Switching between Anion-Binding Catalysis and Aminocatalysis with a Rotaxane Dual-Function Catalyst

Urea-Based [2]Rotaxanes as Effective Phase-Transfer Organocatalysts: Hydrogen-Bonding Cooperative Activation Enabled by the Mechanical Bond

We finely designed a set of [2]rotaxanes with urea threads and tested them as hydrogen-bonding phase-transfer catalysts in two different nucleophilic substitutions requiring the activation of the reactant fluoride anion. The [2]rotaxane bearing a fluorinated macrocycle and a fluorine-containing urea thread displayed significantly enhanced catalytic activity in comparison with the combination of both noninterlocked components. This fact highlights the notably beneficial role of the mechanical bond, cooperatively activating the processes through hydrogen-bonding interactions.

Molecular folding governs switchable singlet oxygen photoproduction in porphyrin-decorated bistable rotaxanes

Rotaxanes are mechanically interlocked molecules where a ring (macrocycle) is threaded onto a linear molecule (thread). The position of the macrocycle on different stations on the thread can be controlled in response to external stimuli, making rotaxanes applicable as molecular switches. Here we show that bistable rotaxanes based on the combination of a Zn(II) tetraphenylporphyrin photosensitizer, attached to the macrocycle, and a black-hole-quencher, attached to the thread, are capable of singlet oxygen production which can be switched on/off by the addition of base/acid. However, we found that only a sufficiently long linker between both stations on the thread enabled switchability, and that the direction of switching was inversed with regard to the original design. This unexpected behavior was attributed to intramolecular folding of the rotaxanes, as indicated by extensive theoretical calculations. This evidences the importance to take into account the conformational flexibility of large molecular structures when designing functional switchable systems.

Dynamic control of circumrotation of a [2]catenane by acid-base switching

Dynamic control of the motion in a catenane remains a big challenge as it requires precise design and sophisticated well-organized structures. This paper reports the design and synthesis of a donor-acceptor [2]catenane through mechanical interlocking, employing a crown ether featuring two dibenzylammonium salts on its side arms as the host and a cyclobis(paraquat-p-phenylene) (CBPQT ⋅ 4PF6) ring as the guest molecule. By addition of external acid or base, the catenane can form self-complexed or decomplexed compounds to alter the cavity size of the crown ether ring, consequently affecting circumrotation rate of CBPQT ⋅ 4PF6 ring of the catenane. This study offers insights for the design and exploration of artificial molecular machines with intricate cascading responsive mechanisms.

Kinetically controlled synthesis of rotaxane geometric isomers

Geometric isomerism in mechanically interlocked systems-which arises when the axle of a mechanically interlocked molecule is oriented, and the macrocyclic component is facially dissymmetric-can provide enhanced functionality for directional transport and polymerization catalysis. We now introduce a kinetically controlled strategy to control geometric isomerism in [2]rotaxanes. Our synthesis provides the major geometric isomer with high selectivity, broadening synthetic access to such interlocked structures. Starting from a readily accessible [2]rotaxane with a symmetrical axle, one of the two stoppers is activated selectively for stopper exchange by the substituents on the ring component. High selectivities are achieved in these reactions, based on coupling the selective formation reactions leading to the major products with inversely selective depletion reactions for the minor products. Specifically, in our reaction system, the desired (major) product forms faster in the first step, while the undesired (minor) product subsequently reacts away faster in the second step. Quantitative 1H NMR data, fit to a detailed kinetic model, demonstrates that this effect (which is conceptually closely related to minor enantiomer recycling and related processes) can significantly improve the intrinsic selectivity of the reactions. Our results serve as proof of principle for how multiple selective reaction steps can work together to enhance the stereoselectivity of synthetic processes forming complex mechanically interlocked molecules.

BP23C7: high-yield synthesis and application in constructing [3]rotaxanes and responsive pseudo[2]rotaxanes

A biphenyl-23-crown-7 ether (BP23C7) is synthesized in 86% yield from commercially available starting materials. BP23C7 forms pseudo[2]rotaxane with a dibenzylammonium ion (DBA+), exhibiting a good association constant value (ka = 1 × 103 M-1). Subsequently, fluorophoric properties of BP23C7 and anthracene terminated axles are blended to create responsive pseudo[2]rotaxanes. The "turn-on" fluorescence response of BP23C7 due to the addition of fluoride and chloride anions to pseudo[2]rotaxane systems has been investigated. Concomitant fluorescence quenching of the anthracene moiety of corresponding axles due to ion-pair formation has been addressed. Furthermore, two variants of [23]crown ethers, i.e. BP23C7 and o-xylene-23-crown-7 ether (X23C7), are applied for constructing homo[3]rotaxane architectures. A half-axle comprising of DBA+ moiety and a terminal olefin is mixed separately with two [23]crown ethers and subjected to self-metathesis using Grubbs' first-generation catalyst.

