Directional Change in Aromatic Ï...Ï Crystal Packing

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Swivel and Tilt Interactions: Directional Change in Aromatic #...# Crystal Packing Roger Bishop, Mohan M. Bhadbhade, Marcia L Scudder, and Jiabin Gao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01673 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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Crystal Growth & Design

Swivel and Tilt Interactions: Directional Change in Aromatic π...π Crystal Packing Published as part of the Crystal Growth & Design virtual special issue In Honor of Professor William Jones

Roger Bishop,*,‡ Mohan M. Bhadbhade,§ Marcia L. Scudder‡ and Jiabin Gao‡ ‡

§

School of Chemistry, The University of New South Wales, UNSW Sydney NSW 2052, Australia Mark Wainwright Analytical Centre, The University of New South Wales, UNSW Sydney NSW

2052, Australia

Bishop, Roger

http://orcid.org/0000-0002-6067-7289

Bhadbhade, Mohan M.

http://orcid.org/0000-0003-3693-9063

Supporting Information

ABSTRACT: Offset interfacial π...π overlap between two aromatic groups of unequal length and breadth often occurs with unidirectional alignment of their molecular long axes. However π...π interaction can also occur with directional change. Analysis of diquinoline crystal structure data reveals π...π swivel angles from 0o (antiparallel alignment) through to 180o (parallel long axes). Swivel angles in the range 0-35o are commonplace and are compatible with formation of linear and layer assemblies. Larger swivel angle values are less frequent and their appearance is encouraged through competition with other weak attractive forces. These higher angular values are associated with formation of zigzag chains, corrugated layers, unusual building blocks, homochiral assemblies, and other less common crystal packing modes. Directional change also occurs in structures where the planes of the interacting π-systems are non-parallel, and the consequences of these tilt angles are described. The principles described herein are applicable to aromatic systems in general.

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INTRODUCTION Interaction between two aromatic groups is one of the more common intermolecular packing arrangements in the organic solid state.1-4 The principal supramolecular synthons5,6 are the edgeface C–H...π (EF)7,8 and the offset face-face π...π (OFF)9-12 motifs and hence, for planar aromatic hydrocarbons, a high degree of crystal packing prediction is often possible.13-15 If two different planar partners have respectively electron-rich and electron-poor characteristics, then eclipsed dipolar π...π stacking may replace the OFF geometry. A striking example is crystalline quinhydrone, the well-known 1:1 complex of hydroquinone and p-benzoquinone.16 When the partners are symmetrical planar molecules, like benzene, then overlap allows no distinction of directionality other than along the π...π stack. Larger aromatic molecules with unequal molecular length and breadth normally overlap with their molecular long axes aligned but, in principle, alternative nonlinear overlap also could occur. More complex molecules, containing aromatic part structures, are often three-dimensional and may have a handed molecular structure. Aromatic pairs created using π...π interaction frequently still overlap with their long axes aligned, since repetition of this dimeric unit leads to linear chains or flat layers that propagate efficiently into a compact crystal structure. However, there is now an increased likelihood of non-linear overlap, because additional competing packing factors (heteroatoms, functionality, substituent groups, dimensionality, stereochemistry, chirality) are also now in play. If this occurs, then an angle of rotation has been introduced between the long dimension axes of the interacting molecules. There is now an inbuilt angular change that may result in formation of a zigzag chain or corrugated layer. We have termed interfacial π...π interaction incorporating such directional change as a swivel interaction.17,18 Structural properties are frequently investigated by utilising compilations of crystallographic information.11 Our studies on diquinoline inclusion hosts19-23 have generated over a hundred crystal structure determinations. Analysis of these data has enabled systematic crystal engineering studies of interactions weaker than the Pauling hydrogen bond to be carried out successfully.24-28 Figure 1 illustrates the generic design of these compounds. The central aliphatic ring provides C2 or pseudoC2 symmetry and a small amount of conformational flexibility to help accommodate guests of varied size. Diquinoline molecules of this general type: (i) have a slightly twisted V-shape, (ii) are three-dimensional rather than planar, and (iii) are handed structures. These racemic compounds usually act as potent inclusion hosts.


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Figure 1. Generic design for V-shaped aromatic inclusion hosts that interact with themselves and guest molecules using only weaker intermolecular forces in the solid state. The three essential design elements are colour-coded: aliphatic linker ring (magenta), aromatic wings (blue), and peripheral halogen substituents X and/or Y (black).

In each individual example there is competition between combinations of several types of weaker intermolecular forces. Substituent halogen atoms on the molecular periphery provide local hotspots for host-host and host-guest association, and the aromatic wings provide opportunities for offset face-face π...π (OFF) and edge-face C-H...π (EF) interaction. All of these molecules offer a choice of exo- or endo-aromatic surfaces for π...π interaction. The interfacial behaviour observed across this large number of crystal structures enables us to establish the first systematic analysis of π...π swivel interaction behaviour.

