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Supramolecular Recognition and Energy Frameworks in Host−Guest Complexes of 18-Crown‑6 and Sulfonamides Ming W. Shi, Sajesh P. Thomas,* George A. Koutsantonis, and Mark A. Spackman* School of Chemistry and Biochemistry, The University of Western Australia, Perth, Western Australia 6009, Australia S Supporting Information *

ABSTRACT: Crystalline molecular complexes of 18-crown-6 with three sulfonamide analogues (methane, benzene, and toluene sulfonamides) have been synthesized and characterized. Among these, the 18-crown-6:benzenesulfonamide complex exhibits three crystal forms, including a polymorphic pair. Interaction energy calculations show that the host−guest binding energies in these complexes are very high (∼92−104 kJ·mol−1). Energy framework analysis identifies the hierarchy of intermolecular interactions and their topology; a trimeric motif formed by two guest molecules and a crown host is found to be a salient structural feature in these complexes. This study establishes the “sulfonamide-crown motif” as a very robust and predictable supramolecular recognition unit.

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INTRODUCTION

Host−guest supramolecular chemistry of small organic compounds is a significant topic of research owing to their exceptional structural features and potential applications.1−8 Among these, crown ethers belong to one of the most prominent classes, as they possess specificity in their supramolecular assembly, such as cation binding.9,10 Crown ethers have found applications in selective transport and separation of metal cations,11,12 phase transfer catalysis,13 and the isolation of radioactive components of nuclear waste.14 Host−guest complexes of 18-crown-6 ethers have been utilized in a broad range of modern materials.15,16 A recent report on the exceptional ferroelectricity observed in the host−guest complex of 18-crown-6 and diisopropylanilinium chlorate17 indicates that they can act as useful building blocks in the design of novel organic ferroelectrics. Hence, 18-crown-6-based compounds and their many modes of supramolecular assembly with cations and neutral moieties remain a topic of continuing interest. While the supramolecular chemistry of ionic complexes of 18crown-6 has been explored in great detail,18,19 the binding preferences of neutral molecules with 18-crown-6 are relatively underexplored. In this paper, we analyze the interaction preferences of neutral molecules with 18-crown-6 in quantitative terms of host−guest binding energies. We discuss the molecular complexes of a series of sulfonamide analogues with 18-crown-6 analyzing the intermolecular interactions present in their crystal structures. The crystal packing in supramolecular complexes of 18-crown-6 with the following guests are discussed: (1) methanesulfonamide (MSA), (2) para-toluenesulfonamide (p-TSA), and (3) benzenesulfonamide (BSA) (Figure 1). Among these, the 18-crown-6:BSA complex (3) exhibits three crystal forms, including two polymorphs. Although the crystal structures of some © XXXX American Chemical Society

Figure 1. Molecular structures of (a) 18-crown-6, (b) MSA, (c) BSA, and (d) p-TSA.

substituted sulfonamides with 18-crown-6 have been reported in the literature, there are few quantitative studies on their host−guest interactions.20,21 The host−guest binding energies in these complexes have been estimated using model energies and energy framework module of CrystalExplorer.22 The accurate interaction energy values (based on molecular wave function calculations) have been used to generate intermolecular interaction topologies in the energy framework analysis. Thus, the hierarchy of various interaction motifs and the interaction topologies in this series of 18-crown-6 complexes have been studied via energy framework analysis. Further, electrostatic complementarity between the Received: September 11, 2015 Revised: October 22, 2015

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DOI: 10.1021/acs.cgd.5b01316 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. Crystallographic and Structure Refinement Details 1 complex formula formula weight temperature (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z density (g cm−3) μ (mm−1) F(000) crystal size 2θ range (deg) hmin, max kmin, max lmin, max R_all, R_obs wR2_all, wR2_obs Δρmin, max (e Å−3)

18-crown-6:MSA (1:2) C12H24O6 2(CH5NO2S) 454.57 100(1) monoclinic P21/n 9.06021(18) 11.41494(18) 10.5969(2) 90 100.2708(18) 90 1078.39(3) 2 1.3998 0.299 488.0 0.360.24 × 0.18 5.46−55.98 −19, 20 −20, 25 −23, 23 0.0274, 0.0297 0.0710, 0.0734 −0.41, 0.31

