals. The fabrication of rotaxane-based crystalline materials with both photoresponsive microscopic and macroscopic dynamic behaviors in solid state can be promising photoactuator devices, and will have implications in emerging fields such as optomechanical microdevices and smart microrobotics.

Collective dynamics of molecular rotors in periodic mesoporous organosilica: a combined solid-state 2H-NMR and molecular dynamics simulation study

Molecular rotors offer a platform to realize controlled dynamics and modulate the functions of solids. The motional mechanisms in arrays of rotors have not been explored in depth. Crystal-like porous organosilicas, comprising p-phenylene rotators pivoted onto a siloxane scaffold, were modelled using molecular dynamics (MD) simulations. Long simulations, on a microsecond scale, allowed to follow the reorientation statistics of rotor collections and single out group configurations and frequency distributions as a function of temperature. The motions observed in the MD simulations support a multiple-site model for rotor reorientations. Computed motional frequencies revealed a complex rotatory phenomenon combining an ultra-fast libration motion (oscillation up to 30°) with a slow and fast 180° flip reorientation. Adopting a multiple-site model provides a more accurate simulation of the ²H-NMR spectra and a rationalization of their temperature dependence. In particular, rotators endowed with distinct rates could be explained by the presence of slower rings locked in a T-shaped conformation.

Impact of Proximity Effect on Uranyl Coordination of Conformationally Variable Weakly-Bonded Cucurbit[6]uril-Bipyridinium Pseudorotaxane

To explore the proximity effect in uranyl coordination of weak-bonded cucurbit[6]uril(CB[6])-bipyridinium ligands, a new pseudorotaxane precursor C7BPCN3@CB[6] containing 1, 1'-(heptyl-1,7-diyl)bis(3-cyanopyridin-1-ium) bromide (C7BPCN3) with elongated alkyl chains and meta-substituted cyano groups,...

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.

Cascade Dynamics of Multiple Molecular Rotors in a MOF: Benchmark Mobility at a Few Kelvins and Dynamics Control by CO2

Achieving sophisticated juxtaposition of geared molecular rotors with negligible energy-requirements in solids enables fast yet controllable and correlated rotary motion to construct switches and motors. Our endeavor was to realize multiple rotors operating in a MOF architecture capable of supporting fast motional regimes, even at extremely cold temperatures. Two distinct ligands, 4,4′-bipyridine (bipy) and bicyclo[1.1.1]pentanedicarboxylate (BCP), coordinated to Zn clusters fabricated a pillar-and-layer 3D array of orthogonal rotors. Variable temperature XRD, ²H solid-echo, and ¹H T₁ relaxation NMR, collected down to a temperature of 2 K revealed the hyperfast mobility of BCP and an unprecedented cascade mechanism modulated by distinct energy barriers starting from values as low as 100 J mol^–1 (24 cal mol^–1), a real benchmark for complex arrays of rotors. These rotors explored multiple configurations of conrotary and disrotary relationships, switched on and off by thermal energy, a scenario supported by DFT modeling. Furthermore, the collective bipy-ring rotation was concerted with the framework, which underwent controllable swinging between two arrangements in a dynamical structure. A second way to manipulate rotors by external stimuli was the use of CO₂, which diffused through the open pores, dramatically changing the global rotation mechanism. Collectively, the intriguing gymnastics of multiple rotors, devised cooperatively and integrated into the same framework, gave the opportunity to engineer hypermobile rotors (10⁷ Hz at 4 K) in machine-like double ligand MOF crystals.

Pb2+-Containing Metal-Organic Rotaxane Frameworks (MORFs)

The metal-organic rotaxane framework (MORF) structures with the advantage of mechanically interlocking molecules (MIMs) have attracted intense interest from the chemical community. In this study, a set of MORFs (i.e., MORF-Pb-1 and MORF-Pb-2) are constructed using Pb2+, a tetraimidazolium macrocycle (Texas-sized molecular box; 14+), and aromatic dicarboxylate (p-phthalate dianions (PTADAs; 2) or 2,6-naphthalene dicarboxylate dianions (3)) via a one-pot three-layer diffusion protocol. In particular, an unusual Pb…Pb weak interaction was shown in MORF-Pb-1 (charactered with distance of 3.656 Å).

Hydrazone exchange: a viable route for the solid-tethered synthesis of [2]rotaxanes

Using a hydrazone exchange methodology, resin beads were functionalised with [2]rotaxanes at up to 80% efficiency—higher than using other dynamic or irreversible synthetic approaches to form self-assembled structures on solid supports.

Structural flexibility in crystalline coordination polymers: a journey along the underlying free energy landscape

In this perspective, we discuss structural flexibility in crystalline coordination polymers. We identify that the underlying free energy landscape unites scientific disciplines, and discuss key areas to advanced the field.