Structural and Spectroscopic Characterizations of Amide–AlCl3-Based

Publication Date (Web): February 5, 2016 ... on the asymmetric splitting of AlCl3 and the coordination site of the amide were investigated by 27Al NMR...
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Structural and Spectroscopic Characterizations of Amide− AlCl3‑Based Ionic Liquid Analogues Pengcheng Hu,† Rui Zhang,† Xianghai Meng, Haiyan Liu, Chunming Xu, and Zhichang Liu* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China S Supporting Information *

ABSTRACT: Several amide−AlCl3-based ionic liquid (IL) analogues were synthesized through a one-step method using three different structure amides as donor molecules. The effects of the steric and inductive effects of the methyl group substituted on the N atom on the asymmetric splitting of AlCl3 and the coordination site of the amide were investigated by 27Al NMR, Raman, in situ IR, and UV−vis spectra for these IL analogues. Bidentate coordination through both the O and N atoms was dominant in the N-methylacetamide−AlCl3- and N,N-dimethylacetamide−AlCl3-based IL analogues because of the inductive effect of the methyl group. By contrast, the acetamide−AlCl3-based IL analogue presented mainly in the form of monodentate coordination via the O atom. Compared with monodentate coordination, bidentate coordination was favorable to the asymmetric splitting of AlCl3 with the same amide−AlCl3 molar ratio. Under the influence of the steric and inductive effects of the methyl group, the ionic species percentages in these IL analogues ranked in the following order: N-methylacetamide > N,N-dimethylacetamide > acetamide. the 27Al NMR analysis of amide−AlCl3-based IL analogues, the Al species at 101.1, 88.6, and 73.6 ppm had been assigned to [AlCl2(amide)2]+, [AlCl4]−, and [AlCl2(amide)]+, respectively.16,17 This assignment was based on the 100% splitting of AlCl3, namely, no existence of neutral species. Subsequently, Coleman et al. determined that amide−AlCl3-based IL analogues were a mixture of anionic, cationic, and neutral complexes in equilibrium.18 However, the effect of the amide structure on the asymmetric splitting of AlCl3 and the coordination site of the amide still lacks an in-depth recognition. For example, Seo et al.19 and Dalibart et al.20 reported that, as a donor molecule, acetonitrile with high dielectric constant and polarity favored the formation of ionic species in the acetonitrile−AlCl3 complex, whereas the use of tetrahydrofuran with low dielectric constant and polarity favored the formation of neutral species in the tetrahydrofuran−AlCl3 complex.21 On the other hand, Campos et al.22 determined that the formamide that coordinated with AlCl3 through both the O and N atoms could form ionic species with bidentate structure; however, the formamide only employed the O atom with monodentate structure to coordinate with ZnCl2 and NiCl2.23 Therefore, the asymmetric splitting of metal halides and coordination sites of donor molecules exhibits considerable dependence on both the donor molecules and metal halides. In the current study, three amide−AlCl3-based IL analogues were synthesized through a one-step method using acetamide,

1. INTRODUCTION As a class of “green” catalysts, ionic liquids (ILs) have been extensively investigated in many catalytic reactions.1,2 However, traditional imidazolium and pyridinium ILs exhibit high toxicity, poor biodegradability, and complex preparation.3,4 IL analogues, also known as deep eutectic solvents, are considered to be better alternatives to ILs because they exhibit excellent performance in many applications.5−8 IL analogues not only have characteristics and properties that are similar to ILs but also overcome the aforementioned disadvantages of ILs.9 As a novel type of liquid catalysts, these IL analogues, prepared by combining metal halides with donor molecules, have attracted significant research attention because of their tunable Lewis acidity and high catalytic activity. Furthermore, these IL analogues, known as liquid coordination complexes, are superior to traditional halometallate ILs given their low cost and ease of preparation.10 Abbott et al.11,12 and Meek and Drago13 reported that the complex of a limited number of metal halides (e.g., ZnCl2, SnCl2, CrCl3, FeCl3, AlCl3, and GaCl3) and donor molecules (e.g., alcohols and amides) could form IL analogues with metalcontaining anions and cations. This process was attributed to the asymmetric splitting of metal halides, forming anions and cations, in which the cations were coordinated with the donor molecules. In particular, amide−AlCl3-based IL analogues with the same amide−AlCl3 molar ratio demonstrated better activity and selectivity than chloroaluminum ILs in both the oligomerization of α-olefins10 and the Friedel−Crafts alkylation of benzene.14 Furthermore, Fang et al.15 developed similar IL analogues based on amines and used for electrodeposition. In © XXXX American Chemical Society

