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Solvation of fluoro and mixed fluoro/chloro complexes of EuIII in the [BMI][PF6] room temperature ionic liquid.
A theoretical studyw
Alain Chaumont, Georges Wipff
To cite this version:
Alain Chaumont, Georges Wipff. Solvation of fluoro and mixed fluoro/chloro complexes of EuIII in
the [BMI][PF6] room temperature ionic liquid. A theoretical studyw. Physical Chemistry Chemical
Physics, Royal Society of Chemistry, 2005, 7, pp.1926-1932. �hal-00016639�
Solvation of fluoro and mixed fluoro/chloro complexes of Eu
IIIin the [BMI][PF
6] room temperature ionic liquid. A theoretical study w
A. Chaumont and G. Wipff*
Laboratoire MSM, UMR CNRS 7551, Institut de Chimie, 4 rue B. Pascal, 67000 Strasbourg, France. E-mail: wipff@chimie.u-strasbg.fr
Received 19th November 2004, Accepted 10th March 2005 First published as an Advance Article on the web 29th March 2005
We report a molecular dynamics study on the solvation of EuFn(3n)complexes in the [BMI][PF6] ionic liquid, composed of 1-butyl-3-methyl-imidazolium1cations and PF6anions. It is found that the most fluorinated complex in the liquid should be the EuF63species. In solution the EuF107and EuF74complexes indeed loose, respectively, 4 and 1 Fanion to form the EuF63complex, while the first solvation shell of the less fluorinated complexes (n¼1 to 5) is completed with 5 to 1 PF6anions to form an octahedral first shell around Eu31. There is one case (simulations with a ‘‘small’’ Fmodel) where the EuF74complex remains stable, and cannot
therefore be fully precluded. The anionic complexes are embedded in a cage formed by 6–9 BMI1cations at ca. 8 A˚, hydrogen-bonded by imidazolium–CH Finteractions. Simulations on the mixed EuFnCl6n3 complexes in solution and in gas phase also reveal the highest stability of EuF63compared to the mixed or the EuCl63
complexes. This is confirmed by free energy perturbation calculations and results from the stronger coordination of F, compared to Clligands, as well as from better solvation of the fluoro complexes by the ionic liquid. In the gas phase, however, QM and MM calculations indicate that EuF63is unstable towards the dissociation of 1 to 2 Fions, which points to the importance of environment and solvation forces on the stability of this octahedrally coordinated lanthanide complex.
Introduction
There is growing interest in room temperature ionic liquids
‘‘RTIL’s’’ whose versatile solvation and physico-chemical properties can be ‘‘tailor made’’ by adequate choice of the organic cation (generally imidazolium, ammonium or phos- phonium derivatives) and of the anion (e.g. halides, AlCl4, PF6, CF3SO3, [CF3SO2)2N]).1–3 Due to their chemical stability, very low vapor pressure and non-flammability, they appear as ‘‘green solvents’’ (see, however, ref. 4) for many applications, ranging from synthesis, catalysis, electrochemis- try, ion separation. RTILs based on hydrophobic components are macroscopically non-miscible with water, and can thus be used for liquid–liquid extraction purposes.5–9 A particularly important field concerns nuclear waste processing, which aims at partitioning cations as a function of their radiotoxicity.10A precise knowledge of coordination properties of actinides and lanthanides is thus particularly important in this context, as well as for understanding their spectroscopic behaviour (e.g., luminescence11–13or EXAFS14,15data). This led our group to undertake computer simulations (quantum mechanics QM and molecular dynamics MD) on the solvation of important ions like uranyl, strontium and trivalent lanthanides Ln31 in RTILs, comparing the [EMI][TCA] and [BMI][PF6] solvents, both based on imidazolium cations.16–18The former solvent is water miscible and can be used for electrochemical processes while [BMI][PF6] is macroscopically non-miscible with water and has been used for liquid–liquid extraction purposes. The solvents were first considered in the ‘‘ideal’’ neat state, but it was shown later that the presence of ‘‘impurities’’ like water19
or Cl anions20 dramatically modifies the nature of the first coordination shell and solvation patterns of the metals.
In this paper, we focus on the fluoro complexes of EuIII, an average-sized lanthanide cation, in the [BMI][PF6] solution.
