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.