Complex Architectures and Topology
Subcomponent self-assembly methods commonly used in the group have been utilised to form complex interlocked molecular topologies.[1,2,3] We have reported the formation of a -catenane via the preformation of two joined Fe₃L₃ helicates, followed by subcomponent exchange in order to form the continuous interlocked structure.2 In addition, we have recently reported the formation of molecular gyroscopes. The sequential addition of 4,4’-dipyridyl napthalenediimide (NDI) and a macrocycle resulted in the generation of a metal-organic host with an encapsulated rotaxane guest.3
Heteroleptic metal-organic architectures, structures which contain more than one ligand-type, can provide cavities with reduced symmetry primed for binding low symmetry guests. Strategies employed to drive the formation of heteroleptic species include the exploitation of steric and π-stacking effects.[1,2] Recently, we reported the selective preparation of a heteroleptic triangular prism, on the basis of a favourable entropic change, which demonstrated the ability to bind a range of low symmetry biologically relevant compounds within its prolate cavity.
Metal-organic cages with specified stereochemistry can provide an internal environment which could be utilised for stereo-selective encapsulation of guests, or even asymmetric catalysis. An enantiopure Fe₄L₄ tetrahedral cage in which there is a Δ configuration at all four metal centres was synthesised by using a chiral amine subcomponent to form the cage as a single diastereomer, followed by displacement of the chiral amines with achiral amines. In addition, the enantioselective formation of Fe₄L₄ tetrahedral cages has been demonstrated by using the stereochemical communication between a racemic mixture of the tetrahedral host and enantiopure cryptophane-111 guests.
A strategy employed in the Nitschke group for the design of self-assembled metal-organic cages is the ‘directional bonding’ approach, in which the pre-organization of coordinate vectors of bridging ligands lead to an energetic preference for a particular architecture. An early example of this is the formation of an Fe₄L₄ tetrahedral cage, in which the self-assembly of three pyridyl-imine ligands around each metal centre favours inter-ligand angles of approximately 60°. More complex examples of this design strategy are the self-assembly of an Fe₈L₁₂ cube, in which a different substitution pattern of a bis(pyridyl imine) ligand biases inter-ligand angles of approximately 90° about each metal centre. The self-assembly of a tetra(phenantholine) ligand with Co(II) likewise leads to inter-ligand angles of approximately 90° about each metal centre. However, as the tetra(phenantholine) ligand may bridge four metal centres, and only two phenanthroline motifs satisfy the coordination sphere of the metal, the architecture favoured is a Co₁₂L₆ cuboctahedron.