We have designed and synthesized a HAB-based receptor with six ethereal oxygens on one face of the central benzene ring by a trimerization of a diarylacetylene in which the ethereal oxygens are tied together with a tetramethylene bridge. This unique amphiphilic receptor allows an efficient binding of a single potassium cation by a synergistic interaction with the polar ethereal fence and with the central benzene ring via cation−π interaction. Furthermore, the ready accessibility of this unique receptor with a bipolar binding pocket will allow the exploration of its usage for developing efficient sensing devices for various metal cations

Lindeman, Sergey V.

Rathore, Rajendra

Shukla, Ruchi

English

epublications@Marquette

Marquette Universitye-Publications@MarquetteChemistry Faculty Research and Publications Chemistry, Department of1-1-2006A Polyaromatic Receptor with an Ethereal Fencethat Directs K+ for Effective Cation−π InteractionRuchi ShuklaMarquette UniversitySergey V. LindemanMarquette University, sergey.lindeman@marquette.eduRajendra RathoreMarquette UniversityAccepted version. Journal of the American Chemical Society, Vol. 128, No. 16 (2006): 5328-5329.DOI. © 2006 American Chemical Society. Used with permission. Marquette University e-Publications@Marquette  Chemistry Faculty Research and Publications/College of Arts and Sciences  This paper is NOT THE PUBLISHED VERSION; but the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation below.  Journal of the American Chemical Society, Vol. 128, No. 16 (2006): 5328-5329. DOI. This article is © American Chemical Society and permission has been granted for this version to appear in e-Publications@Marquette. American Chemical Society does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from American Chemical Society.   A Polyaromatic Receptor with an Ethereal Fence that Directs K+ for Effective Cation−π Interaction  Ruchi Shukla Department of Chemistry, Marquette University, Milwaukee, Wisconsin Sergey V. Lindeman Department of Chemistry, Marquette University, Milwaukee, Wisconsin Rajendra Rathore Department of Chemistry, Marquette University, Milwaukee, Wisconsin   Abstract  We have designed and synthesized a HAB-based receptor with six ethereal oxygens on one face of the central benzene ring by a trimerization of a diarylacetylene in which the ethereal oxygens are tied together with a tetramethylene bridge. This unique amphiphilic receptor allows an efficient binding of a single potassium cation by a synergistic interaction with the polar ethereal fence and with the central benzene ring via cation−π interaction. Furthermore, the ready accessibility of this unique receptor with a bipolar binding pocket will allow the exploration of its usage for developing efficient sensing devices for various metal cations.  The hexaphenylbenzene core has gained considerable attention due to its unique propeller-shaped structure and its usage for the preparation of modern graphitic materials for the applications in the emerging area of molecular electronics and nanotechnology.1 Numerous possibilities exist for future development and use of novel architectures based on hexaarylbenzene (HAB) derivatives in sensor applications and host−guest chemistry, in general, if a controlled synthesis can be developed which will allow the positioning of all the orthosubstituents (such as ethereal oxygens on the peripheral aryl rings) on one face of the central benzene ring. Interestingly, however, the ortho-substituted HAB derivatives exist as a mixture of rotamers due to the restricted rotation around the C−C bonds between the six peripheral aryl rings and the central benzene ring. For instance, it has been established2 that trimerization of bis(2-methoxyphenyl)acetylene leads to the formation of only eight rotamers (i.e., six achiral and one pair of enantiomeric rotamers) of hexakis(2-methoxyphenyl)benzene out of the possible nine rotamers. The only missing rotamer from this synthesis is the one which contains all the ethereal oxygens on one face of the propeller. Such a symmetrical isomer can be potentially useful for directing a metal cation to the central benzene ring with the aid of six ethereal oxygens that are prearranged in a manner akin to an 18-crown-6 geometry.3 Accordingly, in this paper, we now report a new strategy to access this elusive rotamer containing all the ethereal oxygens on one face of the HAB and delineate by NMR spectroscopy and X-ray crystallography that it binds a single potassium cation to the