DOI: 10.1021/cg100831c
Structural Diversity in the Crystalline Complexes of para-Sulfonato-calix[4]arene with Bipyridinium Derivatives
2010, Vol. 10 4542–4549
Oksana Danylyuk,† Barbara Lesniewska,† Kinga Suwinska,†,§ Nabila Matoussi,‡ and Anthony W. Coleman*,‡ †
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Pl-01 224 Warszawa, e Lyon 1, Cnrs Umr 5615, 43 bvd 11 novembre, 69622 Villeurbanne, France, and Poland, ‡LMI, Universit § Faculty of Biology and Environmental Sciences, Cardinal Stefan Wyszynski University, W oycickiego 1/3, PL-01 938 Warszawa, Poland Received June 22, 2010; Revised Manuscript Received August 4, 2010
ABSTRACT: The solid-state structures of eight complexes between para-sulfonato-calix[4]arene (C4S) and bipyridinium derivatives, 4,40 -bipyridine (BP), 1,2-bis(4-pyridyl)ethane (BPE), and 1,3-bis(4-pyridyl)propane (BPP), have been determined. All the complexes show high degrees of solvation, involving water and ethanol or methanol or acetone. The observed host/guest stoichiometry is determined in part by the nature of the solvent. A wide range of noncovalent interactions are observed in the various complexes, with the dominant interaction being hydrogen bonding and aromatic-aromatic stacking.
Introduction The calix[n]arenes are one of the most widely studied classes of organic macrocyclic host systems;1 the diversity in the study of these compounds arises from their ease of synthesis and chemical modification but also from their ease of crystallization and hence structural characterization.2 The acid derivatives, such as the para-sulfonato-, O-phosphonato-, or O-alkylcarboxylatocalix[n]arenes, present the advantage of being hydrosoluble and thus are apt for study with regard to their biological properties3 and their complexation capacities toward molecules of biological interest.4 We and others have determined the solid-state structures of these molecules with a large range of protonatable ligands, including amino-acids,5 amines, and diamines,6 various active pharmaceutical ingredients (APIs),7 and heterocyclic organic molecules.8 Among the rich family of anionic water-soluble calix[n]arenes, the smallest and conformationally constrained derivative, para-sulfonato-calix[4]arene (C4S), is the most widely studied as a supramolecular host or a supramolecular building block in crystal engineering. para-Sulfonato-calix[4]arene has shown a great capacity to generate a wide range of fascinating structural motifs. Most commonly, C4S assembles in an antiparallel up-down fashion, with sulfonate groups covering the surfaces of the bilayers.9 However, spectacular perturbations from bilayer structure are induced by the presence of suitable guest molecules or ions leading to molecular capsules,10 Russian dolls,11 Ferris wheels,12 helical arrays,13 spheroids, and tubules.14 It is evident that such ease in modulating the solid-state chemistry of C4S complexes is due to the remarkable versatility of supramolecular interactions between the molecular subunits, namely, hydrogen bonding, electrostatic, coordination, aromatic-aromatic, and van der Waals interactions. In other words, it is possible to design different supramolecular architectures by subtle manipulation of the nature of the guest molecules or the crystallization media. However, to use this interplay of weak interactions in a more *Corresponding author. Telephone: þ33 4 7243 1027. Fax: þ33 4 7244 0618. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 08/25/2010
rational and reasoned way for the generation of predefined structures, studies in which the various interaction parameters are systematically modified are necessary. Such studies in building up multicomponent para-sulfonato-calix[4-8]arene complexes with organic cations in the presence and in the absence of different metal ions have already been undertaken by several research groups. Makha et al.15 have reported a systematic study on the interplay of phosphonium cations with para-sulfonato-calix[4, 6, and 8]arenes in the presence of lanthanide ions using a combinatorial approach. Ling et al. have reported a certain degree of predictability in self-assembling of pyrrolidinium16 and imidazolium cations17 bearing different terminal alkyl groups with C4S in the presence of various phosphonium cations and lanthanide salts. Liu et al.18 studied the influence of the bispyridinium guest molecules, especially the effects of their terminal groups and spacers, on the solid-state complexes with para-sulfonato-calix[5]arene. A level of control has been achieved with respect to molecular capsule formation between C4S, crown ethers, and