DOI: 10.1021/cg900674p
Anion Binding Induces Helicity in a Hydrogen-Bonding Receptor: Crystal Structure of a 2,6-Bis(anilinoethynyl)pyridinium Chloride
2009, Vol. 9 4247–4249
Charles A. Johnson II, Orion B. Berryman, Aaron C. Sather, Lev N. Zakharov, Michael M. Haley,* and Darren W. Johnson* Department of Chemistry and Materials Science Institute, 1253 University of Oregon, Eugene, Oregon 97403-1253 Received June 17, 2009; Revised Manuscript Received August 24, 2009
ABSTRACT: The synthesis and solid-state of 2,6-bis(anilinoethynyl)pyridine amide 1 is presented. Neutral receptor 1 forms a polymeric chain through hydrogen bonds and π stacking in the solid state. Upon protonation, receptor 1 undergoes significant conformational rearrangement to hydrogen bond to Cl-, forming a helix (H1þCl-). This rare example of anion-induced helicity is detailed in the solid state and initial solution studies are presented. The helix plays a number of important roles in nature.1 The most well-known example certainly comes from the double stranded helix formed when two cDNA molecules twist to accommodate hydrogen bonds formed from their corresponding base pairs.2 The elegant functionality of biological helices has inspired the pursuit of other synthetic counterparts. Synthetically derived helices have largely focused on using metals,3-6 multiple metals,7 polymers,8,9 hydrogen bonds,10 and ion pairing within the ligand strands11-13 to drive helix formation. Interestingly, examples of anion-induced helicity are much less common. An early synthetic example of anion helicates came from de Mendoza and co-workers where bicyclic guanidinium ligands fold around anions into helical assemblies.14 A handful of other exciting examples of anion driven helical assemblies have emerged including a fluoride-directed double helix,15 a dichloride double helicate,16 a quintuple helical coordination polymer with tetrafluoroborate,17 oligoindole helices folding around chloride,18 and recently a triazole oligomer with nontraditional aryl C-H 3 3 3 Clhydrogen bonds that dictate the formation of an anion helicate.19 Only three of these relatively few examples of anion-induced helicity are structurally characterized by single-crystal X-ray diffraction.15-17 We present in this communication a highly conjugated hydrogen-bonding receptor molecule that undergoes a drastic conformational change to form a helical assembly with chloride upon protonation. Additionally, this is the first example of tandem N-H and alkyl C-H hydrogen bonds producing a helical assembly with chloride. We are interested in developing a modular synthesis for highly conjugated receptor molecules as a means to improve sensors for small molecules and ions. Our initial investigations introduced a series of hydrogen bonding sulfonamide receptors that form 2 þ 2 dimers with H2O, HX or both H2O and HX (X=Cl- and Br-).20 Subsequent studies with the corresponding urea receptors afforded 1 þ 1 complexes with HX.21 Our current research endeavors to prepare receptor molecules that are capable of binding heavy metal cations such as arsenic.22 This pursuit led us to the development of 2,6-bis(anilinoethynyl)pyridine amide 1. However, in our exploration of cation binding we observed interesting anion binding properties upon protonation of the receptor. Receptor 1 is synthesized as a white solid in excellent yield from key intermediate 2 in two steps (Scheme 1 and the Supporting Information).21 This modular synthesis was developed to ultimately produce a diverse library of conjugated
*Corresponding author. E-mail:
[email protected] (D.W.J.); haley@ uoregon.edu (M.M.H.).
