Unusual recognition of (n-Bu4N)2SO4 by a cyanuric acid based host via contact ion-pair interactions

Article abstract of DOI:10.1039/c0cc02509g

A tripodal neutral receptor selectively binds tetrabutylammonium sulfate where anion is encapsulated in the cavity and cations are in close contact with the anion via C-H...O interactions. This organic cation capped sulfate-bowl exists both in solid and solution states and the sulfate assisted cap can be opened and closed reversibly with a temperature key.

Full text of DOI:10.1039/c0cc02509g

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Unusual recognition of (n-Bu4N)2SO4 by a cyanuric acid based
host via contact ion-pair interactionsw
I. Ravikumar and Pradyut Ghosh*
Received 12th July 2010, Accepted 27th July 2010
DOI: 10.1039/c0cc02509g
A tripodal neutral receptor selectively binds tetrabutylammonium
sulfate where anion is encapsulated in the cavity and cations are
in close contact with the anion via C–Hꢀ ꢀ ꢀO interactions. This
organic cation capped sulfate-bowl exists both in solid and
solution states and the sulfate assisted cap can be opened and
closed reversibly with a temperature key.
First evidence on the recognition of (n-Bu4N)2SO4 by L via
strong contact ion-pair interactions is obtained from the single
crystal X-ray structural analysis of 1.z The single crystal
of sulfate complex, 1 is grown from dimethyl formamide
(DMF) solution as [L(SO42ꢁ)(n-Bu4N+)2]ꢀDMF in good yield
upon slow evaporation of L in the presence of (n-Bu4N)2SO4.
Complex 1 crystallizes in the monoclinic system with the Cc
space group. Three arms of the tripodal receptor form a cavity
that completely encapsulates the sulfate in its centre via
N–HꢀꢀꢀO hydrogen bonding interactions to all six –NH protons
of the three urea groups of L (Fig. 1).w Interestingly, both
the n-Bu4N+ counter cations are in close contact with the
encapsulated SO42ꢁ via five strong C–Hꢀ ꢀ ꢀO interactions with
its acidic –CH2 group, adjacent to the cationic bridge-head
nitrogen (Fig. 1).w Thus, encapsulated sulfate is in eleven
hydrogen bonding interactions (Fig. 1) which represents an
uncommon eleven coordination number in anion coordination
chemistry.12 Fig. 2 shows the space filling model of 1 which
clearly depicts that the SO42ꢁ is completely encapsulated in the
cavity of L where both the n-Bu4N+ cations act as a cap of the
Ion-pair recognition is important for its great potential in
biological, analytical, and environmental applications.1 It
can also play an important role in controlling the geometry
of self-assembly, enhancement of the binding affinity as the
result of allosteric effects and enhanced electrostatic interactions
between the co-bound ions, and even reversing the binding
selectivity of the host.2 Several strategies have been developed
to achieve recognition of the ionic subunits of the guest salt
where different modes like contact ion-pairs, solvent-bridged
ion-pairs and host-separated ion-pairs have been employed.3
Recently, C–Hꢀ ꢀ ꢀanion interactions involving organic cations
have attracted much attention because of their biological and
chemical relevance.4 However, reports on the interaction
between anions and non-aromatic CH groups as hydrogen
bond donors are rare in the literature.5 On the other hand,
tripodal,6 cyclic,7 macrobicyclic8 and metal–organic9 receptors
are employed for binding and separation of sulfate and have
also been used as templates for the formation of complex
molecular structures.10 Herein we report a tripodal urea
receptor functionalized with pentafluorophenyl side arms on
a rigid cyanuric acid platform as an effective receptor for
tetrabutylammonium sulfate, (n-Bu4N)2SO4 as an associated
ion-pair where sulfate shows a coordination number of eleven.
Further, we also demonstrate selective sulfate assisted contact
ion-pair formation and temperature dependent dissociation/
association of the ion-pair in solution by diffusion NMR
measurements. To the best of our knowledge this report shows
the first example of the binding of (n-Bu4N)2SO4 as a contact
ion-pair by a simple synthetic host in a polar solvent as well as
in the solid state.
2ꢁ
sulfate-bowl. Thus, L, SO4 and two n-Bu4N+ ions assembled
together to an elliptical shaped (major axis of 17 A and minor
axis of 14 A) aggregation in the solid state.
The complex 1 was subjected to ESI mass analysis in
CH3CN (negative mode).w The peak at m/z 1223.3 and its
isotopic distribution pattern suggest the existence of
[L(SO4)(n-Bu4N)]ꢁ species in the gaseous state. The peak at
m/z 981.2 can be attributed to the 1 : 1 sulfate complex of the
receptor L whereas the base peak at m/z 884.0 in the spectrum
can be assigned to the deprotonated receptor and a peak
at m/z 920.6 can be attributed to the chloride complex of L.
The existence of an ion-pair like L(SO4)(n-Bu4N)ꢁ in the
gaseous state is evidence of the strong association of L and
(n-Bu4N)2SO4.
2-Dimensional diffusion ordered spectroscopy (2D-DOSY)
has become an important technique for studying the self-assembly
of supramolecular systems in solution.13 Fig. 3 depicts the
DOSY spectra of 1 in DMSO-d6 at 298 and 333 K. It is evident
from the spectra (Fig. 3A) that all the peaks correlated to the
The synthesis of tripodal urea receptor L based on a
cyanuric acid platform with pentafluorophenyl pendant arms
is shown in Scheme 1. Reaction of the precursor triamine IV
with 3.0 equivalents of pentafluorophenyl isocyanate in tetra-
hydrofuran (THF) results in the tripodal urea receptor, L in
high yield.w The triamine IV is synthesized following the
modified literature procedure.11
Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science, 2A & 2B Raja S. C. Mullick Road
Kolkata 700 023, India. E-mail: icpg@iacs.res.in
w Electronic supplementary information (ESI) available: Experimental
details. CCDC 777533. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0cc02509g
Scheme 1 Synthesis of the receptor, L.
c
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Chem. Commun., 2010, 46, 6741–6743 6741

