Rotaxane Formation Research Articles

Rotaxane Formation from Borate Ion-Containing Crown Ether and Ammonium Ion: Enhancement of Their Association through Ion-Pairing.

We synthesized [2]rotaxanes featuring a catechol borate ion-containing crown ether and secondary ammonium ions. These rotaxane components show both ion-ion interactions and hydrogen bonds. X-ray crystallography and NMR spectroscopy allowed elucidation of the rotaxane structure. Moreover, 1H NMR spectroscopy revealed the rotaxane synthesis can be thermodynamically controlled. The binding affinity between the borate-containing crown ether and ammonium ions is enhanced by ion-pairing.

Conjugated bis(enaminones) as effective templates for rotaxane assembly and their post-synthetic modifications

The development of efficient methods for the synthesis of mechanically interlocked compounds is currently considered a major challenge in supramolecular chemistry. Twofold vinylogous fumaramides, a class of conjugated bis(enaminones), successfully achieve the assembly of hydrogen-bonded amide-based rotaxanes, with a templating ability comparable to that of their parent fumaramide-based systems, showcasing full conversions and impressive yields up to 92%. Computational calculations offer a compelling explanation for the remarkable efficiency of these bis(enaminones) in driving the synthesis of unprecedented rotaxanes. The reactivity of these interlocked species was thoroughly investigated, revealing that a one-step double stopper-exchange process can be successfully performed while preserving the mechanical bond. This approach facilitates the formation of controllable rotaxanes, including a three-station molecular shuttle, whose assembly via a clipping methodology is highly unselective. The internal translational motion of this latter species has been successfully controlled in a reversible way by means of a cycloaddition/retrocycloaddition sequence.

Crown Ether Active Template Synthesis of Rotaxanes**

AbstractRotaxanes are interlocked molecules that consist of a macrocycle encircling a stoppered thread. The ability to control relative component positions makes rotaxanes ideal building blocks for constructing functional and responsive molecular machines. Despite the potential of rotaxanes, their challenging synthesis limits their application. One approach to construct rotaxanes is to use an active template synthesis, where a reaction that forms the thread is accelerated in the cavity of a macrocycle. An emerging method of active template synthesis that exploits the ability of crown ether macrocycles to accelerate simple organic reactions is discussed herein. Crown ether active template synthesis (CEATS) permits the rapid and simple synthesis of rotaxanes containing a wide range of functionality. Integrating rotaxane formation with chemical reaction networks has permitted the construction of molecular machines. The simplification of rotaxane synthesis will facilitate their widespread study and application.

Rotaxane-Functionalized Dyes for Charge-Rectification in p-Type Photoelectrochemical Devices.

A supramolecular photovoltaic strategy is applied to enhance power conversion efficiencies (PCE) of photoelectrochemical devices by suppressing electron-hole recombination after photoinduced electron transfer (PET). Here, the author exploit supramolecular localization of the redox mediator-in close proximity to the dye-through a rotaxane topology, reducing electron-hole recombination in p-type dye-sensitized solar cells (p-DSSCs). Dye PRotaxane features 1,5-dioxynaphthalene recognition sites (DNP-arms) with a mechanically-interlocked macrocyclic redox mediator naphthalene diimide macrocycle (3-NDI-ring), stoppering synthetically via click chemistry. The control molecule PStopper has stoppered DNP-arms, preventing rotaxane formation with the 3-NDI-ring. Transient absorption and time-resolved fluorescence spectroscopy studies show ultrafast (211 ± 7 fs and 2.92 ± 0.05ps) PET from the dye-moiety of PRotaxane to its mechanically interlocked 3-NDI-ring-acceptor, slowing down the electron-hole recombination on NiO surfaces compared to the analogue . p-DSSCs employing PRotaxane (PCE = 0.07%) demonstrate a 30% PCE increase compared to PStopper (PCE = 0.05%) devices, combining enhancements in both open-circuit voltages (VOC = 0.43 vs 0.36V) and short-circuit photocurrent density (JSC = -0.39vs -0.34mA cm-2 ). Electrochemical impedance spectroscopy shows that PRotaxane devices exhibit hole lifetimes (τh ) approaching 1 s, a 16-fold improvement compared to traditional I- /I3 - -based systems (τh = 50ms), demonstrating the benefits obtained upon nanoengineering of interfacial dye-regeneration at the photocathode.

