We have synthesized three simple and versatile terphenyl crowns (TC) receptors containing ethereal canopies that direct a potassium cation for efficient cation–π interactions as established by 1H NMR spectroscopy and by isolation and X-ray crystallography of their K+ salts

Lindeman, Sergey V.

Rathore, Rajendra

Shukla, Ruchi

English

epublications@Marquette

Marquette Universitye-Publications@MarquetteChemistry Faculty Research and Publications Chemistry, Department of1-1-2009Terphenyl Crowns: a New Family of ReceptorsContaining Ethereal Canopies that DirectPotassium Cation onto Benzenoid Platforms forCation–π InteractionsRuchi ShuklaMarquette UniversitySergey V. LindemanMarquette University, sergey.lindeman@marquette.eduRajendra RathoreMarquette UniversityAccepted version. Chemical Communications, No. 37 (2009): 5600-5602. DOI. © 2009 Royal Societyof Chemistry. 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 th citation below.  Chemical Communications, No. 37 (2009): 5600-5602. DOI. This article is © Royal Society of Chemistry and permission has been granted for this version to appear in e-Publications@Marquette. Royal Society of Chemistry does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Royal Society of Chemistry.  Terphenyl Crowns: a New Family of Receptors Containing Ethereal Canopies that Direct Potassium Cation onto Benzenoid Platforms for Cation–π Interactions  Ruchi Shukla Department of Chemistry, Marquette University, Milwaukee, WI Sergey V. Lindeman Department of Chemistry, Marquette University, Milwaukee, WI Rajendra Rathore Department of Chemistry, Marquette University, Milwaukee, WI  Abstract We have synthesized three simple and versatile terphenyl crowns (TC) receptors containing ethereal canopies that direct a potassium cation for efficient cation–π interactions as established by 1H NMR spectroscopy and by isolation and X-ray crystallography of their K+ salts. The noncovalent cation–π interaction has been implicated to play an important role as a stabilizing force in both biological as well as chemical assemblies.1 These interactions have long been recognized in the gas phase and estimated for the benzene–K+complex to amount to ∼16 kcal mol−1.2 However, the importance of cation–π interactions in biological systems is questioned due to the overwhelming need for solvation of the cations in vivo.3 We have recently shown that a deliberately built solvation shell (i.e.a hydrophilic ethereal fence) around a central benzene ring in a hexaarylbenzene derivative allows an efficient binding of a potassium cation to the (hydrophobic) π-cloud of the benzenoid ring.4 Moreover, it was demonstrated that such a polyaromatic system can function as an electrochemical sensor for potassium cation.4b In order to probe the nature and applicability of such cation–π forces in both chemistry and biology, simple and easily modifiable scaffolds are needed which can be readily tailored for binding of different metal cations as well as hold potential to be eventually incorporated into polymeric backbones for development of new functional extractants5 or possibly for the preparation of conducting electrolytes for battery applications.6 Herein, we describe the hitherto unknown class of terphenyl crowns (TC) (see Scheme 1) which meet the criteria of ready availability, versatility, and possible incorporation into polymeric structures. Our interest in these receptors also stems from the fact that they possess an easily modifiable benzenoid platform as well as hydrophilic ethereal canopies which allow the positioning of the metal cations onto the aromatic platforms.    