A Copper(II)-Paddlewheel Metal–Organic Framework with Exceptional Hydrolytic Stability and Selective Adsorption and Detection Ability of Aniline in Water

Copper­(II)-paddlewheel-based metal–organic frameworks (CP-MOFs) represent a unique subclass of MOFs with highly predictable porous structures, facile syntheses, and functional open metal sites. However, the lack of high hydrolytic stability is an obstacle for CP-MOFs in many practical applications. In this work, we report a new CP-MOF, [Cu4(tdhb)] (BUT-155), which is constructed from a judiciously designed carboxylate ligand with high coordination connectivity (octatopic), abundant hydrophobic substituents (six methyl groups), and substituent constrained geometry (tetrahedral backbone), tdhb8– [H8tdhb = 3,3′,5,5′-tetrakis­(3,5-dicarboxyphenyl)-2,2′,4,4′,6,6′-hexamethylbiphenyl)]. BUT-155 shows high porosity with a Brunauer–Emmett–Teller surface area of 2070 m2/g. Quite interestingly, this CP-MOF retains its structural integrity after being treated in water for 10 days at room temperature or in boiling water for 24 h. To the best of our knowledge, BUT-155 represents the first CP-MOF that is demonstrated to retain its structural integrity in boiling water. The high hydrolytic stability of BUT-155 allowed us to carry out adsorption studies of water vapor and aqueous organic pollutants on it. Water-vapor adsorption reveals a sigmoidal isotherm and a high uptake (46.7 wt %), which is highly reversible and regenerable. In addition, because of the availability of soft-acid-type open Cu­(II) sites, BUT-155 shows a high performance for selective adsorption of soft-base-type aniline over water or phenol, and a naked-eye detectable color change for the MOF sample accompanies this. The adsorption selectivity and high adsorption capacity of aniline in BUT-155 are also well-interpreted by single-crystal structures of the water- and aniline-included phases of BUT-155.

基于铜(II)桨轮结构的金属有机框架（Copper(II)-paddlewheel-based metal–organic frameworks，CP-MOFs）是一类独特的金属有机框架（MOFs）子类，具备高度可预测的多孔结构、简便的合成路径以及可发挥功能的开放金属位点。然而，水解稳定性不足是限制CP-MOFs在诸多实际应用场景中发挥价值的关键瓶颈。在本研究中，我们报道了一种新型CP-MOF：[Cu₄(tdhb)]（命名为BUT-155），其构筑基元为经过审慎设计的羧酸配体，该配体具备高配位连接能力（八齿配体）、丰富的疏水取代基（六个甲基）以及受几何约束的取代基构型（四面体骨架），配体的去质子化形式为tdhb⁸⁻，其中H₈tdhb = 3,3′,5,5′-四(3,5-二羧基苯基)-2,2′,4,4′,6,6′-六甲基联苯。BUT-155具备高孔隙率，其布鲁诺尔-埃米特-特勒（Brunauer–Emmett–Teller，BET）比表面积达2070 m²/g。颇为有趣的是，该CP-MOF在室温去离子水中浸泡10天，或在沸水中煮沸24小时后，仍能保持完整的晶体结构。据我们所知，BUT-155是首款被证实可在沸水中保持结构完整性的CP-MOF。得益于BUT-155优异的水解稳定性，我们得以开展其对水蒸气以及水溶液中有机污染物的吸附性能研究。水蒸气吸附实验显示其吸附等温线呈S型，吸附容量高达46.7 wt%，且吸附过程具备良好的可逆性与可再生性。此外，由于存在软酸型开放Cu(II)位点，BUT-155可高效选择性吸附软碱型苯胺，相较于水或苯酚表现出优异的吸附选择性，且该吸附过程伴随可通过肉眼观测到的MOF样品颜色变化。BUT-155对苯胺的高吸附选择性与高吸附容量，可通过其负载水与负载苯胺的单晶结构得到合理解释。