Anomalous Inclusion Behavior Shown by a Thia-Bridged Diquinoline

Feb 16, 2010 - Enhanced Guest Inclusion by a Sulfur-Containing Diquinoline Host. Solhe F. Alshahateet , Roger Bishop , Donald C. Craig , and Marcia L...
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DOI: 10.1021/cg901516j

Anomalous Inclusion Behavior Shown by a Thia-Bridged Diquinoline Derivative

2010, Vol. 10 1842–1847

Solhe F. Alshahateet,† Roger Bishop,* Donald C. Craig,‡ and Marcia L. Scudder School of Chemistry, The University of New South Wales, UNSW Sydney, New South Wales 2052, Australia. †Present address: Department of Chemistry, Mutah University, P.O. Box 7, Mutah 61710, Al Karak, Jordan. E-mail: [email protected]. ‡Deceased May 12, 2009. Received December 3, 2009; Revised Manuscript Received January 27, 2010

ABSTRACT: The sulfur-bridged compound 6,7,14,15-tetrahydro-6,14-thiacycloocta[1,2-b:5,6-b0 ]diquinoline (11) yields a solvent-free crystal form but also produces lattice inclusion compounds containing chloroform, water, and methanol. This latter behavior is in marked contrast to the noninclusion characteristics shown by its corresponding oxygen- and methanobridged analogues. X-ray structures of the three different crystal types formed by 11 reveal that all contain a two-molecule repeat unit in which the S atom of the first is located within the V-shaped cleft of the second. This assembly is reminiscent of a ball and socket joint, and the adoption of this motif is the root cause of the unexpected solid-state inclusion properties shown by 11.

Introduction Crystal engineering seeks to obtain crystalline materials with specific packing arrangements, or functional properties, through deliberate design. Many characteristics such as molecular size, shape, symmetry, and chirality play key roles, in addition to the various types of molecular functionalities that are present. The interplay between these disparate factors can make such syntheses highly problematic. This is particularly so if the intermolecular attractive forces involved do not involve strong hydrogen bonds, which are arguably the best understood and most easily controlled class of interactions at the present time.1,2 The self-assembly of two or more different types of molecules to form a multicomponent crystal is a phenomenon of great fascination to chemists.3 The design of completely new multicomponent systems4 provides a worthwhile challenge in both synthetic chemistry and crystal engineering.5 Such substances generally fall into one of three categories: (i) Lattice inclusion compounds (clathrates), in which a number of host molecules together provide a void space that is occupied by a guest. (ii) Inclusion complexes in which a preformed host receptor structure associates with the guest species. (iii) Cocrystals in which, usually, the components are intimately hydrogen bonded together in a stoichiometric ratio and for which the terms host and guest are no longer meaningful. We have a long-term interest in designing new lattice inclusion hosts that interact with their guests through only weak intermolecular forces rather than strong hydrogen bonds. In one such approach, we have devised a versatile and highly successful synthesis for obtaining diheteroaromatic hosts that form lattice inclusion compounds.6 Using this modular design, two aromatic wings are conjoined with a central alicyclic spacer group in a one-flask reaction. The spacer group causes the diheteroaromatic system to adopt C2-symmetry (or pseudo C2-symmetry) and also provides a *Corresponding author. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 02/16/2010

degree of conformational mobility permitting the potential host to adjust in the presence of different guests. If the racemic test molecule carries two exo-orientated halogens on the central spacer group (e.g., 1,7,8 29) and/or at least four halogen substituents on its aromatic rings (e.g., 3,10 411), then it functions as a clathrate host in 18 of 19 cases tested (96% success rate). These substituted molecules do not pack efficiently by themselves in the solid state due to interruption of potential three-dimensional propagation of the offset aryl π 3 3 3 π (OFF) and edge-face aryl C-H 3 3 3 π (EF) intermolecular attractions.12 Other packing motifs, such as C-H 3 3 3 N interactions,13,14 then can assume equal importance, and it becomes energetically favorable to incorporate guest molecules within their crystal structures. The one exception is a case in which an alternative, and particularly effective, intermolecular attraction was able to dominate this particular crystal structure.15

In contrast, V-shaped diheteroaromatic molecules not meeting the above halogen requirements (e.g., racemic 5,7,8 6,9 7,14 811) yielded solvent-free crystals in 11 of 12 cases tested (92% success rate). This is because these molecules are able to pack together effectively (in the absence of the protruding halogen substituents) by forming OFF and EF interactions r 2010 American Chemical Society

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that propagate unimpeded throughout the crystal. This paper describes the preparation and solid-state characteristics of the one known exception to this behavior, namely, the thiabridged compound 11.

