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Library of Single Crystal Structures of a D3h-Symmetric Hydrocarbon Cyclophane: A Comprehensive Packing Study of Anthraphane from 30 Solvents Marco Servalli,*,† Nils Trapp,‡ Michael Solar,‡ and A. Dieter Schlüter† †

Laboratory of Polymer Chemistry, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland Laboratory of Inorganic Chemistry, Small Molecule Crystallography Center, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland

‡

S Supporting Information *

ABSTRACT: We present a comprehensive single crystal packing study of the D3h-symmetric anthracene-based hydrocarbon cyclophane “anthraphane”. It was crystallized from 30 different solvents, yielding a total of 31 co-crystals (including polymorphs), which were analyzed by SC-XRD. The obtained layered crystal structures were classified in seven different packing motifs according to the type of interactions that the anthracene units of anthraphane are engaged with, namely, CH···π edge-to-face (etf) or π···π face-to-face ( f tf) interactions. Among the seven packing motifs, three were found to be potentially photoreactive in terms of topochemical anthracene dimerization. To the best of our knowledge, this library of crystal structures is the most comprehensive crystal packing study ever performed on a single molecule and could be therefore used to validate the current methods of computational structure prediction, in particular, regarding the influence of the solvent in crystal packings.

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different co-crystals. In the study we were surprised to only find one reactive packing motif that could give rise to formation of dimers. Not satisfied by this, we therefore set out to explore the packing motifs more comprehensively by widening the palette of solvents used for crystallization. These additional 30 solvents gratifyingly furnished 16 more co-crystals and two new potentially reactive packing motifs. The main goal of this work is to provide a comprehensive picture of all packing motifs that compound 1 can be engaged in and try to understand how the solvent used for crystallization influences the crystal packing, in the hope of steering it into either the desired or at least a useful direction, possibly leading to novel photoreactive packings arising from f tf-stacking of at least one of the three anthracene units of 1. Previous studies on the crystallization of anthraphane will be complemented by additional results, providing what we believe to be the final set of packing motifs achievable for this compound. Such an encompassing library of structural data could be used for validation of theoretical packing predictions: crystal structure prediction (CSP) is still a challenge nowadays, even though considerable progress has been made with novel computational methods.17 Several advances in the field have been made by Leusen, Kendrick, and Neumann, whose algorithms were able

INTRODUCTION The recently synthesized trifunctional hydrocarbon macrocycle “anthraphane” 1 carries three anthracene units in a rigid cyclophane framework.1 Anthracenes are well studied reactive groups which upon photoexcitation tend to dimerize across their 9,10-positions, both in solution2−4 and in the solid state.5−8 Such reactions proceed via face-to-face (f tf) stacked pairs of anthracenes, the excimer of which collapses into the covalently bonded dimer. Given its conformational rigidity and the low solubility resulting from it, compound 1 suggested itself for reactivity explorations in the crystal state via topochemical reactions.9−11 Presenting three anthracene units, it potentially can give rise to dimers, linear polymers, 12 and 2D polymers13−16 depending on whether one, two, or three anthracenes of one molecule are involved in dimerization events with neighboring anthraphanes. A prerequisite for bringing about useful chemistry with compound 1 is to crystallize it into layered single crystals in which the anthracene units can assume a f tf-geometry. Figure 1 shows this for the hypothetical case of a 2D polymerization. Despite the progress achieved in crystal engineering, even today it is still a challenging matter to predict the crystal packing of a particular molecule, especially if co-crystals with the solvent used for crystallization are formed. We therefore set out to explore systematically the packing motifs compound 1 assumes upon crystallization and had previously published the results obtained using 43 different solvents resulting in 15 © XXXX American Chemical Society

Received: March 14, 2017 Revised: May 2, 2017 Published: May 8, 2017 A

DOI: 10.1021/acs.cgd.7b00367 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Chemical structure of the potential monomer 1 and the packing in a single crystal it ought to assume in order to enable 2D polymerization.

Figure 2. Seven packing motifs obtained: (a) etf packing 1, (b) etf packing 2, (c) mixed etf/f tf packing 1, (d) mixed etf/f tf packing 2, (e) mixed etf/f tf packing 3, (f) mixed etf/f tf packing 4, (g) no anthracene−anthracene interaction packing. From top to bottom: solvents from which the packing was obtained, top view of a layer in the crystal structures, space group, optical micrograph of one or more representative single crystals, layer arrangement in the crystal structure. In the layer arrangement, solvent molecules are omitted for clarity.

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RESULTS AND DISCUSSION Crystallization. Anthraphane 1 was synthesized on the 500 mg scale according to the literature procedure.1 Due to its designed high structural rigidity and lack of functional groups, the solubility of the compound is very low. In tetrachloroethane (TCE), one of the best solvents for anthraphane, the solubility was found to be as low as 0.2 mg/mL at room temperature. Compound 1 could therefore be best crystallized by slow cooling of nearly saturated solutions in high boiling point solvents. In order to have controlled cooling rates during the crystallization process, a PID controller coupled to a heating plate was used (see Figure S3). A sand bath connected to a thermocouple served as heating medium for the crystallization vials. Screw caps lined with rubber were used to tightly seal the vials, preventing solvent loss and allowing to work slightly

to correctly predict four out of four crystal structures in a blind test organized by the Cambridge Crystallographic Data Center.18,19 The comprehensive set of different crystal structures obtained for compound 1, could therefore be used to validate the current computational methods for structure prediction. This paper complements the previous study and reports the crystallization and single crystal packings of 1 obtained from additional 16 solvents, which led to the discovery of three new packing motifs. Together with the previous study, a total of 73 solvents were screened and 31 cocrystals of anthraphane were obtained, which were classified in seven different packing motifs. B