Controlled and Regioselective Ring-Opening Polymerization for Poly(disulfide)s by Anion-Binding Catalysis

Poly(disulfide)s are an emerging class of sulfur-containing polymers with applications in medicine, energy, and functional materials. However, the constituent dynamic covalent S-S bond is highly reactive in the presence of the sulfide (RS-) anion, imposing a persistent challenge to control the polymerization. Here, we report an anion-binding approach to arrest the high reactivity of the RS- chain end to control the synthesis of linear poly(disulfide)s, realizing a rapid, living ring-opening polymerization of 1,2-dithiolanes with narrow dispersity and high regioselectivity (Mw/Mn ∼ 1.1, Ps ∼ 0.85). Mechanistic studies support the formation of a thiourea-base-sulfide ternary complex as the catalytically active species during the chain propagation. Theoretical analyses reveal a synergistic catalytic model where the catalyst preorganizes the protonated base and anionic chain end to establish spatial confinement over the bound monomer, effecting the observed regioselectivity. The catalytic system is amenable to monomers with various functional groups, and semicrystalline polymers are also obtained from lipoic acid derivatives by enhancing the regioselectivity.

Solvent Acts as the Referee in a Match-Up Between Charged and Preorganized Receptors

The prevalence of anion-cation contacts in biomolecular recognition under aqueous conditions suggests that ionic interactions should dominate the binding of anions in solvents across both high and low polarities. Investigations of this idea using titrations in low polarity solvents are impaired by interferences from ion pairing that prevent a clear picture of binding. To address this limitation and test the impact of ion-ion interactions across multiple solvents, we quantified chloride binding to a cationic receptor after accounting for ion pairing. In these studies, we created a chelate receptor using aryl-triazole CH donors and a quinolinium unit that directs its cationic methyl inside the binding pocket. In low-polarity dichloromethane, the 1 : 1 complex (log K1 : 1 ~ 7.3) is more stable than neutral chelates, but fortuitously comparable to a preorganized macrocycle (log K1 : 1 ~ 6.9). Polar acetonitrile and DMSO diminish stabilities of the charged receptor (log K1 : 1 ~ 3.7 and 1.9) but surprisingly 100-fold more than the macrocycle. While both receptors lose stability by dielectric screening of electrostatic stability, the cationic receptor also pays additional costs of organization. Thus even though the charged receptor has stronger binding in apolar solvents, the uncharged receptor has more anion affinity in polar solvents.

Interplay between anti-anti and syn-anti conformations of thiourea modulating ON-OFF catalysis

The design, synthesis and operation of a readily accessible two-state switch are demonstrated. The switch initially exists in an intramolecularly hydrogen-bonded self-locked state, as evidenced by the solution-state NMR and solid-state structure. The switch can be reversibly altered between anti-anti and syn-anti conformations by adding and removing Cu+ ions, as evidenced by the NMR and crystallographic study. The anti-anti form was found to be catalytically active in the Michael addition reaction, whereas the syn-anti form was catalytically inactive.

An Anion-Switchable Dual-Function Rotaxane Catalyst

In situ switching of the associated anions of a rotaxane catalyst between Cl- and TFPB- exposes its dialkylammonium and imidazolium stations, respectively, thereby selectively catalyzing the reactions of a mixture of trans-cinnamaldehyde and an aliphatic thiol to yield the Michael adduct and the thioacetal product, respectively.

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.

Air-Stable Radical Organic Cages as Cascade Nanozymes for Enhanced Catalysis

The pursuit of single-assembled molecular cage reactors for complex tandem reactions is a long-standing target in biomimetic catalysis but still a grand challenge. Herein, nanozyme-like organic cages are reported by engineering air-stable radicals into the skeleton upon photoinduced electron transfer. The generation of radicals is accompanied by single-crystal structural transformation and exhibits superior stability over six months in air. Impressively, the radicals throughout the cage skeleton can mimic the peroxidase of natural enzymes to decompose H₂ O₂ into OH· and facilitate oxidation reactions. Furthermore, an integrated catalyst by encapsulating Au clusters (glucose oxidase mimics) into the cage has been developed, in which the dual active sites (Au cluster and radical) are spatially isolated and can work as cascade nanozymes to prominently promote the enzyme-like tandem reaction via a substrate channeling effect.

