Terphenyl crowns: A new family of receptors containing ethereal canopies that direct potassium cation onto benzenoid platforms for cation-π interactions

Article abstract of DOI:10.1039/b912796h

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.

Full text of DOI:10.1039/b912796h

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Terphenyl crowns: a new family of receptors containing ethereal canopies
that direct potassium cation onto benzenoid platforms for cation–p
interactionsw
Ruchi Shukla, Sergey V. Lindeman and Rajendra Rathore*
Received (in Austin, TX, USA) 29th June 2009, Accepted 28th July 2009
First published as an Advance Article on the web 20th August 2009
DOI: 10.1039/b912796h
We have synthesized three simple and versatile terphenyl
crowns (TC) receptors containing ethereal canopies that direct
a potassium cation for efficient cation–p interactions as
established by 1H NMR spectroscopy and by isolation and
X-ray crystallography of their K+ salts.
The noncovalent cation–p 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 B16 kcal molꢀ1 2
.
However, the importance of cation–p 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) p-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–p
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.
Accordingly, herein we report the ready preparation of
terphenyl crowns (o-TC, m-TC and p-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.
Scheme 1 A generalized synthetic scheme for o,m,p-terphenyl crowns
and their molecular structures obtained by X-ray crystallography.
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-methoxy-
phenylboronic acid with 1,2-diiodobenzene followed by
demethylation using BBr3 afforded the bis-phenol o-2 in nearly
quantitative yield. A solution of the bis-phenol o-2 in tetra-
hydrofuran 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 (o-TC) in 475% yield.
Using a similar approach, m-TC and p-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
crystallographyz (see Scheme 1 and ESIw for experimental details).
The binding of potassium cation to the para-terphenyl crown
(p-TC) was initially monitored by the changes in 1H NMR
spectrum of p-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).
Department of Chemistry, Marquette University, P. O. Box 1881,
Milwaukee, Wisconsin 53201, USA.
E-mail: rajendra.rathore@marquette.edu; Fax: 01-414-288-7066;
Tel: 01-414-288-2076
w Electronic supplementary information (ESI) available: Synthetic
details of various compounds and NMR spectra. CCDC
738411–738416. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b912796h
ꢁc
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Fig.
2
The ORTEP and space-filling representations of
[o-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–p interaction.8b,c
The hydrogens are omitted for the sake of clarity.
Fig. 1 Partial 1H NMR spectra obtained upon the incremental
addition of KB(C6F5)4 to p-TC in acetone-d6 at 22 1C. The
assignments of the aromatic signals in p-TC are shown by letters
a–e; signal ‘‘a’’ was chosen arbitrarily for tracking of the K+ binding.
Fig. 3 The ORTEP and space-filling representations of [p-TC, K+
]
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 [p-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 (m-TC and p-TC) showed a similarly efficient
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.
1
capture of 1 equiv. of potassium cation by H NMR spectral
titrations with K+B(C6F5)4ꢀ in acetone-d6 (see Fig. S1 and S2
in ESIw). 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 o-TC 4 p-TC 4 m-TC.
Fig.
4 The ORTEP and space-filling representations of
[m-TC, K+]ꢀAsF6 showing that the ethereal canopies hold the K+
onto the central aromatic ring while the two apical openings in m-TC
complex are filled in by a molecule of water and counter anion. The
hydrogens are omitted for the sake of clarity.
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 [o-TC, K+]ꢀBPh4 and
[p-TC, K+]ꢀBPh4 from mixtures of acetonitrile–CH2Cl2 and
acetonitrile–THF, respectively, at 22 1C. Single crystals of
[m-TC, K+], suitable for X-ray crystallography, were obtained
only when hexafluoroarsenate (AsF6ꢀ) was used as the counter
anion. The molecular structures of these complexes obtained
by X-ray crystallography are displayed in Fig. 2–4.y
twisted in the same direction (B751) 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+ꢂꢂꢂCAr
distances which vary from 3.157(1) to 3.422(2) A.8a Furthermore,
one of the phenyls of the BPh4 counter anion makes a short
contact [3.317(1) A] with the K+ through the limited opening
between the crown ether canopy and central benzene ring, see
Fig. 2 (and Fig. S3 in ESIw).8b,c
The X-ray structure of [o-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 A while coordinating to all five ethereal oxygens of
the o-TC crown with the K+ꢂꢂꢂO distances varying in the range
of 2.69–3.04 A.7 The asymmetric (Z3) 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
In contrast, the potassium cation in [p-TC, K+]ꢀBPh4 is