These are important per se, e.g. to investigate the effect of adding fluoride salts to an europium solution in the ionic liquid. Furthermore, Fanions may form upon degradation of anionic components of RTILs like PF6, AsF6or BF4in the presence of water or Lewis acids,4,21,22and may be com- plexed by the LnIIIcation. Our aim is to first determine the preferred stoichiometry of the complex,i.e.how many Fcan bind to EuIIIin this peculiar environment. Indeed, in the solid state, lanthanide fluorides generally display high metal coordi- nation numbers ‘‘CN’’ (nine for LnF3),23which is similar to CNs observed, e.g. with lanthanide hydrates.24 This is also close to the CN of the ‘‘naked’’ Eu31cation simulated in the [BMI][PF6] ionic liquid (10 F atoms arising from 6 PF6 anions).17In the Cambridge Crystallographic Structural Da- tabase,25one finds structures where the lanthanide(III) cation is surrounded by up to 11 or 12 F atoms.26On the other hand, hexafluoro complexes (CN ¼ 6) have been characterized in molten salts of LnF–KF mixtures,27 thus bearing marked analogies with the corresponding chloro complexes which also display an octahedral coordination in molten salts28,29in RTIL solutions,30–32 as well as in solid state structures.31,33,34 We note that a same coordination number of six chlorides was also predicted from theoretical studies in the [BMI][PF6] solu- tion.20,35 On the other hand, recent EXAFS results on euro- pium triflate dissolved in RTILs with PF6
, BF4
and Tf2N anionic components (and thus as a possible source of F anions upon degradation), according to which the coordina- tion number of europium isca. 10 was an important impetus for our study.36 It is thus important to assess how many fluoride anions can bind to LnIII cations in an ionic liquid environment. When compared to the simulation results of different halide complexes in the gas phase, the results allow wElectronic supplementary information (ESI) available: Tables S1–S5
report the characteristics of the complexes simulated by QM in the gas phase, and the results of structural and energy analysis of the com- plexes in the RTIL. Figures S1 and S2 are the colour versions of Figs. 2 and 4. See http://www.rsc.org/suppdata/cp/b4/b417598k/
R E S E A R C H P A P E R
PCCP
www.rsc.org/pccp
DOI:10.1039/b417598k
us to better understand the effect of ionic liquid environment on the preferred stoichiometry and stability of the fluoro- complexes. Another issue addressed in the paper concerns the comparison of chloroversusfluoro complexes, in the RTIL and in the gas phase.
Methods
Molecular dynamics
The different systems were simulated by classical molecular dynamics ‘‘MD’’ using the AMBER 7.0 software37in which the potential energyUis described by a sum of bond, angle and dihedral deformation energy and pairwise additive 1-6-12 (electrostatic and van der Waals) interactions between non- bonded atoms.
U¼X
bonds
kbðbb0Þ2þ X
angles
kyðyy0Þ2
þ X
dihedrals
X
n
Vnð1þcosðnjgÞ
þX
ioj
qiqj Rij
2eij Rij Rij
6
þeij
Rij Rij
12
" #
Cross terms in van der Waals interactions were constructed using the Lorentz–Bertholot rules. The parameters used for the pure liquid were taken from Andrade et al. for the BMI1 cation38while those for the PF6anion originate from the OPLS force field39and have been used by Marguliset al. to simulate ionic liquids.40 These parameters have shown good agreement with experiment. Furthermore, there is good agreement between the AMBER and QM calculated (HF level with a 6-31þG* basis set) on the gas-phase optimized BMI1 PF6dimer (75.9 and 76.2 kcal mol1, respectively). See also ref. 41.
The parameters of the Eu31(R*¼1.852 A˚ ande¼0.05 kcal mol1), F(R*¼1.850 A˚ ande¼0.2 kcal mol1) and Clions (R*¼2.495 A˚ ande¼0.07 kcal mol1) were fitted on their free energies of hydration.42,43 These F and Cl models will be termed ‘‘standard’’. Tests were also performed with ‘‘smaller’’
F and Cl anions, whose parameters came from the PF6 (R*¼1.746 A˚ ande¼0.061 kcal mol1)39and the AlCl4anions (R*¼1.943 A˚ ande¼0.265 kcal mol1),38,44respectively. The 1–
4 van der Waals interactions were scaled down by 2.0 and the 1–4 Coulombic interactions were scaled down by 1.2, as recommended by Cornellet al.45The pure liquids and solutions were simulated with 3D-periodic boundary conditions. Non-bonded interactions were calculated using a 12 A˚ atom-based cut-off, correcting the long-range electrostatics by using the Ewald summation method (particle–particle mesh Ewald approximation).46
The MD simulations were performed at 400 K in order to enhance the diffusion and sampling, but tests performed at 300 K on EuFn3n complexes were found to yield similar results. The dynamics started with random velocities, using the Verlet leapfrog algorithm with a time step of 2 fs to integrate the equations of motion. The temperature was mon- itored by coupling the system to a thermal bath using the Berendsen algorithm47 with a relaxation time of 0.2 ps. All C–H bonds were constrained using the SHAKE algorithm.
