Supramolecular Isomerism of Metal−Organic Frameworks Derived from a Bicarboxylate Linker with Two Distinct Binding Motifs

Two novel metal−organic frameworks (MOFs), Cu(3,4′-bpdc)(H2O)·DMF·2H2O (1) and Cu(3,4′-bpdc)(H2O)·2DMF·4.5H2O (2), have been solvothermally synthesized by the reaction of a unsymmetrically substituted carboxylate ligand 3,4′-biphenyldicarboxylic acid (3,4′-bpdc) and Cu(NO3)2·2.5H2O. X-ray crystal structural analyses reveal that both MOFs possess the binuclear paddle-wheel units [Cu2(O2CR)4], which are linked by the biphenyl connectors, to give two different three-dimensional (3-D) frameworks. The structural diversities are due to the different coordinated arrangements of the two distinct carboxylate groups. In 1, the noncentral symmetrical A−A−B−B arrangement leads to an uncommon twisted Cu-paddlewheel unit, which favors a tetrahedral configuration to generate a 3-fold interpenetrating 3-D framework with a diamond topology. In 2, the paddle-wheel unit constructed by the central symmetrical A−B−A−B arrangement of the ligand acts as a square-planar 4-connected node to give a 2-fold interpenetrating 3-D framework with an NbO topology. These two polymorphs are very rare in MOFs that exhibit similar metal local coordination geometry and the same secondary building units (SBU) but different ligands linking modes. Furthermore, both 1 and 2 possess large one-dimensional channels and show the ability to adsorb H2.

本研究通过溶剂热法，以不对称取代羧酸配体3,4′-联苯二甲酸（3,4′-biphenyldicarboxylic acid, 3,4′-bpdc）与硝酸铜·2.5水合物（Cu(NO₃)₂·2.5H₂O）为反应物，成功合成了两种新型金属有机框架（metal-organic frameworks, MOFs）：Cu(3,4′-bpdc)(H₂O)·DMF（N,N-二甲基甲酰胺）·2H₂O（记为1）与Cu(3,4′-bpdc)(H₂O)·2DMF·4.5H₂O（记为2）。X射线晶体结构分析显示，两种MOFs均含有双核桨轮单元[Cu₂(O₂CR)₄]，该单元通过联苯连接体相互桥联，最终构筑得到两种不同的三维（3D）骨架。二者的结构多样性源于配体上两个独立羧基的配位排布方式差异。在化合物1中，配体采取非中心对称的A-A-B-B排布模式，形成了罕见的扭曲型铜桨轮单元；该单元呈现四面体构型，进而生成具有金刚石拓扑的三重穿插三维骨架。在化合物2中，配体采取中心对称的A-B-A-B排布模式，所构建的桨轮单元作为平面四配位的四连接节点，最终得到具有NbO拓扑的二重穿插三维骨架。这两种同质多晶型物在MOFs体系中十分罕见：它们拥有相似的金属局部配位几何结构与完全一致的次级结构单元（secondary building units, SBU），但配体的连接模式截然不同。此外，化合物1与2均具备较大的一维孔道，且表现出氢气吸附能力。