Selectivity Profiles of 5-(4-Methoxyphenyl)-5H-di benzo[a,d]cyclohepten-5-ol with Aromatic Solvents and Conformational Polymorphism of 10,11-Dihydro-5-(4-methoxyphenyl)-5H-dibenzo[a,d]cyclohepten-5-ol
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 88–94
Luigi R. Nassimbeni,† Gae¨lle Ramon,*,† and Jana H. Taljaard‡ Department of Chemistry, UniVersity of Cape Town, Rondebosch 7701, South Africa, and Department of Chemistry, Nelson Mandela Metropolitan UniVersity, 6031, South Africa ReceiVed May 15, 2007; ReVised Manuscript ReceiVed February 19, 2008
ABSTRACT: The structures of the host 5-(4-methoxyphenyl)-5H-dibenzo[a,d]cyclohepten-5-ol, H1, and its inclusion compounds with benzene, bromobenzene, and p-xylene (H:G ) 1:0.5) and with pyridine (H:G ) 1:1) have been elucidated. The selectivity profiles of H1 with the guests that had the same host:guest ratios were determined for all combination of guests. The structure of the compound 10,11-dihydro-5-(4-methoxyphenyl)-5H-dibenzo[a,d]cyclohepten-5-ol, H2, suffers a phase transformation at 181 K. We compared the polymorphs and concluded that we have obtained a subtle case of conformational polymorphism. Scheme 1. Numbering of the Moleculesa
Introduction Supramolecular chemistry studies how Nature arranges and forms molecular networks by using noncovalent bonds. These bonds, the most documented of which is the hydrogen bond,1,2 have been studied and defined as part of the complex system of variables that triggers molecular recognition between the host and the guest molecules. Thus, these interactions are one of the tools chemists consider when engineering3,4 new compounds. To respond to the ever-growing demand for materials with designated properties and behavior, we study these supramolecular systems to understand the process of molecular recognition. In this article, we describe the formation of host:guest compounds between a bulky and rigid host 5-(4-methoxyphenyl)-5H-dibenzo[a,d]cyclohepten-5-ol (H1) and substituted phenyl solvents. The apohost displayed a structure that is stabilized by hydrogen bonds. We then elucidated the structures of the inclusion compounds formed between H1 and substituted benzene guests to study the effects of the guest substituents on the crystal packing. This was followed by a study of relative selectivities derived by competition experiments. Concomitantly, we studied the related compound, 10,11dihydro-5-(4-methoxyphenyl)-5H-dibenzo[a,d]cyclo hepten-5ol (H2), and its phase transformation at low temperature and compared the structures of these two compounds. Scheme 1 displays the molecules that we used in this work with their numbering. Experimental Section Crystal Structures. Crystals of the R phase for both H1 and H2 were obtained by recrystallization in ethanol. Crystals of the inclusion compounds were obtained by crystallizing saturated solutions of the host H1 in the respective guests, giving rise to H1 · 0.5(BZ), H1 · 0.5(BrBZ), H1 · 0.5(Cl-BZ), H1 · 0.5(p-XYL), and H1 · PYR at 25 °C. In all cases, the host-guest ratios were confirmed by thermal gravimetry (TG), and details of the crystal data, intensity data collections, and refinements are contained in Table 1. Cell dimensions were established from the intensity data measurements on a Nonius Kappa CCD diffractometer using graphite-monochromated Mo KR * To whom the correspondence should be addressed. Tel: +27 21 6502569. Fax: +27 21 6854580. E-mail:
[email protected]. † University of Cape Town. ‡ Nelson Mandela Metropolitan University.
a
In blue are the centers of gravity of the phenyl rings, and in orange are the centers of inversion on which the molecules lie.
radiation. The strategy for the data collections was evaluated using COLLECT software.5 For all structures, data were collected by the standard φ- and ω-scan techniques and were scaled and reduced using DENZO-SMN software.6 The structures were solved by direct methods using SHELX-867 and refined by least-squares with SHELX-978 refining on F2. The program X-Seed9,10 was used as a graphical interface for the structure solution and refinement using SHELX, as well as to produce the packing diagrams. Direct methods yielded the positions of the nonhydrogen atoms. Hydroxyl hydrogen atoms were located in difference electron density maps and refined independently with simple bond length constraints and independent temperature factors. The other hydrogen atoms were placed with geometric constraints and assigned isotropic temperature factors of 1.2 × Ueq of their parent atoms. All nonhydrogen atoms were refined with anisotropic temperature factors. All structures were determined at low temperature (113 K).