various lanthanide cations.19 We wish to report herein our systematic investigation of the solid-state structures of C4S with bipyridinium derivatives, namely, 4,40 -bipyridine (BP), 1,2-bis(4-pyridyl)ethane (BPE), and 1,3-bis(4-pyridyl)propane (BPP). The structural difference in the three diamines allows us to probe the effect of the different spacers between pyridinium rings and the differing molecular flexibility on the interaction modes between host and guest and assembly of the complexes in the solid state. An important part of our study was to investigate the influence of crystallization medium on the resulting complex formation, trying to obtain C4S/bipyridinium crystalline complexes from different solvents. 4,40 -Bipyridine (BP) and its analogous bispyridine type ligands are capable of forming high dimensional supramolecular architectures.20 Atwood et al.21 have reported the crystal structure of C4S complex with 4,40 -bipyridine crystallized at low pH, where the calix[4]arene adopts the so-called 1,3-alternate conformation. Also, Liu et al.22 have prepared the crystalline complex of para-sulfonato-thiacalix[4]arene with 4,40 -bipyridine, where the host adopts a 1,2-alternate conformation. Here, we report for the first time several C4S crystalline r 2010 American Chemical Society
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Table 1. Crystal Data and Refinement Details for Complexes 1-8 chemical formula formula weight crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z crystal color, shape crystal size, mm3 Dcalc (g 3 cm-3) μ (mm-1) θmax (deg) data/restr/param GOF R, wR [I > 2σ(I)] R, wR (all data)
chemical formula formula mass crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z crystal color, shape crystal size, mm3 Dcalc (g 3 cm-3) μ (mm-1) θmax (deg) data/restr/param GOF R, wR [I > 2σ(I)] R, wR (all data)
1
2
3
4
C28H24O16S44-, 2C10H8N22þ, 7H2O 1183.2 triclinic P1 9.8777(3) 12.9265(4) 20.7204(6) 106.506(1) 99.693(2) 93.004(1) 2486.3(1) 2 colorless plate 0.45 0.15 0.1 1.581 0.285 24.3 7994/0/779 1.036 0.041, 0.084 0.053, 0.088
C28H24O16S44-, 2C10H8N22þ, CH4O, 8.7H2O 1240.8 monoclinic C2/c 21.361(1) 18.186(1) 28.865(3) 90 91.363(4) 90 11210(1) 8 colorless prism 0.35 0.28 0.23 1.470 0.260 23.5 8270/115/938 0.921 0.053, 0.131 0.064, 0.139
C28H24O16S44-, 2C10H8N22þ, C2H6O, 8.7H2O 1257.2 monoclinic C2/c 21.5239(4) 18.0942(6) 28.9528(8) 90 91.716(2) 90 11270.8(5) 8 colorless prism 0.3 0.2 0.2 1.482 0.259 24.7 9225/146/979 1.083 0.081, 0.174 0.104, 0.186
C28H24O16S44-, 4C10H8N2þ, C3H6O, 12.5H2O 1626.0 orthorhombic Pbca 16.8539(3) 19.4252(4) 46.020(1) 90 90 90 15066.5(5) 8 colorless prism 0.25 0.23 0.13 1.434 0.218 20.8 7891/346/1206 1.114 0.099, 0.199 0.146, 0.214
5
6
7
8
C28H24O16S44-, 1.25C12H12N22þ, 0.5C2H6O, 3.3H2O, 0.5Al(6H2O)3þ 1122.2 triclinic P1 11.5852(3) 12.3474(3) 17.3879(3) 102.7541(6) 95.0592(7) 93.755(1) 2407.2(1) 2 colorless prism 0.43 0.30 0.25 1.548 0.297 26.4 9816/132/879 1.021 0.039, 0.094 0.043, 0.096
C28H24O16S44-, 1.25C12H12N22þ, 0.5C3H6O, 3.8H2O, 0.5Al(6H2O)3þ 1135.4 triclinic P1 12.2990(2) 20.1641(6) 21.9327(6) 67.339(1) 82.253(1) 76.335(1) 4871.5(2) 4 colorless prism 0.3 0.25 0.25 1.548 0.295 24.7 16466/157/1593 1.033 0.078, 0.160 0.128, 0.181
C28H24O16S44-, 2C13H14N22þ, C3H6O, 3H2O 1253.4 monoclinic P21/c 17.5697(4) 17.2493(4) 18.6606(3) 90 92.368(1) 90 5650.5(2) 4 colorless prism 0.7 0.4 0.4 1.473 0.251 24.7 9533/0/790 1.028 0.049, 0.115 0.063, 0.122
C28H24O16S44-, 2C13H14N22þ, C2H6O, 3H2O
complexes with 4,40 -bipyridine of different stoichiometries and complexation modes in which C4S adopts the pinched cone conformation. Among the N,N0 -donors, flexible 1,2-bis(4-pyridyl)ethane (BPE) and 1,3-bis(4-pyridyl)propane (BPP) represent excellent alternatives for rigid 4,40 -bipyridine in terms of structural research. There are few known crystal complexes of calix-type hosts with these bispyridine compounds.23 A total of eight crystal complexes 1-8 were obtained, in which the influence of the guest type and crystallization medium are compared and discussed. Careful analysis of host-guest binding mode and supramolecular aggregation will help us to better understand the key factors for the construction of functional supramolecular materials. Experimental Section Crystal Growth. Suitable crystals for X-ray diffraction were grown by slow diffusion of a solution of the different guests at 3 equiv in methanol, ethanol, or acetone into a 100 mg/mL aqueous solution of para-sulfonato-calix[4]arene, C4S. The crystallization experiments were carried out at 20 °C in sealed tubes. The suitable crystals grew after several days.