Received: November 30, 2015

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DOI: 10.1021/acs.inorgchem.5b02744 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. IR spectra of propylamine (left) and acetone (right) with different concentrations (0−17 wt %) in the absence (a and c) and presence (b and d) of AlCl3 in the DCM solution. optical resolution was 1 cm−1 and the wavenumber reproducibility 0.2 cm−1. The samples were placed in a well-sealed cuvette to prevent moisture contamination. 2.4. Solubility Measurement by IR. IR spectra over the 4000− 650 cm−1 frequency range were obtained at room temperature and at 8 cm−1 resolution using an in situ IR spectrometer (Mettler-Toledo) equipped with an attenuated total reflectance based a silicon probe and a liquid-nitrogen-cooled mercury−cadmium−tellurium detector. During the measurement, the optical path of the spectrometer was continuously purged with dry N2 at a flow rate of 2 mL/min to eliminate moisture and CO2. The principle of solubility measurement using an in situ IR spectrometer was based on the online monitoring of the variation in the characteristic peaks of the IL analogues and solutes. Solubility measurement was prepared based on the typical procedure.24 The IL analogue (15 g) was placed in a 50 mL two-necked flask equipped with a stirrer. The silicon probe was then inserted into the IL analogue, after which the IR spectra were collected. Next, solute (0.05 g) was added dropwise to the flask, and IR spectra were collected continuously until the characteristic peaks remained constant. The aforementioned steps were repeated until the characteristic peaks did not change with the addition of solute. As an example, Figure S1 shows the variation of the characteristic peaks from ν(NH2) of AA− AlCl3 and ν(CH3) of ethylbenzene.

N-methylacetamide, and N,N-dimethylacetamide as donor molecules. The variations in the number of methyl groups substituted on the N atom in amides make it possible to investigate the effects of the steric and inductive effects of the methyl group on the splitting of AlCl3 and the coordination site of the amide. Identification of the coordination sites of three amide structures was investigated in detail via in situ IR and UV−vis spectroscopy. The effect of the amide structure on the asymmetric splitting of AlCl3 was confirmed by 27Al NMR, Raman spectroscopy, and solubility measurement.

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were used as received without further treatment unless stated otherwise. Ethylbenzene (99%) and dichloromethane (DCM; 99%) were purchased from the Beijing Chemical Reagent Factory, then dried over molecular sieves at 4 Å, and stored in a glovebox. Acetamide (AA; 99.5%), N-methylacetamide (NMA; 99.5%), N,N-dimethylacetamide (DMA; 99.5%), and anhydrous AlCl3 (99.5%) were purchased from the Aladdin Chemistry Company. 2.2. Synthesis of IL Analogues. IL analogues with different amide−AlCl3 molar ratios were prepared in a glovebox under N2 protection. The synthesis of AA−AlCl3 with a molar ratio of 0.65 was used to describe the general procedure for the synthesis of the amide− AlCl3-based IL analogues. AA−AlCl3 was synthesized in a 250 mL two-necked flask placed in a thermostatic oil bath equipped with a stirrer. Anhydrous AlCl3 (26.68 g, 0.2 mol) was placed in the flask, to which AA (7.68 g, 0.13 mol) was added slowly with stirring for 30 min. The mixture was then heated to 80 °C and maintained for 4 h until all solids “dissolved”. All IL analogues were stored in a glovebox. 2.3. Characterization. 27Al NMR spectra were obtained using a Bruker Avance spectrometer comprising 64 scans with a sweep width of 500 ppm and a relaxation delay time of 0.3 s while operating at 156.38 MHz at 25 °C. The samples were placed in a 10 mm standard tube by inserting a well-centered capillary. Thereafter, the entire apparatus was capped and sealed with parafilm. An aqueous solution of Al(NO3)3 (1.0 mol/L) in the capillary was used as an external reference for the 27Al NMR chemical shift; the [Al(H2O)6]3+ ion signal was assigned a value of 0 ppm. Free induction decays were transformed without applying a weighting function. The peak intensities and areas were carefully measured using the Bruker-NMR software package. UV−vis spectra were recorded using a Shimadzu UV-2550 spectrophotometer in quartz cells with 0.1 cm path length. The sample was sealed in a vial prior to use to prevent moisture contamination. Raman spectroscopic measurements were carried out using a LabRAM ARAMIS spectrometer. The excitation light at 532 nm of an argon-ion laser was used with a power of about 300−750 mW. The