We first equilibrated ‘‘cubic’’ boxes of pure [BMI][PF6] liquid of 37.7 A˚ length containing 150 pairs of BMI1 PF6 ions. After equilibration, the solvent density (1.33 g cm3) was in reasonable agreement with experiment (1.36 g cm3).48,49 We then immersed one EuFn3nor one EuFnCl6n3complex in the liquid, adding or removing BMI1 cations to keep the total box neutral, when necessary. Solutions of EuFn3n, 6n F (with n ¼ 4 to 6) were similarly prepared, in order to compare the EuF63 complex with its partially dissociated analogues. Another series of calculations considered the [BMI][PF6,F] liquid which contains an equal amount of PF6
and F solvent anions and in which 4 EuF3 species were immersed. The characteristics of the simulated systems are given in Table 1. Equilibration started with 1500 steps of steepest descent energy minimization, followed by 50 ps with fixed solutes (‘‘BELLY’’ option in AMBER) at constant volume, followed by 50 ps of constant volume without con- straints, and by 50 ps at a constant pressure of 1 atm by coupling the system to a barostat47with a relaxation time of 0.2 ps. Then MD was run for 2 ns in the (NVT) ensemble.
The MD trajectories were saved every 1 ps and analysed with the MDS and DRAW software.50 Typical snapshots were redrawn using the VMD software.51 Insights into energy features were obtained by group component analysis, using a 17 A˚ cut-off distance and a reaction field for the electrostatics.
The average structure of the solvent around Eu31 was char- acterized by the radial distribution functions (RDFs) of the anions (P atoms) and cations (Nbutyl atoms) during the last 0.2 ns. The average coordination number (CN) of the solvent anions and cations and its standard deviation were calculated by integrating the first peak of the RDF.
Free energy calculations: FversusClrelative binding affinities The change in free energy when the X anion is stepwise mutated into Yin solution (either in its free state or within the europium complex) was calculated using the statistical perturbation theory and the TI technique,52based on eqn. (1):
DG¼ Zl2
l1
@U
@l
l
dl ð1Þ
The ‘‘hybrid’’ potential energy U(l) was calculated from a linear combination ofR* andeparameters of the initial (l¼1) and final state (l¼0):
R*l¼lR*1þ(1l)R*0andel¼le1þ(1l)e0
The mutation was achieved in 51 windows (of 20þ30 ps each) and the changes in free energy were averaged from the forward and backward cumulated values.
Quantum mechanical calculations
The EuFn3n(n¼1 to 7) and EuFnCl6n3complexes (n¼0 to 6) were optimized without symmetry constraints by quantum mechanical calculations at the Hartree–Fock (HF) and DFT (B3LYP functional) levels of theory, using the GAUSSIAN 98 software.53 Asf-orbitals do not play a major role in metal–
ligand bonds,54the 46 core and 4f electrons of the Eu were described by quasi-relativistic effective core potential (ECP) of the Stuttgart group.55,56For the valence orbitals, the affiliated (7s6p5d)/[5s4p3d] basis set was used, enhanced by an addi- tionalf-function of exponent 0.591.57The F and Cl atoms were represented by the 6-31þG* basis sets. The final total energies, geometries and atomic charges are given in Table S1.w All complexation energies were corrected for BSSE using the counterpoise method.58
Results
We will first discuss the solvation characteristics of the EuFn3n fluoride complexes in [BMI][PF6]. This is followed by the solvation of mixed halogenated EuFnCl6n3 complexes and by an energy analysis of these systems in solution and in the gas phase. Unless otherwise specified, the results reported in solu- tion are obtained with the ‘‘standard’’ Fand Clmodels at 400 K. The corresponding results are given in Tables 2–4 and in Figs. 1–4. The solvation of the EuFn3ncomplexes turned out to be similar, be they simulated as such (without additional Fanions) or as EuFn3n, 6nFcomplexes (i.e.with a fixed
total number of 6 Fanions,nbeing coordinated to europium and 6nbeing dissociated, atE15 A˚). This is why the latter will be considered only for the purposes of energy analysis within the hexafluoro series.