2 18-crown-6:p-TSA (1:2) ̀ C12H24O6 2(C7H9NO2S) 606.76 100(1) monoclinic P21/n 17.5382(3) 8.96058(15) 19.9529(3) 90 101.2588(14) 90 3075.30(8) 4 1.3104 0.228 1297.7 0.45 × 0.38 × 0.25 6.16−56.00 −29, 22 −14, 15 −33, 32 0.0382, 0.0342 0.0907, 0.0881 −0.46, 0.58

3a

3b

3c

18-crown-6:BSA (1:1) C12H24O6 C6H7NO2S 421.50 100(1) monoclinic P21/n 8.3069(5) 16.5244(8) 15.5695(7) 90 95.397(5) 90 2127.7(2) 4 1.316 0.195 904.0 0.28 × 0.21 × 0.08 3.60−54.00 −10, 10 −20, 21 −19, 16 0.0993, 0.0584 0.1002, 0.1167 −0.39, 0.33

18-crown-6:BSA (1:2) C12H24O6 2(C6H7NO2S) 578.69 100(1) monoclinic P21/c 9.5021(3) 14.5233(5) 9.8682(3) 90 94.637(3) 90 1357.39(8) 4 1.416 0.255 616 0.20 × 0.15 × 0.12 4.30−54.00 −12, 12 −16, 18 −12, 12 0.0768, 0.0491 0.1001, 0.1110 −0.34, 0.33

18-crown-6:BSA (1:2) C12H24O6 2(C6H7NO2S) 578.69 100(1) monoclinic P21/n 16.4214(5) 9.4791(3) 19.0970(5) 90 105.823(3) 90 2860.01(14) 4 1.3439 0.242 1233.7 0.2 × 0.15 × 0.12 3.84−54.00 −23, 24 −13, 14 −27, 27 0.0615, 0.0415 0.1066, 0.0929 −0.41, 0.43

Figure 2. Crystal packing in 1 and 2. (a) Trimer in 1 formed via N−H···O hydrogen bonds and (b) further crystal packing by centrosymmetric C− H···O hydrogen bonds in 1. (c) Centrosymmetric trimer formed in 2 via N−H···O hydrogen bonds. (d) Crystal packing in 2 viewed down the crystallographic c axis.

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host and guest molecules that leads to the supramolecular assembly has been examined based on electrostatic potential (ESP) calculations and Hirshfeld surface analysis.

EXPERIMENTAL SECTION

The codes for the molecular complexes discussed in this study and their formulas are given in Table 1. B



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Figure 3. (a) 1:1 molecular dimers formed by sufonamide-crown motif and stacked via C−H(BSA)···O(crown) hydrogen bonds in 3a. Parts (c) and (e) show the molecular trimers formed by sufonamide-crown motifs in 3b and 3c, respectively. Parts (b), (d), and (f) show further crystal packing in 3a, 3b, and 3c, respectively, supported predominantly by C−H···O interactions. Crystallization. The complexes 2, 3a, and 3c were crystallized by slow evaporation at room temperature (23 °C) from a methanol− ethyl acetate mixture, tetrahydrofuran, and ethanol, respectively. The crystals of complexes 1 and 3b were obtained by fast evaporation from a 1:1 mixture of methanol and ethyl acetate. The crystallization was initially performed in a small sample vial. When the solution became supersaturated, it was transferred to a Petri dish and spread out. Under the microscope, the sudden crystallization process was observed, and crystals with good quality and suitable size were chosen for X-ray diffraction. X-ray Crystallography. The crystals were cooled to 100 K with a liquid nitrogen stream using an Oxford Instruments Cryojet-HT nitrogen gas-stream cooling device. X-ray diffraction data were collected on an Oxford Xcalibur (Mova) diffractometer equipped with an Eos CCD detector using Mo Kα radiation of wavelength 0.71073 Å. The scan width was chosen to be 1° per frame, and the crystal-to-detector distance was fixed at 50 mm during the data collection. Cell refinement, data integration, and reduction were carried out using the program CrysAlisPro.23 Crystal structures were solved by direct methods and refined using SHELXS9724 accessed by the WinGX25 and OLEX2 packages.26 The phenyl ring hydrogen atoms were geometrically fixed, and all other hydrogen atoms were located from the difference Fourier map and in a later stage fixed to

standard X-ray bond length values and refined using a riding atom model.