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Table 1. Crystallographic Data and Refinement Details for the Benzyl Benzoate, 2-Morpholinoethanol, L-Nicotine, L-Carvone, 1,3-Dimethoxybenzene, and 1,2-Dimethoxybenzene Co-Crystals co-crystal

benzyl benzoate

2morpholinoethanol

L-nicotine

L-carvone

1,3dimethoxybenzene

1,2dimethoxybenzene

CCDC No. Empirical formula Formula weight Temperature/K Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° Volume/Å3 Z ρcalcg/cm3 μ/mm−1 F(000) Radiation/Å 2Θ range for data collection/° Reflections collected Independent reflections Data/restraints/parameters GOF R1, wR2 [I ≥ 2σ (I)] R1, wR2 [all data] Largest diff. peak/hole/e Å−3

1505884 C94H54O4 1247.37 100.0(2) triclinic P1̅ 15.5379(3) 15.6519(3) 19.0657(4) 80.8370(10) 85.9920(10) 60.2930(10) 3975.72(14) 2 1.042 0.488 1300 1.54178 4.694 to 133.186 41984 13815 13815/452/878 1.26 0.1218, 0.3432 0.1598, 0.3748 1.54/-0.75

1536767 C95H92N5O9 1447.73 100.0(2) triclinic P1̅ 15.3160(7) 15.4447(7) 18.7437(8) 84.878(3) 88.143(3) 60.401(3) 3839.5(3) 2 1.252 0.636 1538 1.54178 4.734 to 133.434 49959 13300 13300/245/1061 1.032 0.0935, 0.2685 0.1284, 0.3065 1.05/-0.61

1536769 C106H85N8 1470.81 100.0(1) hexagonal P65 15.63140(10) 15.63140(10) 56.1655(3) 90 90 120 11884.91(16) 6 1.233 0.554 4662 1.54184 6.53 to 133.104 608214 13910 13910/795/1059 1.622 0.1144, 0.3280 0.1187, 0.3355 1.09/-0.65

1505885 C66H30 822.9 100.0(2) triclinic P1̅ 13.1888(17) 14.4544(18) 15.4509(17) 93.033(4) 102.262(4) 114.623(4) 2583.2(5) 2 1.058 0.06 852 0.71073 3.14 to 55.132 40186 11746 11746/0/595 1.082 0.0789, 0.2183 0.1110, 0.2312 0.31/-0.35


1505886 C74H40O2 961.06 100.0(2) monoclinic C2/c 20.4490(7) 21.2830(7) 24.6965(8) 90 108.524(3) 90 10191.5(6) 8 1.253 0.572 4000 1.54178 6.166 to 133.182 36572 8897 8897/0/687 1.014 0.0387, 0.0967 0.0551, 0.1063 0.19/-0.19

isophorone, 1-methylnaphthalene, tetramethylurea, tetraethylurea, N,N-diethyl-m-toluamide (DEET), L-nicotine, ethyl 2oxocyclohexanecarboxylate, and 6-carbethoxy-2,2,6-trimethylcyclohexanone. Together with the previously reported crystal structures, a total of 31 co-crystal structures (including polymorphs) were obtained for compound 1. Packing of Anthraphane in the Single Crystal. Anthraphane easily forms yellow single crystals of different morphologies in the size range of 100−500 μm, suitable for inhouse SC-XRD. The different crystal morphologies observed are solvent-dependent and are not indicative of the internal packing; different crystal morphologies can result in the same packing motif: an example in this regard is the crystals obtained from benzyl benzoate, which appear as hexagonal plates, needles, and prisms but all correspond to the same packing (Figure S7). Conversely, cases of polymorphism in which different crystal morphologies obtained from the same solvent resulted in different packings were also observed for 2cyanopyridine. For the packing analysis, the unit cells of the obtained crystals were first verified using SC-XRD, and then a complete data set was measured. In some occasions, with samples involving time-consuming measurements, only the unit cell was determined, and if identical cell parameters (within the standard uncertainties) corresponding to an already existing packing were found, then the crystals were not measured further and their packing was assumed to be the same as those already known. This was the case with the benzonitrile and 2,4,6-collidine solvates. From this study, three new packing motifs were discovered, which combine with the previously reported four for a total of seven different packing motifs. In all cases, anthraphane forms

above the boiling point of the solvent if needed. The crystallization apparatus was operated in a vibration-free environment in order to not disturb the crystallization process. A typical crystallization procedure was carried out as follows: 2−3 mg of anthraphane were put in a clean glass vial equipped with a magnetic stirring bar; 0.5−1.0 mL of solvent was added and the suspension was briefly purged with argon as a precautionary measure to avoid oxidation during the slow cooling process from the high temperatures employed. The vial was then sealed and put in the sand bath for heating at the desired temperature. Once a clear solution was obtained, the stirring was stopped and the solution was let cool down to room temperature without disturbances. Typical cooling rates were 24−36 h. Hot-filtration of the solutions prior to cooling did not result in any evident benefit for crystal quality and size, and due the high temperatures, oxygen free conditions, and small volumes of solvent involved, this challenging procedure was abandoned. If no crystals were present after the cooling process, the vials were left to rest for a few additional days at room temperature and, if needed, stored at 4 °C in the fridge or at −16 °C to promote crystallization. Solvent Choice. A total of 73 solvents were screened for crystallization (for details, see Figures S2, S4 and Table S1). The choice of the solvent was restricted by the solubility of anthraphane. We therefore chose a variety of high-boiling point solvents with different sterical and electronical properties, to see whether the crystal packing could be influenced by the inclusion of the solvent molecules. Single crystals of 1 were obtained from 16 new solvents: 1,2,3-trichloropropane, 1,2dimethoxybenzene, 1,3-dimethoxybenzene, benzonitrile, 2,4,6collidine, benzyl benzoate, 2-morpholinoethanol, L-carvone, C



