Bobo Caoa,
Jiuyao Dua,
Ziping Caoa,
Haitao Sun*a,
Xuejun Sun*a and
Hui Fu*b
aChemistry and Chemical Engineering College, Qufu Normal University, Qufu 273165, Shandong, P. R. China. E-mail: sunhaitao1960@126.com; sxjsunxuejun@163.com; caozp_qfnu@163.com; qufucaobobo@163.com; jwyhbxr@163.com
bState Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum, Qingdao 266580, Shandong, P. R. China. E-mail: fuhui@upc.edu.cn
First published on 13th February 2017
Multiple techniques were used to study the reversibility of a series of imido-based ionic liquids (ILs). DFT (density functional theory) modeling originally indicated that methyl transfer favorably took place at the unsaturated CX bond in the N–CX (N, O and S) fragment. A series of imido-based ILs derived from the N–CX (N, O and S) fragment were studied and characterized using theoretical and experimental methods. Seven imido-based ILs were facilely synthesized in the experiment, which is consistent with the lower energy barriers obtained in the potential energy surface (PES) compared to the other ILs. Their structures were measured in nuclear magnetic resonance (NMR) spectra. The thermal stabilities were further studied by thermogravimetric analysis (TGA). The bond order results indicated that non-covalent interactions were the major driving force in the methyl transfer process. Non-covalent interactions in these ILs were investigated and characterized using atoms in molecules (AIM), reduced density gradient (RDG) and natural bond orbital (NBO) methods.
The distillation tests of ILs can be traced back to an article on the distillation and volatility of a wealth of commonly used aprotic ILs at low pressure without decomposition that was reported by Earle and co-workers.26 Earle et al. forcefully contradicted the widely held belief that ILs were nonvolatile. Unfortunately, the harsh conditions and poor efficiency mean it hardly meets the requirements of industry. Since then, efforts have been devoted to the investigation of the volatility and distillability of ILs.27–30 Jones et al.31 achieved higher distillation rates of [Cnmim][Tf2N] than those obtained by Earle et al.26 under high vacuum conditions. Furthermore, serious efforts were devoted to investigate the distillation of protic ILs, in which the neutral acids and bases were obtained in the distillation process via a proton-transfer mechanism.24,26,32–35 Efforts are still eagerly undertaken to systematically study novel distillable ILs to satisfy many requirements of the decarbonised global economy.
The reversibility and distillation of X-methylated (X = N, O, and S) triflates [TfO]− ILs are studied using experimental and computational methods. Electrostatic potential (ESP) was performed to locate the more favorable position for the methyl transfer reaction of sixteen ILs. Potential energy surface (PES) obtained through energy calculation is helpful in locating the reasonable reaction paths. Among these ILs, seven typical ILs were synthesized, and the structures of them were confirmed by nuclear magnetic resonance (NMR) spectra. Short-term stability and long-term stability of these ILs were investigated by temperature-ramped thermogravimetric analysis (TGA) and isothermal TGA experiments, respectively.36 Mayer bond order (MBO) and Laplacian bond order (LBO) were performed to reveal the nature of the chemical bonds based on two different quantum mechanical properties. The atoms in molecules (AIM) theory combined with the reduced density gradient (RDG) method as well as the natural bond orbital (NBO) theory were used to study the properties of the non-covalent interactions. The emphasis was on investigating the thermal stability of the ILs and revealing the thermodynamics and chemical mechanisms of reversibility.
To answer the question above, ESP was applied for the prediction of the nucleophilic and electrophilic sites.37–39 Since it reflects the specific electron structure features of a molecule, including lone pairs, π electrons, etc., ESP was performed on the van der Waal (vdW) surface. Additionally, it was further quantified and decomposed to the local surfaces corresponding to individual atoms to extract more information.
