Isoreticular Chiral Metal-Organic Frameworks for Asymmetric Alkene Epoxidation: Tuning Catalytic Activity by Controlling Framework Catenation and Varying Open Channel Sizes
Isoreticular Chiral Metal-Organic Frameworks for Asymmetric Alkene Epoxidation: Tuning Catalytic Activity by Controlling Framework Catenation and Varying Open Channel Sizes
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DOI：
10.1021/ja1069773
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发表时间：
2010-11-03
影响因子：
15
通讯作者：
Lin, Wenbin
中科院分区：
文献类型：
--
作者：
Song, Feijie;Wang, Cheng;Lin, Wenbin
一族具有原始立方网络拓扑结构的等网格手性金属有机框架（CMOF）由[Zn-4（mu（4）-O）（O2 CR）（6）]二级结构单元和衍生自手性Mn-Salen催化亚基的系统性细长二羧酸根支柱构建而成。CMOF 1-5是通过在溶剂热条件下将三种不同的手性Mn-Salen支柱直接引入骨架中来合成的，并通过多种方法对其进行了表征，包括单晶X射线衍射、PXRD、TGA和H-1 NMR。尽管CMOFs 1对2和CMOFs 3对4由相同的结构单元构成，但它们分别表现出双重互穿或非互穿结构，这取决于用于生长MOF晶体的溶剂的空间尺寸。对于CMOF-5，由于Mn-Salen衍生的二羧酸盐支柱的极端长度，仅获得三重互穿结构。CMOF系列的开放通道和孔径系统地变化，由于可调的二羧酸支柱和可控的互穿模式。CMOFs 1-5被证明是高效的催化剂，用于各种未官能化的烯烃的不对称环氧化，具有高达92%ee。环氧化反应的速率强烈地依赖于CMOFs开放通道的大小，并且CMOFs 2和4的催化活性接近均相对照催化剂。这些结果表明，尽管大体积烯烃和氧化剂试剂的扩散可能是MOF催化的不对称反应中的限速因素，但具有大开放通道的CMOFs（如本研究中的CMOFs 2和4）的催化活性受到催化分子构建块的固有反应性的限制。CMOF催化剂是可回收和可重复使用的，并且在环氧化反应后保留其骨架结构。这项工作突出了通过将明确定义的均相催化剂直接掺入到MOFs的框架结构中来产生高效的非均相不对称催化剂的潜力。
A family of isoreticular chiral metal-organic frameworks (CMOFs) of the primitive cubic network topology was constructed from [Zn-4(mu(4)-O)(O2CR)(6)] secondary building units and systematically elongated dicarboxylate struts that are derived from chiral Mn-Salen catalytic subunits. CMOFs 1-5 were synthesized by directly incorporating three different chiral Mn-Salen struts into the frameworks under solvothermal conditions, and they were characterized by a variety of methods, including single-crystal X-ray diffraction, PXRD, TGA, and H-1 NMR. Although the CMOFs 1 vs 2 and CMOFs 3 vs 4 pairs were constructed from the same building blocks, they exhibit two-fold interpenetrated or non-interpenetrated structures, respectively, depending on the steric sizes of the solvents that were used to grow the MOF crystals. For CMOF-5, only a three-fold interpenetrated structure was obtained due to the extreme length of the Mn-Salen-derived dicarboxylate strut. The open channel and pore sizes of the CMOF series vary systematically, owing to the tunable dicarboxylate struts and controllable interpenetration patterns. CMOFs 1-5 were shown to be highly effective catalysts for asymmetric epoxidation of a variety of unfunctionalized olefins with up to 92% ee. The rates of epoxidation reactions strongly depend on the CMOF open channel sizes, and the catalytic activities of CMOFs 2 and 4 approach that of a homogeneous control catalyst. These results suggest that, although the diffusion of bulky alkene and oxidant reagents can be a rate-limiting factor in MOF-catalyzed asymmetric reactions, the catalytic activity of the CMOFs with large open channels (such as CMOFs 2 and 4 in the present study) is limited by the intrinsic reactivity of the catalytic molecular building blocks. The CMOF catalysts are recyclable and reusable and retain their framework structures after epoxidation reactions. This work highlights the potential of generating highly effective heterogeneous asymmetric catalysts via direct incorporation of well-defined homogeneous catalysts into framework structures of MOFs.