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

Molecular Compasses for Modulating Electronic Communication in Pillar[5]quinone

Just as a pointer, which moves freely and points to the magnetic North in a compass, affords us with a device for tracking direction on a global scale, a dipole moment in a molecule is capable of aligning itself in a compass-like manner in response to an electric field at the molecular level. Here, we demonstrate that dipole moment pointers, based on amide and ester groups in the dumbbell components of [2]rotaxanes, are susceptible to changes in pole-dipole and dipole-dipole interactions within a redox-active pillar[5]quinone ring component when subjected to redox control. Distinct from free pillar[5]quinone, these molecular compasses exhibit a 1-1-1-1-1 electron-uptake pattern during the first-electron transfers. Density functional theory (DFT) calculations reveal that, upon the reduction of quinoid units in the amide-based molecular compass, the positive end of the dipole moment pointer in the dumbbell component becomes oriented toward the reduced anionic quinone in the ring component, courtesy of hydrogen bonding. The negative end of the dipole moment pointer in the dumbbell component lowers the reduction potentials of the other four quinoid units as a result of electrostatic repulsions, which explain its 1-1-1-1-1 electron-uptake pattern. Our findings highlight how electronic communication between the dipole moment pointers and the quinoid units in the ring component enables the [2]rotaxane to act as a molecular compass, precisely reorienting its dipole moments in response to redox changes.

Multigram Scale Synthesis of Mechanically-Interlocked Derivatives of SWNT using Mechanochemical Methods

The grinding of chemical reagents enables mixing, promotes molecular collisions, and provides the thermal energy required for chemical reactions, while reducing the need for solvent (often to none) and significantly speeding up reactions. This has made mechanochemistry a powerful alternative to traditional solution chemistry. Here, we show that mechanically interlocked derivatives of single-walled carbon nanotubes (MINTs) can be made via mechanochemistry in a multigram scale. Compared to the previously reported method in suspension, mechanochemistry allows us to reduce the amount of solvent by two orders of magnitude and the reaction time from 72 h to 5 min. The mechanochemical synthesis of MINTs is proven to work both with purified (6,5)-SWNTs and affordable TuballTM SWNTs, enabling the cheap, fast, and environmentally friendly multigram scale synthesis of MINTs. With this new synthetic methodology, we open the door to the real-world applications of MINTs in fields such as polymer composites.

Biphasic mechanochemistry of single-chain polymerization

Mechanical forces can induce chemical reactions, produce chemical signals, and alter reaction kinetics. Here, using magnetic tweezers-based single-molecule force spectroscopy, we study the force effects on the ring-opening metathesis polymerization (ROMP) of single-polymer chains, during which nonequilibrium conformational entanglements can form and unravel stochastically. We find a surprising, biphasic force dependence of polymerization kinetics: The single-chain polymerization rate initially slows down with increasing stretching forces, reaching a minimum, and then accelerates at higher forces. Analysis of real-time single-chain growth trajectories allows for dissecting the polymerization process into two distinct regimes, one with and the other without entanglement formation, unveiling the biphasic force dependence in both regimes. Two different mechanisms likely operate for the biphasic dependence: a force-induced entanglement tightening and then splitting and a force-induced catalyst structural distortion that switches the reaction pathway between reactant states of different stability and reactivity. These findings and insights point to opportunities of using force to manipulate polymerization reactions and tune the physiochemical properties of synthetic polymers.

Diastereoselective Polypseudorotaxane Formation with Planar Chiral Pillar[5]arenes via Co-crystallization Processes

As the number of chiral ring molecules in chiral polyrotaxane increases, the number of possible stereoisomers exponentially increases. Consequently, the selective synthesis of a specific stereoisomer becomes much more challenging. To address this problem, we co-crystallized poly(ethylene glycol) and a diastereomeric ring molecule, pillar[5]arene, in the solid state. The co-crystallization formed polypseudorotaxanes with a high diastereomeric excess (ca. 88 % de), meaning that polypseudorotaxanes containing (S, pS) stereoisomer pillar[5]arene rings were synthesized selectively. By contrast, in solution and evaporation systems, the selectivity remained low (ca. 10 % de). The results suggested that the packing effect by the co-crystallization contributed to the denser assembly of ring molecules on the polymeric chain, resulting in the diastereoselective formation. High diastereoselectivity was also observed even in higher-molecular-weight poly(ethylene glycol)s. These selectivities arose from the cooperative effects of the ring molecules on the polymeric chain, which were supported by calculating the stabilization energy.

