Paramagnetic organometallic ionic liquids exhibiting thermochromism based on monomer–dimer equilibrium of cationic half-sandwich complexes

Abstract

We synthesized paramagnetic organometallic ionic liquids [Fe(C5Me4Bu)(SC6H4R)(CO){P(OEt)3}][Tf2N] (R = CF3 (1), H (2), OMe (3)), comprising a cationic half-metallocenium iron-thiolate complex and a bis(trifluoromethanesulfonyl)amide (Tf2N) anion. These liquids exhibited thermochromism based on the monomer-dimer equilibrium of the cation.

Chromic Ionic Liquids

“…It is likely that the presence of an electron-withdrawing heptafluoropropyl group attached to the diketone skeleton is responsible for this transformation, which results in a change in color from light yellow to deep red because of the weakened complexation strength. Another recent development is the observation of thermochromic behavior caused by a dimer-to-monomer transformation of half-sandwich Fe­(III) complex monocations through the cleavage of an intermolecular S–S bond upon heating (Figure b) . A thermochromic response induced by the change in the oligomerization ratio has also been observed in square-planar Rh­(I) complex monocations with four n -butyl isocyanide groups (blue-purple at −100 °C to orange at 100 °C) and square-planar Pt­(II) complex monoanions, [Pt­(CN) 2 L] − , with a bidentate heterocyclic ligand such as 2-( p -tolyl)­pyridine (as HL), benzo­[ h ]­quinoline (as HL), and 1-methyl-3-phenyl-1 H -imidazolium (as H 2 L) .…”

“…Thermal equilibria (a) between discrete ionic and bridged neutral forms in ref (C 6 and C 14 represent the C 6 H 13 and C 14 H 29 groups, respectively; R, R′: t -Bu or C 3 F 7 group), (b) between dimeric and monomeric forms in ref (R = CF 3 , H, or OMe), (c) between hydrated and dehydrated forms in ref , and (d) between closed and open forms associated with intramolecular ether coordination in ref .…”

“…Another recent development is the observation of thermochromic behavior caused by a dimer-to-monomer transformation of half-sandwich Fe(III) complex monocations through the cleavage of an intermolecular S−S bond upon heating (Figure 7b). 66 A thermochromic response induced by the change in the oligomerization ratio has also been observed in square-planar Rh(I) complex monocations with four n-butyl isocyanide groups (blue-purple at −100 °C to orange at 100 °C) 67 and squareplanar Pt(II) complex monoanions, [Pt(CN) 2 L] − , with a bidentate heterocyclic ligand such as 2-(p-tolyl)pyridine (as HL), 68 benzo[h]quinoline (as HL), 69 and 1-methyl-3-phenyl-1H-imidazolium (as H 2 L). 69 The ILs, (C n MI)[Pt(CN) 2 L] (n = 2 or 4), undergo transformation from monomers in the glass state to dimers in the liquid state.…”

Organometallic Ionic Liquids Containing Sandwich Complexes: Molecular Design, Physical Properties, and Chemical Reactivities

“…[34] ILs containing this cation are air-sensitive and are equilibrium mixtures of a paramagnetic monomer and diamagnetic dimer (Figure 11c). [35] IL 8 b (R = CF 3 ) changes from pale purple to purple upon heating from À 40 °C to 80 °C, exhibiting thermochromism based on a temperature-dependent equilibrium shift. The color and monomer ratio (70 %-50 %) vary with substituents.…”

Density, Viscosity, Electrical Conductivity, and Surface Tension of Chelate-Based Ionic Liquids [C10mim][M(hfac)3] (M = Co, Ni, Cu) at Different Temperatures

“…Nockemann et al proved that ILs containing the hexafluoroacetylacetonate anion are immiscible with water, and they exhibit strong metal-complexing ability . Mochida et al prepared metallocenium ILs, and the corresponding research was carried out. − They also prepared vapor-responsive ChILs exhibiting reversible changes in color, ChILs with solvatochromic properties, and other metal-containing ILs. , Our group has also been involved in ChIL research. For instance, we developed a class of ChILs which efficiently catalyzed the solvent-free oxidation of cyclohexene by molecular oxygen, which are highly recyclable, and prepared a series of novel alkanolamine-based ILs showing a highly efficient and excellent reversible capture of CO 2 .…”