The structural convergence of two aromatic inclusion host families

Abstract

The Weber multi-ring aryl hydrocarbons 1–4, dihalo diheteroaryl 6,8, and tetrahalo aryl 10,12 compounds are examples of host molecules from well-known families of lattice inclusion systems. Despite these three systems having a number of features in common, no attempt has been made previously to establish their inter-relationship. In this work, the diheteroaryl ring system of 5 has been substituted with two and four pendant phenyl groups to give the non-halogenated compounds 15 and 17, respectively. In common with typical unsubstituted diheteroaryl compounds (such as 5 and 7), compound 15 shows no host properties. On the other hand, 17 can include several different guests and therefore its behaviour represents a cross-over into the Weber system. Crystal engineering aspects of the X-ray structures of 15, (17)·(chloroform) and (17)·(toluene) are analysed in this light.

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Introduction

For many years, the discovery of new lattice inclusion host molecules was the result of random accidental discovery.¹ As our knowledge of supramolecular chemistry has increased, such discovery has gradually moved towards more targeted molecular design and this evolutionary process continues.² Amongst those who have pursued this understanding of organic host behaviour, the contributions of Weber are particularly noteworthy.^3,4 His research has revealed that a wide spectrum of aromatic compounds show host properties, and that much prediction is possible in this area.^5,6 In particular, aromatic planes that are joined in combinations of edge-edge or edge-face orientations are particularly effective. Fig. 1 illustrates a selection of aromatic hydrocarbon host molecules 1–4 arising from this work.

Fig. 1 Representative examples 1–4 of Weber aromatic hydrocarbon host molecules.

In our own studies,⁷ we have demonstrated that molecules of the C₂-symmetric diheteroaryl family (shown in Fig. 2) can also be effective for generation of lattice inclusion compounds. However, substitution of the molecular periphery is necessary for this property to occur. Thus the parent compounds 5 and 7 do not show inclusion properties, whereas their halogenated derivatives 6 and 8 are very effective hosts.^8,9

Fig. 2 Examples of diheteroaryl molecules that do (6, 8), or do not (5, 7), exhibit lattice inclusion host characteristics.

This pattern of behaviour is also found for members of the tetrahalo aryl host family reported by Bishop^10–13 and by Tanaka and Toda.^14–16 Once again, the parent compounds 9 and 11 are not host molecules, whereas the substituted compounds 10 and 12 are (Fig. 3).

Fig. 3 Members of the tetrahalo aryl host family 10, 12 and their non-host parent substances 9, 11.

It is clear from these findings that the Weber multi-ring aryl hydrocarbons, dihalo diheteroaryl, and tetrahalo aryl hosts comprise closely related families of lattice inclusion compounds. The latter two groups are behaving differently, however, in that halogenation of their parent ring structure is required for inclusion. This paper describes the synthesis and behaviour of two examples of the diheteroaryl family in which the number of aromatic substituents has been systematically increased. The aim of these modifications is to demonstrate that a cross-over in properties from this group into the Weber hydrocarbon family can thereby be achieved.

Results and discussion

Preparation of the diphenyl 15, and tetraphenyl 17, derivatives

Our approach for obtaining new examples of the diheteroaryl compounds has been to conjoin two aromatic wings with a central alicyclic linker group in a one-flask reaction. Thus, two equivalents of commercial 2-aminobenzophenone 13 and one of bicyclo[3.3.1]nonane-2,6-dione¹⁷14 were condensed by means of a double acid-catalysed Friedländer reaction^18,19 to yield the targeted diphenyl derivative 15, as shown in Scheme 1. Kempter²⁰ has prepared this compound previously by heating the diketone with the amine hydrochloride at 150 °C. We find that these extreme conditions are not necessary, and that the condensation proceeds in identical yield in solution at room temperature.

Scheme 1 Synthesis of the diphenyl derivative 15. The small black spheres added to structure 15 indicate the points used for measurement of the molecular fold angle (see Table 2).

The tetraphenyl target molecule 17 was prepared from the known tetrabromide 16¹⁰ by means of quadruple Suzuki-Miyaura coupling²¹ in 58% yield, as shown in Scheme 2.

