Mechanical Properties of Modulative Undulating Layers in Two-Dimensional Metal-Organic Frameworks

Two-dimensional (2D) metal-organic frameworks (MOFs) are a class of materials exhibiting various functionalities based on anisotropic layered structures constructed through strong in-plane connectivity and weak van der Waals interlayer interaction. However, their anisotropic mechanical properties and modulation of 2D-MOF crystals have been rarely investigated. Herein, we report the compression and elastic properties of two 2D-MOFs, [Mn(salen)](2)[Pt(CN)(4)]<middle dot>H2O (1) and [Mn(salen)](2)[PtI2(CN)(4)]<middle dot>H2O (2), composed of undulating layers. These layers were highly compressive due to the undulation changes whose compressibility were much larger than those of other crystalline 2D materials. 1 and 2 incorporated structural differences involving the zigzag angles of undulating layers, leading to opposite trends in anisotropic compressibility caused by compression-induced structural transformation between flattening and rippling of the layers. In addition, by conducting high-pressure experiments for 1 using two different pressure-transmitting media (oils or alcohols), we found that ethanol molecules were introduced into the interlayer spaces, unlike oils. This hyperfilling phenomenon resulted in an anisotropic structural transformation involving an expansion along the layer-stacking direction under high pressures. Furthermore, these compression behaviors were impacted by the crystal morphology, such as single crystals and powder forms. Moreover, the Young's moduli in (110) and (001) directions of 1 and 2 were evaluated by nanoindentation experiments, demonstrating the mechanical flexibility of the wavy cyanido-bridged chains.