3. RESULTS AND DISCUSSION 3.1. IR and UV−Vis Characterization. Amide has two possible donor atoms to coordinate with the Al atom, either through the O atom or through the N atom. As mentioned in the literature, the presence of two coordination sites may form monodentate or bidentate structure.25 Therefore, determination of the coordination sites is important in these amide− AlCl3 IL analogues. The IR spectra can provide information on ν(CO) and ν(NH2) variations during the synthesis of the IL analogues. Accordingly, such information can be used to determine the coordination sites. As a reference, Figure 1 shows the IR spectra of the propylamine (left) and acetone (right) with one donor atom in the absence (a and c) and presence (b and d) of AlCl3 in the DCM solution. N coordination in propylamine−AlCl3 resulted in a significant decrease in the N− H stretching frequency, while O coordination in acetone−AlCl3 resulted in a considerable decrease in the CO stretching frequency. Table 1 lists the variations in the characteristic peaks of three amides in the absence and presence of AlCl3. ν(CO) of AA shifted to lower (decreased by 42 cm−1) wavenumbers in the B

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Inorganic Chemistry Table 1. Variations in ν(CO) and ν(NH2) of the Three Amides in the Absence and Presence of AlCl3 ν(NH), cm−1

electron density in the N atom increased because of the inductive effect of the methyl group substituted on the N atom, which, in turn, increased the possibility of coordination with the Al atom. 3.2. NMR and Raman Characterization. The formation of amide−AlCl3-based IL analogues can be attributed to the asymmetric splitting of Al2Cl6 under induction of the amide. This process generates different Al species, such as ionic and molecular species. 27Al NMR is a good tool to identify these Al species and aids in understanding of the asymmetric splitting behavior. Figure 3 shows the 27Al NMR spectra of NMA−

ν(CO), cm−1

amide

amide

IL

D value

amide

ILs

D value

AA NMA DMA

3317 3304 NA

3489 3396 NA

+172 +92 NA

1704 1653 1643

1662 1649 1641

−42 −4 −2

presence of AlCl3, whereas ν(NH2) of AA shifted to higher (increased by 172 cm−1) wavenumbers. These variations suggested that coordination of AA with the Al atom occurred only through the O atom, as reported in the literature.26−28 By contrast, the offset of ν(CO) and ν(NH2) in NMA and DMA was significantly lower than that in AA. The difference indicated that bidentate coordination through both the O and N atoms to the Al atom might be dominant in the NMA−AlCl3- and DMA−AlCl3-based IL analogues, which favored to reduction of the offset of both ν(CO) and ν(NH2) in the amides. As shown in Figure S2, a new peak ν(Al−N) at 672 cm−1 assigned to ν(Al−N) was detected in propylamine−AlCl3, NMA−AlCl3, and DMA−AlCl3, but this was not observed in AA−AlCl3. Moreover, the results of UV−vis analyses of these IL analogues were used to confirm the difference of the coordination sites further. Figure 2 shows the UV−vis spectra

Figure 3. 27Al NMR spectra of the NMA−AlCl3-based IL analogue with different molar ratios.

AlCl3-based IL analogues with different molar ratios. As shown in Figure 4, the effect of NMA−AlCl3 molar ratio on the percentage of Al species was determined using the normalization method of the peak areas. Three peaks at 102.75, 89.30, and 77.05 ppm were observed in the 27Al NMR spectra of NMA−AlCl3, indicating the existence of three Al species. The

Figure 2. UV−vis spectra of AA−AlCl3- and DMA−AlCl3-based IL analogues (1 wt %) with different amide−AlCl3 molar ratios in the DCM solution.

of AA−AlCl3- and DMA−AlCl3-based IL analogues with different amide−AlCl3 molar ratios in the DCM solvent. A new peak at 305 nm was observed in the UV−vis spectra of AA−AlCl3, which was assigned to the ligand-to-metal chargetransfer (LMCT) absorption resulting from coordination of the O atom with the Al atom.29,30 Therefore, AA−AlCl3 presented mainly in the form of a monodentate structure as a result of the existence of a −δOCCH3NH2+δ resonance structure. However, two peaks at 305 and 354 nm assigned to LMCT absorption were observed in the UV−vis spectra of DMA−AlCl3, indicating that both the N and O atoms coordinated with the Al atom. The same phenomenon was also observed in NMA− AlCl 3 . Therefore, both NMA−AlCl 3 and DMA−AlCl 3 presented mainly in the form of a bidentate structure. The

