Supramolecular organization of calix[4]pyrrole with a methyl- trialkylammonium anion exchanger leads to remarkable reversal of selectivity for sulfate extraction vs. nitrate

Article abstract of DOI:10.1039/c1cc12060c

meso-Octamethylcalix[4]pyrrole (C4P) enhances sulfate selectivity in solvent extraction by Aliquat 336N, an effect ascribed to the supramolecular preorganization and thermodynamic stability imparted by insertion of the methyl group of the Aliquat cation into the cup of C4P in its cone conformation.

Full text of DOI:10.1039/c1cc12060c

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COMMUNICATION
Supramolecular organization of calix[4]pyrrole with a
methyl-trialkylammonium anion exchanger leads to remarkable
reversal of selectivity for sulfate extraction vs. nitratew
a
a
a
b
Christopher J. Borman, Radu Custelcean, Ben P. Hay, Nathan L. Bill,
Jonathan L. Sessler and Bruce A. Moyer*a
bc
Received 11th April 2011, Accepted 24th May 2011
DOI: 10.1039/c1cc12060c
meso-Octamethylcalix[4]pyrrole (C4P) enhances sulfate selectivity
s
in solvent extraction by Aliquat 336N, an effect ascribed to the
interactions that involve both anion and cation recognition by
receptor 1.
supramolecular preorganization and thermodynamic stability
imparted by insertion of the methyl group of the Aliquat cation
into the cup of C4P in its cone conformation.
One of the key challenges within the field of anion recognition
lies in coupling various canonical anion-coordination inter-
actions with favorable coulombic effects to achieve desired
selectivity and affinity. One proven strategy involves creating
molecular frameworks that incorporate multiply charged
1
–3
motifs and coordinating functionalities. However, we suggest
herein that, in the case of anion-directed liquid–liquid extraction,
it may be possible to elicit the same result through an appro-
priate supramolecular organization of independent molecular
The enhancement of liquid–liquid anion-exchange selectivity
by an anion receptor (R) can be usefully viewed as a standard
anion-exchange equilibrium modified by competitive coordi-
4
,5
7
functional units.
nation of the exchanging anions by R. Enhanced exchange of
ꢀ
2ꢀ
In earlier work with liquid–liquid extraction, we proposed
an approach to enhancing the selectivity of lipophilic
anion exchangers, such as fatty quaternary ammonium salts,
by addition of neutral lipophilic anion receptors, such as
a univalent anion (X ) by a divalent anion (Y ) in the
+
presence of a lipophilic cation (Q ) and R may be illustrated
in the following Scheme (for the case of predominant organic-
phase ion-pairing):
6
–8
meso-octamethylcalix[4]pyrrole (C4P, 1).
In exploring
the potential for controlling the selectivity of these
systems through specific synthetic modification of the anion
receptors, we began to question the influence, if any, of the
7
organic-phase cation. Here, we present results of
a
simple extraction experiment, supported by structural and
theoretical results, showing that the organic-phase cation
can indeed play a pivotal role to the extent that the
apparent selectivity of the anion receptor can be reversed. As
detailed below, this effect is ascribed to specific supramolecular
Depending on which anion is more strongly bound by R, mass
action predicts the direction of selectivity enhancement, either
2
ꢀ
to the right or left. If anion Y is monitored, its extraction
would be respectively increased or decreased. In the absence of
an anion receptor, anion exchange from aqueous solutions
into water-immiscible organic diluents tends to follow the
a
Chemical Sciences Division, Oak Ridge National Laboratory,
Oak Ridge, TN 37831-6119, USA. E-mail: moyerba@ornl.gov;
Fax: +1 865 574 4939; Tel: +1 865 574 6718
Department of Chemistry and Biochemistry, Institute for Cellular
b
‘‘Hofmeister bias,’’ a solvation-based selectivity order in which
and Molecular Biology, The University of Texas, Austin,
TX 78712-0165, USA
Department of Chemistry, Yonsei University, Seoul 120-749, Korea
larger, more charge-diffuse anions are thermodynamically
9–11
c
favored over smaller anions.
In accord with this principle,
w Electronic supplementary information (ESI) available: Details of
synthesis, distribution experiments and instrumentation (crystallo-
graphy and radiometric analysis) are included. CCDC 821449. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1cc12060c
a prototypical exchange of charge-dense sulfate from nitrate
without added receptor is found to be very energetically
6
–8
unfavorable (exchange lies to the left). Given the desirability
of attaining controlled selectivity patterns, such as extracting
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7611–7613 7611

