Duplexiphane: A polyaromatic receptor containing two adjoined Δ-shaped cavities for an efficient hopping of a single silver cation

Article abstract of DOI:10.1021/ol7026762

A simple synthesis of a polyaromatic receptor (i.e., duplexiphane) containing two adjoined Δ-shaped cavities is accomplished via an intramolecular (double) McMurry coupling, and its structure is established by X-ray crystallography. The binding of silver

Full text of DOI:10.1021/ol7026762

ORGANIC
LETTERS
2008
Vol. 10, No. 3
389-392
Duplexiphane: A Polyaromatic Receptor
Containing Two Adjoined -Shaped
Cavities for an Efficient Hopping of a
Single Silver Cation
∆
Susanna J. Emond, Paromita Debroy, and Rajendra Rathore*
Department of Chemistry, Marquette UniVersity, P.O. Box 1881,
Milwaukee, Wisconsin 53201
Rajendra.Rathore@Marquette.edu
Received November 4, 2007
ABSTRACT
A simple synthesis of a polyaromatic receptor (i.e., duplexiphane) containing two adjoined
∆-shaped cavities is accomplished via an intramolecular
(double) McMurry coupling, and its structure is established by X-ray crystallography. The binding of silver cation to duplexiphane showed that
it binds only a single silver cation with significantly higher efficiency (>100 times) than a model compound containing only one
like cavity, and the single silver cation hops intramolecularly between the two adjoined cavities in duplexiphane.
π-prismand-
The design and syntheses of macromolecular receptors that
bear two or more aromatic groups in cofacially oriented
arrays continues to attract attention owing to their potential
applications in the areas of electrical conductors and pho-
toresponsive devices.1-3
Among the well-known receptors, π-prismand (1)4a and a
structurally analogous deltaphane (2)4b contain “∆”-shaped
cavities that are especially efficacious for binding a variety
of metal cations, such as Ag+, Tl+, Ga+, etc.4 The X-ray
crystallographic analyses of Ag+ complexes of 1 and 2, in
solid-state, showed that a single silver cation is bound to
only one of the faces of the “∆”-shaped cavities. However,
the simplicity of the 1H/13C NMR spectra of the Ag+
complexes of both 1 and 2, in solution, indicated that the
bound silver cation is highly (kinetically) labile and may
undergo back and forth shuttling through the cylindrical
cavities, e.g., Figure 1.4a,b
(1) (a) Prodi, L.; Bolletta, F.; Montalti, M.; Zaccheroni, N. Coord. Chem.
ReV. 2000, 205, 59. (b) Ikeda, M.; Tanida, T.; Takeuchi, M.; Shinkai, S.
Org. Lett. 2000, 2, 1803. (c) Lindeman, S. V.; Rathore, R.; Kochi, J. K.
Inorg. Chem. 2000, 39, 5707 and references therein.
(2) (a) Heirtzler, F. R.; Hopf, H.; Jones, P. G.; Bubenitachek, P.; Lehne,
V. J. Org. Chem. 1993, 58, 2781. (b) Gano, J. E.; Subramaniam, G.;
Birnbaum, R. J. Org. Chem. 1990, 55, 4760. (c) Yang, R.-H.; Chan,
W.-H.; Lee, A. W. M.; Xia, P.-F.; Zhang, H.-K.; Li, K. J. Am. Chem. Soc.
2003, 125, 2884. (d) Rathore, R.; Chebny, V. J.; Abdelwahed, S. H. J. Am.
Chem. Soc. 2004, 127, 8012.
(3) (a) Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M.;
Suenaga, Y.; Sugimoto, K. Inorg. Chem. 1997, 36, 4903. (b) Munakata,
M.; Wu, L. P.; Ning, G. L.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.;
Macao, Y. J. Am. Chem. Soc. 1999, 121, 4968.
Figure 1. A back and forth shuttling of a single silver cation (pink
ball) through the cylindrical cavities of π-prismand (1) and
deltaphane (2).
10.1021/ol7026762 CCC: $40.75
© 2008 American Chemical Society
Published on Web 12/29/2007

