Article pubs.acs.org/JPCB
Conformational Isomerism Influence on the Properties of Piperazinium Bis(trifluoromethylsulfonyl)imide Xiaoxing Lu,† Qi Cao,† Xi Wu,† Hujun Xie,*,‡ Qunfang Lei,*,† and Wenjun Fang*,† †
Department of Chemistry, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China Department of Applied Chemistry, Zhejiang Gongshang University, No. 18, Xuezheng Street, Hangzhou 310035, China
‡
S Supporting Information *
ABSTRACT: Investigation of conformational isomerism of ring compounds can help us get a clear comprehension of the ring structure and reveal significant structure−activity relationship. In this study, conformational isomerism of the cationic moiety of ionic liquid 1-ethyl-1,4-dimethylpiperazinium bis(trifluoromethylsulfonyl)imide ([C2C1C14pi][NTf2]) has been investigated by means of 1H nuclear magnetic resonance spectra. The energy levels for different conformations of the cationic moiety [C2C1C14pi]+ are obtained via density functional theory calculations. The predominant cis-conformer in [C2C1C14pi][NTf2] at its liquid state is observed under ambient conditions, where the ethyl group locates at the equatorial position of quaternary nitrogen atom, consistent with the calculated results. The trans-conformer minorities in the IL convert to the cisconformers when [C2C1C14pi][NTf2] is well crystallized. Besides, the addition of polar solvents, such as ethanol, leads to a convenient and complete transformation from the trans-form to the recognizable cis-form. The phase-transition behaviors have been measured by means of differential scanning microcalorimetry (DSC), and the DSC results can be highly affected by the initial state of the IL. Density and viscosity measurements for mixtures of [C2C1C14pi][NTf2] with ethanol or 1-propanol at different temperatures T = (293.15 to 323.15) K are performed. Conformational isomerism affects the excess molar volumes of [C2C1C14pi][NTf2] + alcohol systems more significantly than the viscometric property. The behaviors, as comparison, for the mixtures of 1-n-pentyl-1,4-dimethyl-piperazinium bis(trifluoromethylsulfonyl)imide ([C5C1C14pi][NTf2]) with ethanol are observed with the same phenomena as the common binary systems. On the basis of the experimental and calculated results of the ILs, it can be concluded that conformational isomerism in the cation of [C2C1C14pi][NTf2] is quite significant, and it should be taken into account when sensitive properties are evaluated.
■
INTRODUCTION Ionic liquids (ILs), simply defined as salts with melting points below 100 °C,1−3 have been studied extensively in the last two decades. Their appealing features, such as chemical and thermal stabilities, low melting point, nonflammability, and low volatility,1,4−6 make them as promising candidates to replace traditional organic solvents that may be environmentally hazardous to separation or reaction. Nowadays, much research is carried out to focus on their broad uses as alternative solvents and catalysts, and some of the ILs have already been found to act as industrial materials with a wide variety of functions and applications.7 In general, the ILs can be put into two classes: protic and aprotic ILs. The protic ILs always possess strong polarity and are more favorable to be dissolved in polar solvents than the aprotic ones, while they are usually moisture sensitive and their thermal stabilities are poor. On the contrary, the aprotic ILs are hydrophobic, viscous, low-melting, and possess excellent thermal and chemical stabilities because they often contain anions with relatively large spatial structures, such as the widely used bis(trifluoromethylsulfonyl)imide ([NTf2]−) anion.8−10 Among the numerous kinds of ILs, the imidazolium© 2014 American Chemical Society
based ILs with various anions have been widely investigated.10−18 Nevertheless, the tunability in both anions and cations facilitates the unceasing expansion of the IL family. Thus, even though the number of scientific reports about ILs keeps increasing every year, one of the insufficient details about the ILs is the very limited database of their basic properties, which are highly dependent on the structures of the ILs. Conformational isomers for ring compounds may have pronounced discrepancies in the properties and functions. As a large class of ring compounds, heterocycles containing nitrogen atoms have been broadly investigated.19 The conformational isomers at each state were speculated from nuclear magnetic resonance (NMR) results at quite low temperatures down to −100 °C, and the Gibbs free energies of activation for ring inversion were derived. As examples, a series of alkylpiperazinium halides were investigated by 1H and 13C NMR spectra.20 The enthalpy, entropy, and free energy of activation were calculated Received: June 5, 2014 Revised: July 10, 2014 Published: July 11, 2014 9085
dx.doi.org/10.1021/jp505567e | J. Phys. Chem. B 2014, 118, 9085−9095
The Journal of Physical Chemistry B
Article
basis of our syntheses, for a series of novel piperazinium-based ILs bearing the specified [NTf2]− anion and with unsymmetrical heterocycle as cations, in-depth studies are focused on two low-melting ILs, [C2C1C14pi][NTf2] and [C5C1C14pi][NTf2]. Conformational isomerism of IL is possibly caused by the inversion of the methyl group at the piperazinium ring, which may result in the cis−trans isomerism with the equatorial or axial methyl group bonded to the ternary nitrogen atom. Hence, the phenomena of conformational isomerism are hoped to be proved and the influences on physical properties are further discussed to understand the structure−property relationship.
