CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 4 1309-1312
Communications Poly(pseudo)rotaxane-like Network Mediated by Hydrogen Bonds in the Solid-State Structure of 1,7-Phenanthroline Kapildev K. Arora and V. R. Pedireddi* Division of Organic Chemistry, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India Received January 31, 2005;
Revised Manuscript Received April 21, 2005
ABSTRACT: A unique poly(pseudo)rotaxane-like network constituted by C-H‚‚‚N hydrogen bonds in the solid-state structure of 1,7-phenanthroline and a self-inclusion host-guest type assembly in its salt form have been discussed. The assemblies are characterized by single-crystal X-ray diffraction methods. Design and synthesis of functional molecules1 is one of the current frontier research areas, not only because of the tremendous utility of such molecules in various applications but also for the challenges associated with the synthesis of those complex molecules.2 Rotaxanes,3 catenanes,4 calixarenes,5 and self-assembled monolayers (SAMs)6 are a few representative molecular components that have been widely used for the creation of exotic molecular-based devices. Rotaxanes and pseudorotaxanes are of special interest to many research groups due to the wide applicability of those systems in the design of molecular shuttles, molecular machines, and switches.7,8 The basic design element of the rotaxanes is the synthesis of a molecular unit with cavities and insertion of a linear chain through those cavities, i.e., threading and further attaching the bulky groups to the linear chains to avoid dethreading. Such a complex network, in practice, is made into realization through the process of inducing molecular recognition between the two different components, in other words, creation of a supramolecular assembly. In general, the constituent molecular units with voids are the macromolecules consisting of heteroatoms, particularly pyridyl N atoms, constructed by strong covalent or coordinate bonds, following the well-established elegant functional group transformations.3,7 On the other hand, however, such void assemblies also could be obtained through noncovalent procedures, employing small organic molecules, possessing heteroatoms, with the aid of increasing knowledge of the nature of the hydrogen bond.9-11 Observing the close similarity in the topological aspects, especially the prerequisite of voids, between rotaxanes having covalent linkages or molecular complexes with noncovalent linkages, we have been lured for a long time to generate supramolecular assemblies such as calixarenes and rotaxanes by employing hydrogen bonds of different
Figure 1. (a) A square network with a cavity of ∼7 Å in the crystal structure of 1. Dashed lines represent C-H‚‚‚N hydrogen bonds. (b) A rotaxane-like network observed in the lattice of 1.
* To whom correspondence should be addressed. E-mail: pediredi@ ems.ncl.res.in.
types such as O-H‚‚‚O, O-H‚‚‚N, C-H‚‚‚O, etc. Our approach is to expose the ease of synthesis of such complex
10.1021/cg050038t CCC: $30.25 © 2005 American Chemical Society Published on Web 05/18/2005
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Figure 2. (a) A poly(pseudo)rotaxane-like network observed in the crystal lattice of 1. (b) Schematic representation of the poly(pseudo)rotaxane-like network, replacing molecular units in (a) by spheres and the dimeric hydrogen bonds by red cylinders and single hydrogen bonds by blue cylinders. (c) A close-packing drawing of the poly(pseudo)rotaxane-like network shown in (a).
systems, as the conventional methods often may have limitations due to the low yields of final product or long procedures involving multisteps.2b Looking at the versatile utility of moieties such as phenanthroline, bipyridyl, etc.12 in the covalent synthesis of molecular components with voids, ultimately to constitute rotaxanes, we made several attempts to build noncovalent rotaxane networks employing similar ligands in conjunction with various components such as trimesic acid, trithiocyanuric acid, 1,7-phenanthroline, etc.12 However, our attempts were not successful as the voids are often being catenated or filled by suitable guest molecules13,14 rather than permitting anchors of long handles to protrude through the cavity and mimic the topology of rotaxanes. This has compelled us to presume that synthesis of such assemblies may be beyond realization. However, crystal structure of 1,7-phenanthroline (1) that we have determined to evaluate and extend the host features of 1 that we observed in its molecular complex with thiodipropionic acid,14b indeed, formed a rotaxane-like network exclusively formed by hydrogen bonds in its solidstate structure. We report these features in this article along with the salt form of 1, which adopts a self-inclusion complex network. Single crystals of 115 obtained upon crystallization from a methanol solution gave an asymmetric unit with two symmetry-independent molecules. The analysis of threedimensional arrangement in the crystal lattice has revealed quite exotic features of the packing of the molecules. To begin with, the two symmetry-independent molecules (A and B) constitute a four-member square network. A typical network is shown in Figure 1a. In each square, the four molecules are held together by C-H‚‚‚N hydrogen bonds (H‚‚‚N, 2.70 and 2.73 Å), with molecules A and B arranging alternately and also perpendicular to each other. Thus, a cavity of ∼7 Å in dimension is created. Further, the adjacent squares are held together through the formation of a six-member centrosymmetric cyclic C-H‚‚‚N hydrogen-bonding pattern, [R22(6)] by protruding through the cavity (see Figure 1b). The H‚‚‚N distance in the pattern is 2.92 Å. Such an arrangement, indeed, is a rotaxane-like network, considering the linear linkage (dimeric hydrogen-bonding pattern) as a long handle with two phenanthroline molecules (shown in stick mode in
