Exploiting metal-organic cages to control nanoscale properties: from imaging catalytic intermediates to design of novel 3D printed nanomaterials

Metal-organic cages represent a powerful new class of nanoscale materials, with well-defined cavities and pores which allow control of chemical and material properties. These complex molecular architectures can be readily constructed from simple building blocks via metal-ligand self-assembly. Common polyhedral shapes (such as cubes, tetrahedra and octahedra) can all be constructed, with metal ions for the vertices, organic ligands covering the faces or edges, and a well-defined inner void for guest binding. Variation of the metal ions and organic ligands allows control over cage's size (typically 1 to 4 nm), shape, solubility, and most importantly guest binding properties. Cages can be tailored to bind specific targets by controlling a multitude of non-covalent interactions including hydrogen bonding, pi-pi stacking and hydrophobic effects, with the inherent beauty of self-assembly allowing a library of different cages to be accessible in short synthetic sequences. We will exploit the unique properties of these nanomaterials to address chemical and material challenges.

The main objectives:
to generate highly reactive catalytic intermediates which are stabilised in the controlled environment of the cavity, thus allowing full characterisation and elucidation of reactivity;
to produce photoresponsive materials, where the cage influences the nature of the photoresponse;
to formulate the cage-based inks for deposition by inkjet printing;
to access hierarchically porous cage-based materials produced by 3D printing for potential applications

The inner cavity of metal-organic cages provides an environment in which a guest molecule can be isolated from interactions with other guest molecules, bulk solvent, and the environment more broadly. For example, the normally pyrophoric P4 is rendered 'inert' in air once bound (Figure 1a) and the radical initiator AIBN (which is normally unstable to decomposition) has its half-life extended (Figure 1b). However, metal-organic cages remain relatively underused for the study of the structure and properties of reactive intermediates because the metal-organic cage community has an over-reliance on a relatively small set of characterisation techniques, such as 1H NMR spectroscopy, UV-Vis spectroscopy, single crystal X-ray crystallography and mass spectrometry. These techniques either do not directly image the system or have a measurement timescale that is too long to reveal information on fast reaction processes or high energy intermediates. In this project we propose to use techniques such as ultrafast time-resolved infrared spectroscopy coupled with low temperature matrix isolation. In doing so, we will shed new light on the structure and properties of reaction intermediates that may have never been directly observed or imaged before.

This will allow prediction of their reactivity and therefore allow us to plan new catalytic mechanisms in which such intermediates are embedded.

Metal-organic cages are generally constructed and studied in solution phase. There is much less known of their solid-state structure outside of single crystals and this hinders their development into usable materials. Cages have already been shown to be stable in neat liquid form, as gels, and adsorbed to alumina particles; we propose to investigate metal-organic cages as inks for 3D printing. This will allow the construction of material with nanopores (from the cage cavity) and a variety of meso to micro pores (from the additive manufacturing procedure). Metal-organic cages are easily rendered stimuli-responsive, and undergo modification in response to light, heat, or certain classes of chemical signal. This can be used to alter cage shape, binding properties, and cross-link between cages. Such behaviour will enable the bulk material properties of such 3D printed materials to be addressed post printing and may help target key 3D printing issues such as layer adhesion.