I. INTRODUCTION
[C6Trop]+[PF6]− (C14H27NOPF6, see Figure 1), systematic name 8-hexyl-3-hydroxy-8-methyl-nortropanium hexafluorophosphate is a new potential ionic liquid (IL) synthesized by us recently, which is a white powders at room temperature. The skeleton of its cation originates from 3-α-tropanol, which is an important chemical and pharmaceutical raw material (Dewick et al., Reference Dewick2009; Goodman et al., Reference Goodman, Ketchum and Kirby2010). The C6 chain on the N atom lowers the melting point of the salt. Moreover, the solubility of the IL in certain reaction and extraction mixture could be changed greatly with a small-scale variation in temperature, similar to other reported ionic liquids (Li et al., Reference Li, Gu, Yang, Song and Yao2012) with temperature-sensitive properties and broad application prospects. Benzatropine and etybenzatropine are the famous derivatives of tropanol, which is also a building block of atropine, a cholinergic drug prototypical of the muscarinic antagonist class. The title compound [C6Trop]+[PF6]− is also expected to have some biological activities and medicinal value.
Figure 1. Structural formula of [C6Trop]+[PF6]−.
At present, the crystal structure of [C6Trop]+[PF6]− has not been reported.
II. EXPERIMENTAL
A. Sample preparation
The title compound was synthesized as the following procedure (see Figure 2):
(1) 0.05 mol 3-α-tropanol (from Sigma-Aldrich, CAS No. 120-29-6, purity >97%) was dissolved in 50 ml toluene together with 0.05 mol 1-bromohexane. The solution was refluxed and reacted for 6 h, and was concentrated under vacuum to produce a white solid. The product was washed by ethyl acetate for three times to obtain [C6Trop]+[Br]−.
(2) 0.01 mol [C6Trop]+[Br]− and 0.12 mol potassium hexafluorophosphate were dissolved in distilled water. Then the aqueous solution of potassium hexafluorophosphate was added to the aqueous solution of [C6Trop]+[Br]− and stirred thoroughly. Then the solid in the reaction system was filtrated and washed with distilled water to obtain a white powder of [C6Trop]+[PF6]−.
The sample was characterized by high-performance liquid chromatography, as well as by UV, IR, and NMR. UV (CH3CN, 200-400 nm) max: 204 nm. IR (KBr disc, cm-1): 3304 (-OH stretching vibration); 2960, 2931, 2925 (-CH3, -CH2 stretching vibration); 2886 (-CH stretching vibration); 1471, 1466 (-CH2 bending vibration); 1448, 1326 (-CH3 bending vibration); 1261, 1232 (C-N stretching vibration). PNMR (400M, DMSO) ppm: 1.680-1.738 (4H, m, 1,4-H, -CH2×2); 3.949 (2H, brs, 2,3-H, -CH×2); 2.134- 2.198 (4H, m, 5,6-H, -CH2×2); 3.173 (3H, s, 8-H, -CH3); 4.165 (1H, t, 9-H, -CH-O); 3.439 (1H, s, -OH); 3.268-3.310 (2H, t, 11-H, -CH2); 2.732-2.766 (2H, t, 12-H, -CH2); 1.346-1.450 (2H, t, 13-H, -CH2); 0.990 (3H, s, 14-H, -CH3). Eventually, the pure [C6Trop]+[PF6]− was recrystallized in methanol, and its melting point was determined to be 185 °C.
B. Powder diffraction data collection and reduction
The diffraction pattern for the [C6Trop]+[PF6]− powder was collected at room temperature using an X'Pert PRO diffractometer (PANalytical) with an PIXcel 1D detector and CuKα 1 radiation (λ = 1.54056 Å, generator setting: 40 kV and 40 mA). The diffraction data were collected in the angular range from 5° to 50° 2θ with a step size of 0.01313° 2θ and a counting time of 30 s/step. Data evaluation was performed using the software package Material Studio 4.2 (Accelrys Co., Ltd. USA).
The preliminary unit-cell parameters were obtained by analyzing the peak positions in the X-ray powder diffraction (XRD) pattern by the X-Cell method from the “Powder Indexing” tool. The indexing results were then refined with the type of Pawley, which involves assigning the Miller indices (h, k, l) to each observed peak in the experimental powder XRD pattern (Harris, Reference Harris2012).
III. RESULTS
The experimental powder diffraction pattern is depicted in Figure 3. Indexing the results confirmed that [C6Trop]+[PF6]− is triclinic with space group P1 and unit-cell parameters: a = 16.1879(6) Å, b = 11.4271(2) Å, c = 10.3966(5) Å, α = 66.4949(3)°, β = 94.5589(0)°, γ = 93.3092(4)°, unit-cell volume V = 1923.17 Å3, Z = 1, and space group P1 (Table I).
Figure 2. Synthesis procedure of [C6Trop]+[PF6]−.
Figure 3. XRD pattern of the [C6Trop]+[PF6]−, using CuKα 1 radiation (λ = 1.54056 Å).
Table I. Indexed XRD data of 8-hexyl-3-hydroxy-8-methyl-nortropanium hexafluorophosphate, C14H27NOPF6.
Only the peaks with I rel of 1 or greater are presented [a = 16.1879(6) Å, b = 11.4271(2) Å, c = 10.3966(5) Å, α = 66.4949(3)°, β = 94.5589(0)°, γ = 93.3092(4)°, unit-cell volume V = 1923.17 Å3, Z = 1, and space group P1]. All measured lines were indexed and are consistent with the P1 space group. The d-values were calculated using CuKα 1 radiation (λ = 1.54056 Å).
ACKNOWLEDGEMENT
Preparation of this paper was supported by the National Scientific Foundation of China (Grant Nos. 81373284 and 81102344).
