Metal-organic container molecules through subcomponent self-assembly

A variety of different three-dimensional metal-organic container molecules have recently been prepared using subcomponent self-assembly, which relies upon metal template effects to generate complex structures from simple molecular precursors and metal salts. Many of these structures have well defined internal pockets, allowing guest species to be bound and the chemical reactivity of these guests to be modified. Such host molecules have potential applications ranging from the protection of sensitive chemical species to the separation and purification of substrates as diverse as gases, gold compounds, and fullerenes.

Symmetry breaking in self-assembled M4L6 cage complexes

Here we describe the phenomenon of symmetry breaking within a series of M4L6 container molecules. These containers were synthesized using planar rigid bis-bidentate ligands based on 2,6-substituted naphthalene, anthracene, or anthraquinone spacers and Fe(II) ions. The planarity of the ligand spacer favors a stereochemical configuration in which each cage contains two metal centers of opposite handedness to the other two, which would ordinarily result in an S4-symmetric, achiral configuration. Reduction of symmetry from S4 to C1 is achieved by the spatial offset between each ligand's pair of binding sites, which breaks the S4 symmetry axis. Using larger Cd(II) or Co(II) ions instead of Fe(II) resulted, in some cases, in the observation of dynamic motion of the symmetry-breaking ligands in solution. NMR spectra of these dynamic complexes thus reflected apparent S4 symmetry owing to rapid interconversion between energetically degenerate, enantiomeric C1-symmetric conformations.

Transporting and shielding photosensitisers by using water-soluble organometallic cages: a new strategy in drug delivery and photodynamic therapy

Skin photosensitivity remains one of the main limitations in photodynamic therapy. In this Concept article a strategy to overcome this limitation is described, in which the photosensitizer is hidden inside the hydrophobic cavity of a water-soluble organometallic cage. The metallacage not only protects the photosensitizer from light, it also facilitates its delivery to cancer cells.

Dodecanuclear hexagonal-prismatic M12L18 coordination cages by subcomponent self-assembly

Dodecanuclear hexagonal-prismatic M12L18 cages were prepared by a subcomponent self-assembly process with commercially available pyridinecarboxaldehyde, m-xylenediamine, and cadmium(II) perchlorate or manganese(II) perchlorate. The NMR spectrum of the Cd cage shows that there are three independent ligand sets, and the X-ray crystal structure of the Mn cage reveals that the structure has both fac-Δ- and mer-Λ-configured metal centers in a 1:1 ratio. The cage structure also has a large cavity that contains five perchlorate anions.

Size-selective encapsulation of hydrophobic guests by self-assembled M4L6 cobalt and nickel cages

Subtle differences in metal-ligand bond lengths between a series of [M(4)L(6)](4-) tetrahedral cages, where M = Fe(II), Co(II), or Ni(II), were observed to result in substantial differences in affinity for hydrophobic guests in water. Changing the metal ion from iron(II) to cobalt(II) or nickel(II) increases the size of the interior cavity of the cage and allows encapsulation of larger guest molecules. NMR spectroscopy was used to study the recognition properties of the iron(II) and cobalt(II) cages towards small hydrophobic guests in water, and single-crystal X-ray diffraction was used to study the solid-state complexes of the iron(II) and nickel(II) cages.

Constructions of two polycatenanes and one polypseudo-rotaxane by discrete tetrahedral cages and stool-like building units

Mechanically Interlocked molecules, such as catenanes and rotaxanes, are of great interest due to their fascinating structures and potential applications, while such molecules have been mainly restricted to comprising components of interlocked rings or polygons. The constructions of infinite polycatenanes and polyrotaxanes by discrete cages remain great challenge, and only two infinite polycatenanes fabricated by discrete cages have been reported so far, while the structures of polyrotaxanes and polypseudo-rotaxanes fabricated by discrete build units have not been documented to date. Herein we report the first example of a two-dimensional (2D) polypseudo-rotaxane fabricated by stool-like build units, the second example of a one-dimensional (1D) polycatenane, and the second example of a three-dimensional (3D) polycatenane, which were assemblied by discrete tetrahedral cages. The pores of dehydrated 3D polycatenane are dynamic, and display size-dependent adsorption/desorption behaviors of alcohols.

Efficient long-range stereochemical communication and cooperative effects in self-assembled Fe4L6 cages

A series of large, optically active Fe(4)L(6) cages was prepared from linear 5,5'-bis(2-formylpyridines) incorporating varying numbers (n = 0-3) of oligo-p-xylene spacers, chiral amines, and Fe(II). When a cage was constructed from the ligand bridged by one p-xylene spacer (n = 1) and a bulky chiral amine, both a homochiral Fe(2)L(3) helicate and Fe(4)L(6) cage were observed to coexist in solution due to a delicate balance between steric factors. In contrast, when a less bulky chiral amine was used, only the Fe(4)L(6) cage was observed. In the case of larger cages (n = 2, 3), long-range (>2 nm) stereochemical coupling between metal centers was observed, which was minimally diminished as the ligands were lengthened. This communication was mediated by the ligands' geometries and rigidity, as opposed to gearing effects between xylene methyl groups: the metal-centered stereochemistry was not observed to affect the axial stereochemistry of the ligands.

Functional behaviour from controlled self-assembly: challenges and prospects

Many naturally occurring systems show us how multi-component supramolecular assemblies can generate useful functional behaviour. In this article the problems and limitations associated with achieving such behaviour in artificial multi-component assemblies is discussed, together with two examples of functions in artificial supramolecular assemblies based on (i) host-guest chemistry in cavities of cages, and (ii) light-harvesting in multi-chromophore arrays. Important challenges for the future are summarised.

Guanidinium binding modulates guest exchange within an [M4L6] capsule

Take it slow! A metal-organic container molecule has been shown to bind guanidinium cations (blue) between the sulfonate groups on its periphery, as well as accommodating guests such as cyclopentane and cyclohexane in its internal cavity (red). Kinetic studies on the system demonstrated a linear relationship between the amount of bound guanidinium ions and the rate of guest exchange.