Remarkable Structural Diversity between Zr/Hf and Rare-Earth MOFs via Ligand Functionalization and the Discovery of Unique (4, 8)‑c and (4, 12)-connected Frameworks

Ligand modification in MOFs provides great opportunities not only for the development of functional materials with new or enhanced properties but also for the discovery of novel structures. We report here that a sulfone-functionalized tetrahedral carboxylate-based ligand is capable of directing the formation of new and fascinating MOFs when combined with Zr4+/Hf4+ and rare-earth metal cations (RE) with improved gas-sorption properties. In particular, the resulting M-flu-SO2 (M: Zr, Hf) materials display a new type of the augmented flu-a net, which is different as compared to the flu-a framework formed by the nonfunctionalized tetrahedral ligand. In terms of properties, a remarkable increase in the CO2 uptake is observed that reaches 76.6% and 61.6% at 273 and 298 K and 1 bar, respectively. When combined with REs, the sulfone-modified linker affords novel MOFs, RE-hpt-MOF-1 (RE: Y3+, Ho3+, Er3+), which displays a fascinating (4, 12)-coordinated hpt net, based on nonanuclear [RE9(μ3-Ο)2(μ3-ΟΗ)12(−COO)12] clusters that serve as hexagonal prismatic building blocks. In the absence of the sulfone groups, we discovered that the tetrahedral linker directs the formation of new RE-MOFs, RE-ken-MOF-1 (RE: Y3+, Ho3+, Er3+, Yb3+), that display an unprecedented (4, 8)-coordinated ken net based on nonanuclear RE9-clusters, to serve as bicapped trigonal prismatic building units. Successful activation of the representative member Y-ken-MOF-1 reveals a high BET surface area and total pore volume reaching 2621 m2 g–1 and 0.95 cm3 g–1, respectively. These values are the highest among all RE MOFs based on nonanuclear clusters and some of the highest in the entire RE family of MOFs. The present work uncovers a unique structural diversity existing between Zr/Hf and RE-based MOFs that demonstrates the crucial role of linker design. In addition, the discovery of the new RE-hpt-MOF-1 and RE-ken-MOF-1 families of MOFs highlights the great opportunities existing in RE-MOFs in terms of structural diversity that could lead to novel materials with new properties.金属有机框架（metal-organic frameworks, MOFs）的配体修饰，不仅为开发具有全新或增强性能的功能材料提供了绝佳契机，也为新型结构的发现创造了广阔空间。本文报道了一种砜基功能化的四面体羧酸基配体（tetrahedral carboxylate-based ligand），当其与锆离子/铪离子（Zr⁴⁺/Hf⁴⁺）以及稀土金属阳离子（rare-earth metal, RE）结合时，可导向构筑新型且极具研究价值的MOFs，且该类材料具备更优异的气体吸附性能。具体而言，所得M-flu-SO₂（M：Zr、Hf）材料展现出一种新型的扩充型flu-a拓扑网络，与非功能化四面体配体所形成的flu-a骨架截然不同。在性能方面，其二氧化碳吸附量实现了显著提升：在273、298开尔文及1巴压力下，吸附量分别可达76.6%与61.6%。当该配体与稀土金属结合时，砜基修饰的连接体可构筑新型MOFs——RE-hpt-MOF-1（RE：Y³⁺、Ho³⁺、Er³⁺），该材料基于九核[RE₉(μ₃-O)₂(μ₃-OH)₁₂(-COO)₁₂]团簇作为六棱柱型构筑基元，展现出极具吸引力的(4,12)配位hpt拓扑网络。在不含砜基的情况下，我们发现该四面体配体可导向构筑新型稀土MOFs：RE-ken-MOF-1（RE：Y³⁺、Ho³⁺、Er³⁺、Yb³⁺），其基于九核RE₉团簇作为双帽三棱柱型构筑单元，展现出前所未有的(4,8)配位ken拓扑网络。对代表性材料Y-ken-MOF-1进行成功活化后，测得其具备极高的BET比表面积（Brunauer-Emmett-Teller, BET）与总孔体积，分别可达2621 m²·g⁻¹与0.95 cm³·g⁻¹。该数值在所有基于九核团簇的稀土MOFs中位居首位，同时在整个稀土MOFs家族中也处于顶尖水平。本研究揭示了锆/铪基与稀土基MOFs之间独特的结构多样性，证明了配体设计的关键作用。此外，新型RE-hpt-MOF-1与RE-ken-MOF-1系列MOFs的发现，凸显了稀土MOFs在结构多样性方面的巨大潜力，有望助力开发具备全新性能的新型功能材料。