10.1021/cg0704450 CCC: $40.75 2009 American Chemical Society Published on Web 11/19/2008
Selectivity Profiles and Polymorphs of Related Host Molecules
Crystal Growth & Design, Vol. 9, No. 1, 2009 89
Table 1. Crystal Data and Refinement Parameters H1
H1 · 0.5(BZ)
C22H18O2 · 0.5(C6H6) 1:0.5 314.36 353.42 triclinic triclinic P1j P1j 9.4591(1) 7.6611(1) 16.7646(3) 14.8460(3) 11.4853(2) 26.4546(6) 84.629(1) 75.320(1) 66.849(1) 82.590(1) 76.279(1) 76.868(1) 4 6 1626.86(5) 2826.37(9) 1.283 1.246 0.081 0.078 664 1122 113 113 1.3-26.8 1.5-27.2 -11/11; -21/21; -9/9; -19/19; -14/14 -33/33 no. reflections collected 12611 21299 no. unique reflections 6824 12167 Rint ) 0.024 Rint ) 0.051 no. reflections with I > 2σ(I) 4905 3017 data/ parameters refined 6824/443 12167/736 goodness of fit, S 1.03 0.86 final R indices [I > 2σ(I)] R1 ) 0.0425 R1 ) 0.0510 wR2 ) 0.1840 wR2 ) 0.1185 -3 largest diff. peak and hole (e Å ) -0.25 and 0.19 -0.24 and 0.27
molecular formula molar ratio of host:guest molecular mass (g mol-1) crystal symmetry space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) Z V (Å3) Dc (g cm-3) µ(Mo KR) (mm-1) F(000) temp of collection (K) range scanned, θ (°) index ranges (h, k, l)
C22H18O2
H1 · 0.5(Br-BZ)
H1 · 0.5(Cl-BZ)
H1 · 0.5(p-XYL)
H1 · PYR
C22H18O2 · 0.5(C6H5Br) 1:0.5 392.81 monoclinic P21/c 14.0947(3) 7.5643(2) 18.3929(5) 90 103.342(1) 90 4 1908.06(8) 1.359 1.124 806 113 1.5-27.8 -18/18; -9/8; -24/24 8231 4504 Rint ) 0.030 3254 4504/276 0.98 R1 ) 0.0412 wR2 ) 0.1091 -0.34 and 0.54
C22H18O2 · 0.5(C6H5Cl) 1:0.5 370.13 monoclinic P21/c 14.0672(2) 7.5444(1) 18.3900(4) 90 103.433(1) 90 4 1898.31(6) 1.295 0.148 778 113 2.4-27.9 -18/18; -9/9; -24/24 8627 4517 Rint ) 0.034 3043 4517/258 1.05 R1 ) 0.0453 wR2 ) 0.1215 -0.54 and 0.49
C22H18O2 · 0.5(C8H10) 1:0.5 367.44 monoclinic P21/c 14.2697 (3) 7.5721(2) 18.2657(4) 90 102.858(1) 90 4 1924.15(8) 1.268 0.079 780 113 2.9-27.5 -18/18; -9/9; -23/23 25298 4364 Rint ) 0.060 3444 4364/259 1.04 R1 ) 0.0448 wR2 ) 0.1204 -0.28 and 0.27
C22H18O2 · 1(C5H5N) 1:1 393.46 monoclinic P21/c 9.9046(2) 9.1260(1) 23.3473(4) 90 100.460(1) 90 4 2075.28(6) 1.259 0.079 832 113 4.7-29.0 -12/12; -12/12; -31/31 26916 5383 Rint ) 0.049 3843 5383/278 1.02 R1 ) 0.0483 wR2 ) 0.1332 -0.16 and 0.13
H2 (RT) molecular formula molar ratio of host:guest molecular mass (g mol-1) crystal symmetry space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) Z V (Å3) Dc (g cm-3) µ(Mo KR) (mm-1) F(000) temp. of collection (K) range scanned, θ (°) index ranges (h, k, l) no. reflections collected no. unique reflections no. reflections with I > 2σ(I) data/parameters refined goodness of fit, S final R indices [I > 2σ(I)] largest diff. peak and hole (e Å-3)
H2 (LT)
C22H20O2
C22H20O2
316.38 triclinic P1j 9.514(2) 17.057(3) 11.521(2) 88.17(3) 68.90(3) 76.48(3) 4 1693.2(7) 1.241 0.078 672 298 1.9-27.2 -12/11; -21/21; -14/14 11613 7403 Rint ) 0.021 4462 7402/443 1.17 R1 ) 0.0605 wR2 ) 0.1936 -0.30 and 0.58