Results and Discussion Preparation and Crystallization of the Thia-Bridged Diquinoline 11. Friedl€ander condensation16 of 2 equiv of 2-aminobenzaldehyde17 9 with one of racemic 9-thiabicyclo[3.3.1]nonane-3,7-dione18 10 gave the racemic diquinoline adduct 11 in 86% yield as summarized in Scheme 1. Scheme 1. Preparation of the Thia-bridged Diquinoline Derivative 11a

a Only one enantiomer of the compounds 1-8 and 10-11 is illustrated. The black circles added to the molecular structure of 11 designate the three points used for measurement of the fold angles present in the bicyclo[3.3.1]nonane-based crystal structures.

Solvent-free crystals of racemic 11 were obtained on its crystallization from trifluoromethylbenzene. To our surprise, however, crystallization of racemic 11 from chloroform, benzene containing traces of water, or methanol, led to guest inclusion. This behavior was unprecedented in light of our molecular design ideas and the behavior shown by all previous diheteroaromatic analogues such as 5-8. The numerical details relating to the data collection, data processing, and refinement of the X-ray structures of the crystals involving 11 are listed in Table 1.19 Crystal Structure of the Apohost 11. Solvent-free 11 crystallizes in the monoclinic space group C2/c with two independent molecules (A and B) in the asymmetric unit. Homochiral columns containing both A and B molecules run along the b direction. Adjacent columns are of the opposite handedness parallel to c and of the same handedness along a (Figure 1). Edge-face (EF) interactions occur between columns along the short ac diagonal direction, and poor offset face-face (OFF) interactions operate between molecules in adjacent columns along a. The interfacial overlap here is weak but will nonetheless enhance the lattice energy.