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RESULTS AND DISCUSSION Crystal structures of the molecular complexes are described in this section with reference to their intermolecular interactions and the preferred supramolecular synthons. The ORTEPs are given in Figure S1 in the Supporting Information. Crystal structure and refinement details are given in Table 1. The intermolecular interactions are listed in Table S1 (Supporting Information). Color codes used in the diagrams: C-gray; Halmond; O-red; N-blue; S-yellow. 18-crown-6 molecules are shown in green in packing diagrams. Crystal Structure of 18-Crown-6:MSA (1). The asymmetric unit of 1 consists of one MSA molecule and half of the 18-crown-6. MSA, and 18-crown-6 molecules form a centrosymmetric trimer with a MSA molecule on either side of the crown. These trimers are stabilized by the strong N−H··· O hydrogen bonds formed between the sulfonamide −NH2 groups and the crown oxygen atoms. This also results in a short N···N distance of 3.016 Å in this trimer region across the crown ring (Figure 2a). These trimers are further assembled by means C



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one forms C−H(crown)···O(BSA) and C−H(crown)···π(BSA) interactions, while the other exhibits only a C−H(crown)···π(BSA) interaction. These trimers are further interlinked via a variety of C−H···O interaction motifs, including a bifurcated C− H(crown)···O(BSA) interaction. Supramolecular Synthons Involving Sulfonamides. Sulfonamides are known to form dimers (with centosymmetric N−H···O hydrogen bonds) or catemers (N−H···O hydrogen bonded chains).28 Recently, Thomas et al. have reported the significant strength of the sulfonamide dimer synthon in the crystal structure of the diuretic drug acetazolamide, using experimental charge density analysis.29 However, in the structures discussed in this study, the sulfonamide dimer (Figure 4a) or catemer (Figure 4b) synthons are absent, a

of the sulfonamide···sulfonamide dimer motifs (Figure 2b) formed via centrosymmetric C−H···O hydrogen bonds. One of the two sulfone oxygens in MSA acts as a bifurcated C−H···O hydrogen bond acceptor (one toward the MSA dimer and the other toward the crown), while the other oxygen atom offers only very weak borderline cases of C−H···O interactions with the crown (with interaction distances of 2.803 and 3.074 Å). Crystal Structure of 18-Crown-6:p-TSA (2). The asymmetric unit of 2 consists of two p-TSA molecules and one 18-crown-6 (two different half-molecule residues). As in the case of 1, p-TSA and 18-crown-6 form molecular trimers with p-TSA molecules on either side of the crown stabilized by the strong N−H···O hydrogen bonds (amino-crown motif). Both the symmetry-independent crown molecules exhibit this trimer formation (Figure 2c). It may be noted that the C−H··· O hydrogen bonded sulfonamide···sulfonamide motif that interlinks the molecular trimers in 1 is absent in this structure; the crystal packing is further stabilized by the formation of a bifurcated C−H···O hydrogen bond between the sulfone oxygen and the crown C−H groups (Figure 2d). Similarly, two p-TSA molecules are also interlinked via bifurcated C−H··· O hydrogen bonding. These motifs are discussed in terms of their interaction energies in a later section. Crystal Forms of 18-Crown-6:BSA (3). This complex exhibits three crystal forms. The room temperature crystal structure of a 1:1 molecular complex crystallized from tetrahydrofuran solvent (hereafter 3a) was first reported by Buchanan et al.17 This form was recrystallized for the present study in order to obtain the corresponding 100 K data for a comparison with other sulfonamide complexes and for interaction energy calculations. The major structural feature in 3a is that it does not exhibit the host:(guest)2 trimer as in cases of the other complexes discussed in this study. It forms an amino-crown N−H···O hydrogen bonded motif on one side of the BSA molecule, while forming three C−H(BSA)···O(crown) hydrogen bonds on the other side (Figure 3a). It has been shown in the literature that the C−H···O hydrogen bonds, though weak in nature, can act together in a cooperative fashion to offer stable structural motifs,27 and this explains the occurrence of this C−H(benzene)···O(crown) hydrogen bonded motif replacing the amino−crown interaction. The crystal packing in 3a is further supported by C−H(crown)···π(BSA) and C−H(crown)···O(BSA) interactions, which are discussed in detail in a later section in terms of their interaction energies. Figure 3b shows the C−H(crown)···O(BSA) hydrogen bonds that interlink the crown-BSA dimers. Another crystal form of 18-crown6:BSA, a 1:2 complex (hereafter 3b) was reported by Knöchel et al.,20 although that crystal structure determination was performed at room temperature and incomplete. The present crystal structure analysis at 100 K shows the host−guest trimer feature as anticipated. The N−H···O hydrogen bonded aminocrown motif also contains a C−H(crown)···O(BSA) hydrogen bond, which adds to the stability of the host−guest binding. Further crystal packing (Figure 3d) is facilitated by the centrosymmetric C−H···O hydrogen bonded BSA molecular dimers and BSA chains formed via head-to-tail C−H(BSA)··· O(BSA) interactions. New Polymorph of the 18-Crown-6:BSA (3c). In this 1:2 complex (hereafter 3c), the asymmetric unit consists of two BSA molecules and one 18-crown-6 ether (two different halfmolecule residues). The molecular trimer formed via an aminocrown motif is observed here as well. Of the two symmetryindependent BSA molecules forming the amino-crown motifs,

