Enhanced Guest Inclusion by a Sulfur-Containing Diquinoline Host

ARTICLE pubs.acs.org/crystal

Enhanced Guest Inclusion by a Sulfur-Containing Diquinoline Host 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

bS Supporting Information ABSTRACT: The sulfur-bridged compound 7R,15R-dibromo-6,7,14,15tetrahydro-6R,14R-thiacycloocta[1,2-b:5,6-b0 ]diquinoline (10) yields an apohost crystal form plus a series of six lattice inclusion compounds. The diverse range of their X-ray crystal forms is described, and the structures are compared in crystal engineering terms. Compound 10 exhibits none of the anomalous properties shown earlier by its oxygen-bridged and nor-bromo cousins. Further, it demonstrates inclusion capabilities considerably superior to its methano-bridged analogue. Although the sulfur atom does not appear to be actively driving inclusion host behavior, closer analysis reveals that it is always involved. Multiple sulfur interactions are present in all these clathrate structures, most commonly involving bifurcated H 3 3 3 S 3 3 3 H motifs.

’ INTRODUCTION In his recent detailed account of crystalline molecular complexes and compounds, Herbstein has suggested that around 90% of such binary compounds may have been obtained by chance.1 Certainly the design of entirely new inclusion host molecules has been, and largely still is, a problematic synthetic area.2 These difficulties are particularly apparent for lattice inclusion (clathrate) systems in which interactions weaker and less directional than traditional strong hydrogen bonds are employed.3 In one approach to obtaining such compounds, we have prepared and studied V-shaped C2-symmetric diheteroaromatic derivatives such as racemic 1
6 (Figure 1).4 The unsubstituted molecules 1
3 associate efficiently in the solid state by means of aryl offset face
face (OFF),5 edge
face (EF),5 and/or C
H 3 3 3 N interactions6 and exhibit no inclusion properties. In marked contrast, however, their substituted analogues 4
6 do act as potent lattice inclusion (clathrand) hosts. The presence of the halogen substituents attenuates the extent of OFF and EF interactions over three dimensions within their crystals. Furthermore, new opportunities for halogen 3 3 3 halogen,7 halogen 3 3 3 π,8 and other9,10 weak host
host and host
guest interactions, are created. Molecules such as 1
6 can be prepared very simply, and this synthetic approach has proved to be an extremely reliable one across a wide range of structures. The only exceptions to this pattern of behavior have been encountered within the group of oxa- and thiabridged analogues 7
10 shown in Figure 2. Contrary to all expectations, the dibromo compound 8 failed to show molecular inclusion,11 whereas the thia-bridged molecule 9 unexpectedly did act as a host.12 The causes of this anomalous behavior have been investigated, and explanations for its occurrence have been obtained. The crystals of both oxa-bridged compounds 7 and 8 contain an interaction between the oxygen atom of one molecule and r 2011 American Chemical Society

Figure 1. Examples of V-shaped diheteroaromatic molecules that act as nonhosts (1
3) or as lattice inclusion hosts (4
6). Only one enantiomer of these racemic compounds is illustrated.

two 1,3-peri aromatic hydrogens of a second. A number of other examples of this motif are recorded in the Cambridge Structural Database, but the H 3 3 3 O distances present in solid 7 and 8 are the shortest values on record. This results in a very efficient attraction that dominates the crystal structure of 8 and precludes the alternative inclusion behavior.11 The unexpected inclusion Received: June 1, 2011 Revised: August 9, 2011 Published: August 09, 2011 4474

dx.doi.org/10.1021/cg2006937 | Cryst. Growth Des. 2011, 11, 4474–4483

Crystal Growth & Design

Figure 2. The molecular structures of the oxa- and thia-bridged V-shaped diheteroaromatic molecules 7
10. Only one enantiomer of these racemic compounds is illustrated. The black circles added to the structure of 10 designate the three points used for measurement of the fold angles present in its crystal structures.

