Engineering anomalous elastic properties of coordination polymers and their amorphization by employing flexible linkers

Abstract

The common property of all materials is their elasticity, directly related to the chemical components and interactions in their structure. However, the progress in constructing better devices is irreversibly tied to the understanding and designing of static or dynamic structural responses of strained materials. Such structural-property relationships have been explained for polymeric frameworks involving flexible 1,6-hexanediamine (HDA) linkers, with discrete HDA conformers. In the structure of ambient-pressure polymer Cd(HDA)₂(NO₃)₂ the HDA linkers are conformationally disordered. This tetragonal phase α is stable down to 190 K, when the HDA linkers order in different conformations, which triggers a ferroelastic transition to the triclinic phase δ; the flash-cooling of phase α overcools it to 100 K. High pressure induces a ferroelastic transition to triclinic phase β at 1.10 GPa, followed by an isostructural transition to phase γ at 2.00 GPa. All four phases α–δ differ in the configurations of HDA conformers and Cd-coordination involving nitrate linkers. The unusual convex-shaped monotonic compression of Cd(HDA)₂(NO₃)₂ phases α, β and γ has been explained by the mechanism of buckling-sticks: the increasing strain and mounting energy accumulated in the buckling HDA linkers is released by their conversions to shorter conformers. An analogous conformational transition takes place in Cd₂(HDA)₃(NO₃)₄ at 1.50 GPa. In another polymer Cu(HDA)₂(MeCN)₂·2BF₄ the conformational changes induce its very high compressibility and partial amorphization above 1.20 GPa, caused by the non-coordinated conversions of the HDA conformers.