almost ideally placed onto the central benzene ring for an
effective Z6 coordination at a distance of 3.163 A from the
mean plane of the benzene ring with the K+ꢂ ꢂ ꢂCAr distances
varying from 3.38 to 3.56 A. The crown ether moiety in the
ꢁc
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[p-TC, K+] complex makes a rigid arc over the central benzene
ring, as opposed to the flexible cleft in o-TC, with the K+ꢂ ꢂ ꢂO
distances varying from 2.78 to 3.31 A. The K+ is situated
approximately in the mean plane of five ethereal oxygens
which makes a B901 dihedral angle with the plane of the
benzene ring and the bridging p-phenyls are twisted by 56 and
601 in the same direction relative to the plane of the central
benzene ring. It is also noted that the [p-TC, K+]ꢀBPh4
complex is a solvent-separated ion pair in the solid where
the two apical openings in [p-TC, K+] are coordinated
(surprisingly) to two different solvent molecules, i.e. tetra-
hydrofuran and acetonitriley (see Fig. 2).
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 data for o-TC [C26H28O5] (raj4d): M = 420.48, triclinic, P1, a =
9.2824(2), b = 10.4418(2), c = 11.6456(2) A, a = 90.9020(10), b =
98.7730(10), g = 102.9340(10)1, Z = 2, V = 1085.78(4) A3, Dc = 1.286
Mg mꢀ3, 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 [p-TC, K+]ꢀBPh4 [C26H28O5ꢂ
ꢀ
891.95, triclinic, P1,
KBPh4ꢂTHFꢂCH3CN] (raj3ya):
M
=
a
=
10.7324(2), b = 113.8365(3), c = 17.2011(4) A, a = 77.8290(10),
b = 89.3530(10), g = 77.0660(10)1, Z = 2, V = 2431.85(9) A3, Dc =
1.218 Mg mꢀ3, 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 data for [m-TC, K+]ꢀAsF6
ꢀ
[C26H28O5ꢂKAsF6ꢂxH2O] (raj9qa): M = 651.10, triclinic, P1, a =
The X-ray structure of [m-TC, K+]ꢀAsF6 complex showed
that K+ is coordinated by all five oxygens of the crown-ether
canopy with the K+ꢂꢂꢂO distances varying in the range of
2.74–3.14 A. The potassium cation sits asymmetrically onto the
central benzene ring with ideal Z2 coordination with the shortest
K+ꢂꢂꢂC distances of 3.239 and 3.238 A to the central benzene
ring (see Fig. 4). The K+ꢂꢂꢂC distances to other four carbons of
the central benzene ring vary from 3.298 to 3.346 A. Furthermore,
the nucleophilic ꢀAsF6 and a water molecule also make short
FꢂꢂꢂK+ (2.704 A) and OꢂꢂꢂK+ (2.637 A) contacts via the two
apical openings in the [m-TC, K+]ꢀAsF6 complex.9
10.0564(3), b = 15.2834(4), c = 18.9578(5) A, a = 103.571(1),
b = 103.485(2), g = 91.093(1)1, Z = 4, V = 2745.89(13) A3, Dc
=
1.575 Mg mꢀ3, 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 [o-TC, K+]ꢀBPh4
[C26H28O5ꢂKBPh4] (raj3sa): M = 778.79, monoclinic, P21/c, a =
13.1774(5), b = 15.6866(6), c = 19.6322(7) A, b = 94.300(2)1, Z = 4,
V = 4046.7(3) A3, Dc = 1.278 Mg mꢀ3, 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.
y For a discussion of the bent K+ꢂ ꢂ ꢂNC–Me interaction in the
[p-TC, K+] complex, see Fig. S4 and S5 in the ESI.w
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
The X-ray crystallographic analyses of the K+ complexes in
Fig. 2–4 shows that going from o-TC to m-TC to p-TC complex
leads to a progressive stretching of the ethereal canopies. As such
the stretching of the ethereal canopies affects the K+ꢂꢂꢂAr
coordination which varies from asymmetric to symmetric
coordination going from o-TC to m-TC to p-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 [o-TC, K+] complex or at the site
of a pair of apical openings in [m-TC, K+] and [p-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–p 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.
and F. Vogtle, 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+ꢂ ꢂ ꢂO
distances in various potassium/crown complexes have a median
value of B2.85 A.
8 Note that the closest K+ꢂ ꢂ ꢂCAr distance of 3.156 A in [o-TC, K+
]
complex is much shorter than the K+ꢂ ꢂ ꢂCAr distance of 3.38 A 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 [m-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 581 opening in the non-aqueous complex vs. 671 in the
aqueous one, suggesting a significant stability of the Z2-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.
Notes and references
z Crystal structure data for p-TC [C26H28O5] (raj1v): M = 420.48,
monoclinic, P21/n, a = 13.048(5), b = 9.487(3), c = 17.671(6) A,
,
b = 97.031(5)1, Z = 4, V = 2170.9(13) A3, Dc = 1.287 Mg mꢀ3
T = 100 K, 24585 reflections measured, 6832 unique reflections,
11 Introduction to Molecular Electronics, ed. M. C. Petty, M. R. Bryce
and D. Bloor, Oxford Univ. Press, New York, 1995.
Rint = 0.0463, 281 parameters refined, R(all) = 0.1310, wR(all) =
0.2779, S = 1.067. Crystal structure data for m-TC [C26H28O5] (raj4b):
ꢀ
420.48, triclinic, P1,
M
=
a
=
8.8435(5),
b
=
19.9062(10),
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.
c = 26.7874(14) A, a = 111.253(2), b = 98.337(2), g = 93.101(2)1,
Z = 8, V = 4319.3(4) A3, Dc = 1.293 Mg mꢀ3, T = 100 K, 35 410
ꢁc
This journal is The Royal Society of Chemistry 2009
5602 | Chem. Commun., 2009, 5600–5602