Distinctive features and challenges in catenane chemistry

From being an aesthetic molecular object to a building block for the construction of molecular machines, catenanes and related mechanically interlocked molecules (MIMs) continue to attract immense interest in many research areas. Catenane chemistry is closely tied to that of rotaxanes and knots, and involves concepts like mechanical bonds, chemical topology and co-conformation that are unique to these molecules. Yet, because of their different topological structures and mechanical bond properties, there are some fundamental differences between the chemistry of catenanes and that of rotaxanes and knots although the boundary is sometimes blurred. Clearly distinguishing these differences, in aspects of bonding, structure, synthesis and properties, between catenanes and other MIMs is therefore of fundamental importance to understand their chemistry and explore the new opportunities from mechanical bonds.

Anion-π Catalysis Enabled by the Mechanical Bond

We report a series of rotaxane‐based anion–π catalysts in which the mechanical bond between a bipyridine macrocycle and an axle containing an NDI unit is intrinsic to the activity observed, including a [3]rotaxane that catalyses an otherwise disfavoured Michael addition in >60 fold selectivity over a competing decarboxylation pathway that dominates under Brønsted base conditions. The results are rationalized by detailed experimental investigations, electrochemical and computational analysis.

Recent Progress in Light-Driven Molecular Shuttles

Molecular shuttles are typical molecular machines that could be applied in various fields. The motion modes of wheel components in rotaxanes could be strategically modulated by external stimuli, such as pH, ions, solvent, light, and so on. Light is particularly attractive because it is harmless and can be operated in a remote mode and usually no byproducts are formed. Over the past decade, many examples of light-driven molecular shuttles are emerging. Accordingly, this review summarizes the recent research progress of light-driven molecular shuttles. First, the light-driven mechanisms of molecular motions with different functional groups are discussed in detail, which show how to drive photoresponsive or non-photoresponsive molecular shuttles. Subsequently, the practical applications of molecular shuttles in different fields, such as optical information storage, catalysis for organic reactions, drug delivery, and so on, are demonstrated. Finally, the future development of light-driven molecular shuttle is briefly prospected.

Controlling catalyst activity, chemoselectivity and stereoselectivity with the mechanical bond

Mechanically interlocked molecules, such as rotaxanes and catenanes, are receiving increased attention as scaffolds for the development of new catalysts, driven by both their increasing accessibility and high-profile examples of the mechanical bond delivering desirable behaviours and properties. In this Review, we survey recent advances in the catalytic applications of mechanically interlocked molecules organized by the effect of the mechanical bond on key catalytic properties, namely, activity, chemoselectivity and stereoselectivity, and focus on how the mechanically bonded structure leads to the observed behaviour. Our aim is to inspire future investigations of mechanically interlocked catalysts, including those outside of the supramolecular community.

A light-gated regulation of the reaction site by a cucurbit[7]uril macrocycle

On–off competitive inhibition is presented. Photoswitchable pseudorotaxane controls the rate of self-reaction and product selectivity of external reactions.

Modulating the Catalytic Activity by the Mechanical Bond: Organocatalysis with Polyamide [2]Rotaxanes bearing a Secondary Amino Function at the Thread

The modulation of the catalytic activity of degenerate succinamide-based [2]rotaxanes by changes at their macrocyclic component is disclosed herein. These systems, bearing an acyclic secondary amine function at the thread...

Post-Synthetic Macrocyclization of Rotaxane Building Blocks

Although not often encountered, cyclic interlocked molecules are appealing molecular targets because of their restrained tridimensional structure which is related to both the cyclic and interlocked shapes. Interlocked molecules such as rotaxane building blocks may be good candidates for post-synthetic intramolecular cyclization if the preservation of the mechanical bond ensures the interlocked architecture throughout the reaction. This is obviously the case if the modification does not involve the cleavage of either the macrocycle's main chain or the encircled part of the axle. However, among the post-synthetic reactions, the chemical linkage between two reactive sites belonging to embedded elements of rotaxanes still consists of an underexploited route to interlocked cyclic molecules. This Review lists the rare examples of macrocyclization through chemical connection between reactive sites belonging to a surrounding macrocycle and/or an encircled axle of interlocked rotaxanes.

