Structural Characterisation of Metal-Organic Frameworks using Solid-State NMR Spectroscopy

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Microporous and nanoporous solids are one of the most exciting classes of materials in modern chemistry, possessing pores of similar sizes to small molecules and high internal surface areas. This leads to applications in gas storage and separation and drug delivery and the pores can act as nanosize reaction vessels in catalysis, The structure of these materials is intimately connected with any application, and a detailed atomic-level knowledge of this is vital to controlling properties and developing new uses. The sensitivity of NMR spectroscopy to the local environment, without the need for any long-range order, makes it an ideal tool for studying these complex materials. In this project we will focus on the characterisation of metal-organic frameworks, where an organic linker molecule connects nodes composed of metal atoms or metal clusters. These materials exhibit much more structural variety than zeolites, with the topology, size and chemical functionality of the organic linker molecules able to vary, as can the type(s) of metal present. The metal centres affect not only the chemical properties of the final material, but also the structural forms obtained upon adsorption of small molecules.The work proposed will focus on two key areas:(1) Using 17O solid-state NMR spectroscopy to investigate mixed-metal MOFs. This will involve the development of new synthetic procedures for cost-effective and atom-efficient isotopic enrichment of MOFs in 17O (which has a natural abundance of only 0.037%). The work will also require the implementation and optimisation of high-resolution 17O solid-state NMR experiments (and the investigation of methods to improve their sensitivity). These methods will then be employed to investigate the composition and disorder in mixed-metal MOFs, and any structural changes that take place upon the adsorption of guest molecules. (2) NMR spectroscopy of paramagnetic MOFs.Many important MOFs contain paramagnetic metal centres, e.g., Cu2+, Co2+ or Mn2+. NMR spectroscopy of such materials is hampered by the extremely strong coupling between the unpaired electron and the nuclear spin, necessitating specialist experimental approaches for spectral acquisition. This work will develop a protocol for spectral acquisition of paramagnetic MOFs, based on fast sample spinning, rapid signal averaging and the use of spin echoes. This will then be applied to the study of the adsorption of guest molecules, initially in a model Cu-based MOF (HKUST-1), and to the structural characterisation of a series of new functionalised MOFs (based on STAM-1) which display interesting dual adsorption properties.