Stable supramolecular dimer of self-complementary benzo-18-crown-6 with a pendant protonated amino arm

Article abstract of DOI:10.1039/b209235b

The novel 4′-(ammoniummethylene)benzo-18-crown-6 cation was synthesized as a hexafluorophosphate salt and found to exist as a cyclophane-like dimer in the solid state, gas phase and in acetonitrile solution.

Full text of DOI:10.1039/b209235b

Stable supramolecular dimer of self-complementary benzo-18-crown-6
with a pendant protonated amino arm†
Olga P. Kryatova,a Sergey V. Kryatov,a Richard J. Staplesb and Elena V. Rybak-Akimova*a
a Department of Chemistry, Tufts University, Medford, Massachusetts 02155, USA.
E-mail: elena.rybak-akimova@tufts.edu; Fax: 617-627-3443; Tel: 617-627-3413
b Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138,
USA
Received (in Columbia, MO, USA) 19th September 2002, Accepted 23rd October 2002
First published as an Advance Article on the web 8th November 2002
The novel 4A-(ammoniummethylene)benzo-18-crown-6 cat-
ion was synthesized as a hexafluorophosphate salt and found
to exist as a cyclophane-like dimer in the solid state, gas
phase and in acetonitrile solution.
dimerization (the signal at m/z 1119 was identified as
[(1·H)2(PF6)3]2).
The X-ray crystallographic study of the solvate
1·HPF6·0.5MeOH revealed the formation of a centrosymmetric
(Ci) head-to-tail dimeric cation [(1·H)2]2+ in the solid state (Fig.
1).‡ The ammonium group of each component of the cyclic
dimer is coordinated by three oxygens of the complementary
18-crown-6 residue. The average N+…O separation is 2.89 Å,
which is typical for hydrogen bonds between RNH3+ and crown
ethers.10–12 The oxygen atoms in the macrocyclic ring are
approximately in a plane with the mean deviation from the O6
plane at 0.166 Å (maximum deviations are for O3 and O4,
+0.334 and 20.299 Å, respectively). The N atom of the
ammonium group is displaced by 0.764 Å from the mean plane
of the six macrocyclic oxygen atoms. This distance is among the
shortest ammonium—crown ether separations reported in the
Self-complementary (plerotopic) molecules which can form
dimers or oligomers due to weak non-covalent interactions have
been fruitfully used as building blocks for supramolecular
architertures.1–5 For example, Rebek and coworkers developed
molecular capsules based on self-complimentary bowl-shaped
molecules rimmed with urea residues.2 Stoddart and coworkers
designed different types of interlocked systems based on host-
guest interaction of molecules with protonated secondary amino
or 4,4A-dipyridinium functionalities threaded through large (!
24-membered) crown ether macrocycles.3–5
The factors that govern the formation of dimers vs. cyclic or
acyclic oligomers need to be better understood in order to
rationally design target supramolecular aggregates.3–7 Although
structural data are of great value in characterizing plerotopic
dimers, only very few of them have been characterized by X-ray
crystallography.3,4,8
+
literature for RNH3 –18C6 ring systems, which usually are in
the range of 0.8–1.4 Å.10–12 Such a short distance might be
