A Charge-Transfer Salt Featuring a High-Entropy and Plastic Crystal

Ionic plastic crystals (IPCs) bridge the structural rigidity of crystalline solids and the dynamic flexibility of liquids featuring partially disordered molecular orientations within an ordered translational lattice. Herein, we report the charge-transfer salt [DMIm][Ni(mnt)₂] (1; [DMIm]⁺ = 1,3-dimethylimidazolium, mnt^2– = maleonitriledithiolate), which undergoes a crystal-to-plastic crystal phase transition at 414 K upon heating. In the low-temperature crystalline phase, 1 crystallizes in the triclinic space group P-1. Its asymmetric unit contains three distinct ion-pairs, comprising six crystallographically independent components. The [Ni(mnt)₂]⁻ anion radicals form one-dimensional stacks, interconnected via [Ni(mnt)₂]⁻ dimers into two-dimensional layers. This layered structure leads to anisotropic dielectric permittivity along directions parallel and perpendicular to the layers. Three crystallographically distinct cations occupy voids within and between these layers. The presence of multiple crystallographically independent components confers a high configurational entropy on 1, reflected in a large entropy change during its phase transition. 1 also exhibits superior ionic conductivity, reaching 0.42 S cm^–1 in its plastic crystal phase. Magnetic analysis reveals strong antiferromagnetic interactions between neighboring anions, rendering 1 nearly diamagnetic at room temperature. This study provides insights for designing multifunctional molecular materials that integrate high entropy, ionic conductivity, and tailored magnetic properties.