Scheme 1 A generalized synthetic scheme for o,m,p-terphenyl crowns and their molecular structures obtained by X-ray crystallography.  Accordingly, herein we report the ready preparation of terphenyl crowns (oo-TC, mm-TC and pp-TC) and delineate the effective binding of potassium cation to this new class of receptors with the aid of NMR spectroscopy in solution and by X-ray crystallography in the solid state. Syntheses of the various receptors shown above were accomplished in excellent yields by a simple three-step route (see Scheme 1). For example, a Suzuki coupling of 2-methoxyphenylboronic acid with 1,2-diiodobenzene followed by demethylation using BBr3 afforded the bis-phenol oo-2 in nearly quantitative yield. A solution of the bis-phenol oo-2 in tetrahydrofuran was treated with potassium hydride followed by tetraethyleneglycol ditosylate, and the resulting mixture was refluxed for 16 h. A standard aqueous workup and chromatographic purification on alumina (using hexanes and ethyl acetate mixture as eluent) afforded pure ortho-terphenyl crown (oo-TC) in >75% yield. Using a similar approach, mm-TC and pp-TC were obtained in 70 and 74% yields, respectively. The molecular structures of various terphenyl crowns in Scheme 1 were established by 1H/13C NMR spectroscopy and were further confirmed by X-ray crystallography‡ (see Scheme 1 and ESI† for experimental details). The binding of potassium cation to the para-terphenyl crown (pp-TC) was initially monitored by the changes in 1H NMR spectrum of pp-TC in acetone-d6 (0.02 M) by an incremental addition of a solution of potassium perfluorotetraphenylborate(0.08 M) in acetone-d6 (i.e.Fig. 1).    Fig. 1 Partial 1H NMR spectra obtained upon the incremental addition of KB(C6F5)4 to pp-TC in acetone-d6 at 22 °C. The assignments of the aromatic signals in pp-TC are shown by letters a–e; signal “a” was chosen arbitrarily for tracking of the K+binding.  The incremental addition of K+ solution led to considerable shifts of both aliphatic and aromatic signals up to the addition of 1 equiv. of K+ as shown in Fig. 1. It is noteworthy that the 1H NMR spectrum remained unchanged upon further addition of K+solution (i.e. beyond 1 equivalent; see Fig. 1, inset). Unfortunately, an accurate binding constant for the formation of [pp-TC, K+] could not be determined by NMR as it simply showed complete capture of the potassium cation and suggested that the binding constant is too large to be measured by NMR spectroscopy. Moreover, both ortho- and meta-derivatives (mm-TC and pp-TC) showed a similarly efficient capture of 1 equiv. of potassium cation by 1H NMR spectral titrations with K+B(C6F5)4− in acetone-d6 (see Fig. S1 and S2 in ESI†). A comparison of the relative binding of K+ amongst the three terphenyl crown derivatives in Scheme 1 by competition experiments (using 1H NMR spectroscopy) revealed that the binding efficiency of K+ decreases in the order of oo-TC > pp-TC > mm-TC. In order to ascertain that the efficiency of the binding of K+ to various terphenyl-based receptors, in part, arises owing to the placement of K+ onto the benzenoid platform by ethereal canopies, we obtained single crystals of [oo-TC, K+]−BPh4 and [pp-TC, K+]−BPh4 from mixtures of acetonitrile–CH2Cl2 and acetonitrile–THF, respectively, at 22 °C. Single crystals of [mm-TC, K+], suitable for X-ray crystallography, were obtained only when hexafluoroarsenate (AsF6−) was used as the counter anion. The molecular structures of these complexes obtained by X-ray crystallography are displayed in Fig. 2–4.