1239.3 monoclinic P21/c 17.2796(5) 17.4250(5) 18.7121(4) 90 93.019(2) 90 5626.4(3) 4 colorless plate 0.35 0.13 0.06 1.463 0.252 20.8 5875/68/774 1.126 0.108, 0.173 0.134, 0.187
The colorless crystals of complexes 1 and 2 were obtained upon slow solvent diffusion in methanol-water crystallization medium. Surprisingly, two kinds of crystals were formed during a single crystallization trial: triclinic plates of complex 1 and monoclinic prisms of complex 2. The crystals of complexes 3, 5, and 8 grew upon slow solvent diffusion in water-ethanol systems. The crystals of complex 4, 6, and 7 grew from water-acetone systems. The aluminum present in the structures of complexes 5 and 6 arises from the accidental weighing of C4S on aluminum foil; verification of the kinetics of the sold state reaction shows that 24 h is required to reach a situation from which only crystals of 5 and 6 are obtained. The very strong acid nature of C4S led to a solid state extraction of aluminum cations from the weighing foil. It should be mentioned that we observed similar extraction of aluminum cations in the crystalline complex of C4S with phenanthroline.24 The reproducibility of the formation of complexes 5 and 6 was checked by repeated cell dimension determinations on different samples that appeared to be of uniform morphology. Due to the solvent dependent nature of the crystals and their fragility, it was not possible to perform X-ray powder diffraction. Unfortunately, we have not been able so far to isolate crystalline complexes of C4S with BPE in the absence of aluminum. Single-Crystal X-ray Diffraction. The crystals were mounted on the nylon loops and flash-cooled in a nitrogen stream because of the rapid solvent loss and decomposition in air. Uniformity of the samples was checked by cell determination of several crystals for
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each sample. The X-ray data were collected at 100(2) K on a Nonius Kappa CCD diffractometer using Mo KR radiation (λ = 0.71073 A˚). Data were corrected for Lorentz and polarization effects but not for absorption. Structures were solved by direct methods and refined using SHELXL-97.25 Hydrogen atoms were calculated to their idealized positions and were refined as riding atoms with isotropic thermal parameters based upon the corresponding bonding carbon atom (Uiso = 1.2Ueq, Uiso = 1.5Ueq for CH3 and OH hydrogens). Hydrogen atoms of methyl and hydroxyl groups were refined in geometric positions for which the calculated sum of the electron density is the highest (rotating group refinement). Where possible, hydrogen atoms of amino groups and water molecules were located on Fourier difference maps and refined with positional parameters. Some of the calix[4]arene sulfonate groups and guest molecules were found to be disordered over two positions and were refined with a split atom model. Because of the poor diffracting quality of some crystals and disorder, the R and wR values of several structures are high. A summary of crystal data and structure refinements is given in Table 1.
