Guest-driven dimer formation of dibenzo-18-crown-6 Maija Nissinen,a Sari Kiviniemi,b Kari Rissanen*a and Jouni Pursiainen*b a Department of Chemistry, University of Jyväskylä, P.O. Box 35, FIN-40351 Jyväskylä, Finland. E-mail: kari.rissanen@jyu.fi b Department of Chemistry, University of Oulu, P.O. Box 3000, FIN-90014 Oulu, Finland. E-mail: jouni.pursiainen@oulu.fi Received 31st May 2000, Accepted 19th June 2000, Published 23rd June 2000 In the crystalline state, when the biologically important purinium cation (1) is used as a guest, dibenzo-18-crown-6 forms a selfassembled capsule-like dimer, (DB18C6)2·1, via hydrogen bonding, p–p and CH–p interactions. The formation of supramolecular, self-assembled capsules of concave self-complementary molecules has been studied on several occasions during recent years due to their possible applications in designing artificial receptors and transporting and capturing small, organic molecules.1–6 The dimerisation usually requires a suitable guest acting as a template, since an empty capsule would be thermodynamically unfavourable.7 The capsule formation, and hence the complexation of the guest inside the capsule, is driven by several simultaneously acting weak intermolecular interactions, the most typical being the hydrogen bonding.Recently, however, the increasing interest in p-stacking, cation–p, CH–p and charge transfer interactions has revealed their importance for complexation phenomena and also for the capsule formation.8–10 Scheme 1 The preparation of (DB18C6)2·1 and the crystallographic numbering.DOI: 10.1039/b004292g The dibenzo-18-crown-6 (DB18C6) is a fairly rigid, electron rich, bowl-like host and therefore suitable for the complexation of cationic, aromatic, organic guests via p- stacking and charge transfer interactions, as well as via hydrogen bonding. In our earlier studies we have utilised both of these features in preparing the complexes of DB18C6 with small cyclic, biologically important, nitrogen containing cations.11–13 An interesting feature, the formation of one-dimensional arrays in a polar crystal lattice, was observed in all these complexes. The article reports on the X-ray crystallographic and NMR studies of an exceptional, capsule-like dimer of DB18C6 with the nine-atom purinium system as a guest.Purinium, which is vastly important as a component of nucleic acid bases such as adenine and guanine, has also previously been used as a guest in artificial, biomimetic systems.14,15 The crystallisation of the complexes proved to be difficult: a few crystals of the (DB18C6)2·1 complex suitable for Xray analysis (Table 1) were obtained by slow evaporation (several months) of purinium perchlorate and DB18C6 (1 : 1 ratio) in acetonitrile–methanol solution. In the crystalline state the purinium cation was observed to facilitate the formation of the capsule-like dimer (Fig. 1). Purinium is a planar, aromatic, bicyclic cation with two hydrogen bond donating sites located at the opposite sides of the molecule.Therefore a possibility of interacting with both interaction sites of the DB18C6 (the minor site is formed by the O– CH2–CH2–O chains of the crown and the major site issituated between the phenyl rings)12,13 or even with two adjacent hosts simultaneously is obvious. Indeed, both of these features are used when the dimer is formed. The purinium cation connects two adjacent hosts via two moderately strong and one weaker hydrogen bonds [N27···O17B = 2.936(3) Å, N31···O17 = 2.864(3) Å, N31···O20 = 3.034(3) Å]. In addition, the p–p interactions between the guest and the aromatic rings of the crown ethers are observed. The distances between the centroids of the guest and the closest aromatic rings are 3.58 and 3.71 Å, indicating much stronger p-stacking interaction than with smaller cations.11–13 CrystEngComm, 2000, 18Table 1 Crystal data, data collection and refinement parameters for (DB18C6)2·1a Property Formula Formula weight Crystal colour Dimensions /mm Crystal system Space group a/Å b/Å c/Å a/° b/° g/° V/Å3 ZDc/mg m–1 ( mMo-Ka)/mm–1 T/K Measured reflections Unique reflections R1(Fo) / wR2(Fo) a The data was recorded on a Nonius Kappa CCD diffractometer using graphite monochromatised radiation.The structure was