Ready Access to Anhydrous Anionic Lanthanide Acetates by Using Imidazolium Acetate Ionic Liquids as the Reaction Medium

Abstract

Access to lanthanide acetate coordination compounds is challenged by the tendency of lanthanides to coordinate water and the plethora of acetate coordination modes. A straightforward, reproducible synthetic procedure by treating lanthanide chloride hydrates with defined ratios of the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate ([C 2 mim][OAc]) has been developed. This reaction pathway leads to two isostructural crystalline anhydrous coordination complexes, the polymeric [C 2 mim] n [{Ln 2 (OAc) 7 } n ] and the dimeric [C 2 mim] 2 [Ln 2 (OAc) 8 ], based on the ion size and the ratio of IL used. A reaction with an IL : Ln-salt ratio of 5 : 1, where Ln = Nd, Sm, and Gd, led exclusively to the polymer, whilst for the heaviest lanthanides (DyÀ Lu) the dimer was observed. Reaction with Eu and Tb resulted in a mixture of both polymeric and dimeric forms. When the amount of IL and/or the size of the cation was increased, the reaction led to only the dimeric compound for all the lanthanide series. Crystallographic analyses of the resulting salts revealed three different types of metal-acetate coordination modes where η 2 μk 2 is the most represented in both structure types.

Anhydrous vs Hydrated f-Element Acetate Polymers Dictated by the Stoichiometry of Protic Acidic/Basic Azole Mixtures

“…In 1 – 7 the polymeric chains are rather compact, which is characterized by two factors, the total number of bridging acetates and the high proportion of the η 3 μ 2 κ 2 mode, affecting Ln–Ln intrachain distances. In this respect 1 – 5 and 7 have strong analogies with the recently discovered series [C 2 mim]­[Ln 2 (OAc) 7 ] . The latter formally deviates from the 3:1 OAc – :Ln ratio, but one of the acetate groups is nonbridging, in fact substituting for the respective position of the N-/O-donor in 1 – 5 .…”

“…We have hypothesized that by dissolving f-element salts in ILs containing only soft donors (e.g., imidazoles) one could drive off volatiles, including water and carboxylic or mineral acids, leaving no choice but to complex the softer donors of the ILs . We have previously shown that f-element salt hydrates can be dissolved and dehydrated in the ILs with a common anion (e.g., nitrate or acetate). − …”

“…Acetate-based ionic liquids not only serve as a reaction medium but also act as good dehydrating agents, allowing precise control over the course of the reaction . The thermodynamic and electrochemical properties of the acetate-based ionic liquids have been actively investigated recently, particularly in light of CO 2 sorption. − Our recent explorations revealed that particularly imidazolium-based acetate ILs showed perfect performance not only in dissolution of insoluble f-element oxides but also in dehydration of the formed complexes .…”

“…The thermodynamic and electrochemical properties of the acetate-based ionic liquids have been actively investigated recently, particularly in light of CO 2 sorption. − Our recent explorations revealed that particularly imidazolium-based acetate ILs showed perfect performance not only in dissolution of insoluble f-element oxides but also in dehydration of the formed complexes . However, while the formed acetate complexes show huge structural variability, they allow ready incorporation of soft N-donors, opening new experimental pathways and showing sensitivity to the cation size …”

“…21 However, while the formed acetate complexes show huge structural variability, they allow ready incorporation of soft N-donors, 18 opening new experimental pathways and showing sensitivity to the cation size. 20 Because of the difficulty in using transuranic elements, we need to validate our methods first with 4f analogues and we need to find simple one-pot reactions to simplify the handling of the transuranic isotopes. We have chosen Nd 3+ as an analogue for 243 Am 3+ and Ce 3+ /Ce 4+ as analogues for 239 Pu 3+ /Pu 4+ , given their charge to size ratios.…”