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

微孔和纳米孔固体是现代化学中最令人兴奋的一类材料，具有与小分子相似的尺寸和高内表面积的孔。这导致在气体储存和分离以及药物递送中的应用，并且孔可以在催化中充当纳米尺寸的反应容器，这些材料的结构与任何应用密切相关，并且详细的原子级知识对于控制性能和开发新用途至关重要。NMR光谱对局部环境的敏感性，而不需要任何长程有序，使其成为研究这些复杂材料的理想工具。在这个项目中，我们将专注于金属有机框架的表征，其中有机连接分子连接由金属原子或金属簇组成的节点。这些材料表现出比沸石多得多的结构多样性，有机连接分子的拓扑结构、尺寸和化学官能度能够变化，存在的金属的类型也可以变化。金属中心不仅影响最终材料的化学性质，还影响吸附小分子后获得的结构形式。本论文的工作将集中在两个关键领域：（1）利用17 O固体核磁共振谱研究混合金属MOFs。这将涉及开发新的合成程序，以具有成本效益和原子效率的17 O（天然丰度仅为0.037%）中的MOFs同位素富集。这项工作还需要实施和优化高分辨率17 O固态NMR实验（以及研究提高其灵敏度的方法）。然后，这些方法将被用来研究混合金属MOFs的组成和无序，以及吸附客体分子时发生的任何结构变化。(2)顺磁性MOFs的NMR光谱许多重要的MOFs含有顺磁性金属中心，例如，Cu 2+、Co 2+或Mn 2+。这种材料的NMR光谱学受到未成对电子和核自旋之间极强耦合的阻碍，需要专门的实验方法进行光谱采集。这项工作将开发一个协议的顺磁MOFs的光谱采集，快速样品旋转，快速信号平均和使用自旋回波的基础上。然后，这将被应用到客体分子的吸附的研究，最初在一个模型铜为基础的MOF（HKUST-1），并显示有趣的双重吸附性能的一系列新的功能化的MOF（基于STAM-1）的结构表征。

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.