central benzene ring via a synergistic interaction with the hydrophilic ethereal fence and the hydrophobic π-cloud of the central benzene ring4 as follows. It was conceived that preparation of the HAB derivative with oxygens on one face can be accomplished by a trimerization of a diarylacetylene in which the ethereal oxygens are tied together with a polymethylene bridge (see structure 4 in Scheme 1). Unfortunately, the synthesis of such bridged acetylenic derivatives from existing procedures was possible only in very poor yields.5 Therefore, we first developed a practical route to access the bridged diarylacetylene 4 in which the ethereal oxygens are tied with a tetramethylene bridge (see Scheme 1).6 For example, a reaction of 2 equiv of isopropylsalicylaldehyde (1) with 1,4-dibromobutane in the presence of NaOH in refluxing ethanol afforded dialdehyde 2 in 95% yield. The McMurry coupling under mild dilution produced a mixture of cis/trans stilbenes 3 in >85% yield on a 20 g scale reaction, which were readily converted to the corresponding acetylene 4 (in >80% yield) by a simple bromination and dehydrobromination reaction sequence (see Supporting Information).  Scheme 1.  Synthesis of HAB-based Receptor 5Sa a Conditions:  (a) NaOH, EtOH, reflux; (b) TiCl4, Zn, Py, THF, reflux; (c) Br2, AcOH; (d) KOtBu, THF, 22 °C; (e) Co2(CO)8, dioxane, reflux.  A Co2(CO)8-catalyzed trimerization of 4 in refluxing dioxane for 12 h produced a mixture which contained roughly 25% trimer and ∼75% of the starting acetylene 4. A quick chromatographic separation on silica gel (using hexanes and ethyl acetate mixture as eluent) afforded pure symmetrical isomer 5s and the unsymmetrical isomer 5u in ∼1:2 ratio in >95% yield based on the recovered 4. The identity of 5s was readily established by the simplicity of its 1H/13C NMR spectra in comparison to that of 5u and was further confirmed by FAB mass spectrometry (see Supporting Information). The binding of K+ cation to 5s was monitored by 1H NMR spectroscopy. Thus, an incremental addition of potassium perfluorotetraphenylborate solution (0.08 M) to a solution of 5s in acetone-d6 (0.02 M) showed the appearance of a new set of signals in addition to the initial signals due to the uncomplexed 5s. The 1H NMR signals due to the 5s were completely replaced by the new signals upon addition of 1 equiv of K+, as shown in Figure 1. It is noteworthy that the 1H NMR spectrum remained unchanged upon further addition of K+ solution (i.e., beyond 1 equiv). Such a spectral change is readily reconciled as arising from the complexation of K+ to 5s. Moreover, it is noted that the potassium cation is held strongly in the [5s, K+] complex because of the fact that it does not undergo an exchange reaction with the uncomplexed 5s on the NMR time scale (at 22 °C). Unfortunately, an accurate binding constant for the formation of [5s, K+] could not be determined by NMR method as it simply showed complete capture of the K+ and suggested that the binding constant is too large to be measured by NMR spectroscopy.7  Figure 1 Partial 1H NMR spectra obtained upon the incremental addition of KB(C6F5)4 to 5s in acetonitrile−CH2Cl2 at 22 °C.  To ascertain the structure of the [5s, K+] complex, we attempted the crystallization of [5s, K+] -B(C6F5)4 by a slow diffusion of hexanes in an acetonitrile solution of the complex that resulted in poor quality crystals. However, by changing the counteranion from -B(C6F5)4 to -BPh4, the formation of an excellent crop of single crystals of the [5s, K+] -BPh4 from an acetonitrile−CH2Cl2mixture at 22 °C resulted. The structure of the complex [5s, K+ CH3CN] -BPh4 is shown in Figure 2.  Figure 2 Structure of [5s, K+] -BPh4 with a single CH3CN molecule complexed to K+. The hydrogens and the anion are omitted.  The X-ray structure in Figure 2 reveals that a single potassium cation sits in a shallow cavity with an (hydrophobic) aromatic bottom (i.e., the central benzene ring) while effectively interacting with the (hydrophilic) polar fence formed by six ethereal oxygens. The depth (i.e., the distance between the mean planes of the central benzene ring carbons and the ethereal fence) and the radius of the cavity are 2.22 and 2.97 Å, respectively. Such a bipolar nature of the cavity in 5s allows a tight van der Waals fit for a single K+ cation as discussed below. The most striking feature of the structure in Figure 2 is the symmetrical η6-coordination of the K+by the central benzene ring of 5s. The distance between the mean plane of the