Reactivity of Rare-Earth Oxides in Anhydrous Imidazolium Acetate Ionic Liquids

Abstract

Rare-earth metal sesquioxides (RE2O3) are stable compounds that require high activation energies in solid-state reactions or strong acids for dissolution in aqueous media. Alternatively, dissolution and downstream chemistry of RE2O3 have been achieved with ionic liquids (ILs), but typically with additional water. In contrast, the anhydrous IL 1-butyl-3-methylimidazolium acetate [BMIm][OAc] dissolves RE2O3 for RE = La–Ho and forms homoleptic dinuclear metal complexes that crystallize as [BMIm]2[RE2(OAc)8] salts. Chloride ions promote the dissolution without being included in the compounds. Since the lattice energy of RE2O3 increases with decreasing size of the RE3+ cation, Ho2O3 dissolves very slowly, while the sesquioxides with even smaller cations appear to be inert under the applied conditions. The Sm and Eu complex salts show blue and red photoluminescence and Van Vleck paramagnetism. The proton source for the dissolution is the imidazolium cation. Abstraction of the acidic proton at the C2-atom yields an N-heterocyclic carbene (imidazole-2-ylidene). The IL can be regenerated by subsequent reaction with acetic acid. In the overall process, RE2O3 is dissolved by anhydrous acetic acid, a reaction that does not proceed directly.

Ionometallurgy: an academic exercise or promising approach?

“…Since the latter can be regenerated to the initial IL with acetic acid, the overall process comes down to the dissolution of RE 2 O 3 in anhydrous acetic acid, a reaction that does not proceed directly. 54…”

Facile Synthesis of Anhydrous Rare‐Earth Trichlorides from their Oxides in Chloridoaluminate Ionic Liquids

“…However, in these studies, the starting materials are usually dissolved in strong acids, such as nitric acid, and then the further separation in ILs starts from the nitrates [17] . Only a few investigations are devoted to the direct dissolution of RE 2 O 3 in (mostly anhydrous) task‐specific ILs, such as [Hbet][NTf 2 ] (betaine bis(trifluoromethylsulfonyl)imide) [18] or [BMIm][OAc] (1‐butyl‐3‐methylimidazolium acetate) [19] . There are several reports on synthesizing rare‐earth fluorides in ILs by treating the oxides or the hydroxides with fluorinating agents, such as HF, NH 4 F, or NH 4 HF 2 [20–22] .…”

“…[17] Only a few investigations are devoted to the direct dissolution of RE 2 O 3 in (mostly anhydrous) task-specific ILs, such as [Hbet][NTf 2 ] (betaine bis(trifluoromethylsulfonyl)imide) [18] or [BMIm][OAc] (1butyl-3-methylimidazolium acetate). [19] There are several reports on synthesizing rare-earth fluorides in ILs by treating the oxides or the hydroxides with fluorinating agents, such as HF, NH 4 F, or NH 4 HF 2 . [20][21][22] Since the more electronegative fluorine atom can replace an oxygen atom at the RE 3 + cation, the presence of water is less problematic.…”