Article pubs.acs.org/JPCA
Structures and Electronic Properties of Transition Metal-Containing Ionic Liquids: Insights from Ion Pairs Weihong Wu,† Yunxiang Lu,*,† Yingtao Liu,‡ Haiying Li,† Changjun Peng,† Honglai Liu,† and Weiliang Zhu‡ †
Key Laboratory for Advanced Materials and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China ‡ Drug Discovery and Design Center, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China S Supporting Information *
ABSTRACT: Transition metal-containing ionic liquids (TM-ILs) have attracted a great deal of attention in recent years, due to their unique physical and chemical properties. In this work, several representative TM-ILs, such as the cations consisting of silver(I) center coodinated by two n-alkylimidazole ligands ([(Cnim)Ag(mim)]+) and the anions involving mercury(II) (HgCl3−), zinc(II) (ZnCl3−), and rhenium(VII) (ReO4−), were investigated using density functional theory calculations. First, the structural and energetic properties of the ion pairs for these TM-ILs have been examined in detail and compared with properties for conventional ILs. It was found that the interactions between the cations and anions, including hydrogen bonds and electrostatic interactions, in TM-ILs become weaker in strength than those in traditional ILs. In particular, the calculated geometric and energetic features compare fairly well with the experimental results, such as melting points and X-ray crystal structures of these TM-ILs. Then, the structures and energetics of ion-pair dimers for three ILs containing HgCl3−, ZnCl3−, and ReO4− were also explored, to gain a deeper understanding of the properties of TM-ILs. Finally, a survey of the Cambridge Structural Database (CSD) was undertaken to provide some crystallographic implications of TM-ILs.
1. INTRODUCTION Ionic liquids (ILs) are a class of novel compounds composed exclusively of organic cations (usually imidazolium, pyrimidinium, ammonium and phosphonium) and inorganic anions (e.g., X−, BF4−, PF6−, Tf2N−, etc).1−5 Owing to the unique physical properties and a widely tunable character, ILs have been used as green solvents in organic synthesis and good catalysts in catalysis chemistry.6−11 In addition, ILs are also becoming increasingly important in the electrodeposition of metals as well as in applications such as electrolytes in batteries and supercapacitors.12−15 However, the solubility of metal salts in conventional ILs is often rather limited, due to the weak coordination ability of common cations and anions. Incorporation of metals into the cations or anions clearly can improve the dissolution of metal salts into ILs.16−18 In recent years, transition metal-containing ionic liquids (TM-ILs) have attracted intensive research interest, because of their many advantages over traditional ILs, such as a high solubility of metal salts, an unexpectedly strong response to an additional magnetic field, and a wonderful color rendering property.19,20 Development of TM-ILs thus provides a great opportunity for task-specific ILs, which are promising alternatives to conventional ILs. In 2011, a series of mercury(II) ILs ([Cnmim][HgX3]), where [Cnmim] is nalkyl-3-methylimidazolium with n = 3, 4 and X = Cl, Br, were synthesized and structurally characterized via single-crystal Xray diffraction.21 Subsequently, Binnemans and co-workers © 2014 American Chemical Society
studied heterolepic ILs with the cations consisting of silver(I) center coorinated by two different n-alkylimidazole ligands ([(Cnim)Ag(Cmim)]+), and they found that these new ILs have low melting points and are useful in the electrodeposition of silver coating.22−25 More recently, Lin and Vasam have reviewed the important role of ILs and ionic liquid crystals of imidazolium salts composed of various transition and main group metals in organic transformations and material science.19 However, to the best of our knowledge, no theoretical studies concerning TM-ILs have been conducted so far. Considering the importance of these ILs in a wide range of chemical fields, a deep understanding of microstructures and intermolecular interactions of TM-ILs at molecular level should be of great value. In this work, three heteroleptic silver-containing ILs, i.e., [Ag(mim)2][Tf2N], [(C2im)Ag(mim)][Tf2N], and [(C4im)Ag(mim)][Tf2N], together with three ILs involving transition metals in the anions ([C1mim][HgCl3], [C1mim][ZnCl3], and [C1mim][ReO4]) were considered. The crystal structures and physicochemical properties of these TM-ILs have been reported in previous papers.21,22,26,27 In addition, two conventional ILs, [C1mim][Tf2N] and [C1mim][BF4], were also chosen for comparison. First, all possible conformations of the Received: December 20, 2013 Revised: March 13, 2014 Published: March 14, 2014 2508
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Figure 1. ESP surfaces of isolated cations and anions.
ion pairs for the studied TM-ILs were simulated, and different molecular interactions, such as hydrogen bonds (HBs) and electrostatic forces, between the cations and anions were examined. Then, the structures and energetics of two ion pairs for the ILs containing HgCl3−, ZnCl3−, and ReO4− were also explored, to gain more insight into the properties of these TMILs. Finally, the implications of the determined geometric and energetic features of ion pairs on the design of TM-ILs were discussed. This work could be very helpful for tuning the properties of TM-ILs and developing novel task-specific ILs.
and the electrostatic potential (ESP) analysis was undertaken by Multiwfn 2.4 program,43 using the wave functions generated with B3LYP/aug-cc-pVDZ(-PP).
3. RESULTS AND DISCUSSION Isolated Cations and Anions. The B3LYP-optimized structures of isolated cations and anions under study are shown in Figure 1. It is clear that the two imidazole rings in the cations containing silver are almost perpendicular to each other, as a result of steric effects and metal−ligand bonds. A comparison between the calculated geometric data and the X-ray crystal structures reveals that the calculated structures of [(Cnim)Ag(mim)]+ (n = 1, 2, 4) compare well to the X-ray structures.22 For example, the computed Ag−N distances are within 0.03 Å of the X-ray values; the N−Ag−N angles are predicted to be about 175°, quite similar to those in crystal structures.22 The introduction of transition metals into the cations or anions leads to some obvious changes in electrostatic potential (ESP) distribution, as also depicted in Figure 1. First, the ESPs for the O/Cl atoms in ReO4−, HgCl3−, and ZnCl3− become less negative than the F atoms in BF4−, consistent with the smaller VS,min (the most negative surface ESPs) for the O/Cl atoms (cf. Figure 1). Moreover, the ESPs over the plane of the anions involving transition metals appear to be less negative compared with that for the BF4 anion. Second, for the [(Cnim)Ag(mim)] cations, the H(C2) atoms exhibit the most positive ESPs; however, these atoms have smaller values of VS,max (the most positive surface ESPs) with respect to the [C1mim] cation. Similarly, relative to [C1mim]+, the ESPs over the imidazole rings become less positive for [(Cnim)Ag(mim)]+. On the basis of these, the presence of transition metals into the cations or anions should give rise to weaker intermolecular interactions, including HBs and electrostatic forces, in ion pairs (vide infra). Additionally, as the alkyl chain of [(Cnim)Ag(mim)]+ increases, VS,max for the H(C2) atoms tends to decrease gradually, in
2. THEORETICAL METHODS The geometries of all the ion pairs under study were fully optimized by means of the hybrid B3LYP functional,28,29 which has been commonly utilized in the study of cation−anion interactions in ILs.30−36 The aug-cc-pVDZ-PP basis set, which uses pseudopotentials to describe the inner core orbitals, was employed for transition metals, whereas for the remaining atoms aug-cc-pVDZ was applied. Very recently, we have successfully employed the B3LYP/aug-cc-pVDZ(-PP) method to investigate intermolecular interactions in halogenated ILs.37 During geometry optimizations no symmetry or geometry constraint was imposed. Frequency calculations performed at the same theoretical level indicated that all the structures obtained correspond to energetic minima without imaginary frequency. All the calculations reported in this work were carried out with the Gaussian 09 suite of programs.38 The interaction energy was estimated as the difference between the total energy of the ion pair and the sum of the total energy of the minimum geometry of the cation and anion. The standard counterpoise method of Boys and Bernardi was employed to correct the basis set superposition error (BSSE).39 Atomic charges and canonical orbitals were computed via the natural bond order (NBO) method implemented in Gaussian 09.40,41 The quantum theory of atoms in molecules (QTAIM) analysis was performed with the help of AIM 2000 software42 2509
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Figure 2. Favorable locations for the anions around the imidazolium cation and four initial configurations for the ion pairs involving silver.