RESULTS AND DISCUSSION Molecular axes and interfacial interaction types The π-electron density of a quinoline molecule is unequal across its two fused rings, with the benzo ring being relatively more electron-rich than the pyridine ring. Quinoline derivatives therefore have differing electron density along their aromatic long axis. These π-electron density differences are indicated by the relative partial charges (δ-) and (δ+), respectively. The antiparallel alignment of two quinoline-derived molecules has the relatively electron-rich ring of one positioned over the electron-poor ring of the second. This favourable arrangement is defined as a swivel angle of 0o (Figure 2). Parallel alignment of the aromatic long axes can also occur (180o). This much less common arrangement has the electron-rich rings positioned over each other, and also the electron-



poor rings situated on top of each other. All swivel angle values between 0 and 180 degrees impart directional change to the π...π interaction. The numerical value of the swivel interaction is the rotation angle necessary to re-establish the antiparallel alignment, and this is obtained quantitatively by measuring the torsion angle from the experimental X-ray structural data. Directional change also occurs in structures where the planes of the associated π-systems are non-parallel and tilted with respect to each other. The tilt angle value is that between the normal to the mean aromatic plane of each component. These cases do not require dissimilar axis lengths.

Figure 2. Left: Examples of swivel angles resulting from π...π overlap of two quinoline molecules (red and black). The molecular long axes are shown in green, with the arrowhead indicating the more electron rich (δ-) region of the molecule. Antiparallel overlap (0o) is the most favoured. All angular values between 0 and 180o result in rotational deviation from linear packing, as seen for the central 60o example. Right: Hypothetical non-parallel packing of two quinoline molecules with a tilt angle of 15o. The normal (blue) to the ring plane is shown for both interacting molecules.

The principal types of face-face interaction observed in the crystal structures of our V-shaped diquinoline molecules26-28 are shown diagrammatically in Figure 3. Endo,endo-facial assembly Type I is very common. Most frequently, a pair of molecules interact by means of one OFF and two EF interactions to create a motif known as the parallel fourfold aromatic embrace (P4AE).29,30 This is normally located around an inversion centre and therefore the two aromatic long axes are antiparallel. A close relative of the P4AE is the pi-halogen dimer (PHD), which is produced when X and Y in Figure 1 are halogen and hydrogen respectively.31-33 The two molecules mutually rotate to replace the two EF interactions by four pi-halogen contacts. This is favoured since the electron-rich halogen atoms become positioned directly over the electron-poor pyridine rings. Once again, the PHD motif is centrosymmetric and so the long axes are antiparallel (swivel angle 0o).


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The exo,exo-overlap assembly Type II is by far the most common type of interfacial π...π interaction in our crystal structures and it can occur between molecules of either the same, or different, chirality. Relatively small swivel angles do not reduce the area of π...π overlap or result in serious steric packing difficulties. Endo,exo-facial packing is the rarest of the three facial interactions, but it can occur as the alternative assembly Types III-VI. These tend to have large swivel angles and are usually associated with less conventional crystal packing arrangements. The endo,exo-interaction VI is particularly noteworthy since molecules of the same handedness stack into eclipsed columns. Both aromatic wings overlap and hence there is perfect linear directionality along each column. The aromatic long axes are parallel and the swivel angle is 180o. Alternating enantiomers could, in principle, also pack into columns of this general configuration. However, the mismatch between neighbours would rule out full eclipsing and result in a more convoluted assembly.



Figure 3. Diagrammatic representation of the aromatic interfacial π...π assembly Types I-VI observed in crystals of the racemic V-shaped diquinoline molecules. In principle, each of these arrangements could involve either homochiral or heterochiral molecules.


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Analysis of interfacial aromatic interactions in selected diquinoline crystal structures Nine crystal structures, utilising the eight diquinoline molecules 1-8 (Figure 4), have been chosen for our analysis of aromatic swivel interactions. A common atom colour code has been used for all Figures that illustrate the individual swivel interactions (A-Q): Br brown, C grey, H off-white, and N blue. Bond centroids used to define the aromatic long axes are indicated by red spheres, and the axes themselves as green arrows. The head (δ−) and tail (δ+) of each arrow indicates the relatively electron-rich benzo ring and the electron-poor pyridine ring, respectively.



Figure 4. Molecular structures of the diquinoline derivatives 1-8 whose crystal structures are discussed. Only one enantiomer of the racemic substance is shown in each case.

Crystal structure of (1).(chloroform)34,35 This crystal structure contains both the Type I endo,endo- A and Type II exo,exo- B face-face interactions

illustrated in Figure 5. Both motifs contain opposite enantiomers of 1, are

centrosymmetric, and have antiparallel aromatic long axes (swivel angle = 0o). The P4AE motif A contains distal quinoline nitrogens (ring separation 3.46 Å), whereas the exo,exo-arrangement B has proximal nitrogen atoms (ring separation 3.40 Å). Aromatic wings surrounding an inversion centre must have parallel planes and therefore the tilt angle for both interactions A and B is 0o.