properties of the thia-bridged molecule 9 also arose from an unanticipated molecular interaction. In the apohost and inclusion structures of 9, a sulfur atom of one molecule is located within the aromatic V-shaped cleft of a second. This creates a ball and socket type assembly, which results in an awkwardly shaped repeat unit with a strong concomitant tendency to include guest molecules.12 The importance of intermolecular S 3 3 3 π interactions in solid state packing has been realized for some time, and considerable analysis of these motifs has been carried out.13 This present account investigates the crystallization properties of the dibromo thia-bridged derivative 10 and explores whether or not unexpected molecular interactions such as the ones above are involved in its behavior.

’ RESULTS Preparation and Crystallization of the Thia-Bridged Diquinoline 10. Benzylic bromination of the thia-bridged diquino-

line 9 was carried out using N-bromosuccinimide in refluxing carbon tetrachloride (Wohl
Ziegler reaction14). As usual for such diheteroaromatic systems, this free radical process took place with both high regio- and stereospecificity15 and afforded the target dibromo compound 10 in 82% yield. Crystallization from a range of solvents at room temperature yielded solvent-free material and also crystals containing chloroform, furan, benzene, dichloromethane, dibromomethane, or water. Numerical details of the solution of their X-ray structures are presented in Table 1. Crystal Structure of Solvent-free 10. Crystallization of the dibromide 10 from trifluoromethylbenzene gave crystals of the apohost 10 in the monoclinic space group P21/c. Molecules of alternating handedness associate into columns running along the c direction by means of a bromine atom of one molecule being positioned in the cleft of its neighbor. The shortest Br 3 3 3 C distances are 3.53 and 3.92 Å in these Br 3 3 3 π interactions. This is an analogue of the ball and socket motif encountered for the non-brominated compound 912 but where Br has replaced S. The aromatic wings of adjacent columns are linked by exo,exo-facial interactions between opposite enantiomers of 10 (labeled OFF in Figure 3). The apparent endo,endo-facial interaction in the diagram is an artifact of projecting the crystal structure onto the ac plane. Along b, 21-related molecules take part in a chain of EF interactions (shortest C
H 3 3 3 C 3.69 Å), and a Br 3 3 3 N

ARTICLE

interaction (3.34 Å) operates between translation related molecules of 10. The bridging sulfur atom is linked to two further molecules of 10 (one in the adjacent column along b and the other in an adjacent column along a) by means of an Ar
H 3 3 3 S 3 3 3 H
Ar motif (d = 3.01 and 3.07 Å). This and the other sulfur atom environments in the various 10 compounds are compared later. Crystal Structure of the Chloroform Inclusion Compound. Crystallization of 10 from chloroform yielded (10) 3 (chloroform) in the monoclinic space group P21/c. Molecules of 10 form centrosymmetric endo,endo-facial dimers through a combination one aryl OFF and two aryl EF interactions. This is a parallel 4-fold aryl embrace (P4AE) interaction using the terminology of Dance and Scudder.16 These units associate further into chains by means of interdimer aryl OFF interactions, and parallel chains pack further as host layers in the bc plane (Figure 4, upper). Each molecule of 10 in a layer employs one of its nitrogen atoms to link with another molecule of opposite handedness in the adjacent layer by means of a centrosymmetric edge
edge double C
H 3 3 3 N interaction,17 and since these interactions operate on either side of the layer, the result is a three-dimensional array. The second nitrogen atom of each host molecule is hydrogen bonded to a chloroform guest molecule (Figure 4, lower), whereas the bridging sulfur atom participates in three interactions: S 3 3 3 Cl (3.78 Å, to guest), S 3 3 3 π aromatic (ca. 3.9 Å, to host, same enantiomer), and S 3 3 3 H
Ar (3.22 Å, to host, opposite enantiomer). The crystal structure of (10) 3 (chloroform) therefore comprises layers of host molecules with the chloroform guests located between these (Figure 5, upper). The bromine atoms of the layers are located on their outer surfaces and hence the solid comprises alternating layers of hosts and halogens in the bc plane. No guest
guest Cl 3 3 3 Cl interactions are present, but a very effective network of host
host Br 3 3 3 Br and host
guest Br 3 3 3 Cl interactions is created (Figure 5, lower) that cements the layers together. Crystal Structures of the Furan and Benzene Inclusion Compounds. Crystallization of 10 from furan or benzene yielded the isostructural compounds (10) 3 (guest) in triclinic space group P1. In (10) 3 (furan), there are two crystallographically independent furan molecules, each of which is disordered about a different center of inversion. These types (indicated as orange or pink in Figure 6) alternate along channels in the b direction and are linked by aryl EF interactions. The pink guest acts as a double H-donor, and the orange guest acts as double H-acceptor, in these C
H 3 3 3 π interactions. Pairs of host molecules form centrosymmetric molecular pens18,19 around the chain of guests by enclosing the pink furan guests but not the orange ones. This outcome is rather similar to threading a string of beads to form a necklace (Figure 6, center). Host
guest stabilization is provided by an EF interaction (two EF in the benzene case) between each enclosed furan and the endo-surfaces of the surrounding host pair (Figure 6, lower). One aromatic wing of each host molecule takes part in an exo, exo-facial OFF interaction with a neighboring pen along c and there is also a Br 3 3 3 Br attraction (3.57 Å) between two molecules of 10 of the same handedness but belonging to adjacent pens along a. In addition, an edge
edge dimeric C
H 3 3 3 N contact is present along the a direction between two host aromatic wings. However, its interatomic separations (d = 3.24, D = 4.22 Å) are too long for this motif to have energetic significance. Typical effective interactions of this type have 4475

