Control of self-assembly and functionalisation of coordination cages

Polyhedral coordination cages are a class of complex that have become of interest for two major reasons.

(i) Their attractive, highly-symmetric structures can be prepared by self-assembly methods in which simple components combine under the correct conditions to give remarkably elaborate multi-component species (cf. biological assembly of virus coats from hundreds of protein subunits). Accordingly they act as a 'test-bed' for our ability to perform self-assembly in artificial systems.

(ii) Their large central cavities can act as size- and shape-specific hosts for a variety of small guest species, giving access to a range of possible functions (sensing, catalysis, transport) that are based on recognition and binding of a particular molecules.

So far however there are almost no examples of the development of cage synthesis beyond the simple self-assembly-of-labile-components approach. This proposal aims to expand our ability to synthesis specific cage-type species, with useful functional behaviour, in two distinct and complementary ways.

Firstly, we will use the techniques of 'subcomponent self-assembly' to develop ways to assembly cages in a step-by-stap manner using partially pre-formed building blocks ('subcomponents'). These subcomponents may be based on triangular faces which can be cross-linked in different ways to give complete polyhedra, just as a triangular panels can be combined in different ways to give tetrahedral, trigonal prisms, and so on. Alternatively these subcomponents may be based on very stable metal complex fragments whose geometry ensures that they will be incorporated into specific vertex sites of a polyhedral array, allowing formation of mixed-metal cages with metal ions of one type at some vertices and metal ions of a different type at the other vertices. Such stepwise assembly from pre-fabricated subcomponents would greatly extend our ability to control the synthesis of cage complexes of desired shapes and sizes and with built-in functionality (luminescence, redox activity, paramagnetism) at specific positions.

Secondly, we will investigate ways to add useful properties and functional behaviour to cages after their assembly is complete, by covalent attachment of additional units to the external surface of the cages which will have appropriate externally-directed reactive groups (hydroxyl, amine). This will require initial synthesis of ligands bearing these hydroxyl or amine substituents and their use to form cages which will contain up to 48 externally-directed sites for attachment of other units. Covalent attachment of a wide range of groups will be investigated including:

(i) fluorescent species (to give an array of numerous luminescent units around a central cage core);

(ii) redox-active fragments (to allow the charge on the cage to be massively altered by simple oxidation / reduction of the array of external units);

(iii) hydrogen-bonding sites (nucleobases) or metal-ion binding sites (pyridyl units) to allow formation using crystal-engineering methods of arrays of hollow capsules in the solid state which will perform solid-state host-guest chemistry; and

(iv) oligopeptide sequences which have known cellular recognition sites, to allow uptake of luminescent cages into cells and their binding at the specific recognition sites, permitting luminescence-based imaging of specific targeted regions of a cell and possibly delivery of a guest 'payload' to a specific cell site.

This project is fundamental research but in a highly topical area which is attracting worldwide attention, and will have a substantial academic impact on our efforts to exploit self-assembly methods to prepare new compounds and materials that are unavailable using more predictable covalent-bond chemistry. They key to major new advances in supramolecular chemistry is to be able to design and predict a particular assembly pathway to give a useful product, and this is exactly the area of this project.

Improvements in our ability to design, synthesise and functionalise supramolecular assemblies with a high degree of control will have wide general utility, not only in supramolecular chemistry research itself but also for many applications. Self-assembled edificies based on metal coordination complexes are known to have applications across the whole spectrum of chemical, materials and biological sciences. These include
- size and shape-selective uptake of gas molecules in pores of mesoporous materials;
- catalysis inside the constrained environments of cage complexes;
- non-linear optical properties of acentric infinite networks;
- diagnostic and cellular imaging agents based on DNA- and peptide-functionalised spherical assemblies which target specific cell receptor sites;
- light harvesting arrays for solar cells;
... and countless more. All such applications will benefit from the ability to exercise a greater degree of control over the design and synthesis of the assemblies which provide the function.

The impact will therefore be academic in the first instance but with extensive possibilities for downstream applications as our ability to prepare functional supramolecular assemblies improves; longer-term impact may be expected in all of the field listed above. Selection of the 'control of self-assembly' field as an EPSRC signpost, and the current work of the Grand Challenge network 'Directed Assembly of Extended Structures with Targeted Properties' in defining a roadmap for self-assembly over the next several decades, are both indications of the high potential academic impact of this project.