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Figure 5. The endo,endo- A and exo,exo- B face-face interactions present in the crystal structure of (1).(chloroform) (CCDC no. 1237040, refcode PORCOO). Swivel colour code: Br brown, C grey, H off-white, and N blue. The red spheres are the bond centroids defining the aromatic long axes. Both motifs have antiparallel aromatic long axes represented by the green arrows (swivel angle 0o).

Crystal structure of (2)2.(methylchloroform)34-36 If the P4AE unit (endo,endo-interaction) is expanded hypothetically to form a penannular structure enclosing a guest molecule, then the original inversion centre may be either lost or retained. In both cases, all four aromatic surfaces of the resulting molecular pen interact with their neighbours by means of exo,exo-interactions and form a layer structure. Two representative examples are described here. Compound (2)2.(methylchloroform) forms chirally pure layers that alternate in handedness along c.34-36 The symmetry elements present in this structure reveal that all four exo,exo-interactions C subtended by a given pen are identical, with a ring separation of 3.46 Å (Figure 6). This overlap occurs with a swivel angle of 33.8o, but the ‘self-correcting’ alternating clockwise and counter-clockwise rotations across the layer result in only minor corrugation of the layer topology.



Figure 6. Crystal structure of (2)2.(methylchloroform) (CCDC no. 137825, refcode LICGIN).34-36 Upper: Projection in the ab plane of one layer of homochiral molecular pens. Each pen is surrounded by four identical exo,exo-facial interactions C. One disordered methylchloroform guest molecule occupies each molecular pen. Centre: The aromatic long axes of the exo,exo-interaction C subtend a swivel angle of 33.8o. Lower: The two near-parallel aromatic planes of interaction C exhibit a tilt angle of 2.5o.

Crystal structure of (2)2.(dioxane)37 The second penannular compound, (3)2.(dioxane), is structurally more typical of this series of inclusion compounds. The inversion centres are retained, but its crystals contain two independent molecules (A and B) and their enantiomers (A* and B*). Hence each layer comprises a combination of (A+A*, blue) and (B+B*, green) pens. There are now two different exo,exo-interactions D and E (Figure 7).


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Figure 7. Upper: One complete (A+A*, blue) molecular pen in the structure (2)2.(dioxane) (CCDC no. 189727, refcode MUSDUZ), surrounded by parts of its four neighbouring pens (green) and with the inversion centres (orange spheres) indicated. The different exo,exo-interactions D (swivel angle 31.6o, tilt angle 4.9o, ring separation 3.59 Å) and E (swivel angle 33.2o, tilt angle 5.1o, ring separation 3.55 Å) are indicated. Lower: The exo,exo-interaction D between the molecular pens.

Crystal structure of (3)4.(benzene)38,39 The racemic tetrabromodiquinoline 3 crystallises as a unidirectional molecular staircase assembly, and many different guest molecules can occupy the interstitial spaces between parallel staircases. Structure (3)4.(benzene) contains two independent molecules of 3 (A and B) and their enantiomers (A* and B*). Pairs of host molecules form pi-halogen dimer (PHD) motifs14 (A+A*, green) and (B+B*, blue) around inversion centres. The Type I endo,endo-facial interactions F and F’ therefore have swivel angles of 0o. These dimers then complete the staircase structure through use of the enantiomeric A/B and A*/B* Type II exo,exo-facial associations (Figure 8). The aromatic long axes of this interaction G have a swivel angle of 23.8o.




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Figure 8. Upper: Part of a molecular staircase assembly in the crystal structure of (3)4.(benzene) (CCDC no. 190589, refcode MUTREY) showing the independent molecules A/A* (blue) and B/B* (green), plus the π...π interactions F, F’ and G. Centre: The centrosymmetric endo,endo-PHD interactions F and F’ have swivel angles of 0o. Lower: The exo,exo-interaction G has a swivel angle of 23.8o.

Figure 9 shows the molecular staircase of Fig. 8 rotated by ninety degrees around the vertical b axis to highlight the orientations of the aromatic planes. The A/A* and B/B* interactions surround inversion centres and therefore their planes are parallel (tilt angle 0o). This is not the case for the A/B contact (and its enantiomer A*/B*) where there is a significant tilt angle of 6.8o between the interacting (orange and green) aromatic planes. This diagram illustrates further how a linear staircase is generated along b through presence of alternating +6.8o and -6.8o tilt angles at the latter enantiomeric sites.

Figure 9. Orientation of the aromatic planes along a molecular staircase in solid (3)4.(benzene). The interacting orange and green aromatic planes of the enantiomeric A/B and A*/B* contacts show opposed tilt angles of 6.8o. ACS Paragon Plus Environment


Crystal structure of (4).(acetophenone)17 Host cations 4 assemble into homochiral columns within the crystal structure of (4).(acetophenone). The principal association within each column is a Type II exo,exo-facial interaction H between the protonated ring of one molecule and the unprotonated ring of its neighbour. Hence both aromatic wings participate in identical interactions along the linear column (Figure 10). This π...π interaction has a swivel angle value of 50.1o. Although the molecular columns run along the b direction, they are clearly sinusoidal in shape, and this is reflected by the larger value of the observed swivel angle value. In contrast, the observed tilt angle is only 1.3o.