Figure 3. Optimized structures of ion pairs containing transition metals in anions. Distances are in angstroms.
the back of the cation (back D), and the top of the cation (top E), as shown in Figure 2. Here it should be pointed out that the interaction in the conformer E can be defined as an anion−π interaction that shows some directionality.49 In recent years, this interaction involving cationic aromatic rings has been studied from both experimental and theoretical viewpoints.50,51 For the ion pairs [(Cnim)Ag(mim)][Tf2N], four initial configurations (A, B, C, and D) were considered on the basis of the ESP analysis and crystal structures. Moreover, two arrangements of the imidazole rings, i.e., cisoid and transoid arrangements, in these ion pairs were taken into account
accordance with the trend of interaction energies of corresponding ion pairs (see below). Structures of Single Ion Pairs. Ionic liquids are composed of ions completely so that the ion pairs as the basic structural units are very important for studying ILs.44−48 Given multiple stable structures of the ion pairs under study, we initially defined five favorable locations for the anions (BF4−, ReO4−, HgCl3−, and ZnCl3−) around the imidazolium cation, that is, the front of the C2−H moiety (front A), the site between the C2−H group and the methyl chain (meth-front B), the site between the methyl chain and the C4−H group (meth-back C), 2510
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Figure 4. Optimized structures of ion pairs [C1mim][Tf2N] and [Ag(mim)2][Tf2N]. Distances are in angstroms.
very similar to [C1mim][BF4]. Obviously, although the structures and interaction motifs are somewhat different, HBs are the explicit structural features for both conventional ILs and TM-ILs. When the transition metal was introduced into the cations, the conformations become much more complicated compared with conventional ILs, as shown in Figures 4 and 5. Two kinds of configurations, i.e., cisoid and transoid arrangements of the two imidazole rings in the cations, were attained for the ion pairs [(Cnim)Ag(mim)][Tf2N]. Particularly, in all transoid configurations the two imidazole rings lie nearly in the same plane, consistent with the X-ray structure of corresponding ILs also displayed in Figure 4.22 However, the two imidazole rings in cisoid conformer C appear to be not in the same plane. This difference may be explained by the fact that the weaker N/O··· H(C4)/H(C5) HBs in this conformer cannot keep the two rings in the same plane, whereas much stronger N/O···H(C2) interactions are presented in other cisoid conformers (A and B) and all transoid structures. In general, the interactions between the cations and the [Tf2N] anion are characterized by multiple HBs between the electronegative O/N atoms and the H atoms of the imidazole ring or the methyl chain. Moreover, in the ion pairs containing silver, the anion is involved in several HBs with the H atoms from both imidazole rings, in line with the X-ray structure of corresponding ILs.22 As expected, relative to [Ag(mim)2][Tf2N], more conformers were attained for the ion pairs [(C2im)Ag(mim)][Tf2N] and [(C4im)Ag(mim)][Tf2N], due to two different alkylimidazole ligands coordinated to silver center in the latter systems. In addition, all the N···H(C2) distances in the ion pairs containing silver are predicted longer than those in [C1mim][Tf2N], which implies weaker N··· H(C2) HBs in these ion pairs (see below).
(Figure 2), albeit the two rings in the crystal structures of corresponding ILs always exhibit transoid arrangement. In addition, for comparison purposes, four initial configurations (front A, meth-front B, meth-back C, and back D) were also calculated for the ion pair [C1mim][Tf2N]. In common, a geometric criterion for HB is the distance between the proton in the donor group and the acceptor atom, which is less than the sum of their van der Waals (vdW) radii.52 It is interesting that for [C1mim][BF4] only one stable structure (E), in which the anion is located over the imidazole ring and also forms two F···H HBs with the C2−H moiety, was obtained at the level of B3LYP/aug-cc-pVDZ(-PP), whereas the other two structures (A and D) have one imaginary frequency (about 20i cm−1). However, as demonstrated previously, three stable minima (C, D, and E) were attained for [C4mim][BF4] through the B3LYP/6-31++(d,p) method.36 Apparently, the longer alkyl chain (butyl group) tends to stabilize the structures with higher energy. The optimized structures of [C1mim][ZnCl3] and [C1mim][HgCl3] are quite similar to each other: in structure A, two Cl···H interactions occur between the Cl atom and the H(C2) atom of the ring as well as the H atom of the alkyl chain, and moreover, the anion lies perpendicular to the imidazole ring, in good agreement with the X-ray structure of corresponding IL also shown in Figure 3.21 In conformer C the anion is involved in three or four HBs with the C4−H/C5− H moiety and the alkyl chain, whereas the anion resides over the imidazolium cation in conformer E. For the ion pairs [C1mim][ReO4], four stable configurations were obtained: in structure A, two symmetrical O···H interactions (about 2.1 Å) take place between two O atoms and the H(C2) atom, and moreover, the two O atoms lie in the plane of the imidazole ring. In structures C and D, the anion forms two HBs with the C4−H/C5−H moiety or the alkyl chain, whereas conformer E is 2511
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Figure 5. Optimized structures of ion pairs [(C2im)Ag(mim)][Tf2N] and [(C4im)Ag(mim)][Tf2N]. Distances are in angstroms.
Table 1. Interaction Energies (kcal/mol) of the Ion Pairs Calculated with B3LYP/aug-cc-pVDZ(-PP) conformers A B C D E conformers A B C D A′ B′ C′ D′
[C1mim][BF4]
[C1mim][ZnCl3]
[C1mim][HgCl3]
[C1mim][ReO4]
−82.14
−73.67
−73.24
−78.77
−66.29
−65.63
−71.87 −83.66 [C1mim][Tf2N]
−71.90 [Ag(mim)2][Tf2N]
−75.23
−67.31
−66.97 −67.76
−60.97 −64.71 −62.45
Interaction Energies of Single Ion Pairs. The interaction energy is often seen as one of the most important parameters to correlate with the properties of ILs. Table 1 lists the interaction energies with corrected values of BSSE for the ion pairs under study. It is evident that the calculated interaction energies of the ion pairs including ReO4−, HgCl3−, and ZnCl3− are somewhat less negative than those of [C1mim][BF4]. As a consequence, intermolecular interactions in these ion pairs become much
−72.81 [(C2im)Ag(mim)][Tf2N]
−69.22 −70.88 −77.50 [(C4im)Ag(mim)][Tf2N]
−66.90 −66.70 −60.59
−66.50 −65.52 −60.29
−64.25 −64.10 −60.71 −60.08
−63.82 −63.72 −61.49 −61.47
weaker in strength, in accordance with the results of the ESP analysis (vide supra). In fact, transition metal-containing ILs ([C4mim][HgCl3] and [C4mim][ZnCl3]) show a relatively low melting point (about 90 °C).21,26 For the ion pairs involving HgCl3− and ZnCl3−, the interaction energy of conformer A is computed to be the largest in absolute value (about −73.2 kcal/ mol), thus indicating the stronger acidity of H(C2) with respect to H(C4)/H(C5) and the H atom of the methyl chain. 2512
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Table 2. Properties of the Electron Density at BCPs for Selected Ion Pairsa ion pairs [C1mim][BF4] (E)
[C1mim][ZnCl3] (A) [C1mim][ZnCl3] (C)
[C1mim][ZnCl3] (E)
a
HBs
ρc
∇2ρc
Hc
B1 B2 B3 B1 B2 B1 B2 B3 B1 B2 B3
0.012 0.013 0.024 0.021 0.008 0.010 0.017 0.009 0.010 0.011 0.010
0.042 0.057 0.081 0.055 0.021 0.036 0.064 0.038 0.036 0.043 0.038
0.0005 0.0016 0.0000 0.0009 0.0007 0.0007 0.0009 0.0013 0.0008 0.0014 0.0008
ion pairs [C1mim] [Tf2N](A)
[C1mim] [Tf2N](C) [(C2im)Ag(mim)] [Tf2N](A′)
[(C2im)Ag(mim)] [Tf2N](C′)
HBs
ρc
∇2ρc
Hc
B1 B2 B3 B2 B3 B1 B2 B3 B1 B2
0.017 0.029 0.017 0.031 0.015 0.011 0.022 0.013 0.018 0.017
0.072 0.124 0.071 0.134 0.060 0.047 0.089 0.057 0.070 0.074
0.0007 0.0019 0.0007 0.0019 0.0004 0.0005 0.0017 0.0008 0.0012 0.0010
All values are given in au.