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Table 2. Crystallographic Data and Refinement Details for the 1-Methylnaphthalene, 1,2,3-Trichloropropane, Isophorone, and Tetramethylurea Co-Crystals co-crystal

1-methylnaphthalene

1,2,3-trichloropropane

isophorone

tetramethylurea

CCDC No. Empirical formula Formula weight Temperature/K Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° Volume/Å3 Z ρcalcg/cm3 μ/mm−1 F(000) Radiation/Å 2Θ range for data collection/° Reflections collected Independent reflections Data/restraints/parameters GOF R1, wR2 [ I ≥ 2σ (I) ] R1, wR2 [all data] Largest diff. peak/hole/e Å−3

1505888 C77H39 964.08 100.0(2) triclinic P1̅ 13.1858(4) 14.5908(5) 15.5997(6) 77.023(3) 75.331(3) 63.147(2) 2569.15(17) 2 1.246 0.541 1002 1.54178 8.378 to 133.5 31028 8711 8711/173/705 1.659 0.1247, 0.3943 0.1660, 0.4347 1.62/-0.73

1536770 C66H30 822.9 100.0(1) triclinic P1̅ 13.2374(2) 14.3502(2) 15.2230(2) 91.9080(10) 102.8130(10) 115.301(2) 2522.21(7) 2 1.084 0.472 852 1.54184 6.886 to 140.144 94464 9553 9553/0/595 1.044 0.0578, 0.1459 0.0779, 0.1567 0.25/-0.32


1536771 C66H30 822.9 100.0(1) monoclinic C2/c 36.017(7) 15.3186(2) 26.5998(14) 90 130.926(5) 90 11088(2) 8 0.986 0.429 3408 1.54184 6.496 to 136.494 87330 10143 10143/0/595 1.097 0.0649, 0.1863 0.0815, 0.2029 0.25/-0.29

Table 3. Crystallographic Data and Refinement Details for the Tetraethylurea, 6-Carbethoxy-2,2,6-trimethylcyclohexanone (CETMC), N,N-Diethyl-m-toluamide (DEET), and Ethyl-2-oxocyclohexanecarboxylate (EOCHC) Co-Crystals co-crystal

tetraethylurea

CETMC

DEET

EOCHC

CCDC No. Empirical formula Formula weight Temperature/K Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° Volume/Å3 Z ρcalcg/cm3 μ/mm−1 F(000) Radiation/Å 2Θ range for data collection/° Reflections collected Independent reflections Data/restraints/parameters GOF R1, wR2 [I ≥ 2σ (I)] R1, wR2 [all data] Largest diff. peak/hole/e Å−3

1536772 C66H30 822.9 100.0(2) monoclinic C2/c 36.0151(15) 15.3013(5) 26.6097(9) 90 131.017(4) 90 11064.2(9) 8 0.988 0.43 3408 1.54178 6.506 to 133.434 72683 9789 9789/0/595 1.037 0.0447, 0.1112 0.0696, 0.1234 0.13/-0.20

1536773 C66H30 822.9 100.0(2) monoclinic C2/c 35.6258(10) 15.3844(5) 26.4510(13) 90 129.111(2) 90 11248.8(8) 8 0.972 0.423 3408 1.54178 6.752 to 133.428 39618 9834 9834/0/595 1.071 0.0439, 0.1232 0.0563, 0.1316 0.17/-0.14

1536774 C66H30 822.9 100.0(1) monoclinic I2/a 27.4027(3) 15.4051(2) 26.4129(3) 90 95.9160(10) 90 11090.6(2) 8 0.986 0.429 3408 1.54184 6.48 to 134.16 65782 9919 9919/0/595 1.079 0.0522, 0.1389 0.0643, 0.1460 0.19/-0.19

1536775 C75H44O3 993.1 100.0(2) monoclinic P21/n 18.8347(5) 15.5816(4) 19.4211(6) 90 90.418(2) 90 5699.5(3) 4 1.157 0.539 2072 1.54178 6.514 to 134.12 60875 10049 10049/553/856 1.12 0.0885, 0.2801 0.1281, 0.3081 0.47/-0.48

layered crystal structures. The results are summarized in Figure 2, where the packings are listed, together with the solvents from

which they are obtained, the characteristic space groups, and some representative optical micrographs of the single crystals. D



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Figure 3. Detailed view from the top of a single layer in the crystal structure of the ODCB solvate of anthraphane (left). Each anthracene unit acts as hydrogen bond donor (C−H σ bonds on the long edge) and acceptor (π surface), donating and receiving three hydrogen bonds from its two neighbors. The anthracene units thus associate into a triplex of mutual and in-plane CH···π hydrogen bonds (top right). Solvents from which this packing is obtained (bottom right).