Precise assembly/disassembly of homo-type and hetero-type macrocycles with photoresponsive and non-photoresponsive dynamic covalent bonds

Dynamic covalent macrocycles offer the advantage of tunable ring-opening/ring-closure and structural transformation, but their control with precision remains a daunting task due to the labile nature of reversible bonds. Herein we demonstrate the precise formation/scission of covalent macrocycles with varied sizes by contrasting the reactivity, stability, and degradability of light-active and light-inactive dynamic covalent bonds. The incorporation of photoswitchable and non-photoresponsive aldehyde sites into one single dialdehyde component afforded the creation of [1 + 1] type macrocycles with primary diamines of suitable lengths. The manipulation of light and acid/base stimuli allowed on-demand breaking/remaking of macrocycles, achieving the interconversion between macrocyclic and linear skeletons. Moreover, a combination of the dialdehyde, primary diamines, and secondary diamines enabled the construction of hetero-type [2 + 1 + 1'] macrocycles via enhanced discrimination and hierarchical assembly. Light-induced kinetic locking/unlocking of dynamic bonds further afforded macrocycle-to-macrocycle conversion when needed. Through leveraging controllable covalent connection/disconnection, switchable formation/disintegration of mechanically interlocked catenanes was further accomplished. The results described showcase the potential of photoinduced dynamic covalent chemistry for preparing complex architectures and should set the stage for molecular recognition, dynamic assemblies, synthetic motors, and responsive materials.

Scalable Mechanochemical Synthesis of Biotin[6]uril

Biotin[6]uril, a chiral, water-soluble and anion binding macrocycle, is formed via dynamic covalent chemistry. In this study, we present a scalable and high-yielding synthesis of biotin[6]uril via a mechanochemical solid-state approach. The optimized protocol involves mechanical grinding of solid d-biotin with paraformaldehyde in the presence of 0.3 equivalents of 48 % aqueous HBr, which functions as a catalyst, template, and liquid grinding additive. This mechanochemical process is carried out in a shaker or planetary mill, followed by aging at an elevated temperature to produce biotin[6]uril with an HPLC yield of up to 96 %. The condensation and macrocyclization reaction was successfully scaled up 82-fold, producing nearly 20 g of biotin[6]uril with a high 92 % isolated yield and 91 % purity. Compared to conventional solution-based method, this mechanochemical approach offers several advantages, including significantly higher yields, shorter reaction times, enhanced scalability, simpler operational requirements, and substantially lower process mass intensity.

Cucurbit[8]uril-Mediated Supramolecular Heterodimerisation and Photoinduced [2+2] Heterocycloaddition to Generate Unexpected [2]Rotaxanes

In contrast to the self-assembly of homosupramolecules, the self-assembly of heterosupramolecules is more challenging and significant in various fields. Herein, we design and investigate a cucurbit[8]uril-mediated heterodimerisation based on an arene-fluoroarene strategy. Furthermore, the heteroternary complex is found to be able to undergo a photoinduced [2+2] heterocycloaddition, resulting in the formation of an unexpected [2]rotaxane. This work demonstrates a novel supramolecular heterodimerisation system that not only contributes to the development of photoisomerisation systems, but also enriches synthetic methods for mechanically interlocked molecules.

Mechanocatalytic Synthesis of Ammonia by Titanium Dioxide with Bridge-Oxygen Vacancies: Investigating Mechanism from the Experimental and First-Principle Approach

Mechanochemical ammonia (NH3) synthesis is an emerging mild approach derived from nitrogen (N2) gas and hydrogen (H) source. The gas-liquid phase mechanochemical process utilizes water (H2O), rather than conventional hydrogen (H2) gas, as H sources, thus avoiding carbon dioxide (CO2) emission during H2 production. However, ammonia yield is relatively low to meet practical demand due to huge energy barriers of N2 activation and H2O dissociation. Here, six transition metal oxides (TMO) such as titanium dioxide (TiO2), iron(III) oxide (Fe2O3), copper(II) oxide (CuO), niobium(V) oxide(Nb2O5), zinc oxide (ZnO), and copper(I) oxide (Cu2O) are investigated as catalysts in mechanochemical N2 fixation. Among them, TiO2 shows the best mechanocatalytic effect and the optimum reaction rate constant is 3.6-fold higher than the TMO-free process. The theoretical calculations show that N2 molecules prefer to side-on chemisorb on the mechano-induced bridge-oxygen vacancies in the (101) crystal plane of TiO2 catalyst, while H2O molecules can dissociate on the same sites more easily to provide free H atoms, enabling an alternative-way hydrogeneration process of activated N2 molecules to release NH3 eventually. This work highlights the cost-effective TiO2 mechanocatalyst for ammonia synthesis under mild conditions and proposes a defect-engineering-induced mechanocatalytic mechanism to promote N2 activation and H2O dissociation.

