Cation extraction by 18-crown-6 to a room-temperature ionic liquid: The effect of solvent humidity investigated by molecular dynamics simulations

Abstract

We report a molecular dynamics study of the solvation of 18-crown-6 (''18C6'') and of its K 1 , Cs 1 and Sr 21 complexes in a room-temperature ionic liquid (IL) based on 1-butyl-3-methyl-imidazolium 1 , PF 6 À . ''Dry''models of the IL are compared, demonstrating the importance of solvent humidity on the solvation properties. Upon ''dissolution'' of a piece of crystal, 18C6 is found to undergo a conformational change from C i to D 3d , mainly due to enhanced interactions with the BMI 1 solvent cations and H 2 O molecules, when present. The complexes were first studied with dissociated counterions. In the dry IL, the complexed K 1 and Sr 21 cations are locked at the center of the crown by 1 þ 1 (K 1 ), 1 þ 2 or 1 þ 3 (Sr 21 ) PF 6 À anions in facial positions, respectively. The Cs 1 cation is perched over the crown, solvated by 3 PF 6 À anions. In the humid IL, the complexed K 1 also binds to 1 þ 1 PF 6 À facial anions only (no water), whereas Sr 21 is asymmetrically coordinated to at least 3 H 2 O molecules. When co-complexed with Cl À or NO 3 À counterions, Sr 21 is shielded from the dry IL, but coordinates up to 3 additional H 2 O molecules in the humid IL, while K 1 is not hydrated. The solvation of the ''naked'' K 1 , Cs 1 and Sr 21 ions also markedly depends on the solvent humidity. K 1 is coordinated to 4 PF 6 À anions in the dry IL and by 2 PF 6 À plus 3-5 H 2 O in the humid IL. The most spectacular difference concerns Sr 21 , whose first shell is purely anionic (5 PF 6 À ) in the dry IL, but all neutral (8 H 2 O) in the humid IL. According to an energy component analysis, the 18C6 crown, the cations and their complexes are better solvated by the humid than by the dry IL. Finally, we report simulations of 18C6 and on its Sr C 18C6(NO 3 ) 2 complex at the aqueous interface with the ionic liquid, showing enhanced solvent mixing, compared to the interface with classical organic liquids. The microscopic views obtained by these simulations show the active role of the ionic and aqueous components of the liquid on the solvation of the free crown, the free cations and their complexes.

Solvation of Uranium Hexachloro Complexes in Room-Temperature Ionic Liquids. A Molecular Dynamics Investigation in Two Liquids

“…Our results are in agreement with experimental observations: the UCl 6 2species is stable toward hydrolysis in ionic liquids, and its spectroscopic properties, 14 like those of the PuCl 6 2and NpCl 6 2analogues, 19 are not significantly affected upon addition of a large amount of water. The small effect of IL humidity on the solvation of the UCl 6 ncomplexes contrasts with the spectacular effect observed experimentally [15][16][17][18] and computationally 27,33 with Ln 3+ or UO 2 2+ cations that become hydrated in humid ILs. 33 Arguably, the U-Cl bonds may be stronger than Ln-Cl bonds, due to a larger involvement of bonding f-orbitals with actinides.…”

“…It is thus important to understand under which circumstances water binds to the complexed metal. These issues led us to undertake molecular dynamics (MD) simulations in order to gain microscopic insight into the solvation of key uranium complexes (UCl 6 2-, UCl 6 -, and UCl 6 3-) ILs, MD simulations have been reported on the structure and dynamics of neat ionic liquids 20-23 and of their mixtures with gases or water 24,25 or to study the solvation of complexant molecules [26][27][28][29] and of f-ions. The "naked" Eu 3+ and UO 2 2+ cations have been simulated in ILs such as [BMI][PF 6 ], 30,31 also investigating the effect of added Clor Fanions, 12,32 or the effect of IL humidity.…”

Strontium Nitrate Extraction to Ionic Liquids by a Crown Ether: A Molecular Dynamics Study of Aqueous Interfaces with C4mim+- vs C8mim+-Based Ionic Liquids

“…25 PF 6 -anions that stabilize the first shell of C 4 mim + or C 8 mim + cations, leading to onion-type alternation of IL ions, as observed around alkali, alkaline earth, or lanthanide ions. [44][45][46] Energy Analysis of the Different Complexes. The results of an energy component analysis of the different systems are reported in Table 3.…”

Role of water in complexation of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) with Li+ and K+ in hydrophobic 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquid

“…To clarify what role water molecules play in the complexation between crown ether and metal ions in hydrophobic ILs, it is essential to make clear the complexation in hydrophobic IL system with and without water. Although a molecular dynamics simulation has given the effect of water in hydrophobic ILs on the complex structure and the binding affinities of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6, 18C6) [ 10 ], any experimental evidence has not yet been reported. Hence, we tried to elucidate the effect of water on the complexation of 18C6 with Li + and K + in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C 2 mim][TFSA]) solutions at 298.2 K under both dry and humid conditions using 1 H and 13 C NMR techniques.…”