Danylyuk et al. Scheme 1. Structural Formulas for para-Sulfonato-calix[4]arene (C4S), 4,40 -Bipyridine (BP), 1,2-Bis(4-pyridyl)ethane (BPE), and 1,3-Bis(4-pyridyl)propane (BPP)
Results and Discussion The structural formulas of para-sulfonato-calix[4]arene (C4S) and the bipyridinium compounds 4,40 -bipyridine (BP), 1,2-bis(4-pyridyl)ethane (BPE), and 1,3-bis(4-pyridyl)-propane (BPP) are given in Scheme 1. Crystal Structure of Complex 1. The asymmetric unit of complex 1 comprises one crystallographically distinct C4S, one BP included into the calix[4]arene cavity, two halves of BP lying on the inversion centers, and seven water molecules. All nitrogen atoms of BP guests are protonated in order to satisfy the charge neutrality of the complex. The calix[4]arene adopts a cleft-shaped pinched cone conformation of approximate C2v symmetry which is particularly suitable for the inclusion of planar aromatic guest species.26 The dihedral angles between the opposite phenolic rings of the calix[4]arene are 36.21 and 88.85°. One of the BP molecules is slantways included into the C4S cavity. The angle made by the immersed pyridinium ring with the plane of the calix[4]arene CH2 carbon atoms is 74.17°. The penetration depth of the pyridinium ring into the cavity measured by the distance of the centroid of the aromatic ring from the plane of the calix[4]arene CH2 carbon atoms is equal to 3.882 A˚. Such deep penetration is possible due to strong N-H 3 3 3 π interaction (3.039 A˚) between the protonated nitrogen atom of the included pyridinium ring and one of the C4S phenyl rings. The inclusion is additionally stabilized by one π 3 3 3 π interaction (3.823 A˚). A dimeric arrangement of host-guest stoichiometry 2:2 is observed in which the nitrogen atom of the pyridinium ring that points out of the cavity is linked to the sulfonate group of the symmetry related C4S by an N-H 3 3 3 O hydrogen bond (2.633 A˚) (Figure 1). Taking the relative position of two face-to-face calix[4]arenes into account, complex 1 can hardly be described as a molecular capsule or even a slipped capsule because the cavity of one C4S resides over the edge of another C4S molecule. The BP dimer is arranged in a face-to-face manner with an interplanar separation of 3.318 A˚ and a centroid-centroid distance of 3.833 A˚. The BP dimer is further trapped by π-stacking with two C4S molecules (centroid-centroid distance 3.887 A˚). Such complicated and compact interaction between guest and host leads to a head-to-tail chain arrangement of C4S anions propagating along the a direction in which calix[4]arenes are connected by a strong hydrogen bond between the hydrogen atom of the hydroxyl group of one C4S and the sulfonate oxygen atom of another C4S molecule (2.799 A˚). Differing strongly from the typical bilayer arrangement, complex 1 presents a relatively complicated extended structure.
The chains of two neighboring dimers run in opposite directions and are connected by π 3 3 3 π interaction between the calix[4]arenes, forming a layer that comprises both the calix[4]arenes and included BP cations, and then form the 3-D structure via peripheral π 3 3 3 π interactions of BP located between the layers of C4S (Figure 2). These BP cations complexed exo to the host cavity are hydrogen bonded to water molecules, thus forming a hydrophilic layer. In the complex, three walls of each C4S are surrounded by the BP cations, while the fourth one is adjacent to the antiparallel C4S and linked by a π 3 3 3 π interaction. There are no distinct hydrophobic regions in the structure because of the penetration of hydrated BP cations between the C4S tetraanions. The water molecules are involved in the extensive hydrogen bonding between themselves, with the calix[4]arene sulfonate groups and BP cations, stabilizing the overall structure of the complex. Crystal Structure of Complexes 2 and 3. The structure of complex 2 is very similar to that of complex 3; the difference is in the inclusion of either methanol (2) or ethanol (3) into the C4S molecular cavity, and there is also a slight difference in the position of some disordered water molecules. The position and interaction modes between the main components of the complexes, namely the C4S tetraanion and BP cations, are the same; in view of this, we will discuss only the crystal structure of complex 3. In the asymmetric unit there are one C4S tetraanion, one fully occupied BP (named Y), two halves of BP (cation Z lies on the 2-fold axis, and X is located close to the inversion center and is disordered), one ethanol molecule disordered over two positions, and 8.7 water molecules of crystallization. Several water molecules and three of the four sulfonate groups are disordered over two positions. The dihedral angles between the opposite phenolic rings of the calix[4]arene are 58.04 and 77.80°. All the nitrogen atoms of BP molecules are protonated in order to retain the electroneutrality of the complex.
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Figure 3. View of polymeric capsules observed in complex 3.
Figure 1. Dimeric arrangement of C4S and BP cations in 1.
Figure 2. Overall structure of 1 along the a direction.