Figure 1b) as spheres of the dumbell and cyclic network (square) as a macrocyclic analogue with a cavity. Further, in three-dimensional arrangement, a poly(pseudo)rotaxanelike network is established as shown in Figure 2. To the best of our knowledge, the solid-state structure of 1 is the first of its kind to form the rotaxane-like network formed by hydrogen bonds, that too through weak C-H‚‚‚N hydrogen bonds, employing only organic entities in contrast to the metal complexes known in the literature.3g Rotaxane structures are often influenced by pH of the reaction medium,3,7,8 so we continued our further studies on 1 to evaluate its ability to retain the rotaxane structure under acidic conditions. Thus, we carried out crystallization of 1 from HCl as it can form the corresponding salt very easily. The structure analysis revealed that the overall topological arrangement with respect to the cavities remained unaltered to a large extent, but the resultant assembly did not generate a rotaxane; however, it gave a self-inclusion host-guest system as detailed below. The single crystals of 1 from HCl (hereafter 216) gave an asymmetric unit consisting of a neutral and a charged phenanthroline moiety along with a Cl- ion and two H2O molecules. The molecular arrangement in the crystal lattice reveals that each type of phenanthroline moiety forms, independently, an identical cyclic network, but with varied hydrogen-bonded distances. The networks are shown in Figure 3. In each network, the adjacent aza molecules are separated by either two H2O molecules, which are in turn held together by a strong O-H...O hydrogen bond with a H‚‚‚O distance of 1.86 Å, or by a Cl- ion. As a result, each aza molecule forms two C-H‚‚‚Cl- bonds (H‚‚‚Cl-, 2.75/2.85; 2.79/2.84 Å) with a Cl- ion and C-H‚‚‚O (H‚‚‚O, 2.71/2.95 Å) as well as O-H‚‚‚N (H‚‚‚N, 1.99 Å) or N+-H‚‚‚O (H‚‚‚ O, 1.87 Å) hydrogen bonds with H2O molecules. Thus, a cavity of 8 × 11 Å is formed in each cyclic network (see Figure 3). In the three-dimensional arrangement, these two independent networks (Figure 3a,c) are arranged as an interwoven lattice to yield a self-inclusion host-guest type network, as shown in Figure 4. Comparing the cavities in 1 and 2, it appears that the increase of dimension in 2 perhaps precluded the formation of rotaxane as other phenanthroline molecules with a dimension of 6 × 8 Å
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Figure 3. Two-dimensional arrangement of (a) neutral phenanthroline moiety and (c) protonated form in the crystal lattice of 2. (b) and (d) correspond to the same as shown in (a) and (c), respectively, but in a close packing representation.
of the core molecules to obtain complex architectures of supramolecular assemblies. Experimental Section Good quality single crystals, carefully chosen using a microscope equipped with a CCD camera, were used to collect X-ray intensity data on a Bruker diffractometer (APEX CCD area detector). The data were collected at room temperature (293 K) for 1 while it was collected at 120 K for 2. The crystals were quite stable and did not show any changes in morphology or transparency during the process of data collection. The data were processed using Bruker suite of programs (SAINT).17a Structures determination and refinements were carried out using the SHELXTL package.17b All the intermolecular interactions were calculated using PLATON.17c Figure 4. Three-dimensional arrangement in the crystal structure of 2 forming a self-inclusion host-guest lattice.
could pass through it unlike in 1 wherein the dimension of the cavity is too small. Thus, it may be concluded that rotaxanes synthesis through hydrogen bonds may be quite facile as long as the dimension of the guest molecules exceeds the cavity being created and they possess appropriate functional groups to interact while protruding through the cavity. Thus, the structures of 1 and 2 exhibit the remarkable features of quite simple organic compounds in the solid state by adopting exotic and complex modes of packing of molecules in the crystal lattices. Also, we believe that this study has shed more light on the further tuning of the intermolecular interactions with appropriate substitution
Acknowledgment. We thank Professor Judith Howard (University of Durham, UK) for her help in the determination of the crystal structure of 2 and also Dr. S. Sivaram as well as Dr. K. N. Ganesh for their encouragement. This work is supported by Department of Science and Technology, New Delhi. Supporting Information Available: X-ray data with details of refinement procedures (cif files), ORTEP diagrams, lists of bond parameters (bond lengths and angles), structure factors of 1 and 2, a table of characteristics of hydrogen bonds. This material is available free of charge via the Internet at http://pubs.acs.org.
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