Figure 4. Synthons involving the sulfonamide functional group: (a) dimer, (b) catemeric chain, (c) C−H···O dimer, and (d) “aminocrown synthon”.

consequence of the preference for the amino-crown motif formed by the N−H(guest)···O(host) hydrogen bonds in these host−guest complexes. Especially in the case of MSA, the −CH3 group was anticipated to bind to the crown via three C− H(guest)···O(host) interactions facilitating the sulfonamide dimer on the other side. Instead, it prefers the amino-crown motif. In all five crystal structures, the amino-crown motifs are extended further via a variety of C−H···O interactions. Notably, complexes 1 and 3b form the sulfonamide C−H···O hydrogen bonded dimers (Figures 4c; 1-II and 3b-II in Figures 5 and 6). The specific synthon preferences and the hierarchy of intermolecular interactions are examined in quantitative terms of interaction energies and energy frameworks in the next section. Interaction Energies of the Molecular Dimers. We have recently described an efficient procedure for obtaining accurate model energies for intermolecular interactions in molecular crystals.30 The approach uses electron densities of unperturbed monomers to obtain accurate estimates of electrostatic, polarization, and repulsion energies, and these are combined with Grimme’s D2 dispersion corrections31 to derive a family of energy models by scaling the separate energy components to fit dispersion-corrected DFT energies for a large number of molecular pairs extracted from organic and inorganic molecular crystals. Here, we use the best performing modeldenoted CE-B3LYPwhich was shown to reproduce B3LYP-D2/631G(d,p) counterpoise-corrected energies with a mean absolute deviation (MAD) of just over 1 kJ·mol−1 but in considerably less computation time. It also performed surprisingly well (MAD of 2.5 kJ·mol−1) for a combined data set of CCSD(T)/CBS benchmark energies for 152 molecular D



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Figure 5. Molecular dimers in 1 and 2 and their interaction energies in kJ·mol−1.