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Figure 10. Upper: Packing of two enantiomeric homochiral columns of 4 (black or orange) in the crystal structure of (4).(acetophenone) (CCDC no. 249410, refcode QETYIY). Bridging bromide ions are shown as orange spheres. Lower: The exo,exo-facial interaction H has a swivel angle of 50.1o. Crystal structure of 517 The racemic dichlorodibromodiquinoline 5 crystallises as a structure containing highly corrugated zigzag layers. Enantiomers (black or orange) alternate along each zigzag chain and the molecules of 5 associate using two types of aromatic interfacial interactions (Figure 11, Upper). The centre of each linear section of the chain comprises a Type II exo,exo-OFF interaction I around an inversion centre (swivel and tilt angles = 0o). Hence the apex of one V-shaped molecule points in an upward direction and the other downwards. Their second aromatic wings form an aromatic endo,exointeraction At first sight this appears to be of Type III, but closer examination reveals it to be more complex. All the molecules of 5 form eclipsed homochiral stacks along b and adjacent enantiomeric stacks interdigitate at each turning point of the zigzag chain. This results in formation of a Type V interaction. The enantiomeric stacks do not interleave in a parallel manner and there is considerable directional change at each turning point. The resulting endo,exo-interaction J has a swivel angle of 89.4o (Figure 11, Centre). Interaction I plays a major role in this crystal structure even though it causes no angular change. It doubles the distance between the neighbouring turning points J and thus increases the corrugation amplitude of the zigzag chain. The interdigitated homochiral columns of 5 (Figure 11, Lower) exhibit the very high tilt angle value of 13.8o.



Figure 11. Upper: The crystal structure of 5 (CCDC no. 249415, refcode QETZEV) showing the π...π interactions I (0o) and J. Centre: The endo,exo-interaction J has a swivel angle = 89.4o. Lower: Interdigitation of the homochiral columns of 5 (orange and black) with the large aromatic interplanar tilt angle of 13.8o emphasised. ACS Paragon Plus Environment

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Crystal structure of (6)4.(benzene)340 The inclusion compound (6)4.(benzene)3 contains two independent molecules of 6 (A and B) and their enantiomers (A* and B*). Pairs of host molecules (A/B* and A*/B) associate by means of a Type I endo,endo-facial π...π interaction K with a swivel angle of 104.9o (Figure 12, upper). This combination results in a dimeric building block that is the host repeat unit in this crystal structure. The orthogonal view of the dimer (Figure 12, lower) shows the significant intra-dimer associations as dashed lines. In addition to the π...π interaction (K, blue), a suite of six further attractive forces connects the two molecules of 6. These comprise two CBr-H...N (black), two C-Br...π (thin green) and two Br...Br interactions (thick green dashes). The tilt angle between the host aromatic planes is 8.7o.



Figure 12. Upper: Part of the crystal structure of (6)4.(benzene)3 (CCDC no. 1582720) showing one A/B* dimeric building block and its swivel angle of 104.9o. Lower: An orthogonal view of the dimer showing the significant intra-dimer associations as dashed lines. Crystal structure of 741 Racemic 7 crystallises as enantiomerically-pure layers that alternate in handedness along the c direction (Figure 13, upper). Each layer is constructed from infinite parallel columns in which the diquinoline molecules are stacked along b in a fully eclipsed manner. Type VI endo,exo-interactions L in each aromatic wing therefore have long axes running parallel to each other (swivel angle 180o and tilt angle 0o) as illustrated (Figure 13, lower).


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Figure 13. Upper: Crystal structure of racemic 7 (CCDC no. 1506267, refcode TAYGEI) showing the enantiomerically-pure layers (black or orange) in the ab plane. Lower: Just two molecules comprising part of one infinite homochiral column along b. Both aromatic wings overlap in a fully eclipsed manner to yield identical endo,exo-interactions L with parallel aromatic long axes (swivel angle 180o).

Crystal structure of (8)3.(chlorobenzene)242 This substance has a complex crystal structure that is particularly instructive in the context of swivel interaction behaviour. Its crystals contain three independent molecules (A, B and C) and their enantiomers (A*, B* and C*), and these form layers in the ab plane. One type of layer, illustrated in Figure 14, contains A*, B, and C molecules. Its neighbouring layers along c are enantiomeric, and so are built from A, B* and C* molecules. Each layer is constructed using three different π...π interaction Types.