Figure 4. Comparison of the solvates of the etf packing 1 in terms of porosity of their crystal structures. In the space-fill view, the solvent has been omitted for clarity to show the presence of the channels. The channels’ diameters range from 9 to 11 Å. From left to right: diphenylacetone, DMPU, benzyl benzoate, 2-morpholinoethanol, L-nicotine. The ODCB, TCE, and bromoform solvates do not have channels in their crystal structures.

relationship, interacting with each other by means of CH···π interactions. In the context of hydrogen bonding, it is noteworthy that each anthracene unit is involved in a twobody interaction, acting as both a hydrogen bond acceptor (through its π-surface) and donor (through its σ C−H bonds on the long edge), with two anthracene neighbors, also acting as donors and acceptors and forming the observed triangular arrangement (or triplex) of mutual CH···π hydrogen bonds seen in the crystal structure (see Figure 3). The voids between the anthraphane molecules are efficiently filled with crystallographically resolved solvent molecules. This packing is obtained with o-dichlorobenzene (ODCB), tetrachloroethane (TCE), bromoform, a polymorph of 2-cyanopyridine (needles), 1,3-dimethyltetrahydropyrimidin-2(1H)-one (DMPU), 1,3-diphenylacetone, benzyl benzoate, 2-morpholinoethanol, and L-nicotine. Interestingly, while the in-layer packing motif is the same in every co-crystal structure, the way the layers are stacked in the crystal structure can vary: depending on the solvent used, channels can be formed (Figure 4).

The packings are named according to the type of interactions between the anthracene units, namely, edge-to-face (etf) or face-to-face (f tf), and the three new packings are the etf/f tf packing 2, etf/f tf packing 3, and etf/f tf packing 4. In the following paragraphs, the packings will be discussed and compared to one another. When necessary, to describe the CH···π interactions, the distance between the H atoms and the centroids of the aromatic rings (dC−H···cn), and the distance between the H atoms and the nearest C atoms (dC−H···C) are used. For π···π interactions, the distances between the 9 and 10 positions of anthracene pairs are examined. The crystallographic data and refinement details are summarized in Table 1, Table 2, and Table 3. Additional information on the etf packing 1, etf packing 2, etf/f tf packing 1, and the no anthracene− anthracene interaction packing can be found in the previously reported study.1 etf Packing 1. The crystal system for this packing is triclinic belonging to the P1̅ space group, an exception being the Lnicotine solvate, which is hexagonal with a P65 space group. In this packing, the anthracene units are all in an exclusive etf E



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solvates can be found in the SI. Due to the exclusive etf relationship of the anthracenes, this packing can be considered nonphotoreactive. etf Packing 2. This packing is obtained exclusively when using diphenyl ether as solvent. It has a triclinic crystal system with the P1̅ space group. Anthraphane packs in layers, exclusively exhibiting etf interactions. It is characterized by a quadruplex of mutual CH···π interactions (marked in red color in Figure 6). Solvent molecules are filling the voids between the cyclophanes, in some cases interacting by π···π stacking (as seen with the blue colored anthracene units). Similarly to the case of etf packing 1, the arrangement of layers in this crystal structure forms a microporous structure with channels of approximately 6 Å in diameter. Due to the lack of f tf-stacked anthracenes, photoinduced dimerizations in this packing can be excluded. etf/f tf Packing 1. This is the most common packing found for 1 and can be obtained from nitrobenzene, 1,2,4trichlorobenzene, one polymorph of 2-cyanopyridine (platelets), o-cresol, 1,3-dimethoxybenzene, 1,2-dimethoxybenzene, benzonitrile, 1-methylnaphthalene, quinoline, 2,4,6-collidine, Lcarvone, λ-butyrolactone, ε-caprolactone, and 1,2,3-trichloropropane. The crystals are in the triclinic system with a P1̅ space group. An exception is the 1,2-dimethoxybenzene solvate, which is monoclinic in space group C2/c; the packing motif can be however considered the same as the other solvates (see Figure S26 for more details). As its name suggests, there are some anthracene units which are stacking f tf, making this packing potentially photoreactive. It is characterized by a quadruplex of CH···π interactions (marked in red and pink color in Figure 7), in which two out of three anthracene moieties are involved. An interesting feature of this packing is the pink anthracenes units in the quadruplex, which are interacting with each other through a parallel displaced π···π interaction: the distance between the 9 and 10 positions of the anthracenes ranges 4.267−4.539 Å, rendering a photodimerization between this pair unlikely for all the solvates.21 However, the remaining anthracene units in the packing (blue in Figure 7) are well paired with each other by ftf π···π interactions, with distances between the 9 and 10 positions ranging 3.638−4.000 Å. The minor displacement between these pairs would make

It seems in this case that the bulkier is the solvent, the higher is the chance to obtain channels in the structure. With ODCB, TCE, and bromoform, which are less voluminous than the other solvents, nonporous crystal structures were obtained. Bulkiness alone is not sufficient though; conformation seems also to play a role: in order to create channels, a solvent molecule able to interpenetrate the layers can be advantageous; in other words a solvent molecule has to be elongated enough to be present in at least two layers at the same time as can be seen in Figure 5 for benzyl benzoate.

Figure 5. Solvent molecules (here, benzyl benzoate, in light blue color) that can interpenetrate the layers can help to create channels. Elongated conformations are advantageous.

There is however an exception encountered with DMPU. Despite being a small molecule, the DMPU solvate contains channels as big as the ones observed with the benzoyl benzoate solvate. This implies that there must be very favorable interactions between the DMPU molecules, especially along the direction of the channels. As a general observation, the maximal diameter that the channels can have corresponds to approximately 11 Å, which corresponds to the size of the cavities between the cyclophanes. Formation and size of the channels only depends on the way the layers in the crystal structure are stacked. As a comparison, the cavities in the prototype porous material MOF-520 are approximately 10 Å in diameter. For the diphenylacetone solvate, the pore diameter is approximately 9 Å: in this specific case, the packing is however slightly different from the others, as the layers in the structures are slightly offset (see Figure S62). Details on the single

Figure 6. Detailed view from the top of a single layer in the diphenyl ether solvate of anthraphane. The typical CH···π quadruplexes are marked in red, whereas the anthracene units which are π···π stacking with the solvent molecules are marked in blue (left). Detailed view along the channels without solvent molecules in the space fill model (right). The pore diameter is approximately 6 Å. F



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Figure 7. Types of interactions in the mixed etf/f tf packing 1. The anthracenes involved in the CH···π quadruplexes are displayed in red and pink color whereas the anthracenes involved in the f tf π···π stacking are displayed in blue. Solvent molecules π···π stack with the pink and blue sets of anthracenes, efficiently filling the voids (left). Solvents from which the packing can be obtained (right).