sulfonamide −NH2 protons (typical pKa values of sulfonamides and amines are ∼16 and 30, respectively).38−40 The presence of the highly polarized sulfone group in sulfonamide also adds to the electrostatic component of the interaction energy. Although ether oxygen atoms are known to be weak hydrogen bond acceptors, in cyclic ethers such as 18-crown-6 cooperatively they offer stronger host−guest binding motifs. In addition, the large molecular size of the crown contributes to the dispersion component of the interaction energy (as revealed in the energy partitioning; see the Supporting Information). Further, the crystal packing is facilitated by a variety of C− H···O hydrogen bonds, as described in the previous sections, and the associated interaction energies are also given in Figures 5 and 6 and Table S4. Among these, notable cases are the C− H···O hydrogen bonded dimers in 1 and 3b, as they are similar to the secondary synthon given in Figure 4c and possess significant interaction energies (>26 kJ·mol−1 for 1-II and 3bII). 3a, the only structure in this series that does not form the host−bis(guest) trimer, is characterized by an interaction motif where the BSA molecules form three C−H···O hydrogen bonds with the crown oxygen atoms. This preference can be rationalized in terms of the significantly large interaction energy for the corresponding dimer, 3a-II, around 53 kJ·mol−1. A common packing feature in all the structures (except 3a) is the N···N short contact distances between the two guest molecules of the host−bis(guest) trimer, across the crown ring. The N···N distances vary from as short as 2.880 Å in complex 2 to 3.553 Å in complex 3b (2-VI, 3b-IV in Figures 5 and 6). Although these guest molecule pairs seem to be repulsive, the interaction energy calculations show otherwise. All such dimers,

dimers from the literature. CrystalExplorer22 was used to obtain CE-B3LYP energies using the crystal structures from the experimental charge density studies, but with bond lengths to hydrogen atoms normalized to standard neutron values. Figures 5 and 6 show various molecular groups for which CE-B3LYP interaction energies were calculated, and Table S4 in the Supporting Information lists the interaction energy values partitioned into electrostatic, repulsion, dispersion, and polarization terms. Interestingly, all the five crystal structures show very similar interaction energies for the sulfonamidecrown host−guest dimers, irrespective of the difference in guest molecule sizes. This is indicative of the modular nature of the sulfonamide-crown motif with a high value of interaction energy. The seven sulfonamide-crown dimers (1-I, 2-I, 2-II, 3aI, 3b-I, 3c-I, and 3c-II) evaluated in this series show a mean binding energy of −97 kJ·mol−1, indicating the robustness of the host−guest binding. This is remarkable as it is far greater than the corresponding dimer energy values for most of the common supramolecular synthons.32 It may be noted that the so-called strong synthons, such as carboxylic acid dimer, amide dimer, etc., have interaction energies around 60−70 kJ·mol−1.32 A well-known supramolecular recognition unit involving 18crown-6 and organic moieties is the ammonium cation:18crown-6 motif.33−35 The interaction energies for these motifs have been found to be around 200−270 kJ·mol−1.36 While these very high interaction strengths are due to the cationic binding, the corresponding neutral amine:18-crown-6 binding strengths have been found to be in the range 45−70 kJ·mol−1.37 The high binding strengths of sulfonamides as compared to amines can be understood in terms of the higher acidity of E



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Figure 6. Molecular dimers in three crystal forms of 3 (3a, 3b, and 3c) and their interaction energies in kJ·mol−1.

formed by MSA, BSA, and p-TSA molecules, are found to form attractive interactions, and as stable as 9−18 kJ·mol−1. It should be noted that the guest molecules in these dimers are related by a center of inversion, and hence, their electrostatic moments interact in a stabilizing manner (it is straightforward to imagine their individual molecular dipole moments oriented in antiparallel fashion). Similar is the case of 2-VII given in Figure 5. It has become quite common in the literature to classify such dimers as arising from “H···H interaction”. The interaction energy value of −11 kJ·mol−1 suggests that these motifs are to be visualized in terms of molecule−molecule interaction (which results predominantly from their whole-ofmolecule Coulombic and dispersion terms) rather than as atom−atom based interactions (H···H, F···F, etc.) with any directional specificity. While this is applicable to all the dimeric motifs in general, the stabilization in motifs which are bound by weak interactions like C−H···O hydrogen bonds originates predominantly from such whole-of-molecule interactions. These motifs may be viewed as consequences of stronger interactions in the crystal packing, as in the case of the molecular dimers formed by N···N short contacts that exhibit negative interaction energy values. Hence, such interactions must be distinguished from realistic interaction motifs that have consequences in crystal engineering, i.e., supramolecular motifs (synthons) with certain degrees of directional specificity, modularity, and having a predictable range of interaction energy values. In fact, the quantitative analysis in the present study itself shows that the sulfonamide-crown motif is one such modular unit, which could serve as a predictable supramolecular recognition unit. This has been further verified by a Cambridge Structural Database (CSD) analysis of the crystal structures