Scheme 2 Synthesis of the tetraphenyl diquinoline derivative 17. The small black spheres added to structure 17 indicate the points used for measurement of the molecular fold angle (see Table 2).

Crystal structure of the diphenyl diquinoline derivative 15

As expected from our earlier molecular design for diheteroaryl systems,⁷ the non-halogenated compound 15 showed no host properties on recrystallisation from a range of solvents. Solvent-free crystals of 15 suitable for single crystal X-ray determination were obtained in space groupP2₁/c on slow evaporation of a chloroform solution. Numerical details of the solution and refinement of this structure are presented in Table 1.

Table 1 Numerical details of the solution and refinement of the three crystal structures

Compound	15	17 and chloroform	17 and toluene
Formula	C35H26N2	(C47H34N2).(CHCl3)	(C47H34N2).(C7H8)
Formula mass	474.6	746.2	718.9
Space group	P21/c	P1̄	P1̄
a/Å	9.603(2)	10.342(4)	10.340(4)
b/Å	26.188(4)	13.641(6)	13.730(4)
c/Å	10.334(2)	14.134(6)	14.370(5)
α/°	90	103.21(3)	102.34(2)
β/°	109.838(3)	93.07(2)	92.60(2)
γ/°	90	103.41(3)	102.99(2)
V/Å^3	2445(1)	1876(1)	1933(1)
T/K	150(1)	294(1)	294(1)
Z	4	2	2
D calc./g cm^–3	1.29	1.32	1.24
Radiation, λ/Å	Mo Kα, 0.7107	Mo Kα, 0.7107	Mo Kα, 0.7107
µ/mm^–1	20.070	0.284	0.071
Scan mode	θ/2θ	θ/2θ	θ/2θ
2θmax./^°	56	50	50
No. of intensity meas.	5822	6597	6783
Criterion for obs. ref.	I/σ(I) > 2	I/σ(I) > 2	I/σ(I) > 2
No. of indep. obsd. ref.	4718	3303	3085
No. of reflections (m),	4718	3303	3085
Variables (n) in final ref.	267	323	322
R = Σ^m|ΔF|/Σ^m|Fo|	0.052	0.060	0.063
R w = [Σ^mw|ΔF|^2/Σ^mw|Fo|^2]^1/2	0.067	0.066	0.067
s = [Σ^mw|ΔF|^2/(m–n)]^1^/2	1.65	1.68	1.65
Crystal decay	none	none	none
R for mult. meas.	0.028	0.035	0.028
Largest peak in final diff. map/e Å^–3	0.51	0.75	0.62

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The structural arrangement of solid 15 is illustrated in Fig. 4. Pairs of opposite enantiomers assemble into endo-face to endo-face brick-like dimers that are arranged as layers in the bc plane. These bricks are translated along the c direction, such that molecules of the same chirality also form chains along b. Adjacent chains have opposite handedness.

Fig. 4 Part of the crystal structure of the diphenyl diquinoline derivative 15 projected on the bc plane. Atom codes: C green (opposite enantiomers are light or dark), N dark blue, H light blue. The brick-like dimers are highlighted by ellipses.

The dimeric brick formed by 15 is generally similar, but not identical, in structure to that formed by its 7α,15α-dibromo analogue (see Scheme 1 for atom numbering) described by us earlier.²² Both bricks are closely related in overall shape, size and external morphology. Three different aryl edge-face (EF) interactions^23–26 are present around an inversion centre, thereby producing an (EF)₆ dimer. In this new brick, EF₁ operates between aromatic wings of two molecules of 15 at the corners, EF₂ between a wing and a pendant phenyl group across the centre of the brick, and EF₃ between two pendant phenyl groups (Fig. 5). The principal difference between this new (EF)₆ brick and its previous version is the different rotation of the two phenyl groups protruding from the small faces (see below) of the brick. The two central phenyl rings are tilted towards the N region of the opposing molecule. However this N^⋯H distance is too long (3.06 Å) to be considered significant. Although these two phenyls are parallel, they are too displaced from each other to constitute an offset face-face (OFF) interaction.^23–26

Fig. 5 The centrosymmetric (EF)₆ brick, projected here on the bc plane, that constitutes the building-block present in solid 15, and shown in both space-filling and diagrammatic representations.