dx.doi.org/10.1021/cg2006937 |Cryst. Growth Des. 2011, 11, 4474–4483

4476

4% 0.35, 0.49

0.015

0.67

822454

R for mult meas

largest peak in final diff map/e Å
3

CCDC number

3330 I/σ(I) > 2

no. of intensity meas. criterion for obs. ref.

1.69

50

2θmax/o

crystal decay min, max trans coeff

θ/2θ

scan mode

s = [∑mw|ΔF|2/(m
n)]1/2

4.374

μ/mm
1

0.051

MoKR, 0.7107

radiation, λ/Å

Rw = [∑mw|ΔF|2/∑mw|Fo|2]1/2

1.74

Dcalc/g cm
3

0.038

4

Z

130

1901.3(10) 294(1)

V/Å3 T/K

R = ∑m|ΔF|/∑m|Fo|

90

γ/°

variables (n) in final ref.

98.93(2)

β/°

2542

90

R/°

2542

13.205(5)

c/Å

no. of reflections (m)

θ/2θ

7.719(3)

b/Å

no. of indep. obsd. ref.

3.944

18.882(8)

a/Å

822449

1.18

0.021

none 0.56, 0.69

1.30

0.044

0.036

150

2391

2391

4037 I/σ(I) > 2

50

MoKR, 0.7107

1.78

4

2299(2) 294(1)

90

121.11(2)

90

17.779(8)

15.382(4)

9.820(7)

617.6 P21/c

498.2 P21/c

formula mass space group

(C22H14Br2N2S) 3 (CHCl3)

(10) 3 (chloroform)

C22H14Br2N2S

10

formula

compound

822452

0.72

0.016

7% 0.47, 0.63

1.79

0.086

0.045

283

2869

2869

4025 I/σ(I) > 2

50

θ/2θ

3.624

MoKR, 0.7107

1.64

2

1143.6(9) 294(1)

107.55(2)

91.20(3)

101.67(2)

11.986(5)

10.430(5)

9.835(4)

566.3 P1

173085

0.98

0.028

none 0.58, 0.73

1.29

0.039

0.032

274

2180

2180

4102 I/σ(I) > 2

50

θ/2θ

3.556

MoKR, 0.7107

1.64

2

1169(1) 294(1)