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Each column is constructed from molecular pairs comprised of one A and one B molecule, in which the sulfur atom of an A molecule is positioned within the V-shaped cleft of a B molecule of 11 with the same chirality (Figure 2a). The spatial relationship of the S and cleft is reminiscent of a balland-socket joint. The distance between the S atoms of the two molecules in the pair is 6.13 A˚. The inner S is located over both aromatic wings of the outer molecule at distances of 3.67 and 3.71 A˚. Each column is generated by repetition of these molecular pairs, related by a 21 axis along b (colored blue and orange in Figure 2c). The motif present between adjacent pairs is a second ball-and-socket dimer, but this time the location of the S “ball” is asymmetric with respect to the two aromatic wings of the “socket” (Figure 2b). The S atoms are now 8.02 A˚ apart, and the inner S atom is near to only one aromatic wing of the outer molecule (3.8 A˚). Projection of the chains in the ac plane results in the rhomboidal column cross-section seen in Figure 1. Crystal Structure of (11)4 3 (chloroform)3. Crystallization of the racemic diquinoline 11 yielded crystals of composition (11)4 3 (chloroform)3 in the triclinic space group P1, again containing two independent molecules of 11 (A and B) in the asymmetric unit. Once again, the arrangement of 11 is such that the repeat unit comprises pairs of molecules of the same chirality with a B molecule positioned within the cleft of a second of type A to form a similar ball and socket motif (Figure 3). The relative orientation of the two partner molecules is similar to the asymmetric apohost case shown in Figure 2b, with a slightly reduced S 3 3 3 S distance of 7.53 A˚ and the sulfur of the B molecule about 4 A˚ from one aromatic ring of the A molecule. The molecular pairs are arranged in homochiral layers in the ab plane (Figure 4, upper). Two such layers of opposite handedness associate at c = 1/2 via both EF (between one A and one B type molecule) and also edge-edge (EE) dimers (Figure 4, lower). This unusual C-H 3 3 3 N motif involves the bridgehead methine hydrogen of the central linker group of molecule 11 (type A). The participating hydrogen atom is weakly acidic due to the influence of the adjacent bridging sulfur atom. Layers are further linked on their other surface by centrosymmetric OFF interactions between pairs of B molecules. The result is a scaffold arrangement which leaves vacant channels at b=1/2, c=0 parallel to a, and also cavities at the unit-cell corners. Two independent chloroform guest molecules are present. One (within the channels) is disordered over two sites with occupancies 0.72 and 0.28, while the other (within the cavities) is disordered about a center of symmetry. Guests lying within the channels take part in Cl 3 3 3 N (3.12 A˚), Cl 3 3 3 π (shortest Cl 3 3 3 C distances 3.41 and 3.56 A˚), and C-H 3 3 3 Cl (H 3 3 3 Cl 3.25 and 3.27 A˚) interactions with the host molecules. The second guest molecule participates in host-guest C-H 3 3 3 N (d=2.25 A˚, D= 3.24 A˚, C-H 3 3 3 N 169°) and Cl 3 3 3 N (3.37 A˚) interactions. Crystal Structure of (11)6 3 (water). When the racemic diquinoline 11 was crystallized from benzene containing traces of water, then crystals of composition (11)6 3 (water) in hexagonal space group R3 were obtained. The ball-andsocket molecular pair is also present in this third type of crystal structure, but it has undergone significant modification. Each pair is again oriented so that the sulfur atom of one molecule is located approximately symmetrically within the cleft of the other. These two molecules are now, however, of the opposite handedness (Figure 5). Despite this difference in chirality of the two molecules, the metrics of the

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Table 1. Numerical Details of the Solution and Refinement of the Crystal Structures compound

11

formula formula mass space group a/A˚ b/A˚ c/A˚ R/o β/o γ/o V/A˚3 T/°C Z Dcalc/g cm-3 radiation, λ/A˚ μ/mm-1 scan mode 2θmax./o no. of intensity measurements criterion for observed reflection no. of indep. obsd. reflections no. of reflections (m) and variables (n) in final refinement R = Σm|ΔF|/Σm|Fo| Rw = [Σmw|ΔF|2/Σmw|Fo|2]1/2 s = [Σmw|ΔF|2/(m - n)]1/2 crystal decay R for multiple measurements largest peak in final diff. map/ e A˚-3 CCDC no.

C22H16N2S 340.4 C2/c 25.075(8) 15.408(4) 21.745(7) 90 126.40(2) 90 6762(3) 21(1) 16 1.34 MoKR, 0.7107 0.20 θ/2θ 50 5637 I/σ(I)>2 2807 2807 222 0.050 0.051 1.28 none 0.027 0.46 754910

(11)4 3 (CHCl3)3 (C22H16N2S)2 3 (CHCl3)1.5 860.0 P1 11.480(8) 13.028(9) 14.061(9) 94.12(3) 96.43(3) 101.91(2) 2035(2) 21(1) 2 1.40 MoKR, 0.7107 0.46 θ/2θ 36 2352 I/σ(I)>2 1688 1688 218 0.078 0.116 1.95 68% 0.009 0.66 754909

(11)6 3 (H2O) (C22H16N2S)6 3 (H2O) 2060.7 R3 27.857(4) 27.857(4) 11.053(3) 90 90 120 7428(2) 21(1) 3 1.38 MoKR, 0.7107 0.19 θ/2θ 50 2893 I/σ(I)>2 1957 1957 117 0.042 0.052 1.56 none 0.013 0.33 754908

(11)6 3 (CH3OH) (C22H16N2S)6 3 (CH4O) 2074.7 R3 27.820(4) 27.820(4) 11.145(3) 90 90 120 7470(2) 21(1) 3 1.38 MoKR, 0.7107 0.19 θ/2θ 50 2925 I/σ(I)>2 1782 1782 231 0.040 0.047 1.35 none 0.014 0.99 173086