by measurements of the rate of ring inversion and the coalescence temperatures of NMR signals. It is discovered that the ring inversion rate is quite high for the salts composed of 1,4-dimethylpiperazinium and its derivatives. Thereafter, a few reports about conformational isomerism of ILs are presented, which has been discovered in 1-alkyl-3-methylimidazolium-based ILs.21−24 The conformational isomerism usually occurs in the alkyl chain substituents at the nitrogen atom in the imidazolium ring with respect to diverse dihedral angles between adjacent carbon atoms, and it is similar to the situation of liner or branched hydrocarbon compounds. There exist several crystalline polymorphs that differ in the alkyl chain conformations, which results in different spatial arrangements of the ions. The relative stable conformers of the imidazolium-based ILs can be isolated by suitable methods,23 and their detailed information on the structures can be inferred with the assistances of Raman spectroscopy, X-ray diffraction analysis, and quantum calculations. Taking the relationship between structure and property into account, conformational isomerism should be well studied to get a clear elucidation of its effects on basic properties, such as density and viscosity, which are necessary for design and evaluation in industrial procedures. In general, thermophysical information can be obtained by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). As typical examples, Wasserscheid et al.25 reported the density, viscosity, and hydrolysis data of 1-n-butyl-3-methylimidazolium octylsulfate and found that the IL often stays at a supercooled state under ambient conditions. Fredlake and coworkers26 systematically investigated the thermophysical properties of a total of 13 imidazolium-based ILs. Different thermal behaviors were observed with the assistance of DSC. Jacquemin et al.13 measured the density and viscosity of several pure and water-saturated imidazolium-based ILs consisting of the [NTf2]− anion. It is shown that the water content in the water-saturated IL is small with mass fraction lower than 3.00%, but it affects the viscosity values greatly, while the densities of the systems are not changed significantly. Oliveira et al.27 reported similar experimental results with pyridinium−NTf2 ILs. The effects of both the structural and positional isomerism of ILs are discussed at the same time. In our previous work,28,29 some kinds of protic piperaziniumbased ILs have been employed to evaluate their extractive performance of aromatic species from hydrocarbon-based fuels. The data of density and viscosity for N-ethylpiperazinium propionate ([C2pi][C2H5CO2]) and the binary mixtures of it with n-alcohols were determined.30 The interactions in the extraction systems and the mechanism of extraction were also discussed on the basis of quantum calculations.31,32 In this work, on the
■
EXPERIMENTAL SECTION Materials. 1,4-Dimethylpiperazine (C1C1pi, >0.980), 1iodopentane (>0.980), and bis(trifluoromethylsulfonyl)imide lithium (Li[NTf2], 0.9995 trace metals basis) were purchased from Aldrich. Methanol (>0.998), ethanol (>0.995), 1-propanol (>0.997), iodoethane (>0.990), dichloromethane (>0.998) and ethyl acetate (>0.998) were purchased from Sigma-Aldrich. All of these agents were used without further purification. A Millipore Q3 system was used to produce ultrapure water. Preparation and Characterization of ILs. The IL, 1-ethyl-1,4-dimethylpiperazinium bis(trifluoromethylsulfonyl)imide ([C2C1C14pi][NTf2]), was synthesized in this laboratory