CRYSTAL GROWTH & DESIGN
Selective Enclathration of Picolines
2005 VOL. 5, NO. 1 379-382
Susan A. Bourne,† Kirsten C. Corin,† Luigi R. Nassimbeni,*,† and Fumio Toda‡ Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa and Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Riaicho, Okayama 700-0005, Japan Received May 6, 2004;
Revised Manuscript Received July 19, 2004
ABSTRACT: The host 1,1,2,2-tetraphenyl-1,2-ethane diol has been employed to separate the isomers 2-picoline (2-pic), 3-picoline (3-pic), and 4-picoline (4-pic) by selective enclathration. The crystal structures are all stabilized by (host)-O-H‚‚‚N(guest) hydrogen bonds. The competition experiments show that enclathration preferentially takes place in the order 4-pic > 3-pic > 2-pic. The effect of benzene and methanol as “neutral” solvents shows that the former is incorporated as a guest in the 4-pic structure, thus enhancing the enclathration of 4-pic over 3-pic. Methanol displays no such effect. Introduction The process of selective enclathration depends on the efficiency of molecular recognition between host and guest molecules. This in turn relies on an appropriate fit between the surfaces of the molecular components and may be strongly aided by functional groups that enhance secondary attractive interactions such as hydrogen bonding. Host molecules may be classified into two main types:1 (i) those that form molecular complexes by fitting convex guests into the concave cavity of the host. These give rise to “endo-binding”2 and examples include cyclodextrins, cyclophanes, and various carcerands; (ii) those that form lattice inclusion compounds by packing in such a manner as to leave spaces between the host molecules, which may accommodate the guests. The latter display “exo-binding” and give rise to systems that are more flexible but not necessarily more selective. The separation of a given compound from a mixture by enclathration is industrially important when the individual components have similar boiling points, rendering distillation inefficient or unusable. This is often the case with isomers, and we have employed a variety of organic hosts to separate such systems. Thus the host 1,1-bis(4-hydroxyphenyl)cyclohexane has been employed in the separation of phenylenediamines, benzenediols, picolines, and aliphatic alcohols.3 The host 1,4-bis(9-hydroxy-9-fluorenyl)benzene displays selective enclathration of lutidines,4 while the compound 1,1,6,6tetraphenylhexa-2,4-diyne-1,6-diol5 is an efficient host in the separation of the isomers of lutidine, aminobenzonitrile and substituted pyridines. A p-tert-butylcalix[4]arene has been used to separate a variety of volatile guests,6 while β-cyclodextrin has proved useful in the separation of a large number of terpenes and aromatics.7 In this work, we present the results of the competition experiments carried out between the isomers of picoline using the host 1,1,2,2-tetraphenyl-1,2-ethane diol, shown in Scheme 1. We discuss the results in terms of the structure of the inclusion compounds and the effect of adding different solvents. * To whom correspondence should be addressed. E-mail: xrayluig@ science.uct.ac.za. Tel +27 21 6502569. Fax +27 21 6854580. † University of Cape Town. ‡ Okayama University of Science.
Scheme 1
Experimental Procedures Preparation. Suitable crystals of compounds 1 and 2 were grown by slow evaporation of the host H in solutions of the respective picolines (183.0 mg of host in 1.4 mL of picoline). Equal volume mixtures of 4-picoline/R,R,R-trifluorotoluene and 4-picoline/benzene were employed for the crystallization of compounds 3 and 4. The R,R,R-trifluorotoluene was added to improve the quality of the ensuing crystals which otherwise exhibited a large mosaic spread and yielded a structure which would not refine between R1 ) 18%. Data Collection. X-ray intensity data were collected on a Nonius Kappa-CCD diffractometer using graphite monochromated MoKR radiation. Temperature was controlled by an Oxford Cryostream cooling system (Oxford Cryostat). The strategy for the data collections was evaluated using the COLLECT software and data were scaled and reduced with DENZO-SMN software.8 All structures were solved by direct methods using SHELX-869 and refined employing full-matrix least-squares with the program SHELX-97 refining on F2.10 All non-hydrogen atoms were treated anisotropically. All hydrogen atoms appeared in a difference electron density map, and the H atoms bound to carbon were refined with appropriate geometric constraints. The hydroxyl hydrogens were allowed to refine independently and with individual isotropic temperature factors. Packing diagrams were produced using the program PovRay included in the graphic interface X-seed.11 All structures were determined at low temperature, 198K. Competition Experiments. For any pair of guests, competition experiments were carried out by setting up 11 vials containing the host and mixtures of the two guests such that the mole fraction of a given guest varied systematically from 0 to 1, in steps of 0.1, with the (total guest)/host ratio kept at 30. The crystals obtained by slow evaporation were filtered, dried, and warmed gently to release the guests. The relative
