A CuI6L4 Cage Dynamically Reconfigures to Form Suit[4]anes and Selectively Bind Fluorinated Steroids.

Simple organic ligands can self-assemble with metal ions to generate metal-organic cages, whose cavities bind guests selectively. This binding may enable new methods of chemical separation or sensing, among other useful functions. Here we report the preparation of a CuI6L4 pseudo-octahedral metal-organic cage, the ligands of which self-assemble from simple organic building blocks. Temperature, solvent, and the presence of different guests governed which structure predominated from a dynamic mixture of cage diastereomers with different arrangements of right- or left-handed metal vertices. Dissolution in dimethyl sulfoxide or the binding of tetrahedral guests led to a chiral tetrahedral T-symmetric framework, whereas low temperatures favored the achiral S4-symmetric diastereomer. Tetrahedral guests with long arms were encapsulated to form mechanically bonded suit[4]anes, with guest arms protruding out through host windows. The cage was also observed to bind fluorinated steroids, an important class of drug molecules, but not non-fluorinated steroids, providing the basis for new separation processes.

A Diverse Array of Large Capsules Transform in Response to Stimuli.

The allosteric regulation of biomolecules, such as enzymes, enables them to adapt and alter their conformation to fit specific substrates, expressing different functionalities in response to stimuli. Different stimuli can also trigger synthetic coordination cages to change their shape, size, and nuclearity by reconfiguring the dynamic metal-ligand bonds that hold them together. Here we demonstrate an abiological system consisting of different organic subcomponents and ZnII metal ions, which can respond to simple stimuli in complex ways. A ZnII20L12 dodecahedron transforms to give a larger ZnII30L12 icosidodecahedron through subcomponent exchange, as an aldehyde that forms bidentate ligands is displaced in favor of one that forms tridentate ligands together with a penta-amine subcomponent. In the presence of a chiral template guest, the same system that produced the icosidodecahedron instead gives a ZnII15L6 truncated rhombohedral architecture through enantioselective self-assembly. Under specific crystallization conditions, a guest induces a further reconfiguration of either the ZnII30L12 or ZnII15L6 cages to yield an unprecedented ZnII20L8 pseudo-truncated octahedral structure. The transformation network of these cages shows how large synthetic hosts can undergo structural adaptation through the application of chemical stimuli, opening pathways to broader applications.

A Double-Walled Tetrahedron with AgI 4 Vertices Binds Different Guests in Distinct Sites.

A double-walled tetrahedral metal-organic cage assembled in solution from silver(I), 2-formyl-1,8-naphthyridine, halide, and a threefold-symmetric triamine. The AgI 4 X clusters at its vertices each bring together six naphthyridine-imine moieties, leading to a structure in which eight tritopic ligands bridge four clusters in an (AgI 4 X)4 L8 arrangement. Four ligands form an inner set of tetrahedron walls that are surrounded by the outer four. The cage has significant interior volume, and was observed to bind anionic guests. The structure also possesses external binding clefts, located at the edges of the cage, which bound small aromatic guests. Halide ions bound to the silver clusters were observed to exchange in a well-defined hierarchy, allowing modulation of the cavity volume. The principles uncovered here may allow for increasingly more sophisticated cages with silver-cluster vertex architectures, with post-assembly tuning of the interior cavity volume enabling targeted binding behavior.

A Hydrogen-Bonded Ravel Assembled by Anion Coordination.

Anion-coordination-driven assembly (ACDA) is showing increasing power in the construction of anionic supramolecular architectures. Herein, by expanding the anion centers from oxoanion (phosphate or sulfate) to organic tris-carboxylates, an Archimedean solid (truncated tetrahedron) and a highly entangled, double-walled tetrahedron featuring a ravel topology have been assembled with tris-bis(urea) ligands. The results demonstrate the promising ability of tris-carboxylates as new anion coordination centers in constructing novel topologies with increasing complexity and diversity compared to phosphate or sulfate ions on account of the modifiable size and easy functionalization character of these organic anions.

Enantioselective fullerene functionalization through stereochemical information transfer from a self-assembled cage.

The regioselective functionalization of C60 remains challenging, while the enantioselective functionalization of C60 is difficult to explore due to the need for complex chiral tethers or arduous chromatography. Metal-organic cages have served as masks to effect the regioselective functionalization of C60. However, it is difficult to control the stereochemistry of the resulting fullerene adducts through this method. Here we report a means of defining up to six stereocentres on C60, achieving enantioselective fullerene functionalization. This method involves the use of a metal-organic cage built from a chiral formylpyridine. Fullerenes hosted within the cavity of the cage can be converted into a series of C60 adducts through chemo-, regio- and stereo-selective Diels-Alder reactions with the edges of the cage. The chiral formylpyridine ultimately dictates the stereochemistry of these chiral fullerene adducts without being incorporated into them. Such chiral fullerene adducts may become useful in devices requiring circularly polarized light manipulation.

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 chiral interlocking auxiliary strategy for the synthesis of mechanically planar chiral rotaxanes.

Rotaxanes can display molecular chirality solely due to the mechanical bond between the axle and encircling macrocycle without the presence of covalent stereogenic units. However, the synthesis of such molecules remains challenging. We have discovered a combination of reaction partners that function as a chiral interlocking auxiliary to both orientate a macrocycle and, effectively, load it onto a new axle. Here we use these substrates to demonstrate the potential of a chiral interlocking auxiliary strategy for the synthesis of mechanically planar chiral rotaxanes by producing a range of examples with high enantiopurity (93-99% e.e.), including so-called 'impossible' rotaxanes whose axles lack any functional groups that would allow their direct synthesis by other means. Intriguingly, by varying the order of bond-forming steps, we can effectively choose which end of an axle the macrocycle is loaded onto, enabling the synthesis of both hands of a single target using the same reactions and building blocks.

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.

Simplicity in the Design, Operation, and Applications of Mechanically Interlocked Molecular Machines.

Mechanically interlocked molecules are perhaps best known as components of molecular machines, a view further reinforced by the Nobel Prize in 2016 to Stoddart and Sauvage. Despite amazing progress since these pioneers of the field reported the first examples of molecular shuttles, genuine applications of interlocked molecular machines remain elusive, and many barriers remain to be overcome before such molecular devices make the transition from impressive prototypes on the laboratory bench to useful products. Here, we discuss simplicity as a design principle that could be applied in the development of the next generation of molecular machines with a view to moving toward real-world applications of these intriguing systems in the longer term.