according to Scheme 1. To a solution of iodoethane (0.02 mol) in ethyl acetate (20 mL) was dropwise added a mixture of 1,4-dimethylpiperazine (0.02 mol) in ethyl acetate (20 mL) in a 100 mL roundbottomed flask. The mixture was magnetically stirred for 24 h at room temperature and then filtered. The white precipitate was recrystallized in the mixed solvent of methanol and ethyl acetate and subsequently washed with ethyl acetate. A colorless crystalline compound, 1-ethyl-1,4-dimethylpiperazinium iodide ([C2C1C14pi]I), as the desired intermediate product was obtained to give a yield of 80%, and its structure was identified by NMR spectroscopy and elemental analysis. Afterward, [C2C1C14pi]I (0.02 mol) was dissolved in water and mixed with an aqueous solution of bis(trifluoromethylsulfonyl)imide lithium (0.02 mol). Dichloromethane was subsequently added to the mixture to extract the product. The mixture was magnetically stirred for 12 h under room temperature and settled for phase separation. The organic layer was washed with water until the I− anion was no longer detected with a silver nitrate solution. After evaporation of the volatile components, the liquid was further purified by freeze-drying (Peking Sihuan Scientific Instrument, LGJ-10C) to remove water and other impurities thoroughly. Eventually, the
Scheme 1. Syntheses of Piperazinium-Based Ionic Liquids of 1-Ethyl-1,4-dimethylpiperazinium Bis(trifluoromethylsulfonyl)imide ([C2C1C14pi][NTf2]) and 1-n-Pentyl-1,4-dimethyl-piperazinium Bis(trifluoromethylsulfonyl)imide ([C5C1C14pi][NTf2])
9086
dx.doi.org/10.1021/jp505567e | J. Phys. Chem. B 2014, 118, 9085−9095
The Journal of Physical Chemistry B
Article
Table 1. Chemical Shifts of 1H NMR Spectra for the Piperazinium-Based ILs with CDCl3 as Solvent at 25 °C [C2C1C14pi][NTf2]
[C5C1C14pi][NTf2] δH/ppm
δH/ppm
groups
cis-
trans-
groups
N+−C−CH3 N−CH3 N+−C−CH2−N N+−CH3 N+−CH2−C-N N+−CH2−C
1.40, t, 3H 2.38, s, 3H 2.68 and 2.78, m, 4H 3.05, s, 3H 3.37 and 3.41, m, 4H 3.46, q, 2H
1.36, t, 3H 2.36, s, 3H
−C−C−C−CH3 −C−C−CH2−C −C−CH2−C−C −CH2−C−C−C N−CH3 N+−CH3 N+−C−CH2−N N+−CH2−C−N N+−CH2−C
3.00, s, 3H
cis0.92, 1.37, 1.37, 1.74, 2.38, 3.06, 2.76, 3.42, 3.29,
t, 3H m, 2H m, 2H m, 2H s, 3H s, 3H m, 4H m, 4H t, 2H
Figure 1. 1H NMR spectra changes for mixtures of [C2C1C14pi][NTf2] (1) + ethanol (2) at different concentrations with CDCl3 as solvent in 6 days. Mole fraction: (a) x1 = 1.0, (b) x1 = 0.9, (c) x1 = 0.5, and (d), x1 = 0.1.
product, [C2C1C14pi][NTf2], was obtained as a crystal at its stable state. It should be noted that the solid [C2C1C14pi][NTf2] can be easily melted and the liquid at the metastable (supercooled) state can remain for a very long time. Thus, both samples of [C2C1C14pi][NTf2] at the solid and the liquid states are prepared. For the synthesis of 1-n-pentyl-1,4-dimethyl-piperazinium bis(trifluoromethylsulfonyl)imide ([C5C1C14pi][NTf2]), the solvent ethyl acetate was replaced with methanol during the preparation of 1-n-pentyl-1,4-dimethyl-piperazinium iodide ([C5C1C14pi]I), the crude product [C5C1C14pi]I was washed with n-hexane and further purified under vacuum (50 °C,
24 h), and the following steps were the same as those of [C2C1C14pi][NTf2]. The synthesized ILs were characterized by 1H NMR spectra carried out on a Bruker AVANCE III 500 MHz NMR spectrometer using CDCl3 as the solvent and tetramethylsilane (TMS) as the internal standard at 25 °C. A coulometer (Mettler Toledo, C20) was applied to determine the water content in the prepared IL samples by a Karl Fisher titration. The water mass fractions were
