摘要:
One of the goals of crystal engineering is to obtain crystals that have or lack a particular symmetry element. In this context, it is useful to consider relationships between molecular and supramolecular symmetry under three categories: (1) the symmetry of the molecule is lowered in the crystal; (2) the crystal symmetry is higher than the molecular symmetry; (3) the molecular and crystal symmetries are matched. The first of these categories is very common but not so useful in crystal design,1being merely a restatement of Kitaigorodskii's closepacking principle.2A common example in the second category is when carboxylic groups with 1symmetry are hydrogen bonded as dimers with 1&cmb.macr;symmetry. But, such symmetry increase has not been explored systematically. The third category listed above, symmetry matching, is very useful in crystal engineering. However, structural chemistsand crystal engineers are used to thinking of the closepacking principle as being of nearuniversal applicability, and, therefore, such studies of symmetry carryover of higherfold symmetry elements have generally not been pursued.Here, we discuss the packing features of three diverse molecules,1–3, with threefold symmetry that crystallize in space groups with 3axes. In all of these threeC3molecules – 2,4,6tris(5chloro3pyridyloxy)1,3,5triazine31, 1,3,5tris(3methylphenyl)benzene42and 1,3,5tripyrrolylbenzene53– the symmetry of the molecule coincides with a crystal symmetry element.Triazine1when crystallized from 1∶1 hexane–EtOAc adopts the centrosymmetric space groupR3&cmb.macr; (no. 148) (Table 1). The asymmetric unit contains 1/3 molecule. TheC3isymmetric Piedfort Unit7(C3iPU) formed by an assembly of two stacked molecules of1may be identified (Fig. 1). The two molecules in theC3iPU are related by a 3&cmb.macr;-axis and are held together by weak intermolecular interactions,8π⋯π, C–H⋯O and C–H⋯N (Table 2). TheC3iPUs are stacked along [00 1] in a staggered manner to produce a columnar structure, which in turn is stabilized by short N⋯Cl interactions (3.18 Å, 148.5°). The PU stacks are themselves related by 31axes being connected with C–H⋯N hydrogen bonds (Fig. 2). This helical arrangement is finally stabilized by C–H⋯Cl interactions. Both the formation of theC3iPUsviaπ⋯π stacking and the interconnection of the PUsviaC–H⋯N hydrogen bonds are important in preserving the supramolecular threefold symmetry in1.Stacking of triazine1molecules to form aC3iPiedfort Unit. Molecules at different levels are shaded differently. C–H⋯O and C–H⋯N interactions are shown as dashed lines.C–H⋯N (pyridine) hydrogen bonds (dashed lines) connect Piedfort Units in a threefold helical arrangement in the crystal structure of1. Notice the synthon (void) symmetry. Click image orhereto access a 3D representation.Summary of crystal data for1–3,,Parameter123All data were collected atT⊕=⊕120 K using a MADNESMesserschmidt and Pflugrath diffractometer with MoKα radiation (λ⊕=⊕0.71073 Å).Fullmatrix, least squares refinement onF2using SHELXL97.6Clickherefor full crystallographic data (CCDC 164454–164456).Empirical formulaC18H9N6O3Cl3C27H24C18H15N3M463.66348.46273.33Crystal systemHexagonalHexagonalHexagonalSpace groupR3&cmb.macr;P63R3&cmb.macr;ca/Å21.189(3)12.3060(17)19.420(3)b/Å21.189(3)12.3060(17)19.420(3)c/Å7.1030(14)7.5710(15)6.6760(13)V/Å32761.8(8)992.9(8)2180.4(6)Z626Dc/Mg m−31.6731.1661.166F(000)1404372864μ/mm−10.5350.0660.076&thetas;Range for data collection/°3.08–28.553.63–27.483.30–27.48Index ranges−27⊕≤⊕h⊕≤⊕26; −23⊕≤⊕k⊕≤⊕25; −9⊕≤⊕l⊕≤⊕80⊕≤⊕h⊕≤⊕15; −13⊕≤⊕k⊕≤⊕0; 0⊕≤⊕l⊕≤⊕90⊕≤⊕h⊕≤⊕25; −21⊕≤⊕k⊕≤⊕0; −8⊕≤⊕l⊕≤⊕8Reflections collected41688181020Unique reflections1456818543R10.04250.06130.0547wR20.10600.16310.1396Geometrical parameters of interactions in the crystal structure of triazine1InteractionD/Åd/Å&thetas;/°C–H bonds are neutron normalized to 1.08 Å.