Elastic hydrogen-bonded ionic framework

Elasticity, as an emerging phenomenon of crystals, endows the newfangled properties on crystals owing to the altered local crystallinity in the deformed state, and hence attracts increasing research endeavors. However, only a few molecular crystals and a limited number of one-dimensional coordination polymer crystals have exhibited such fantastic elastic response under mechanical stress. Herein, we report the first example of elastic hydrogen-bonded ionic framework (HIF) of {(CN₃H₆)₂[Ti(μ₂-O)(SO₄)₂]}_n, assembled from one-dimensional negatively charged inorganic [Ti(μ₂-O)(SO₄)₂]_n²ⁿ⁻ chains and positively charged organic guanidinium cations via hydrogen bonds and electrostatic interactions. The slender prismatic single crystal exhibits remarkable elasticity with an optimal elastic bending strain (ε) of 2.5%. Impressively, the crystals give rise to two-dimensional elasticity owing to the equivalent crystallographic planes of the exposed faces and an unusual elastic response at liquid nitrogen temperature. The in-depth crystallographic analyses reveal hydrogen bonds and electrostatic interactions between anion chains and cations function like adhesive glue and account for such specific elastic properties, owing to the flexible and dynamic attributes of hydrogen bonds as they can work in a range of distance and orientation. And the channel in HIF provides space for bending with reduced strain. Incorporating these factors into low-dimensional crystals could be a general guidance for designing elastic crystals. Elasticity ganged with other intrinsic properties of HIF materials could inspire their newfangled applications in the near future.