solved by direct methods (SHELXS-9720) and refinement, based on F2, was made by full-matrix least-squares techniques (SHELXL-9721). The hydrogen atoms were calculated to their idealised positions with isotropic temperature factors (1.2 or 1.5 times the C temperature factor) and refined as riding atoms.Click here for full crystallographic data (CCDC no. 1350/24). (a) (b) Fig. 1 Molecular structure of dimer (DB18C6)2·1 shown as (a) stick and (b) VDW presentations. The hydrogen bonds are shown as dashed lines. Solvent molecules and the anion are excluded for clarity. Click images or here for a 3D representation. Value 2 C20H24O6·C5H5N4+ClO4–·3 CH3CN·H2O 1082.56 Colourless 0.05 × 0.10 × 0.20 Triclinic 1 P 14.1174(6) 14.1970(6) 14.8625(5) 94.349(3) 95.682(3) 114.886(2) 2666.1(2) 21.346 0.150 173.0(1) 14023 9346 0.0592 / 0.1184 Although simultaneous hydrogen bonding to two adjacent hosts was also observed with imidazolium and triazolium cations,12,13 the remarkable difference arises from the orientation of the adjacent hosts. With five-membered cations the hosts pack head-to-tail fashion while with purinium the hosts orient head-to-head, i.e., the cavities are facing each other and form a molecular capsule.The reason for the capsulation is the larger size of the guest and the bigger distance between hydrogen bonding sites [the distance between the hydrogen bond donating N–H groups in imidazolium and triazolium is 2.11 Å while in purinium the respective distance is 3.92 Å]. Therefore the most favourable and the closest packing of the complex in this case is not the columnar packing with 90° turn between the adjacent, head-to-tail oriented hosts11–13 but head-to-head oriented hosts with aromatic rings facing each other.The formation of the strong p–p interactions between the guest and two hosts causes the hosts to be not exactly facing but slightly shifted (Fig. 1). Therefore also complex-stabilising CH–p interactions are observed between the dimer forming hosts, the closest distances between the aromatic rings being 3.65–4.00 Å. Since the aromatic parts of the capsule are facing each other, there is an open side suitable for intermolecular interaction between the crown ether chains of the dimer forming host molecules (Fig. 2). This causes that dimers themselves are also self-complementary to each other, i.e., the ether chains form loops suitable to fit the open side of the dimer and interact with the purinium cation inside the capsule.Therefore intercapsular CH···N interactions are observed between N29···C16B* = 3.53 Å and N34···C23** = 3.51 Å, thus stabilising the packing. Perchlorate anions, acetonitrile and water molecules are filling the interstice between the dimeric packages in the crystal lattice.(a) (b) Fig. 2 Self-complementary packing of (DB18C6)2·1 capsules shown as (a) stick and (b) VDW presentations. Hydrogen atoms, solvent molecules and anions are excluded for clarity. Click images or here for a 3D representation. When association constant determinations of DB18C6·1† are performed in dilute acetonitrile solution, the stoichiometry of the complex is 1 : 1 and the complex is much more stable (K = 154 M–1) than the earlier studied corresponding cation–p tropylium complex (K = 6 M–1),16 indicating strong hydrogen bonding effects in the former complex.At higher concentrations, a strong deviation from the linearity of the Benesi–Hildebrand plot was observed. The result is in agreement with the 2 : 1 host–guest stoichiometry observed in the crystal structure of the complex. For DB24C8·1 complex, which has been studied as a comparison to the DB18C6·1 complexes, the increase in the concentration did not affect the 1 : 1 ratio. The reason for this is probably the fact that during the complexation the larger DB24C8 folds completely over the guest17 and no 2 : 1 complex can be formed. However, a strong enhancement of the stability of the DB24C8·1 complex via hydrogen bonding was observed also in this case (K = 107 M–1 vs.10 M–1 of the respective tropylium complex16). The attempts to obtain crystals from the DB24C8·1 for X-ray crystal analysis were not successful. Preliminary studies by ESI mass spectrometry in acetonitrile solution support the observed stoichiometries of the