Giant Negative Linear Compression along the Stacking Direction of a Highly Compressive Two-Dimensional Coordination Polymer

Abstract

Negative linear compression (NLC) represents one of the most counterintuitive and elusive mechanical responses in crystalline solids. Herein, we report the first example of giant NLC along the layer-stacking direction (c axis) in a cyanido-bridged two-dimensional (2D) coordination polymer, [Mn(salen)]2[ReN(CN)4] (MnReN), with a compressibility coefficient of −24.8(3) TPa–1 at 0.05–0.37 GPa, accompanied by pronounced contraction of undulating layers (a and b axes: +82.8(9) TPa–1). The observed NLC arises because the pressure-induced increase in the thickness of the undulating layers, driven by the rotation of the Mn(salen) units, outweighs the contraction of the interlayer spacing. This mechanism differs from conventional topological hinge-like deformation, revealing a new correlation unique to 2D systems, which does not conform to the previously established relationship between NLC and negative thermal expansion. Under pressures above 0.4 GPa, MnReN undergoes a reversible transformation to a poorly crystalline phase. Furthermore, the presence of lattice water critically modulates these mechanical responses, enabling the switching between positive linear compression and NLC while simultaneously stabilizing the framework via hydrogen bonding. In addition, we demonstrate the metal-ion dependence of the mechanical responses using a Mn analog, [Mn(salen)]2[MnN(CN)4], which gradually transitions into an amorphous-like state as the pressure increases. These findings establish the undulating-layer motif as a new structural platform for achieving pressure-responsive mechanical flexibility in 2D coordination materials.