Rotaxane Formation of Multicyclic Polydimethylsiloxane in a Silicone Network: A Step toward Constructing “Macro‐Rotaxanes” from High‐Molecular‐Weight Axle and Wheel Components

AbstractRotaxanes consisting of a high‐molecular‐weight axle and wheel components (macro‐rotaxanes) have high structural freedom, and are attractive for soft‐material applications. However, their synthesis remains underexplored. Here, we investigated macro‐rotaxane formation by the topological trapping of multicyclic polydimethylsiloxanes (mc‐PDMSs) in silicone networks. mc‐PDMS with different numbers of cyclic units and ring sizes was synthesized by cyclopolymerization of a α,ω‐norbornenyl‐functionalized PDMS. Silicone networks were prepared in the presence of 10–60 wt % mc‐PDMS, and the trapping efficiency of mc‐PDMS was determined. In contrast to monocyclic PDMS, mc‐PDMSs with more cyclic units and larger ring sizes can be quantitatively trapped in the network as macro‐rotaxanes. The damping performance of a 60 wt % mc‐PDMS‐blended silicone network was evaluated, revealing a higher tan δ value than the bare PDMS network. Thus, macro‐rotaxanes are promising as non‐leaching additives for network polymers.

Rotaxane Formation of Multicyclic Polydimethylsiloxane in a Silicone Network: A Step toward Constructing "Macro-Rotaxanes" from High-Molecular-Weight Axle and Wheel Components.

Rotaxanes consisting of a high-molecular-weight axle and wheel components (macro-rotaxanes) have high structural freedom, and are attractive for soft-material applications. However, their synthesis remains underexplored. Here, we investigated macro-rotaxane formation by the topological trapping of multicyclic polydimethylsiloxanes (mc-PDMSs) in silicone networks. mc-PDMS with different numbers of cyclic units and ring sizes was synthesized by cyclopolymerization of a α,ω-norbornenyl-functionalized PDMS. Silicone networks were prepared in the presence of 10-60 wt % mc-PDMS, and the trapping efficiency of mc-PDMS was determined. In contrast to monocyclic PDMS, mc-PDMSs with more cyclic units and larger ring sizes can be quantitatively trapped in the network as macro-rotaxanes. The damping performance of a 60 wt % mc-PDMS-blended silicone network was evaluated, revealing a higher tan δ value than the bare PDMS network. Thus, macro-rotaxanes are promising as non-leaching additives for network polymers.

Cooperative Capture Synthesis of Functionalized Heterorotaxanes─Chemical Scope, Kinetics, and Mechanistic Studies.

The self-assembly of molecularly interlocked molecules offers new opportunities for creating bioactive molecules for applications in medicine. Cooperative capture synthesis of heterorotaxanes in water is an attractive methodology for developing multifunctional supramolecular imaging agents or drugs, but derivatizing the rotaxane scaffold with biologically active vectors like peptides and proteins, or reporter probers like radioactive metal ion complexes and fluorophores, requires the installation of reactive functional groups. Here, we explored the chemical scope of β-cyclodextrin (β-CD) derivatization on the cucurbit[6]uril (CB[6])-mediated cooperative capture synthesis of hetero[4]rotaxanes with the objective of identifying which reactive groups can be used for further functionalization without compromising the efficiency of rotaxane synthesis. Nine β-CD derivatives featuring an electrophilic leaving group (tosylate), aliphatic amines, a carboxylic acid, aliphatic azides, anilines, and aryl isothiocyanate were evaluated in the synthesis of hetero[4]rotaxanes. Experimental measurements on the kinetics of rotaxane synthesis were combined with detailed computational studies using the density functional theory to elucidate the mechanistic pathways and rate determining step in the cooperative capture process. Computational studies on the structure and bonding also revealed why intermolecular interactions between the β-CD and CB[6] macrocycles improve the rate and efficiency of rotaxane formation through cooperative capture. Understanding the mechanistic details and synthetic scope will facilitate broader access to functionalized hetero[4]rotaxanes for applications in biomedicine and beyond.

Speed Tuning of the Formation/Dissociation of a Metallorotaxane.

Switching between the formation/dissociation of rotaxanes is important to control the function of various types of rotaxane-based materials. We have developed a convenient and simple strategy, the so-called "accelerator addition", to make a static rotaxane dynamic without apparently affecting the chemical structure. As an interlocked molecule that enables tuning of the formation/dissociation speed, a metallorotaxane was quantitatively generated by the complexation of a triptycene-based dumbbell-shaped mononuclear complex, [PdL2 ]2+ (L=2,3-diaminotriptycene), with 27C9. As a result of the inertness of the Pd2+ -based coordination structure, the metallorotaxane was slowly formed (the static state). This rotaxane formation was accelerated 27 times simply by adding Br- as an accelerator (the dynamic state). A similar drastic acceleration was also demonstrated during the dissociation process when Cs+ was added to the metallorotaxane to form the free axle [PdL2 ]2+ and the 27C9-Cs+ complex.