Figure 1. The apohost crystal structure of 11 projected onto the ac plane and showing the homochiral columns along b in cross-section and packed parallel to each other. Atom codes: C green (opposite enantiomers light or dark), H light blue, N dark blue, and S yellow.

arrangement otherwise closely resemble those of the motif shown in Figure 2a found in pure 11. The S 3 3 3 S distance is 6.87 A˚ and the S atom is located adjacent to both wings of the outer molecule, and at a distance of about 4 A˚ from each. Since there is just one molecule in the asymmetric unit of this structure, each molecule must take part in two such motifs, one as the ball and another as the socket. This is accomplished in this crystal lattice by forming a cycle of six molecules of 11 with alternating chirality creating a hexamer of oblate spheroidal shape. The center of the hexamer is the 3 site which is a small cavity containing the disordered water molecule. An alternative analysis of this spheroid is that three (þ)-molecules of 11 assemble as a propeller-shaped arrangement at the top of the spheroid and three (-)molecules form a similar propeller of opposite handedness at the bottom (Figure 6, left). The six sulfur atoms surrounding the 3 site are approximately coplanar (Figure 6, right). In each propeller half-structure, the three homochiral

Figure 2. (a) The molecular pair repeat unit present in the crystal structure of pure 11 showing the ball (S atom from A molecule) located approximately symmetrically within the socket (V-shaped cleft of B molecule). (b) The asymmetric motif between adjacent pairs shown in (a). (c) Part of one column running along the b direction. All molecules of 11 have the same handedness within any given column but are alternately of types A and B. Alternating molecular pairs from part (a) around the 2-fold screw axis have their carbon atoms colored blue or orange in this diagram.

molecules associate by means of three aryl edge-face interactions (EF)3 located close to the propeller hub. There is no direct host-host EF interaction across the 3 site since this is the guest location, but the N2 atoms are oriented to provide N2 3 3 3 O hydrogen bonding (3.16 A˚) with one orientation of the disordered water molecule.

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Figure 3. The ball-and-socket molecular pair present in solid (11)4 3 (chloroform)3, showing the sulfur atom (the ball) of the B molecule located near one aromatic wing of the V-shaped cleft (the socket) of the A molecule of 11. This arrangement is similar to that shown in Figure 2b for the apohost.

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Figure 5. The ball and socket molecular pair repeat unit present in solid (11)6 3 (water) and involving opposite enantiomers of the diquinoline 11. This arrangement is similar to that shown in Figure 2a for pure 11.

Figure 6. Top (left) and side (right) views of the oblate spheroid assembly present in the compound (11)6 3 (water). The disordered water guest molecule is located at its center.

Figure 4. Upper: The packing arrangement present in (11)4 3 (chloroform)3. Additional color coding: Cl brown, chloroform C pink or black for the full and half occupancy guests, respectively. Lower: The centrosymmetric C-H 3 3 3 N edge-edge interaction present between layers of opposite handedness in (11)4 3 (chloroform)3.

The unit cell packing of the structure is illustrated in Figure 7 (upper). Where the spheroids come together along

Figure 7. Upper: The packing in (11)6 3 (water) viewed along the c direction. All six locations for the disordered guest water molecule, each indicated as a red sphere, are shown. Lower: The centrosymmetric pair of aza-1,3-peri aromatic hydrogen interactions in (11)6 3 (water). The C-H 3 3 3 N distances are d = 2.58 and 2.85 A˚.