complexes indicating a higher tendency for the 2 : 1 complexation of DB18C6. According to the intensities of the ESI MS peaks, when the crown ether/purinium ratio of 1 : 1 was used, the relative amount of the 2 : 1 complex with respect to the 1 : 1 complex was 12% for DB18C6 and under 1% for DB24C8 complex. In conclusion, the purinium cation acts as a suitable template for capsule-like dimer formed by two dibenzo-18- crown-6 molecules.The capsule formation is stabilised not only via the moderately strong hydrogen bonds but also via p–p interactions between the purinium guest and the host crown ethers. In addition, the CH–p interactions between the aromatic rings of the capsule forming hosts enhance the dimeric complexation. Acknowledgements Financial support by the Finnish Academy (SK) and the Ministry of Education of Finland (MN) is gratefully acknowledged. We thank Dr Jorma Jalonen and Mrs Päivi Joensuu for producing the ESI mass spectra. References 1 M. M. Conn and J. Rebek, Jr., Chem. Rev., 1997, 97, 1647. 2 J. de Mendoza, Chem. Eur. J., 1998, 4, 1373. 3 K. N. Rose, L. J. Barbour, G. W. Orr and J. L. Atwood, Chem. Commun., 1998, 407.4 J. J. González, R. Ferdani, E. Albertini, J. M. Blasco, A. Arduini, A. Pochini, P. Prados and J. de Mendoza, Chem. Eur. J., 2000, 6, 73. 5 S. K. Körner, F. C. Tucci, D. M. Rudkevich, T. Heinz and J. Rebek, Jr., Chem. Eur. J., 2000, 6, 187. 6 K. Kobayashi, T. Shirasaka, K. Yamaguchi, S. Sakamoto, E. Horn and N. Furukawa, Chem. Commun., 2000, 41. 7 R. G. Chapman and J. C. Sherman, Tetrahedron, 1997, 53, 15911. 8 J. C. Ma and D. A. Dougherty, Chem. Rev., 1997, 97, 1303. 9 K. Murayama and K. Aoki, Chem. Commun., 1998, 607. 10 A. Shivanyuk, E. F. Paulus and V. Böhmer, Angew. Chem., Int. Ed., 1999, 38, 2906. 11 M. Lämsä, J. Huuskonen, K. Rissanen and J. Pursiainen, Chem. Eur. J., 1998, 4, 84. 12 S. Kiviniemi, A. Sillanpää, M.Nissinen, K. Rissanen, M. T. Lämsä and J. Pursiainen, Chem. Commun., 1999, 897. 13 S. Kiviniemi, M. Nissinen, M. T. Lämsä, J. Jalonen, K. Rissanen and J. Pursiainen, New J. Chem., 2000, 24, 47. 14 H.-J. Schneider, T. Blatter, B. Palm, U. Pfingstag, V. Rüdinger and I. Theis, J. Am. Chem. Soc., 1992, 114, 7704. 15 J. Rebek, Jr., Angew. Chem., Int. Ed. Engl., 1990, 29, 245 and references therein. 16 M. Lämsä, J. Pursiainen, K. Rissanen and J. Huuskonen, Acta Chem. Scand., 1998, 52, 563. 17 M. Lämsä, T. Suorsa, J. Pursiainen, J. Huuskonen and K. Rissanen, Chem. Commun., 1996, 1443. 18 D. H. Bonsor, B. Borah, R. L. Dean and J. L. Wood, Can. J. Chem., 1976, 54, 2458. 19 H. A. Benesi and J. H. Hildebrand, J. Am.Chem. Soc., 1949, 71, 2703. 20 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, A46, 467. 21 G. M. Sheldrick, SHELXL-97 - A program for crystal structure refinement, 1997, University of Göttingen, Germany.Footnote Experimental procedure and selected spectroscopic data †Purinium perchlorate was prepared according to a published procedure.18 The complexes were prepared by mixing the solid guest and host (1 : 1) and then dissolving the mixture in acetonitrile. Solid complexes precipitated after the addition of diethyl ether. The stability constants were measured by 1H NMR titration in a CD3CN solution at 30 °C using the Benesi–Hildebrand least square linefitting procedure.19 1H NMR spectra were recorded on a Bruker DPX200 spectrometer. The ESI mass spectra were obtained on a LCT (Micromass Ltd) time-of-flight mass spectrometer with an OpenLynx3 data system using a desolvation temperature of 120 °C and nitrogen as nebuliser and desolvation gas. The sample cone voltage was 22 V and the instrument resolution 5000. The sample mixtures were at 0.3 mM range. Selected data for 1: Yield 58%. 1H NMR (CD3CN, 25 oC) d : 8.84 (s, 1H), 9.17 (s, 1H), 9.27 (s, 1H). Selected data for (DB18C6)2·1: Yield 65%, 1H NMR (CD3CN, 25 oC) d : 3.85 (s, OCH2, 16H), 4.10 (m, OCH2, 16H), 6.89 (s, arene, 16H), 8.82 (s, purinium, 1H), 9.16 (d, purinium, 1H), 9.30 (d, purinium, 1H). Selected data for DB24C8·1: Yield 70%, 1H NMR (CD3CN, 25 oC) d : 3.75 (s, OCH2, 8H), 3.82 (m, OCH2, 8H), 4.02 (m, OCH2, 8H), 6.79 (s, arene, 8H), 8.78 (s, purinium, 1H), 9.07 (d, purinium, 1H), 9.27 (d, purinium, 1H). CrystEngComm © The Royal Society of Chemistry 2000