Table 3. Electron Donor Orbitals, Electron Acceptor Orbitals, and the Second-Order Interaction Energies E2 (kcal/mol) of Selected Ion Pairs ion pair [C1mim][BF4] (E)
[C1mim][ReO4] (C) [C1mim][ReO4] (D) [C1mim][ReO4] (E)
donor (i) B1:LP B2:LP B3:LP B1:LP B2:LP B1:LP B2:LP B1:LP B2:LP B3:LP
(F) (F) (F) (O) (O) (O) (O) (O) (O) (O)
acceptor (j) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1)
C7−H C2−H C2−H C5−H C6−H C4−H C5−H C7−H C2−H C6−H
E2 2.22 2.24 7.57 5.47 7.20 1.63 16.19 0.31 5.92 1.07
ion pair
donor (i)
[C1mim][Tf2N] (A)
[C1mim][Tf2N] (C)
[(C2im)Ag(mim)] [Tf2N] (A′)
[(C2im)Ag(mim)] [Tf2N] (C′)
B1:LP B2:LP B3:LP B1:LP B2:LP B3:LP B1:LP B2:LP B3:LP B1:LP B2:LP
(O) (N) (O) (O) (N) (O) (O) (N) (O) (N) (O)
acceptor (j) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1)
C6−H C2−H C7−H C7−H C4−H C4−H C6−H C2−H C4′-H C4−H C2′-H
E2 6.21 16.05 6.18 5.52 15.30 0.43 3.50 9.36 2.53 6.42 3.79
representative ion pairs are summarized in Table 2. As can be seen, the interactions between the cations and anions are all closed shell interactions with ρc and ∇2ρc in the range of normal HB (ρc < 0.200 au and 0.020 au < ∇2ρc < 0.139 au). Moreover, the total electron energy densities (Hc) at the BCPs were evaluated to be positive, in good agreement with previous AIM results of imidazolium-based ILs.54 Here it is worth mentioning that the calculated interaction energies of present ion pairs contain not only effects of HB formation but also Coulombic attraction of two opposite charges; Coulombic forces dominate interactions in ILs and the contribution of HBs is small but non-negligible.31 In fact, the energies of HBs in several imidazolium-based ILs were recently determined to vary from −2.8 to −3.8 kcal/mol, using combined X-ray crystallographic, Infrared/Raman spectroscopic, and DFT methods.54 In conformer E of [C1mim][BF4] and [C1mim][ZnCl3], three BCPs are identified for F···H interactions, and moreover the values of ρc for [C1mim][ZnCl3] are estimated to be smaller than [C1mim][BF4]. Therefore, intermolecular interactions in the ion pairs containing transition metals become weaker, consistent with the energetic features of these ion pairs (vide supra). As expected, in conformer A of [C1mim][ZnCl3], ρc for the Cl···H(C2) HB is calculated to be larger with respect to the Cl···H(methyl chain) interaction (0.021 au vs 0.008 au); the computed ρc for the three HBs in conformer C is smaller than the Cl···H(C2) interaction in structure A. These AIM results accord well with the energetic properties of these ion pairs as demonstrated above. From Table 2, it is also seen that ρc for the N···H interactions in [C1mim][Tf2N] and [(C2im)Ag(mim)][Tf2N] is evaluated somewhat greater than other HBs. Moreover, the calculated ρc
Similarly, for [C1mim][ReO4] conformer A exhibits the most negative interaction energy (about −77.8 kcal/mol), due to the occurrence of four HBs and the strong acidity of H(C2). From Table 1, it is also seen that, as compared to the case for [C 1 mim][Tf 2N], less negative interaction energies are predicted for the ion pairs containing silver. Therefore, cation−anion interactions in these ion pairs also become weaker, accordant with the results of the ESP analysis (see above). Particularly, the computed interaction energies of transoid structures A and B for [(Cnim)Ag(mim)][Tf2N] are smaller in absolute value with respect to the corresponding cisoid ones, although the two imidazole rings in the crystal structures of corresponding ILs exhibit transoid arrangement. To some extent, this can be attributed to the fact that the anion in cisoid configurations resides in the front of the C2−H moiety of both imidazole rings, whereas in transoid configurations the anion only lies close to the C2−H group of one ring. Therefore, both the C2−H moieties could form stronger HBs with the electronegative atoms in the anion in cisoid configurations. Generally, the interaction energies of silver-containing ion pairs slightly decrease as the alkyl chains increase, which coincides with the trend of VS,max for the H(C2) atoms in isolated cations as pointed out above. In fact, the melting points of the three ILs, [Ag(mim) 2 ][Tf2N], [(C 2 im)Ag(mim)][Tf 2 N] and [(C4im)Ag(mim)][Tf2N], were reported to be 87, 35, and 30 °C, respectively.22 AIM and NBO Analyses of Ion Pairs. The bond properties between each pair of atoms were analyzed in detail using the AIM theory, which is based on the topological analysis of electron density (ρc) and its Laplacian (∇2ρc) at bond critical points (BCP).53 The calculated AIM data of some 2513
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Figure 6. FMOs energy-order diagram for selected structures.
and ∇2ρc for the N···H interactions in [(C2im)Ag(mim)][Tf2N] are less than those in [C1mim][Tf2N], which indicates weaker N···H HBs in silver-containing ion pairs as noted above. In addition, the AIM analysis also reveals several secondary O··· H interactions between the O atom and the H atoms in the imidazole ring. It has been established that the general n(Y) → σ*(X−H) donor−acceptor HB interaction (X−H···Y) can be evaluated by the perturbation theory to calculate the delocalization energy (E2) as follows: 2
E = ΔEij = qi
Charge Distribution and Orbital Analysis. The NPA charges of isolated cations, anions, and some typical ion pairs are summarized in Table S1 in Supporting Information. Relative to the isolated [C1mim] cation, the charge on the two N atoms remains almost unchanged in conformer E of [C1mim][BF4] and [C1mim][ReO4]. However, the positive charge on H(C2) and C2 atoms both increases by about 0.03 au, while the positive charge on H(C4)/H(C5) atoms decreases and C4/C5 atoms carry more negative charge. These indicate that partial π electrons transfer from the C2−H group to the H(C4) and H(C5) atoms in the two structures, due to the repulsion between the anion and the π electrons on the ring. Additionally, upon the formation of the ion pairs, a magnitude of charge transfer (about 0.05 au) occurs from the anions to the imidazolium cation. NPA analysis also shows that the incorporation of silver into the cation influences the charge distribution to a large degree. Most of the positive charge concentrates upon the Ag atom, which results in an obvious decrease in the positive charge of C2−H, C4−H, and C5−H groups. Moreover, the N3 atom in [(Cnim)Ag(mim)]+ exhibits considerably more negative charge than that in [C1mim]+ due to silver coordination. Similarly, upon ion pair formation, a small amount of charge transfer (about 0.07 au) takes place from the [Tf2N] anion to the cations. It is well-known that the canonical molecular orbital analysis can provide an essential understanding of the charge transfer. Figure 6 depicts the frontier molecular orbitals (FMOs) energyorder diagram for four representative structures of the ion pairs, i.e., conformer E of [C1mim][BF4] and [C1mm][ReO4] and conformer A of [C1mim][Tf2N] and [Ag(mim)2][Tf2N]. As can be seen, the HOMO of the [BF4]/[ReO4] anions is lone pair orbitals of four F/O atoms and the orbital energies are −0.166 and −0.131 au, respectively, whereas the LUMO of the [C1mim] cation is the anti-π (π*) bond orbital with four energetic nodes and the orbital energy is −0.961 au. Notably, the FMOs of [C1mim][BF4] and [C1mm][ReO4] exhibit distinct interaction between the lone pair of the O/F atoms and the lowest empty π*(C2−H) orbital of the cation. Nevertheless, the HOMO of [C1mim][ReO4] is mainly located on the anion, whereas in [C1mim][BF4] the HOMO resides largely on the imidazole ring of the cation. Figure 6 also shows that the HOMO of the [Tf2N] anion is composed of lone pair orbitals of the N/O/S atoms (the orbital energy E = −0.149 au),
(Fij)2 ei − ej
(1)
where qi is the donor orbital occupancy, ei and ej are diagonal elements (orbital energies), and Fij is the off-diagonal NBO Fock matrix element. The E2 energies of selected ion pairs obtained from the NBO analysis, which represent the different capacities of forming HBs, are listed Table 3. In conformer D of [C1mim][ReO4], the calculated values of E2 for the donor− acceptor orbitals, n(O) → σ*(C5−H) and n(O) → σ*(C4− H), are 16.19 and 1.63 kcal/mol, respectively, consistent with the much shorter O···H(C5) distance compared with O··· H(C4) (1.88 Å vs 2.08 Å) as shown in Figure 3. Additionally, in conformer E of [C1mim][BF4] and [C1mim][ReO4], E2 for the donor−acceptor orbital, n(F/O) → σ* (C2−H), is computed to be 7.57 and 5.92 kcal/mol, respectively, which further confirms weaker HB capacities in the ion pairs involving transition metals. Table 3 also shows that in [C1mim][Tf2N] and [(C2im)Ag(mim)][Tf2N], the n(N) → σ*(C−H) orbital interactions exhibit the largest E2, and moreover, these interactions are much stronger in [C1mim][Tf2N] with respect to [(C2im)Ag(mim)][Tf2N]. For instance, the values of E2 for n(N) → σ*(C2−H) in conformer A of [C1mim][Tf2N] and [(C2im)Ag(mim)][Tf2N] are evaluated to be 16.05 and 9.36 kcal/mol, respectively, much larger than other orbital interactions in this conformer. These NBO results agree well with the geometric and AIM properties of these ion pairs. In fact, for silvercontaining ion pairs, there exists a good correlation between electron densities at the BCPs and orbital interaction energies (R = 0.92). Additionally, albeit the calculated E2 for n(O) → σ*(C−H) is relatively smaller, these HBs should also influence the structures of silver-containing ion pairs. 2514
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Figure 7. Optimized structures of ion-pair dimers. Distances are in angstroms.