Figure 14. Part of one ab layer in the (8)3.(chlorobenzene)2 crystal structure (CCDC no. 219112, refcode IQIJUN) built from A* (green), B (blue) and C (red) molecules. The layer construction utilises three different π...π interaction Types (I, II and IV), and five different aromatic π...π interactions M-Q. A* and B molecules associate as a PHD unit31-33 by means of the Type I endo,endo-facial interaction M. The PHD motif is nearly always centrosymmetric (swivel angle 0o), but here the two molecules surround a pseudo-inversion centre and so the swivel and angles become 0.5o and 0.4o, ACS Paragon Plus Environment


respectively (Figure 15). The B and C molecules associate as a Type IV assembly. If this comprised two identical homochiral molecules then the swivel angle would be 180o. Here, however, the homochiral molecules are crystallographically independent and also there is loss of C2 symmetry in 8. This results in the two endo,exo-interactions N (swivel 179.0o, tilt 1.7o) and O (swivel 177.9o, tilt 6.2o. The A* and C molecules form a double Type II assembly using both aromatic wings. Since these aromatic π...π interactions surround pseudo-inversion centres, their swivel angles change from the ideal value of 0o to exo,exo-interactions P (swivel 1.1o, tilt 1.4o) and Q (swivel 2.7o, tilt 1.4o).


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Figure 15. The three aromatic interaction Types and five π...π interactions present in the crystal structure of (8)3.(chlorobenzene)2, with their swivel angles indicated. Upper: endo,endo-Type I, M (0.5o); Centre: endo,exo-Type IV, N (179.0o) and O (177.9o); and Lower: exo,exo-Type II: P (1.1o) and Q (2.7o).

Swivel angles and interaction Types I-VI Our sample of nine racemic diquinoline crystal structures revealed the seventeen aromatic π...π interactions A-Q summarised in Table 1. Values of the swivel angle ranged all the way from 0o (antiparallel long axes) to 180o (parallel long axes). Descriptions and analyses of the molecular interactions already appear in the published literature, except for compound (6)4.(benzene)3. Crystal structures, especially those of racemic compounds, very much favour inversion centres in their packing.43 This is generally the case for the assembly Type I endo,endo-interactions that employ either H...π or Br...π (PHD) contacts. These motifs, with swivel angle 0o, are very common. The highly anomalous Type I interaction K in compound (6)4.(benzene)3 shows, however, that exceptions can and do occur. As already noted, K is a consequence of competition with six other weak interactions operating between two molecules of the diquinoline 6 (Figure 12). The outcome is formation of an unusual dimer structure with an endo,endo-facial swivel angle of 104.9o. This acts as the host repeat in the asymmetric unit of the crystal. Assembly Type II exo,exo-interactions are the most frequently encountered π...π contacts. Examples surrounding an inversion centre (0o) are not rare, but swivel angles around 20-35o are much more common. A modest mutual rotation may not seriously reduce the extent of π...π overlap or result in serious steric packing difficulties. Indeed, such rotation may assist with other aspects of the overall crystal packing. It should be noted, however, that linear or planar assemblies are produced due to ‘self-correction’. In other words, numerically equivalent clockwise and counterclockwise interactions alternate throughout the greater assembly (e.g. see Fig. 9). This makes sense in light of crystal packing strongly preferring linear and layer arrangements over circular or tubular ones. The endo,exo-interaction is the least common π-facial interaction, but it has greater structural variability through its alternative assembly Types III-VI. When this facial interaction does occur, its swivel angle values are high and represent cases where the competing interactions are dominant. Large swivel angles are associated with less common crystal packing arrangements such as highly corrugated layers, unusual building blocks, and extensive regions of homochirality. It should be emphasised that these large swivel angles are a symptom, rather than the cause, of such behaviour. Perhaps the most striking case is the crystal structure of racemic diquinoline 7, in which ACS Paragon Plus Environment


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considerable enantiomer ordering44 has taken place to produce homochiral layers. The fully eclipsed stacked columns of homochiral 7 require endo,exo-interactions with parallel aromatic long axes (swivel angle 180o).41

Table 1. The aromatic π...π facial interactions A-Q with their swivel and tilt angles Type

Facial

Swivel

Tilt

interaction

angle (o)

angle (o)

Compound

Refcode or CCDC no.

A

I

endo,endo-

0.0

0.0

(1).(chloroform)

PORCOO

B

II

exo,exo-

0.0

0.0

(1).(chloroform)

PORCOO

C

II

exo,exo-

33.8

2.5

(2)2.(methylchloroform)

LICGIN

D

II

exo,exo-

31.6

4.9

(2)2.(dioxane)

MUSDUZ

E

II

exo,exo-

33.2

5.1

(2)2.(dioxane)

MUSDUZ

F, F’

I

endo,endo-PHD 0.0

0.0

(3)4.(benzene)

MUTREY

G

II

exo,exo-

23.8

6.8

(3)4.(benzene)

MUTREY

H

II

exo,exo-

50.1

1.3

(4).(acetophenone)