Figure 4. Effect of the NMA−AlCl3 molar ratio on the integral area percentage of Al species in the NMA−AlCl3-based IL analogue. C

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based IL analogue, as shown in Figure S5. Raman analyses confirmed further that a dynamic equilibrium between [Al2Cl7]− and [AlCl4]− did exist in the amide−AlCl3-based IL analogues. Moreover, the [Al2Cl7]− signal was always accompanied by the [AlCl4]− signal in 27Al NMR and Raman spectra when the NMA−AlCl3 molar ratio ranged from 0.65 to 0.95. This phenomenon indicated that the asymmetric splitting reaction of AlCl3 was stronger than the reaction of [AlCl4]− with AlCl3 generating [Al2Cl7]−. The anionic Al species was only [Al2Cl7]− if the reaction of [AlCl4]− with AlCl3 generating [Al2Cl7]− was stronger than the asymmetric splitting reaction of AlCl3 because once the asymmetric splitting reaction of AlCl3 occurred, [AlCl4]− would be completely converted into [Al2Cl7]− with excess AlCl3. Figure 6 shows the integral area percentage of 102.75 ppm in three different amide−AlCl3-based IL analogues as a function of

integral area percentage of 89.30 ppm increased linearly with increasing NMA−AlCl3 molar ratio, whereas those of 102.75 and 77.05 ppm decreased linearly. Meanwhile, the increased integral area percentage of 89.30 ppm was approximately equal to the sum of the decreased integral area percentage of 102.75 and 77.05 ppm. As shown in Figures S3 and S4, the same phenomena were also observed in DMA−AlCl3 and AA−AlCl3, respectively. Similar phenomena had also been reported by the Abbott group in the literature.17 These IL analogues exhibited a balance between one molecular Al species [AlCl3Ln] and two ionic Al species (cationic [AlCl2Ln]+ and anionic [AlCl4]−). This balance could be broken with the addition of amide or AlCl3, as shown in formulas 1 and 2. Therefore, the peak at 89.30 ppm should be assigned to the molecular Al species [AlCl3Ln], and the integral area percentage of 89.30 ppm increased with a decrease of those of 102.75 and 77.05 ppm. [AlCl 2Ln]+ + [AlCl4]− + n L ⇄ 2[AlCl3Ln]

(1)

[AlCl3Ln] + AlCl3 ⇄ [AlCl 2Ln]+ + [AlCl4]−

(2)

Theoretically, the asymmetric splitting of Al2Cl6 under induction of the amide will generate the same amount of [AlCl2Ln]+ and [AlCl4]−. However, the integral area percentage of 102.75 ppm was higher than that of 77.05 ppm. Such a phenomenon was attributed to the further reaction of [AlCl4]− with AlCl3 generating [Al2Cl7]− because the integral area represented the number of Al nucleus and [Al2Cl7]− had two Al nuclei. Therefore, the peaks at 102.75 and 77.05 ppm should be assigned to the anionic Al and cationic Al species, respectively. Furthermore, the ratio of the integral area percentage of 102.75 ppm to that of 77.05 ppm remained between 1 and 2 at the same NMA−AlCl3 molar ratio, which indicated that the anionic Al species was a dynamic equilibrium between [Al2Cl7]− and [AlCl4]−. Figure 5 shows the Raman spectra of the NMA− AlCl3-based IL analogue with different molar ratios. Two peaks assigned to [AlCl4]− (349.2 cm−1) and [Al2Cl7]− (315.1 cm−1) anions were observed in the Raman spectra, and the intensity of [AlCl4]− increased with the decreasing intensity of [Al2Cl7]−. The same phenomena were also observed in the AA−AlCl3-

Figure 6. Integral area percentage of 102.75 ppm in the three amide− AlCl3-based IL analogues as a function of the molar ratio.

the molar ratios. The integral area percentage of 102.75 ppm of amide−AlCl3 with the same molar ratios ranked in the following order: NMA > DMA > AA. The results of the aforementioned IR and UV−vis characterizations indicated that NMA−AlCl3 and DMA−AlCl3 presented mainly in the form of a bidentate structure, whereas AA−AlCl3 presented mainly in the form of a monodentate structure. The formation of monodentate and bidentate structures can be expressed in the general formulas 3 and 4, respectively. Al 2Cl 6 + 2L ⇄ [AlCl4]− + [AlCl 2L 2]+ −

+

Al 2Cl 6 + L ⇄ [AlCl4] + AlCl 2L]

(3) (4)