explained by the ‘chelate effect’ of two simultaneous crown–
ammonim interactions and the p–p stacking between the
aromatic rings of the monomers (interplanar separation 3.40 Å,
centroid–centroid separation 3.87 Å). An example of a ‘push
Here, we report a new self-complementary molecule based
on a simple, well-known host–guest motif, namely 18-crown-6
+
effect’ was previously found for the H2NNH3 complex with
+
and protonated primary amino-group (RNH3 ). Even in the
18C6, where a set of H-bonds between the ‘upper rim’ oxygens
and the NH2-group induced a nested configuration of the NH3-
group interacting with the ‘lower rim’ oxygens and displaced by
only 0.11 Å from the mean O6 plane.11
absence of threading, a dimeric supramolecule is formed from
the crown ether with a pendant protonated amino arm in the gas
phase, in the solid state, and in solution, as evidenced by mass
1
spectrometry, X-ray crystallography, and H NMR spectros-
The dimer [(1·H)2]2+ may be considered as a supramolecular
cyclophane (Fig. 1). The interplanar separation of the aromatic
rings in [(1·H)2]2+ (3.40 Å) is close to that observed in the
classic [3.3]paracyclophane (3.30 Å).13
copy, respectively. Hydrogen bonding supplemented by p-
stacking interactions stabilized the dimer (as opposed to a
monomer lacking both of these interactions, or linear oligomers
lacking p-stacking interactions).
The analysis of packing of the supramolecular dimers
revealed the formation of a ‘herring bone’ structure composed
of dimer pairs with methanol as a co-crystallized solvent.† The
network is organized by packing effects rather than specific
interactions. In spite of a small value of the interplanar
separation between the benzene rings of adjacent dimers (3.30
Å), the large shift of the benzene rings (centroid–centroid
separation 5.09 Å) indicates the absence of p–p stacking
between the (1·H)22+ dimers.
Working on the synthesis of ‘molecular tweezers’ for
diammonium cations,9 we prepared the benzo-18-crown-6 with
a pendant aminomethylene arm (1) and its hexafluorophosphate
salt (1·HPF6) (Scheme 1).†‡
1
The simple nature of the H NMR spectrum of 1·HPF6 in
DMSO-d6 at 295 K indicates the absence of aggregation (Fig.
Scheme 1
The electrospray mass spectrum of 1·HPF6 taken in positive
mode revealed intense peaks at m/z 683 and 829 that were
identified as the supramolecular ions [(1·H)(1)]+ and
[(1·H)2(PF6)]+, respectively, indicating the dimerization of the
molecule in the gas phase. No higher order oligomers were
found according to the mass spectrum, and the signal of the
monomer [(1·H)]+ was weak (its intensity is ca. 13% of the base
peak of [(1·H)(1)]+). The ESMS in negative mode also showed
† Electronic supplementary information (ESI) available: synthetic proce-
dures, NMR and X-ray diffraction data. See http://www.rsc.org/suppdata/
cc/b2/b209235b/
Fig. 1 Molecular structure of the [(1·H)2]2+ dimer. Selected hydrogen
bonding distances: N+–H…O (Å): N1(a)–O(2) 2.878, N(1a)–O(4) 2.884,
N(1a)–O(6) 2.962.
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CHEM. COMMUN., 2002, 3014–3015
This journal is © The Royal Society of Chemistry 2002