Figure 8. Types of interactions in the mixed etf/f tf packing 2. The anthracenes involved in the f tf π···π stacking are displayed in blue color, while the other anthracenes are in red and pink color (left). Detailed view of the isophorone quadruplex in the crystal structure (bottom right). The solvent molecules are interacting via CH···π hydrogen bonds with the red and pink anthracene units. The green dotted lines represent the distances to nearest carbon atom dC−H···C.

them a potential candidate for photodimerization. Details on the single solvates can be found in the SI. etf/f tf Packing 2. This novel packing motif is obtained when using isophorone as solvent. Single crystals grow as clear yellow prismatic needles and belong to the triclinic crystal system, with the P1̅ space group. The molecules pack with both etf and f tf interactions between the anthracene units (see Figure 8). Similarly to the etf/f tf packing 1, there is a pair of anthracenes (blue color) engaged in a displaced f tf π···π interaction. This pair is slightly displaced but could in principle photodimerize topochemically as the distances between the 9 and 10 positions of the anthracenes are in a suitable range around 3.827 Å. The most interesting feature of this packing is the role of the solvent, which packs and interacts with the cyclophane in two different ways. In the first one, as seen in the etf/f tf packing 1, one solvent molecule (disordered) is sandwiched between the blue anthracene pairs efficiently filling the voids between the cyclophanes (similarly to the etf/f tf

packing 1). In the second case, the solvent molecules are organized in a quadruplex, which is encircled by the red and pink anthracene units. Through the methylene and methyl moieties, the solvent molecules are interacting with the anthracenes by CH···π hydrogen bond, forming a network of interactions as depicted in Figure 8. Typical distances ranges are dC−H···cn = 2.637−3.634 Å, and dC−H···C = 2.774−3.079 Å. When stacking, the layers do not form any channel in the crystal structure. etf/f tf Packing 3. This packing motif closely resembles the etf/f tf packing 1 and can be obtained from tetramethylurea, tetraethylurea, N,N-diethyl-m-toluamide (DEET), and 6carbethoxy-2,2,6-trimethylcyclohexanone. Single crystals grow as clear yellow rectangular prisms and belong to the monoclinic crystal system, with the C2/c space group for tetramethylurea, tetraethylurea, and 6-carbethoxy-2,2,6-trimethylcyclohexanone, and the I2/a space group for the DEET solvate. The packing is characterized by the usual quadruplex of CH···π interactions G



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Figure 9. Types of interactions in the mixed etf/f tf packing 3. The anthracenes involved in the displaced ftf π···π stacking are displayed in blue color, whereas the anthracenes involved in the CH···π quadruplexes are displayed in red and pink color (left). Front view showing the anthracene displacement of the blue pair (top right). Solvents from which this packing can be obtained (bottom right).

Figure 10. Interactions in the mixed etf/f tf packing 3. The anthracenes involved in the displaced f tf π···π stacking are displayed in blue color, whereas the anthracenes involved in the CH···π quadruplexes are displayed in red and pink colors (left). Front view showing the anthracene displacement of the pink pair (bottom right).

group. One out of three anthracene units (blue color in Figure 10) are engaged in mutual f tf π···π stacking, with distances between the 9 and 10 positions of 3.948 Å, making these pairs potentially photoreactive. In fact, accidental prolonged exposure of the crystals to light induced partial dimerization (approximately 33%) in the blue pairs (see SI, Figure S54). The remaining pink and red anthracene units both interact with the blue anthracene units via etf CH···π interactions, with typical distances ranges dC−H···cn = 2.753−2.814 Å, and dC−H···C = 3.055−3.114 Å for the red units and dC−H···cn = 3.869−3.895 Å, and dC−H···C = 2.877−3.111 Å (acetylenic carbons) for the pink units, respectively. The pink anthracene units are significantly too displaced to one another to potentially photoreact, having distances between their 9 and 10 positions of approximately 6.816 Å. The voids between the anthraphane molecules are filled with solvent molecules, some of which could not be modeled due to severe disorder, resulting in what appears to be

(marked in red and pink color in Figure 9) and by the parallel displaced π···π interaction of the blue and pink anthracene pairs. In this case, however, the displacement of these pairs is way too big to allow for any topochemical dimerization: the anthracene faces are not in contact with each other and the distances between the 9 and 10 positions are ranging between 6.345 and 6.624 Å for the blue pair and 5.374-5.676 Å for the pink pair, respectively. Solvent molecules once again occupy the voids between the anthraphanes, but unfortunately due to severe disorder they could not be modeled and were therefore masked. When stacking, the solvent filled cavities form channels of approximately 6 Å in diameter (Figures S41, S45, S49, and S53). etf/f tf Packing 4. This peculiar packing is exclusively obtained when using ethyl 2-oxocyclohexanecarboxylate as solvent. Single crystals grow as clear yellow oval plates and belong to the monoclinic crystal system, with the P21/m space H



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Figure 11. Crystal packing of 1 in the NMP solvate (left). The anthracene units are not interacting with each other; instead they are surrounded by solvent molecules with which they interact through CH···π interactions (top right). De facto, every anthraphane molecule is surrounded by a shell of 12 NMP molecules (in blue color), completely isolating it from the neighboring cyclophanes (bottom right).