Speed Tuning of the Formation/Dissociation of a Metallorotaxane

AbstractSwitching between the formation/dissociation of rotaxanes is important to control the function of various types of rotaxane‐based materials. We have developed a convenient and simple strategy, the so‐called “accelerator addition”, to make a static rotaxane dynamic without apparently affecting the chemical structure. As an interlocked molecule that enables tuning of the formation/dissociation speed, a metallorotaxane was quantitatively generated by the complexation of a triptycene‐based dumbbell‐shaped mononuclear complex, [PdL2]2+ (L=2,3‐diaminotriptycene), with 27C9. As a result of the inertness of the Pd2+‐based coordination structure, the metallorotaxane was slowly formed (the static state). This rotaxane formation was accelerated 27 times simply by adding Br− as an accelerator (the dynamic state). A similar drastic acceleration was also demonstrated during the dissociation process when Cs+ was added to the metallorotaxane to form the free axle [PdL2]2+ and the 27C9‐Cs+ complex.

Mechanochemical Enhancement of the Structural Stability of Pseudorotaxane Intermediates in the Synthesis of Rotaxanes.

Mechanochemical syntheses of rotaxanes have attracted considerable attention of late because of the superior reaction rates and higher yields associated with their production compared with analogous reactions carried out in solution. Previous investigators, however, have focused on the demonstration of the mechanochemical syntheses of rotaxanes per se, rather than on studying the solid-phase host-guest molecular interplay related to their rapid formation and high yields. In this investigation, we attribute the lower yields of rotaxanes prepared in solution to the limited concentration and a desolvation energy penalty that must be compensated for by host-guest interactions during complexation that precedes the templation leading to rotaxane formation. It follows that, if the desolvation energy can be removed and higher concentrations can be attained, even weak host-guest interactions can drive the complexation of host and guest molecules efficiently. In order to test this hypothesis, we chose two host-guest pairs of permethylated pillar[5]arene/1,6-diaminohexane and permethylated pillar[5]arene/2,2'-(ethylenedioxy)bis(ethylamine) for the simple reason that they exhibit extremely low binding constants (2.7 ± 0.4 M-1 when 1,6-diaminohexane is the guest and <0.1 M-1 when 2,2'-(ethylenedioxy)bis(ethylamine) is the guest in CDCl3; i.e., ostensibly no pseudorotaxane formation is observed). We argue that the amount of pseudorotaxanes formed in the solid state is responsive to mechanical treatments or otherwise and changes in temperature during stoppering reactions. Compared to the amount of pseudorotaxanes that can be obtained in solution, large quantities of pseudorotaxanes are formed in the solid state because of concentration and desolvation effects. This mechanochemical enhancement of pseudorotaxane formation is referred to as a self-correction in the current investigation. Rotaxanes based on permethylated pillar[5]arene/1,6-diaminohexane and permethylated pillar[5]arene/2,2'-(ethylenedioxy)bis(ethylamine) have been synthesized in much higher yields compared to those obtained in solution, aided and abetted by self-correction effects during mechanical treatments and heating at a mild temperature of 50 °C.

An Operative Electrostatic Slipping Mechanism along Macrocycle Flexibility Accelerates Guest Sliding during pseudo-Rotaxane Formation.

A pseudo‐rotaxane is a host−guest complex composed of a linear molecule encircled by a macrocyclic ring. These complexes can be assembled by sliding the host over the guest terminal groups. If there is a close match between the molecular volume of the flanking groups on the guest and the cavity size of the macrocycle, the slipping might occur slowly or even become completely hindered. We have previously shown that it is possible to overcome the restraints imposed by steric effects on the sliding process by integrating electrostatic attractive interactions during the slipping step. In this work, we extend our electrostatically assisted slipping approach (EASA) to a new host−guest system featuring a flexible macrocyclic ring and a series of asymmetric guests containing a cyclic tertiary ammonium group. Compelling evidence for pseudo‐rotaxane formation is presented, along with thermodynamic and kinetic data. Experimental results suggests that the higher conformational flexibility of 24‐crown‐8 significantly increases the sliding rate, compared with the more rigid dibenzo‐24‐crown‐8, without affecting complex stability. Furthermore, by combining the EASA and macrocycle flexibility, we were capable to slip a large eight‐membered cyclic group across the 24‐crown‐8 annulus, setting a new limit on the ring molecular size that can pass through a 24‐membered crown ether.

