NMR-Enhanced Crystallography Aids Open Metal–Organic Framework Discovery Using Solvent-Free Accelerated Aging

We demonstrate the combined use of NMR-enhanced crystallography and solvent-free synthesis by accelerated aging (AA), for the discovery and structural characterization of a novel cadmium-based open metal–organic framework (MOF) belonging to the class of zeolitic imidazolate frameworks (ZIFs). Although solid-state NMR spectroscopy has been used to assist in structural characterization of crystalline solids by powder X-ray diffraction (PXRD), typically through quantification of the contents of the asymmetric unit, this work highlights how it can take a more active role in guiding structure determination, by elucidating the coordination environment of the metal node in a novel MOFs. Exploration of AA reactions of cadmium oxide (CdO) and 2-methylimidazole (HMeIm) enabled the synthesis of not only the previously reported yqt1-topology framework but also a new material (1) exhibiting a Cd/MeIm ratio of 1:3, contrasting the 1:2 ratio expected for a ZIF. Structural characterization of 1 was enabled by using 111Cd solid-state nuclear magnetic resonance (SSNMR) to provide information on the coordination environment of the cadmium node. Specifically, 111Cd SSNMR experiments were conducted on a series of model compounds to correlate the cadmium coordination environment to the observed isotropic chemical shift, δiso(111Cd), followed by multinuclear SSNMR experiments on 1 to determine the nature of the metal coordination environment and the number of distinct chemical sites. This information was used in refinement of the molecular level structure from the available PXRD data, a technique termed NMR-enhanced crystallography, revealing that 1 is an open diamondoid (dia) topology Cd­(MeIm)2 framework based on Cd2+ ions tetrahedrally coordinated with MeIm– ligands and additional HMeIm guest molecules within the framework pores. Although AA was initially devised as a clean, mild route for making MOFs, these results provide a proof-of-principle of how, by combining it with SSNMR spectroscopy as a means to overcome limitations of PXRD structure determination, it can be used to screen for new solid phases in the absence of solvents, high temperatures, or mechanical impact that are inherent to other thermally-, solution-, or mechanochemically-based techniques.本研究展示了将核磁共振（Nuclear Magnetic Resonance, NMR）增强晶体学与加速老化（accelerated aging, AA）无溶剂合成法相结合，用于发现并结构表征一种新型镉基开放金属有机框架（metal–organic framework, MOF），该框架属于沸石咪唑酯骨架（zeolitic imidazolate frameworks, ZIFs）类材料。尽管此前固态核磁共振（solid-state NMR）光谱学常通过定量非对称单元的组分，借助粉末X射线衍射（powder X-ray diffraction, PXRD）辅助解析晶体固体的结构，但本研究凸显了其可通过阐明新型金属有机框架中金属节点的配位环境，在指导结构解析中发挥更积极的作用。通过对氧化镉（cadmium oxide, CdO）与2-甲基咪唑（2-methylimidazole, HMeIm）的加速老化反应进行探索，我们不仅合成了此前已报道的yqt1拓扑框架，还得到了一种新型材料（1），其Cd/MeIm比例为1:3，与沸石咪唑酯骨架典型的1:2比例截然不同。借助111Cd固态核磁共振（solid-state nuclear magnetic resonance, SSNMR）获取镉节点的配位环境信息，我们完成了材料1的结构表征。具体而言，我们首先对一系列模型化合物开展111Cd固态核磁共振实验，将镉的配位环境与观测到的各向同性化学位移δiso(111Cd)建立关联；随后对材料1开展多核固态核磁共振实验，以确定金属配位环境的类型以及不同化学位点的数量。我们将这些信息用于基于现有粉末X射线衍射数据的分子级结构精修，该技术即核磁共振增强晶体学。结果显示，材料1属于开放金刚石型（diamondoid, dia）拓扑结构的Cd(MeIm)2框架，其Cd²+离子与MeIm⁻配体形成四面体配位，框架孔道中还存在游离的HMeIm客体分子。尽管加速老化法最初被设计为一种清洁温和的金属有机框架合成路径，但本研究结果证明了：将其与固态核磁共振光谱学相结合以克服粉末X射线衍射结构解析的局限后，即可在无需溶剂、高温或机械冲击（其他热法、溶液法、机械化学法固有的操作条件）的前提下，用于筛选新型固相产物。