It was conjectured that such receptors, if assembled, in a
continuous electronically coupled array, may allow the long-
range transport of a metal ion for the potentially interesting
applications in the emerging areas of molecular electronics
and nanotechnology.5
Unfortunately, neither 1 nor 2 is easily accessible nor
readily amenable to structural modifications for such appli-
cations.6 Therefore, we envisioned a structure with two
adjoined π-prismand-like (or ∆-shaped) cavities that can be
easily accessed using standard synthetic procedures (see
structure 3 in Scheme 1) and which would hold the potential
cavity, that it binds only a single silver cation which most
likely hops between the two adjoined cavities.8
The synthesis of 3 was accomplished in five steps (Scheme
1) using readily available starting materials in excellent
overall yield. Thus, following the procedure of Hart and co-
workers,9 a reaction of freshly prepared Grignard reagent
from protected 4-bromohexanophenone 5 (4 equiv) with 2,6-
dibromoiodobenzene9 in tetrahydrofuran at 22 °C for 24 h
followed by a quenching of the reaction mixture with iodine
at 0 °C afforded 2,6-diaryliodobenzene 6, in a one-pot
procedure, in 66% yield. The diaryliodobenzene 6 was
subjected to a Suzuki coupling reaction with 1,4-benzene-
diborate ester in refluxing dimethoxyethane in the presence
of aqueous sodium carbonate and a catalytic amount (2 mol
%) of Pd(PPh3)4 for 24 h.
Scheme 1. Synthetic Scheme for the Preparation of 3
The crude 7, from above, was hydrolyzed to the corre-
sponding tetraketone 8 in a 2:1 mixture of dichloromethane/
acetone containing 5% (v/v) conc. hydrochloric acid at 22
°C, and the resulting material was purified by column
chromatography to afford 8 in 68% overall yield in two steps.
The tetraketone 8, whose structure was confirmed by X-ray
crystallography, was then subjected to an intramolecular
(double) McMurry coupling under mild dilution to afford
the desired duplexiphane (3) in excellent yield (72%). The
1
molecular structure of 3 was easily established by H and
13C NMR spectroscopy and further confirmed by high-
resolution mass spectrometry (see the Supporting Informa-
tion).
The unequivocal structural confirmation of 3 was obtained
by X-ray crystallography as shown by an ORTEP diagram
and by a space-filling representation in Figure 2. A com-
to be readily tailored for incorporation into polymeric
structures or for the deposition onto various surfaces by
changing the “R” groups. Furthermore, the two electronically
coupled, ∆-shaped cavities in 3, which is referred to hereafter
as duplexiphane,7 will allow us to probe the fluxionality of
the silver cation in the [3, Ag+] complex.
Figure 2. Molecular structure of 3 shown as an ORTEP diagram
(left) and as a space-filling representation (right).
Thus, herein we will describe a versatile synthesis and
X-ray crystallographic structural characterization of duplex-
iphane (3) and delineate, with the aid of 1H NMR spectros-
copy and a model compound containing only one “∆”-shaped
parison of the geometrical parameters of 3 with 1 and 2
indicated that the size of the internal ∆-shaped cavity is
characteristically similar in all three compounds, i.e., an
average distance from its center to the centers of the three
benzene rings is ∼2.6 Å. Moreover, the two ∆-shaped
cavities in 3 are geometrically identical as the molecule
occupies a crystallographic center of symmetry. On the basis
of the structural similarity of the cavities in 3 with that of 1
and 2, one would advocate that it should bind Ag+ with more
or less similar efficiency as that of 1 and 2.
(4) (a) Pierre, J. L.; Baret, P.; Chautemps, P.; Armand, M. J. Am. Chem.
Soc. 1981, 103, 2986. (b) Kang, H. C.; Hanson, A. W.; Eaton, B.;
Boelelheide, V. J. Am. Chem. Soc. 1985, 107, 1979. (c) Pierre, G.; Baret,
P.; Chautemps, P.; Pierre, J. L. Electrochim. Acta 1983, 28, 1269. (c)
Schmidbaur, H.; Hager, R.; Huber, B.; Muller, G. Angew. Chem., Int. Ed.
Engl. 1987, 26, 338. (d) Probst, T.; Steigelmann, O.; Riede, J.; Schmidbaur,
H. Angew. Chem., Int. Ed. Engl. 1990, 29, 1397.
(5) (a) Petty, M. C.; Bryce, M. R. In Introduction to Molecular
Electronics; Bloor, D., Ed.; Oxford University Press: New York, 1995.
(b) Maiya, B. G.; Ramasarma, T. Curr. Sci. 2001, 80, 1523.
(6) Compare: Heirtzler, F. R.; Hopf, H.; Jones, P. G.; Bubenitschek, P.
Tetrahedron Lett. 1995, 36, 1239.
(8) For other examples of metal-ion hopping, see: (a) Ohseto, F.; Sakaki,
T.; Araki, K.; Shinkai, S. Tetrahedron Lett. 1993, 34, 2149. (b) Walker, A.
V.; Tighe, T. B.; Cabarcos, O. M.; Reinard, M. D.; Haynie, B. C.; Uppili,
S.; Winograd, N.; Allara, D. L. J. Am. Chem. Soc. 2004, 126, 3954. (c)
Berg, D. J.; Sun, J.; Twamley, B. Chem. Commun. 2006, 4019.
(7) The term “duplexiphane” was coined based on the suggestion by
our colleague James R. Kincaid who pointed out that the structure of 3
conceptually resembles that of a “duplex”, a house partitioned to accom-
modate two residents.
(9) Du, C. J. F.; Hart, H.; Ng, K. K. D. J. Org. Chem. 1986, 51, 3162.
390
Org. Lett., Vol. 10, No. 3, 2008