The color-code ESP on the vdW surfaces of N,N-dimethylformamide (DMF)/N,N-dimethylformimidamide (DMFA)/N,N-dimethylmethanethioamide (DMSF), their two possible methylated products and relative energies was calculated at the M06-2X/6-311++G** level and is shown in Fig. 1. The detailed results are listed in Table S1 (ESI†). CX is an electron-withdrawing group conjugated to the nitrogen atom that tends to make the nitrogen atom less nucleophilic in the N–CX fragment. Due to the n–π* electron delocalization, the average of the ESP calculated on the surface of the oxygen atom (−33.06 kcal mol−1) was more negative than the nitrogen atom (−1.17 kcal mol−1) in DMF. Additionally, the negative (red) regions and maximal value (−43.69 kcal mol−1) of the whole molecular surface were distributed around the oxygen atom on the color-code ESP surface (Fig. 1), where it is easier to attract the positive methyl carbon (blue regions). The same situations were found in DMSF and DMFA. It suggests that X instead of N in the N–CX fragment is more likely to react with the alkylating agents due to its higher nucleophilicity. It can be clearly seen from Fig. 1 that the X-alkylated products are more stable than their N-alkylated counterparts, with an energy difference of 22.15, 10.67, 24.91 kcal mol−1 for the methylated DMSF, DMF and DMFA cations, respectively. Considering the higher nucleophilicity and more stable methylated products, the X atom in the N–CX fragment should be the more favorable position for the methyl transfer reaction.
The [TfO]− anion has been used to study reversible and distillable ILs for many years due to its excellent performance in [Bmim][TfO].40,41 The distillation of TfO-based ILs has been systemically studied by both experimental and computational methods.27,28,30,31,42 Based on the analyses above, reversible and distillable ILs containing a series of methylated N–CX cations and the [TfO]− anion were designed by replacing the branched chain. The synthetic route of these ILs is typically depicted in Fig. 2 (see the Experimental section for details).
Fig. 3 Reaction paths for the methyl transfer in ILs. (a), (b) and (c) [MMU][TfO], [MtriMSU][TfO] and [MDTG][TfO], respectively. The color coding of atoms is depicted on the right side. |
Species | R | IM | TS | IL | ΔEa | ΔEb |
---|---|---|---|---|---|---|
[MDMF][TfO] | 0 | −6.02 | 48.57 | −9.79 | 54.59 | 58.36 |
[MMA][TfO] | 0 | −7.03 | 49.38 | −8.98 | 56.41 | 58.36 |
[MIpIb][TfO] | 0 | −6.65 | 49.07 | −7.84 | 55.72 | 56.91 |
[MDMP][TfO] | 0 | −6.59 | 48.50 | −10.99 | 55.09 | 59.49 |
[MDMA][TfO] | 0 | −7.43 | 47.82 | −11.73 | 55.16 | 59.55 |
[MMU][TfO] | 0 | −9.29 | 44.05 | −13.49 | 53.34 | 57.54 |
[MU][TfO] | 0 | −8.41 | 45.56 | −11.79 | 53.97 | 57.35 |
[MDMSF][TfO] | 0 | −6.97 | 44.61 | −17.70 | 51.58 | 62.31 |
[MMSU][TfO] | 0 | −10.54 | 39.72 | −21.40 | 50.26 | 61.12 |
[MMMSU][TfO] | 0 | −11.48 | 38.72 | −24.28 | 50.20 | 63.00 |
[MtriMSU][TfO] | 0 | −10.04 | 37.84 | −26.42 | 47.88 | 64.26 |
[MtMSU][TfO] | 0 | −8.66 | 39.59 | −25.67 | 48.25 | 65.26 |
[MSU][TfO] | 0 | −7.91 | 43.67 | −18.89 | 51.58 | 62.56 |
[MtMG][TfO] | 0 | −6.02 | 42.25 | −41.21 | 51.27 | 83.46 |
[MDPG][TfO] | 0 | −7.15 | 44.62 | −30.81 | 51.77 | 75.43 |
[MDTG][TfO] | 0 | −10.79 | 38.72 | −44.55 | 49.51 | 83.27 |
Fig. 4 PES of sixteen ILs calculated at M06-2X/6-311++G** level. ΔEa and ΔEb are energy barriers for synthesis and decomposition of ILs, respectively. |
As shown in Fig. 3a, the methyl of the methyl triflates transfers to the unsaturated CO bond by forming a TS where the methyl carbon atom lies almost midway between the two O atoms. Additionally, the IM corresponding to the TS structure also refers to the reaction path. The bond distance of CF3SO3⋯CH3 extends from 1.47 Å in the IM to 2.21 Å in the TS; simultaneously, the CO⋯CH3 bond begins to form with a distance of 2.28 Å. Finally, the neutral molecules transform to ILs, in which molecular-type hydrogen bonds transform to ionic-type hydrogen bonds. The fragments obtain (lose) methyls to become cations (anions). For the decomposition process, the path can be traced back from the ILs to the neutral molecules, and more energy will be consumed to overcome the energy barrier.