QETYIY

I

II

exo,exo-

0.0

0.0

5

QETZEV

J

V

endo,exo-

89.4

13.8

5

QETZEV

K

I

endo,endo-

104.9

8.6

(6)4.(benzene)3

1582720

L

VI

endo,exo-

180.0

0.0

7

TAYGEI

M

I

endo,endo-PHD 0.5

0.4

(8)3.(chlorobenzene)2

IQIJUN

N

IV

endo,exo-

179.0

1.7


IQIJUN

O

IV

endo,exo-

177.9

6.2


IQIJUN

P

II

exo,exo-

1.1

1.4


IQIJUN

Q

II

exo,exo-

2.7

1.4


IQIJUN

Aromatic tilt angles The tilt angles observed for the aromatic interactions A-Q are listed in Table 1. Motifs surrounding an inversion centre (A, B, F, F,’ and I), plus the fully eclipsed stack L, have parallel aromatic planes and hence values of 0o. The remainder have tilt angles of up to 13.8o, though values of up to around 5.0o are more typical. The individual π...π motifs A-Q vary considerably in their detailed structure: not just in swivel and tilt angles, but also relating to their atomic interactions, overlap area, π-separation, and mutual orientation. The centrosymmetric motifs have short interplanar aromatic distances, C-H...π (A 3.46 Å, B 3.40 Å, I 3.56 Å) and Br...π (F 3.48 Å, F’ 3.55 Å). Ring separation increases with greater swivel angle: the 104.9o motif K 3.62-4.00 Å, and 180o motif L (3.63-3.71 Å). ACS Paragon Plus Environment

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The area of ring overlap is greatest for the 0 and 180o cases, with smaller values for intermediate swivel angles. There has been considerable discussion of the term π-stacking in the literature,911,45,46

but the Type VI assembly L is the only example here where infinite stacks of the same

orientation occur.

Crystallographically independent molecules Crystallographically independent molecules are frequently encountered in the solid state. It is usually far from clear why this additional complexity is necessary, or what precise role these play in the net crystal structure. Compound (8)3.(chlorobenzene)2 contains three independent molecules that result in the five π...π interactions M-Q. It is therefore particularly pleasing that the swivel and tilt angle values obtained (Table 1, Figure 15) provide a new method of obtaining quantitative data on this fascinating phenomenon.

Tilting and aromatic edge interactions Overlap of the edges, rather than the centroids, of aromatic rings can occur through C-H...π associations that are encouraged by the tilt angles of non-parallel π-systems. This is explored for the ‘turning point’ motif J in the crystal structure of 5. Here, two enantiomerically pure columns of 5, of opposite handedness, are interdigitated (Figure 11). There would be little incentive for π...π stacking here, and interaction occurs instead with large swivel and tilt angles of 89.4o and 13.8o, respectively. Neighbouring inter-enantiomer aromatic C...C distances are 3.26, 3.33, 3.76, 3.85, 4.28 and 4.33 Å, but these values are reversed between the next adjacent pair of enantiomers (see Figure 11). Thus tilting of the aromatic planes results in favourable contacts being produced on opposite ends of both faces. The compound (6)4.(benzene)3 contains two types of independent benzene guests (yellow and red) in different environments. Both types, however, participate in π...π interactions. Consideration of the tilting mechanism in 5 provides some rationalisation of the benzene guest inclusion behaviour. Yellow benzenes occupy an inversion centre between two A and two A* host molecules, to which they are connected by π...π (Ar-H...C, d 3.02 Å) and Ar-H...Br (d 3.18 Å) interactions (Figure 16, upper). The tilt angle is 13.6o. Red benzenes have twice the occupancy, and are linked to all four types of host molecule 6 (A, A*, B and B*) by means of Ar-H...π, π...π, and Ar-H...Br interactions. The π...π tilt angles present in the π...π part of this complex are illustrated in Figure 16, lower. This reveals the host A (light green)/B* (dark blue) value of 8.7o, and the A/red benzene 12.4o and B*/red benzene 18.4o tilt angles. Once again, tilting provides stabilisation on opposite sides of both faces of either benzene type. ACS Paragon Plus Environment


Figure 16. Upper: Part of the crystal structure of (6)4.(benzene)3 (CCDC no. 1582720) showing the centrosymmetric benzene guest (yellow) tilted at an angle of 13.6o with respect to the host molecules A and A*. Lower: The π...π tilt angles between host A and host B* (8.7o), host-guest A/red benzene 12.4o, and host-guest B*/red benzene 18.4o.


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CONCLUSION Aromatic π...π association can occur together with directional change due to non-parallel alignment of the aromatic long axes (swivel angle), and also due to the presence of non-parallel aromatic planes (tilt angle). Moderate values of swivel angles are compatible with formation of linear and planar molecular packing. Large swivel angles are less frequent and are a consequence of rarer assemblies such as zigzag chains, highly corrugated layers, and homochiral domains. Further aromatic fine-tuning results from the tilt angle. In particular, tilting can cause a switch from bimolecular to multimolecular assembly by subtending attractive forces from both faces of the aromatic ring. This data analysis was derived from our collection of diquinoline X-ray crystal structures.18,26-28 However, the various crystallographic databases catalogue many tens of thousands of additional aromatic assemblies. Swivel and tilt interactions play significant roles throughout these wider groups of materials including, for example, DNA stacking, unfolding, and intercalation.47-49

ASSOCIATED CONTENT Supporting Information X-ray crystallographic information file (CIF) for the structure of (6)4.(benzene)3 CCDC 1582720. This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

DEDICATION This paper celebrates the outstanding career of Bill Jones and his pioneering research into solidstate organic chemistry.