107.83(2)

92.09(3)

102.37(3)

12.032(7)

10.565(6)

9.952(5)

576.3 P1

(C6H6)

(C22H14Br2N2S) 3

(C22H14Br2N2S) 3 (C4H4O)

(10) 3 (benzene)

(10) 3 (furan)

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

822450

0.67

0.035

5%

1.68

0.058

0.053

156

2336

2336

3968 I/σ(I) > 2

50

θ/2θ

3.902

MoKR, 0.7107

1.72

4

2258(2) 294(1)

90

110.26(3)

90

10.470(7)

23.061(6)

9.969(6)

583.2 P21/c

(C22H14Br2N2S) 3 (CH2Cl2)

(10) 3 (dichloromethane)

822453

2.40

0.045

none

1.70

0.074

0.060

136

1222

1222

3180 I/σ(I) > 2

46

θ/2θ

7.051

MoKR, 0.7107

1.94

4

2296(2) 294(1)

90

111.38(3)

90

10.601(5)

23.143(8)

10.050(4)

672.1 P21/c

(C22H14Br2N2S) 3 (CH2Br2)

(10) 3 (dibromomethane)

822451

0.90

0.024

none 0.44, 0.62

1.42

0.047

0.040

139

1787

1787

3451 I/σ(I) > 2

50

θ/2θ

4.206

MoKR, 0.7107

1.75

4

1962.7(13) 294(1)

90

118.19(2)

90

18.869(8)

12.439(3)

9.487(4)

516.3 P21/c

(C22H14Br2N2S) 3 (H2O)

(10) 3 (water)

Crystal Growth & Design ARTICLE

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ARTICLE

Figure 3. The apohost crystal structure of 10. Upper: Association of two opposite enantiomers by means of Br 3 3 3 π interaction. Lower: Projection on the ac plane, with the aryl interfacial interactions marked OFF and EF, and the Br 3 3 3 N and Ar
H 3 3 3 S 3 3 3 H
Ar interactions indicated by dashed lines. Br 3 3 3 π interactions are marked with arrows. Atom code: C green (opposite enantiomers light or dark), H light blue, N dark blue, S yellow, and Br brown.

C 3 3 3 N distances within the range D = 3.37
3.79 Å.17 Pairs of opposite enantiomers of 10 are linked by a centrosymmetric motif that incorporates both their S atoms. Each sulfur forms a bifurcated Ar
H 3 3 3 S 3 3 3 H
Ar motif (S 3 3 3 H = 3.05 and 3.21 Å) involving two molecules of 10 with opposite handedness. The concomitant S 3 3 3 S separation is 3.30 Å. A description of the isostructural inclusion compound (10) 3 (benzene) has already been published.20 Crystal Structures of the Dichloromethane and Dibromomethane Inclusion Compounds. Crystallization of 10 from dichloromethane or dibromomethane yielded the isostructural inclusion compounds (10) 3 (guest) in monoclinic space group P21/c. The guest molecule is disordered with two alternate overlapping sites. This discussion, and the associated figures, illustrate only the major component. Host molecules related by a glide plane (and therefore of alternating chirality) form an open canal along c. Adjacent canals along b alternate in the direction of the canal opening and are linked by exo-exo OFF interactions to give an undulating sheet. Adjacent layers are translated along a, such that a series of guest-containing channels is formed between them. Three sides of each channel, seen here projected in the ab plane, are created by one host layer and the fourth side by the adjacent host layer (Figure 7). The crystal structure of (10) 3 (dichloromethane) involves many halogen 3 3 3 halogen associations (Figure 8).7 A sinusoidal chain of dichloromethane molecules runs along each channel,

Figure 4. Upper: Part of a layer of host molecules 10 projected onto the bc plane in the solid (10) 3 (chloroform). This shows several centrosymmetric P4AE interactions, one of which is highlighted by an ellipse and their further association into chains within the layer. Lower: Centrosymmetric aryl edge
edge C
H 3 3 3 N dimer interactions such as the one shown (dashed lines) join adjacent layers of 10. The second nitrogen atom of each host molecule is hydrogen bonded to a chloroform guest. Atom code: chloroform C magenta and Cl orange.

