Thermochromic Magnetic Ionic Liquids from Cationic Nickel(II) Complexes Exhibiting Intramolecular Coordination...

Among the various thermochromic materials, liquid thermochromic materials are comparatively rare. To produce functional thermochromic liquids, we have designed ionic liquids based on cationic nickel complexes with ether side chains, [Ni(acac)(Me₂ NC₂ H₄ NR¹ R² )]Tf₂ N ([1]Tf₂ N: R¹ =C₃ H₆ OEt, R² =Me; [2]Tf₂ N: R¹ =C₃ H₆ OMe, R² =Me; [3]Tf₂ N: R¹ =R² =C₃ H₆ OMe), where acac=acetylacetonate and Tf₂ N=(F₃ CSO₂ )₂ N^- . The side chains (R¹ , R² ) can moderately coordinate to the metal center, enabling temperature-dependent coordination equilibria in the liquid state. [1]Tf₂ N is a liquid at room temperature. [2]Tf₂ N is obtained as a solid (T_m =352.7 K) but remains liquid at room temperature after melting. [3]Tf₂ N is a solid with a high melting point (T_m =422.3 K). These salts display thermochromism in the liquid state, appearing red at high temperatures and orange, light-blue, or bluish-green at lower temperatures, and exhibiting concomitant changes in their magnetic properties. This phenomenon is based on temperature-dependent equilibrium between a square-planar diamagnetic species and a paramagnetic species with intramolecular ether coordination.