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Table 2. Fold Angle Values Present in the Crystal Structures of the Diheteroaromatic Bicyclo[3.3.1]nonane Derivativesa compound

fold angle (degrees) 80.0 89.8 96.0 (A) 80.4 (B) 90.2 (A) 88.6 (B) 85.9 82.4 82.5

5 6 8 11 (11)4 3 (chloroform)3 (11)6 3 (water) (11)6 3 (methanol)

a The symbols A and B indicate crystallographically independent molecules.

the c axis, there is a 6-fold concert of EF phenyl 3 3 3 phenyl interactions. The neighboring (host)6 spheroids are linked by means of C-H 3 3 3 N interactions. Opposite enantiomers of 11 associate as centrosymmetric pairs by utilizing two identical aza-1,3-peri aromatic hydrogen interactions (Figure 7, lower). Crystal Structure of (11)6 3 (methanol). Crystallization of racemic 11 from methanol afforded crystals of the lattice inclusion compound (11)6 3 (methanol) also in the hexagonal space group R3. This material is isostructural with the hydrate compound described above.19 Conclusions Our V-shaped diheteroaromatic molecules are designed to have a degree of conformational flexibility, thereby allowing their compatibility with potential guest molecules or important intermolecular attractions. One measure of such molecular adjustment is the value of the fold angle, defined in Scheme 1, which can lie between extremes of 67.0° and 141.4° based on our previous work. Fold angle values are listed in Table 2. The angle of the oxa-bridged compound 8 is slightly higher at 96.0°, compared to the methano-bridged 5,6 values of 80.0° and 89.8°, respectively. All six fold angles for the thiabridged molecule 11 compounds lie within this latter range of 80-90°, thereby demonstrating that the anomalous behavior of 11 is not based on conformational changes initiated by the sulfur atom. The four structures formed by 11 involve three different packing types which have a common feature. In all three types, two molecules of 11 form repeat units reminiscent of a ball and socket joint, in which the sulfur atom (ball) of one molecule is positioned within the V-shaped cleft (socket) of the other. The variables in this assembly are the relative chiralities of the two molecules, the relative directionality of their aromatic wings, and the positioning of the inner molecule with respect to the outer one (Figure 8). In the apohost and chloroform inclusion cases, there are two molecules of 11 in the asymmetric unit, which together constitute the homochiral ball and socket repeat unit. This awkwardly shaped unit can pack either with or without the inclusion of solvent depending on the case. The isostructural water and methanol inclusion compounds have only one molecule in the asymmetric unit that thus provides both the ball and the socket functionalities. Here, adjacent molecules are of opposite handedness in what must be an endless string of molecules - achieved in this lattice by a cyclic hexamer. In the symmetric motif, found in the apohost (Figure 2a) and the water inclusion case (Figure 5), the inner sulfur atom is located about 3.8-4.0 A˚ from both aromatic wings of the outer molecule. Since the two molecules of 11 are rotated with respect to each other, this suggests S 3 3 3 π interaction with one

Figure 8. Comparison of the four different ball and socket motifs. Each component molecule, 11, is represented diagrammatically by its central sulfur atom and the two outer carbon atoms of each aromatic wing. The two views are orthogonal and show the variation in the relative locations and orientations of the inner molecule (orange) with respect to the outer one (gray). The four parts are (a) the apohost symmetric motif, (b) the apohost asymmetric motif, (c) the chloroform inclusion compound, and (d) the water inclusion case.

ring of the outer molecule and C-H 3 3 3 π interaction with the other. This combination is much more clearly visible for the asymmetric arrangement found in the apohost (Figure 2b) and the chloroform inclusion compound (Figure 3). It is this persistence of the ball and socket arrangement, in one form or another, that causes the diheteroaromatic molecule 11 to behave so differently to its close analogues 5-8. The methano-bridged 5 utilizes aromatic OFF and EF intermolecular packing motifs,8 and the oxa-bridged analogue 8 prefers to form an ether;1,3-peri aromatic hydrogen interaction15 (cf. its aza-analogue in Figure 7), to the exclusion of ball and socket motif formation in either case. There are also significant differences in the types of edge-edge C-H 3 3 3 N interactions20 observed. The analogues 5 and 8 both utilize typical centrosymmetric Ar-H 3 3 3 N dimer EE interactions,8,15 but these are absent in the ball and socket structures formed by 11. The apohost structure contains no EE interactions, but the chloroform compound contains the unusually modified variant illustrated in Figure 4 (lower) and the water/methanol compounds instead utilize the EE aza;1,3-peri aromatic hydrogen interaction shown in Figure 7 (lower). The structure of (11)4.(chloroform)3 is the most complex, involving two independent host molecules, two independent guest molecules, plus guest disorder. It is likely that these compromises are required since the chloroform molecule is too large to be accommodated in the center of the hexamer present in the crystal structure adopted by the water and methanol compounds.