with Cl 3 3 3 Cl interactions (3.77 and 4.01 Å) linking adjacent guests. These guest chains are further stabilized by host
guest Br 3 3 3 Cl (4.02 and 4.08 Å) and bifurcated N 3 3 3 Cl (3.52 and 3.59 Å) interactions. Two host
host Br 3 3 3 Br attractions occur: Br1 3 3 3 Br2 (3.56 Å) between molecules translated along a to form a chain, and Br1 3 3 3 Br2 (4.10 Å) between centrosymmetric pairs. The latter contact helps create triply interacting dimers that incorporate an S 3 3 3 S interaction (3.36 Å). Each sulfur atom also participates in a bifurcated Ar
H 3 3 3 S 3 3 3 H
Ar interaction 4477

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Figure 6. Upper: Part of a chain of furan guest molecules occupying a channel within the solid (10) 3 (furan). The independent guest C atoms are colored orange or pink, and O atoms are red. Only one disorder component is shown at each site. Center: A different perspective of the same guest chain but with each pink furan molecule now enclosed within a molecular pen formed by two molecules of the host 10. Lower: The crystal structure of (10) 3 (furan) projected onto the ac plane and showing the centrosymmetric host molecular pens edge-on. Here each chain of furan guests is oriented along the b direction (only one disorder component is shown at each site). One OFF interaction is indicated at the center of the diagram.

Figure 5. Upper: Projection of part of the crystal structure of (10) 3 (chloroform) in the ab plane, and showing three host layers seen edge-on with a layer of halogen atoms (host Br and guest Cl) between them. Lower: The extensive network of host
host Br 3 3 3 Br and host
guest Br 3 3 3 Cl halogen interactions (both indicated by dashed lines) present in the bc plane and lying between two host layers.

(S 3 3 3 H = 2.98 and 3.14 Å). One of the latter is part of the centrosymmetric motif, while the other involves a second host molecule of opposite handedness. As in the structures with benzene and furan described above, a centrosymmetric edge
edge dimeric C
H 3 3 3 N contact is present between two host aromatic wings. However, again, its interatomic separations (d = 3.53, D = 4.50 Å) are too long for this motif to have energetic significance.

Crystal Structure of the Monohydrate Compound. Crystallization of 10 from either acetone or tetrahydrofuran (containing adventitious water) yielded the hydrate compound (10) 3 (water) in monoclinic space group P21/c. Host molecules of identical handedness form chains, linked by Br 3 3 3 π interactions to one wing only, along the b direction. An oblique EF accompanies the Br 3 3 3 π interaction and is probably a secondary consequence only (Figure 9, upper). In addition, each host molecule forms a centrosymmetric endo,endo-facial dimer through a combination of one aryl OFF and two aryl EF associations, namely, the P4AE parallel 4-fold aryl embrace interaction to create a layer structure in the bc plane (Figure 9, center). The aromatic nitrogen atoms present in our V-shaped molecules are capable of accepting hydrogen bonds, but this behavior is infrequently encountered.4 It does occur in (10) 3 (water) 4478

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ARTICLE

Figure 8. Diagrammatic representation of the halogen 3 3 3 halogen interactions (dashed lines) present in solid (10) 3 (dichloromethane). A chain of dichloromethane guests, linked by Cl 3 3 3 Cl attractions, runs along c (shown horizontally). The molecules of host 10 are reduced to just their bromine atoms for simplicity. Atom code: Br brown and Cl orange.

different layers take the form of interdimer aryl exo,exo-facial OFF interactions along [101]. The bridging sulfur atom again interacts with two H atoms, but this time by an Ar
H 3 3 3 S 3 3 3 H
CBr motif (S 3 3 3 H = 3.17 and 3.20 Å) that involves only one other molecule of 10.