Figure 12. etf/f tf Packing 1 with solvent stacking between the anthracene units. For 2-cyanopyridine and nitrobenzene, it might seem that there is donor−acceptor relationship with anthraphane. However, the same packing is also obtained with strong donors such as 1-methylnaphthalene.

an empty space between the red anthracene units in Figure 10. Upon stacking, the layers do not form any channel in the crystal structure. No Anthracene−Anthracene Interaction Packing. This packing is obtained when using NMP as solvent and is characterized by the fact that the anthracene units of 1 are not interacting with each other (Figure 11). Every anthracene is sandwiched between two NMP molecules. Additionally, every edge of an anthracene is blocked by further two NMP molecules, so that each anthraphane is tightly surrounded by 12 NMP molecules in total. This effectively prevents all interactions between anthracenes. Aliphatic CH···π interactions between anthracene and NMP are present, similarly to the ones observed in the etf/f tf packing 2, with isophorone as solvent. Considerations on the Packings. From this library of crystal structures, two clear considerations can be made: (a) apart from the different interactions the anthracene units can be engaged with each other, in every solvate, anthraphane 1 always crystallizes in well-defined layers; (b) the layers are stacking

with each other and in no case solvent molecules were found in between them. Regarding the systematic approach that we used in order to understand the solvent influence on the packings, it is difficult to identify clear trends in the obtained results. In this section we will try to relate the different packings to each other and try to define and understand some general trends regarding the solvent-packing relationship. etf Packing 1 vs etf/f tf Packing 1. The first aspect that becomes evident is that the two most common packings encountered are the etf packing 1 (with 9 solvates) and the etf/ f tf packing 1 (with 14 solvates). If we neglect completely the solvent molecules, by looking at the etf/f tf packing 1, one can see that the cyclophanes pack in the densest way possible (for a discussion on packing densities of objects with the same symmetry as anthraphane, please refer to the work of Murray22and its Supporting Information), fitting together by minimizing any unnecessary void space. In the etf packing 1, instead, the cyclophanes prefer to assemble predominantly through CH···π interactions between the anthracene units, I



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Figure 13. Formal transition from the etf/f tf packing 1 to the etf packing 2. The latter can be seen as a solvent-induced anomaly of the former.

two) in the form of two polymorphs: needles and cylindrical plates. The etf packing 1 is the result of well-defined and directional CH···π interactions; one could therefore cautiously argue that if a solvent is not able to interfere with said interactions (such as the halogenated bromoform or tetrachloroethane), this packing would be the preferred one. An exception is 1,2,3-trichlorobenzene, which prefers the etf/f tf packing 1. In the etf packing 1, the voids between the cyclophanes are bigger and can therefore accommodate more than one solvent molecule. As a consequence, the most interesting feature of this packing are the channels that can be present in the structure according to the solvate. However, in contrast to other porous crystals such as MOF-5 (and other MOFs), here the solvent molecules cannot be removed from the channels, by, for instance, vacuum,24 as the boiling point of the solvents is too high. Moreover, the intermolecular forces within and between the layers are only of weak nature (CH···π vs coordinate covalent bond in MOFs), so that removal of the solvent from the crystals by, for instance, supercritical CO2,24 would most likely result in the collapse of the structure. The same argument holds for solvent exchange in the channels; the lack of strong intermolecular forces renders the crystals brittle: already by manipulating them outside their mother liquor and trying to rinse them with different solvents, degradation and loss of crystallinity are observed. Solvent-Induced Anomalies: etf Packing 2, etf/f tf Packing 2, etf/f tf Packing 3, and No Anthracene−Anthracene Interactions Packing. Having ascertained the preference of anthraphane to pack in the etf packing 1 and the etf/f tf packing 1, what can be said for the other packings obtained, namely, the etf packing 2, etf/f tf packing 2, 3, and 4, and the special case where no anthracene−anthracene interactions occur at all? These packings have to be considered as solvent-induced anomalies, or better said, special variations of the main two packings due to specific solvent−anthraphane interactions. Let us take, for example, the etf packing 2 obtained with diphenyl ether as solvent: despite the apparent complexity of the structure, it can be related to the usual etf/f tf packing 1. Figure 13 displays this relation, where the same color code of the anthracene units has been retained in the two packings for better clarity: starting from the etf/f tf packing 1, the nitrobenzene molecules depicted in orange color are substituted with diphenyl ether; more precisely, in each cavity

forming the triangular motif discussed previously, and leaving considerable void space between the molecules. In terms of electrostatics, it seems reasonable to assume that the typical quadrupolar moment of the anthracenes is preventing the desired all-f tf packing. By looking at how the solvent is arranged in the etf/f tf packing 1, namely, stacking between the anthracene units, one could tentatively assume that this arrangement is particularly favored when using aromatic solvents showing positive electrostatic surface potentials (acceptor character) such as nitrobenzene and 2-cyanopyridine, which form donor−acceptor (D−A) complexes with the anthracene units. Such donor−acceptor interactions are in fact often responsible for f tf stacking due to the favorable interactions of the quadrupolar moments.23 However, this donor−acceptor relationship does not seem important here for two reasons: first, a very favorable D−A interaction would likely result in a crystal structure in which D and A units perfectly alternate themselves in a D-A-D-A-D fashion, something which is not observed here, where the sequence observed is D-D-A-DD-A (Figure 12); second, the same packing is obtained with milder acceptors such as quinoline and even donors such as dimethoxybenzene and 1-methylnaphthalene. Moreover, solvents lacking aromaticity such as 1,2,3-trichloropropane, GBL, ε-caprolactone, and L-carvone also produce the etf/f tf packing 1. It seems therefore that electronical factors such as polarity and aromaticity of the solvent do not play an important role for this kind of packing. Instead the important factor here is sterics, in terms of both size and shape of the solvent molecule: it seems that solvents which are flat or can easily assume a flat conformation and are small enough to fit into the cavities between the cyclophanes have a preference for this packing. As a general observation, only one solvent molecule per cavity can be accommodated. Regarding the etf packing 1, it is again not easy to find clear patterns. It seems that the argument of flatness and size of the solvent holds, as there is a preference for this packing with nonflat aliphatic solvents, such as tetrachloroethane, bromoform, 2-morpholinoethanol, and nonflat and bulky solvents, such as 1,3-diphenylacetone, L-nicotine, and benzoyl benzoate. There are a few exceptions, such as DMPU and ODCB which are flat and small and could in principle crystallize in the etf/f tf packing 1. Interestingly, 2-cyanopyridine produced both packings (indicating a small energy difference between the J



