Metal-organic frameworks (MOFs) are emerging materials with many current and potential applications due to their unique properties. One critical feature is that the physical and chemical properties of MOFs are tunable. One of the methods for tuning MOF properties is to introduce defects by design for desired applications. Characterization of MOF defects is important, but very challenging due to the local nature and short-range ordering. In this work, we have introduced the ordered vacancies (the defects) in the form of the coordinatively unsaturated sites (CUSs) into the framework of MOF MIL-120(Al). The creation of ordered vacancies is achieved by replacing one quarter of the BTEC (1,2,4,5-benzenetetracarboxylate) with BDC (benzene-1,4-dicarboxylate) linkers. Both parent and defective MOFs were characterized by multinuclear solid-state NMR spectroscopy. ¹H MAS NMR is used to characterize the hydrogen bonding in these MOFs, whereas ¹³C CP MAS NMR confirms unambiguously that the BDC is incorporated into the framework. One-dimensional ²⁷Al MAS NMR provides direct evidence of the coordinatively unsaturated Al sites (the defects). Furthermore, ²⁷Al 3QMAS experiments at 21.1 ​T allow direct identification of one penta-coordinated and three chemically inequivalent octahedral Al sites in the defective MIL-120(Al). Two of the above-mentioned octahedral Al sites are in the domain which appears defect-free. The third octahedral Al site is near the defective site. This work clearly demonstrates the power of solid-state NMR spectroscopy for characterization of defective MOFs.

金属有机框架 (MOF) 是新兴材料，由于其独特的特性，具有许多当前和潜在的应用。一个关键特征是 MOF 的物理和化学性质是可调的。调整 MOF 特性的方法之一是通过设计为所需应用引入缺陷。MOF 缺陷的表征很重要，但由于局部性质和短程排序，因此非常具有挑战性。在这项工作中，我们以配位不饱和位点 (CUS) 的形式将有序空位（缺陷）引入到 MOF MIL-120(Al) 的框架中。通过将四分之一的 BTEC（1,2,4,5-苯四羧酸盐）替换为 BDC（苯-1,4-二羧酸盐）接头，可以产生有序空位。¹ H MAS NMR 用于表征这些 MOF 中的氢键，而¹³ C CP MAS NMR 明确证实 BDC 被纳入框架中。一维²⁷ Al MAS NMR 提供了配位不饱和 Al 位点（缺陷）的直接证据。此外，在 21.1 T 下进行的²⁷个 Al 3QMAS 实验可以直接识别有缺陷的 MIL-120(Al) 中的一个五配位和三个化学不等价的八面体 Al 位点。上述八面体 Al 位点中的两个位于看起来无缺陷的域中。第三个八面体 Al 位点靠近缺陷位点。这项工作清楚地证明了固态 NMR 光谱在表征缺陷 MOF 方面的能力。