’ DISCUSSION

Figure 7. The crystal structure of (10) 3 (dichloromethane) projected in the ab plane. Upper: Sinusoidal layers of the host 10 run along b, with one complete and one partial layer being shown here. Guest-containing voids are formed between adjacent layers. Lower: Space filling representation of one guest-containing void. Atom code: Host C green (light or dark for opposite enantiomers), dichloromethane guest C magenta and Cl orange.

where pairs of water molecules link pairs of host molecules via N 3 3 3 OW 3 3 3 OW 3 3 3 N bridges within the layer structure. The unusual involvement of two water molecules provides an ideal bridge length in this case. The water oxygen atom is close to two host nitrogen atoms in different molecules: N1 3 3 3 O (2.87 Å) and a somewhat longer value for N2 3 3 3 O (3.34 Å). The hydrogen atoms are disordered. Figure 9 (lower) and Table 2 show the arrangement in just one disorder component. Links between

Comparison of Crystal Structure Parameters. The calculated density values for the seven crystal structures are listed in Table 3. Comparison of these densities is problematical, however, due to the differences in elements present across the series of guests. The packing coefficients of the seven crystal structures lie within the narrow range of 67.4
70.2%. Molecule 10 has a slightly twisted V-shaped structure with the potential for conformational adaptation to different circumstances of host packing or guest inclusion. One means of structural comparison is the fold-angle present in the various compounds. This is defined by the angle subtended at three points indicated by black circles on the molecular structure of 10 in Figure 2. All of these values, with one exception, lie within the very tight range of 86.2
89.0 deg. The fold angle of 92.5° for (10) 3 (chloroform) is an outlier value, but nonetheless is not markedly different. This difference presumably is related to the lower energy obtained through formation of N 3 3 3 H
CCl3 host
guest hydrogen bonding in this compound. Otherwise, host conformational variation is a minor factor within this particular series of structures. Analysis of the Crystal Structures. The diheteroaromatic derivative 10 yielded crystals of the apohost when it was crystallized from trifluoromethylbenzene solution. We have commented previously on the remarkable ability of this solvent to give both high quality crystals and also to be excluded from crystals of known diheteroaromatic hosts.21 The tally now is 10 hosts tested: resulting in formation of seven solvent-free crystalline solids and only three inclusion compounds. Several of these apohost crystals were only obtainable from this enigmatic solvent. An exo-bromine atom of one molecule was inserted within the V-shaped cleft of a second molecule of 10 in the apohost crystal structure (Figure 3), but this arrangement was not 4479

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Table 2. Hydrogen Bonding Parameters in (10) 3 (water) D
H (Å)

a

Figure 9. Upper: A P4AE dimer with two adjacent host molecules associating via the Br 3 3 3 π (arrows) and oblique EF interactions in (10) 3 (water). Center: Chains of host molecules formed by aryl exo,exofacial OFF interactions between P4AE dimers. Lower: One disorder component of the hydrate (10) 3 (water) structure, showing the resulting hydrogen bonding pattern. The homochiral chains are oriented horizontally (b direction), and their handedness alternates vertically (c direction).

OW1
H1OW1 3 3 3 N1 OW1
H1OW1 3 3 3 OW1a OW1
H20 OW1 3 3 3 N2b

1.0 1.0 1.0

D
H 3 3 3 A (Å) 1.87 179 2.34

D3 3 3A (Å)

D
H 3 3 3 A (o)

2.87(1)

179

2.79(1)

179

3.34(1)

180

1
x, 1
y,
z. b 1
x, 1/2 + y, 1/2
z.