containing sulfonamides and compounds containing the 18crown-6 framework (CSD v5.36, February 2015). It is remarkable that 23 of the 25 structures (only organics) in the CSD form the sulfonamide-crown motifs showing the preference for this interaction pattern, indicating the existence of a sulfonamide-crown synthon. These crystal structures include those of the sulfonamide drugs such as acetazolamide and hydrochlorothiazide, pointing to the possible utilization of the sulfonamide-crown synthon in the design of novel multicomponent crystal forms of active pharmaceutical ingredients (APIs) with sulfonamide functionality. Further, it is possible to visualize the relative strengths of these interactions in terms of their topology. The energy framework analysis41 recently introduced by us provides a means to visualize the intermolecular interaction topology in molecular crystals. In this method, the pairwise intermolecular interaction energies are visualized as cylinders joining the centers of mass of the interacting molecules. The thickness of each cylinder is proportional to the relative strength of interaction energies. The topology of intermolecular interactions thus obtained is termed as an “energy framework” corresponding to the crystal structure. We have shown that energy frameworks correlate well with the known mechanical properties of the crystals.41 The interaction anisotropy in bending/shearing crystals manifests in their energy frameworks as columns or slabs. Similarly, it may also be employed to visualize the differences and similarity in crystal structures exhibiting polymorphism and isostructurality.42 Furthermore, it is possible to visualize the energy frameworks corresponding to the separate electrostatic and dispersion components involved in individual interactions. In the present study, the analysis was F



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Figure 7. Energy frameworks of 1 representing electrostatic, dispersion, and total interaction energy terms.

Figure 8. Energy frameworks of 2 and the three crystal forms of 3.

Figure 9. Hirshfeld dnorm surfaces for the host (a) and guest (b) molecules in 18-crown-6:MSA. The corresponding ESP maps are shown as (c) for the host and (d) for the guest molecules (± 0.05 au).

complexes are relatively weak and are formed by nearly similar contributions from electrostatic and dispersion terms. Thus, in analogy with the metal−organic-framework terminology, the energy framework topology in these complexes (apart from 3a) may be regarded as “zero-dimensional”. In the context of MOFs, a zero-dimensional MOF means a crystal structure for which the metal−ligand binding occurs to form a complex which does not covalently extend in any direction, whereas, in the context of energy frameworks, a “zero-dimensional” energy framework implies a crystal structure for which the strongest interactions occurs between a few molecules (in this case, the host−guest trimer), and such assemblies are further packed by relatively weaker interactions. In order to make the relevance of such dimensionality considerations clear, it is worth mentioning that many of the bending crystals studied by us exhibited onedimensional energy frameworks (molecular columns) and the shearing crystals exhibited two-dimensional energy frameworks (molecular sheets).41 Hirshfeld Surface Analysis and Host−Guest Electrostatic Complementarity. The high host−guest binding

applied to the 18-crown-6:sulfonamide complexes, which resulted in energy frameworks that clearly highlight the primary structural motif in these complexes. The strong host:(guest)2 trimeric motif, a salient feature in these complexes, manifests as the capsule-like feature in the energy frameworks of all the complexes, except that of 3a (Figures 7 and 8). The energy partitioning into components shows that this motif is dominated by the electrostatic term and supported by the dispersion contribution (see Figure S4 in the Supporting Information for the electrostatic and dispersion energy frameworks for the rest of the complexes). The host−guest dimer found in complex 3a manifests as a “half-capsule” appended by a smaller cylindrical extension. While the larger half-capsule represents a sulfonamide-crown motif, the smaller extension represents the crown-phenyl C−H interaction motif (3a-II in Figure 8). The latter has only half the strength of the former (−53 kJ·mol−1 and −104 kJ·mol−1, respectively), but it permits the p-TSA molecule to act as a bridge between the strongly bound dimers, thus creating a one-dimensional chain. The interactions that bind the trimeric capsules in these G