the methyl chains. Not surprisingly, the configurations of [C1mim][ZnCl3] are roughly analogous to [C1mim][HgCl3], on account of the same geometric symmetry of the two anions. The interactions between different fragments of ion-pair dimers were studied systematically, including the total interaction energy (ΔEtot) of the whole dimers and the interactions energy (ΔE1/ΔE2) of two ion-pair fragments. ΔEtot was evaluated as the difference between the total energy of ionpair dimer and the sum of the total energy of isolated cations and anions, i.e., ΔEtot = Edimer − 2 × (Ecation + Eanion), and ΔE1 and ΔE2 represent the interactions energy between two ion-pair fragments, which were taken from the whole system. The corresponding ion-pair fragments are defined in Figure 7 and the interaction energies mentioned are summarized in Table 4. As can be seen, the total interaction energy of the dimers involving transition metals is estimated somewhat less negative than [C1mim][BF4], which further confirms weaker cation−
whereas the orbital of Ag contributes significantly to the LUMO of the [Ag(mim)2] cation. However, the HOMO of [Ag(mim)2][Tf2N] resides mostly on one imidazole ring in the cation, whereas in [C1mim][Tf2N] the HOMO is mainly located on the anion. Clearly, the presence of transition metals into the cations or anions influences the orbital overlap of ion pairs significantly. Structures and Energetics of Ion-Pair Dimers. The above description of cation−anion interactions in ILs was solely based on the single ion-pair unit, and therefore, it neglected the interactions between the ion pairs. To gain a deeper understanding of the properties of TM-ILs, the study of larger clusters containing up to four component units has been undertaken in this work. It should be pointed out that here only ion pairs of ReO4−, HgCl3−, and ZnCl3− were investigated, as a result of the computational cost and much complex of the systems involving silver. Various chemically reasonable structures of ion-pair dimers have been calculated at the B3LYP/aug-cc-pVDZ(-PP) level of theory, and all possible cation−anion interaction modes were taken into account during the design of initial geometries. According to our calculations, several stable geometries of two ion pairs were obtained, as displayed in Figure 7. For [C1mim][BF4], the cations and anions form a ring-like structure with multiple HBs, such as C4/5−H···F and C6/7− H···F interactions; the two imidazole rings adopt a roughly antiparallel geometry. Configuration I of [C1mim][ReO4] is quite similar to [C1mim][BF4], but the bond length H···O is generally longer than the corresponding H···F distance due to the larger vdW radius of the O atom. Configuration II of [C1mim][ReO4] is a standard layer structure composed of two units of conformer A of the ion pair. In configuration III of [C1mim][HgCl3], four symmetrical HBs occur between two Cl atoms and the H(C4)/H(C5) atoms with the distances of about 2.45 Å, whereas configuration I can be considered as the stacking of conformer A of the ion pair. In configuration II of [C1mim][HgCl3], the anion forms several secondary HBs with
Table 4. Total Interaction Energies, the Interaction Energies of Two Ion-Pair Fragments, and the Average Interaction Energies for Ion-Pair Dimersa conformers
ΔEtot
ΔE1
ΔE2
ΔEaver
[C1mim][BF4] [C1mim][ZnCl3] (I) [C1mim][ZnCl3] (II) [C1mim][HgCl3] (I) [C1mim][HgCl3] (II) [C1mim][HgCl3] (III) [C1mim][ReO4] (I) [C1mim][ReO4] (II)
−183.62 −166.57 −165.77 −165.87 −163.24 −159.52 −170.94 −178.99
−38.06 −40.55 −33.28 −23.95 −31.22 −33.99 −34.80 −20.15
−37.88 −24.57 −29.73 −41.47 −31.08 −31.59 −39.22 −29.11
−37.97 −31.73 −32.33 −32.71 −31.15 −32.79 −37.01 −24.63
All values are given in kcal/mol. ΔE1 is the interaction energy between fragment (P1 + P2) and fragment (P3 + P4) in the dimers, ΔE2 is the interaction energy between fragment (P1 + P4) and fragment (P2 + P3) in the dimers, and ΔEaver is the average interaction energy of ΔE1 and ΔE2. The fragments (P1, P2, P3, and P4) are defined in Figure 7. a
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anion interactions in TM-ILs. Additionally, for [C1mim][ReO4] the computed total interaction energy of configuration II is greater than configuration I (−178.99 kcal/mol vs −170.94 kcal/mol), due to the stronger HBs between the anion and the C2−H group in former structure. This also indicates that though the electrostatic attraction is still dominant in ion-pair dimers, HB interactions cannot be negligible. Notably, the average interaction energy of two ion-pair fragments of TM-ILs is predicted to be smaller in absolute value than conventional IL, consistent with the tendency observed in single ion pairs. Thus, the physicochemical properties of conventional ILs or TMILs, which may relate to the so-called binding energy, would be determined not only by the interactions between the cations and anions but also by the interactions between neutral ion pairs. Further CSD Study. The Cambridge Structural Database (CSD) is a convenient and reliable storehouse for structural information.55 The utility of small molecule crystallography and the CSD in analyzing geometric features has been well established. To provide some crystallographic implications of TM-ILs studied in this work, a survey of the CSD (version 5.34, updates November 2012) was undertaken. Only crystal structures with no disorder and errors as well as R-factor less than 0.1 were considered. Herein two types of salts were taken into account: (1) the imidazolium cations and transition metal halide anions MXny− (X = F, Cl, Br); (2) the [(Cnim)Ag(Cmim)] cations and anions. According to our survey of the CSD, 91 crystal structures of type 1, together with 56 crystal structures of type 2, were retrieved, as shown in Supporting Information. For the crystal structures of type 1, a wide range of transition metals, such as Pd, Co, Cu, Fe, Zn, Pt, Hg, and Mn, are presented. In particular, 76% of these crystal structures contain transition metal chloride anions, 20% include bromide anions, and only four structures contain fluoride anions. For the crystal structures of type 2, various kinds of anions, such as NO3−, F3CSO3−, PF6−, BF4−, SbF6−, and Tf2N−, are presented. Moreover, approximately half of the cations in these crystal structures include two or more Ag atoms and show a ring structure. On the basis of these findings, TM-ILs involving transition metal chloride anions or poly coordinated cations could be easily attained and characterized.
plane, consistent with the X-ray crystal structure of corresponding ILs. In general, the interactions between the cations and the [Tf2N] anion are characterized by multiple HB interactions between the electronegative N/O atoms and the H atoms of both imidazole rings. Notably, the interaction energies of silver-containing ion pairs slightly decrease as the alkyl chains increase, which coincides with the trend of the melting points of corresponding ILs. The NBO delocalization energy reveals multiple HBs in TMILs, and the critical point from the AIM analysis demonstrates weaker HBs in TM-ILs with respect to conventional ILs. These NBO and AIM results agree well with the ESP, structural, and energetic features of TM-ILs. Furthermore, HBs are very important in locating the positions of ions as well as in determining the properties of ILs, based on the NBO and AIM analyses. The canonical orbital analysis reveals that the presence of transition metals into the cations or anions influences the orbital overlap of ion pairs significantly. For the ion-pair dimers, the average interaction energies of TM-ILs are calculated to be less negative than conventional ILs, in good agreement with the tendency observed in single ion pairs. Although present calculations were performed on ion pairs in gas phase, the results obtained herein allow us to provide some useful insight. First, intermolecular interactions become weaker with the introduction of transition metals into the cations or anions, and therefore, TM-ILs could have relatively low melting points. Second, the microstructures of TM-ILs are much more complex compared with conventional ILs, due to the coordination ability of transition metals. TM-ILs thus could possess certain unique properties, such as high dissolution of metal salts or even CO2 and good catalytic ability. Additionally, according to our survey (the CSD analysis), a number of X-ray crystal structures of salts containing [(Cnim)Ag(Cmim)] cations or transition metal halide anions were extracted. On the basis of the CSD analysis, TM-ILs involving transition metal chloride anions or polycoordinated cations could be easily attained and characterized.
4. CONCLUSIONS AND IMPLICATIONS In this work, the geometric and energetic properties of several ILs containing transition metals in cations or anions were systematically investigated by means of the DFT/B3LYP method. For comparison, two conventional ILs, i.e., [C1mim][Tf2N] and [C1mim][BF4], were also considered. For the ion pair [C1mim][BF4], the anion prefers to reside over the imidazole ring and also forms HBs with the C2−H group. This typical structure can also be obtained for the ion pairs [C1mim][ZnCl3], [C1mim][HgCl3], and [C1mim][ReO4]. Nonetheless, in comparison with the interactions in [C1mim][BF4], cation−anion interactions become weaker in the ion pairs involving transition metals, consistent with the results of the ESP analysis. When transition metals were introduced into the cations, the conformations become much more complicated with respect to conventional ILs. Two kinds of configurations, i.e., cisoid and transoid arrangements of the two imidazole rings in the cations, were attained for [(Cnim)Ag(mim)][Tf2N]. In all transoid configurations the two imidazole rings lie nearly in the same
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ASSOCIATED CONTENT
S Supporting Information *
The results of the CSD survey and a table displaying NPA charges of selected cations and ion pairs. This information is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Y. Lu: tel/fax, 86-21-64252767; e-mail, [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Shanghai (11ZR1408700), the National Natural Science Foundation of China (21103047), the National Basic Research Program of China (2009CB219902), and the Fundamental Research Funds for the Central Universities of China.