1. Stability and solvation of EuFn3ncomplexes (n= 1–7 and 10) in [BMI][PF6] solution
We started our simulations with the EuF107complex whose structure has been extracted from the simulation in [BMI][PF6] where the ‘‘naked’’ Eu31cation is ‘‘solvated’’ by 10 F atoms of the first shell PF6anions.17This complex turned out to be unstable and to lose 4 Fanions at the minimisation stage to form the EuF63complex, which remained stable for 2 ns. This
complex is embedded in a cage of 8 BMI1cations atca. 7.5 A˚, while the 4 Fanions which decomplexed sit in a second shell (atca. 7.5–9.0 A˚) connectedviaH-bonds to the BMI1‘‘cage’’.
We then studied the EuF74complex, which remained stable for about 0.7 ns, but then lost one Fanion to form again the EuF63 species. This is why we decided to further simulate EuF63which remained stable during the whole dynamics. As seen from the radial distribution functions (RDF’s) (Fig. 1), this complex is embedded in a cage of 7.4 BMI1(see Table 2), while PF6anions sit in its second shell.
When similar simulations where repeated with the ‘‘small’’
Fmodel,39the EuF107and EuF85complexes also lost 4 and 2 Fanions, respectively, to form EuF63as with the ‘‘stan- dard’’ Fmodel. For the EuF74complex, however, a different behavior was observed, as it remained bound during the whole dynamics (2 ns), as well as after additional 3 ns of simulation.
Less halogenated EuFn3n complexes (n ¼ 1 to 5) were similarly simulated in solution, and all remained stable during the whole dynamics. Their first shell was completed by 5 to 1 PF6 anions, respectively, forming an octahedral solvation structure. Typical solvation patterns of these complexes can be seen in Fig. 2. Around the EuF52and EuF4complexes the PF6 anions coordinate monodentate to Eu31, leading to a coordination number of 6 F atoms. Around the EuF21, EuF21
and EuF3complexes, the PF6anions are either monodentate or bidentate, as can be seen from the splitting of the first peak of the P–Eu RDF’s, leading to a coordination number of 9 F (1 Fand 8 FPF6) for EuF21, 8 F (2 Fþ6 FPF6) for EuF21
, and 7 F atoms (3 Fþ4 FPF6) for EuF3. All these complexes are embedded in a cage of 9 to 11 BMI1cations, at about 10 A˚.
As expected, the average Eu–F distances increase with the numbernof Fligands (Eu–F¼2.32, 2.33, 2.34, 2.36, 2.38 and 2.40 A˚, respectively, when n increases from 1–6), due to enhanced intrashell repulsions between the anions.
2. Solvation of mixed EuFnCl6n3complexes (n= 3,4,5) in [BMI][PF6] solution
We further simulated EuFnCl6n3 (n ¼ 3,4,5) hexa-halo complexes in the [BMI][PF6] solution, comparing thecisand Table 2 EuIIIcomplexes (‘‘standard’’ Fmodel) in [BMI][PF6] solu-
tion at 400 K Characteristics of the first peak of the solvent RDFs around Eu31; coordination number (first line). Distance (A˚) of the first maximum and minimum (second line). Averages over the last 0.2 ns of MD
PPF6 NBMI
EuF21 5.0 10.5
3.6 (4.0); 4.6 6.4; 9.7
EuF21
4.0 11.5
3.6 (4.0); 4.6 6.4; 9.8
EuF3 3.0 6.6
3.6 (4.1); 5.0 6.3; 8.1
EuF4 2.0 5.0
4.1; 4.9 6.1; 7.1
EuF52 1.0 6.2
4.1; 5.0 5.5; 7.1
EuF63 19.6 7.4
9.2; 12.1 5.4; 7.0
EuF5Cl3 7.0 11.6
8.2; 9.0 5.8; 9.7
EuF4Cl23 17.2 10.0
9.1; 11.7 5.7; 9.0
EuF3Cl33 (trans)
5.4 8.6
8.5; 9.3 5.8; 7.9
EuF3Cl33
(cis) 10.2 10.3
8.8; 10.0 6.1; 8.7
EuCl63 21.6 11.1
8.6; 12.5 6.0; 9.6
Table 3 EuFn3n complexes (‘‘standard’’ F model) in [BMI][PF6] solution at 400 K: Average interaction energies (kcal mol1) of the complex with the liquid and its anionic and cationic components.