The most astonishing feature of complex 3 is the formation of multicomponent “molecular capsules”. The two cavities of C4S face each other across an inversion center, forming a dimeric arrangement whose height is about 15.9 A˚ (the distance between the top and bottom planes defined by the four phenolic oxygen atoms of C4S anions). The cavity of each C4S is occupied by a disordered ethanol molecule. The pyridinium ring of disordered BP cation X protrudes into the capsule through the pocket made by two neighboring sulfonate groups of the C4S and donates hydrogen bonds to oxygen atoms of ethanol or disordered water molecules also present inside the capsule. In other words, the protonated nitrogen atom points to the capsule center, which is made hydrophilic by the presence of ethanol hydroxyl groups and disordered water molecules. The protonated nitrogen atom of another pyridinium ring of the BP is hydrogen bonded to disordered water molecules from the neighboring capsule. The sulfonate groups of two opposite calix[4]arenes are linked together by O-H 3 3 3 O hydrogen bonds, with several water molecules acting as bridges between two C4S caps and stabilizing the capsule. The C-H 3 3 3 O hydrogen bond between the aromatic carbon atom of BP and the sulfonate oxygen of C4S (3.025 A˚) also joins two of the calix[4]arene caps together. The disordered BP cation acts as a linker holding neighboring capsules together along the equatorial orientation, as shown in Figure 3. Similar 1-D polymerization of capsules was recently observed by Liu et al.27 in the complex with benzyl viologen, in which the guest acts both as the subunit to induce the formation of the capsule and as the self-linker to hold the capsules equatorially together without any third component such as metal ion.28 But in our case the resulting capsule is multicomponent: it consists of two calix[4]arene caps, two ethanol molecules, disordered water in the capsule center, and protruded
pyridinium rings of BP cation X. The third type of BP counterion Y connects the parallel chains of polymeric capsules into a 2-D arrangement. One of the protonated nitrogen atoms of BP Y acts as a hydrogen bond donor to a sulfonate oxygen of C4S from one chain while the other one is hydrogen bonded to a water molecule that serves as a bridge joining two C4S caps from the adjacent parallel chain. For the extended structure of complex 3, C4S assembles into the well-known bilayer arrangement, in which three of the four walls of C4S are surrounded by calix[4]arene themselves, while the fourth one is adjacent to BP cation Z (Figure 4). Thus, one type of BP cation intercalates vertically into the calix[4]arene region, making the hydrophobic layer looser. The bilayer arrangement incorporating molecules other than calix[4]arenes has been observed in many multicomponent C4S complexes with positively charged guests and is welldocumented.15-17,29 There are no aromatic-aromatic interactions between neighboring calix[4]arenes (the three π 3 3 3 π distances are in the range 4.140-4.255 A˚). The intercalated Z cation is parallel to the fourth wall of the C4S and π-stacks with aromatic rings of two calix[4]arenes (3.792 A˚), forming an exo sandwich complex. The protonated nitrogen atoms of the cation Z project out from the hydrophobic layer and are hydrogen bonded to water molecules from the hydrophilic layer (distance 2.721 A˚). Most water molecules are positioned close to the plane of the sulfonate groups, forming hydrogen bonds between themselves, with the sulfonate groups and protonated BP nitrogen atoms. Surprisingly, we have found some water molecules residing deep in the hydrophobic core of the bilayer. These water molecules are situated in close proximity to intercalated BP cation in small holes between the BP and six neighboring calix[4]arenes and are acceptors of relatively strong C-H 3 3 3 O hydrogen bonds (3.141 and 3.281 A˚) from aromatic carbon atoms of the intercalated BP. As a result, the final calix[4]arene bilayer is less dense and less hydrophobic. Crystal Structure of Complex 4. The asymmetric unit comprises one C4S molecule, four monoprotonated BP molecules (one of which is disordered over the two positions) named X, Y, M, G, and Z, one acetone molecule, and 12.5 water molecules from the crystallization. The calix[4]arene adopts the pinched cone conformation with dihedral angles between the opposite phenolic rings 39.04 and 77.17°. In complex 4 (Figure 5) acetone wins the competition with BP for the inclusion into the C4S cavity. It is held inside by one weak C-H 3 3 3 O interaction and van der Waals forces. All BP cations are situated outside the cavity, forming four infinite chains, each composed of one type of crystallographically distinct monocation. These tetrachains intercalate between calix[4]arenes and cut the typical bilayers into single 1-D tapes parallel to the b direction composed of π 3 3 3 π stacked C4S tetraanions in up-down fashion. Similar cutting was previously observed in the C4S complex with 4-(6-methoxyquinoline),30 but in our case BP cations hold the C4S tapes far apart from each other; the distance between two sulfur
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Figure 4. View of the bilayer arrangement along the [110] direction (polymeric chains run perpendicular to the plane of the page) in complex 3.