The brick comprises a parallelepiped with one small, and two large, external surfaces. For the first large surface, the exo-face of one aromatic wing forms a centrosymmetric OFF interaction with an adjacent brick. The other exo-face, on the small surface, behaves very differently (due to the protruding phenyls) and forms aryl EF interactions with two neighbouring translated molecules of opposite handedness. These two EF contacts result in undulating chains of molecules of 15 which are linked into layers by the OFF interactions of the first large surface. The second large surface of the brick interacts with adjacent layers by means of C–H^⋯N (d = 2.66 and 2.87 Å), and further aryl EF interactions.

Crystallisation of the tetraphenyl derivative 17

Attempts to crystallise 17 from a number of solvents resulted in success for the cases of dimethyl sulfoxide, 1,2-dimethoxyethane, chloroform and toluene. The crystals of the former two cases initially appeared suitable for X-ray analysis, but turned out to be flawed when examined by diffractometry. The latter two crystals proved to be isostructural 1 ∶ 1 inclusion compounds in space groupP1̄. Numerical details of the solution and refinement of these structures are also presented in Table 1.

Crystal structure of (17)·(CHCl₃)

The structural arrangement in solid (17)·(CHCl₃) is illustrated in Fig. 6. Once again, pairs of opposite enantiomers of the diquinoline derivative assemble into brick-like dimers that are assembled into layers, this time in the ab plane. These bricks are translated along the a direction, and molecules of the same chirality form chains along b. Adjacent chains have opposite handedness.

Fig. 6 Cell diagram for the crystal structure of (17).(CHCl₃) projected on the ab plane. Additional atom codes: Cl yellow, guest C purple.

The chloroform guest molecules occupy interstitial cavities present between the pendant phenyl groups and lie above and below the layers of bricks, thereby separating them from each other. The chloroform guest is oriented so that its H atom is directed towards one of the pendant phenyl rings and is approximately equidistant from all C atoms (range 2.69–3.09 Å). Pairs of guests are associated by means of two different Cl^⋯Cl interactions (3.79, 3.95 Å). There is a single C–H^⋯Cl interaction (d = 3.66, D = 4.60 Å, angle at H 159.2°) but there are no other host–guest interactions of significance.

The structure of the dimeric brick formed by the host 17 is illustrated in Fig. 7. This brick is again centrosymmetric, but there are now (Fig. 7 lower) four phenyl groups on top of the brick (red) and four below (purple). As previously, it is constructed from three different aromatic edge-face interactions EF₁–EF₃. The nature of EF₃ has changed, however. Formerly (Fig. 5), it was edge (phenyl on left wing) to face (phenyl on wing at lower edge). For 17, it is now edge (phenyl on lower edge) to face (phenyl on left wing). The directionality of this interaction is not quite as good, but it is still acceptable.

Fig. 7 The centrosymmetric (EF)₆ host brick unit in solid (17)·(CHCl₃) showing the three types of edge-face interactions EF₁-EF₃ present. This should be compared to the equivalent structure for compound 15 in Fig. 5.

There is a fundamental difference in the (EF)₆ brick construction employed by molecules 15 and 17. In the case of 15 (Fig. 5), the two wings forming a corner of the brick are rather similar and use two directionally opposed edge-face interactions (EF₁ and EF₃), but in solid 17 (Fig. 7) these operate in the same direction. This is a consequence of the different torsion angles present between the planes of the aromatic wings and the phenyl substituent groups, as discussed later in the Conclusion section. This results in the very different shapes, sizes and external morphologies adopted by individual molecules of 15 and 17 illustrated in Fig. 8.

Fig. 8 The conformations and shapes adopted by one molecule only in the dimeric (EF)₆ brick structures present in the crystal structures of 15 (left) and (17)·(CHCl₃) (right). In the latter, the two phenyl rings attached to each wing of the same molecule are approximately orthogonal.