π is the centroid of the aromatic ring.orthoC–H⋯O3.3092.73112.8orthoC–H⋯N3.5562.58149.6paraC–H⋯N3.6632.61163.4paraC–H⋯Cl3.8713.09129.3π⋯π3.345The next compound of interest is2. This hydrocarbon crystallizes in the noncentrosymmetric space groupP63(no. 173) (Table 1). The asymmetric unit contains 1/3 molecule. The 63-related molecules are connected by C–H⋯π interactions (2.94 Å, 3.914 Å, 150.1°) involving the CH3groups as donors9(Fig. 3). Further stabilization is provided by C(phenyl)–H⋯π interactions (2.78 Å, 3.791 Å, 154.3°). Additionally, adjacent CH3groups of threefold-related molecules form a hydrophobic core (methyl pool) that effectively defines the supramolecular void in this packing. All these interactions are combined to generate thehexagonal network structure.Stereoview of the crystal structure of hydrocarbon2. Molecules shaded differently are located on 3axes and are connected to 63-related molecules by C–H⋯π interactions. Notice the methylrich hydrophobic core enclosing a void withC3symmetry. Click image orhereto access a 3D representation.When heterocycle3is crystallized from chlorobenzene, it crystallizes in the centrosymmetric hexagonal space groupR3&cmb.macr;c(no. 167) (Table 1).The nonlinear hyperpolarisability (β) of3measured at 1.064 µm in CHCl3is (1.9⊕±⊕0.4)⊕×⊕10−30 esu.The asymmetric unit contains 1/6 molecule. Molecules are stacked at a distance of 3.34 Å and, remarkably, with no offset (Fig. 4). The π–π stacking interactions are very significant. They arise from the electrondeficient nature of the aromatic rings10and could be major determinantsin the generation of a hexagonal crystal structure, in that there is no offset. The reader should note that the existence of any lateral offset would necessarily break the hexagonal symmetry.Dimer (left) and complete structure (right) of heterocycle3. H atoms are removed in the right view for clarity. Notice how the absence of lateral offset in π–π stacking is compatible with supramolecular hexagonal symmetry.Finally, it should be noted that for none of the compounds studied here is the full molecular symmetry transferred into the crystal. For example, in the tripyrrolyl derivative3, there is the possibility that the molecule could crystallize withZ′⊕=⊕1/12 with the molecule being planar. In the observed structure,Z′⊕=⊕1/6 and the heterocyclic rings are inclined at an angle of 31° to the plane of the central phenyl ring. This is not surprising. The ubiquity and nearuniversality of the close-packing principle is such that even partial retention of molecular symmetry in the crystal is noteworthy.This study shows that matching of molecular and supramolecular symmetry is achieved when the symmetry of the molecule matches the symmetry of the intermolecular voids, in other words the supramolecular synthon11symmetry. Synthon symmetry follows from the nature, number and relative positioning of intermolecular interactions; by no means need these be connected to the molecular symmetry, at least in a general sense. For example, closely related analogues of compounds1–3crystallize in low symmetry, closepacked structures.12These difficulties do not apply for the most part in substituted tetraphenylmethanes where there is a facile symmetry carryover between molecules and crystals. In these compounds,S4molecular symmetry relates to tetragonal crystal symmetry because of the characteristic tetraphenyl embrace,13which encloses a tetrahedral void. Even the space groups of these compounds may be roughly predicted to be eitherP4&cmb.macr;21corI4&cmb.macr;.14In trigonal molecules, such symmetry carryover is more difficult because there is no clear classification of the pertinent supramolecular synthons in terms of their symmetry properties. The present work attempts to identify some synthons that connectC3molecular symmetry and hexagonal crystal packing. The next step would be to identify more molecules that crystallize with these synthons, thus establishingrobustness.
ISSN:1466-8033
DOI:10.1039/b104431c
出版商:RSC
年代:2001
数据来源: RSC