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REFERENCES
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Structures and Electronic Properties of Transition Metal-Containing Ionic Liquids: Insights from Ion Pairs Weihong Wu,† Yunxiang Lu,*,† Yingtao Liu,‡ Haiying Li,† Changjun Peng,† Honglai Liu,† and Weiliang Zhu‡ †
Key Laboratory for Advanced Materials and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China ‡ Drug Discovery and Design Center, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China S Supporting Information *
ABSTRACT: Transition metal-containing ionic liquids (TM-ILs) have attracted a great deal of attention in recent years, due to their unique physical and chemical properties. In this work, several representative TM-ILs, such as the cations consisting of silver(I) center coodinated by two n-alkylimidazole ligands ([(Cnim)Ag(mim)]+) and the anions involving mercury(II) (HgCl3−), zinc(II) (ZnCl3−), and rhenium(VII) (ReO4−), were investigated using density functional theory calculations. First, the structural and energetic properties of the ion pairs for these TM-ILs have been examined in detail and compared with properties for conventional ILs. It was found that the interactions between the cations and anions, including hydrogen bonds and electrostatic interactions, in TM-ILs become weaker in strength than those in traditional ILs. In particular, the calculated geometric and energetic features compare fairly well with the experimental results, such as melting points and X-ray crystal structures of these TM-ILs. Then, the structures and energetics of ion-pair dimers for three ILs containing HgCl3−, ZnCl3−, and ReO4− were also explored, to gain a deeper understanding of the properties of TM-ILs. Finally, a survey of the Cambridge Structural Database (CSD) was undertaken to provide some crystallographic implications of TM-ILs.
1. INTRODUCTION Ionic liquids (ILs) are a class of novel compounds composed exclusively of organic cations (usually imidazolium, pyrimidinium, ammonium and phosphonium) and inorganic anions (e.g., X−, BF4−, PF6−, Tf2N−, etc).1−5 Owing to the unique physical properties and a widely tunable character, ILs have been used as green solvents in organic synthesis and good catalysts in catalysis chemistry.6−11 In addition, ILs are also becoming increasingly important in the electrodeposition of metals as well as in applications such as electrolytes in batteries and supercapacitors.12−15 However, the solubility of metal salts in conventional ILs is often rather limited, due to the weak coordination ability of common cations and anions. Incorporation of metals into the cations or anions clearly can improve the dissolution of metal salts into ILs.16−18 In recent years, transition metal-containing ionic liquids (TM-ILs) have attracted intensive research interest, because of their many advantages over traditional ILs, such as a high solubility of metal salts, an unexpectedly strong response to an additional magnetic field, and a wonderful color rendering property.19,20 Development of TM-ILs thus provides a great opportunity for task-specific ILs, which are promising alternatives to conventional ILs. In 2011, a series of mercury(II) ILs ([Cnmim][HgX3]), where [Cnmim] is nalkyl-3-methylimidazolium with n = 3, 4 and X = Cl, Br, were synthesized and structurally characterized via single-crystal Xray diffraction.21 Subsequently, Binnemans and co-workers © 2014 American Chemical Society
studied heterolepic ILs with the cations consisting of silver(I) center coorinated by two different n-alkylimidazole ligands ([(Cnim)Ag(Cmim)]+), and they found that these new ILs have low melting points and are useful in the electrodeposition of silver coating.22−25 More recently, Lin and Vasam have reviewed the important role of ILs and ionic liquid crystals of imidazolium salts composed of various transition and main group metals in organic transformations and material science.19 However, to the best of our knowledge, no theoretical studies concerning TM-ILs have been conducted so far. Considering the importance of these ILs in a wide range of chemical fields, a deep understanding of microstructures and intermolecular interactions of TM-ILs at molecular level should be of great value. In this work, three heteroleptic silver-containing ILs, i.e., [Ag(mim)2][Tf2N], [(C2im)Ag(mim)][Tf2N], and [(C4im)Ag(mim)][Tf2N], together with three ILs involving transition metals in the anions ([C1mim][HgCl3], [C1mim][ZnCl3], and [C1mim][ReO4]) were considered. The crystal structures and physicochemical properties of these TM-ILs have been reported in previous papers.21,22,26,27 In addition, two conventional ILs, [C1mim][Tf2N] and [C1mim][BF4], were also chosen for comparison. First, all possible conformations of the Received: December 20, 2013 Revised: March 13, 2014 Published: March 14, 2014 2508
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Figure 1. ESP surfaces of isolated cations and anions.
ion pairs for the studied TM-ILs were simulated, and different molecular interactions, such as hydrogen bonds (HBs) and electrostatic forces, between the cations and anions were examined. Then, the structures and energetics of two ion pairs for the ILs containing HgCl3−, ZnCl3−, and ReO4− were also explored, to gain more insight into the properties of these TMILs. Finally, the implications of the determined geometric and energetic features of ion pairs on the design of TM-ILs were discussed. This work could be very helpful for tuning the properties of TM-ILs and developing novel task-specific ILs.
and the electrostatic potential (ESP) analysis was undertaken by Multiwfn 2.4 program,43 using the wave functions generated with B3LYP/aug-cc-pVDZ(-PP).
3. RESULTS AND DISCUSSION Isolated Cations and Anions. The B3LYP-optimized structures of isolated cations and anions under study are shown in Figure 1. It is clear that the two imidazole rings in the cations containing silver are almost perpendicular to each other, as a result of steric effects and metal−ligand bonds. A comparison between the calculated geometric data and the X-ray crystal structures reveals that the calculated structures of [(Cnim)Ag(mim)]+ (n = 1, 2, 4) compare well to the X-ray structures.22 For example, the computed Ag−N distances are within 0.03 Å of the X-ray values; the N−Ag−N angles are predicted to be about 175°, quite similar to those in crystal structures.22 The introduction of transition metals into the cations or anions leads to some obvious changes in electrostatic potential (ESP) distribution, as also depicted in Figure 1. First, the ESPs for the O/Cl atoms in ReO4−, HgCl3−, and ZnCl3− become less negative than the F atoms in BF4−, consistent with the smaller VS,min (the most negative surface ESPs) for the O/Cl atoms (cf. Figure 1). Moreover, the ESPs over the plane of the anions involving transition metals appear to be less negative compared with that for the BF4 anion. Second, for the [(Cnim)Ag(mim)] cations, the H(C2) atoms exhibit the most positive ESPs; however, these atoms have smaller values of VS,max (the most positive surface ESPs) with respect to the [C1mim] cation. Similarly, relative to [C1mim]+, the ESPs over the imidazole rings become less positive for [(Cnim)Ag(mim)]+. On the basis of these, the presence of transition metals into the cations or anions should give rise to weaker intermolecular interactions, including HBs and electrostatic forces, in ion pairs (vide infra). Additionally, as the alkyl chain of [(Cnim)Ag(mim)]+ increases, VS,max for the H(C2) atoms tends to decrease gradually, in
2. THEORETICAL METHODS The geometries of all the ion pairs under study were fully optimized by means of the hybrid B3LYP functional,28,29 which has been commonly utilized in the study of cation−anion interactions in ILs.30−36 The aug-cc-pVDZ-PP basis set, which uses pseudopotentials to describe the inner core orbitals, was employed for transition metals, whereas for the remaining atoms aug-cc-pVDZ was applied. Very recently, we have successfully employed the B3LYP/aug-cc-pVDZ(-PP) method to investigate intermolecular interactions in halogenated ILs.37 During geometry optimizations no symmetry or geometry constraint was imposed. Frequency calculations performed at the same theoretical level indicated that all the structures obtained correspond to energetic minima without imaginary frequency. All the calculations reported in this work were carried out with the Gaussian 09 suite of programs.38 The interaction energy was estimated as the difference between the total energy of the ion pair and the sum of the total energy of the minimum geometry of the cation and anion. The standard counterpoise method of Boys and Bernardi was employed to correct the basis set superposition error (BSSE).39 Atomic charges and canonical orbitals were computed via the natural bond order (NBO) method implemented in Gaussian 09.40,41 The quantum theory of atoms in molecules (QTAIM) analysis was performed with the help of AIM 2000 software42 2509
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Figure 2. Favorable locations for the anions around the imidazolium cation and four initial configurations for the ion pairs involving silver.