Fig. 9 is a space filling version of (17)·(CHCl₃) (Fig. 6) but with the solvent omitted. This highlights the multitude of EF and OFF interactions present across the top surface of each host layer. Along the a direction (at b = 0) there are EF-EF-EF- interactions which occur between bricks. The double EF on the top surface of each brick is along b (at a = 0). In addition there is an OFF interaction between rings of homochiral molecules near a = b = 0.

Fig. 9 Space filling version of a host layer in (17)·(CHCl₃) (cf.Fig. 6), emphasising the multiple aryl EF and OFF interactions present. All guest molecules are omitted.

Crystal structure of (17)·(C₇H₈)

The toluene inclusion compound formed by 17 is isostructural with the chloroform case, and the cell diagram of (17)·(C₇H₈) is illustrated in Fig. 10. Despite the guest molecule being aromatic, there are no significant host–guest interactions. The guest molecule is located directly adjacent to one of the pendant phenyl rings, but the separation between the two is too great (>4.4 Å) to be considered as an OFF interaction. As in (17)·(CHCl₃), the pairs of guest molecules are located near each other, but again the distance between them (> 4.6 Å) does not suggest interaction.

Fig. 10 Part of one layer present in the crystal structure of (17)·(C₇H₈) when projected on the ab plane. The guest carbon atoms are coloured purple. This compound is isostructural with (17)·(CHCl₃) shown in Fig. 6.

Comparisons and conclusions

The molecular structure of these diquinolines has inbuilt flexibility due to the central bicyclic group, and the value of the fold angle is a measure of this property. Diquinolines with a bicyclo[3.3.1]nonane linker usually have fold angles between 80 and 115°, so the values recorded in Table 2 are unexceptional. The two values found for 17 are, however, significantly less than for 15. This reflects the ability of the molecules to adapt their conformation according to the needs imposed by host–host and host–guest packing. The packing coefficients of the inclusion compounds of 17 are also rather less than for pure 15, despite the incorporation of guest molecules in the former two structures. This is consistent with the observation that the host–guest and guest-guest interactions were not particularly significant (see above) in these compounds.

Table 2 Comparison of the three crystal structures

Properties/Compound	15	(17)·(CHCl3)	(17)·(C7H8)
a Measured between the points indicated by the black spheres added to the structures of 15 and 17 in Schemes 1 and 2. The torsion angles are measured starting at the edge of the pendant phenyl ring that is exo- to the fold of the molecule, and then heading towards the linker group. b For rings on the aromatic wing edges not containing N. c For rings on the aromatic wing edges containing N.
Diquinoline fold angle^a	108.3	96.2	94.8
Packing coefficient (%)	71.0	66.0	66.3
Torsion angles	69.1, 86.2	49.8, 130.1^b	48.8, 128.1^b
		141.4, 48.7^c	146.2, 49.1^c

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The torsion angles found in the three structures are also listed in Table 2. Those observed for 15 are approximately 70 and 90°; the orientations of the two rings being different from each other but not by a large amount. In contrast, the corresponding brominated molecules typically have torsion angles of 90 and 105°, implying that the absence of the Br substituent allows the rings to twist towards the central linker group. The torsion angles found for both host molecules of 17 presented here show that the two pendant phenyl rings on each wing are approximately orthogonal to each other (see Fig. 8).

The most significant difference found in this work is the change in inclusion properties between 15 and 17. Compound 15 is a typical member of the diheteroaryl family in not showing any host behaviour. This immediately changes, however, when bromine atoms are placed at its 7α and 15α positions.²² In marked contrast, compound 17 has been shown here to behave as a lattice inclusion host without any such requirement for halogen substitution. The halogen atoms function partly as hot spots for association with aryl rings and other halogens, and partly as spoiler groups that prevent the pure molecule packing efficiently in the crystal just by itself. Their replacement by additional aryl rings continues to provide the latter function,^3,4 such that guest inclusion is required to achieve efficient crystal packing. This molecular structure has thus crossed over into the Weber multi-ring aromatic host family. These findings represent the first experimental proof of the family relationship between these two groups of aromatic host molecules, and reveal that their different behaviour depends on the number of aromatic planes present in the test molecule.