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Table 1. Crystal Data and Structure Refinements molecular formula host/guest ratio molecular mass temperature, K λ, Å cryst syst space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Dc, mg m-3 µ, mm-1 F(000) approx cryst size, mm θmax for collected data, deg index ranges min/max h,k,l reflns collected independent reflns Rint data/parameters refined R (F) [I > 2σ(I)] no. of reflns with I > 2σ(I) wR (all F2) largest diff peak and hole, e/Å-3
1
2
3
4
C26H22O2‚2(C6H7N) 1:2 552.69 193 0.71073 triclinic P1 h 9.2879(1) 11.5562(1) 15.4264(2) 89.8000(10) 79.0700(10) 69.0180(10) 1514.18(3) 2 1.212 0.074 588 0.20 × 0.30 × 0.40 27.5 -12/12, -14/14, -19/19 12649 6839 0.016 5538/384 0.0438 5538 0.1166 -0.31 and 0.25
C26H22O2‚2(C6H7N) 1:2 552.69 193 0.71073 triclinic P1 h 9.2661(1) 12.0847(1) 15.4879(2) 97.1410(10) 100.9390(10) 111.4240(10) 1549.16(3) 2 1.185 0.073 588 0.30 × 0.50 × 0.50 27.9 -12/12, -15/15, -19/20 13572 7322 0.014 5914/384 0.0458 5914 0.1297 -0.21 and 0.26
C26H22O2‚2(C6H7N) 1:2 552.69 193 0.71073 monoclinic P21/c 25.0856(1) 14.4001(1) 18.5263(2) 90 111.54(3) 90 6225.0(13) 8 1.179 0.072 2352 0.10 × 0.30 × 0.30 27.5 -32/32, -18/18, -24/24 52367 14274 0.040 10025/766 0.0596 10025 0.1548 -0.25 and 0.28
C26H22O2‚C6H7N‚0.5(C6H6) 1:1:0.5 498.62 193 0.71073 monoclinic P21/n 13.1223(3) 15.1017(3) 14.0733(4) 90 98.2440(10) 90 2760.07(11) 4 1.200 0.073 1060 0.30 × 0.30 × 0.50 27.5 -17/17, -18/19, -18/18 11688 6272 0.034 3659/347 0.0510 3659 0.1508 -0.24 and 0.55
proportions of the guests were determined by gas chromatography using a Varian 3300 instrument with a Supelcowax 10 column (0.2 mm diameter, 30 m length) and a Varian SP4290 integrator. For the experiments involving “neutral” solvent S, (MeOH or benzene), the molar ratios of H/total guest/S was 1:15:15. The experiment was extended to analyze the simultaneous competition by all three picoline isomers. Initial mixtures of the three guests were prepared by careful choice of mole fractions located on a triangular diagram the apexes of which represent a pure guest. The equi-mixture comprising a mole fraction 1/3 of each guest and represented at the center of the triangle was also analyzed. Thermal Analysis. Differential scanning calorimetry (DSC) was performed on a Perkin-Elmer PC7 series system and thermal gravimetry (TG) was performed on a Mettler Toledo TGA/SDTA 851e system. Fine powdered specimens, obtained by crushing the sample, were dried in air and placed in crimped, vented aluminum DSC sample pans or open aluminum TG sample pans. Sample masses in each case were 2-5 mg, and the samples were purged by a stream of nitrogen flowing at 30 mL min-1. All experiments were carried out over a temperature range of 25-350 °C at a constant heating rate of 20 °C min-1.
Table 2. Hydrogen Bonding Details donor acceptor compound (D) (A) D‚‚‚A/Å D-H/Å H‚‚‚A/Å D-H‚‚‚A/° 1 2 3
4
O1 O2 O1 O2 O1 O2 O3 O4 O1
N1G2 N1G1 N1G1 N1G2 N1G4 N1G2 N1G3 N1G1 N1G1
2.843(2) 2.984(2) 2.832(2) 2.911(2) 2.855(2) 2.911(3) 2.915(2) 2.781(2) 2.854(2)
0.84 0.84 0.84 0.84 0.84 0.84 0.84 0.84 0.84
2.02 2.32 2.02 2.08 2.06 2.10 2.10 1.99 2.03
167 137 161 169 158 164 165 158 165
Compound 3 crystallizes in the space group P21/c with Z ) 8. Thus there are two host and four guests in general positions, stabilized by host-guest hydrogen bonds, with metrics given in Table 2. The packing is shown in Figure 2, a projection along [001], in which the guests are shown to be in channels running parallel to c. Compound 4 has a stoichiometry of H/G/benzene of 1:1:0.5. Thus the host and 4-picoline guest are located in general positions, while symmetry requires the
Results and Discussion Structures. Crystal data for all compounds are shown in Table 1. For 1, the space group is P1 h with Z ) 2. Thus both the host and guest molecules are in general positions. The structure is stabilized by (host)-O-H‚‚‚N-(guest) hydrogen bonds, with d O‚‚‚N ) 2.843(2) and 2.984(2) Å. Details of all hydrogen bonding are shown in Table 2. The packing is shown in Figure 1, a projection along [100], in which the guest molecules are located in channels parallel to a and the host molecules are shown with van der Waals radii. For 2, the structure is similar to that of 1, with hostguest hydrogen bonding and guests located in channels parallel to [100].
