Unlocking New Topologies in Zr-Based Metal–Organic Frameworks by Combining Linker Flexibility and Building Block Disorder

The outstanding diversity of Zr-based frameworks is inherently linked to the variable coordination geometry of Zr-oxo clusters and the conformational flexibility of the linker, both of which allow for different framework topologies based on the same linker–cluster combination. In addition, intrinsic structural disorder provides a largely unexplored handle to further expand the accessibility of novel metal–organic framework (MOF) structures that can be formed. In this work, we report the concomitant synthesis of three topologically different MOFs based on the same M6O4(OH)4 clusters (M = Zr or Hf) and methane-tetrakis­(p-biphenyl-carboxylate) (MTBC) linkers. Two novel structural models are presented based on single-crystal diffraction analysis, namely, cubic c-(4,12)­MTBC-M6 and trigonal tr-(4,12)­MTBC-M6, which comprise 12-coordinated clusters and 4-coordinated tetrahedral linkers. Notably, the cubic phase features a new architecture based on orientational cluster disorder, which is essential for its formation and has been analyzed by a combination of average structure refinements and diffuse scattering analysis from both powder and single-crystal X-ray diffraction data. The trigonal phase also features structure disorder, although involving both linkers and secondary building units. In both phases, remarkable geometrical distortion of the MTBC linkers illustrates how linker flexibility is also essential for their formation and expands the range of achievable topologies in Zr-based MOFs and its analogues.

锆基框架的卓越多样性本质上与锆氧簇（Zr-oxo clusters）的可变配位几何构型以及连接配体（linker）的构象灵活性密切相关，二者均可基于相同的簇-配体组合衍生出不同的框架拓扑结构。此外，本征结构无序为进一步拓展可合成的新型金属有机框架（metal–organic framework, MOF）结构的可及性提供了尚未被充分探索的调控手段。本研究中，我们报道了基于相同M₆O₄(OH)₄簇（M=Zr或Hf）与四(对联苯基苯甲酸)甲烷（methane-tetrakis­(p-biphenyl-carboxylate), MTBC）连接配体的三种拓扑结构不同的MOF的同步合成。基于单晶衍射分析，我们提出了两种全新的结构模型，分别为立方相c-(4,12)-MTBC-M₆与三方相tr-(4,12)-MTBC-M₆，二者均包含12配位的簇单元与4配位的四面体连接配体。值得注意的是，该立方相具有基于簇取向无序的全新骨架结构，这一结构特征对其形成至关重要；我们通过对粉末与单晶X射线衍射数据的平均结构精修与漫散射分析相结合的手段，对该无序结构进行了解析。该三方相同样存在结构无序现象，不过其无序同时涉及连接配体与次级结构单元。在两种物相中，MTBC连接配体均呈现出显著的几何畸变，这表明连接配体的灵活性对两种物相的形成均不可或缺，同时也拓展了锆基MOF及其同系物可实现的拓扑结构范围。