REFERENCES (1)

Robertson, J. M. Organic Crystals and Molecules: Theory of X-Ray Structure Analysis With Applications to Organic Chemistry, Cornell University Press: Ithaca, 1953.

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Robertson, J. M. Proc. Roy. Soc. London, Ser. A 1951, 207, 101-110. ACS Paragon Plus Environment


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Desiraju, G. R. Angew. Chem. Int. Ed. Engl. 1995, 34, 2328-2361.

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Mukherjee, A.; Dixit, K.; Sarma, S. P.; Desiraju, G. R. IUCrJ 2014, 1, 228-239.

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Nishio, M.; Umezawa, Y.; Suezawa, H.; Tsuboyama, S. The CH/π Hydrogen Bond: Implication in Crystal Engineering, in The Importance of Pi-Interactions in Crystal Engineering, Tiekink, E. R. T.; Zukerman-Schpector, J., Eds., Wiley: Chichester; 2012, Chapter 1, pp. 1-39.

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Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-5534.

(10) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885-3896. (11) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J. Chem. Soc., Perkin Trans. 2 2001, 651-669. (12) Loots, L.; Barbour, L. J., A Rudimentary Method for Classification of π...π Packing Motifs for Aromatic Molecules, in The Importance of Pi-interactions in Crystal Engineering: Frontiers in Crystal Engineering, Tiekink, E. R. T.; Zukerman-Schpector, J., Eds.; Wiley: Chichester, 2012; Chapter 4, pp. 109-124. (13) Gavezzotti, A.; Desiraju, G. R. Acta Crystallogr., Sect. B 1988, 44, 427-434. (14) Desiraju, G. R.; Gavezzotti, A. Acta Crystallogr., Sect. B 1989, 45, 473-482. (15) Desiraju, G. R.; Gavezzotti, A. J. Chem. Soc., Chem. Commun. 1989, 621-623. (16) Sakurai, T. Acta Crystallogr., Sect. B 1968, 24, 403-412. (17) Structure (4).(acetophenone), refcode QETYIY and structure 5, refcode QETZEV: Ashmore, J.; Bishop, R.; Craig, D. C.; Scudder, M. L. Cryst. Growth Des. 2007, 7, 47-55. (18) Bishop, R. New Aspects of Aromatic π...π and C-H...π Interactions in Crystal Engineering, in The Importance of Pi-interactions in Crystal Engineering: Frontiers in Crystal Engineering, Tiekink, E. R. T.; Zukerman-Schpector, J., Eds.; Wiley: Chichester, 2012; Chapter 2, pp. 4177. (19) Fischer, F. R.; Wood, P. A.; Allen, F. H.; Diederich, F. PNAS 2008, 105, 17290-17294. (20) Childs, S. L.; Wood, P. A.; Rodriguez-Hornedo, N.; Reddy, L. S.; Hardcastle, K. I. Cryst. Growth Des. 2009, 9, 1869-1888. (21) Giangreco, I.; Cole, J. C.; Thomas, E. Cryst. Growth Des. 2017, 17, 3192-3203.