316.38 triclinic P1j 9.4472(1) 16.8789(3) 22.8926(4) 88.295(3) 67.612(3) 76.542(3) 8 3275.6(1) 1.283 0.081 1344 173 1.5-26.7 -11/11; -21/21; -28/28 24712 13569 Rint ) 0.033 8562 13569/885 0.85 R1 ) 0.0458 wR2 ) 0.1421 -0.25 and 0.20
For the study of the conformational polymorphism, we monitored the cell parameters with temperature to follow the single crystal to single crystal transition. The method employed was to allow the crystal to equilibrate to a given temperature for 10 min in a stream of cold nitrogen supplied by an Oxford Cryostat and to carry out a limited data collection of 600 reflections (20 frames; varying the Φ circle by 1°; exposure time, 22 s; and 1.4 kW Mo KR radiation). Thermal Analysis. Differential scanning calorimetry (DSC) was performed on a Perkin-Elmer PC7 series system and TG on a Mettler Toledo TGA/SDTA 851e system. Finely powdered, air-dried specimens (2-5 mg) were placed in crimped, vented aluminum DSC pans or open aluminum TG sample pans. Dinitrogen was used as the purging gas at a flow rate of 30 mL/min. All experiments were carried out over temperature ranges of 30-300 °C for DSC and 30-350 °C for the TG experiments at a constant heating rate of 20 °C/min.
Competition Experiments. Competition experiments were set up to determine the selectivity of the host compound toward the series of guests from which it crystallized. In the case of two-component competition, the mixtures of the two guests were prepared in vials in which the mole fraction of a given guest varied from 0 to 1 in steps of 0.1. A fixed amount of host compound was added and dissolved by heating and stirring. The ratio of the host compound to the total guest was at least 1:20 to ensure that there was sufficient guest of lower mole fraction for efficient competition. In the case of three-component competition, the mixtures prepared were represented on an equilateral triangle whose apexes represent the pure guest compounds. The starting mixture compositions were defined from an inner equilateral triangle as well as from the center of the triangle, which represents the equimolar mixture of the guests (0.33 mol fraction for each).
90
Crystal Growth & Design, Vol. 9, No. 1, 2009
Nassimbeni et al. Table 2. Metrics of the Hydrogen Bonds and of C-H · · · π Hydrogen Bonds in the Isostructural H1 and H2 Complexes H1
H2 (RT)
hydrogen bonds donor-H acceptor d D-H (Å) d H · · · A (Å) d D · · · A (Å) angle (°) (D-H · · · A)
O(1)-H(1) O(23) 0.89(2) 1.99(2) 2.877(1) 172(2)
O(1)-H(1) O(23) 0.85(3) 2.16(3) 3.005(2) 174(3)
O(23)-H(23) Cg(1) 1 + x, -1 + y, z
O(23)-H(23) Cg(1) x, y, z
2.44(2) 3.173(1) 139(2)
2.55(3) 3.245(2) 139(3)
O-H · · · π hydrogen bonds
Figure 1. Stabilization of a pair of H1 hosts is by a hydrogen bond and an O-H · · · π hydrogen bond.