hydrogen-bonding receptors in which to explore sensors for small molecules and ions. The solid-state conformation of neutral receptor 1 was investigated by single crystal X-ray diffraction. Colorless single crystals of 1 were harvested from slowly evaporating CH2Cl2 solutions. Receptor 1 crystallizes in the Pc space group with two symmetrically independent molecules per unit cell.23 Each molecule adopts a conformation with both amide functionalities on the opposite side of the molecule with respect to the pyridine nitrogen. The N 3 3 3 O distances between the amide nitrogens of one receptor and the amide oxygens of another receptor are in the range of 2.862(6) to 3.039(6) A˚, typical for amide N 3 3 3 O hydrogen bonds. The receptor conformation is stabilized by both hydrogen bonds between the amide nitrogens of one receptor and the amide oxygens of the next (2.855 and 3.009 A˚) and π-stacking between the pyridine ring and an alkyne functionality on an adjacent molecule (3.381-3.494 A˚). These interactions propagate throughout the crystal structure forming a one-dimensional chain (Figure 1). The empty spaces created between chains within the crystal structure are filled by dichloromethane solvent molecules. We were initially interested in studying the capability of 1 to bind heavy metals. Once deprotected, the thiol functionality along with the amide and pyridine nitrogens could fulfill a number of binding sites for a large cation. In an effort to deprotect 1 in situ to form a metal complex, we added receptor 1 to a nonanhydrous THF solution of AsCl3. To our surprise, this solution immediately turned bright yellow.24 Upon slow evaporation of the THF solution, yellow single crystals suitable for X-ray diffraction were obtained. Unexpectedly, the solid- state structure revealed the helical assembly of protonated 1 wrapping around chloride, H1þCl- (Figure 2). H1þCl- crystallizes in the P1 space group with two H1þCl- helical assemblies and four THF solvent molecules per unit cell.25 The helical assembly is stabilized intramolecularly by three strong N-H 3 3 3 Cl- hydrogen bonds to the chloride anion with N 3 3 3 Cl- contacts ranging from 3.004(3) A˚ (pyridinium NH) to 3.271(3) A˚ (amide NH) and two weaker alkyl C-H 3 3 3 Cl- interactions (3.631(4) and 3.644(4) A˚). An additional intermolecular alkyl C-H 3 3 3 Cl- interaction (3.639(4) A˚) is formed between adjacent molecules to complete the coordination sphere around the Cl- in the solid state. The helical pitch of the assembly is approximately 7.05 A˚ (defined as the distance the between the two methylene carbons in each molecule). Interestingly, AsCl3 is not required to form this helical assembly. Simply passing HCl gas through an organic solution of receptor 1 or adding a few drops of conc. HCl is sufficient to
r 2009 American Chemical Society
Published on Web 09/02/2009
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Figure 1. Stick representation of receptor 1 in the crystalline state highlighting (a) the intermolecular hydrogen bonds between 1 and solvent and (b) the extended packing structure. Non-hydrogen-bonding hydrogens have been removed for clarity; hydrogen bonds are depicted as black dashes.
Figure 2. (A) Stick (top) and CPK (bottom) representations of the helical assembly that forms between receptor 1 and chloride. (B) Wireframe representation showing the packing of two molecules of H1þCl- across the crystallographic inversion center to form a racemic dimer. Carbon is depicted as gray, nitrogen is blue, oxygen is red, sulfur is yellow, hydrogen is white, and THF solvent molecules are removed for clarity.
Scheme 1. Synthesis of Conjugated Hydrogen-Bonding Receptor
form the helical assembly. Upon addition of conc. HCl, colorless solutions of receptor 1 in CDCl3 turn bright yellow, suggesting protonation of the pyridine nitrogen. The 1H NMR spectrum of 1 in the presence of HCl shows marked changes from the neutral receptor 1. Significant resolution of the aromatic signals as well as substantial downfield shift of the amide protons is observed upon protonation. We were also interested in whether this conformation persists in solution. The 1H NMR spectrum of H1þCl- in CD2Cl2, d7-DMF and d8-toluene at room temperature all showed a singlet for the methylene protons indicating either coincidental overlap of the signals for the two possible helices, rapid interconversion of the two helices or no helix formation. To
support our hypothesis that the helices interconvert rapidly on the 1H NMR time scale, we performed low-temperature studies. A notable broadening of the methylene signals was observed when the CD2Cl2 and d8-toluene solutions of H1þClwere cooled to 205 and 222 K, respectively. The broadening of the methylene signal at low temperature may indicate that the two helices are interconverting in solution. However, the lack of complete decoalescence of the methylene signal at low temperatures suggests that interconversion barrier between the two helical enantiomers is less than 9 kcal mol-1 (see the Supporting Information). The synthetic control of helix formation is an exciting prospect in supramolecular chemistry. Nature has developed many
Communication
Crystal Growth & Design, Vol. 9, No. 10, 2009
essential uses for helices and we are exploring whether control over drastic conformational changes will result in improved sensors for anions. We are also currently investigating the electronic properties of these highly conjugated receptors and their assemblies with anions as well as cations. Future studies will be performed to examine the generality of helix formation for this class of receptor with other guests such as larger anions, metal cations, or chiral guests to induce formation of a single enantiomer. Acknowledgment. This work was funded by the National Science Foundation (NSF-0718242) and the University of Oregon (UO). OBB acknowledges the NSF for an Integrative Graduate Education and Research Traineeship (DGE-0549503). C.A.J. thanks UO for a Doctoral Research Fellowship. D.W.J. is a Cottrell Scholar of Research Corporation and gratefully acknowledges the NSF for a CAREER award. The authors would also like to thank Calden N. Carroll for additional synthetic work during the revision of this manuscript. Supporting Information Available: Synthetic details and variable-temperature 1H NMR data (PDF); crystallographic information (TXT). This material is available free of charge via the Internet at http://pubs.acs.org/.
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