In-Situ Crystal Growth and Properties of the Magnetic Ionic Liquid [C2mim][FeCl4]

[C2mim][FeCl4] behaves like a paramagnetic liquid at room temperature. Specific heat measurements indicate that there is a magnetic ordering below 3.8 K. The released entropy is far from the expected value for a J= 5/2 system, implying that the magnetic state is partly disordered. Full magnetic polarization was reached in fields above 4 T, and the anomaly in the magnetic susceptibility can be fully suppressed in higher B fields. Hence, the suggested magnetic ground state is superparamagnetic. In order to better understand the magnetic nature of this compound, structural investigations were carried out. Raman and IR-spectroscopy as well as UV–vis absorption measurements reveal the features characteristic for isolated [FeCl4]−anions in the liquid and solid state. The formation of dimeric [Fe2Cl7]−units could be not be evidenced. Thermal investigations show that the compound melts at 283 K. A solid–solid phase transition is observed at 261 K. [C2mim][FeCl4] decomposes above 623 K. By in situ crystal growth, it was possible to trap the two solid polymorphs of [C2mim][FeCl4]. The high temperature form of [C2mim][FeCl4] (1) crystallizes in the centric, monoclinic space group P21/c(No. 14) (a= 9.4124(19) Å, b= 14.645(3) Å, c= 12.444(4) Å, β = 129.841(18)°, V= 1317.1(6) Å3, Z= 4, ρ = 1.557 mg/m3), and the low temperature modification (2) in the non-centrosymmetric, monoclinic space group P21(No. 4) (a= 6.1337(12) Å, b= 14.362(3) Å, c= 7.1111(14) Å, β = 91.33(3)°, V= 626.3(2) Å3, Z= 2, ρ = 1.638 mg/m3). Both polymorphs show the characteristic structural features of the [C2mim]+cation and isolated complex [FeCl4]−anions. However, they deviate in the conformation of the ethyl-chain of the cation and more importantly the relative arrangements of cation and anion. Interestingly, the low temperature modification shows only weak hydrogen bonds, whereas the high temperature modification shows an extended hydrogen-bonding network.