The binding of silver cation to 3 was monitored by the
changes in the 1H NMR spectrum of 3 in chloroform-d (0.015
M) by an incremental addition of substoichiometric amounts
of a solution of silver trifluoromethanesulfonate (0.23 M)
in 1:1 mixture of chloroform-d and methanol-d4. The addition
of the increments of Ag+ solution showed considerable
changes (i.e., broadening and shifts) in the aromatic signals
only up to the addition of one equivalent of Ag+ solution,
Scheme 2. Intermolecular Exchange between [3, Ag+] and
Uncomplexed 3
1
as shown in Figure 3. It is noteworthy that the H NMR
equimolar amounts of [3, Ag+] and uncomplexed 3 were
mixed. Such an observation of slow inter-molecular exchange
between [3, Ag+] and uncomplexed 3 suggests that a single
silver cation is held tightly by 3 and thus prevents a rapid
inter-molecular exchange with the uncomplexed 3.
1
More interestingly, the observation of a symmetrical H
NMR spectrum of [3, Ag+] complex (i.e., the same number
of sharp signals as in the uncomplexed 3) suggests that the
single silver cation must be shuttling back and forth among
the two adjoined cavities with a speed faster than the NMR
time scale, e.g., Scheme 3.12
Scheme 3. Intramolecular Exchange of Ag+ among the Two
Adjoined Cavities of 3
Figure 3. Partial 1H NMR spectra of 3 obtained upon an
incremental addition of CF3SO3Ag in CDCl3-CD3OD at 22 °C.
spectrum remained unchanged upon further addition of Ag+
solution (i.e., beyond 1 equiv).
1
A careful examination of the H NMR spectra in Figure
3, revealed that near the 0.5 equiv point (of Ag+ titrations),
the NMR signals in Figure 3 are considerably broadened,
which however, become sharp when a complete equivalent
of Ag+ solution is added. Such a broadening of the 1H NMR
signals near the 0.5 equiv point suggests that the intermo-
lecular exchange between [3, Ag+] and uncomplexed 3 is
rather slow at 20 °C, i.e., Scheme 2.10
In order to probe that a single silver cation undergoes intra-
molecular hopping in 3 (Scheme 3), we have synthesized13
a model compound (i.e., 9) containing only one ∆-shaped
cavity (R ) n-pentyl) and confirmed that the shape and size
of the cavity in 9 is identical to that of the pair of adjoined
cavities in duplexiphane (3) by X-ray crystallography.13
The intermolecular exchange process between [3, Ag+]
and uncomplexed 3 was further confirmed by the observation
of the identical broadened NMR signals (Figure 3) when
(11) Sanders, J. K. M.; Hunter, B. K. Modern NMR Spectroscopy: A
Guide for Chemists; Oxford University Press: New York, 1993, pp 205-
234.
(12) The chemical shifts of the two resonances involved in the fast
exchange are unknown. We only observed a weight average of the two
states. Therefore, we estimated that on a 300 MHz NMR instrument, based
on the chemical shifts of Ag+-bound and unbound 3, the chemical shift
difference between these two resonances is roughly 200 Hz. Accordingly,
at 22 °C where little chemical exchange was observed, the exchange rate
should be at least a couple of orders of magnitude larger than 400 Hz. At
-95 °C, the lowest temperature accessible to us, we observed significant
line broadening, indicating that the exchange rate at this temperature
(10) For chemical exchanges, the line shape of the NMR resonances
depends on the chemical shift difference between the states involved in the
exchange process. In a two-site exchange, coalescence occurs when the
x
exchange rate kex ) π∆ν/ 2 ) 2.22∆ν, where ∆ν is the chemical shift
difference in Hz.11 In this study, ∆ν is around 100 Hz on a 300 MHz
instrument. Thus, the slow intermolecular exchange rate is expected to be
in the vicinity of ∼200 s-1 at 22 °C, since we observed only one broad
peak for each proton. It should be contrasted that an intermolecular
exchange, akin to Scheme 2, with deltaphane (2) was noted4b to be much
faster than NMR time scale at 22 °C, i.e. >200 Hz.
decreases to ∼400 s-1
.
(13) The synthetic details and X-ray crystallographic structural charac-
terization of the model receptor 9 will be published separately.
Org. Lett., Vol. 10, No. 3, 2008
391