Fig. 4 shows that the energy barriers within each family are similar; however, the situation is discriminatory between the three families. All the energy barriers of decomposition (ΔEb) are obviously larger than those of the synthesis (ΔEa). As shown in Table 1, the ΔEa of ILs are in the order of [MMU][TfO] (53.34) < [MU][TfO] (53.97) < [MDMF][TfO] (54.59) < [MDMP][TfO] (55.09) < [MDMA][TfO] (55.16) < [MMA][TfO] (56.41) < [MIpIb][TfO] (55.72), and the ΔEb are in the order of [MIpIb][TfO] (56.91) < [MU][TfO] (57.35) < [MMU][TfO] (57.54) < [MDMF][TfO] (58.36) = [MMA][TfO] (58.36) < [MDMP][TfO] (59.49) < [MDMA][TfO] (59.55 kcal mol−1). The ΔEa and ΔEb of [MMU][TfO] and [MU][TfO] are about 54 and 57 kcal mol−1, respectively. It may indicate that their synthesis/decomposition processes can take place under mild conditions, which has been further demonstrated in our laboratory. Similar discussions and analyses can be extended to other ILs and similar conclusions can be obtained. Besides, Table 1 shows that the synthesis/decomposition processes of ILs are endothermal/exothermal reactions.
[MtriMSU][TfO] and [MtMSU][TfO] in the N–CS family are easier to prepare due to their ΔEa of around 48 kcal mol−1 [MMU][TfO] and [MU][TfO] in the N–CO family are easier to decompose because of their ΔEb of about 57 kcal mol−1 [MtMG][TfO], [MDPG][TfO] and [MDTG][TfO] in the N–CN family are also easy to prepare since their ΔEa are around 50 kcal mol−1. However, their larger ΔEbs (up to 75 kcal mol−1) makes them difficult to decompose. The seven kinds of ILs were chosen to further study their short-term and long-term thermal stabilities by using temperature-ramped TGA and isothermal TGA experiments, respectively.
Fig. 5 TG curves (a) and their TG-DTG curves for [MMU][TfO] (b), [MU][TfO] (c), [MtriMSU][TfO] (d) and [MtMSU][TfO] (e). |
Considering its advantages of lower ΔEa and energy consumption, the long-term stability of [MMU][TfO] was further investigated by isothermal TG experiment with a temperature gradient of 15 °C in the range of 300–225 °C, which is depicted in Fig. 6a. Distinctly, the isothermal TG curve shows a similar behavior where a linear weight loss was obtained for the entire duration of the experiment. Since the weight loss is about 12% after 6 h of isothermal TG experiment at 240 °C, this distillable IL can be used for different purposes because of its relative thermal stability.
Fig. 6 Isothermal TG curves of [MMU][TfO] at different temperatures (a) and the variation of weight% as a function of temperature after 6 h of isothermal thermogravimetry (b). |
Isothermal TG is shown in Fig. 6b, in which the weight loss of [MMU][TfO] is a function of temperature. Distinctly, the weight loss increases gradually in the temperature range of 225–270 °C; thereafter, it increases rapidly within the temperature range of 270–300 °C. It can be easy to conclude that [MMU][TfO] exhibits substantial (6 h) long-term thermal stability up to 270 °C with less than 20% weight loss. The Tonset determined using the temperature-ramped TGA method is useful to serve as the limiting value for practical application. This is consistent with the results of PES analyses above.