Halides as versatile anions in asymmetric anion-binding organocatalysis

This review intends to provide an overview on the role of halide anions in the development of the research area of asymmetric anion-binding organocatalysis. Key early elucidation studies with chloride as counter-anion confirmed this type of alternative activation, which was then exploited in several processes and contributed to the advance and consolidation of anion-binding catalysis as a field. Thus, the use of the halide in the catalyst–anion complex as both a mere counter-anion spectator or an active nucleophile has been depicted, along with the new trends toward additional noncovalent contacts within the HB-donor catalyst and supramolecular interactions to both the anion and the cationic reactive species.

New horizons for catalysis disclosed by supramolecular chemistry

The adoption of a supramolecular approach in catalysis promises to address a number of unmet challenges, ranging from activity (unlocking of novel reaction pathways) to selectivity (alteration of the innate selectivity of a reaction, e.g. selective functionalization of C-H bonds) and regulation (switch ON/OFF, sequential catalysis, etc.). Supramolecular tools such as reversible association and recognition, pre-organization of reactants and stabilization of transition states upon binding offer a unique chance to achieve the above goals disclosing new horizons whose potential is being increasingly recognized and used, sometimes reaching the degree of ripeness for practical use. This review summarizes the main developments that have opened such new frontiers, with the aim of providing a guide to researchers approaching the field. We focus on artificial supramolecular catalysts of defined stoichiometry which, under homogeneous conditions, unlock outcomes that are highly difficult if not impossible to attain otherwise, namely unnatural reactivity or selectivity and catalysis regulation. The different strategies recently explored in supramolecular catalysis are concisely presented, and, for each one, a single or very few examples is/are described (mainly last 10 years, with only milestone older works discussed). The subject is divided into four sections in light of the key design principle: (i) nanoconfinement of reactants, (ii) recognition-driven catalysis, (iii) catalysis regulation by molecular machines and (iv) processive catalysis.

N,N-Dialkylhydrazones as Versatile Umpolung Reagents in Enantioselective Anion-Binding Catalysis

An enantioselective anion-binding organocatalytic approach with versatile N,N-dialkylhydrazones (DAHs) as polarity-reversed (umpolung) nucleophiles is presented. For the application of this concept, a highly ordered hydrogen-bond (HB) network between a carefully selected CF3 -substituted triazole-based multidentate HB-donor catalyst, the ionic substrate and the hydrazone in a supramolecular chiral ion-pair complex was envisioned. The formation of such a network was further supported by both experimental and computational studies, which showed the crucial role of the anion as a template unit. The asymmetric Reissert-type reaction of quinolines as a model test reaction chemoselectively delivered highly enantiomerically enriched hydrazones (up 95:5 e.r.) that could be further derivatized to value-added compounds with up to three stereocenters.

Functional Rotaxanes in Catalysis

Mechanically interlocked molecules (MIMs) have gained attention in the field of catalysis due to their unique molecular properties. Central to MIMs, rotaxanes are highly promising and attractive supramolecular catalysts due to their unique three-dimensional structures and the flexibility of their subcomponents. This Minireview discusses the use of rotaxanes in organocatalysis and transition-metal catalysis.

Mechanical bonding activation in rotaxane-based organocatalysts

Interlocked organocatalysts show enhanced catalytic performance when compared with their non-interlocked threads.The ring cooperatively activates the substrates, facilitating the formation and stabilization of catalytically active intermediates.

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.

Insight into the Folding and Cooperative Multi-Recognition Mechanism in Supramolecular Anion-Binding Catalysis

H-bond donor catalysts able to modulate the reactivity of ionic substrates for asymmetric reactions have gained great attention in the past years, leading to the development of cooperative multidentate H-bonding supramolecular structures. However, there is still a lack of understanding of the forces driving the ion recognition and catalytic performance of these systems. Herein, insight into the cooperativity nature, anion binding strength, and folding mechanism of a model chiral triazole catalyst is presented. Our combined experimental and computational study revealed that multi-interaction catalysts exhibiting weak binding energies (≈3-4 kcal mol^-1 ) can effectively recognize ionic substrates and induce chirality, while strong dependencies on the temperature and solvent were quantified. These results are key for the future design of catalysts with optimal anion binding strength and catalytic activity in target reactions.