Figure 3. Optimized structures of ion pairs containing transition metals in anions. Distances are in angstroms.
the back of the cation (back D), and the top of the cation (top E), as shown in Figure 2. Here it should be pointed out that the interaction in the conformer E can be defined as an anion−π interaction that shows some directionality.49 In recent years, this interaction involving cationic aromatic rings has been studied from both experimental and theoretical viewpoints.50,51 For the ion pairs [(Cnim)Ag(mim)][Tf2N], four initial configurations (A, B, C, and D) were considered on the basis of the ESP analysis and crystal structures. Moreover, two arrangements of the imidazole rings, i.e., cisoid and transoid arrangements, in these ion pairs were taken into account
accordance with the trend of interaction energies of corresponding ion pairs (see below). Structures of Single Ion Pairs. Ionic liquids are composed of ions completely so that the ion pairs as the basic structural units are very important for studying ILs.44−48 Given multiple stable structures of the ion pairs under study, we initially defined five favorable locations for the anions (BF4−, ReO4−, HgCl3−, and ZnCl3−) around the imidazolium cation, that is, the front of the C2−H moiety (front A), the site between the C2−H group and the methyl chain (meth-front B), the site between the methyl chain and the C4−H group (meth-back C), 2510
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Figure 4. Optimized structures of ion pairs [C1mim][Tf2N] and [Ag(mim)2][Tf2N]. Distances are in angstroms.
very similar to [C1mim][BF4]. Obviously, although the structures and interaction motifs are somewhat different, HBs are the explicit structural features for both conventional ILs and TM-ILs. When the transition metal was introduced into the cations, the conformations become much more complicated compared with conventional ILs, as shown in Figures 4 and 5. Two kinds of configurations, i.e., cisoid and transoid arrangements of the two imidazole rings in the cations, were attained for the ion pairs [(Cnim)Ag(mim)][Tf2N]. Particularly, in all transoid configurations the two imidazole rings lie nearly in the same plane, consistent with the X-ray structure of corresponding ILs also displayed in Figure 4.22 However, the two imidazole rings in cisoid conformer C appear to be not in the same plane. This difference may be explained by the fact that the weaker N/O··· H(C4)/H(C5) HBs in this conformer cannot keep the two rings in the same plane, whereas much stronger N/O···H(C2) interactions are presented in other cisoid conformers (A and B) and all transoid structures. In general, the interactions between the cations and the [Tf2N] anion are characterized by multiple HBs between the electronegative O/N atoms and the H atoms of the imidazole ring or the methyl chain. Moreover, in the ion pairs containing silver, the anion is involved in several HBs with the H atoms from both imidazole rings, in line with the X-ray structure of corresponding ILs.22 As expected, relative to [Ag(mim)2][Tf2N], more conformers were attained for the ion pairs [(C2im)Ag(mim)][Tf2N] and [(C4im)Ag(mim)][Tf2N], due to two different alkylimidazole ligands coordinated to silver center in the latter systems. In addition, all the N···H(C2) distances in the ion pairs containing silver are predicted longer than those in [C1mim][Tf2N], which implies weaker N··· H(C2) HBs in these ion pairs (see below).
(Figure 2), albeit the two rings in the crystal structures of corresponding ILs always exhibit transoid arrangement. In addition, for comparison purposes, four initial configurations (front A, meth-front B, meth-back C, and back D) were also calculated for the ion pair [C1mim][Tf2N]. In common, a geometric criterion for HB is the distance between the proton in the donor group and the acceptor atom, which is less than the sum of their van der Waals (vdW) radii.52 It is interesting that for [C1mim][BF4] only one stable structure (E), in which the anion is located over the imidazole ring and also forms two F···H HBs with the C2−H moiety, was obtained at the level of B3LYP/aug-cc-pVDZ(-PP), whereas the other two structures (A and D) have one imaginary frequency (about 20i cm−1). However, as demonstrated previously, three stable minima (C, D, and E) were attained for [C4mim][BF4] through the B3LYP/6-31++(d,p) method.36 Apparently, the longer alkyl chain (butyl group) tends to stabilize the structures with higher energy. The optimized structures of [C1mim][ZnCl3] and [C1mim][HgCl3] are quite similar to each other: in structure A, two Cl···H interactions occur between the Cl atom and the H(C2) atom of the ring as well as the H atom of the alkyl chain, and moreover, the anion lies perpendicular to the imidazole ring, in good agreement with the X-ray structure of corresponding IL also shown in Figure 3.21 In conformer C the anion is involved in three or four HBs with the C4−H/C5− H moiety and the alkyl chain, whereas the anion resides over the imidazolium cation in conformer E. For the ion pairs [C1mim][ReO4], four stable configurations were obtained: in structure A, two symmetrical O···H interactions (about 2.1 Å) take place between two O atoms and the H(C2) atom, and moreover, the two O atoms lie in the plane of the imidazole ring. In structures C and D, the anion forms two HBs with the C4−H/C5−H moiety or the alkyl chain, whereas conformer E is 2511
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Figure 5. Optimized structures of ion pairs [(C2im)Ag(mim)][Tf2N] and [(C4im)Ag(mim)][Tf2N]. Distances are in angstroms.
Table 1. Interaction Energies (kcal/mol) of the Ion Pairs Calculated with B3LYP/aug-cc-pVDZ(-PP) conformers A B C D E conformers A B C D A′ B′ C′ D′
[C1mim][BF4]
[C1mim][ZnCl3]
[C1mim][HgCl3]
[C1mim][ReO4]
−82.14
−73.67
−73.24
−78.77
−66.29
−65.63
−71.87 −83.66 [C1mim][Tf2N]
−71.90 [Ag(mim)2][Tf2N]
−75.23
−67.31
−66.97 −67.76
−60.97 −64.71 −62.45
Interaction Energies of Single Ion Pairs. The interaction energy is often seen as one of the most important parameters to correlate with the properties of ILs. Table 1 lists the interaction energies with corrected values of BSSE for the ion pairs under study. It is evident that the calculated interaction energies of the ion pairs including ReO4−, HgCl3−, and ZnCl3− are somewhat less negative than those of [C1mim][BF4]. As a consequence, intermolecular interactions in these ion pairs become much
−72.81 [(C2im)Ag(mim)][Tf2N]
−69.22 −70.88 −77.50 [(C4im)Ag(mim)][Tf2N]
−66.90 −66.70 −60.59
−66.50 −65.52 −60.29
−64.25 −64.10 −60.71 −60.08
−63.82 −63.72 −61.49 −61.47
weaker in strength, in accordance with the results of the ESP analysis (vide supra). In fact, transition metal-containing ILs ([C4mim][HgCl3] and [C4mim][ZnCl3]) show a relatively low melting point (about 90 °C).21,26 For the ion pairs involving HgCl3− and ZnCl3−, the interaction energy of conformer A is computed to be the largest in absolute value (about −73.2 kcal/ mol), thus indicating the stronger acidity of H(C2) with respect to H(C4)/H(C5) and the H atom of the methyl chain. 2512
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Table 2. Properties of the Electron Density at BCPs for Selected Ion Pairsa ion pairs [C1mim][BF4] (E)
[C1mim][ZnCl3] (A) [C1mim][ZnCl3] (C)
[C1mim][ZnCl3] (E)
a
HBs
ρc
∇2ρc
Hc
B1 B2 B3 B1 B2 B1 B2 B3 B1 B2 B3
0.012 0.013 0.024 0.021 0.008 0.010 0.017 0.009 0.010 0.011 0.010
0.042 0.057 0.081 0.055 0.021 0.036 0.064 0.038 0.036 0.043 0.038
0.0005 0.0016 0.0000 0.0009 0.0007 0.0007 0.0009 0.0013 0.0008 0.0014 0.0008
ion pairs [C1mim] [Tf2N](A)
[C1mim] [Tf2N](C) [(C2im)Ag(mim)] [Tf2N](A′)
[(C2im)Ag(mim)] [Tf2N](C′)
HBs
ρc
∇2ρc
Hc
B1 B2 B3 B2 B3 B1 B2 B3 B1 B2
0.017 0.029 0.017 0.031 0.015 0.011 0.022 0.013 0.018 0.017
0.072 0.124 0.071 0.134 0.060 0.047 0.089 0.057 0.070 0.074
0.0007 0.0019 0.0007 0.0019 0.0004 0.0005 0.0017 0.0008 0.0012 0.0010
All values are given in au.