encountered often for the various inclusion compounds, although a somewhat asymmetric version was found in (10) 3 (water). The isostructural compounds (10) 3 (furan) and (10) 3 (benzene) contain a novel guest arrangement in which two independent molecules alternate along guest chains. One of these independent molecules is enclosed within a molecular pen18 formed by two surrounding molecules of host 10, and the other is not. The resulting “string of beads” construction is the supramolecular equivalent of a multiple rotaxane structure (Figure 6). When two V-shaped molecules of 10 associate, they do so most frequently by means of exo,exo-facial OFF interaction. The apohost crystal structure of 10 contains an example of this motif, as do the structures of (10) 3 (dichloromethane) and (10) 3 (dibromomethane). The endo,endo-facial OFF assembly mode, as present in the structures of (10) 3 (chloroform) and (10) 3 (water), is the next most common arrangement. The molecular assembly used in the isostructural compounds (10) 3 (dichloromethane) and (10) 3 (dibromomethane) is unprecedented among our diheteroaromatic structural studies.4 Compounds (10) 3 (CH2Cl2) and (10) 3 (CH2Br2), as illustrated in Figure 7, appear to employ an exo,endo-facial OFF interaction between two hosts of opposite chirality to produce a section of the U-shaped canal. Closer inspection, however, reveals that the base of the canal does not involve OFF overlap (even though the two aromatic wings are parallel). The remaining aromatic wings create the walls of the canal and the dimeric units are linked to their neighbors along b by standard exo,exo-facial OFF interactions. Each guest molecule is linked to both partners of the dimer by means of a bifurcated N 3 3 3 halogen 3 3 3 N motif. This is almost certainly the driving force behind the adoption of this unusual type of construction motif. Figure 10 illustrates three views of the U-shaped dimeric unit and its associated guest molecule. The sulfur bridged diheteroaromatic compound 10 did not repeat the anomalous behavior shown by the oxygen bridged derivative 8. It included a wide range of organic solvents, with four distinct crystal forms being produced across its six inclusion compounds. Its carbocyclic analogue 4, in contrast, only yielded two crystal forms from the three inclusion compounds isolated.17,22 The increased versatility of host 10 must be associated with its sulfur atom, which is similar in volume to a CH2 group. Quite often, the analogous S and CH2 compounds behave in exactly the same manner and produce isostructural crystals. This is the case for the structure of (10) 3 (chloroform) reported here and the earlier compound (4) 3 (chloroform).17 In such instances, the sulfur atom can be regarded as mimicking the behavior of the methylene group. Further comparisons cannot be made here, however, since host 4 is not known to include the other guests trapped by 10. On other occasions, entirely different outcomes result between the analogous S and CH2 compounds. We have described examples of both these outcomes being 4480

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Table 3. Comparison of Crystal Structure Parameters compound Dcalc/g cm
3

a

10 1.74

(10) 3 (CHCl3) 1.78

(10) 3 (C4H4O)

(10) 3 (C6H6)

1.64

1.64

(10) 3 (CH2Cl2) 1.72

(10) 3 (CH2Br2) 1.94

(10) 3 (H2O) 1.75

packing coefficient/%

68.7

68.8

68.2

69.6

67.4

67.9

70.2

host fold anglea/°

89.0

92.5

86.3

87.3

88.1

87.9

86.2

The angle at the S atom subtended by the sites marked by black circles on structure 10 in Figure 2.

Figure 10. Details of the construction of the unusual repeat unit present in (10) 3 (dichloromethane). Left: Opposite enantiomers of 10 associate by means of an apparent exo,endo-facial OFF interaction to form a section of the canal. Center: The base of the canal utilizes parallel aromatic wings but these do not overlap. Right: A chlorine atom of the dichloromethane guest is joined (dashed lines) to both molecules of 10 by means of a bifurcated N 3 3 3 Cl 3 3 3 N (3.52 and 3.59 Å) halogen bond motif. Atom code: Br brown and Cl orange.