Guest-Reaction Driven Cage to Conjoined Twin-Cage Mitosis-Like Host Transformation

We report here a guest-reaction-induced mitosis-like host transformation from a known Pd₄ L₂ cage 1 to a conjoined Pd₆ L₃ twin-cage 2 featuring two separate cavities. The encapsulation of 1-hydroxymethyl-2-naphthol (G1), a known ortho-quinone methide (o-QMs) precursor, within the hydrophobic cavity of cage 1 is found crucial to realize the cage to twin-cage conversion. Confined G1 molecules within the nanocavity undergo self-coupling dimerization reaction to form 2,2'-dihydroxy-1,1'-dinaphthylmethane (G2) which then triggers the cage to twin-cage mitosis. The same conversion also proceeds, in a much faster rate, via the direct templation of G2, confirming the induced-fit transformation mechanism. The structure of the (G2)₂ ⊂2 host-guest complex has been established by X-ray crystallographic study, where cis- to trans- conformational switch on one bridging ligand is revealed.

Construction of pillar[4]arene[1]quinone-1,10-dibromodecane pseudorotaxanes in solution and in the solid state

We report novel pseudorotaxanes based on the complexation between pillar[4]arene[1]quinone and 1,10-dibromodecane. The complexation is found to have a 1:1 host–guest complexation stoichiometry in chloroform but a 2:1 host–guest complexation stoichiometry in the solid state. From single crystal X-ray diffraction, the linear guest molecules thread into cyclic pillar[4]arene[1]quinone host molecules in the solid state, stabilized by CH∙∙∙π interactions and hydrogen bonds. The bromine atoms at the periphery of the guest molecule provide convenience for the further capping of the pseudorotaxanes to construct rotaxanes.

Time Programmable Locking/Unlocking of the Calix[4]arene Scaffold by Means of Chemical Fuels

In this work, we report that 2-cyano-2-phenylpropanoic acid and its p-Cl, p-CH₃ and p-OCH₃ derivatives can be used as chemical fuels to control the geometry of the calix[4]arene scaffold in its cone conformation. It is shown that, under the action of the fuel, the cone calix[4]arene platform assumes a "locked" shape with two opposite aromatic rings strongly convergent and the other two strongly divergent ("pinched cone" conformation). Only when the fuel is exhausted, the cone calix[4]arene scaffold returns to its resting, "unlocked" shape. Remarkably, the duration of the "locked" state can be controlled at will by varying the fuel structure or amount. A kinetic study of the process shows that the consume of the fuel is catalyzed by the "unlocked" calixarene that behaves as an autocatalyst for its own production. A mechanism is proposed for the reaction of fuel consumption.

Cross dehydrogenative C-O coupling catalysed by a catenane-coordinated copper(i)

Catalytic activity of copper(i) complexes supported by phenanthroline-containing catenane ligands towards a new C(sp³)–O dehydrogenative cross-coupling of phenols and bromodicarbonyls is reported. As the phenanthrolines are interlocked by the strong and flexible mechanical bond in the catenane, the active catalyst with an open copper coordination site can be revealed only transiently and the stable, coordinatively saturated Cu(i) pre-catalyst is quickly regenerated after substrate transformation. Compared with a control Cu(i) complex supported by non-interlocked phenanthrolines, the catenane-supported Cu(i) is highly efficient with a broad substrate scope, and can be applied in gram-scale transformations without a significant loss of the catalytic activity. This work demonstrates the advantages of the catenane ligands that provide a dynamic and responsive copper coordination sphere, highlighting the potential of the mechanical bond as a design element in transition metal catalyst development.

The use of a catenane-supported copper(i) complex for the cross dehydrogenative C–O coupling of phenols and bromodicarbonyls is described.

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.

Precision Molecular Threading/Dethreading

The general principles guiding the design of molecular machines based on interlocked structures are well known. Nonetheless, the identification of suitable molecular components for a precise tuning of the energetic parameters that determine the mechanical link is still challenging. Indeed, what are the reasons of the "all-or-nothing" effect, which turns a molecular "speed-bump" into a stopper in pseudorotaxane-based architectures? Here we investigate the threading and dethreading processes for a representative class of molecular components, based on symmetric dibenzylammonium axles and dibenzo[24]crown-8 ether, with a joint experimental-computational strategy. From the analysis of quantitative data and an atomistic insight, we derive simple rules correlating the kinetic behaviour with the substitution pattern, and provide rational guidelines for the design of modules to be integrated in molecular switches and motors with sophisticated dynamic features.