Table 3. Electron Donor Orbitals, Electron Acceptor Orbitals, and the Second-Order Interaction Energies E2 (kcal/mol) of Selected Ion Pairs ion pair [C1mim][BF4] (E)
[C1mim][ReO4] (C) [C1mim][ReO4] (D) [C1mim][ReO4] (E)
donor (i) B1:LP B2:LP B3:LP B1:LP B2:LP B1:LP B2:LP B1:LP B2:LP B3:LP
(F) (F) (F) (O) (O) (O) (O) (O) (O) (O)
acceptor (j) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1)
C7−H C2−H C2−H C5−H C6−H C4−H C5−H C7−H C2−H C6−H
E2 2.22 2.24 7.57 5.47 7.20 1.63 16.19 0.31 5.92 1.07
ion pair
donor (i)
[C1mim][Tf2N] (A)
[C1mim][Tf2N] (C)
[(C2im)Ag(mim)] [Tf2N] (A′)
[(C2im)Ag(mim)] [Tf2N] (C′)
B1:LP B2:LP B3:LP B1:LP B2:LP B3:LP B1:LP B2:LP B3:LP B1:LP B2:LP
(O) (N) (O) (O) (N) (O) (O) (N) (O) (N) (O)
acceptor (j) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1)
C6−H C2−H C7−H C7−H C4−H C4−H C6−H C2−H C4′-H C4−H C2′-H
E2 6.21 16.05 6.18 5.52 15.30 0.43 3.50 9.36 2.53 6.42 3.79
representative ion pairs are summarized in Table 2. As can be seen, the interactions between the cations and anions are all closed shell interactions with ρc and ∇2ρc in the range of normal HB (ρc < 0.200 au and 0.020 au < ∇2ρc < 0.139 au). Moreover, the total electron energy densities (Hc) at the BCPs were evaluated to be positive, in good agreement with previous AIM results of imidazolium-based ILs.54 Here it is worth mentioning that the calculated interaction energies of present ion pairs contain not only effects of HB formation but also Coulombic attraction of two opposite charges; Coulombic forces dominate interactions in ILs and the contribution of HBs is small but non-negligible.31 In fact, the energies of HBs in several imidazolium-based ILs were recently determined to vary from −2.8 to −3.8 kcal/mol, using combined X-ray crystallographic, Infrared/Raman spectroscopic, and DFT methods.54 In conformer E of [C1mim][BF4] and [C1mim][ZnCl3], three BCPs are identified for F···H interactions, and moreover the values of ρc for [C1mim][ZnCl3] are estimated to be smaller than [C1mim][BF4]. Therefore, intermolecular interactions in the ion pairs containing transition metals become weaker, consistent with the energetic features of these ion pairs (vide supra). As expected, in conformer A of [C1mim][ZnCl3], ρc for the Cl···H(C2) HB is calculated to be larger with respect to the Cl···H(methyl chain) interaction (0.021 au vs 0.008 au); the computed ρc for the three HBs in conformer C is smaller than the Cl···H(C2) interaction in structure A. These AIM results accord well with the energetic properties of these ion pairs as demonstrated above. From Table 2, it is also seen that ρc for the N···H interactions in [C1mim][Tf2N] and [(C2im)Ag(mim)][Tf2N] is evaluated somewhat greater than other HBs. Moreover, the calculated ρc
Similarly, for [C1mim][ReO4] conformer A exhibits the most negative interaction energy (about −77.8 kcal/mol), due to the occurrence of four HBs and the strong acidity of H(C2). From Table 1, it is also seen that, as compared to the case for [C 1 mim][Tf 2N], less negative interaction energies are predicted for the ion pairs containing silver. Therefore, cation−anion interactions in these ion pairs also become weaker, accordant with the results of the ESP analysis (see above). Particularly, the computed interaction energies of transoid structures A and B for [(Cnim)Ag(mim)][Tf2N] are smaller in absolute value with respect to the corresponding cisoid ones, although the two imidazole rings in the crystal structures of corresponding ILs exhibit transoid arrangement. To some extent, this can be attributed to the fact that the anion in cisoid configurations resides in the front of the C2−H moiety of both imidazole rings, whereas in transoid configurations the anion only lies close to the C2−H group of one ring. Therefore, both the C2−H moieties could form stronger HBs with the electronegative atoms in the anion in cisoid configurations. Generally, the interaction energies of silver-containing ion pairs slightly decrease as the alkyl chains increase, which coincides with the trend of VS,max for the H(C2) atoms in isolated cations as pointed out above. In fact, the melting points of the three ILs, [Ag(mim) 2 ][Tf2N], [(C 2 im)Ag(mim)][Tf 2 N] and [(C4im)Ag(mim)][Tf2N], were reported to be 87, 35, and 30 °C, respectively.22 AIM and NBO Analyses of Ion Pairs. The bond properties between each pair of atoms were analyzed in detail using the AIM theory, which is based on the topological analysis of electron density (ρc) and its Laplacian (∇2ρc) at bond critical points (BCP).53 The calculated AIM data of some 2513
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Figure 6. FMOs energy-order diagram for selected structures.
and ∇2ρc for the N···H interactions in [(C2im)Ag(mim)][Tf2N] are less than those in [C1mim][Tf2N], which indicates weaker N···H HBs in silver-containing ion pairs as noted above. In addition, the AIM analysis also reveals several secondary O··· H interactions between the O atom and the H atoms in the imidazole ring. It has been established that the general n(Y) → σ*(X−H) donor−acceptor HB interaction (X−H···Y) can be evaluated by the perturbation theory to calculate the delocalization energy (E2) as follows: 2
E = ΔEij = qi
Charge Distribution and Orbital Analysis. The NPA charges of isolated cations, anions, and some typical ion pairs are summarized in Table S1 in Supporting Information. Relative to the isolated [C1mim] cation, the charge on the two N atoms remains almost unchanged in conformer E of [C1mim][BF4] and [C1mim][ReO4]. However, the positive charge on H(C2) and C2 atoms both increases by about 0.03 au, while the positive charge on H(C4)/H(C5) atoms decreases and C4/C5 atoms carry more negative charge. These indicate that partial π electrons transfer from the C2−H group to the H(C4) and H(C5) atoms in the two structures, due to the repulsion between the anion and the π electrons on the ring. Additionally, upon the formation of the ion pairs, a magnitude of charge transfer (about 0.05 au) occurs from the anions to the imidazolium cation. NPA analysis also shows that the incorporation of silver into the cation influences the charge distribution to a large degree. Most of the positive charge concentrates upon the Ag atom, which results in an obvious decrease in the positive charge of C2−H, C4−H, and C5−H groups. Moreover, the N3 atom in [(Cnim)Ag(mim)]+ exhibits considerably more negative charge than that in [C1mim]+ due to silver coordination. Similarly, upon ion pair formation, a small amount of charge transfer (about 0.07 au) takes place from the [Tf2N] anion to the cations. It is well-known that the canonical molecular orbital analysis can provide an essential understanding of the charge transfer. Figure 6 depicts the frontier molecular orbitals (FMOs) energyorder diagram for four representative structures of the ion pairs, i.e., conformer E of [C1mim][BF4] and [C1mm][ReO4] and conformer A of [C1mim][Tf2N] and [Ag(mim)2][Tf2N]. As can be seen, the HOMO of the [BF4]/[ReO4] anions is lone pair orbitals of four F/O atoms and the orbital energies are −0.166 and −0.131 au, respectively, whereas the LUMO of the [C1mim] cation is the anti-π (π*) bond orbital with four energetic nodes and the orbital energy is −0.961 au. Notably, the FMOs of [C1mim][BF4] and [C1mm][ReO4] exhibit distinct interaction between the lone pair of the O/F atoms and the lowest empty π*(C2−H) orbital of the cation. Nevertheless, the HOMO of [C1mim][ReO4] is mainly located on the anion, whereas in [C1mim][BF4] the HOMO resides largely on the imidazole ring of the cation. Figure 6 also shows that the HOMO of the [Tf2N] anion is composed of lone pair orbitals of the N/O/S atoms (the orbital energy E = −0.149 au),
(Fij)2 ei − ej
(1)
where qi is the donor orbital occupancy, ei and ej are diagonal elements (orbital energies), and Fij is the off-diagonal NBO Fock matrix element. The E2 energies of selected ion pairs obtained from the NBO analysis, which represent the different capacities of forming HBs, are listed Table 3. In conformer D of [C1mim][ReO4], the calculated values of E2 for the donor− acceptor orbitals, n(O) → σ*(C5−H) and n(O) → σ*(C4− H), are 16.19 and 1.63 kcal/mol, respectively, consistent with the much shorter O···H(C5) distance compared with O··· H(C4) (1.88 Å vs 2.08 Å) as shown in Figure 3. Additionally, in conformer E of [C1mim][BF4] and [C1mim][ReO4], E2 for the donor−acceptor orbital, n(F/O) → σ* (C2−H), is computed to be 7.57 and 5.92 kcal/mol, respectively, which further confirms weaker HB capacities in the ion pairs involving transition metals. Table 3 also shows that in [C1mim][Tf2N] and [(C2im)Ag(mim)][Tf2N], the n(N) → σ*(C−H) orbital interactions exhibit the largest E2, and moreover, these interactions are much stronger in [C1mim][Tf2N] with respect to [(C2im)Ag(mim)][Tf2N]. For instance, the values of E2 for n(N) → σ*(C2−H) in conformer A of [C1mim][Tf2N] and [(C2im)Ag(mim)][Tf2N] are evaluated to be 16.05 and 9.36 kcal/mol, respectively, much larger than other orbital interactions in this conformer. These NBO results agree well with the geometric and AIM properties of these ion pairs. In fact, for silvercontaining ion pairs, there exists a good correlation between electron densities at the BCPs and orbital interaction energies (R = 0.92). Additionally, albeit the calculated E2 for n(O) → σ*(C−H) is relatively smaller, these HBs should also influence the structures of silver-containing ion pairs. 2514
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Figure 7. Optimized structures of ion-pair dimers. Distances are in angstroms.
