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

There are various thermochromic materials, though liquid thermochromic materials are less common. To produce functional thermochromic liquids, we have designed ionic liquids based on cationic nickel complexes with ether side chains, [Ni(acac)(Me2NC2H4NR1R2)]Tf2N ([1]Tf2N: R1 = C3H6OEt, R2 = Me, [2]Tf2N: R1 = C3H6OMe, R2 = Me, [3]Tf2N: R1 = R2 = C3H6OMe), where acac = acetylacetonate and Tf2N = (F3CSO2)2N−. The side chains (R1, R2) can moderately coordinate to the metal center, enabling temperature-dependent coordination equilibrium in the liquid state. [1]Tf2N is a liquid at room temperature. [2]Tf2N is obtained as a solid (Tm = 352.7 K) but remains liquid at room temperature after melting. [3]Tf2N is a solid with a high melting point (Tm = 422.3 K). These salts display thermochromism in the liquid state, colored red at high temperatures and orange, light blue, or bluish green at lower temperatures, exhibiting concomitant changes in the magnetic properties. This phenomenon is based on temperature-dependent equilibrium between a square-planar diamagnetic species and a paramagnetic species with intramolecular ether coordination.