Adaptisorption of Nonporous Polymer Crystals

Although our knowledge and understanding of adsorptions in natural and artificial systems has increased dramatically during the past century, adsorption associated with nonporous polymers remains something of a mystery, hampering applications. Here we demonstrate a model system for adaptisorption of nonporous polymers, wherein dative B-N bonds and host-guest binding units act as the kinetic and thermodynamic components, respectively. The coupling of these two components enables nonporous polymer crystals to adsorb molecules from solution and undergo recrystallization as thermodynamically favored crystals. Adaptisorption of nonporous polymer crystals not only extends the types of adsorption in which the sorbate molecules are integrated in a precise and orderly manner in the sorbent systems, but also provides a facile and accurate approach to the construction of polymeric materials with precise architectures and integrated functions.

Solution and Solvent-Free Stopper Exchange Reactions for the Preparation of Pillar[5]arene-containing [2] and [3]Rotaxanes

Diamine reagents have been used to functionalize a [2]rotaxane building block bearing an activated pentafluorophenyl ester stopper. Upon a first acylation, an intermediate host-guest complex with a terminal amine function is obtained. Dissociation of the intermediate occurs in solution and acylation of the released axle generates a [2]rotaxane with an elongated axle subunit. In contrast, the corresponding [3]rotaxane can be obtained if the reaction conditions are appropriate to stabilize the inclusion complex of the mono-amine intermediate and the pillar[5]arene. This is the case when the stopper exchange is performed under mechanochemical solvent-free conditions. Alternatively, if the newly introduced terminal amide group is large enough to prevent the dissociation, the second acylation provides exclusively a [3]rotaxane. On the other hand, detailed conformational analysis has been also carried out by variable temperature NMR investigations. A complete understanding of the shuttling motions of the pillar[5]arene subunit along the axles of the rotaxanes reported therein has been achieved with the help of density functional theory calculations.

Diastereoselective Rotaxane Synthesis with Pillar[5]arenes via Co-crystallization and Solid-State Mechanochemical Processes

Chiral rotaxanes have attracted much attention in recent decades for their unique chirality based on their interlocked structures. Thus, selective synthesis methods of chiral rotaxanes have been developed. The introduction of substituents with chiral centers to produce diastereomers is a powerful strategy for the construction of chiral rotaxanes. However, in case of a small energy difference between the diastereomers, diastereoselective synthesis is extremely difficult. Herein, we report a new diastereoselective rotaxane synthesis method using solid-phase diastereoselective [3]pseudorotaxane formation and mechanochemical solid-phase end-capping reactions of the [3]pseudorotaxanes. By co-crystallization of stereodynamic planar chiral pillar[5]arene with stereogenic carbons at both rims and axles with suitable end groups and lengths, the [3]pseudorotaxane with a high diastereomeric excess (ca. 92% de) was generated in the solid state because of higher effective molarity with aid by packing effects and significant energy differences between [3]pseudorotaxane diastereomers. In contrast, the de of the pillar[5]arene was low in solution (ca. 10% de) because of a small energy difference between diastereomers. Subsequent end-capping reactions of the polycrystalline [3]pseudorotaxane with high de in solvent-free conditions successfully yielded rotaxanes while maintaining the high de generated by the co-crystallization.

Pillar[5]arene based water-soluble [3]pseudorotaxane with enhanced fluorescence emission for cell imaging and both type I and II photodynamic cancer therapy

Water-soluble [3]pseudorotaxane with enhanced fluorescence emission was successfully constructed and applied in cell imaging and photodynamic cancer therapy.

Comparison of the Conventional and Mechanochemical Syntheses of Cyclodextrin Derivatives

Many scientists are working hard to find green alternatives to classical synthetic methods. Today, state-of-the-art ultrasonic and grinding techniques already drive the production of organic compounds on an industrial scale. The physicochemical and chemical behavior of cyclodextrins often differs from the typical properties of classic organic compounds and carbohydrates. The usually poor solubility and complexing properties of cyclodextrins can require special techniques. By eliminating or reducing the amount of solvent needed, green alternatives can reform classical synthetic methods, making them attractive for environmentally friendly production and the circular economy. The lack of energy-intensive synthetic and purification steps could transform currently inefficient processes into feasible methods. Mechanochemical reaction mechanisms are generally different from normal solution-chemistry mechanisms. The absence of a solvent and the presence of very high local temperatures for microseconds facilitate the synthesis of cyclodextrin derivatives that are impossible or difficult to produce under classical solution-chemistry conditions. Although mechanochemistry does not provide a general solution to all problems, several good examples show that this new technology can open up efficient synthetic pathways.