Dendrimeric Tectons in Halogen Bonding-Based Crystal Engineering Pierangelo Metrangolo,*,† Franck Meyer,† Tullio Pilati,‡ Davide M. Proserpio,§ and Giuseppe Resnati*,†
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 654–659
NFMLab c/o Department of Chemistry, Materials, and Chemical Engineering “G. Natta”, Politecnico di Milano Via L. Mancinelli 7, 20131 Milan, Italy, CNR-Institute of Molecular Science and Technology UniVersity of Milan, Via Golgi 19, 20133 Milan, Italy, and Department of Structural Chemistry and Inorganic Stereochemistry, UniVersity of Milan, Via Venezian 21, 20133 Milan, Italy ReceiVed September 11, 2007
ABSTRACT: The use of DAB dendrimers in halogen bonding-based crystal engineering is reported. The DAB-dendr-(NHC6F4I)22 self-assembles with (E)-1,2-bis-(4-pyridyl)-ethylene in a 1:2 supramolecular adduct, whose crystal lattice is composed of 5-fold interpenetrated sql networks, which is the highest degree for a square topology reported so far. The synthesis and coordination properties of higher generation dendrimers are also described, both in the liquid state and in solution. Introduction Dendritic molecules have attracted massive interest because of their unique structures, properties, and potential for applications in fields as diverse as materials chemistry, biomedicine, and nanotechnology.1 When involved in molecular recognition and self-assembly processes, dendrimers tend to give a complex pattern of intra- and intermolecular interactions of comparable relative strength, whose balanced cooperation determines the structure of the resulting assembling system. As a probable consequence of the difficult control of such complex patterns of interactions, the use of dendrimers as tectons in crystal engineering, namely, as molecules whose interactions are dominated by particular associative forces that induce their selfassembly into organized networks with specific and preestablished architectural or functional features,2 is rare. This article describes the synthesis of the poly(propylenimine) dendrimer DAB-dendr-(NH-C6F4I)22 3a that is decorated at the periphery with four iodotetrafluorophenyl groups. These groups dominate the pattern of intra- and intermolecular interactions in this dendrimer. In particular, they promote a net of intramolecular hydrogen bonds that effectively preorganize the dendrimeric module to the point that the single crystal X-ray structure of the DAB-dendr-(NHC6F4I)22 3a is obtained. Halogen bonding (XB) is the noncovalent interaction wherein halogens work as electrophilic sites.3 The iodotetrafluorophenyl groups in 3a work as effective XB-donors, to the point that 3a functions as tetratopic tecton and self-assembles with (E)-1,2bis-(4-pyridyl)-ethylene (6), a ditopic XB-acceptor (Scheme 1). The 1:2 supramolecular adduct 7 is formed, whose single crystal X-ray structure, showing the presence of very large twodimensional (2D) square networks, is described. The overall lattice of 7 is composed of 5-fold interpenetrated square layers (sql) translationally related,4 which is the highest interpenetration mode reported for a sql topology.5 Self-assembly processes are usually a balance between enthalpic gain and entropic loss, so that the formation of heteromeric aggregates requires strong interactions and/or * To whom correspondence should be addressed. E-mail: pierangelo.
[email protected] (P.M.);
[email protected] (G.R.). Fax: +390223993180. † Politecnico di Milano. ‡ CNR. § University of Milan.
Scheme 1. Synthesis of the Supramolecular Network 7 from 3a and 6
preorganized starting modules. In particular, introducing fluorine into the chemical formula of dendritic molecules has been shown to result in a total change in the self-assembly and selforganization patterns with respect to their nonfluorinated analogues;6 interesting applications of fluorinated dendrimers are emerging in the fields of catalysis, new materials, and biology.7 In our search for dendrimers that can work as tectons and drive the formation of well-ordered crystal lattices, we focused on DAB-dendr-(NH2)2n+1 as they are commercially available and can be easily functionalized. To date, only two single crystal X-ray structures of DAB-dendr derivatives are described in the literature.8 In these rare cases, the mobility of dendritic arms is greatly reduced by strong intramolecular hydrogen bonds,8a or through diprotonation of core nitrogens.8b On the assumption that the dendritic arms’ flexibility should be decreased to make the dendrimers work as tectons, the derivatization of terminal amine groups of DAB-dendr-(NH2)2n+1 has to play a double role. First, it has to rigidify and preorganize the dendrimeric
10.1021/cg700870t CCC: $40.75 2008 American Chemical Society Published on Web 12/15/2007
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Figure 1. Structure and asymmetric unit numbering scheme of the DAB-dendr-(NHC6F4I)22 in 3a (left) and in the supramolecular complex 7 (right); views nearly along the C1-C2 bond. Colors are as follows: carbon, black; hydrogen, sky blue; nitrogen, blue; fluorine, green; iodine, purple. N-H · · · N hydrogen bonds as dotted black lines.