observed for a common host in an earlier diheteroaromatic study.23 It is well-known that sulfur atoms can participate in many types of intermolecular attractions, but these are generally less understood, and less widely applied, by crystal engineers.24 Frequently, where different crystal structures are recorded between S and CH2 analogues, the sulfur atom does not appear to be playing a dominant role in determining the outcome of the crystal packing. Consequently, we have examined carefully the role played here by the S atom. Figure 11 compares the sulfur atom environment and its participation in intermolecular attractions across our range of seven crystal structures. Descriptions and numerical values for these appear under the individual materials. It is immediately apparent that sulfur participates in every case by means of multiple interactions, always involving S 3 3 3 H
C,25 and sometimes S 3 3 3 S26 or S 3 3 3 halogen19,27 as well. These interactions are all bifurcated or trifurcated in nature. There is striking similarity between the sulfur motifs used in the dichloromethane/ dibromomethane and furan/benzene pairs, even though the host molecules are arranged quite differently in the two cases. The Pauling electronegativity values of H, C, S, N, and O are 2.20, 2.55, 2.58, 3.04, and 3.44, respectively.28 Solely on this basis it might be expected that S 3 3 3 H
C interactions would be strongly disfavored relative to the now well established N 3 3 3 H
C and O 3 3 3 H
C attractions.6,9 Being larger than these other atoms, however, endows sulfur with greater polarizability. It therefore is somewhat stickier in the context of supramolecular contact than first appears apparent. We have previously reported that the (bifurcated) thioether—1,3-peri interaction does occur in crystal structures,20 although less frequently than its oxygen counterpart.11 This present work indicates that the more general bifurcated C
H 3 3 3 S 3 3 3 H
C motif actually is a rather favorable interaction. This parallels our earlier observation that double C
H 3 3 3 N motifs were often more effective in crystal engineering than the simple single C
H 3 3 3 N interaction. These could be either centrosymmetric C
H 3 3 3 N dimer or C
H 3 3 3 N 3 3 3 H
N motifs.6

It should be emphasized that the crystallization outcomes in this work, and our earlier studies of diheteroaromatic hosts, result from competition between many combinations of weak intermolecular attractions, rather than being driven by stronger hydrogen bonding. Consequently, the lowest energy structure depends on the best compromise between the possible interactions. It is clear that sulfur can play a very active, though not usually dominant, role in this type of process.

’ EXPERIMENTAL SECTION NMR data were recorded using a Bruker ACF300 instrument at 25 °C (1H 300 MHz, 13C 75.4 MHz) 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 HRMS results were determined at the Australian National University, Canberra.

7r,15r-Dibromo-6,7,14,15-tetrahydro-6r,14rthiacycloocta[1,2-b:5,6-b0 ]diquinoline (10). A solution of N-bromosuccinimide (0.33 g, 1.84 mmol) and the diquinoline 912 (0.25 g, 0.74 mmol) in carbon tetrachloride (40 mL) was refluxed overnight. Succinimide was filtered from the cooled mixture, washed with a small amount of CCl4, and then the combined filtrate evaporated under reduced pressure to give a light yellow precipitate. Crystallization from ethyl acetate gave the colorless dibromide 10 (0.75 g, 82%), mp 158
160 °C. HRMS m/z (FAB, EtOAc) (MH)+: Found: 500.929237, 498.929748, 496.929912; Calc. for (C22H15N2SBr2)+: 500.928175, 498.930222, 496.932268. νmax (paraffin mull) 1290w, 1230m, 1150s, 1100w, 1040w, 950w, 910w, 860w, 740s cm
1; 1H NMR (CDCl3) δ: 4.75 (d, J 2.3 Hz, 2H), 5.84 (d, J 2.3 Hz, 2H), 7.38
7.44 (m, 2H), 7.59
7.67 (m, 4H), 7.96 (d, J 8.7 Hz, 2H), 8.00 (s, 2H); 13C NMR (CDCl3) δ: 48.5 (CH), 51.5 (CH), 127.4 (CH), 127.5 (CH), 127.9 (C), 128.5 (C), 128.9 (CH), 131.0 (CH), 140.9 (CH), 147.6 (C), 152.0 (C); m/z (M+ and >20%): 500 (M+, 81Br/81Br,