Unveiling the Dynamic Bond Switching and Continuous Structural Phase Transitions in a Cadmium(II)-Nitrate Crystal

Stimuli-actuated dynamic bond switching enables the modulation of chemical and physical properties in a precise and coherent way, rendering molecular materials broad applications in molecule-based motors, sensors, and energy storage devices. To date, the majority of dynamic bond switching occurs between neutral ligands and cationic metal ions. Herein, we report a new dynamic crystal CdII(L)2(NO3) (1; L = 4-morpholineethanamine) that features the reversible bond switching between the anionic nitrate ion and the cationic cadmium ion. Single-crystal X-ray diffraction analysis indicated that one of the Cd–O1(nitrate) bonds experienced unprecedented elongation from 2.489 (3) Å at 300 K to 2.739 (2) Å, while the rest of the Cd–O2(nitrate) bond showed insignificant variation from 2.467 (3) Å at 300 K to 2.325 (2) Å at 120 K. The anomalous bond length variations lead to the transformation from symmetric one in high-temperature phase (300 K) to asymmetric coordination mode in low-temperature phase (120 K). Such unusual dynamic bond variations also caused continuous structural phase transitions from phase I (above 235 K) to phase II (from 235 to 145 K), and then phase III (below 145 K), wherein the transition from phase I to phase II is of the first order. Compound 1 also exhibited the anisotropic negative thermal expansion (NTE) effect in phase II, which is managed by the deformation of Cd-centered polyhedron and rearrangement of intermolecular interactions. This work demonstrates that utilizing flexible nitrate units is a feasible approach in designing novel dynamic crystalline compounds with switchable structures and functions.