Figure 3. Single crystal X-ray structure of the supramolecular complex 7; 2D square layers with a 44 topology are formed. Figure 2. The crystal lattice stabilization of 3a rests on the π · · · π stacking between centrosymmetrically related aromatic groups as the four aromatic rings at the periphery are pinned to the perfluoroarenes of four other modules by π · · · π interactions.
module via strong intramolecular interactions; second, it has to deliver the binding sites that drive the self-assembly process at the dendrimer periphery. Being expected to perform both roles, the p-iodotetrafluorophenyl moiety was chosen as the derivatizing group. The strong electron withdrawing ability of fluorine dramatically boosts the acidity of polyfluorophenylamines. It is expected the core of halotetrafluorophenyl-substituted DAB dendrimers is rigidified by strong intramolecular hydrogen bonds, with the arms being locked in an extended conformation and thus ready for the XB-based exo recognition of suitable telechelic modules. The formation of supramolecular networks can be anticipated, wherein the high directionality of XB translates the tetragonal arrangement of the first generation DAB-dendr tectons to the overall network, inducing the formation of an extended lattice with large cavities, which can be filled by solvent molecules and/or through interpenetration. Experimental Section Materials and Methods. Commercial HPLC-grade solvents were used without further purification. Starting materials were purchased from Sigma-Aldrich, Acros Organics, and Apollo Scientific. Reactions were carried out in oven-dried glassware under a nitrogen atmosphere. 1H NMR and 19F NMR spectra were recorded at ambient temperature with a Bruker AC 250 spectrometer or a Bruker AV 500. Unless otherwise stated, CDCl3 was used as both solvent and internal standard for 1H NMR spectra. For 19F NMR spectra, CDCl3 was used as solvent and CFCl3 as internal standard. Multiplicity in 19F NMR and 13C NMR
spectra is referred to as fluorine-fluorine and carbon-fluorine couplings, respectively. IR spectra were obtained using a Nicolet Nexus FT-IR spectrometer equipped with UATR unit. The values are given in wave numbers and are rounded to 1 cm-1 upon automatic assignment. Chromatographic separations were performed on silica gel 230–400 mesh. Melting points were determined with a Reichert instrument by observing the melting and crystallizing process through an optical microscope. Mass spectra were obtained in electrospray ionization (ESI) mode by using a Bruker Esquire 3000+ machine. Samples were infused into the source by a syringe pump. Single crystals of 3a and of the complex 7 were analyzed at room temperature on a Bruker SMART 1000 CCD diffractometer using graphite monochromatized Mo-KR radiation (λ ) 0.71073 Å). The structure were solved by direct methods using SIR20029 and refined full-matrix least-squares on F2 by SHELXL97.10 Complete structural data were deposited with the Cambridge Crystallographic Data Centre as CCDC 641309 (3a) and 641308 (7). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Typical Procedure for the Reaction of DAB-dendr-(NH2)22 1a with Iodoperfluorobenzene 2a. A mixture of 100 mg (0.31 mmol) of DAB-dendr-(NH2)22 (1a), 0.33 mL (2.52 mmol) of pentafluoroiodobenzene (2a), and 196 mg (1.42 mmol) of K2CO3 is stirred in 1.00 mL of refluxing CH3CN for 24 h. Then the reaction is cooled down to room temperature, and the solid is filtered. After evaporation of the solvent, the crude material is chromatographed on silica gel with CH2Cl2 then CH2Cl2/MeOH 93/7 as the eluting system. DAB-dendr-(NH-C6F4I)22 C40H36N6F16I4 3a. After chromatography, 19 F NMR spectra of 3a shows the peaks corresponding to both paraiodotetrafluorobenzene pendants (two doublets of equal intensity with 3 JF,F) and ortho-iodotetrafluorobenzene pendants (two doublets of equal intensity with 3JF,F and two triplets of equal intensity with 3JF,F) (see onward). No peak is observed that corresponds to meta-iodotetrafluorobenzene pendants (expectedly, one singlet, two doublets of equal intensity with 3JF,F, and one triplet of equal intensity with 3JF,F). The area ratio between the peaks of para- and ortho-pendants is 95:5. On
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Figure 4. Schematic view of the 5-fold interpenetrated network 7 showing the superimposed interpenetration vector [0,1,0], which repeated five times originates the overall entanglement.