Averages and RMS fluctuations (in parentheses) taken over the last 0.2 ns
BMI1 PF6 [BMI][PF6]
EuF21 920 (21) 1461 (21) 541 (14)
EuF21 460 (11) 754 (13) 294 (13)
EuF3 52 (6) 84 (7) 137 (8) EuF4
637 (9) 475 (10) 162 (11) EuF52
1223 (15) 930 (17) 292 (15) EuF63
1859 (24) 1266 (18) 593 (20)
Table 4 EuFnCl6n3(n¼0, 3, 4, 5, 6) complexes (‘‘standard’’ Fand Clmodels) in [BMI][PF6] solution at 400 K: Average interaction energies (kcal mol1) of the complex with the solvent and its anionic and cationic components and internal energy of the complex. Averages and RMS fluctuations (in parentheses) taken over the last 0.2 ns
BMI1 PF6 [BMI][PF6] ESOLUTE
EuCl63 1715 (18) 1250 (18) 465 (17) 949 (2) EuF3Cl33(trans) 1797 (19) 1285 (18) 513 (16) 1033 (3) EuF3Cl33(cis) 1817 (20) 1283 (19) 533 (18) 1032 (3) EuF4Cl23(cis) 1783 (20) 1230 (20) 552 (16) 1056 (3) EuF5Cl3 1873 (26) 1303 (21) 570 (21) 1079 (3) EuF63
1859 (24) 1266 (18) 593 (20) 1098 (3) Table 1 Main characteristics of the simulated systems
[BMI][PF6] BMI1 PF6 Box length/A˚ Time/ns
EuF21 200 202 42.0 2
EuF21 200 201 42.0 2
EuF3 200 200 41.6 2
EuF4 201 200 41.6 2
EuF52 202 200 41.6 2
EuF63 203 200 41.7 2
EuF4, 2 F 203 200 41.7 2
EuF52, F 203 200 41.7 2
EuF74 204 200 41.7 2
EuF107 207 200 42.0 2
EuCl63 294 291 47.2 2
EuF3Cl33 203 200 41.8 2
EuF4Cl23 203 200 41.7 2
EuF5Cl3 203 200 41.7 2
[BMI][PF6,F] BMI1 PF6 F Box length/A˚ Time/ns
4 EuF3 402 201 201 49.9 2.5
transisomers forn¼3 and 4. They all remained stable during the whole dynamics, except for thetransEuF4Cl23complex, which rearranged to thecisform during the minimisation. In these complexes the Eu–F bonds are shorter than the Eu–Cl bonds (2.40 A˚versus ca. 2.85 A˚, respectively, on average). The complexes are 3 charged and surrounded by a positively charged cage formed by 9 to 11 BMI1cations, at about 10 A˚, as can be seen from the RDF’s (Fig. 3 and Table 2). These BMI1cations are hydrogen bonded to the For Clligands via their imidazolium C2H, C4H or C5H protons, while their alkyl chains are more remote. Typical snapshots of the first solvation shell can be found in Fig. 4.
3. Energy analysis of the fluoro and chloro europium complexes in the gas phase and in solution
Stepwise complexation of F. The stability of the EuIII complexes was first investigated in the gas phase via the stepwise F complexation by Eu31 (eqn. (1)) as a reference to better understand their solution behaviour.
EuFn(3n)þF-EuFn11(2n) (3) The energy results obtained by molecular mechanics (compar- ing the two models for F) and by QM methods at the HF/
6-31þG* and DFT B3LYP/6-31þG* levels are presented in Fig. 5. Except for a small stability inversion between the EuF4 and EuF52 complexes, the three approaches yield similar conclusions, as found for the corresponding chloro com- plexes.20This is why we only discuss the DFT results which area priorimost satisfactory. They show that the addition of the first F anions to Eu31 is favourable, but becomes less exothermic asnincreases from 1 to 3 (332,232 and103 kcal mol1, respectively). For the complexation from 4 to 5 F, there is a smaller energy difference (þ32 kcal mol1) and for n¼6 and 7, the reaction is clearly endothermic and unfavour- able (þ121 andþ232 kcal mol1, respectively). Thus, in the gas phase, EuF74, EuF63and EuF52complexes are intrinsically unstable and should loose from three to one fluoride anions, respectively. Concerning the structure of the optimized com-
plexes (Table S1w), it is seen that HF and DFT optimized distances are the same within 0.01 A˚, and intermediate between MM optimized distances obtained with the ‘‘small’’ and
‘‘standard’’ F models. As expected, the Eu–F distances increase with the numbernof Fligands (byca. 0.5 A˚ in the studied series).