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(22) Herbstein, F. H. Crystalline Molecular Complexes and Compounds: Structures and Principles, Oxford University Press: Oxford, 2005. (23) Bishop, R., Synthetic Clathrate Systems, in Supramolecular Chemistry: From Molecules to Nanomaterials, Gale, P. A.; Steed, J. W., Eds.; Wiley: Chichester, 2012; pp. 3033-3056. (24) Bishop, R. Design of Clathrate Compounds that Use Only Weak Intermolecular Attractions, Aust. J. Chem. 2012, 65, 1361-1370. (25) Bishop, R. Organic Crystal Engineering Beyond the Pauling Hydrogen Bond, CrystEngComm 2015, 17, 7448-7460. (26) Bishop, R. Supramolecular Host-Guest Chemistry of Heterocyclic V-shaped Molecules, in Heterocyclic Supramolecules II, Matsumoto, K.; Hayashi, N., Eds. Top. Heterocycl. Chem. 2009, 18, 37-74. (27) Bishop, R.; Gao, J.; Djaidi, D.; Bhadbhade, M. M., A Clathrate Uncertainty Principle, Trans. Amer. Crystal. Assn. 2012, 43, 34-44; http://www.amercrystalassn.org/content/pages/maintransactions. (28) Alshahateet, S. F.; Bhadbhade, M. M.; Bishop, R.; Craig. D. C.; Scudder, M. L. CrystEngComm 2015, 17, 9111-9122. (29) Dance, I.; Scudder, M. Chem. Commun. 1995, 1039-1040. (30) Scudder, M. L.; Goodwin, H. A.; Dance, I. G. New J. Chem. 1999, 23, 695-705. (31) Bishop, R.; Scudder, M. L.; Craig, D. C.; Rahman, A. N. M. M.; Alshateet, S. F. Mol. Cryst. Liq. Cryst. 2005, 440, 173-186. (32) Rahman, A. N. M. M.; Bishop, R.; Craig, D. C.; Scudder, M. L. CrystEngComm 2003, 5, 422428. (33) Rahman, A. N. M. M.; Bishop, R.; Craig, D. C.; Scudder, M. L. CrystEngComm 2002, 4, 510513. (34) Structure (1).(chloroform), refcode PORCOO: Marjo, C. E.; Bishop, R.; Craig, D. C.; O’Brien, A.; Scudder, M.L. J. Chem. Soc., Chem. Commun. 1994, 2513-2514. (35) Marjo, C. E.; Scudder, M.L.; Craig, D. C.; Bishop, R. J. Chem. Soc, Perkin Trans. 2 1997, 2099-2104 (36) Structure (2)2.(methylchloroform), refcode LICGIN: Rahman, A. N. M. M. Bishop, R.; Craig, D. C.; Scudder, M. L. Chem. Commun. 1999, 2389-2390. (37) Structure (2)2.(dioxane), refcode MUSDUZ: Rahman, A. N. M. M. Bishop, R.; Craig, D. C.; Scudder, M. L. Eur. J. Org. Chem. 2003, 72-81. (38) Structure (3)4.(benzene), refcode MUTREY: Marjo, C. E.; Rahman, A. N. M. M., Bishop, R.; Scudder, M. L.; Craig, D. C. Tetrahedron 2001, 57, 6289-6293. (39) Rahman, A. N. M. M., Bishop, R.; Craig, D. C.; Marjo, C. E.; Scudder, M. L. Cryst. Growth ACS Paragon Plus Environment


Des. 2002, 2, 421-426. (40) Structure (6)4.(benzene)3, CCDC no. 1582720): Gao, J. Ph.D. thesis, The University of New South Wales, Australia, 2013. (41) Structure 7, refcode TAYGEI: Gao, J.; Djaidi, D.; Marjo, C. E.; Bhadbhade, M. M.; Ung, A. T.; Bishop, R. Aust. J. Chem. 2017, 70, 538-545. (42) Structure (8)3.(chlorobenzene)2, refcode IQIJUN: Rahman, A. N. M. M.; Bishop, R.; Craig, D. C.; Scudder, M. L. Org. Biomol. Chem. 2004, 2, 175-182. (43) Kitaigorodskii, A. I., Molecular Crystals and Molecules, Academic Press, New York, 1973. (44) Bishop, R. Enantiomer Ordering and Separation During Molecular Inclusion, in Separations and Reactions in Organic Supramolecular Chemistry, Toda, F.; Bishop, R., Eds., Wiley: Chichester, 2004, Chapter 2, pp. 33-60. (45) Grimme, S. Angew. Chem. Int. Ed. 2008, 47, 3430-3434. (46) Martinez, C. R.; Iverson, B. L. Chem. Science 2012, 3, 2191-2201. (47) Matta, C. F.; Castillo, N.; Boyd, R. J. J. Phys. Chem. B 2006, 110, 563-578. (48) Yakovchuk, P.; Protozanova, E.; Frank-Kamenetskii, M. D. Nucleic Acids Res. 2006, 34, 564–574. (49) Barone, G.; Guerra, C. F.; Gambino, N.; Silvestri, A.; Lauria, A.; Almerico, A. M.; Bickelhaupt, F. M. J. Biomol. Struct. Dynamics 2008, 26, 115-129.


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For Table of Contents Use Only

Swivel and Tilt Interactions: Directional Change in Aromatic π...π Crystal Packing Roger Bishop,* Mohan M. Bhadbhade, Marcia L. Scudder and Jiabin Gao

Offset aromatic π...π overlap frequently occurs with antiparallel alignment of the molecular long axes (swivel angle 0o). Moderate values (20-35o) are also common, but high values (50-180o) usually indicate crystal structures containing sinusoidal chains, corrugated layers, unusual building blocks, or homochiral assemblies. Directional change also occurs when the planes of the interacting π-systems are mutually tilted and non-parallel. The consequences of both types are discussed.



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ILLUSTRATED CONTENTS ENTRY

Swivel and Tilt Interactions: Directional Change in Aromatic π ...π Crystal Packing Roger Bishop,* Mohan M. Bhadbhade, Marcia L. Scudder and Jiabin Gao

Offset aromatic π...π overlap frequently occurs with antiparallel alignment of the molecular long axes (swivel angle 0o). Moderate values (20-35o) are also common, but high values (50-180o) usually indicate crystal structures containing sinusoidal chains, corrugated layers, unusual building blocks, or homochiral assemblies. Directional change also occurs when the planes of the interacting π-systems are mutually tilted and non-parallel. The consequences of both types are discussed.

N

N δ−

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N 60o swivel


N 15 o tilt

Directional Change in Aromatic Ï...Ï Crystal Packing

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