Experimental

NMR data were recorded using a Bruker ACF300 instrument (300 MHz for ¹H, 75.4 MHz for ¹³C) as solutions in CDCl₃ at 25 °C. Proton coupling constants are given in hertz (Hz) and carbon substitution was determined using the DEPT procedure. HRMS data were recorded by Mr N. W. Proschogo using a Bruker CMS47 FTICR instrument. The microanalytical results were determined at the Australian National University, Canberra.

8,16-Diphenyl-6,7,14,15-tetrahydro-6,14-methanocycloocta[1,2-b:5,6-b']diquinoline 15

2-Aminobenzophenone 13 (2.02 g, 10.24 mmol) and bicyclo[3.3.1]nonane-2,6-dione¹⁷14 (0.71 g, 4.66 mmol) were dissolved in methanol (25 mL), and HCl (10 M; 1.0 mL) was added. The mixture was stirred at rt overnight. The resulting cream coloured solid was filtered, and washed with a small volume of methanol to yield the diquinoline 15 as a white powder (2.14 g, 97%), mp 309–311 °C (dec). Microanalytical sample recrystallised from CF₃–C₆H₅ Calc. for C₃₅H₂₆N₂ (FW = 474.6) C 88.58, H 5.52, N 5.90; Found: C 88.34, H 5.53, N 5.74%. IRν_max (paraffin mull) 3055w, 3026w, 1610w, 1569m, 1398m, 1378m, 1364w, 1351w, 1327w, 1281m, 1247m, 1221w, 1153w, 1141w, 1133w, 1072m, 1031m, 966w, 952w, 923w, 881w, 849w, 824w, 766s, 749s, 706s, 654m, 615m cm^–1; ¹H NMR (CDCl₃) δ 2.47 (br s, 2H, bridging methano), 3.13 and 3.18 (d, J_AB = 17.3 Hz, 2H), 3.21–3.29 (dd, J_AB = 17.3 Hz, J_AX = 5.7 Hz, 2H), 3.80 (br s, 2H, bridgehead), 6.93–7.00 (m, 2H), 7.18–7.24 (m, 2H), 7.25–7.30 (m, 4H), 7.36–7.44 (m, 4H), 7.45–7.52 (m, 2H), 7.54–7.60 (m, 2H), 8.02 (d, J = 8.7 Hz, 2H); ¹³C NMR (CDCl₃) δ 28.17 (CH₂), 36.67 (CH), 37.51 (CH₂), 126.09 (CH), 126.19 (CH), 126.29 (C), 127.31 (C), 128.19 (CH), 128.31 (CH), 129.02 (CH), 129.05 (CH), 129.15 (CH), 129.18 (CH), 136.49 (C), 146.31 (C), 148.76 (C), 158.26 (C), 161.34 (C); HRMS (ESI, m/z⁺): Calc. for ¹²C₃₃¹³C₂H₂₇N₂ 477.223537, Found 477.223650; Calc. for ¹²C₃₄¹³CH₂₇N₂ 476.220259, Found 476.218446; Calc. for ¹²C₃₅H₂₇N₂ 475.216757, Found 475.213985.

1,4,9,12-Tetraphenyl-6,7,14,15-tetrahydro-6,14-methanocycloocta[1,2-b:5,6-b″]diquinoline 17