[MMU][TfO], [MtriMSU][TfO] and [MDTG][TfO] are chosen to show the bond order variation from IMs, TSs and ILs. The bond orders are listed in Table 2. The LBOs are distinctly smaller than the MBOs, which is attributed to the fact that the definition of the LBO is a covalent bond order rather than total bond order and no contribution of non-covalent interactions is included. In [MMU][TfO], the trend of the methyl transfer elongates O12–CH3 from 1.47 Å (in IM) to 2.21 Å (in TS) and, subsequently, the cleavage of O12–CH3 and the formation of O23–CH3 (1.33 Å in IL). Simultaneously, the MBO (LBO) varies from 0.802 (0.174), 0.300 (0.002) and 0.858 (0.277), respectively. It is worth it to note that both of the bond distances between the transferring methyl and two fragments are more than 2.20 Å, and their MBOs are 0.300 and 0.274, respectively. In contrast, their corresponding LBOs are almost zero (0.002). It can be concluded that non-covalent interactions instead of covalent interactions are the major driving force between the transferring methyl with two fragments. As expected, the increase in the CO bond distance from 1.23 Å (in IM) and 1.26 Å (in TS) to 1.33 Å (in IL) and the decrease of their corresponding MBO (LBO) from 1.727 (1.126) and 1.495 (0.995) to 1.108 (0.657) were found where a double bond reduced to a single bond. The MBO (total bond order) and the LBO (covalent bond order) decreased by 0.619 and 0.469, respectively. This indicates that the covalent bond order included in the total bond order is extremely weakened in this process. The same results can be obtained from the variation of the bond distance and bond orders in [MtriMSU][TfO] and [MDTG][TfO].
Species | Bond distance | MBO | LPO | ||
---|---|---|---|---|---|
[MMU][TfO] | IM | O12–CH3 | 1.47 | 0.802 | 0.174 |
O12–S11 | 1.61 | 0.920 | 0.381 | ||
O23C1 | 1.23 | 1.727 | 1.126 | ||
TS | O12–CH3 | 2.21 | 0.300 | 0.002 | |
O12–S11 | 1.52 | 1.278 | 0.612 | ||
O23–CH3 | 2.28 | 0.274 | 0.002 | ||
O23–C1 | 1.26 | 1.495 | 0.995 | ||
IL | O12–S11 | 1.48 | 1.605 | 0.767 | |
O23–CH3 | 1.44 | 0.858 | 0.277 | ||
O23–C1 | 1.33 | 1.108 | 0.657 | ||
[MtriMSU][TfO] | IM | O13–CH3 | 1.47 | 0.804 | 0.177 |
O13–S10 | 1.61 | 0.925 | 0.384 | ||
S25C1 | 1.70 | 1.466 | 1.121 | ||
TS | O13–CH3 | 2.19 | 0.282 | 0.002 | |
O13–S10 | 1.52 | 1.285 | 0.609 | ||
S25–CH3 | 2.80 | 0.430 | 0.001 | ||
S25–C1 | 1.73 | 1.287 | 1.042 | ||
IL | O13–S10 | 1.49 | 1.497 | 0.715 | |
S25–CH3 | 1.84 | 0.952 | 0.571 | ||
S25–C1 | 1.78 | 1.114 | 0.896 | ||
[MDTG][TfO] | IM | O13–CH3 | 1.46 | 0.808 | 0.183 |
O13–S7 | 1.61 | 0.916 | 0.377 | ||
N3–C1 | 1.28 | 1.760 | 1.667 | ||
N3–H4 | 1.02 | 0.819 | 0.687 | ||
TS | O13–CH3 | 2.18 | 0.304 | 0.002 | |
O13–S7 | 1.53 | 1.263 | 0.602 | ||
N3–CH3 | 2.36 | 0.321 | 0.001 | ||
N3–C1 | 1.30 | 1.564 | 1.563 | ||
N3–H4 | 1.02 | 0.811 | 0.690 | ||
IL | O13–S7 | 1.48 | 1.537 | 0.750 | |
N3–CH3 | 1.47 | 0.922 | 0.675 | ||
N3–C1 | 1.35 | 1.202 | 1.230 | ||
N3–H4 | 1.01 | 0.792 | 0.691 |
Table 2 shows that the LBOs of the X–CH3 bond in [MMU][TfO], [MtriMSU][TfO] and [MDTG][TfO] are 0.277, 0.571 and 0.675, respectively. It indicates that the thermal stabilities are in the order of [MMU][TfO] < [MtriMSU][TfO] < [MDTG][TfO], which is consistent with the results of TGA experiments.