2(a)). The spectrum in acetonitrile-d3 at 300 K has broad signals
of aromatic protons shifted up-field (Fig. 2(d)), which can be
attributed to an association process. Decreasing the temperature
resolved the spectra of two exchanging species in a ratio of 4+1,
which stays approximately the same in the temperature range
238–273 K and concentration range 1024–1022 M (Fig. 2).
Both sub-spectra have signals of aromatic protons below 6.8
ppm indicating significant p–p interaction between the catechol
rings that might result from the offset face-to-face stacking of
the two aromatic units in the cyclic head-to-tail dimer [(1·H)2]2+
(Fig. 1).
Stoddart and co-workers demonstrated that three ster-
eoisomeric dimers (a meso-form and a pair of enantiomers)
could be formed upon self-threading of an alkylamino-
substituted DB24C8, resulting in a two-component low-
temperature NMR.5 The same interpretation is applicable to the
association of 1·HPF6 reported herein. Even though only one
stereoisomer (a centrosymmetric meso form) was found in the
crystal, the second form (an enantiomeric pair) may co-exist
with the meso form in acetonitrile solution.§
MHz) upon complexation with alkylammonium salts (CD3CN,
295 K).9 Slow exchange in 1·HPF6 solutions in acetonitirile at
low temperatures, and intermediate exchange at room tem-
perature, can be attributed to decreased decomplexation rates
due to a ‘chelate effect’ of dimerization reinforced by the
stacking interactions. Slow exchange is typical for threaded
complexes of ammonium cations with large crowns,3–5 but is
unusual for the perched complexes with medium-sized crown
ethers.14
In conclusion, [(1·H)2]2+ is the first characterized self-
complementary supramolecular dimer based on the host–guest
interaction of protonated primary amino-groups and 18-crown-
6 residues. The dimer has unusually high thermodynamic and
kinetic stability in solution. The dimer can be considered as a
supramolecular cyclophane, in which two aromatic rings are
fixed in a p-stacking geometry by non-covalent interaction.
This work was supported by Tufts University (startup funds
and faculty research award to E. R. A.).
Increasing the temperature of the acetonitrile-d3 solution of
1
1·HPF6 (1024–1022 M) results in a simple H NMR spectrum
Notes and references
containing ‘averaged’ signals with well-resolved fine pattern
(Fig. 2(c)). The signals of the aromatic protons remain upfield-
shifted (6.55, 6.72 and 6.96 ppm at 343 K), indicating that p-
stacking in the dimers is not destroyed. Treatment of the
solution with an excess of KI at 295 or 343 K gives spectra in
which the signals of the catechol protons of 1·HPF6 lie in the
usual region (6.9–7.5 ppm) and their positions are similar to the
spectrum of the monomer in DMSO-d6 (Fig. 2). It can be
concluded that self-association of 1·H+, still evident at 343 K in
dilute acetonitrile solution (1024 M), is destroyed by the
competitive complexation of K+ with the crown ether.¶
Remarkably, the dimer formed from 1·HPF6 is even some-
what more stable than the analogous ‘threaded’ crown–
ammonium self-associate observed by Stoddart and co-work-
ers.4,5 The dimer [(1·H)2]2+ is the predominant form in the
acetonitrile solution over a broad concentration and temperature
ranges (1022–1024 M, 238–343 K), while the ‘threaded’ dimer
of alkylamino-substituted DB24C8 dissociates into the mono-
meric form upon heating of the 1022 M acetonitrile solution to
343 K or upon dilution to 1024 M at room temperature.5 The
relatively high stability of [(1·H)2]2+ could be explained by
stronger association between the primary ammonium group and
B18C6 recognition sites (K ~ 104 M21 in acetonitrile at 295
K)9,14 compared to the ‘threading’ of DB24C8 with secondary
ammonium cations (K ~ 102–103 M21 under similar condi-
tions).14,15
‡ Compound 1·HPF6 was obtained by the neutralization of 1 with HPF6 in
ethanol. 13C NMR (DMSO-d6) ppm: 148.0, 147.6, 126.1, 121.6, 113.6,
112.7, 69.9, 69.8, 69.6, 68.6, 67.9, 42.2. 1H NMR (300 MHz, DMSO-d6)
ppm: 7.88 (3H, br s), 7.07 (1H, s), 6.95–6.85 (2H, m), 4.08–4.03 (4H, m),
3.94 (2H, q, J 7 Hz), 3.75–3.80 (4H, m), 3.62–3.52 (8H, m), 3.52 (4H, s).
Single crystals of the solvate 1·HPF6·0.5MeOH were obtained by slow
diffusion of diethyl ether into MeOH–MeCN (1+1 v/v) solution. Crystal
data: C17.50H30F6NO6.50P, M = 503.9, monoclinic, space group C2/c, a =
24.348(6), b = 7.9883(18), c = 24.881(6) Å, b = 110.589(5)°, U =
4530.3(18) Å3, Z = 8, T = 293(2) K, m(Mo-Ka) = 1.22 cm21, 12246
reflections collected, 3991 independent (Rint = 0.0697), R1 and wR2 (all
data) are 0.1782 and 0.2847, respectively. Hydrogens were calculated by
geometrical methods and refined as a riding model. CCDC 194286. See
http://www.rsc.org/suppdata/cc/b2/b209235b/ for crystallographic data in
CIF or other electronic format.
§ An alternative interpretation, the existence of adducts with different
degree of oligomerization, is less likely because the dilution of the
acetonitrile-d3 solution of 1·HPF6 does not influence the relative concentra-
tions of the two species.
¶ The 1H NMR spectra of benzo crown ethers experience only minor
changes upon alkali metal or alkylammonium ion binding.9,14
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Fig. 2 Partial 1H NMR spectra of 1·HPF6: (a) 1022 M solution in DMSO-d6
at 295 K; (b) 1024 M solution in acetonitrile-d3 with excess KI added at 295
K; (c)–(f) 1022 M solution in acetonitrile-d3 at different temperatures.
Signals of the aromatic protons are assigned according to Scheme 1.
CHEM. COMMUN., 2002, 3014–3015
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