Figure 5. The peculiar sequence of π-π interactions between nearly parallel iodotetrafluoroarenes and pyridines in the supramolecular complex 7 favors the observed interpenetration mode. the assumption that fluoroarylation of single pendants all occurs with the same para/ortho selectivity, the fluoroarylation reaction selectivity is 95:5. This implies DAB-dendr-(NH-p-C6F4I)22 (3a) is an isomer mixture, whose relative abundance is statistically determined by the 95:5 selectivity of fluoroarylation reaction: DAB-dendr-(NH-C6F4I)22 3a tetra-para isomer, 81.5%; DAB-dendr-(NH-C6F4I)22 3a ortho-tripara isomer, 17.1%; DAB-dendr-(NH-C6F4I)22 3a diortho-dipara isomer, 1.3%; DAB-dendr-(NH-C6F4I)22 3a para-triortho isomer, 0.05%; DABdendr-(NH-C6F4I)22 3a tetra-ortho isomer, 6.2 × 10-4%. DAB-dendr-(NH-C6F4Br)22 C40H36N6F16Br4 4a. Similar to 3a, 19F NMR spectra of 4a shows, after chromatography, only the peaks corresponding to para-bromotetrafluorobenzene pendants and orthobromotetrafluorobenzene pendants (see onward). No peak is observed that corresponds to meta-bromotetrafluorobenzene pendants. The area ratio between the peaks of para- and ortho-pendants, namely, the fluoroarylation reaction selectivity, is 88:12. As in 3a, DAB-dendr(NH-p-C6F4Br)22 4a is an isomer mixture, whose composition is statistically determined by the fluoroarylation reaction selectivity. Typical Procedure for the Reaction of DAB-dendr-(NH2)23 1b with Iodoperfluorobenzene 2a. A mixture of 100 mg (0.13 mmol) of DAB-dendr-(NH2)23 (1b), 0.30 mL (2.08 mmol) of pentafluoroiodobenzene (2a), and 161 mg (1.17 mmol) of K2CO3 were stirred in 0.50 mL of refluxing CH3CN for 48 h. Then, the solution is filtered, and
Metrangolo et al. the solvent is evaporated. The crude material was chromatographed on silica gel, eluting system CH2Cl2 then CH2Cl2/MeOH 8/2. DAB-dendr-(NH-C6F4I)23 C88H88N14F32I8 3b. Similar to 3a, 19F NMR spectra of 3b shows, after chromatography, the peaks corresponding to both para-iodotetrafluorobenzene pendants and orthoiodotetrafluorobenzene pendants (see onward). No peak is observed that corresponds to meta-iodotetrafluorobenzene pendants. The area ratio between the peaks of para- and ortho-pendants, namely, the fluoroarylation reaction selectivity, is 95:5. DAB-dendr-(NH-C6F4I)23 3bis therefore an isomer mixture, whose composition is statistically determined by the fluoroarylation reaction selectivity. DAB-dendr-(NH-C6F4Br)23 C88H88N14F32Br8 4b. Similar to 3b, 19F NMR spectra of 4b shows, after chromatography, the peaks corresponding to both para-bromotetrafluorobenzene pendants and orthobromotetrafluorobenzene pendants (see onward). No peak is observed that corresponds to meta-bromotetrafluorobenzene pendants. The area ratio between the peaks of para- and ortho- pendants, namely, the fluoroarylation reaction selectivity, is 88:12. DAB-Dendr-(NHC6F4Br)23 4b is therefore an isomer mixture, whose composition is statistically determined by the fluoroarylation reaction selectivity. Typical Procedure for the Reaction of DAB-dendr-(NH2)24 1c with Iodoperfluorobenzene 2a. A mixture of 100 mg (0.06 mmol) of DAB-dendr-(NH2)24 (1c), 0.25 mL (1.90 mmol) of pentafluoroiodobenzene (2a), and 141 mg (1.02 mmol) of K2CO3 were stirred in 0.50 mL of refluxing THF for 48 h. Then, the solution is filtered, and the solvent is evaporated. The crude material was