Figure 1. van der Waals representation of structure 1, showing the guest molecules (sticks) are located in channels running along [100].
Selective Enclathration of Picolines
Crystal Growth & Design, Vol. 5, No. 1, 2005 381
Figure 2. van der Waals representation of structure 3, showing the guest molecules (sticks) are located in channels running along [001].
Figure 3. Representation of 4 viewed along [100] showing the guest molecules with their van der Waals radii and the host molecules as sticks. Table 3. Thermal Analysis Data inclusion compound H: G ratio TG results
DSC results
calc % mass loss exp % mass loss 1 2 total Peak A T/°C Peak B T/°C Peak Melt T/°C
1
2
3
4
1:2 33.7 11.1 22.2 33.3 87 164 200
1:2 33.7 22.7 10.4 33.1 82 170 193
1:2 33.7 7.4 26.9 34.3 116 157 199
1:1:0.5 26.6 27.3 27.3 118 211
benzene to lie on a center of inversion at Wyckoff position b. In this structure, we have effectively replaced one 4-picoline guest by half a benzene molecule. The host now displays only one (host)-O-H‚‚‚N-(guest) hydrogen bond, while the second hydroxyl moiety is not hydrogen bonded. The structure is characterized by columns of 4-picoline (P) and benzene (B) molecules stacked in columns: ‚‚‚PPBPPB‚‚‚ running along [001]. This is shown in Figure 3, where the benzene (orange) and the 4-picoline (blue) are shown with van der Waals radii and the host molecules as sticks. Thermal Analysis and Competition Experiments. Thermal analysis results are summarized in Table 3. All three compounds decompose in two distinct steps, and the total measured mass loss is in good agreement with the calculated value in each case. Each decomposition step is accompanied by a concomitant
Figure 4. Plot showing the results of the Competition experiments.
endotherm in the DSC, but the decompositions are complex and do not allow us to compare onset temperatures to the boiling points of the guests directly. The results of the competition experiments are shown in Figure 4. For a pair of competing guests, A and B, one may define12 a selectivity coefficient KA:B ) ZA/ZB*XA/XB, (XA + XB) ) 1. In the 3-pic vs 4-pic competition, 4-pic is favored over the whole concentration range, with a mean K4-pic:3-pic ) 7.5, as shown by the selectivity curve. In the 4-pic vs 2-pic competition, 4-pic is again preferentially included, K4-pic:2-pic ) 46.4. The 2-pic vs 3-pic experiment shows practically no selectivity of the host for these two isomers, with the experimental points lying close to the diagonal line representing K2-pic:3-pic ≈ 1. The results of the three-component experiment shows the starting composition of the mixtures as represented by the black squares in the equilateral triangle, which after enclathration move to the red area. This result is entirely consistent with the general observation that enclathration takes place in the preference order: 4-pic > 3-pic > 2-pic. Effect of “Neutral” Solvent. In a previous work,13 we had noted the effects of changes in selectivity when the competition experiments were carried out in different solvents. Thus the competition experiment for 4-picoline vs 3-picoline was carried out in the presence of both benzene and methanol as “neutral” solvents. The results are shown in Figure 5, where the curves are plotted using the calculated values of K4-pic:3-pic as well as showing the original data points for each case. In both cases, the experiments were carried out at 25 °C with host/total (3-pic + 4-pic)/solvent as 1:14.4:14.4. In the benzene case, the selectivity curve in favor of 4-picoline is enhanced toward the latter dramatically with K4-pic:3-pic increasing from 7.5 to 61.1. This result is attributed to the fact that benzene is not a “neutral” solvent but is incorporated into the crystal structure with the 4-picoline. The result with methanol however show practically no change (K4-pic:3-pic changes from 7.5 to 6.4). This is consistent with the fact that it was shown not to be enclathrated in any of the inclusion compounds. Methanol therefore does indeed behave as a
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References
Figure 5. Plot showing the results of the “neutral” solvent experiments.
noncompetitive solvent, i.e., neutral, with no significant influence on the selectivity profile of the picolines. Acknowledgment. The financial assistance of the University of Cape Town (UCT) and the Department of Labour (DoL) towards this research is hereby acknowledged. Opinions expressed, and conclusions arrived at, are those of the author and are not necessarily to be attributed to either UCT or the DoL. Supporting Information Available: Crystallographic information files are available free of charge via the Internet at http://pubs.acs.org.
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