Odonor-H with host (x, y, z) Cg(J) symmetry operation leading to the molecule including Cg(J) d H · · · Cg(J) (Å) d O · · · Cg(J) (Å) angle (°) [C-H · · · Cg(J)]
Figure 2. Packing of H1 R-form down [100] showing the antiparallel chains. The resulting crystals were removed from the mother liquor and dried over filter paper. They were placed in airtight glass vials with silicone seals incorporated into a screw-on lid while being dissolved in a minimum amount of acetone. These solutions as well as the starting solutions were analyzed by gas chromatography (GC). These analyses were carried out using a Varian 3900 gas chromatograph connected to a computer and equipped with a CP-1177 split/ splitless injector, an FID detector, a Varian fused silica column (15 m length and 0.25 mm diameter) and a CP-8400 AutoSampler. The chromatograms obtained were recorded and analyzed using the Galaxie GC WS software. The method used for all of the measurements was the following: injection volume, 1 µL; injector temperature, 200 °C; column temperature, 70 °C; detector temperature, 250 °C; and carrier gas, helium (flow rate, 1.6 mL min-1). Before each set of competition experiments, the GC analyzer was calibrated by analyzing the starting mixture of known mole fraction of the guests. The reproducibility was checked by repeating the measurements three times, made possible by the use of the autosampler, which ensured the injection of consistent amounts of sample.
Results Crystal Structures. The apohost H1 crystallizes in the triclinic crystal system, in the centrosymmetric space group P1j. There are two molecules of the host compound lying in general positions in the asymmetric unit and Z ) 4. The host molecules are hydrogen bonded in pairs via (host)O-H · · · O(host) bonds with d O · · · O ) 2.877(2) Å. Within the pairs, the host molecules are stabilized by O-H · · · π hydrogen bonds (Figure 1). The packing of the host compound is formed by antiparallel chains and gives rise to a nonporous material (Figure 2). The compound H2 has a similar structure to that of the H1 host compound. The point of difference between these two molecules resides in the seven-membered ring, which contains a double bond in H1 but not in H2. For this reason, the sevenmembered ring in H1 presents a torsion angle τ ∼ 0 ° about the C8-C9 bond, which enables the flanking benzene rings to be almost symmetrical (mirrored). The molecular shapes of H1 and H2 give rise to similar crystal structures, which display corresponding stabilizing intermolecular interactions (Table 2). H1 · 0.5(BZ) crystallizes in the space group P1j. There are six host molecules and three guest molecules in the unit cell. One
Figure 3. (a) Hydrogen-bonded pair of host compound in the packing of H1 · 0.5(BZ) via double (host)O-H · · · O(host) and (b) C-H · · · π hydrogen-bonded pairs of host compounds forming ribbons in the packing of H1 · 0.5(BZ). In this packing, there are two types of benzene guest compounds: Some are stabilized by a C-H · · · π hydrogen bond, while some are not. Table 3. Metrics of the Hydrogen Bonds and of C-H · · · π Hydrogen Bonds in the Isostructural H1 Complexes H1 · 0.5(Br-BZ) H1 · 0.5(Cl-BZ) H1 · 0.5(p-XYL) hydrogen bonds donor-H scceptor d D-H (Å) d H · · · A (Å) d D · · · A (Å) sngle (°) (D-H · · · A)
O(1)-H(1) O(19) 0.89(3) 2.07(4) 2.891(3) 154(2)
O(1)-H(1) O(19) 0.92(3) 2.02(3) 2.885(2) 156(2)
O(1)-H(1) O(19) 0.90(2) 2.10(2) 2.944(3) 156(2)
C-H · · · π hydrogen bonds Cdonor-H with host (x, y, z) C(18)-H(18) C(18)-H(18) Cg(J) Cg(3) Cg(3) symmetry operation 1 - x, 1/2 + y, -x, 1/2 + y, leading to the molecule 1/2 - z -1/2 - z including Cg(J) d H · · · Cg(J) (Å) 2.79(3) 2.78(2) d C · · · Cg(J) (Å) 3.596(2) 3.583(2) angle (°) [C-H · · · Cg(J)] 143(2) 143(1)
C(18)-H(18) Cg(3) 2 - x, 1/2 + y, 1/2 - z 2.80(1) 3.603(2) 142(1)
of the guests lies on a center of inversion (in orange in Scheme 1) at g. The host compounds are stabilized by hydrogen bond pairs via (host)O-H · · · O(host) bonds (Figure 3a). The packing of H1 · 0.5(BZ) is stabilized by C-H · · · π hydrogen bonds (Figure 3b). The host molecules 1 and 2 (in blue) are hydrogen bonded via a double (host)O-H · · · π hydrogen-bonded forming ribbons. There are also C-H · · · π hydrogen bonds associated with these pairs of host compounds between the first host molecule and the first guest molecule. The second type of ribbon is formed by the host molecule 3. The latter is situated near a center of inversion at Wyckoff
Selectivity Profiles and Polymorphs of Related Host Molecules
Crystal Growth & Design, Vol. 9, No. 1, 2009 91
Figure 4. (a) Packing of H1 · 0.5(Br-BZ): The host compounds are hydrogen bonded forming a helix along [010], and the guest molecules intercalate in the voids. (b) Within the helix, the packing is stabilized by C-H · · · π hydrogen bonds. (c) The same helix as presented in part a (with the guest omitted for clarity) is presented to illustrate the saw tooth packing along 010 with the screw axis running in the opposite direction.
