The structures of Zr and Hf metal–organic frameworks (MOFs) are very sensitive to small changes in synthetic conditions. One key difference affecting the structure of UiO MOF phases is the shape and nuclearity of Zr or Hf metal clusters acting as nodes in the framework; although these clusters are crucial, their evolution during MOF synthesis is not fully understood. In this paper, we explore the nature of Hf metal clusters that form in different reaction solutions, including in a mixture of DMF, formic acid, and water. We show that the choice of solvent and reaction temperature in UiO MOF syntheses determines the cluster identity and hence the MOF structure. Using in situ X-ray pair distribution function measurements, we demonstrate that the evolution of different Hf cluster species can be tracked during UiO MOF synthesis, from solution stages to the full crystalline framework, and use our understanding to propose a formation mechanism for the hcp UiO-66(Hf) MOF, in which first the metal clusters aggregate from the M₆ cluster (as in fcu UiO-66) to the hcp-characteristic M₁₂ double cluster and, following this, the crystalline hcp framework forms. These insights pave the way toward rationally designing syntheses of as-yet unknown MOF structures, via tuning the synthesis conditions to select different cluster species.

中文翻译：

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通过原位 X 射线对分布函数分析探索簇形成在 UiO 家族 Hf 金属-有机框架中的作用

Zr 和 Hf 金属有机骨架 (MOF) 的结构对合成条件的微小变化非常敏感。影响 UiO MOF 相结构的一个关键区别是作为框架中节点的 Zr 或 Hf 金属簇的形状和核度；尽管这些簇是至关重要的，但它们在 MOF 合成过程中的演变尚不完全清楚。在本文中，我们探讨了在不同反应溶液中形成的 Hf 金属簇的性质，包括在 DMF、甲酸和水的混合物中。我们表明，UiO MOF 合成中溶剂和反应温度的选择决定了簇的身份，从而决定了 MOF 的结构。就地使用X 射线对分布函数测量，我们证明了在 UiO MOF 合成过程中可以跟踪不同 Hf 簇物种的演化，从溶液阶段到全晶框架，并利用我们的理解提出hcp UiO-66的形成机制(Hf) MOF，其中首先金属簇从 M ₆簇（如在fcu UiO-66 中）聚集到hcp特征的 M ₁₂双簇，然后形成结晶hcp框架。这些见解通过调整合成条件以选择不同的簇种类，为合理设计未知 MOF 结构的合成铺平了道路。