Hu Gaoli, Liu Qi, Deng Hexiang
Key Laboratory of Biomedical Polymers, Ministry of Education, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China.
College of Chemistry, Chemical Engineering and Materials, Soochow University, Suzhou 215123, China.
Acc Chem Res. 2025 Jan 7;58(1):73-86. doi: 10.1021/acs.accounts.4c00633. Epub 2024 Dec 13.
ConspectusThe past decades have witnessed the proliferation of porous materials offering high surface areas and the revolution in gas storage and separation, where metal-organic frameworks (MOFs) stand out as an important family. Alongside the pursuit of higher surface area, the increase in the size of guests, such as nanoparticles and biomolecules, has also led to the demand for larger space defined by the pores and cages within the MOF structure, from the conventional micropore regime (<2 nm) toward the mesopore regime (2-50 nm). Among the essential elements in the design of MOFs, molecular building blocks, their coordination and spatial arrangement, the chemistry for molecular design, and coordination bonds have become relatively mature, offering precise control of the shape and environment of the molecularly defined 3D cages; however, the correlation between the geometrical parameters and the size of polyhedrons describing the cages, concerning the spatial arrangement of building blocks, is much less explored.In this Account, we made efforts to associate actual cage size with the critical geometrical components, vertices, edges, connectivity, rings, and underlying polyhedrons, as well as the combination of components of various types in the design of MOFs. Several trends were found, such as influence from connectivity and expansion efficiency, offering insights into the construction of 3D cages in MOFs. This enables the creation of extremely large mesoporous cages in MOFs with an internal diameter up to 11.4 nm from relatively small building blocks. Furthermore, we discuss a strategy of partial removal or replacement of organic linkers to construct mesoporous cages from readily known topologies.All of the above efforts urged us to ask the following questions: Is there any limit in the sculpting of the 3D space from molecules? How large an area can one chemical bond support? The answer to these questions will deepen the knowledge of efficient utilization of chemical bonds in the sculpting of 3D spaces and guide the design of larger mesopores. Several general geometrical principals emerged: (1) Expansion efficiency and radius are positively correlated with the number of vertices. (2) Increase in the number of vertices and decrease of their connectivity favor the construction and expansion of large cages. (3) The boundary of the 3D space constructed by the chemical bonds is related to the polyhedron type and determined by the energy involved in crystallinity. Such principals are likely to be applicable also in the design of isolated cages in supramolecular chemistry. In addition to the structural design and synthesis, the applications of these mesoporous cages in MOFs are also summarized.
概述
在过去几十年中，具有高比表面积的多孔材料大量涌现，气体储存和分离领域也发生了变革，其中金属有机框架（MOF）作为一个重要的材料家族脱颖而出。在追求更高比表面积的同时，客体分子（如纳米颗粒和生物分子）尺寸的增大，也引发了对MOF结构中由孔和笼所限定的更大空间的需求，即从传统的微孔范围（<2nm）向介孔范围（2 - 50nm）发展。在MOF设计的基本要素中，分子构建单元、它们的配位和空间排列、分子设计化学以及配位键已经相对成熟，能够精确控制分子定义的三维笼的形状和环境；然而，关于构建单元的空间排列，描述笼的多面体几何参数与尺寸之间的相关性却很少被研究。
在本综述中，我们致力于将实际的笼尺寸与关键的几何组成部分（顶点、边、连接性、环以及潜在的多面体）以及MOF设计中各种类型组成部分的组合联系起来。发现了几个趋势，比如连接性和扩展效率的影响，这为MOF中三维笼的构建提供了见解。这使得能够用相对较小的构建单元在MOF中创建内径高达11.4nm的超大介孔笼。此外，我们还讨论了一种通过部分去除或替换有机连接体从已知拓扑结构构建介孔笼的策略。
从分子构建三维空间是否存在限制？一个化学键能支撑多大的面积？这些问题的答案将加深对化学键在三维空间构建中高效利用的认识，并指导更大介孔的设计。出现了几个通用的几何原理：（1）扩展效率和半径与顶点数量呈正相关。（2）顶点数量增加且连接性降低有利于大笼的构建和扩展。（3）由化学键构建的三维空间边界与多面体类型有关，并由结晶过程中涉及的能量决定。这些原理可能也适用于超分子化学中孤立笼的设计。除了结构设计和合成，还总结了这些介孔笼在MOF中的应用。