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Experimental Section NMR data were recorded using a Bruker ACF300 instrument at 25 °C and carbon substitution information was determined using the DEPT procedure. MS data (EI) were recorded by Dr. J. J. Brophy using a VG Quattro triple quadrupole instrument. The microanalytical data were determined at the Australian National University, Canberra. 6,7,14,15-Tetrahydro-6,14-thiacycloocta[1,2-b:5,6-b0 ]diquinoline (11). 2-Aminobenzaldehyde17 (9) (2.50 g, 20.66 mmol, freshly prepared) and 9-thiabicyclo[3.3.1]nonane-2,6-dione18 (10) (1.60 g, 9.4 mmol, freshly sublimed) were dissolved in methanol (40 mL) with stirring. After this mixture was cooled in ice, aq. NaOH (2 M; 4 mL) was added dropwise. The solution was allowed to warm to room temperature and left stirring overnight. The precipitated solid was filtered and washed with a small amount of cold methanol. Recrystallization of the crude diquinoline gave 11 as its inclusion compound (2.75 g, 86%), mp 218-220 °C. IR (paraffin mull) 1435 m, 1380 m, 1305 m, 1220 m, 1155w, 1010w, 985s, 910w, 850w, 795m, 785s, 775s, 770s, 750s cm-1. 1H NMR (CDCl3) δ 3.61 and 3.67 (d, JAB 16.9 Hz, 2H), 3.88 and 3.94 (dd, JAB 16.9 Hz, JAX 4.5 Hz, 2H), 4.66 (d, J 5.7 Hz, 2H), 7.38-7.44 (m, 2H), 7.59-7.65 (m, 4H), 7.75 (s, 2H), 7.99 (d, J 8.7 Hz, 2H);.13C NMR (CDCl3) δ 39.4 (CH), 40.1 (CH2), 126.3 (CH), 126.8 (CH), 127.3 (C), 127.7 (C), 128.4 (CH), 129.2 (CH), 137.5 (CH), 146.3 (C), 157.2 (C). MS m/z (>10%) 341 (14%), 340 (Mþ, 80), 308 (14), 307 (100), 305 (32), 295 (49), 198 (26), 179 (14), 169 (12), 167 (14), 153 (48), 139 (18), 115 (10), 43 (11). Calc. for (C22H16N2S)6 3 (CH3OH) requires C, 76.99; H, 4.86; N, 8.10. Found: C, 76.92; H, 4.93; N, 7.97%. The other crystalline samples were obtained by recrystallization of the crude solid from the appropriate solvent. Solution and Refinement of the Crystal Structures. Reflection data were measured at 294 K with an Enraf-Nonius CAD-4 diffractometer in θ/2θ scan mode using graphite monochromated molybdenum radiation (λ 0.7107 A˚). The crystal quality for (11)4 3 (chloroform)3 was poor, with data only extending to θ=18°. In addition, the crystal suffered significant decomposition, for which corrections were applied. Data were not corrected for absorption for any structure. For each structure, the positions of all atoms in the asymmetric unit were determined by direct phasing (SIR92)21 and hydrogen atoms were included in calculated positions. There were two independent chloroform guest molecules in (11)4 3 (chloroform)3. One was disordered over two sites of occupancies 0.72 and 0.28, while the other was disordered about a center of symmetry. Each was refined as a rigid group. The methanol and water guests in (11)6 3 (methanol) and (11)6 3 (water) were disordered about a 3 site. Full details of refinement22 can be found in the Supporting Information.

Acknowledgment. We thank the Australian Research Council for financial support of this work. Supporting Information Available: X-ray crystallographic information files (CIF) for the three new structures CCDC 754908-754910 (see Table 1). This information is available free of charge via the Internet at http://pubs.acs.org/.

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