Figure 5. Packing diagram of 4 viewed along the b direction (C4S tapes and BP tetrachains run perpendicular to the plane of the page).
atoms of the closest calix[4]arene tapes is 15.136 A˚. Two opposite walls of each C4S in the tape are π 3 3 3 π stacked with the neighbors (distance between centroids 3.554 A˚), while the remaining two are involved in weak π 3 3 3 π interactions with the BP cations Y (violet) (3.759 A˚) on one side and disordered Z or G (green) (3.401 A˚ or 3.758 A˚) on the other side. The interaction of C4S with BP chains is further facilitated by weak C-H 3 3 3 π hydrogen bonds between methylene bridging carbons of C4S and the aromatic rings of the BP cations. The BP in chains are held together via strong N-H 3 3 3 N hydrogen bonding between the protonated nitrogen of one molecule and the free nitrogen atom of another, with the distances ranging from 2.680 to 2.718 A˚. The chains are further π-stacked with each other or with phenyl rings of C4S molecules (centroid-centroid distances range from 3.642 to 3.767 A˚). The stacking interactions are realized between protonated and unprotonated aromatic rings of BP monocations from neighboring chains. The overall structure of 4 presents some characteristics of a layer type arrangement; however, it differs completely from the traditional bilayer assembly. The striking feature of the complex is unusually large separation of calix[4]arene strands from each other through cocrystallization with BP in one layer. Here, the hydrophobic layers are composed of the hydrophobic cores of C4S tapes π-stacked with BP tetracation chains. The water molecules are expelled into the thin hydrophilic layer and are hydrogen bonded between themselves and with sulfonate oxygen atoms of C4S anions. Crystal Structures of Complexes 5 and 6. The asymmetric unit of 5 contains one crystallographically independent C4S,
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one BPE fully occupied partially included into the C4S molecular cavity, and another BPE located inside the channels in the crystal lattice, and it was found to be in substitutional disorder with ethanol and water molecules: about half of the channels are occupied with BPE cations and the other half with solvent molecules. Moreover, this half occupied BPE cation lies close to the inversion center and was refined as disordered over two positions with 0.25 site occupancy factors. Additionally, half of the aluminum cation resides on the inversion center, a total of 6.3 water molecules of crystallization some of which are disordered over two positions, and half of an ethanol molecule are present in the asymmetric unit. The structure of complex 6 is a superstructure of complex 5 with the unit cell about twice as big as that in 5. In 6 there are two crystallographically independent inclusion complexes of BPE with two C4S entities that differ slightly in the interaction mode between the host and guest. The charge balance in both complexes is achieved through the protonation of all nitrogen atoms of BPE molecules and cocrystallization of aluminum cations. The aluminum cation is hexacoordinated with water molecules of octahedral geometry, with the metaloxygen distances being in the range 1.868-1.899 A˚. It should be noted that an aluminum(III) Keggin ion has been characterized with the same calix[4]arene as the counterion in the presence of 18-crown-6.31 As noted in the Experimental Section, the aluminum cations are present as a result of solid-solid reaction over 24 h between C4S and the aluminum foil used for weighing. The reaction is fully reproducible and is of interest for the introduction of other metals into complexes with C4S. C4S adopts a pinched cone conformation with two opposite phenyl rings directed toward the plane of the pyridinium moiety, while the other two phenyl rings are splayed apart (the dihedral angles between the opposite phenolic rings are 33.20 and 88.09°). The angle made by the immersed pyridinium ring with the plane of the calix[4]arene CH2 carbon atoms is 88.39°. The protonated nitrogen atom of the included pyridinium ring acts as donor of the N-H 3 3 3 π hydrogen bond to one of the C4S phenyl rings, with the N-centroid distance equal to 3.16 A˚. Because of the weaker N-H 3 3 3 π interaction compared to that of complex 1 with BP (N-H 3 3 3 π distance 3.039 A˚), the guest penetration into the cavity with a depth equal to 4.135 A˚ (4.141 A˚ and 4.116 A˚ in 6) is shallower than that in complex 1. Such shallow inclusion does not afford the possibility of a π 3 3 3 π stacking interaction between the included guest and the internal walls of the calix[4]arene host. As shown in Figure 6, there are two C-H 3 3 3 O interactions between the aromatic carbon atoms from both pyridinium rings of BPE and the sulfonate oxygen atoms of C4S: 3.308 A˚ for the included ring and 3.169 A˚ for the ring pointing out of the cavity. Additionally, the exo pyridinium ring donates a C-H 3 3 3 O hydrogen bond to the sulfonate oxygen atom of C4S from the adjacent layer, leading to the dimeric arrangement of host-guest stoichiometry 2:2 that can be described as a slipped capsule. The aluminum hexa-aqua cation was found to reside near the upper rims and links two C4S caps from one capsule and further connects neighboring capsules into chains propagating along the b direction via the AlOH2 3 3 3 OS hydrogen bonds with distances in the range 2.592-2.770 A˚. The pyridinium ring of BPE that protrudes out from the capsule volume is involved in a complicated interaction with two C4S anions from the adjacent layer: it is π-stacked with the external wall of one C4S (the centroid-centroid distance
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Figure 6. Interaction mode between BPE and neighboring C4S in complex 5.
