Isoreticular Chiral Metal−Organic Frameworks for Asymmetric Alkene Epoxidation: Tuning Catalytic Activity by Controlling Framework Catenation and Varying Open Channel Sizes,Journal of the American Chemical Society

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Isoreticular Chiral Metal−Organic Frameworks for Asymmetric Alkene Epoxidation: Tuning Catalytic Activity by Controlling Framework Catenation and Varying Open Channel Sizes
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2010-11-03 , DOI: 10.1021/ja1069773
Feijie Song ₁ , Cheng Wang ₁ , Joseph M. Falkowski ₁ , Liqing Ma ₁ , Wenbin Lin ₁

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A family of isoreticular chiral metal-organic frameworks (CMOFs) of the primitive cubic network topology was constructed from [Zn(4)(μ(4)-O)(O(2)CR)(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 (1)H 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.

中文翻译：

用于不对称烯烃环氧化的等网状手性金属-有机骨架：通过控制骨架链和改变开放通道尺寸来调节催化活性

由 [Zn(4)(μ(4)-O)(O(2)CR)(6)] 二级构建单元和系统地构建原始立方网络拓扑的一系列等网状手性金属有机框架 (CMOF)衍生自手性 Mn-Salen 催化亚基的延长的二羧酸盐支柱。CMOFs 1-5 是通过在溶剂热条件下将三种不同的手性 Mn-Salen 支柱直接掺入骨架中而合成的，并通过多种方法对其进行表征，包括单晶 X 射线衍射、PXRD、TGA 和 (1)核磁共振。尽管 CMOFs 1 vs 2 和 CMOFs 3 vs 4 对是由相同的构建块构建的，但它们分别表现出双重互穿或非互穿结构，具体取决于用于生长 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，仅获得了三重互穿结构。由于可调节的二羧酸支柱和可控的互穿模式，CMOF 系列的开放通道和孔径大小有系统地变化。CMOF 1-5 被证明是一种高效的催化剂，可用于多种非官能化烯烃的不对称环氧化，ee 高达 92%。环氧化反应的速率很大程度上取决于 CMOF 开放通道的尺寸，并且 CMOF 2 和 4 的催化活性接近均相控制催化剂的催化活性。这些结果表明，虽然大烯烃和氧化剂的扩散可能是 MOF 催化的不对称反应的限速因素，具有大开放通道的 CMOF（如本研究中的 CMOF 2 和 4）的催化活性受到催化分子结构单元的固有反应性的限制。CMOF 催化剂可回收和重复使用，并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结合到 MOF 的框架结构中来产生高效非对称催化剂的潜力。

更新日期：2010-11-03

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