Obtaining chiral metal-organic frameworks via a prochirality synthetic strategy with achiral ligands step-by-step.

Although some achievements of constructing chiral metal-organic frameworks (MOFs) with diverse achiral ligands have been made, there is still a lack of full understanding of the origin and formation mechanism of chirality, as well as the reasonable principles for the design and construction of chiral frameworks. The concept of prochirality in organic molecules and complex systems inspires us to explore the synthetic strategy of chiral MOFs based on achiral sources. Here, an achiral compound [Cu(en)][(VO3)2] (1) was isolated in the CuCl2/NH4VO3/en system, while further chiral frameworks [Cu(en)(Im)2][(VO3)2] (2a and 2b) were obtained by the reaction between compound 1 and another achiral ligand Im (ethanediamine = en and imidazole = Im). In the present system, compound 1 has the characteristic of a quasi-plane structure unit. Further reaction of compound 1 and the achiral ligand (Im) induced the formation of chiral Λ/Δ Cu centers, and then a pair of chiral frameworks containing one-dimensional (1D) helical chains was formed. The chiral symmetry breaking phenomenon of compounds 2a and 2b can also be expected and explained based on this kind of prochirality synthetic strategy.