benzene ring and the K+ is 2.742 Å, a distance that is much shorter than the sum of van der Waals/ionic radii of carbon and K+ (i.e., 3.22 Å) and surpasses any of the previous experimental observations of the cation−arene contacts.8 One of the main factors favoring such a close K+···Ar coordination is the synergistic coordination of potassium cation by the all six ethereal oxygens prearranged in a geometry similar to that of the 18-crown-6. Another feature of the structure is that the K+ sits slightly (i.e., ∼0.52 Å) above the coordination plane of the ethereal fence, as pictured below in Figure 3. A simple geometrical consideration showed that the possible perpendicular orientation of the peripheral aryl rings in 5s could provide a more coplanar coordination of potassium cation with the ethereal fence with an almost normal K+···O separation of ∼2.84 Å (as opposed to the observed K+···O separation of 2.83−3.21 Å or av. 3.02 Å). However, such a structural alteration of the HAB propeller may only occur at the expense of substantial repulsive interaction among the peripheral aryl rings.9  Figure 3 Molecular structure of [5s, K+] showing the effective overlap of the polar ethereal fence with K+ while it rests on the aromatic bottom via cation−π interactions. The methyls of isopropyl groups and CH3CN are omitted.  In summary, we have designed and synthesized a HAB-based receptor that contains a bipolar receptor site that allows a remarkably efficient binding of a single potassium cation because it synergistically interacts with the polar ethereal fence and with the central benzene ring via cation−π interaction a phenomenon that is well established in gas phase10 and in solid state 8and is known to play an important role in the stabilization of tertiary structures of various proteins.11 We believe that the development of this unique receptor with an amphiphilic binding pocket and the electronically coupled peripheral aryl groups12 will allow its usage for the development of practical sensors for various metal ions. We are currently exploring the selectivity of the binding of various metal cations to receptor 5s as well as its usage for sensing devices. Acknowledgment We thank the National Science Foundation (Career Award) and Marquette University for financial support. This paper is dedicated to Prof. S. Chandrasekaran (I. I. Sc., Bangalore, India) on the occasion of his 60th birthday. References 1(a) Introduction to Molecular Electronics; Petty, M. C., Bryce, M. R., Bloor, D., Eds.; Oxford University Press:  New York, 1995. (b) Watson, M. D.; Fechtenkötter, A.; Müllen, K. Chem. Rev. 2001, 101, 1267. 2Pepermans, H.; Willem, R.; Gielen, M.; Hoogzand, C. J. Org. Chem. 1986, 51, 301. 3Compare:  Trueblood, K. N.; Knobler, C. B.; Maverick, E.; Helgeson, R. C.; Brown, S. B.; Cram, D. J. J. Am. Chem. Scoc. 1981, 103, 5594. 4Via a cation−π interaction, for example, see:  (a) Gokel, G. W.; De Wall, S. L.; Meadows, E. S. Eur. J. Org. Chem. 2000, 2967. (b) Ma, J. C.; Dougherty, D. A. Chem. Rev. 1997, 97, 1303 and references therein. 5Brizius, G.; Billingsley, K.; Smith, M. D.; Bunz, U. H. F. Org. Lett. 2003, 5, 3951 and references therein. 6Complete synthetic details for the preparation of various bridged diarylacetylenes will be presented separately. 7Competition experiments indicated that 5s and 18-crown-6 have a comparable affinity for K+. 8(a) Atwood, J. L.; Crissinger, K. D.; Rogers, R. D. J. Organomet. Chem. 1978, 155, 1. (b) King, B. T.; Noll, B. C.; Michl, J. Collect. Czech. Chem. Commun. 1999, 64, 1001. (c) Compare:  Hu, J.; Barbour, Gokel, G. W. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5121 and references therein. 9Note that dihedral angles between peripheral aryl rings and central benzene ring vary between 55 and 70°; see CCD Database. 10Guo, B. C.; Purnell, J. W.; Castlman, A. W., Jr. Chem. Phys. Lett. 1990, 168, 155 and references therein. 11(a) Burley, S. K.; Petsko, G. A. FEBS Lett. 1986, 203, 139. (b) Sakurai, Y.; Mizuno, T.; Hiroaki, H.; Gohda, K.; Oku, J.; Tanaka, T. Angew. Chem., Int. Ed. 2005, 44, 6180 and references therein. 12Rathore, R.; Burns, C. L.; Abdelwahed, S. H. Org. Lett. 2004, 6, 1689.  

A Polyaromatic Receptor with an Ethereal Fence that Directs K\u3csup\u3e+\u3c/sup\u3e for Effective Cation−π Interaction

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A Polyaromatic Receptor with an Ethereal Fence that Directs K\u3csup\u3e+\u3c/sup\u3e for Effective Cation−π Interaction

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