is equal to 3.557 A˚), pushing the C4S molecules further apart in the layer, and it donates a N-H 3 3 3 O hydrogen bond to a sulfonate group of another C4S anion (2.707 A˚). In complex 6 the corresponding interaction between the exo pyridinium ring of molecule Y and the closest external wall of C4S is very weak: the centroid-centroid distance is equal to 3.796 A˚, but the rings are far from parallel, with the dihedral angle equal to 19.21°. To sum up, each BPE cation interacts with a total of four C4S moieties. The solid-state structure is generated by a stepped layer structural motif of the calix[4]arene molecules similar to the previously observed C4S arrangement in the complex with triethylamine.32 As shown in Figure 7, two antiparallel C4S anions that form one step interact with each other by π 3 3 3 π stacking (3.644 A˚) and hydrogen bonding between the phenolic oxygen atom of one C4S and the sulfonate oxygen atom of another C4S (2.627 A˚), in contrast to the structure with triethylamine where no such calixarene-calixarene interactions exist. Such a stepped array is probably induced by the aromatic-aromatic interaction of pyridinium rings that protrude out from the “capsules” with the external walls of neighboring capsules and thus prevent the traditional π 3 3 3 π stacking between neighboring calix[4]arenes from the same layer. There are no interactions between neighboring steps in one stepped layer, but there is hydrogen bonding between a phenolic oxygen atom of C4S from one layer and a sulfonate oxygen atom from an adjacent layer (2.911 A˚). In complex 6 each step is formed by two crystallographically distinct C4S tetra-anions. The packing of the chains of molecular capsules results in channels in the crystal lattice which run in the b direction (Figure 8). Channels with diameter of approximately 8.3 A˚ are formed between two steps from adjacent layers and are filled with disordered BPE and/or ethanol (acetone in 6) together with disordered water molecules. The “top” and “bottom” of the channel consist of the hydroxyl groups of two lower rim to lower rim oriented C4S entities from adjacent layers, and the “sides” of the channel are formed by sulfonate groups. In complex 6 two distinct hydrophilic channels running along the a direction are formed that differ by the acetone and BPE cation orientation inside. The resulting negatively charged and highly hydrophilic channels are similar to those previously observed in supramolecular arrays of para-sulfonato-calix[8]arene33 and in the structure of the C4S complex with 40 -(pyridyl)terpyridine.10b Crystal Structure of Complexes 7 and 8. The asymmetric units contain one crystallographically independent C4S tetraanion, two independent BPP dications, three water molecules,
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Figure 7. View along the b axis of the molecular packing in complex 5: noncovalent chains (running into and out from the page) pack into a stepped layer arrangement.
Figure 8. View along the b axis showing the channels within the crystal lattice in 5 (occupying disordered BPE cations and solvent molecules omitted for clarity).
and one acetone molecule (complex 7) or one ethanol molecule disordered over two positions (complex 8). C4S adopts cone conformation with the dihedral angles between opposite phenolic rings equal to 67.92 and 72.52° in complex 7 and 67.08 and 69.82° in complex 8. The host molecular cavity is occupied by acetone or ethanol molecules. Interestingly, we have found that BPP cations assemble into dimers consisting of crystallographically independent BPP via bimolecular aryl embracing.34 The supramolecular motif called “phenyl embrace” introduced by Ian Dance consists of a concerted set of interactions between phenyl groups through a combination of offset face-to-face π 3 3 3 π stacking and C-H 3 3 3 π interactions.35 Since the nitrogen heteroatom within the ring is an electron-withdrawing perturbation and the protonation of the heteroatom further enhances the electronwithdrawing effect, leading to a decrease of π-electron repulsion,36 these aryl embraces are attractive.37 However, the presence of cationic charges on the aryl rings together with geometrical restrictions prevents perfect face-to-face π-stacking, leading to the slipped arrangements of pyridinium rings with only partial ring-plane overlap (the plane-plane angles are 16.65° and 28.16°) with the closest N-C atom approaches of 3.324 A˚ and 3.353 A˚ (Figure 9). Evidently, the presence of a π-rich electron cavity of C4S prevents further multimolecular aromatic BPP interactions; thus, the outer pyridinium surfaces of the dimer are involved in π 3 3 3 π interactions with the external walls of the C4S tetraanions. The BPP dimer interacts with four neighboring calix[4]arenes by π-stacking and two calix[4]arenes via hydrogen bonding between two protonated nitrogen atoms, one from each BPP, and the sulfonate oxygen of C4S. The remaining two pyridinium rings donate hydrogen bonds to water molecules present in the structure.