the methyl chains. Not surprisingly, the configurations of [C1mim][ZnCl3] are roughly analogous to [C1mim][HgCl3], on account of the same geometric symmetry of the two anions. The interactions between different fragments of ion-pair dimers were studied systematically, including the total interaction energy (ΔEtot) of the whole dimers and the interactions energy (ΔE1/ΔE2) of two ion-pair fragments. ΔEtot was evaluated as the difference between the total energy of ionpair dimer and the sum of the total energy of isolated cations and anions, i.e., ΔEtot = Edimer − 2 × (Ecation + Eanion), and ΔE1 and ΔE2 represent the interactions energy between two ion-pair fragments, which were taken from the whole system. The corresponding ion-pair fragments are defined in Figure 7 and the interaction energies mentioned are summarized in Table 4. As can be seen, the total interaction energy of the dimers involving transition metals is estimated somewhat less negative than [C1mim][BF4], which further confirms weaker cation−
whereas the orbital of Ag contributes significantly to the LUMO of the [Ag(mim)2] cation. However, the HOMO of [Ag(mim)2][Tf2N] resides mostly on one imidazole ring in the cation, whereas in [C1mim][Tf2N] the HOMO is mainly located on the anion. Clearly, the presence of transition metals into the cations or anions influences the orbital overlap of ion pairs significantly. Structures and Energetics of Ion-Pair Dimers. The above description of cation−anion interactions in ILs was solely based on the single ion-pair unit, and therefore, it neglected the interactions between the ion pairs. To gain a deeper understanding of the properties of TM-ILs, the study of larger clusters containing up to four component units has been undertaken in this work. It should be pointed out that here only ion pairs of ReO4−, HgCl3−, and ZnCl3− were investigated, as a result of the computational cost and much complex of the systems involving silver. Various chemically reasonable structures of ion-pair dimers have been calculated at the B3LYP/aug-cc-pVDZ(-PP) level of theory, and all possible cation−anion interaction modes were taken into account during the design of initial geometries. According to our calculations, several stable geometries of two ion pairs were obtained, as displayed in Figure 7. For [C1mim][BF4], the cations and anions form a ring-like structure with multiple HBs, such as C4/5−H···F and C6/7− H···F interactions; the two imidazole rings adopt a roughly antiparallel geometry. Configuration I of [C1mim][ReO4] is quite similar to [C1mim][BF4], but the bond length H···O is generally longer than the corresponding H···F distance due to the larger vdW radius of the O atom. Configuration II of [C1mim][ReO4] is a standard layer structure composed of two units of conformer A of the ion pair. In configuration III of [C1mim][HgCl3], four symmetrical HBs occur between two Cl atoms and the H(C4)/H(C5) atoms with the distances of about 2.45 Å, whereas configuration I can be considered as the stacking of conformer A of the ion pair. In configuration II of [C1mim][HgCl3], the anion forms several secondary HBs with
Table 4. Total Interaction Energies, the Interaction Energies of Two Ion-Pair Fragments, and the Average Interaction Energies for Ion-Pair Dimersa conformers
ΔEtot
ΔE1
ΔE2
ΔEaver
[C1mim][BF4] [C1mim][ZnCl3] (I) [C1mim][ZnCl3] (II) [C1mim][HgCl3] (I) [C1mim][HgCl3] (II) [C1mim][HgCl3] (III) [C1mim][ReO4] (I) [C1mim][ReO4] (II)
−183.62 −166.57 −165.77 −165.87 −163.24 −159.52 −170.94 −178.99
−38.06 −40.55 −33.28 −23.95 −31.22 −33.99 −34.80 −20.15
−37.88 −24.57 −29.73 −41.47 −31.08 −31.59 −39.22 −29.11
−37.97 −31.73 −32.33 −32.71 −31.15 −32.79 −37.01 −24.63
All values are given in kcal/mol. ΔE1 is the interaction energy between fragment (P1 + P2) and fragment (P3 + P4) in the dimers, ΔE2 is the interaction energy between fragment (P1 + P4) and fragment (P2 + P3) in the dimers, and ΔEaver is the average interaction energy of ΔE1 and ΔE2. The fragments (P1, P2, P3, and P4) are defined in Figure 7. a
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anion interactions in TM-ILs. Additionally, for [C1mim][ReO4] the computed total interaction energy of configuration II is greater than configuration I (−178.99 kcal/mol vs −170.94 kcal/mol), due to the stronger HBs between the anion and the C2−H group in former structure. This also indicates that though the electrostatic attraction is still dominant in ion-pair dimers, HB interactions cannot be negligible. Notably, the average interaction energy of two ion-pair fragments of TM-ILs is predicted to be smaller in absolute value than conventional IL, consistent with the tendency observed in single ion pairs. Thus, the physicochemical properties of conventional ILs or TMILs, which may relate to the so-called binding energy, would be determined not only by the interactions between the cations and anions but also by the interactions between neutral ion pairs. Further CSD Study. The Cambridge Structural Database (CSD) is a convenient and reliable storehouse for structural information.55 The utility of small molecule crystallography and the CSD in analyzing geometric features has been well established. To provide some crystallographic implications of TM-ILs studied in this work, a survey of the CSD (version 5.34, updates November 2012) was undertaken. Only crystal structures with no disorder and errors as well as R-factor less than 0.1 were considered. Herein two types of salts were taken into account: (1) the imidazolium cations and transition metal halide anions MXny− (X = F, Cl, Br); (2) the [(Cnim)Ag(Cmim)] cations and anions. According to our survey of the CSD, 91 crystal structures of type 1, together with 56 crystal structures of type 2, were retrieved, as shown in Supporting Information. For the crystal structures of type 1, a wide range of transition metals, such as Pd, Co, Cu, Fe, Zn, Pt, Hg, and Mn, are presented. In particular, 76% of these crystal structures contain transition metal chloride anions, 20% include bromide anions, and only four structures contain fluoride anions. For the crystal structures of type 2, various kinds of anions, such as NO3−, F3CSO3−, PF6−, BF4−, SbF6−, and Tf2N−, are presented. Moreover, approximately half of the cations in these crystal structures include two or more Ag atoms and show a ring structure. On the basis of these findings, TM-ILs involving transition metal chloride anions or poly coordinated cations could be easily attained and characterized.
plane, consistent with the X-ray crystal structure of corresponding ILs. In general, the interactions between the cations and the [Tf2N] anion are characterized by multiple HB interactions between the electronegative N/O atoms and the H atoms of both imidazole rings. Notably, the interaction energies of silver-containing ion pairs slightly decrease as the alkyl chains increase, which coincides with the trend of the melting points of corresponding ILs. The NBO delocalization energy reveals multiple HBs in TMILs, and the critical point from the AIM analysis demonstrates weaker HBs in TM-ILs with respect to conventional ILs. These NBO and AIM results agree well with the ESP, structural, and energetic features of TM-ILs. Furthermore, HBs are very important in locating the positions of ions as well as in determining the properties of ILs, based on the NBO and AIM analyses. The canonical orbital analysis reveals that the presence of transition metals into the cations or anions influences the orbital overlap of ion pairs significantly. For the ion-pair dimers, the average interaction energies of TM-ILs are calculated to be less negative than conventional ILs, in good agreement with the tendency observed in single ion pairs. Although present calculations were performed on ion pairs in gas phase, the results obtained herein allow us to provide some useful insight. First, intermolecular interactions become weaker with the introduction of transition metals into the cations or anions, and therefore, TM-ILs could have relatively low melting points. Second, the microstructures of TM-ILs are much more complex compared with conventional ILs, due to the coordination ability of transition metals. TM-ILs thus could possess certain unique properties, such as high dissolution of metal salts or even CO2 and good catalytic ability. Additionally, according to our survey (the CSD analysis), a number of X-ray crystal structures of salts containing [(Cnim)Ag(Cmim)] cations or transition metal halide anions were extracted. On the basis of the CSD analysis, TM-ILs involving transition metal chloride anions or polycoordinated cations could be easily attained and characterized.
4. CONCLUSIONS AND IMPLICATIONS In this work, the geometric and energetic properties of several ILs containing transition metals in cations or anions were systematically investigated by means of the DFT/B3LYP method. For comparison, two conventional ILs, i.e., [C1mim][Tf2N] and [C1mim][BF4], were also considered. For the ion pair [C1mim][BF4], the anion prefers to reside over the imidazole ring and also forms HBs with the C2−H group. This typical structure can also be obtained for the ion pairs [C1mim][ZnCl3], [C1mim][HgCl3], and [C1mim][ReO4]. Nonetheless, in comparison with the interactions in [C1mim][BF4], cation−anion interactions become weaker in the ion pairs involving transition metals, consistent with the results of the ESP analysis. When transition metals were introduced into the cations, the conformations become much more complicated with respect to conventional ILs. Two kinds of configurations, i.e., cisoid and transoid arrangements of the two imidazole rings in the cations, were attained for [(Cnim)Ag(mim)][Tf2N]. In all transoid configurations the two imidazole rings lie nearly in the same
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ASSOCIATED CONTENT
S Supporting Information *
The results of the CSD survey and a table displaying NPA charges of selected cations and ion pairs. This information is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Y. Lu: tel/fax, 86-21-64252767; e-mail, [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Shanghai (11ZR1408700), the National Natural Science Foundation of China (21103047), the National Basic Research Program of China (2009CB219902), and the Fundamental Research Funds for the Central Universities of China.
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