Hydrogen-Mediated Noncovalent Interactions in Solids: What Can NMR Crystallography Tell About?

Hydrogen atoms play a crucial role in the aggregation of organic (bio)molecules through diverse number of noncovalent interactions that they mediate, such as electrostatic in proton transfer systems, hydrogen bonding, and CH-π interactions, to mention only the most prominent. To identify and adequately describe such low-energy interactions, increasingly sensitive methods have been developed over time, among which quantum chemical computations have witnessed impressive advances in recent years. For reaching the present state-of-the-art, computations had to rely on a pool of relevant experimental data, needed at least for validation, if not also for other purposes. In the case of molecular crystals, the best illustration for the synergy between computations and experiment is given by the so-called NMR crystallography approach. Originally designed to increase the confidence level in crystal structure determination of organic compounds from powders, NMR crystallography is able now to offer also a wealth of information regarding the noncovalent interactions that drive molecules to pack in a given crystalline pattern or another. This is particularly true for the noncovalent interactions which depend on the exact location of labile hydrogen atoms in the system: in such cases, NMR crystallography represents a valuable characterization tool, in some cases complementing even the standard single-crystal X-ray diffraction technique. A concise introduction in the field is made in this mini-review, which is aimed at providing a comprehensive picture with respect to the current accuracy level reached by NMR crystallography in the characterization of hydrogen-mediated noncovalent interactions in organic solids. Different types of practical applications are illustrated with the example of molecular crystals studied by our research group, but references to other representative developments reported in the literature are also made. By summarizing the major concepts and methodological progresses, the present work is also intended to be a guide to the practical potential of this relatively recent analytical tool for the scientists working in areas where crystal engineering represents the main approach for rational design of novel materials.

Transformation Network Culminating in a Heteroleptic Cd6L6L'2 Twisted Trigonal Prism

Transformations between three-dimensional metallosupramolecular assemblies can enable switching between the different functions of these structures. Here we report a network of such transformations, based upon a subcomponent displacement strategy. The flow through this network is directed by the relative reactivities of different amines, aldehydes, and di(2-pyridyl)ketone. The network provides access to an unprecedented heteroleptic Cd6L6L'2 twisted trigonal prism. The principles underpinning this network thus allow for the design of diverse structural transformations, converting one helicate into another, a helicate into a tetrahedron, a tetrahedron into a different tetrahedron, and a tetrahedron into the new trigonal prismatic structure type. The selective conversion from one host to another also enabled the uptake of a desired guest from a mixture of guests.

Functionalized Macrocycles in Supramolecular Organocatalysis

Supramolecular organocatalysis has emerged as a novel research field in the context of homogeneous catalysis. In particular, the use of functionalized macrocycles as supramolecular catalysts is highly promising, as these systems are oftentimes easily accessible and offer distinct advantages in catalysis. Macrocyclic catalysts can provide defined binding pockets, such as hydrophobic cavities, and can thus create a reaction microenvironment for catalysis. In addition, macrocycles can offer a preorganized arrangement of functional groups, such as binding sites or catalytically active groups, thus enabling a defined and possibly multivalent binding and activation of substrates. The aim of this Minireview is to provide an overview of recent advances in the area of supramolecular organocatalysis based on functionalized macrocycles (including cyclodextrins, calixarenes, and resorcinarenes), with a focus on those examples where certain catalytically active groups (such as hydrogen bond donors/acceptors, Brønsted acid or base groups, or nucleophilic units) are present in or have been installed on the macrocycles.

N-Heterocyclic Carbene Copper(I) Rotaxanes Mediate Sequential Click Ligations with All Reagents Premixed

We have prepared NHC-Cu^I complexes with a rotaxane structure and used them as sterically sensitive catalysts for one-pot sequential copper-catalyzed azide/alkyne cycloadditions in solutions containing all of the coupling partners premixed in unprotected form. Most notably, a photolabile and sterically encumbered complex first catalyzed the coupling of a less bulky azide/alkyne pair; after removing the protective macrocyclic component from the rotaxane structure, through irradiation with light, the exposed dumbbell-shaped NHC-Cu^I complex catalyzed the second click reaction of a bulkier azide/alkyne pair. Using this approach, we obtained predominantly, from a single sealed pot, a bis-triazole product (84 %) from a mixture of two sterically distinct azides and a diyne.