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molecular dimers provides insights into the secondary interaction motifs that facilitate further crystal packing. Further, ESP maps (on Hirshfeld surfaces) have provided a means to visualize the fundamental origin of host−guest supramolecular chemistry in these systems, i.e., the electrostatic complementarity between the host and guest molecules. An important outcome of this study is that it establishes the sulfonamidecrown motif as a predictable supramolecular recognition unit, which can be utilized as a design element in the crystal engineering of novel solids with neutral guest molecules and 18-crown-6.

energies of the sulfonamide-crown motif need to be understood apart from the interaction energies and the hydrogen bonds formed between the host and guest. Hence, we set out to examine the role of host−guest electrostatic complementarity in the formation of these motifs. Hirshfeld surface analysis43 has been carried out in order to visualize the directional features of the host−guest interactions. It is evident that the predominant intermolecular interactions are seen as H···O contacts, encompassing both C−H···O and N−H···O hydrogen bonds. The N−H···O hydrogen bonds that bind the host and guest can be seen as red regions in Figure 9a. Further, Hirshfeld fingerprint breakdown analysis44,45 provides the contributions from these interaction types (Supporting Information, Table S2). Thus, it is also seen that there is a notable contribution from C−H···π interactions in structures with p-TSA and BSA. Although fingerprint analysis is commonly used in the literature to compare polymorphic structures, recently, it has been shown that, in chemical analogues, the contributions from analogous intermolecular interactions are roughly conserved.46 The present series of structures when subjected to fingerprint analysis loosely show such a trend. Further, electrostatic potentials mapped on the Hirshfeld surfaces47,48 were calculated and mapped to visualize the fundamental origin of host−guest chemistry in these complexes. A representative case of complex 1 is given in Figure 9 (see the Supporting Information for similar maps for all the complexes). It can be seen that, in the observed crown-like molecular conformation of the host, three oxygen atoms of the crown pointing up interact with one guest molecule, while the remaining three oxygen atoms interact with the another guest molecule approaching from the other side of the ring. Thus, the red region in the Hirshfeld ESP surface of the host represents the electronegative regions around these three oxygen atoms. On the other hand, the guest molecule shows blue regions near the electropositive −NH2 group of the sulfonamide and red regions around sulfonate oxygens. It may be noted that there is remarkable complementarity between the ESP surfaces of the host and guest molecules. In other words, the blue regions of the guest molecules overlap with the red regions of the host molecule. This is a common feature in all the host−guest dimers presented here. Hence, it may be assumed that the host−guest supramolecular assembly in crown ether systems is strongly driven by this electrostatic complementarity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01316. CCDC numbers 1013770−1013771, 1015054−1015056. Crystallographic data for 1, 2, 3a, 3b, and 3c (CIF) ORTEPs, intermolecular interaction tables, Hirshfeld surface analysis, and energy frameworks (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.P.T.). *E-mail: [email protected] (M.A.S.). Notes

The authors declare no competing financial interest

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ACKNOWLEDGMENTS This work was supported in part by the Australian Research Council (DP130103304) and the Danish National Research Foundation (DNRF93) Center for Materials Crystallography. The authors acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments.

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CONCLUSIONS Structural analysis of the molecular complexes formed by sulfonamides and 18-crown-6 presented in this study has provided clear insights into the preferred host−guest binding mode. Evaluation of intermolecular interaction energies in the series of complexes shows the predominance of the “sulfonamide-crown” motif, due to its large interaction energy. Thus, it is shown to be superior to most of the supramolecular synthons commonly used in crystal engineering. This is substantiated by a CSD analysis of the multicomponent crystal structures containing sulfonamides and 18-crown-6, where an unambiguous preference for this motif has been observed. All five crystal structures studied here exhibit this motif, and it results in host−bis(guest) trimers (except in the case of 3a, where it results in a host−guest 1:1 dimer). The energy framework analysis clearly highlights the importance of these trimeric host−guest assemblies in discussing the crystal structures of 18-crown-6 with neutral molecules. The hierarchy of intermolecular interactions thus obtained for various H



Article

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