Rotaxane formation by an allosteric pseudomacrocyclic anion receptor utilising kinetically labile copper(I) coordination properties.

Rotaxanes, which are composed of ring and axle components, are important interlocked molecules with wide applications such as molecular machines and switchable catalysts. The construction of interlocked structures targeting anions is an important issue, as evidenced by the fact that anionic groups are usually abundant in many biomacromolecules. We now report an allosteric pseudomacrocyclic anion receptor as a ring that spontaneously generates a rotaxane in an auto-clipping way, which does not require the successive ring forming reaction like usual clipping, in the presence of an axle with an anionic station. We designed a linear ligand 1 bearing three anion recognition moieties, i.e., one thiourea group at the centre and two urea groups near the 2,2'-bipyridine ends of 1. The complexation of 1 with Cu+ proceeded in an intramolecular manner that quantitatively led to a macrocyclic structure [1·Cu]+. Compared to 1, the anion binding ability of [1·Cu]+ was significantly larger (positive allosteric effect) due to the macrocyclization arrangement of the three anion recognition moieties in a cyclic fashion and electrostatic interaction. In addition, the kinetically labile but thermodynamically stable coordination properties of the pseudomacrocyclic ring unit promoted the spontaneous rotaxane formation with a phosphate axle at room temperature.

Whither Second-Sphere Coordination?

Whither Second-Sphere Coordination?

Ru(II)Porphyrinate-based molecular nanoreactor for carbene insertion reactions and quantitative formation of rotaxanes by active-metal-template syntheses

Selectivity in N–H and S–H carbene insertion reactions promoted by Ru(II)porphyrinates currently requires slow addition of the diazo precursor and large excess of the primary amine and thiol substrates in the reaction medium. Such conditions are necessary to avoid the undesirable carbene coupling and/or multiple carbene insertions. Here, the authors demonstrate that the synergy between the steric shielding provided by a Ru(II)porphyrinate-based macrocycle with a relatively small central cavity and the kinetic stabilization of otherwise labile coordinative bonds, warranted by formation of the mechanical bond, enables single carbene insertions to occur with quantitative efficiency and perfect selectivity even in the presence of a large excess of the diazo precursor and stoichiometric amounts of the primary amine and thiol substrates. As the Ru(II)porphyrinate-based macrocycle bears a confining nanospace and alters the product distribution of the carbene insertion reactions when compared to that of its acyclic version, the former therefore functions as a nanoreactor.

Rotaxa-polymeric-gelation of acrylamides with vinyl-β-cyclodextrin

Previously, we have reported a single-step preparation method for a topological gel by copolymerizing isoprene and vinyl-modified β-cyclodextrin (V-β-CyD). In the prepared gel, the vinyl part of V-β-CyD consists a part of the copolymer chain, and the β-CyD part includes another copolymer chain, thus V-β-CyD works as a cross-linker. We call the procedure rotaxa-polymeric-gelation (RPG) because polymerization, rotaxane formation, and gelation proceed simultaneously. In this study, we applied acrylamide (AAm) instead of isoprene for RPG, and successfully obtained highly swellable hydrogel. By contrast, a combination of V-β-CyD and N-isopropylacrylamide (NIPAAm) was not suitable for RPG because NIPAAm and cavity of β-CyD were mismatched in size. In addition, we prepared three-component gels. Among of them, gels composed of NIPAAm, isoprene, and V-β-CyD showed thermo-responsiveness by reflecting the character of polyNIPAAm. These results provide an insight for usage of vinyl monomers for RPG strategy.

Change in the rate of pseudo[1]rotaxane formation by elongating the alkyl-chain-substituted diphenylethynylene linked to permethyl α-cyclodextrin

Herein, we report the kinetics of pseudo[1]rotaxane formation from permethyl α-cyclodextrin attached to a flexible-chain-substituted diphenylethynylene. When the chain is an alkyl group, the rate of formation shows different trends over three regions of chain length: deceleration (chain length = 1–3), acceleration (4–8), and re-deceleration (>12). This behavior is driven by a relative decrease in the ΔH‡ of the transition.

