Enantioseparation of Racemic Organic Molecules by a Zeolite Analogue

Abstract

The rational design and construction of hybrid organic ± inorganic zeolite analogues (HOIZAs) for enantioseparation and catalysis is of intense current interest. These solids have shown their potential to control the host ± guest chemistry of the framework by tailoring or tuning the chemical building blocks from which the network is assembled. [1±6] Through the incorporation of appropriate organic ligands the polarity of an intraframework void may be tuned to attract a range of guest molecules. This potential is not available with pure inorganic zeolites. [1] The inclusion of enantiomerically pure chiral building blocks into microporous materials (zeotypes) for enantiomer separation, as well as chiral synthesis and catalysis, has been a continuing challenge and represents an ambitious goal for chemists. [2±4] Currently known materials capable of offering enantiomeric separation and catalysis tend to be 2D layer-type structures that are able to intercalate guests. [1,5] Attempts to resolve enantiomers using zeolite beta, one polymorph of which crystallizes in the chiral space group P4 1 22, have as yet been unsuccessful. [6] It is known that some 3D metal coordination polymers, such as chiral coordination polymers, can reversibly exchange guest molecules while maintaining crystallinity. [7±22] These materials, however, are constructed from achiral building blocks. As a consequence, the product is a racemic mix of crystals, although their single crystals are enantiomeric. Enantiomerically pure HOIZAs with intraframework voids capable of accommodating guest molecules are highly desirable. Ideally, such a 3D framework should have relatively high thermal stability and rigidity, and possess channels that permit reversible inclusion of guests. To the best of our knowledge the existence of chiral 3D HOIZAs capable of resolving racemic guest molecules by the reversible inclusion/removal of guests is unknown. Here we report a robust 3D enantiopure HOIZA capable of resolving racemic mixtures of small organic molecules.Our strategies employed in preparing such a material involve the rational design and use of functionalized chiral ligands that are capable of bridging metal centers to form an open framework. Specifically, such a framework is produced in a self-assembly process by linking cationic metal centers and an enantiopure chiral bridging ligand containing the following functional groups: 1. A carboxylate binding group that makes the ligand anionic and the framework neutral. This ensures that intraframework voids are not occupied by counterions. 2. A mix of hydrophobic and hydrophilic groups that induce the inclusion of a variety of guests. 3. A pyridyl-type group that is able to satisfactorily complete the coordination sphere of the metal center in the generation of a 3D diamondoid coordination polymer. [23±27] One candidate that fulfils these requirements is quitenine (6'-methoxyl-(8S,9R)-cinchonan-9-ol-3-carboxylic acid (HQA)). The enantiopure chiral building block HQA, which is prepared from the off-the-shelf ...

Engineering chiral porous metal-organic frameworks for enantioselective adsorption and separation

“…Chiral MOF catalysts with uniform active sites for enantioselective reactions have been developed based on functionalized chiral molecular (pre)catalysts [21][22][23][24][25][26][27][28][29][30] . In a handful of latest studies, the resolution of enantiomers (racemic alcohols and sulphoxides) with chiral MOFs via chromatography [31][32][33][34] , crystallization [35][36][37][38][39][40] and membrane 41,42 has been reported, but only few exhibit good chiral separation performance.…”

Manganese and Cadmium Benzoate Complexes Having 8-Aminoquinoline Ancillary Ligand

“…These binding modes complicate selective synthesis of metal carboxylate complexes [1][2][3][4][5][6][7][8][9][10][11][12]. A general approach to rationalize selective synthetic procedures for polynuclear carboxylate complexes would require extensive structural study on carboxylate complexes prepared via different synthetic routes [13][14][15][16][17][18][19][20][21][22][23][24][25]. Retro-analysis of structures of polynuclear metal complexes provides information on the smallest mononuclear building blocks to form assemblies [26].…”

“…It is necessary to choose ligand that has versatility to form different types of metal complexes [27][28][29][30][31][32][33]. There is vast literature on mononuclear and polynuclear metal carboxylate complexes [13][14][15][16][17][18][19][20][21][22][23][24][25]34], thus the metal carboxylate complexes would be suitable for such study. The rationalization of the structural aspect through co-ordination capability of the metal ions can be another approach to standardize synthetic procedures.…”

Crystal structure of catena-poly[μ2-2,2′-(1,3-phenylene)diacetato-κ4 O,O′:O′′,O′′′)-(μ2-1,6-bis(2-methyl-1H-benzo[d]imidazol-1-yl)hexane-κ2 N:N′)cadmium(II)], C32H34CdN4O4

“…Great attention in the design and construction of coordination networks has been attracted owing to their intriguing structural motifs and potential applications [1][2][3][4]. The design of effective ligands are crucial to the structural tuning of the resulting complexes [5][6][7][8].…”