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2ꢁ
Fig. 1 Binding of SO4 with L and tetrabutylammonium groups.
Non acidic hydrogen atoms and lattice DMF are omitted for clarity.
Fig. 2 Space filling model of complex 1. Lattice DMF is omitted for
clarity. Major axis: C62–H62Bꢀ ꢀ ꢀH46B–C46 E 17 A; minor axis
C42–H42Aꢀ ꢀ ꢀH22A–C22 E 14 A. Blue: carbon, brown: hydrogen,
green: fluorine and red: oxygen.
Fig. 3 1H-DOSY (500 MHz) spectra of complex 1 in DMSO-d6 at
signals in the chemical shift dimensions are in a horizontal
line. Thus both sets of signals attributed to L and n-Bu4N+
cation have the same diffusion coefficients. Further, the downfield
(A) 298 K and (B) 333 K.
Table 1 Diffusion coefficient, D and hydrodynamic radii, rs in
DMSO-d6 at different temperatures
shifted signals of urea –NH resonances (–NHc and –NHd) are
2ꢁ
D/10ꢁ10 m2 sꢁ1
rs/A
supposed to represent the encapsulated SO4
which has
System
T/K
the same diffusion coefficient as L and n-Bu4N+ ions. This
supports the fact that L and (n-Bu4N)2SO4 exist as a contact
ion-pair in solution. Addition of (n-Bu4N)2SO4 to the solution
of L in DMSO-d6 showed a DOSY spectrum similar to thꢁat
(n-Bu4N+)2SO4
L
298
298
298
313
333
298
298
298
298
10.72 ꢂ 0.02
8.805 ꢂ 0.01
7.127 ꢂ 0.01
7.752 ꢂ 0.01
7.858 ꢂ 0.02
8.112 ꢂ 0.01
8.258 ꢂ 0.01
8.521 ꢂ 0.01
8.289 ꢂ 0.01
10.22 ꢂ 0.02
12.45 ꢂ 0.01
15.39 ꢂ 0.01
14.14 ꢂ 0.01
13.95 ꢂ 0.02
13.52 ꢂ 0.01
13.28 ꢂ 0.01
12.87 ꢂ 0.01
13.23 ꢂ 0.01
2ꢁ
Complex 1
of 1 whereas the addition of n-Bu4N+ salts of H2PO4
,
L + n-Bu4N+H2PO4
ꢁ
CH3COOꢁ or Clꢁ to the DMSO-d6 solution of L showed
two different sets of peaks with different diffusion coefficients,w
representing that these anions do not assist the formation of
contact ion-pairs. However, downfield shifted signals of urea
–NH resonances are observed due to anion binding to L. The
hydrodynamic radii (rs) calculated from the Stokes–Einstein
relation, using experimentally obtained diffusion coefficients
for L, (n-Bu4N)2SO4, L + n-Bu4NH2PO4, L + n-Bu4NCH3
COO, L + n-Bu4NNO3, L + n-Bu4NCl and 1 at different
temperatures, are tabulated in Table 1. It is evident that 1
represents maximum hydrodynamic radii of 15.39 A, indicating
the bigger size of the aggregated species compared to L (12.45 A)
or (n-Bu4N)2SO4 (10.22 A) or anion bound L. The hydro-
dynamic radii obtained from diffusion measurements for 1 is
in good agreement with the average of the dimensions (15.5 A)
calculated from single crystal X-ray analysis in the solid state
(Fig. 2). A similar comparison has recently been made in the
literature.14 Variable temperature DOSY experiments of 1
clearly show that the hydrodynamic radius of 1 is sensitive
to temperature (Fig. 3B, Table 1). Variable temperature
L + n-Bu4N+CH3COOꢁ
ꢁ
L + n-Bu4N+NO3
L + n-Bu4N+Clꢁ
Hydrodynamic radii are calculated from the experimentally obtained
diffusion coefficients using the Stokes–Einstein equation.
experiments were performed at 298 K, 313 K and 333 K
respectively in DMSO-d6. With the increase in temperature
the diffusion coefficient increases along with the development
of two different sets of signals which might be attributed to
[L + SO42ꢁ] and n-Bu4N+ as separated ions at higher
temperatures in the solution. Notably, the one-dimensional
1H-NMR spectra do not display any variations in the chemical
shifts of urea –NH resonances and do not show broadening of
the resonance signals upon changing the temperature, indicating
that the sulfate ion remains intact in the cleft even at the
elevated temperatures. When the temperature of the solution is
cooled down to 298 K, the DOSY spectrum exactly matches
with the spectrum of the associated ion-pair, 1. This indeed
justifies the temperature dependent reversible ion-pairing and
c
6742 Chem. Commun., 2010, 46, 6741–6743
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performed in the DST funded National Single Crystal X-ray
Facility at IACS.
