Structural Consequences of Strong and Weak Interactions to Binary Benzoic Acid/Bipyridine Supramolecular Assemblies Jeffrey R. Bowers,† Gregory W. Hopkins,† Glenn P. A. Yap,‡ and Kraig A. Wheeler*,†
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 2 727-736
Department of Chemistry, Delaware State University, Dover, Delaware 19901, and Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716 Received July 28, 2004;
Revised Manuscript Received September 9, 2004
W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: The crystal chemistry of a family of bimolecular complexes constructed from benzoic acid/bipyridine and benzoic acid/bipyridine-N-oxide components is described. The building blocks used in this investigation follow a retrosynthetic approach based on the complementary nature of carboxyl‚‚‚pyridine/pyridine-N-oxide supramolecular synthons. These strong interactions largely form predictable discrete heterotrimers; however, the weaker, no less important, secondary interactions (e.g., C-H‚‚‚acceptor and π‚‚‚π stacking) control alignment of each heterotrimer resulting in a variety of packing motifs. The observed interplay of strong and weak interactions supports the need to examine and design molecular frameworks based on the full range of known contacts responsible for crystal cohesion. By introducing a structural family with similar topological features, the current study provides a means of exploring the interdependency of structural features to molecular alignment in competitive crystal environments. Introduction Single-crystal diffraction methods continue to provide a vital characterization tool for ordered solid-state materials. Of the numerous literature reports that present crystallographic data, a significant portion examine only solitary structures or a small sampling of relatively unrelated compounds. Although such structural contributions hold intrinsic value for assessing fundamental chemical details, for example, molecular connectivity and stereochemical assignments, the transferability of observed conformational and packing motifs remains largely indeterminate unless evaluated with a sufficient number of related structures. For example, 4,4′-dimethoxyselenobenzophenone (WIDJAU)1 crystallizes with Z′ ) 1 and pendant methoxy groups in anti conformations. By comparison, the structures of the oxo (DMBOPN)2 and thio (CELDEC)3 homologues reveal syn methoxy functions with two symmetry-independent molecules. It is interesting to note that only DMBOPN and CELDEC form weak CdO/S‚‚‚H-C interactions via intermolecular contacts between the methoxy and carbonyl/thiocarbonyl groups. The absence of analogous interactions in WIDJAU possibly suggests a diminished role of the selenocarbonyl group as an acceptor of hydrogen bonds. Because no additional selenobenzophenone structures exist in the literature, attempts to identify factors responsible for the observed conformational and nonbonded differences currently lack any practical means of assessment. Clearly one would cause serious oversight by extending the observed crystal packing patterns for WIDJAUsthe only known sele* Corresponding author. Fax: (302)-857-6539. E-mail: kwheeler@ desu.edu. † Delaware State University. ‡ University of Delaware.
nobenzophenone structuresas a general template for all future selenobenzophenone structures. With this in mind, it is not surprising that most strategies that seek to understand and exploit persistent structural motifs rely on information gleaned from sizable sets of related crystallographic data. Both the study of newly synthesized families of compounds4 and the use of the Cambridge Structural Database (CSD)5 provide fertile ground by which to understand the construction of molecular assemblies. The current study combines these approaches by (i) investigating a family of binary cocrystals consisting of benzoic acid (1-2)/ bipyridine (3-7) components, (ii) assessing common
structural features, and (iii) comparing any prominent nonbonded contacts to those found in the CSD. Despite this seemingly modular approach for probing supramolecular assembly, we recognize that even slight chemical modifications might mean drastic changes to
10.1021/cg0497391 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/28/2004
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Table 1. Crystallographic Data for a Carboxylic Acid Heterodimer and Bipyridine Complexes crystal data
1‚2
1‚3
molecular formula MW (g‚mol-1) crystal size (mm3) crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Z′ Dcalcd (g‚cm-3) F(000) µ(Mo KR) (mm-1) temp (K) 2θ scan range (deg) reflns collected unique reflns data/param ratio Rint R/R2ω (obs data) R/R2ω (all data) ∆Fmax/min (e‚Å-3) S
C14H12N2O6 304.26 0.35 × 0.22 × 0.03 monoclinic P21/c (No. 14) 9.233(1) 6.1926(8) 23.143(3) 90 91.548(2) 90 1323.2(3) 4 1 1.527 632 0.122 158(2) 1.72-20.77 14114 3061 12.9 0.0277 0.0638/0.1537 0.0708/0.1584 0.453/-0.276 1.18