Fig. 1 EuFn3n complexes in [BMI][PF6] solution at 400 K: radial distribution functions of the [BMI][PF6] ionic liquid around the Eu atom. Distances (on the abscissa) are in A˚.
Fig. 2 EuFn3ncomplexes in [BMI][PF6] at 400 K: Represenation of the first solvation shell of anions only, cations only and anionsþ cations. A colour version of the figure is given in Fig. S1.w
Fig. 3 EuFnCl6n3ncomplexes in [BMI][PF6] at 400 K: radial dis- tribution functions of the [BMI][PF6] ionic liquid around the Eu atom.
Distances (on the abscissa) are in A˚.
When simulated in the ionic liquid all EuFn(3n)complexes (n ¼ 1 to 6) remain ‘‘stable’’ (no dissociation) and display attractive interaction energiesEsolvwith the solvent (Table 3).
Furthermore, Esolvincreases with the absolute charge of the complex, and EuF63is better solvated than the less fluori- nated analogues. It is interesting to compare the gain in solvation energies DEsolv when the number of F-ligands in- creases (Table 3), with the change in intrinsic energyDEcof the complex, as obtained, e.g.from DFT calculations in the gas phase (Fig. 5). The EuF3to EuF4complexation is favoured byDEsolvas well as byDEc. Further complexation of one and two F is disfavored by DEc (by 32 and 121 kcal mol1, respectively), but this is more than compensated for byDDEsolv (130 and300 kcal mol1, respectively).
The above analysis concerns solutes of different composi- tions. An alternative approach consists of comparing the
‘‘solvation energy’’ of EuFn3n, 6n F solutes therefore
including the contribution of the 6 nfree Fanions. This was achievedviatwo different procedures. (i) By simply adding theEsolvcontribution of F(125 8 kcal mol1) obtained from a simulation of this anion in solution, and (ii) by independent simulations on the EuFn3n, 6nFsolutes with 6ndissociated anions (n¼4, 5, 6). The results confirm the sequence of increasing solvation energies from n ¼ 4 to 6:
412, 542 and 595 10 kcal mol1, respectively, with procedure (i) and433,480,59510 kcal mol1, respec- tively, with procedure (ii). This confirms that the intrinsically unstable EuF63 and EuF52 complexes are stabilized by antagonistic solvation forces in the ionic liquid.
An interesting comparison concerns complexes with the same absolute charge but of opposite sign (EuF21
versus EuF4
; EuF21 versus EuF52
), showing that the positively charged complexes are better solvated than their negatively charged analogues (by 132 and 249 kcal mol1for the1 and 2 charged complexes, respectively). This is mostly due to the interaction of first shell PF6anions which stabilise the posi- tively charged complexes but destabilise the negatively charged ones. This is also consistent with the Born solvation model (DGsolv ¼ 166 (Ze)2[1 1/e]/ r; in kcal mol1),59 if one considers that the cations have a smaller ‘‘radius’’r than the anions of same absolute chargeZe.