Figure 6. Transition of the cell dimensions between H2 (RT) and H2 (LT) for the cell parameters, the angles, and the volume. The data obtained from the cooling (squares) and the data obtained from the heating (dots) coincide for all of the parameters studied. Table 4. Thermal Expansion Coefficients
Figure 5. (a) Host molecule to guest molecule hydrogen bond with d O · · · N ) 2.765(2) Å in H1 · (PYR) inclusion compound. (b) C-H · · · π hydrogen bonds between host compounds forming a helix in the packing of H1 · (PYR).
position a, which generates a host pair (similar to the one formed by host molecules 1 and 2). This pair displays two C-H · · · π hydrogen bonded to another pair forming a second type of ribbon (in gray). There are no bonding interactions between this second type of ribbon and the guest compounds. Hence, in the packing of H1 · 0.5(BZ), there are two types of benzene guest compounds depending on the presence or the absence of stabilizing interactions with the host compounds. The packing of H1 · 0.5(BZ) gives rise to cavities in which the guests are located. H1 · 0.5(Br-BZ) crystallizes in the monoclinic crystal system, in the space group P21/c. The unit cell of H1 · 0.5(Br-BZ) consists in four host molecules lying in general positions and two guest molecules disordered over a center of inversion at Wyckoff position h.
parameters
before transition temperature
after transition temperature
a-axis b-axis c-axis R angle β angle γ angle volume
6.6 × 10-5 K-1 6.8 × 10-5 K-1 2.5 × 10-5 K-1 9.0 × 10-5 K-1 -1.9 × 10-5 K-1 -2.4 × 10-5 K-1 6.1 × 10-5 K-1 -6.4 × 10-6 K-1 -1.1 × 10-5 K-1 2.3 × 10-4 K-1
Thehostmoleculesarehydrogenbondedvia(host)O-H · · · O(host) bonds forming a helix that is coincident with the screw axis located at (1/2 and 1/4). The packing within the helix is stabilized by C-H · · · π hydrogen bonds (Figure 4). The packing of H1 · 0.5(Br-BZ) gives rise to cavities in which the guest molecules are situated. H1 · 0.5(Cl-BZ) and H1 · 0.5(p-XYL) are isostructural to H1 · 0.5(Br-BZ). Despite their similarities, we note that for H1 · 0.5(Cl-BZ), the guest lies on a center of inversion at Wyckoff position a, and the center of the phenyl ring is coincident with the center of symmetry; hence, the chlorine atom is disordered over two positions.
92
Crystal Growth & Design, Vol. 9, No. 1, 2009
Nassimbeni et al.
