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Assembly of Chiral Cluster-Based Metal–Organic Frameworks and the Chirality Memory Effect during their Disassembly

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journal contribution
posted on 28.06.2021, 21:29 by Guocheng Deng, Boon K. Teo, Nanfeng Zheng
Many metal clusters are intrinsically chiral but are often synthesized as a racemic mixture. By taking chiral Ag14(SPh­(CF3)2)12(PPh3)4(DMF)4 (Ag14) clusters with bulky thiolate ligands as an example, we demonstrate herein an interesting assembly disassembly (ASDS) strategy to obtain the corresponding, optically pure crystals of both homochiral enantiomers, R-Ag14m and S-Ag14m. The ASDS strategy makes use of two bidentate linkers with different chiral configurations, namely, (1R,2R,N1E,N2E)-N1,N2-bis­(pyridin-3-ylmethylene)­cyclohexane-1,2-diamine (LR) and the corresponding chiral analogue LS. For comparison, we also use the racemic mixture of equimolar of LR and LS (LRS). Three three-dimensional (3D) Ag14-based metal–organic frameworks (MOFs) were characterized by X-ray crystallography to be [Ag14(SPh­(CF3)2)12(PPh3)4(LR)2]n (Ag14-LR), [Ag14(SPh­(CF3)2)12(PPh3)4(LS)2]n (Ag14-LS), and [Ag14(SPh­(CF3)2)12(PPh3)4(LRS)2]n (Ag14-LRS), respectively. As expected, the building blocks in Ag14-LR or Ag14-LS are homochiral R-Ag14 or S-Ag14, respectively. In contrast, Ag14-LRS is achiral and crystallizes with a diamond-like structure containing alternate R-Ag14 and S-Ag14 clusters. During the assembly process, the racemic Ag14 clusters were converted to homochiral building blocks, namely, R-Ag14 for Ag14-LR and S-Ag14 for Ag14-LS. Subsequently, the chiral linkers were removed from the crystals of Ag14-LR and Ag14-LS via hydrolysis with water, and from the disassembled solid material Ag14-DR and Ag14-DS, optically pure enantiomers R-Ag14m and S-Ag14m were obtained. It is hoped that this simple assembly strategy can be used to construct cluster-based chiral assemblage materials and that the subsequent disassembly protocol can be used to obtain optically pure chiral cluster molecules from as-prepared racemic mixtures.