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Figure 14. Formal transition from the etf/f tf packing 1 to the etf/f tf packing 2. The latter can be seen as an expanded version of the former.

Figure 15. Formal transition from the etf/f tf packing 1 to the etf/f tf packing 3. The latter can be seen as an expanded version of the former.

Figure 16. Transition from the etf packing 1 to the packing without anthracene−anthracene interactions. The triangular motif of CH···π interactions is disrupted by the NMP molecules, which themselves form CH···π hydrogen bonds with the anthracene units.

a molecule of nitrobenzene is substituted with two molecules of diphenyl ether. The quadruplex of CH···π interactions of the pink and red colored anthracenes is retained in both packings. To complete the transition, a diphenyl ether molecule (green color) inserts itself and π···π stacks between the blue anthracene pairs at the site marked by the green dots, creating an array of ftf π···π interactions in the structure (for comparison and details on the interactions of the solvent molecules with the

same color code, please refer to Figure S65 in the SI). Thus, the etf packing 2 can be seen as a derivative of the etf/f tf packing 1. Let us now consider the etf/ftf packing 2 obtained with isophorone. This packing can also be seen as a derivative of the etf/f tf packing 1 and this time the relation is more straightforward as can be seen in Figure 14. By substituting the orange-colored nitrobenzene molecules with the quadruplex of isophorone molecules the transition between the K



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Figure 17. CH···π interactions in the ODCB, NMP, and isophorone solvates.

packings occurs. The etf/f tf packing 2 is de facto an expanded version of the etf/f tf packing 1. The relation between the etf/f tf packing 1 and etf/f tf packing 3 is also straightforward. By slightly shifting the rows of cyclophanes marked in green in Figure 15 with respect to one another, the transition between the packings occurs. This results in slightly bigger cavities in the etf/f tf packing 3, able to accommodate more sterically demanding solvent molecules such as DEET. Regarding the etf/f tf packing 4, there does not seem to be a trivial and direct relation to the other packings, rendering it the most exotic one. It is however interesting to note how a subtle structural difference in the crystallization solvent can lead to such a dramatic packing change: by removing three methyl groups from 6-carbethoxy-2,2,6-trimethylcyclohexanone, one ends up in the etf/f tf packing 4 instead of the etf/f tf packing 3. Conversely, in other cases, small structural changes in the solvent molecules do not affect the crystal packings at all such as in tetramethylurea and tetraethylurea, whereas in other cases very different solvents can result in the same packing motif. Additionally, by adding the phenomenon of polymorphism (observed for 2-cyanopyridine), one can perhaps better appreciate the complexity of this fascinating matter. Finally, by analyzing the NMP solvate, in which the anthracene units are not interacting with each other, one can see that there is a relation with the etf packing 1. In Figure 16 the etf packing 1 is displayed with its typical triangular motif of CH···π interactions (the solvent has been omitted for clarity). By inserting three NMP molecules at the site marked by the orange circle, the triangular network of CH···π interactions is disrupted, pulling apart and isolating the anthracene units from each other. The etf packing 1 is thus converted to the packing with no anthracene−anthracene interactions, which is basically an expanded version of the former. One should perhaps say a few words about the CH···π interactions that occur between aliphatic moieties of the solvent and the anthracene units: it is seemingly because of these interactions that two packing anomalies such as the NMP and isophorone solvates were discovered (Figure 17). It seems that there is a preference of the anthracene π surface to interact with the sp3-hybridized CH3 and CH2 moieties of the solvents rather than the aromatic sp2 CH moieties of the neighboring anthracenes as seen for instance in the etf packing 1. According to Nishio,25 such kind of interactions could arise due to a favorable entropic term of the CH···π hydrogen bond. These considerations should be however taken with great care: GBL, ε-caprolactone, and L-carvone also contain aliphatic moieties, but anomalous packing motifs could not be observed in their solvates. It might well be that the observed shortcontacts are simply the result of the best spatial arrangement of