§    Fig. 2 The ORTEP and space-filling representations of [oo-TC, K+]−BPh4 showing that the ethereal canopy holds the K+ onto the central aromatic ring while a single opening is protected by a phenyl group of the −BPh4 counter anion via cation–π interaction.8b,c The hydrogens are omitted for the sake of clarity.     Fig. 3 The ORTEP and space-filling representations of [pp-TC, K+] showing that the ethereal canopy holds the K+ onto the central aromatic ring while the two apical openings in the complex are plugged-in by solvent molecules. The hydrogens are omitted for the sake of clarity.     Fig. 4 The ORTEP and space-filling representations of [mm-TC, K+]−AsF6 showing that the ethereal canopies hold the K+ onto the central aromatic ring while the two apical openings in mm-TC complex are filled in by a molecule of water and counter anion. The hydrogens are omitted for the sake of clarity.  The X-ray structure of [oo-TC, K+]−BPh4 revealed that a single potassium cation is situated asymmetrically onto the central benzene ring with a distance from the mean plane of the benzene ring of 2.958 Å while coordinating to all five ethereal oxygens of the oo-TC crown with the K+⋯O distances varying in the range of 2.69–3.04 Å.7 The asymmetric (η3) coordination of K+ to the central benzene ring is not surprising, considering the asymmetric shape of the receptor in which two ortho-phenyl groups are twisted in the same direction (∼75°) relative to the central benzene ring and thereby shifting the main axis of the crown-ether moiety together with the complexed K+ away from the center of the benzene ring. The asymmetric placement of K+ onto the central benzene ring is also gauged by the K+⋯CAr distances which vary from 3.157(1) to 3.422(2) Å.8a Furthermore, one of the phenyls of the BPh4 counter anion makes a short contact [3.317(1) Å] with the K+ through the limited opening between the crown ether canopy and central benzene ring, see Fig. 2 (and Fig. S3 in ESI†).8b,c In contrast, the potassium cation in [pp-TC, K+]−BPh4 is almost ideally placed onto the central benzene ring for an effective η6coordination at a distance of 3.163 Å from the mean plane of the benzene ring with the K+⋯CAr distances varying from 3.38 to 3.56 Å. The crown ether moiety in the [pp-TC, K+] complex makes a rigid arc over the central benzene ring, as opposed to the flexible cleft in oo-TC, with the K+⋯O distances varying from 2.78 to 3.31 Å. The K+ is situated approximately in the mean plane of five ethereal oxygens which makes a ∼90° dihedral angle with the plane of the benzene ring and the bridging p-phenyls are twisted by 56 and 60° in the same direction relative to the plane of the central benzene ring. It is also noted that the [pp-TC, K+]−BPh4 complex is a solvent-separated ion pair in the solid where the two apical openings in [pp-TC, K+] are coordinated (surprisingly) to two different solvent molecules, i.e.tetrahydrofuran and acetonitrile§ (see Fig. 2). The X-ray structure of [mm-TC, K+]−AsF6 complex showed that K+ is coordinated by all five oxygens of the crown-ethercanopy with the K+⋯O distances varying in the range of 2.74–3.14 Å. The potassium cation sits asymmetrically onto the central benzene ring with ideal η2 coordination with the shortest K+⋯C distances of 3.239 and 3.238 Å to the central benzene ring (see Fig. 4). The K+⋯C distances to other four carbons of the central benzene ring vary from 3.298 to 3.346 Å. Furthermore, the nucleophilic −AsF6 and a water molecule also make short F⋯K+ (2.704 Å) and O⋯K+ (2.637 Å) contacts via the two apical openings in the [mm-TC, K+]−AsF6 complex.9 The X-ray crystallographic analyses of the K+ complexes in Fig. 2–4 shows that going