Isoreticular Chiral Metal−Organic Frameworks for Asymmetric Alkene Epoxidation: Tuning Catalytic Activity by Controlling Framework Catenation and Varying Open Channel Sizes

Abstract

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.

Formation and characterisation of Mn-MIL-100

“…To the best of our knowledge, no MOF has been reported up to now based on inorganic nodes built up from Mn 3+ and carboxylate ions, although Mn 3+ was incorporated as part of the metal-organic linker molecule. 26,27 Herein, we report the synthesis of the Mnbased analogue of the MIL-100 structure that could be obtained using Mn(II)nitrate as the metal source. The divalent ions are 5 oxidized in-situ in methanolic solution.…”

Metal-Organic Frameworks as Selective or Chiral Oxidation Catalysts

“…Evaluation of the latter in the asymmetric epoxidation of 2,2-dimethyl-2H-chromene gave very high TON values (nearly 4000) 123 Other Salen based MOFs were developed by the group of Lin who synthesized an isoreticular series of Mn-Salen based Zn-MOFs, denoted as CMOF-1 to CMOF-5 (Scheme 13c) (entry 2, Table 5). This family of isoreticular chiral MOFs was examined in the epoxidation of different olefins affording good to excellent yields and ee's 124 . Furthermore they noticed that the rates of the epoxidation reaction strongly depend on the channel sizes.…”

Crystal structure of (4-(1H-imidazol-5-yl)benzoic acid-κN) (4-(1H-imidazol-5-yl)benzoato-κN)silver(I), C20H15N4O4Ag

“…The design and synthesis of metal-organic frameworks (MOFs) have attracted considerable attention because of their potential applications as functional materials as well as their structural diversity and intriguing variety of topologies [1][2][3][4]. The organic linkers with N and O donors often have been employed as e ective building blocks in the construction of MOFs [5][6][7].…”