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

Many metal clusters are intrinsically chiral but are often synthesized as a racemic mixture. By taking chiral Ag-14(SPh-(CF3)(2))(12)(PPh3)(4)(DMF) (4) (Ag-14) 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-Ag-14m and S-Ag-14m. The ASDS strategy makes use of two bidentate linkers with different chiral configurations, namely, (1R,2R,(NE)-E-1,(NE)-E-2)-N-1,N-2-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) Ag-14-based metal-organic frameworks (MOFs) were characterized by X-ray crystallography to be [Ag-14(SPh(CF3)(2))(12)(PPh3)(4)(LR)(2)](n) (Ag-14-LR), [Ag-14(SPh-(CF3)(2))(12)(PPh3)(4)(LS)(2)] (n) (Ag-14-LS), and [Ag-14(SPh(CF3)(2))(12)(PPh3)(4)(LRS)(2)](n) (Ag-14-LRS), respectively. As expected, the building blocks in Ag-14-LR or Ag-14-LS are homochiral R-Ag-14 or S-Ag-14, respectively. In contrast, Ag-14-LRS is achiral and crystallizes with a diamond-like structure containing alternate R-Ag-14 and S-Ag-14 clusters. During the assembly process, the racemic Ag-14 clusters were converted to homochiral building blocks, namely, R-Ag-14 for Ag-14-LR and S-Ag-14 for Ag-14-LS. Subsequently, the chiral linkers were removed from the crystals of Ag-14-LR and Ag-14-LS via hydrolysis with water, and from the disassembled solid material Ag-14-DR and Ag-14-DS, optically pure enantiomers R-Ag(14)m and S-Ag(14)m 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.

Automated summary

This study demonstrates an interesting assembly disassembly strategy to obtain optically pure crystals of metal clusters, which were synthesized as racemic mixtures. The metal-organic frameworks characterized by X-ray crystallography exhibit different structural features, reflecting the effectiveness and importance of the assembly disassembly strategy in constructing chiral materials.