from oo-TC to mm-TC to pp-TCcomplex leads to a progressive stretching of the ethereal canopies. As such the stretching of the ethereal canopies affects the K+⋯Ar coordination which varies from asymmetric to symmetric coordination going from oo-TC to mm-TC to pp-TC. Furthermore the structural analysis provides important clues as to the potential of the terphenyl crowns for the development of a whole new family of crown receptors for practical applications. For example, incorporation of electro/photo-active groups either at the site of single opening in the [oo-TC, K+] complex or at the site of a pair of apical openings in [mm-TC, K+] and [pp-TC, K+] complexes should lead to the next-generation of sensing scaffolds for various metal ions using electrochemical/optical techniques. In summary, we have designed and synthesized simple terphenyl-based receptors containing (polar) ethereal canopies that direct a potassium cation to the central benzene ring for cation–π interaction—a phenomenon that is well established in the gas phase2 and is known to play an important role in the stabilization of tertiary structures of various proteins.10 We are actively exploring the potential of this new family of crowns for the preparation of functional devices for potential applications in emerging areas ranging from molecular electronics and nanotechnology11 to solar energy storage.12 We thank the National Science Foundation for financial support.  Notes and references 1. (a) G. W. Gokel, W. M. Leevy and M. E. Weber, Chem. Rev., 2004, 104, 2723 ; (b) Comprehensive Supramolecular Chemistry, ed. J.-M. Lehn, J. L. Atwood, J. E. D. Davies, D. D. MacNicol and F. Vögtle, Pergamon, Oxford, 1996 ; (c) Supramolecular Chemistry, ed. J. W. Steed and J. L. Atwood, Wiley, New York, 2000, vol. 9 ; (d) A. Joy, L. S. Kaanumalle and V. Ramamurthy, Org. Biomol. Chem., 2005, 3, 3045  , and references cited therein. 2. B. C. Guo, J. W. Purnell and A. W. Castlman, Jr, Chem. Phys. Lett., 1990, 168, 155  , and references cited therein. 3. (a) J. C. Ma and D. A. Dougherty, Chem. Rev., 1997, 97, 1303 ; (b) G. W. Gokel, S. L. De Wall and E. S. Meadows,Eur. J. Org. Chem., 2000, 2967  , and references cited therein. 4. (a) R. Shukla, S. V. Lindeman and R. Rathore, J. Am. Chem. Soc., 2006, 128, 5328 ; (b) R. Shukla, S. V. Lindeman and R. Rathore, Org. Lett., 2007, 9, 1291 . 5. A. Aydogan, D. J. Coady, S. K. Kim, A. Akar, C. W. Bielawski, M. Marquez and J. L. Sessler, Angew. Chem., Int. Ed., 2008, 47, 9648  , and references cited therein. 6. P. G. Bruce, Dalton Trans., 2006, 1365  , and references cited therein. 7. According to the Cambridge Structural Database, the K+⋯O distances in various potassium/crown complexes have a median value of ∼2.85 Å. 8. Note that the closest K+⋯CAr distance of 3.156 Å in [oo-TC, K+] complex is much shorter than the K+⋯CAr distance of 3.38 Å in a K+/toluene complex, see: (a) B. T. King, B. C. Noll and J. Michl, Collect. Czech. Chem. Commun., 1999, 64, 1001 ; (b) G. W. Gokel, L. J. Barbour, R. Ferdani and J. Hu, Acc. Chem. Res., 2002, 35, 878 ; (c) Q. Q. Munro and N. Pearson, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2003, 59, m407 . 9. The X-ray structure analysis of the [mm-TC, K+]AsF6− complex showed that there were two symmetrically independent molecules with one of them containing a coordinated water molecule with only 32% population. The inclusion of a coordinated water molecule in the cleft between the ethereal arc and aromatic ring does not change the K+ coordination to aromatic ring but does alter the cleft angle slightly, i.e. a 58° opening in the non-aqueous complex vs. 67° in the aqueous one, suggesting a significant stability of the η2-coordination of the K+ similar to that found for various Ag+ analogs. 