F versus Cl ligands in hexa-halo complexes. We now compare the fluoroversuschloro hexahalogenated complexes, i.e.the EuF63, EuF5Cl3, EuF4Cl23, EuF3Cl33, EuF2Cl43, EuFCl53and EuCl63series. In the gas phase, the exchange of one Clanion by one Fanion, modelled by the reaction (2)
EuF(6n)Cln3þF-EuF(7n)Cl(n1)3þCl (2) stabilizes the complexes forn¼6, 5 and 4, byDE2¼ 37.8, 34.3 and 30.7 kcal mol1, respectively (HF results) or DE2 ¼ 32.6, 30.4 and 27.0 kcal mol1, respectively (DFT results). In the case of the EuF4Cl23 and EuF5Cl3 complexes, the coordinated Cl anions decomplexed during the optimization process to form EuF4and EuF52, respec- tively, indicating the lack of affinity of the latter for a Clanion in the gas phase. The exchange reaction (3) of the six anions is also exothermic and favours the fluoro complex, by DE3 ¼ 159 kcal mol1 (HF calculations), 138 kcal mol1 (DFT calculations) and150 kcal mol1(AMBER). Note the good agreement between the QM and force field results obtained with ‘‘standard’’ anion models. The ‘‘smaller’’ anion models are less satisfactory.60
EuCl63þ6 F-EuF63þ6 Cl (3) When simulated in the ionic liquid solution the EuFnCl(6n)3 complexes display attractive interaction energiesEsolvwith the solvent (see Table 4). The main contributions stem from the first shell BMI1cations which overcompensate for the repul- sions with PF6anions. TheEsolvenergies are more attractive for fluoro than for chloro complexes and increase byca. 20 kcal mol1 per Cl - F anion substitution, mainly due to stronger interactions with BMI1solvent cations. Furthermore, as in the gas phase, the internal energy of the complexes becomes more negative when the number of F anions in- creases, by 149 kcal mol1when one moves from EuCl63to EuF63. We note that this number is very close to the corres- ponding gas phase energy DE3, which also validates the
‘‘standard’’ force field models. Using the ‘‘smaller’’ Clmodel (see Table S3w) yields the same trends in solvation energies as with the ‘‘standard’’ model (Table 4). This, however, clearly underestimates the difference in internal energies between the EuF63and EuCl63solutes (33 kcal mol1) compared to the QM results. Comparing now EuF63 with the ‘‘standard’’
versus‘‘small’’ Fmodels (Table S3w) also yields very similar
‘‘solvation energies’’DEsolv and stabilizes the latter complex, due to reduced intrashell repulsions.
Fig. 4 EuFnCl6n3 complexes in [BMI][PF6] at 400 K: Represen- tation of the first solvation shell of anions only, cations only and anionsþcations. A colour version of the figure is given in Fig. S2.w
Fig. 5 Energy changes (kcal mol1) as a function ofn for the F complexation reaction: EuFn3n
þF-EuFn112n. QM calculated DFT-B3LYP/ 6-31þG* level (’), HF/6-31þG* level (K) and MM/
AMBER calculations with the ‘‘small’’ Fmodel (m) and with the
‘‘standard’’ Fmodel (E). QM energies are corrected for BSSE.
The higher stability of the F complex, compared to its chloro analogue is confirmed be free energy perturbation calculations in the [BMI][PF6] liquid. The mutation of EuCl63 to EuF63 is energetically favorable (DG1 ¼ 222 5 kcal mol1, as an average over-independent forward and backward calculations), as is the mutation of one uncomplexed Clto F anion (DG2¼ 11 kcal mol1). As a result, the change in free energy for reaction (3) in solution isDG16DG2E156 kcal mol1. This is much larger than the uncertainty (ca. 10 kcal mol1) estimated, e.g. from the difference between the two- ways mutation on the complexes.
Discussion and conclusions
We report a theoretical study of the nature and solvation of fluoro complexes of EuIIIin the [BMI][PF6] ionic liquid. The MD simulations are based on a simple force field representa- tion of the potential energy of the system, assuming that intermolecular interactions are essentially stericþelectrostatic by nature, thus following most current methodologies used to simulate RTILs.8,38,41,44,61–67
Although polarization effects may be quite important,68 simulations of molten salts also generally employ additive pair-potentials.69–72 This issue has been discussed in previous papers.17,19,20,35
The lack of explicit representation of polarization and charge transfer effects in the studied systems should be most critical for the interactions of the ‘‘hard’’ EuIIIspecies with its ‘‘soft’’ anionic environment.
For instance, according to our QM calculations on hexa- halogenated complexes, the EuIII charge is closer to þ1.5 e (Table S1w) than toþ3e, as assumed in our force field. We note however that our force field based on aþ3 charge representa- tion of the cation is capable of accounting for energy changes upon successive Fcomplexation by the metal (see Fig. 5), as well as for the relative stabilities of the EuCl63versusEuF63
complexes in the gas phase. The fixed charged model used here also accounts for the free energy of hydration of EuIIIas well as for its main hydration characteristics.42 The parameters used for the ionic liquid also satisfactorily fit the interaction between its ionic components in the gas phase, as well as for the main solvent properties.73A recent analysis can be found in ref. 41.