Notes and references
z Crystallographic data for 1: C65H97F15N12O11S, M = 1539.61,
monoclinic, space group Cc,
a = 32.4(12), b = 12.936(5),
c = 23.066(15) A, b = 126.879(8)1, V = 7733.0(6) A3, Dc = 1.322
g cmꢁ3, Z = 4, l = 0.71073 A, T = 100(2) K, 25 676 reflections,
12 215 independent (Rint = 0.0773), and 9835 observed reflections
[I > 2s(I)], 971 refined parameters, R = 0.0545, wR2 = 0.1468.
CCDC number: 777533.
Fig. 4 Proposed modes of ion-pair association and dissociation
of L and tetrabutylammonium sulfate.
dissociation of L and (n-Bu4N)2SO4 in the solution state as
shown in Fig. 4.
1 (a) B. D. Smith, in Ion Pair Recognition by Ditopic Receptors,
Macrocyclic Chemistry: Current Trends and Future, ed. K. Gloe
and B. Antonioli, Kluwer, London, 2005, pp. 137–152;
(b) P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40,
486.
Detailed binding properties of L with different anions in the
solution state were also investigated by 1H NMR experiments
in DMSO-d6 in the presence of various anions as their
ꢁ
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n-Bu4N+Xꢁ salts (wꢁhere Xꢁ = Fꢁ, Clꢁ, Brꢁ, Iꢁ, H2PO4
,
HSO4ꢁ, ClO4ꢁ, NO3 and CH3COOꢁ).w Substantial changes
in chemical shifts are observed for the ureaꢁprotons (–NHc and
–NHd) of L with Clꢁ, H2PO4ꢁ, HSO4 and CH3COOꢁ,
indicating the participation of –NH protons in the solution
state binding events. To evaluate the binding of these anions
with L, 1H-NMR titrations were carried out in DMSO-d6. The
titration curve showed the best fit for a 1 : 1 binding model
for host to guest, in agreement with Job’s plots,w and the
association constants were calculated using WINEQNMR 2.0
for these anions.15 The association constants are summarized
in Table 2. The stability constants and change in free energy
data show that L binds very strongly toward SO42ꢁ compareꢁd
with other anions with log K of 5.74 Mꢁ1. However H2PO4
also displays a significant binding with L whereas the binding
4 L. O. Abouderbala, W. J. Belcher, M. G. Boutelle, P. J. Cragg,
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constant of L with CH3COOꢁ and Clꢁ is comparable. Thus,
2ꢁ
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the binꢁding order of different anions toward L is SO4
c
H2PO4 > CH3COOꢁ > Clꢁ. Higher binding affinity in the
case of SO42ꢁ compared to H2PO4ꢁ could be due to the existence
of ion-pair interaction, without loss of electrostatic energy
arising from charge separation in the former case.
¨
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Table 2 Binding constants and free energy change of L with different
anions in DMSO-d6 at 298 K (errors o5%)
2ꢁ
ꢁ
SO4
H2PO4
CH3COOꢁ
Clꢁ
log K/Mꢁ1
5.74
ꢁ7.82
4.39
ꢁ5.98
3.41
ꢁ4.65
3.38
ꢁ4.61
DG/kcal molꢁ1
7 (a) Z. Rodriguez-Docampo, S. I. Pascu, S. Kubik and S. Otto,
J. Am. Chem. Soc., 2006, 128, 11206; (b) M. A. Hossain,
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In conclusion, we have unambiguously demonstrated that
encapsulated sulfate selectively assists ion-pair formation with
the aliphatic chain of Bu4N+ cation via strong C–Hꢀ ꢀ ꢀO
interactions by single crystal X-ray structural analysis. Both
1H-NMR and DOSY NMR support the recognition of
(n-Bu4N)2SO4 in a polar solvent like DMSO. Variable tempera-
ture DOSY experiments show the reversible assembly/
dis-assembly of the n-Bu4N+ sulfate assisted cap. The cyanuric
acid platform and the pentafluorophenyl units in the pendant
arms of the receptor might have played a crucial role toward
sulfate selective/assisted formation of contact ion-pair recognized
assembly.
PG thanks the Department of Science and Technology
(DST) for financial support through a Swarnajayanti Fellowship.
The single crystal X-ray structure analysis of complex 1 was
14 N. Giuseppone, J.-L. Schmitt, L. Allouche and J.-M. Lehn, Angew.
Chem., Int. Ed., 2008, 47, 2235.
15 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 6741–6743 6743