C O M M U N I C A T I O N S
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.
propeller may only occur at the expense of substantial repulsive
interaction among the peripheral aryl rings.9
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 synergisti-
cally interacts with the polar ethereal fence and with the central
benzene ring via cation-π interactionsa phenomenon that is well
established in gas phase10 and in solid state 8 and 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.
Figure 1. Partial 1H NMR spectra obtained upon the incremental addition
of KB(C6F5)4 to 5s in acetonitrile-CH2Cl2 at 22 °C.
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.
Figure 2. Structure of [5s, K+] -BPh4 with a single CH3CN molecule
complexed to K+. The hydrogens and the anion are omitted.
Supporting Information Available: Preparation, spectral data for
1-5, and the X-ray data. This material is available free of charge via
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.
References
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(3) Compare: Trueblood, K. N.; Knobler, C. B.; Maverick, E.; Helgeson, R.
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
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Å (as opposed to the observed K+‚‚‚O separation of 2.83-3.21 Å
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(6) Complete synthetic details for the preparation of various bridged diaryl-
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(7) Competition experiments indicated that 5s and 18-crown-6 have a
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(9) Note that dihedral angles between peripheral aryl rings and central benzene
ring vary between 55 and 70°; see CCD Database.
(10) Guo, B. C.; Purnell, J. W.; Castlman, A. W., Jr. Chem. Phys. Lett. 1990,
168, 155 and references therein.
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(12) Rathore, R.; Burns, C. L.; Abdelwahed, S. H. Org. Lett. 2004, 6, 1689.
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