The tetrabromide 16 (0.54 g, 0.85 mmol) was dissolved in toluene (10 mL). This solution was sparged with Ar to remove O₂. To this was added a solution of phenylboronic acid (0.71 g, 5.82 mmol) in ethanol (10 mL), which had also been sparged with Ar. An aqueous solution of Na₂CO₃ (10 mL; 2 mol L^–1) in boiled water (to remove O₂) was added to the mixture. The reaction flask was flushed for 30 min with Ar prior to the addition of Pd(PPh₃)₄ (0.08 g, 0.069 mmol; 8 mol%). The reaction mixture was refluxed for 72 h with stirring. After this time, the cooled solution was extracted with CH₂Cl₂ (3 × 75 mL). The combined extracts were dried (Na₂SO₄) and the CH₂Cl₂ was evaporated from the filtrate under reduced pressure. The dark toluene solution was allowed to stand. Upon cooling to rt, off-white crystals formed. These crystals were filtered and recrystallised from toluene to yield white needles of 17 (0.31 g, 0.50 mmol, 58%), mp 279–280 °C (dec). Microanalytical sample recrystallised from CH₃–C₆H₅ Calc. for (17)·(toluene)·(H₂O)_0.5 C₅₄H₄₃N₂O_0.5 (FW 728.0) C 89.10, H 5.95, N 3.85; Found: C 89.27, H 6.01, N 3.90%. IR (ν_max (paraffin mull) 1599(w), 1570 (w), 1397(m), 1377(s), 1341(m), 1236(w), 1216(s), 1180(w), 1153(m), 1072(m), 1011(s), 1031(w), 967(m), 932(m), 911(w), 848(s), 817(m), 814(m), 756(s), 694(s), 667(m), 611(w) cm^–1; ¹H NMR (300 MHz, CDCl₃) δ 2.41 (br s, 2H, bridging methano), 3.18 and 3.22 (d, ²J_AB= 16.6 Hz, 2H), 3.35–3.42 (dd, ²J_AB = 16.6 Hz, ³J = 4.5 Hz, 2H), 3.70 (d, ³J = 3.0 Hz, 2H, bridgehead), 7.38–7.56 (m, 18H, ArH), 7.69 (d, ³J = 7.5 Hz, 2H, ArH), 7.80–7.86 (m, 6H, ArH); ¹³C NMR (75.5 MHz, CDCl₃) δ 29.01 (CH₂), 36.62 (CH), 38.29 (CH₂), 126.48 (C), 126.54 (CH), 127.30 (CH), 127.69 (CH), 127.96 (CH), 128.61 (C), 128.66 (CH), 129.13 (CH), 130.28 (CH), 131.34 (CH), 134.36 (CH), 139.20 (C), 139.44 (C), 140.01 (C), 145.05 (C), two aromatic quaternary obscured; HRMS (ESI, m/z⁺): Calc. for ¹²C₄₇H₃₅N₂ 627.279519, Found 627.276235; Calc. for ¹²C₄₆¹³CH₃₅N₂ 628.282781, Found 628.281205; Calc. for ¹²C₄₅¹³C₂H₃₅N₂ 629.286027, Found 629.287634; Calc. for ¹²C₄₇H₃₄N₂Na 649.261472, Found 649.259992; Calc. for ¹²C₄₆¹³CH₃₄N₂Na 650.264717, Found 650.263694; Calc. for ¹²C₄₅¹³C₂H₃₄N₂Na 651.267978, Found 651.267304.

Structure determinations

Reflection data for 15 were measured with a Bruker SMART CCD at 150 K while those for the structures of 17 were recorded on an Enraf-Nonius CAD-4 diffractometer at 294 K. No absorption corrections were applied to the data. The positions of all atoms in the asymmetric unit were determined by direct phasing (SIR92)²⁷ with hydrogen atoms included in calculated positions with thermal motion equivalent to that for the bonded atom. For 15 all non-hydrogen atoms were refined anisotropically. For both compounds of 17, the four phenyl rings were refined as rigid groups with a 15 parameter TLX thermal group (where T is the translation tensor, L is the libration tensor and X is the origin of libration) used to the define the thermal motion of each. The remaining atoms of the hosts were refined as individual anisotropic atoms, and guest molecules were refined as rigid groups of the appropriate symmetry with TLX thermal motion. Full details of refinement²⁸ can be found in the supplementary information.†

Acknowledgements

We gratefully acknowledge financial support from the UNSW Faculty Research Grants Program.

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Footnote

† CCDC reference numbers 660521–660523. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b713854g

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