Chiral Metal Self-Assemblies of Zirconium-Tetrahedra and Their Second Harmonic Generation Activity.

Abstract

The chirality of metal-organic cages holds enormous potential for novel applications in diverse fields, while it is relatively rare to employ such asymmetric units for the construction of noncentrosymmetric materials. Herein, by self-assembling the 4,4',4''-nitrilotribenzoic acid (H3NBA) with bis(cyclopentadienyl)-zirconium dichloride (Cp2ZrCl2, Cp=η5-C5H5) in different solvent conditions, we have obtained three hierarchical packing modes of metallo-tetrahedra with distinct spatial symmetry groups (designated as Zr-α, Zr-β, and Zr-γ). Among them, Zr-α employs a simple cubic arrangement and is a common centrosymmetric superstructure, which consists of a pair of equimolar metallo-tetrahedra enantiomers in its unit cell. While Zr-β results in conglomerates with spontaneous resolution without using any resolving agents, giving rise to two enantiopure entities separately (Zr-β-P, Zr-β-M). More importantly, Zr-γ breaks the inversion center of symmetry and crystallizes into a racemic yet non-centrosymmetric superstructure with face-centered cubic packing mode. Based on the non-centrosymmetric nature, the hierarchical superstructure Zr-γ displayed good second harmonic generation activities. This work presents a successful instance wherein the reaction solvent induces the modulation of intermolecular packing mode to afford non-centrosymmetric solid materials, which can greatly promote the development of noncentrosymmetric solid (NCS) materials.