C12H11N2O2 215.23 0.45 × 0.43 × 0.32 monoclinic P21/c (No. 14) 15.8195(9) 4.3087(4) 16.4660(8) 90 107.173(5) 90 1072.3(1) 4 1 1.333 452 0.093 293(2) 2.54-28.69 3431 2475 7.8 0.0269 0.0573/0.1138 0.1417/0.1442 0.165/-0.210 1.00
the recognition profile of each bimolecular system. The inherent complexities in attempting structure prediction, even of relatively simple supramolecular motifs, are apparent when considering the collective contribution of each known interaction to crystal stabilization. Consequently, it is not surprising that the majority of studies that seek to understand self-assembly processes examine strong and weak interactions separately. Our investigation builds on the known associative behavior of carboxyl and pyridine/pyridine-N-oxide functions6 and attempts to assess the relative importance of both strong and weak interactions together for a structural family with related topological features. Experimental Section General Considerations. All chemicals and solvents were purchased from the Aldrich Chemical Co. or Acros Chemicals and used as received without further purification. Melting point data were determined using a Melt-Temp apparatus and are uncorrected. Recrystallization experiments were conducted at room temperature by slow evaporation using spectroscopic grade solvents. The crystal structures of 1‚47 and 1‚68 were previously reported in the literature; original atomic numbering schemes were altered to reflect those employed in the current study. General Synthesis of Benzoic Acid/Bipyridine Bimolecular Compounds. Cocrystalline samples were prepared by dissolution of benzoic acid (1 or 2, 1.29 mmol) and bipyridine (3-7, 0.647 mmol) in warm solvent. Compound 1‚2 was prepared from an equimolar mixture of 1 and 2. The resulting homogeneous solutions were allowed to recrystallize by slow evaporation at room temperature. After several days, crystalline samples were collected, assessed for quality using polarizing microscopy, and mounted for subsequent crystallographic investigation. 4-Aminobenzoic Acid/4-Nitrobenzoic Acid, 1‚2. Mp 228-229 °C; recrystallization solvent 1:2:3 CHCl3/hexane/2propanone. 4-Aminobenzoic Acid/2,2′-Bipyridine, 1‚3. Mp 151-152 °C; recrystallization solvent methanol. Attempted Synthesis of 4-Aminobenzoic Acid/2,2′Bipyridine-N,N′-dioxide, 1‚5. Repeated attempts to grow
1‚47 C12H11N2O2 215.23 monoclinic P21/n (No. 14) 8.261 5.4962 23.641 90 95.39 90 1068.65 4 1 1.331
2‚3
2‚4
C12H9N2O4 245.21 0.56 × 0.14 × 0.08 monoclinic C2/c (No. 15) 29.210(3) 3.8089(5) 23.2503(15) 90 118.019(4) 90 2283.4(4) 8 2 1.427 1016 0.110 293(2) 2.83-27.50 3551 2594 8.6 0.0347 0.0547/0.1125 0.1145/0.1374 0.150/-0.227 1.02
C12H9N2O4 245.21 0.70 × 0.22 × 0.10 monoclinic P21/n (No. 14) 7.8386(4) 6.8499(4) 20.793(1) 90 92.559(4) 90 1115.3(1) 4 2 1.460 508 0.112 293(2) 2.74-27.50 3618 2565 8.6 0.0296 0.0554/0.1236 0.1139/0.11519 0.174/-0.248 1.01
crystals of 1‚5 from varying solvents and conditions only resulted in starting materials as indicated by melting point and preliminary crystallographic studies. Since carboxyl and N-oxide groups are known to generate robust motifs, the lack of observed complexation was unexpected. Results from the recrystallization experiments suggested incompatible solubilities of 4-aminobenzoic acid and 2,2′-bipyridine-N,N′-dioxide as the source of experiment failure. 4-Aminobenzoic Acid/4,4′-Bipyridine-N-oxide, 1‚7. Mp 120-121 °C; recrystallization solvent methyl tert-butyl ether. 4-Nitrobenzoic Acid/2,2′-Bipyridine, 2‚3. Mp 180-181 °C; recrystallization solvent methanol. 4-Nitrobenzoic Acid/4,4′-Bipyridine, 2‚4. Mp 238-239 °C; recrystallization solvent methanol. 4-Nitrobenzoic Acid/2,2′-Bipyridine-N,N′-dioxide, 2‚5. Mp 208-210 °C; recrystallization solvent 1:1 methanol/H2O. 4-Nitrobenzoic Acid/4,4′-Bipyridine-N,N′-dioxide, 2‚6. Mp 271-271 °C; recrystallization solvent methanol. 4-Nitrobenzoic Acid/2,2′-Bipyridine-N-oxide, 2‚7. Mp 225-226 °C; recrystallization solvent 1:1:2 methanol/acetone/ H2O. Although several methods and solvent systems were employed, no suitable crystals could be retrieved for crystallographic analysis. Crystallography. Crystallographic details for compounds prepared for this study are summarized in Tables 1 and 2. The X-ray data for compounds 1‚3, 1‚7, 2‚3, 2‚4, and 2‚6 were collected at 25 °C on a Siemens P4 diffractometer using a graphite monochromatic Mo KR radiation (λ ) 0.710 73 Å) and XSCANS software package.9 Structures of 1‚2 and 2‚5 were collected at -153 and -138 °C, respectively, on a Bruker AXS Apex CCD diffractometer equipped with an LT-3 low-temperature device using graphite monochromatized Mo KR radiation (λ ) 0.710 73 Å). Data sets were corrected for Lorentz and polarization effects. No absorption corrections were applied since the absorption coefficient, µ, was small and crystal geometry was favorable in each case. Crystal stabilities were monitored by comparing 50 CCD frames collected (APEX CCD) with the same angles at the start and end of the data collection or by measuring three standard reflections every 97 reflections (P4) with no significant variations (
