# Three-Dimensional Chiral Metal–Organic Frameworks: Synthesis and Structural Transformations

## TL;DR

This paper reports the synthesis of four new chiral metal-organic frameworks and their structural and adsorption properties.

## Contribution

The study introduces new chiral MOFs with solvent-induced structural transformations and size-selective adsorption capabilities.

## Key findings

- The MOFs show phase transitions between open and squeezed frameworks while maintaining network connectivity.
- Adsorption tests revealed a preference for benzene over cyclohexane in [Zn2(camph)2(bpa)].
- Thermal stability varied with the metal cation, from 225°C for Zn2+ to 375°C for Ni2+.

## Abstract

Four new porous homochiral metal–organic frameworks (MOFs), [M2(camph)2(bpa)]∙Solv (M = Co(II), Ni(II), Cu(II) and Zn(II)), based on (+)-camphoric acid (H2camph) and 1,2-bis(4-pyridyl)ethane (bpa) were synthesized and characterized. The crystal structures of [Ni2(camph)2(bpa)] and [Zn2(camph)2(bpa)] were established by single-crystal X-ray diffraction analysis. Powder X-ray data prove the phase purity and isostructural nature of all four compounds. The thermal stability of [M2(camph)2(bpa)] was found to depend on the electronic configuration, as well as on the redox properties of the metal cation, and varied from 225 °C (M = Zn2+) to 375 °C (M = Ni2+). The reversible, solvent-induced sponge-like dynamics of the coordination frameworks was thoroughly investigated. Changes in the positions of reflexes, related to the length of the flexible bpa linker, were observed by powder XRD, pointing to transitions between an open-framework phase and a squeezed, non-porous phase in a crystal-to-crystal manner, while the integrity and connectivity of the coordination network were maintained. Size-selective adsorption from a benzene–cyclohexane 1:1 mixture on [Zn2(camph)2(bpa)] was studied by 1H NMR analysis. The benzene-favorable composition of guest molecules (C6H6:C6H12 = 5:1) occluded within the host crystalline sponge revealed a preferable adsorption affinity towards smaller benzene compared with larger cyclohexane. High framework stability in various solvents, as well as successful molecular separation in the liquid state, validates the potential utilization of chiral porous metal(II) camphorate MOFs in important stereoselective applications.