10. (a) S. K. Burley and G. A. Petsko, FEBS Lett., 1986, 203, 139 ; (b) Y. Sakurai, T. Mizuno, H. Hiroaki, K. Gohda, J. Oku and T. Tanaka, Angew. Chem., Int. Ed., 2005, 44, 6180  , and references cited therein. 11. Introduction to Molecular Electronics, ed. M. C. Petty, M. R. Bryce and D. Bloor, Oxford Univ. Press, New York, 1995 . 12. F. D'Souza, R. Chitta, S. Gadde, M. E. Zandler, A. L. McCarty, A. S. D. Sandanayaka, Y. Araki and O. Ito, Chem.–Eur. J., 2005, 11, 4416  , and ref. 6. Footnotes † Electronic supplementary information (ESI) available: Synthetic details of various compounds and NMR spectra. CCDC 738411–738416. For ESI andcrystallographic data in CIF or other electronic format see DOI: 10.1039/b912796h ‡ Crystal structure data forpp-TC [C26H28O5] (raj1v): M = 420.48, monoclinic, P21/n, a = 13.048(5), b = 9.487(3), c = 17.671(6) Å, β = 97.031(5)°, Z = 4, V = 2170.9(13) Å3,Dc = 1.287 Mg m−3, T = 100 K, 24585 reflections measured, 6832 unique reflections, Rint = 0.0463, 281 parameters refined, R(all) = 0.1310, wR(all) = 0.2779, S = 1.067.Crystal structure data formm-TC [C26H28O5] (raj4b): M = 420.48, triclinic, P, a = 8.8435(5), b = 19.9062(10), c = 26.7874(14) Å, α = 111.253(2), β = 98.337(2), γ = 93.101(2)°, Z = 8, V = 4319.3(4) Å3, Dc = 1.293 Mg m−3, T = 100 K, 35 410 reflections measured, 12 685 unique reflections, Rint = 0.0247, 1119 parameters refined,R(all) = 0.0417, wR(all) = 0.1105, S = 1.044. Crystal structure dataforoo-TC [C26H28O5] (raj4d): M = 420.48, triclinic, P , a = 9.2824(2), b = 10.4418(2), c = 11.6456(2) Å,α = 90.9020(10), β = 98.7730(10), γ = 102.9340(10)°, Z = 2, V = 1085.78(4) Å3, Dc = 1.286 Mg m−3, T = 100 K, 8959 reflections measured, 3184 unique reflections, Rint= 0.0164, 393 parameters refined, R(all) = 0.0281, wR(all) = 0.0684, S = 0.992. Crystal structure data for [pp-TC, K+]−BPh4 [C26H28O5·KBPh4·THF·CH3CN] (raj3ya): M = 891.95, triclinic, P , a = 10.7324(2), b = 113.8365(3), c = 17.2011(4) Å, α = 77.8290(10), β = 89.3530(10), γ = 77.0660(10)°, Z = 2, V = 2431.85(9) Å3, Dc = 1.218 Mg m−3, T= 100 K, 20552 reflections measured, 7178 unique reflections, Rint = 0.0141, 843 parameters refined, R(all) = 0.0319, wR(all) = 0.0869, S = 1.016. Crystal structure datafor [mm-TC, K+]−AsF6 [C26H28O5·KAsF6·xH2O] (raj9qa): M = 651.10, triclinic, P , a = 10.0564(3), b = 15.2834(4), c = 18.9578(5) Å, α = 103.571(1), β = 103.485(2), γ = 91.093(1)°, Z = 4, V = 2745.89(13) Å3, Dc = 1.575 Mg m−3, T = 100 K, 8874 reflections measured, 6702 unique reflections, Rint = 0.0164, 393 parameters refined, R(all) = 0.0589, wR(all) = 0.1603, S = 1.039. Crystal structure data for [oo-TC, K+]−BPh4 [C26H28O5·KBPh4] (raj3sa): M = 778.79, monoclinic, P21/c, a = 13.1774(5), b = 15.6866(6), c = 19.6322(7) Å, β = 94.300(2)°, Z = 4, V = 4046.7(3) Å3, Dc = 1.278 Mg m−3, T = 100 K, 25942 reflections measured, 6139 unique reflections, Rint = 0.0180, 716 parameters refined, R(all) = 0.0284, wR(all) = 0.0774, S = 1.009. § For a discussion of the bent K+⋯NC–Me interaction in the [pp-TC, K+] complex, see Fig. S4 and S5 in the ESI.†   

Terphenyl Crowns: a New Family of Receptors Containing Ethereal Canopies that Direct Potassium Cation onto Benzenoid Platforms for Cation–π Interactions

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Terphenyl Crowns: a New Family of Receptors Containing Ethereal Canopies that Direct Potassium Cation onto Benzenoid Platforms for Cation–π Interactions

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