Highly Connected Three-Dimensional Metal−Organic Frameworks Based on Polynuclear Secondary Building Units

A series of new highly connected MOFs [Cd(L)(PBEA)0.5]·H2O (1), [Cd2(μ3-OH)(L)(PBEA)] (2), [Cd3(μ3-OH)(L)2(PBEA)1.5]·H2O (3), and [Mn(μ2-H2O)(L)(PBEA)0.5] (4) have been synthesized by hydrothermal reactions of 3,5-di(imidazol-1-yl)benzoic acid (HL) and 1,4-benzenediacetic acid (H2PBEA) with different metal salts, and these compounds were fully characterized by X-ray diffraction, IR, elemental analysis, TGA. The photoluminescence of 1−4 and magnetic property of 4 were investigated. Complex 1 is a binodal (3,8)-connected tfz-d net based on the binuclear [Cd2(COO)2] secondary building units (SBU1); while 2 is a binodal (3,10)-connected 3D net based on the tetranuclear motif [Cd4(μ3-OH)2(COO)6] (SBU2). Complex 3 is the first example of trinodal (3,8,10)-connected net based on SBU1 and SBU2, which can be considered as combination of the structural features presented in 1 and 2. Complex 4 is an unprecedented (3,8)-connected net based on the binuclear species [Mn2(μ2-H2O)(COO)2] (SBU3), which showed weak antiferromagnetic interactions between the metal ions. Complexes 1−3 were synthesized under the same experimental conditions except that different Cd(II) salts were used in the syntheses. Notably, although the counteranions of Cd(II) salts are absent from the resulting structures, yet they show subtle influence on the self-assembly process to provide different complexes. The approach of incorporating polynuclear metal cluster into highly connected frameworks and linking the SBUs by organic ligands provides opportunities for the design of new MOFs.

本研究通过3,5-二(咪唑-1-基)苯甲酸（3,5-di(imidazol-1-yl)benzoic acid, HL）与1,4-苯二乙酸（1,4-benzenediacetic acid, H2PBEA）分别与不同金属盐进行水热反应，成功合成了系列新型高连接金属有机框架（Metal-Organic Framework, MOF）：[Cd(L)(PBEA)0.5]·H2O (1)、[Cd2(μ3-OH)(L)(PBEA)] (2)、[Cd3(μ3-OH)(L)2(PBEA)1.5]·H2O (3)以及[Mn(μ2-H2O)(L)(PBEA)0.5] (4)。上述化合物均通过X射线衍射（X-ray Diffraction）、红外光谱（Infrared Spectroscopy, IR）、元素分析及热重分析（Thermogravimetric Analysis, TGA）完成了全面表征。此外，本研究探究了配合物1~4的光致发光（Photoluminescence）性能，以及配合物4的磁学性质。

配合物1为基于双核[Cd2(COO)2]次级结构单元（Secondary Building Unit, SBU1）的双节点(3,8)-连接tfz-d拓扑网络；配合物2则是以四核基元[Cd4(μ3-OH)2(COO)6]（SBU2）为构筑基础的双节点(3,10)-连接三维网络。配合物3是首例基于SBU1与SBU2的三节点(3,8,10)-连接拓扑网络，可视为配合物1与2的结构特征的结合体。配合物4为基于双核物种[Mn2(μ2-H2O)(COO)2]（SBU3）的前所未有的(3,8)-连接网络，其金属离子间存在弱反铁磁相互作用。

配合物1~3的合成实验条件完全一致，仅在反应中使用了不同的Cd(II)盐。值得注意的是，尽管Cd(II)盐的抗衡阴离子并未出现在最终产物的结构中，但它们对自组装过程存在微妙影响，进而得到了结构各异的配合物。

将多核金属簇引入高连接框架，并通过有机配体连接次级结构单元的策略，为新型金属有机框架的设计与构筑提供了新的可行路径。