Compared with the formation of a monodentate structure under the same amide−AlCl3 molar ratio, the double mole of AlCl3 occurred to the asymmetric splitting in the formation of a bidentate structure, generating the double mole of [AlCl4]− and [AlCl2Ln]+ correspondingly. Although the peak area at 102.75 ppm represented a sum of the [Al2Cl7]− and [AlCl4]− concentrations, all [Al2Cl7]− originated from [AlCl4]−. Such a result explained the phenomenon that the integral area percentage of 102.75 ppm of both NMA−AlCl3 and DMA− AlCl3 was higher than that of AA−AlCl3 with the same AlCl3− amide molar ratio. Moreover, the integral area percentage of

Figure 5. Raman spectra of the NMA−AlCl3-based IL analogue as a function of the molar ratio. D

DOI: 10.1021/acs.inorgchem.5b02744 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 77.05 ppm of both NMA−AlCl3 and DMA−AlCl3 was also higher than that of AA−AlCl3 with the same amide−AlCl3 molar ratio, as shown in Figure S6, which indicated further that more asymmetric splitting occurred in both NMA−AlCl3 and DMA−AlCl3 than AA−AlCl3. Although the inductive effect of the methyl group substituted on the N atom favored the formation of a bidentate structure, more methyl groups would lead to a strong steric effect,31,32 which did not favor the formation of a bidentate structure. Therefore, the integral area percentage at 102.75 ppm of NMA−AlCl3 was higher than that of DMA−AlCl3 with the same amide−AlCl3 molar ratio owing to the steric effect. Figure 7 shows Raman spectra of three different amide− AlCl3-based IL analogues with a molar ratio of 0.65. The

Figure 8. Integral area percentage of ionic species in the three amide− AlCl3-based IL analogues as a function of the molar ratio.

Figure 7. Raman spectra of three different amide−AlCl3-based IL analogues with a molar ratio of 0.65.

intensity ratio of [Al2Cl7]− to [AlCl4]− (I315.1/I349.2) was analyzed for each amide−AlCl3-based IL analogue. NMA− AlCl3 showed the highest I315.1/I349.2 ratio among three amide− AlCl3-based IL analogues, followed by DMA−AlCl3 and then AA−AlCl3. This phenomenon indicated that more [AlCl4]− was converted into [Al2Cl7]−, so more Al nuclei were added into anion Al species in NMA−AlCl3. This was another reason that the integral area percentage of 102.75 ppm of NMA−AlCl3 was highest. 3.3. Solubility of Ethylbenzene in IL Analogues. The sum of the integral area percentages of 102.75 ppm (anion) and 77.05 ppm (cation) can be used to represent the ionic species percentage in these IL analogues, which also followed the order of NMA > DMA > AA, as shown in Figure 8. The ionic species percentage influenced the physicochemical properties of these IL analogues, such as the solubility, polarity, and catalytic activity. To understand the effect of the ionic species percentage on the physicochemical properties of these IL analogues, the ethylbenzene solubility in these three amide− AlCl3-based IL analogues was studied as an example. In situ IR spectroscopy as a technically feasible tool was successfully employed to measure the solubility with sufficient accuracy and precision.24 Figure 9 shows the ethylbenzene solubility in these three amide−AlCl3-based IL analogues with different molar ratios at 30 °C. The ethylbenzene solubility in NMA−AlCl3 and DMA−AlCl3 was lower than that in AA−AlCl3 with the

Figure 9. Ethylbenzene solubility in the three amide−AlCl3-based IL analogues with different molar ratios at 30 °C.

same amide−AlCl3 molar ratio. This result can be attributed to the fact that AA−AlCl3 had the least percentage of ionic species and increased the physical dissolution of ethylbenzene. However, the ethylbenzene solubility in DMA−AlCl3 was lower than that in NMA−AlCl3, which was just consistent with the ionic species percentage in these two amide−AlCl3-based IL analogues. This abnormal phenomenon indicated that the ionic species percentage in these IL analogues was not the only influencing factor for the ethylbenzene solubility. To determine the other influencing factors, the ethylbenzene solubility in the Et3NHCl−AlCl3 IL (100% ionic species) with different molar ratios was also measured at 30 °C, as shown in Figure S7. The ethylbenzene solubility in the Et3NHCl−AlCl3 IL decreased with increasing molar ratio, indicating that the [Al2Cl7]− concentration might be another influencing factor for the ethylbenzene solubility. As shown in Figure 9, the same phenomenon was observed in the three amide−AlCl3-based IL analogues. Furthermore, the [Al2Cl7]− concentration in DMA− AlCl3 was lower than that in NMA−AlCl3 because of the steric effect of the methyl group substituted on the N atom, with consideration of the results of 27Al NMR analysis and the measurement of the [Al2Cl7]− concentration using probe E