H-Bonds | Distance/Å | Angle/deg. | ρ/a.u. | ∇2ρ/a.u. | H/a.u. | |
---|---|---|---|---|---|---|
[MMU][TfO] | N20–H21⋯O18 | 1.77 | 149.4 | 0.0318 | 0.1794 | −0.00042 |
N2–H19⋯O18 | 1.86 | 147.5 | 0.0278 | 0.1430 | −0.00084 | |
[MtriMSU][TfO] | N24–H5⋯O11 | 1.71 | 160.0 | 0.0341 | 0.2079 | −0.00041 |
C15–H18⋯O13 | 2.27 | 150.0 | 0.0125 | 0.0522 | 0.00030 | |
[MDTG][TfO] | N5–H43⋯O11 | 1.79 | 155.3 | 0.0306 | 0.1693 | −0.00075 |
N2–H38⋯O11 | 1.87 | 150.2 | 0.0269 | 0.1374 | −0.00079 |
Topological parameters, such as electron density (ρ), Laplacian of electron density (∇2ρ) as well as energy density (H), are commonly extracted from interested bond critical points (BCPs) at interaction regions.49 It can be seen from Table 3, two types of intermolecular hydrogen bonds are found, i.e., N–H⋯O and C–H⋯O. The former type is distinctly stronger than the latter due to their ideal geometrical parameters, such as short bond distance, large bond angles and topological parameters. In [MMU][TfO], the intermolecular hydrogen bond distances of N20–H21⋯O18 and N2–H19⋯O18 are 1.77 and 1.86 Å, and their corresponding bond angles are 149.4 and 147.5 deg., respectively. As expected, the positive values of the Laplacian of the electron density (∇2ρ) at their bond critical points (BCP) demonstrate that these bonds are strong hydrogen bonds, and the electron density (ρ) value of N20–H21⋯O18 (0.0318 a.u.) is larger than N2–H19⋯O18 (0.0278 a.u.). Besides, the negative energy density (H) of them indicates that the partial covalent is included. In [MtriMSU][TfO], the intermolecular hydrogen bond distances of N24–H5⋯O11 and C15–H18⋯O13 are 1.71 and 2.27 Å, and their corresponding bond angles are 160.0 and 150.0 deg., respectively. Considering the small value of ρ (0.0125 a.u.) and∇2ρ (0.0522 a.u.) at BCPs, C15–H18⋯O13 was determined to be a weak hydrogen bond. Simultaneously, its positive H value (0.0003 a.u.) also indicates that no covalent component exists in this weak hydrogen bond. Similar conclusions can be obtained in [MDTG][TfO].
The BCP criterions in the AIM analysis are hard to use to distinguish weak interaction types. The RDG method can be regarded as an extension of the topological theory and has been performed as a suitable tool to further differentiate weak interactions from repulsive and attractive. As predicted, all N/C–H⋯O intermolecular hydrogen bonds were classified as attractive by the RDG method; however, the intensity of them cannot be distinguished quantitatively. Fig. 7a shows the scatter graphs of [MMU][TfO], in which two RDG spikes corresponding to N20–H21⋯O18 and N2–H19⋯O18 were found, and the sign(λ2)ρ were −0.0318 and −0.0278 a.u., respectively. Indeed, their disc-shape regions between the O and H atoms are also shown in a deep blue color in the RDG isosurfaces. Both of these results indicated the two interactions were strong intermolecular hydrogen bonds, coinciding with the geometrical parameters in Table 3. As shown in Fig. 7b, both the RDG spike and disc-shape region corresponding to N24–H5⋯O11 were similar to that of N20–H21⋯O18 and N2–H19⋯O18. Thus, the attraction between the O11 and H5 atoms was reasonably determined to be a strong hydrogen bond. In contrast, the RDG spike of C15–H18⋯O13 was located at a sign(λ2)ρ of −0.0125 a.u., and the disc-shape region between O13 and H18 was drawn in a light green color. This indicated that the C15–H18⋯O13 contact was a very weak hydrogen bond or even a van der Waals interaction, which can successfully confirm the results obtained from the geometrical results. The similar analysis can be extended to other ILs studied in this work.
