Gated Channels in a Honeycomb-like Zinc−Dicarboxylate−Bipyridine Framework with Flexible Alkyl Ether Side Chains
Porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) represent a divers and highly tunable family of inorganic-organic hybrid materials exhibiting high specific surface areas and huge pore volumes. The targeted synthesis of novel network topologies with specific functions is of general interest for the design of MOFs for particular tasks in molecular recognition, gas separation, or sensing applications. The highly variable MOF system [M2L2P]n (M = Cu, Zn; L = linear dicarbxylate linker; P = neutral pillar) has been extensively studied in the literature. All known [M2L2P]n derivatives exhibit either a tetragonal or a Kagomé-type topology. We successfully synthesized a novel derivative of the composition [Zn2(BME-bdc)2(bipy)]n (BME-bdc = 2,5-bis(2-methoxyethoxy)-1,4-benzene dicarboxylate; bipy = 4,4'-bipyridine) which crystallizes in an unexpected honeycomb-like topology with one-dimensional cylindrical channels. The channels are populated by the flexible 2-methoxyethoxy groups of the BME-bdc linkers, which act as gates for incoming guest molecules. This polar environment in the porous channels of [Zn2(BME-bdc)2(bipy)]n is responsible for an exceptionally high sorption selectivity towards CO2 over N2 and CH4.
The key for the construction of the honeycomb-like structure is the disubstitution of the 1,4-benzene dicarboxylate linker, which has a major impact on the conformational freedom of the carboxylate groups. Our concept is substantiated by the syntheses of other isoreticular derivatives of [Zn2(BME-bdc)2(bipy)]n from differently disubstituted bdc derivatives.
Sebastian Henke, Roland A. Fischer, J. Am. Chem. Soc., 2011, 133, 2064-2067.
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