Figure 8. Detail of the point of difference shown in Figure 7. Table 6. Selectivity Constants of H1 Toward Pairs of Solvents
Figure 7. Comparison of the polymorphic packings after superimposing pairs of the compounds in blue for H2 (RT) and in purple for H2 (LT). Table 5. Torsion Angles in H2 for the Molecules Presented in Figure 7
τ1 τ2 τ3
A1 (°) -4.4(2) 61.5(2) 2.2(3)
B1 (°) -1.7(3) 48.8(4) 0.3(3)
τ1 τ2 τ3
A2 (°) -3.6(2) 60.2(2) -0.4(2)
B2 (°) 3.8(3) 59.8(2) -2.8(3)
τ1 τ2 τ3
A4 (°) 0.1(2) 61.8(2) 5.2(3)
B1′ (°) 1.7(3) -48.8(4) -0.3(3)
τ1 τ2 τ3
A3 (°) 2.8(2) -57.3(2) 1.6(2)
B2′ (°) -3.8(3) -59.8(2) 2.8(3)
For H1 · 0.5(p-XYL), the guest lies on a center of inversion at Wyckoff position c. As the p-xylene is centrosymmetric, it is not disordered. Table 3 reports the values of the hydrogen bonds and of C-H · · · π hydrogen bonds in the isostructural inclusion compounds of H1. H1 · (PYR) crystallizes in the monoclinic crystal system, in the space group P21/c. The asymmetric unit consists of one molecule of host compound and one molecule of guest lying in general positions so that Z ) 4. The host molecule is hydrogen
solvent 1
solvent 2
K1:2
K2:1
benzene benzene benzene p-xylene p-xylene chlorobenzene
p-xylene chlorobenzene bromobenzene chlorobenzene bromobenzene bromobenzene
2.40 0.60 0.58 0.44 0.47 0.98
0.42 1.67 1.72 2.27 2.13 1.02
bonded to the guest molecule via (host)O-H · · · N(guest) bond with d O · · · N ) 2.765(2) Å. The packing of H1 · (PYR) is stabilized by C-H · · · π hydrogen bonds between host molecules forming a helix along [010], which is coincident with the screw axis located at (1/2 and 1/4) (Figure 5). The packing of H1 · (PYR) gives rise to cavities in which the guests are located. Conformational Polymorph. When a crystal of H2 was cooled from 298 to 113 K, it underwent a phase transformation such that the volume of the cell almost doubled. In particular, the cell parameters a, b, R, γ, and β remained practically unchanged, while c is doubled. The only noteworthy change is in the angle β whose value alters from 68.9 to 67.7° (1.2° difference), and we note the small volume change from 3320 to 3299 Å3 (21 Å3 difference) upon cooling below 181 K. We monitored the cell parameters at regular temperature intervals, and the results are shown in Figure 6, in which the cell parameters have been normalized to the low temperature (LT) cell for direct comparison. The transition occurs at 181 ( 1 K and is reversible, in that the same cell parameters were obtained at all steps while cooling and reheating. The matrix used for the comparison between the two polymorphs is the following:
[ ] [ ][ ] a b c
1 0 0 ) 0 1 0 0 0 2 H2(LT)
a b c
(1) H2(RT)
Similar results have been obtained in the phase transitions in the inclusion compounds of 9-phenylfluoren-9-ol with methylcyclohexylamine12 and of 4,4′-bis(diphenylhydroxymethyl)diphenyl with 4-picoline.13 We have calculated the linear coefficients of thermal expansion for the unit cell over the two temperature ranges before (182-296 K) and after the transition temperature (113-180 K). The results are shown in Table 4 and vary over a narrow range. Comparison of the packing diagrams of the room temparature (RT) with the LT structure shows that the differences are subtle. This kind of polymorphism is a conformational polymorphism where changes in the molecules occur in the torsion angles, which are presented in Table 5. In Figure 7, we compare the asymmetric unit of the LT structure with that of the RT structure. The RT structure contains two molecules per asymmetric unit, here labeled B1 and B2, and shown with their centrosymmetric pair B1′ and B2′. In contrast, the LT structure contains four
Selectivity Profiles and Polymorphs of Related Host Molecules
Crystal Growth & Design, Vol. 9, No. 1, 2009 93
Figure 9. Three-way competition experiments for H1 host compound. Table 7. Classification of the Inclusion Compounds According to the Host:Guest Ratio and the Type of Intermolecular Interactions Involved in the Packing
H1 · 0.5(BZ)
H1 · 0.5(Br-BZ) H1 · 0.5(Cl-BZ) H1 · 0.5(p-XYL)
host-host OH · · · O 2.789(4) 2.798(5) 2.815(5) pair (double bonded) channels
host-host OH · · · O 2.891(3) 2.885(2) 2.944(1) ribbons channels
H1 R-form interactions OH · · · A type d O · · · A (Å)
host-host OH · · · O 2.877(2)
host topology voids topology
pair (single bonded) non porous