the solvent molecules in the packing rather than specific and directional CH···π hydrogen bonds. Toward an All-f tf Packing. The previous discussion on the packings uncovered the heart of the matter: despite the systematic approach employed, it is not possible to clearly predict how anthraphane will pack according to the solvent used for crystallization. While it has been demonstrated that there seems to be a preference for the etf packing 1 and etf/f tf packing 1, an accurate packing prediction is not possible just by qualitatively assessing the potential interactions that the solvent might entertain with the cyclophane. For another anthracenebased D3h-symmetric cyclophane, similar problems were encountered and the desired crystal packing could not be obtained.26 As such, so far, the only reliable method for predicting a packing would involve computational structural prediction.17 Apart from resorting to rather extended (and expensive) computational methods, co-crystallization might help to achieve the desired all-f tf packing. Such packing is characterized by a typical honeycomb structure with voids of a diameter of approximately 1.4 nm. Since anthraphane 1 cannot selftemplate, it could be co-crystallized with a suitable guest molecule able to fill the voids. We considered fullerenes as possible candidates for co-crystallization due to their size and shape, since they are known to form a variety of inclusion compounds27,28 and also possess an electron acceptor character,29 which would be of advantage in terms of electrostatic interactions with the electron donor anthraphane. Regrettably, the co-crystallization experiments only yielded homocrystals of fullerene and anthraphane (confirmed by SCXRD), but no co-crystals at all. Additional analysis by confocal Raman spectroscopy on the single crystals of 1 obtained from the experiments excluded any inclusion of C60 in the crystals (Figure S68). Details on the co-crystallization procedures can be found in the Supporting Information for this paper. Even if these first results might not seem very promising, by fine-tuning the crystallization conditions and finding the appropriate solvent, there might still be a chance to obtain the desired fullerene co-crystals. Nevertheless, this procedure would involve a screening process and considerable work. As a final remark, it has to be pointed out that alternative crystallization methods such as sublimation were tried, but unfortunately exposure of anthraphane for several days to high vacuum conditions at 2 × 10−6 mbar and temperatures up to 310 °C did not yield any sublimate; instead, at temperatures above 280 °C the compound started decomposing. Crystallization from exotic and bulky solvents such as ionic liquids30,31 was also performed. However, in the two most common and apolar 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate L



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initiate program optimizations beneficial to the field of crystal engineering, where the role of the solvent in crystallization is often neglected.33,34

([bmim][PF6]), anthraphane remained completely insoluble. Further combinations of cation and anion were not investigated.

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CONCLUSIONS This work describes and compares presumably all possible packing motifs of anthraphane 1. It was driven by the wish to discover all potentially reactive packings this intriguing compound with its three photoreactive anthracene units can be engaged in. In principle three types of such packings could be envisaged depending on whether one, two, or all three of the anthracene units of compound 1 can involve themselves in solid-state photochemically induced dimerization reactions. Accordingly, dimers, linear polymers, and 2D polymers could be expected. Anthraphane 1 was therefore crystallized from 30 solvents, representing virtually the entire spectrum of possible high boiling solvents in terms of their availability, electronical character, size, and shape. It was shown that regardless of the solvent employed this compound exclusively crystallizes in layers and that the choice of solvent only influences the packing within the layers. This is a remarkable finding likely reflecting a layered arrangement to be the only way to accommodate such a propeller-shaped rigid compound in a crystal ensuring highest possible density. Of the 31 co-crystals, only two main packing motifs (etf packing 1 and etf/f tf packing 1) and five solvent specific packings (etf packing 2, etf/f tf packing 2, 3, 4, noanthracene interaction packing) were found, all characterized by different types of interaction motifs among the anthracene units of 1. In the etf/f tf packing 1, partial f tf π···π along with etf CH···π interactions were found. This packing, in which anthraphane packs in the densest way possible, was the one most observed when using solvents which are small and/or can easily assume a flat conformation, such as aromatics. In the etf packing 1 instead, the dominant interactions between anthraphane molecules were all etf CH···π; this packing seems to be preferred when using bulky, nonflat solvents. The etf packing 2, etf/f tf packing 2, 3, and the packing without anthracene−anthracene interactions were found to be variations of the two main packings due to solvent− anthraphane specific interactions. While the importance of the choice of solvent for crystallization was demonstrated, an empirical method to predict or influence the packing of anthraphane could not be developed. Nevertheless, three potentially photoreactive patterns were discovered: the etf/f tf packing 1, 2, and 4 could produce dimers and the etf/ftf packing 1 could in principle also produce a novel ladder linear polymer. As a matter of fact, accidental exposure to sunlight of the etf/f tf packing 4 already gave an indication about the photoreactivity of this packing, in which approximately 33% of the anthracene pairs dimerized. Research in this regard will be reported in due course. Packings that could be a starting point for 2D polymerization were not observed, while they were fortuitously furnished by similar investigations for other monomers.16,32 To have disclosed a library of packings of a given compound not only has relevance to synthetic issues but could also be of more fundamental value for crystal engineering. In fact, to the best of our knowledge, this is the most comprehensive crystal packing study ever performed on a single molecule and to our eyes it would therefore suggest itself to validate the available computational methods for structure prediction regarding their ability and robustness to correctly propose the impact of even subtle changes in the solvent on the packing. This could

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00367. NMR data of compound 1; details on the solvent screening procedure and crystallization procedure; crystallographic data for every co-crystal, with optical micrographs and additional information on the structures; preliminary co-crystallization experiments with fullerenes (PDF) Accession Codes

CCDC 1505883−1505889, 1536767, and 1536769−1536775 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Marco Servalli: 0000-0001-8520-7868 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was supported by the ETH Zürich, Switzerland (grant number ETH-26 10-2). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Kirill Feldman and Prof. Jan Vermant (Laboratory of Polymer Technology, ETH Zürich) for access to the optical microscopy equipment; We thank Dr. Thomas Schweizer (Institute of Polymer Chemistry, ETH Zürich) for providing the PID-controller and crystallization apparatus. We thank Feng Shao and Prof. Renato Zenobi (Laboratory of Organic Chemistry, ETH Zurich) for the confocal Raman spectroscopy measurements. Many thanks go to Dr. Michael Wörle (Small Molecule Crystallography Center, ETH Zurich), Prof. Hans-Beat Bürgi (Department of Chemistry, UZH) and Dr. Marcus Neumann (Avant Garde Materials Simulation Deutschland GmbH) for constructive and helpful discussions.

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