Improved Acid Resistance of a Metal-Organic Cage Enables Cargo Release and Exchange between Hosts

The use of di(2-pyridyl)ketone in subcomponent self-assembly is introduced. When combined with a flexible triamine and zinc bis(trifluoromethanesulfonyl)imide, this ketone formed a new Zn4 L4 tetrahedron 1 bearing twelve uncoordinated pyridyl units around its metal-ion vertices. The acid stability of 1 was found to be greater than that of the analogous tetrahedron 2 built from 2-formylpyridine. Intriguingly, the peripheral presence of additional pyridine rings in 1 resulted in distinct guest binding behavior from that of 2, affecting guest scope as well as binding affinities. The different stabilities and guest affinities of capsules 1 and 2 enabled the design of systems whereby different cargoes could be moved between cages using acid and base as chemical stimuli.

Luminescent Anion Sensing by Transition-Metal Dipyridylbenzene Complexes Incorporated into Acyclic, Macrocyclic and Interlocked Hosts

A series of novel acyclic, macrocyclic and mechanically interlocked luminescent anion sensors have been prepared by incorporation of the isophthalamide motif into dipyridylbenzene to obtain cyclometallated complexes of platinum(II) and ruthenium(II). Both the acyclic and macrocyclic derivatives 7⋅Pt, 7⋅Ru⋅PF6 , 10⋅Pt and 10⋅Ru⋅PF6 are effective sensors for a range of halides and oxoanions. The near-infra red emitting ruthenium congeners exhibited an increased binding strength compared to platinum due to the cationic charge and thus additional electrostatic interactions. Intramolecular hydrogen-bonding between the dipyridylbenzene ligand and the amide carbonyls increases the preorganisation of both acyclic and macrocyclic metal derivatives resulting in no discernible macrocyclic effect. Interlocked analogues were also prepared, and preliminary luminescent chloride anion spectrometric titrations with 12⋅Ru⋅(PF6 )2 demonstrate a marked increase in halide binding affinity due to the complementary chloride binding pocket of the [2]rotaxane. 1 H NMR binding titrations indicate the interlocked dicationic receptor is capable of chloride recognition even in competitive 30 % aqueous mixtures.

Synthesis of a Mechanically Planar Chiral Rotaxane Ligand for Enantioselective Catalysis

Rotaxanes are interlocked molecules in which a molecular ring is trapped on a dumbbell-shaped axle because of its inability to escape over the bulky end groups, resulting in a so-called mechanical bond. Interlocked molecules have mainly been studied as components of molecular machines, but the crowded, flexible environment created by threading one molecule through another has also been explored in catalysis and sensing. However, so far, the applications of one of the most intriguing properties of interlocked molecules, their ability to display stereogenic units that do not rely on the stereochemistry of their covalent subunits, termed "mechanical chirality," have yet to be properly explored, and prototypical demonstration of the applications of mechanically chiral rotaxanes remain scarce. Here, we describe a mechanically planar chiral rotaxane-based Au complex that mediates a cyclopropanation reaction with stereoselectivities that are comparable with the best conventional covalent catalyst reported for this reaction.

Enhancing the selectivity of prolinamide organocatalysts using the mechanical bond in [2]rotaxanes

The synthesis of a pair of switchable interlocked prolinamides and their use as organocatalysts in three different enamine-activated processes are reported. A diacylaminopyridine moiety was incorporated into the thread for directing [2]rotaxane formation further allowing the association of complementary small molecules. The rotaxane-based systems were tested as organocatalysts in asymmetric enamine-mediated processes, revealing a significantly improved catalytic ability if compared with the non-interlocked thread. The presence of an electron-withdrawing nitro group at the macrocycle helps to achieve high conversions and enantioselectivities. These systems are able to interact with N-hexylthymine as a cofactor to form supramolecular catalysts displaying a divergent catalytic behaviour. The presence or absence of the cofactor controls the chemoselectivity in competitive reactions.

Temporal control in tritylation reactions through light-driven variation in chloride ion binding catalysis – a proof of concept

A proof-of-concept on temporal control in the tritylation reactions has been demonstrated using a designed tripodal triazole-linked azo(hetero)arene-based photoswitchable catalyst.