Vapochromic Ionic Liquids from Metal-Chelate Complexes Exhibiting Reversible Changes in Color, Thermal, and Magnetic Properties

Vapor- and gas-responsive ionic liquids (ILs) comprised of cationic metal-chelate complexes and bis(trifluoromethanesulfonyl)imide (Tf2N) have been prepared, namely, [Cu(acac)(BuMe3en)][Tf2N] (1?a), [Cu(Bu-acac)(BuMe3en)][Tf2N] (1?b), [Cu(C12-acac)(Me4en)][Tf2N] (1?c), [Cu(acac)(Me4en)][Tf2N] (1?d), and [Ni(acac)(BuMe3en)][Tf2N] (2?a) (acac=acetylacetonate, Bu-acac=3-butyl-2,4-pentanedionate, C12-acac=3-dodecyl-2,4-pentanedionate, BuMe3en=N-butyl-N,N',N'-tetramethylethylenediamine, and Me4en=N,N,N',N'-trimethylethylenediamine). These ILs exhibited reversible changes in color, thermal properties, and magnetic properties in response to organic vapors and gases. The CuII-containing ILs are purple and turn blue-purple to green when exposed to organic vapors, such as acetonitrile, methanol, and DMSO, or ammonia gas. The color change is based on the coordination of the vapor molecules to the cation, and the resultant colors depend on the coordination strength (donor number, DN) of the vapor molecules. The vapor absorption caused changes in the melting points and viscosities, leading to alteration in the phase behaviors. The IL with a long alkyl chain (1?d) transitioned from a purple solid to a brown liquid at its melting point. The NiII-containing IL (2?a) is a dark red diamagnetic liquid, which turned into a green paramagnetic liquid by absorbing vapors with high DN. Based on the equilibrium shift from four- to six-coordinated species, the liquid exhibited thermochromism and temperature-dependent magnetic susceptibility after absorbing methanol.