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in the form of monodentate coordination via the O atom. The Al NMR spectra of amide−AlCl3-based IL analogues with different molar ratios indicated that the asymmetric splitting degree of AlCl3 decreased and ionic species transformed into molecular species with increasing amide−AlCl3 molar ratio. A comparison of the ionic species percentage in the amide−AlCl3based IL analogues further indicated that bidentate coordination was more favorable to the asymmetric splitting of AlCl3 than monodentate coordination. Meanwhile, the steric effect of the methyl group inhibited the formation of bidentate coordination. Finally, measurement of the ethylbenzene solubility showed that the structure of the amides affected the [Al2Cl7]− concentration and ionic species percentages in the amide−AlCl3-based IL analogues, which were determined as the two contradictory factors influencing the ethylbenzene solubility.

molecule titration. This result can be used to explain why the ethylbenzene solubility in DMA−AlCl3 was lower than that in NMA−AlCl3. On the other hand, the UV−vis spectra of AA−AlCl3 with a molar ratio of 0.65 in the absence and presence of ethylbenzene were measured to understand the relationship between the ethylbenzene solubility and [Al2Cl7]− concentration. Figure 10

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02744. Trend of the characteristic peaks in the 3682−2718 cm−1 range for the addition of ethylbenzene to the AA−AlCl3based IL analogue, IR spectra of propylamine, AA, NMA, and DMA in the absence and presence of AlCl3, effects of the amide−AlCl3 molar ratio on the integral area percentages of Al species in DMA−AlCl3- and AA− AlCl3-based IL analogues, Raman spectra of the AA− AlCl3-based IL analogue, integral area percentage of 77.05 ppm in the three amide−AlCl3-based IL analogues, and the ethylbenzene solubility in the Et3NHCl−AlCl3 IL with different molar ratios (PDF)

Figure 10. UV−vis spectra of the AA−AlCl3-based IL analogue with different concentrations (0−1 wt %) in the absence (a) and presence (b) of ethylbenzene in the DCM solution.

shows the UV−vis spectra of AA−AlCl3 with a molar ratio of 0.65 in the absence and presence of ethylbenzene in the DCM solution. Two new peaks at 261 and 448 nm were observed in the UV−vis spectrum of AA−AlCl3 in the presence of ethylbenzene; these peaks were assigned to the π−π* aromatic ring transition (B band) of ethylbenzene and metal-to-ligand charge-transfer (MLCT) absorption, respectively.33 Compared with LMCT absorption, MLCT absorption was characterized by low transition energy and high wavelength, resulting from electron transfer from the molecular orbital of [Al2Cl7]− to the π* empty orbital of ethylbenzene. The peak at 448 nm was also observed in the Et3NHCl−AlCl3 IL with a molar ratio of 0.65, but it disappeared when the mole ratio of AA−AlCl3 or Et3NHCl−AlCl3 increased to 1. The UV−vis experiment further confirmed that an interaction occurred between ethylbenzene and [Al2Cl7]−, which increased the “chemical dissolution” of ethylbenzene in these IL analogues. Therefore, the [Al2Cl7]− concentration and ionic species percentage were the two contradictory factors that influenced the ethylbenzene solubility.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-10-8973-1252. Fax: +8610-6972-4721. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support of the Natural Science Foundation of China (Grants 21425626, 21436001, 21276275, 21206193, and 21036008), the National Science and Technology Support Program of China (Grant 2014BAE13B01), the Program for New Century Excellent Talents in the University of China (Grant NCET-12-0970), and the Science Foundation of China University of Petroleum, Beijing (Grant 2462015QZDX03).

4. CONCLUSIONS In this study, three different structure amides were used as donor molecules to synthesize amide−AlCl3-based IL analogues via a one-step method. The influence of the steric and inductive effects of the methyl group substituted on the N atom on the asymmetric splitting of AlCl3 and coordination sites of amides in these IL analogues was investigated. The in situ IR and UV−vis spectra of these IL analogues showed that the inductive effect of the methyl group resulted in the existence of two coordination sites through both the O and N atoms in the NMA−AlCl3- and DMA−AlCl3-based IL analogues. By contrast, the AA−AlCl3-based IL analogue presented mainly

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02744 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02744 Inorg. Chem. XXXX, XXX, XXX−XXX