Alternative approaches consist of reducing the BMI1 and PF6ion charges (e.g.0.9 instead of1, as reported in ref.
61) in order to somewhat mimic the anion to cation electron transfer in the solvent, but this turns out to have modest effects on the simulated solvent properties. Another important issue concerns the effect of temperature and sampling on the solva- tion of the complexes in solution. The comparison of the EuFn3n complexes (n¼ 3 to 6) at 400 K (Tables 2 and 3) versus 300 K (Tables S4 and S5w) yields the same trends concerning the changes in first shell solvation of the complexes and their interactions with the liquid. As expected, the first shell BMI1 cations are less mobile at 300 K than at 400 K, leading to sharper peaks in the RDFs, with stronger imidazo- lium–CH hydrogen bonds to the fluoro ligands. We thus believe that the results of the MD simulations are meaningful.
One important finding is that in the studied ionic liquid EuIII coordinates up to six F anions only to form the EuF63
complex, thus bearing strong analogies with the corresponding chloro-complexes.19,35This is fully consistent with the results of Raman spectroscopy studies of LnF3-KF binary melts (Ln¼ La;Ce;Nd;Sm;Dy;Yb) in which EuF63 octahedra has been characterized.27 In our simulations, complexes with higher coordination numbers are found to dissociate in the ionic liquid, the only exception being the EuF74 complex which remains stable when simulated with ‘‘small’’ F ions, and cannot therefore be fully precluded. As a consequence, the experimental EXAFS36observation of CNs of 9–10 for EuIIIin ionic liquids based on anions such as PF6, BF4, CF3SO3, [CF3SO2]2Ncannot be attributed to the coordination of F anions produced, e.g. by degradation of the liquid. Intrinsi-
cally, however (e.g.in the gas phase), complexes like EuF74, EuF63
or EuF52
are shown to be unstable towards the dissociation of three to one Fanions, respectively. This is because the attraction (if any) between the resulting EuF4
species and additional anionic ligands is not strong enough to compensate for the increase in intrashell repulsions. The ob- servation of the negatively charged complex EuF63complex in the ionic liquid is thus due to solvation forces which compensate for this intrinsic instability, mainly due to the first shell BMI1 solvent cations. Concerning the EuF63 versus EuF74 comparison, no firm conclusion can be made from the energy component analysis alone, as the change in internal energy upon Fcomplexation (þ230 kcal mol1; DFT results) is in the order of magnitude of the change in ‘‘solvation energies’’Esolv(200 kcal mol1) between EuF74(1060 kcal mol1) and EuF63 þ dissociated F (860 kcal mol1) calculated with the ‘‘standard’’ Fmodel. Furthermore, from a thermodynamic point of view, other energy contributions (e.g. changes in solvent–solvent interactions) and entropy effects also contribute to the status and solvation free energies of the complexes. Further insights could be obtained,e.g.by potential of mean force calculations, but the latter are quite computer demanding, due to the sampling issues and slow relaxation dynamics of the ionic liquid. In reality, there is an equilibrium between different fluorinated species, which evolves with the EuIIIand Fconcentrations and cannot be predicted by computations only. This is illustrated by a MD simulation we performed on the [BMI][PF6,F] ionic liquid containing an equal amount of PF6 and F anions and, initially, 4 EuF3complexes as solute. Three of them sponta- neously formed 1 EuF4þ 1 EuF52þ 1 EuF63complexes after 2.5 ns of dynamics at 400 K. An interesting feature of this system is the strong hydrogen bonding interactions formed between Fanions and imidazolium protons, consistent with QM results in the gas phase.74These interactions increase the viscosity of the liquid and compete with the Fcomplexation by EuIII. Thus, the nature of ionic liquid anion and its hydrogen bonding capabilities with imidazolium are also likely to influence the stability and stoichiometry of the europium fluoride complexes.
Concerning now the ClversusFcomparison, we find that fluoro complexes are more stable, due to both their internal energies and better solvation, thus following well-known trends observed in other solvents.75We hope that these studies will stimulate experimental investigations on fluoride complexation in RTILs.
Acknowledgements
The authors are grateful to IDRIS, CINES, Universite´ Louis Pasteur, and PARIS for computer resources and to E. Engler for his kind assistance.
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