Supramolecular Conformational Control of Aliphatic Oligoketones by Rotaxane Formation.

Conformational control of aliphatic oligoketones bearing two 1,3-diketone subunits is achieved by molecular recognition with pillar[5]arene. Pillar[5]arene binds to aliphatic ketones, with association constants K of ∼10 M-1, to form pseudorotaxanes. The pseudorotaxanes are locked by BF2 complexation at the 1,3-diketone sites through quasi-solid-state reactions. X-ray crystallography reveals linear conformations for the axis molecules. The effect of supramolecular conformational restriction is evaluated using an alkyl-linked dyad chromophore system that shows solvatochromism upon intramolecular aggregation.

Hypervalent Iodine Based Reversible Covalent Bond in Rotaxane Synthesis

AbstractReversible covalent bonds play a significant role in achieving the high‐yielding synthesis of mechanically interlocked molecules. Still, only a handful of such bonds have been successfully employed in synthetic procedures. Herein, we introduce a novel approach for the fast and simple preparation of interlocked molecules, combining the dynamic bond character of bis(acyloxy)iodate(I) anions with macrocyclic bambusuril anion receptors. The proof of principle was demonstrated on rotaxane synthesis, with near‐quantitative yields observed in both the classical and “in situ” approach. The rotaxane formation was confirmed in the solid‐state and solution by the X‐ray and NMR studies. Our novel approach could be utilized in the fields of dynamic combinatorial chemistry, supramolecular polymers, or molecular machines, as well inspire further research on molecules that exhibit dynamic behavior, but owing to their high reactivity, have not been considered as constituents of more elaborate supramolecular structures.

A Molecular Dual Pump

Artificial molecular machines (AMMs) built from mechanically interlocked molecules (MIMs) can use energy ratchets to control the unidirectional motion of their component parts. These energy ratchets are operated by the alteration of kinetic barriers and thermodynamic wells, which are, in turn, determined by the switching on and off of noncovalent interactions. Previously, we have developed artificial molecular pumps (AMPs) capable of pumping rings consecutively onto a collecting chain as part of a molecular dumbbell, leading to the formation of rotaxanes. Here, we report a molecular dual pump (MDP) consisting of two individual AMPs linked in a head-to-tail fashion, wherein a single ring is pumped, in a linear manner, on and off a dumbbell involving a [2]rotaxane intermediate by exploiting the redox properties of the two pumps. This MDP, defined by the finely tuned noncovalent interactions and fueled by either chemicals or electricity, utilizes an energy ratchet mechanism to capture a ring and subsequently release it back into solution. The unidirectional motion and the resulting controlled capture and release of the ring were followed by 1D and 2D 1H NMR spectroscopy and supported by control experiments. This molecular dual pump may be considered to be a forerunner of AMMs that are capable of pumping rings across a membrane in a way similar to how bacteriorhodopsin transports protons from one side of a membrane to the other under the influence of light. Such extensive multicomponent AMMs can lead potentially to molecular transporting platforms with positional and directional control of cargo uptake and release when, and only when, instructed.

Conformer Distribution in Rotaxanes Containing Nonsymmetric Threads: A Systematic Approach

Rotaxanes are designated as molecular machines due their different movements. Systematic studies regarding the different conformations adopted by these systems and the factors that lead to the distribution of the conformations, in both solution and the solid state, have not been widely explored, especially for rotaxanes with nonsymmetric stoppers. Therefore, in this study we have investigated three novel [2]rotaxanes containing threads derived from nonsymmetric succinamides [R1R2NC(O)‐CH2CH2‐C(O)NR2R1, with R1/R2 = Bu/Bn, Bu/2‐furylmethyl, and 5‐methylisoxazol‐3‐yl/2‐furylmethyl]. The proportions of rotamers were investigated for threads and rotaxanes by solution and solid‐state NMR spectroscopy as well as by single‐crystal and powder X‐ray diffraction. In solution, the threads present different proportions of conformer, with the E,Z conformation prevailing, whereas only one conformer is observed in the solid state. For the rotaxanes, only one conformer prevails in the single crystal, whereas the solution and solid (bulk) states present more than one rotamer. These proportions are modified when the threads are incorporated into the macrocycle during rotaxane formation. The intramolecular interactions in each rotamer were investigated by QTAIM and variable‐temperature 1H NMR experiments. The changes in conformational population between the threads and respective rotaxanes can be explained by a set of different intramolecular interactions, with trifurcated hydrogen bonds responsible for most of the stabilization energy.