The binding of a single silver cation to model receptor 9
was confirmed by the changes in the H NMR spectrum of
suggesting that the coalescence temperature for the hopping
process in Scheme 3 was much lower than -95 °C. As such
this observation allows us to roughly estimate that the
dynamic process in Scheme 3 must occur at a rate faster
than ∼10,000 Hz.12
1
9 in chloroform-d (0.023 M) by an incremental addition of
substoichiometric amounts of a solution of silver trifluo-
romethanesulfonate (0.37 M) in 1:1 mixture of chloroform-d
and methanol-d4. The addition of the increments of Ag+
solution showed considerable changes in the aromatic signals
of 9, without any broadening, only up to the addition of one
equivalent of Ag+ solution. Interestingly, however, when a
CDCl3-CD3OD solution of [9, Ag+] complex was exposed
to an equimolar amount of duplexiphane 3, it quantitatiVely
transferred its bound silver cation to 3 as established by 1H
NMR spectroscopy, i.e., Scheme 4 (and Figure S1 in the
Supporting Information).
In summary, we have designed and synthesized a versatile
receptor (3) with two adjoined ∆-shaped cavities with an
expanded (and continuous) π -surface which allows a rapid
hopping of a single silver cation among the two electronically
coupled cavities with a single aromatic wall, as shown in
Scheme 5.
Scheme 5. Possible Mechanism of intramolecular Shuttling of
a Single Silver Cation between the Two Adjoined,
Electronically Coupled Cavities in Duplexiphane 3
Scheme 4. Quantitative Transfer of Silver Cation from [9,
Ag+] to Equimolar 3
The dramatically enhanced association constant of 3 with
Ag+ in comparison to that of the model receptor 9, that
contains only one cavity, may be reconciled by the fact that
there is a considerable entropic gain due to the rapid hopping
of Ag+ on the continuous π-surface of 3. Furthermore, it
can be envisioned that the molecules containing multiple
adjoined ∆-shaped cavities would hold potential for the long-
range transport of metal cations akin to the natural ion-
transport channels found in bilayer membranes.14 We are
actively exploring the preparation of a number of such
molecules containing multiple (adjoined) receptor sites for
their potential materials’ applications.
In another experiment, [3, Ag+] complex was exposed up
to 4 equiv of model compound 9, under otherwise identical
conditions as in Scheme 4, and its 1H NMR spectrum showed
no broadening of the 1H NMR signals of either uncomplexed
9 or the [3, Ag+] complex.
These experiments clearly demonstrate that duplexiphane
3 binds a single silver cation with an efficiency that is at
least 100 times greater than its model compound 9, contain-
ing a single cavity of similar dimensions as the pair of
cavities in duplexiphane 3. Moreover, the [3, Ag+] complex
holds its bound Ag+ tightly and does not release to the model
receptor 9 even when it is present in large excess, indicating
that the hopping of a single silver cation in duplexiphane 3
most likely occurs intramolecularly.
Acknowledgment. We thank the National Science Foun-
dation (CAREER Award) for financial support, Dr. Sergey
V. Lindeman (Marquette University) for X-ray crystal-
lography, and Dr. Sheng Cai (Marquette University) for the
assistance with variable-temperature NMR spectroscopy.
Supporting Information Available: Synthetic details,
1H/13C NMR data, and X-ray structural data of 3 and 8. This
material is available free of charge via the Internet at
http://pubs.acs.org.
In order to estimate the rate of the hopping of the Ag+
cation in Scheme 3, we attempted variable-temperature NMR
spectroscopy. For example, the 1H NMR spectra of [3, Ag+]
complex in CD2Cl2 containing 5% CD3OD showed consider-
able broadening of the signals only at -95 °C, thus
OL7026762
(14) (a) Gokel, G. W.; Leevy, W. M.; Weber, M. E. Chem. ReV. 2004,
104, 2723. (b) Weber, M. E.; Gokel, G. W.; Leevy, W. M.; Hughes-Strange,
G. B.; Daranciang, D. D.; Ferdani, R.; Gokel, M. R. Org. Biomol. Chem.
2005, 3, 1647.
392
Org. Lett., Vol. 10, No. 3, 2008