Fig. 8 NBO 3D overlap images for donor–acceptor orbital interactions in intermolecular hydrogen bonds. (a), (b) and (c) respect [MMU][TfO], [MtriMSU][TfO] and [MDTG][TfO], respectively. |
Species | Donor (i) | Acceptor (j) | E(2)/(kcal mol−1) | E(j) − E(i)/(a.u.) |
---|---|---|---|---|
[MMU][TfO] | LP(3)O18 | σ*(1)N20–H21 | 12.15 | 0.72 |
LP(1)O18 | σ*(1)N20–H21 | 7.60 | 1.17 | |
LP(3)O18 | σ*(1)N2–H19 | 7.10 | 0.72 | |
LP(2)O18 | σ*(1)N2–H19 | 7.07 | 0.69 | |
LP(2)O18 | σ*(1)N20–H21 | 4.59 | 0.69 | |
LP(1)O18 | σ*(1)N2–H19 | 4.11 | 1.17 | |
[MtriMSU][TfO] | LP(3)O11 | σ*(1)N24–H5 | 19.20 | 0.69 |
LP(1)O11 | σ*(1)N24–H5 | 10.35 | 1.15 | |
LP(2)O11 | σ*(1)N24–H5 | 4.57 | 0.68 | |
LP(1)O13 | σ*(1)C15–H18 | 2.10 | 1.12 | |
[MDTG][TfO] | LP(3)O11 | σ*(1)N5–H43 | 15.73 | 0.70 |
LP(2)O11 | σ*(1)N2–H38 | 13.82 | 0.72 | |
LP(1)O11 | σ*(1)N5–H43 | 6.43 | 1.17 | |
LP(1)O11 | σ*(1)N2–H38 | 4.42 | 1.17 | |
LP(2)O11 | σ*(1)N5–H43 | 2.28 | 0.72 |
As shown in Table 4, there were strong orbital interactions between the antibonding orbital of the proton donors σ*N–H and the lone pairs of the proton acceptors LPO. Coinciding with the geometrical and topological results, the weak orbital interaction between σ*(1)C15–H18 and LP(1)O13 was obtained in the NBO analysis. In Table 4, three types of electron transfer are found from the LPO18 to the σ*C15–H18, i.e., LP(1)O18 → σ*(1)N20–H21, LP(2)O18 → σ*(1)N20–H21 and LP(3)O18 → σ*(1)N20–H21, respectively. The E(2)s are in the order of LP(3)O18 → σ*(1)N20–H21 (12.15 kcal mol−1) > LP(1)O18 → σ*(1)N20–H21 (7.60 kcal mol−1) > LP(2)O18 → σ*(1)N20–H21 (4.59 kcal mol−1). This indicated that the orbital interaction intensity reduced in the order above. Besides, substantial overlaps were also found between LP(1)O13 with σ*(1)N20–H21/σ*(1)N2–H19 in Fig. 8a. In contrast, only the small E(2) value of 2.10 kcal mol−1 (Table 4) and the small overlap (Fig. 8b) in the boundaries of the orbitals of LP(1)O13 and σ*(1)C15–H18 were found, which indicated the existence of weak orbital interaction. Similar results can be extended to other ILs. The NBO analysis in this work is consistent with the strength of the intermolecular hydrogen bonds mentioned above.
It is the further aim of this work to extend studies to more reversible and distillable ILs and their applications. A subsequent paper on radioiodine capture and carbohydrate polymer dissolution in these reversible and distillable ILs is being prepared.
Thermogravimetric measurements were conducted on a TGA apparatus Henven HCT-1. Unless otherwise stated, N2 atmosphere was used as the insert carrier gas. Thermal stabilities of the distillable ILs were studied by both temperature-ramped TG and isothermal TG experiments. Temperature-ramped TG experiments were performed on 10 mg of each sample from room temperature to 600 °C with a heating rate of 10 °C min−1 by using open aluminium pans. Isothermal TG was performed on 20 mg of each sample at a certain temperature for 6 h.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra00008a |
This journal is © The Royal Society of Chemistry 2017 |