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Symmetry expansion around the C4S tetraanion reveals that the calix[4]arene hydrophobic core sits in the cage formed by four surrounding BPP dimers (Figure 10). Three of the four aromatic interactions between C4S and BPP are quite strong (the centroid-centroid distances are 3.421, 3.509, and 3.677 A˚), and one aromatic-aromatic contact is very weak, with the centroid-centroid distance equal to 4.042 A˚ and aryl rings far from being parallel (the dihedral angle is 22.63°). The anionic upper rim of the C4S interacts with another set of BPP dimers by two N-H 3 3 3 O hydrogen bonds (2.705 A˚ and 2.765 A˚) and numerous C-H 3 3 3 O interactions. Such an interaction mode between the host and BPP dimers leads to the head-to-tail C4S arrangement into separate columns composed of C4S-acetone repeating units running along the c direction which are separated by the columns of
Figure 9. Space filling representation of the BPP embrace.
Figure 10. C4S in the cage formed by four BPP dimers.
Danylyuk et al.
BPP dimers (Figure 11). In the C4S column, every second calix[4]arene is reflected across the mirror plane and translated 1/2 in the c direction in relation to the previous one (the calix[4]arene lies about the glide plane perpendicular to b direction). The neighboring columns are linked through the set of C-H 3 3 3 π interactions between antiparallel calixarenes. The guest molecules are embedded in the columnar array and stabilize the structure by hydrogen bonding and extensive π 3 3 3 π interactions. The lattice water molecules additionally stabilize the supramolecular arrangement via hydrogen bonding with oxygen atoms of the C4S upper rim, BPP cations, and acetone. Conclusion In summary, we have shown the remarkable structural diversity in the C4S/bipyridinium solid-state complexes induced both by the differing guest flexibility induced by different spacer lengths between pyridinium rings and by the choice of crystallization medium. In the system C4S/4,40 -bipyridine the crystal structures of four new pseudopolymorphs have been determined; all of them are highly solvated in contrast to the complex reported by Atwood et al.16 The crystal structures of four C4S complexes with flexible bipyridinium guests 1,2bis(4-pyridyl)ethane and 1,3-bis(4-pyridyl)propane have also been determined. Such rich pseudopolymorphism demonstrates that solvents are active components of C4S supramolecular assemblies with pyridinium guests; they compete with pyridinium rings for the inclusion into the host macrocyclic cavity and are active in hydrogen bonding with both C4S and nitrogen-containing molecules. The careful selection of solvent gives the possibility to generate solid-state complexes of different C4S/guest stoichiometry due to either mono- (1:4 ratio in complex 4) or diprotonation (1:2 ratio in complexes 1-3 and 5-8) of bipyridine derivatives. It should be mentioned that water is present in considerable amounts in every structure and plays an important role in the complex formation through extensive hydrogen bonding with C4S sulfonate groups and nitrogen atoms of bipyridinium guests. The effect of cosolvents on the solubility of cyclodextrins and their complexation behavior has previously been described and the effects attributed to how solvents modify water structure at differing concentrations.38 The structure of the C4S/bipyridinium complexes is mainly guided by a complicated array of hydrogen bonding
Figure 11. Stick (left) and space filling (right) representations of the columnar array viewed from the c direction.
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complemented by additional supramolecular interactions such as π-stacking and electrostatics. It is evident that faceto-face aromatic interactions between the electron-rich C4S cavity and pyridinium rings play an important role in the structuring of C4S/bipyridinium complexes; however, it is still difficult to predict, despite extended structural research conducted by both us and other groups, the preferential interaction between the host and the pyridinium rings of the guest: the inclusion into the macrocyclic cavity or interaction with the external walls of the C4S cavity or both these modes simultaneously. It is evident that by subtle manipulation of guest choice and crystallization medium it is possible to reduce the traditional C4S bilayer to a one-dimensional motif: tapes composed of back-to-back oriented C4S in complex 4 or columns composed of head-to-tail oriented C4S molecules in complexes 7 and 8. To conclude, the control and predictability in constructing complex structures of C4S with bipyridinium compounds remains somewhat problematic because of active competition of solvent molecules with pyridinium groups for inclusion in the macrocyclic cavity, and it is a significant challenge. The design strategy based on the use of space filling phosphonium cations and templating lanthanide metal ions in building up multicomponent architectures lends insight into this problem and has been demonstrated in many examples of incorporating imidazolium and pyrrolidinium cations within the C4S cavity in a controllable way15-17 and may prove useful in gaining some control in constructing complex structures of C4S with bipyridinium derivatives in future studies.
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Supporting Information Available: X-ray crystallographic files in CIF format and figures showing structural details. This material is available free of charge via the Internet at http://pubs.acs.org.
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