Phase-Dependent Responsiveness in Soft Porous Crystals Assembled from Flexible Metal-Organic Cages

Abstract

Soft porous crystals (SPCs) based on flexible metal-organic cages (MOCs) remain elusive due to challenges in preserving both dynamic behavior and crystallinity after desolvation. Here, we report an SPC assembled from flexible metal-organic cages (MOCs), in which the resulting porous properties, either conventional microporous filling or gate-opening behavior, depend on the activation process. The flexible MOCs are constructed by a covalent linking strategy; amine-functionalized dirhodium paddlewheel complexes serve as building blocks and are connected with aldehyde units through Schiff base condensation. By systematically tuning ligand substituents, axial pyridine coordination, and activation conditions, we achieve a delicate balance between crystallinity and flexibility. Notably, one compound provided two distinct activated phases under different activation conditions. Their structures were determined by three-dimensional electron diffraction (3D ED). One phase displays a conventional type I CO2 isotherm at 195 K, confirming the rigid MOC assembly. In contrast, the other phase exhibits reversible stepwise CO2 adsorption accompanied by hinge-like ligand flipping and packing rearrangements, as confirmed by in situ powder X-ray diffraction (PXRD) and atomistic simulations. In this system, the gasresponsive behavior is governed by both molecular conformation and packing, providing a unique strategy for developing a novel SPC class based on the flexible MOCs.