Fe(OAc)[EMIM]: Novel Iron-Based Acetate EMIM Ionic Compound.

We synthesized and characterized a novel iron(II) aceto EMIM coordination compound, which has a simplified empirical formula Fe(OAc)[EMIM], in two different hydration forms: as anhydrous monoclinic compound and triclinic dihydrate Fe(OAc)[EMIM]·2HO. The dihydrate compound is isostructural with recently reported Mn(OAc)[EMIM]·2HO, while the anhydrate is a superstructure of the Mn counterpart, suggesting the existence of solid solutions. Both new Fe compounds contain chains of Fe octahedrally coordinated exclusively by acetate groups. The EMIM moieties do not interact directly with the Fe and contribute to the structural framework of the compound through van der Waals forces and C-H···O hydrogen bonds with the acetate anions. The compounds have a melting temperature of ∼94 °C; therefore, they can be considered metal-containing ionic liquids. Differential thermal analysis indicates three endothermic transitions associated with melting, structural rearrangement in the molten state at about 157 °C, and finally, thermal decomposition of the Fe(OAc)[EMIM]. Thermogravimetric analyses indicate an ∼72 wt % mass loss during the decomposition at 280-325 °C. The Fe(OAc)[EMIM] compounds have higher thermal stability than their Mn counterparts and [EMIM][OAc] but lower compared to iron(II) acetate. Temperature-programmed desorption coupled with mass spectrometry shows that the decomposition pathway of the Fe(OAc)[EMIM] involves four distinct regimes with peak temperatures at 88, 200, 267, and 345 °C. The main species observed in the decomposition of the compound are CH, HO, N, CO, OC-CH, OH-CO, HC-CO-CH, and HC-O-CO-CH. Variable-temperature infrared vibrational spectroscopy indicates that the phase transition at 160-180 °C is associated with a reorientation of the acetate ions, which may lead to a lower interaction with the [EMIM] before the decomposition of the Fe(OAc)[EMIM] upon further heating. The Fe(OAc)[EMIM] compounds are porous, plausibly capable of accommodating other types of molecules.