molecules in the asymmetric unit, labeled A1, A2, A3, and A4. Table 5 compares the torsion angles τ1, τ2, and τ3 in the four molecules in each structure. For the RT structure, the centrosymmetric molecules necessarily have torsion angles that are equal and opposite. We note that we have a pseudo center of symmetry in the LT structure, represented by the symbol Ø. The value of the τ2, which defines the conformation of the molecules, changes sign for the pair A2 (+60.2) and A3 (-57.3) but retains the value for the pair A1 (61.5) and A4 (61.8). This is the principal difference in the two structures at RT and LT, which accompanies the phase transformation. Figure 8 allows the observation of the point of difference between the two packings. Competition Experiments. The crystal structures allowed the identification of four inclusion compounds with identical host:guest ratio (1:0.5), three of them being isostructural with respect to the host position (bromobenzene, chlorobenzene, and p-xylene). We selected these inclusion compounds to carry out all of the two-component competition experiments, which were extended to three-component competition experiments. The results (blue curves) are presented in Figure 9, the zero selectivity curves (purple lines) being presented for comparison. The graphs show the mole fraction Z of a given solvent in the crystals as a function of the mole fraction X of the same solvent
H1 · PYR host-guest OH · · · N 2.765(2)
channels
in the initial solution. For the three-component experiments, the blue area is the result obtained from the starting middle center triangle. There were six possible two-component experiments when combining the four solvents. For each competition experiment, the selectivity constant K was calculated following Ward.14 These constants are presented in Table 6. From this table, it is possible to predict how the H1 host compound will include the various guest compounds when in presence of a mixture. Therefore, the trend for inclusion will be p-xylene < benzene < chlorobenzene ∼ bromobenzene. In Figure 9a, the resulting area (in blue) shifted according to predictions. The selectivity of H1 is increasing, following the trend p-xylene < benzene < chlorobenzene, and the crystals enclathrated preferably the latter. In the same way, for the guest combination benzene, p-xylene and bromobenzene presented in Figure 9b, the results observed were of the same trend as described previously with chlorobenzene. It appeared when calculating the selectivity constants that chlorobenzene and bromobenzene had approximately the same behavior toward the other solvents. For this reason, the shifts observed follow the same tendency. Meanwhile, when bromobezene is present, the shifts are more homogeneous and the original triangular shape is maintained.
94
Crystal Growth & Design, Vol. 9, No. 1, 2009
In Figure 9c,d, the competition experiments demonstrate the influence of the third solvent on the crystal constitution when working with two solvents for which the host compound has a similar selectivity. With benzene as the third solvent and guest, the constitution of the crystals does not change drastically if compared with the starting solutions. In fact, the contents of the enclathrated guests obtained all fitted in the starting triangle, only showing a slight increase in the concentration of chlorobenzene and bromobenzene. With p-xylene, for which the H1 host compound has an even weaker selectivity, the resulting crystals demonstrate a more significant increase in the concentration of chlorobenzene and bromobenzene when compared to the starting solutions. Conclusion We obtained three inclusion compounds that are isostructural with respect to the host compound positions with bromobenzene, chlorobenzene, and p-xylene as guests. The topology of these crystals exhibits infinite ribbons of hydrogen-bonded host molecules. The structure with benzene as guest demonstrates the same host:guest ratio, but the arrangement of the host molecules is significantly different as the host molecules are stabilized by two separate hydrogen bonds forming a dimer of host molecules. Nevertheless, despite this difference in the packings of the inclusion compounds, all of the guest molecules are situated in channels that motivated our study on the selectivity of the host compound for the four solvents. Threecomponent competition experiments were carried out for all of the combinations of solvents. We found the general trend of the selectivity of the host compound as being: p-xylene < benzene