chromatographed on silica gel, eluting system CH2Cl2 then CH2Cl2/MeOH 7/3. DAB-dendr-(NH-C6F4I)24 C184H192N30F64I16 3c. Similar to 3a,b, 19F NMR spectra of 3c shows, after chromatography, only the peaks corresponding to para-iodotetrafluorobenzene pendants and orthoiodotetrafluorobenzene pendants (see onward). No peak is observed that corresponds to meta-iodotetrafluorobenzene pendants. The area ratio between the peaks of para- and ortho- pendants, namely, the fluoroarylation reaction selectivity, is 95:5. DAB-dendr-(NH-C6F4I)24 3c is therefore an isomer mixture, whose composition is statistically determined by the fluoroarylation reaction selectivity. DAB-dendr-(NH-C6F4Br)24 C184H192N30F64Br16 4c. Similar to 4a,b, 19 F NMR spectra of 4c shows, after chromatography, only the peaks corresponding to para-bromotetrafluorobenzene pendants and orthobromotetrafluorobenzene pendants (see onward). No peak is observed that corresponds to meta-bromotetrafluorobenzene pendants. The area ratio between the peaks of para- and ortho- pendants, namely, the fluoroarylation reaction selectivity, is 88:12. DAB-dendr-(NH-C6F4Br)24 4c is therefore an isomer mixture, whose composition is statistically determined by the fluoroarylation reaction selectivity. Crystallization Experiments. Colorless single crystals of 3a were obtained in five days by the slow evaporation of an isopropyl ether solution of dendrimer 3a (mp 100–103 °C). The supramolecular complex 7 was obtained by dissolving at room temperature, in a vial of clear borosilicate glass, 1 equivt of DABdendr-(NH-C6F4I)22 3a and 2 equiv of (E)-1,2-bis-(4-pyridyl)-ethylene (6) in chloroform. The open vial was placed in a closed cylindrical wide-mouth bottle containing paraffine oil. The slow evaporation of the solvent at room temperature provided colorless crystals after seven days (mp 162–165 °C). Characterization of Supramolecular Complex 7. IR νmax (cm-1) ) 3414, 3186, 2950, 2873, 2816, 1639, 1598, 1488, 1290, 1140, 944, 782 cm-1. 1H and 19F NMR spectra corresponded to a 1:2 mixture of DAB-dendr-(NHC6F4I)22 and (E)-1,2-bis-(4-pyridyl)-ethylene.
Results and Discussion The SNAr attack of DAB-dendr-(NH2)2n+1 (1a-c) on iodopentafluorobenzene 2a afforded three different generations of DAB-dendr-(NHC6F4I)2n+1 [with n ) 1 (3a), n ) 2 (3b), n ) 3 (3c) (Scheme 2 in Supporting Information) in 68-50% isolated yields, after chromatographic purification (dichloromethane/methanol as eluent)]. 19F, 1H, and 13C NMR clearly indicates that reactions invariably occur with very high regioselectivity (95% ca.) on the fluorine atom para to the iodine atom of 2a, as typical of this kind of reaction.11 The slow evaporation of an isopropyl ether solution of compound 3a has provided crystals suitable for X-ray diffraction
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Table 1. Crystal Data and Structure Refinement Details for 3a and 7
empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) β (°) volume (Å3) Z, Fcalc (g cm-3) crystal size (mm3) absorption coefficient (mm-1) θ range (°) max/min transmission no. reflections collected Rave independent, I > 2σ(I) reflections no. refined parameters final R1, wR2 [I > 2σ(I)] final R1, wR2 [I > 2σ(I)] goodness-of-fit ∆F min, max (e Å3)
3a
7
C40H36F16I4N6 1412.35 monoclinic P21/n 14.246(2) 11.5917(17) 15.810(2) 111.28(2) 2432.8(6) 2, 1.928 0.10 × 0.13 × 0.32 2.659