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8
sulfate from nitrate, a major challenge for the design of anion
receptors for liquid–liquid extraction and other separations
5
,11,12
has been to effect non-Hofmeister selectivity patterns.
To
achieve this aim by design for enhanced anion exchange, it is
apparent from the Scheme that the role of the cation must be
explicitly considered.
The present experiment was devised to test whether a
+
change in the structure of Q while keeping the same receptor
(
R) would have a significant effect on the apparent receptor
selectivity. Previously, we observed that the addition of 1 to
s
methyltrialkyl(C8–10)ammonium nitrate (2aꢁNO
3
, Aliquat
3
36N) in chloroform resulted in enhanced exchange for sulfate
2ꢀ
(
SO
4
) from a solution of sodium nitrate as measured
Fig. 1 Normalized sulfate distribution ratio (DSO ,Q+1/DSO ,Q)
4 4
plotted as a function of receptor concentration [C4P] in the presence
by an increase in the sulfate distribution ratio (D
=
SO4
2
ꢀ 2ꢀ
SO4 ]org/[SO4 ] ). Equilibrium modeling showed that
aq
7
of three anion exchangers (Q), (K) 2a, (’) 2b, or (E ) 2c. Aqueous
[
phase (all experiments): 10 mM Na
QNO , 6 to 25 mM C4P in experiments with 2b and 2c, and 12 to
5 mM C4P with 2a. DSO4 is defined as the ratio of the sulfate
concentration in the organic phase to that in the aqueous phase
2 4
SO . Organic phase: 10 mM
the enhanced exchange corresponds to a shift to the right in
7
the Scheme, nitrate binding being relatively weak. Unexpectedly,
3
2
enhancement with 1 was stronger than with its beta-
octafluorinated analog, which normally binds anions much
more strongly in aprotic organic solvents, presumably owing
42ꢀ
42ꢀ
(
DSO4 = [SO
]
org/[SO
]aq). The normalized sulfate distribution
ratio is calculated by dividing DSO4 determined from equilibrium
1
3,14
to stronger hydrogen-bond (H-bond) donation than 1.
experiments with both anion exchanger and receptor (DSO ,Q+1) by
4
Receptor 1 has been shown to function as an ion-pair receptor
in its cone conformation, with pyrrole NH-anion H-bonding
facilitating anion recognition and the cup-like cavity formed
from the four pyrrolic subunits accepting the cationic species.
In the case of methyl quaternary ammonium cations, the
cation has been shown to be bound with one methyl group
DSO4 obtained from experiments with exchanger only (D
SO ,Q
).
4
the methyl groups from TMA residing inside the electron-rich
‘‘cup’’ of 1. Four NH–O hydrogen bonds from the pyrrole NH
moieties serve to stabilize the bound sulfate anion. A second
TMA counter cation is present in the lattice but does not
interact directly with 1. While both the anion recognition and
cation complexation aspects of this structure have antecedents
1
2–16
inserted into the ‘‘cup’’.
We postulated that such mediated
ion-pairing effects could be relevant under conditions of
enhanced liquid–liquid anion exchange, indeed explaining
13–16
in prior structurally characterized calix[4]pyrrole complexes,
the unexpected strength of 1 vs. its beta-octafluorinated analog
7
. How-
the present X-ray structure is noteworthy in that it represents the
2ꢀ
in enhancing sulfate exchange by exchanger 2aꢁNO
3
first with sulfate (SO
4
) and the first 1 : 1 ion-pair complex
ever, it would follow that a reduced effect would be noted in
the case of 2b and 2c, both of which lack the proposed
required methyl group on the quaternary ammonium center.
To test these expectations, the sulfate distribution ratio
DSO4 was determined for extraction of aqueous 10 mM
Na SO solutions using 1,2-dichloroethane (1,2-DCE) phases
involving an anion with multiple negative charges.
The extent of the methyl group penetration (dN4–N), defined
as the distance between the mean plane formed by the four
˚
pyrrolic N atoms and the N atom of TMA, is 3.96 A. This
˚
distance is similar to the analogous value of 3.91 A found in
1
the TMACl complex of 1. In the present instance, one of the
3
2
4
containing variable concentrations of 1 in combination with
0 mM 2aꢁNO , 2bꢁNO , or 2cꢁNO
As revealed by an inspection of Fig. 1, a marked dependence
O atoms from the sulfate anion interacts with the pyrrolic NH
1
3
3
3
.
on the cation structure of the quaternary ammonium anion
exchanger is observed in which the enhancement due to 1
favors or disfavors sulfate in competition with nitrate. Here,
the log of the normalized sulfate distribution ratio for each of
the three test ammonium cations 2a, 2b, and 2c is plotted as a
function of the concentration of 1. In the case of 2a, a positive
effect on the sulfate extraction is seen. In contrast, an
antagonistic effect (downward trend in Fig. 1) is seen in the
case of 2c. Thus, a receptor (C4P, 1) that is apparently weak or
even antagonistic to competitive sulfate binding in the
presence of a symmetrical long-chain quaternary ammonium
cation is ‘‘turned on’’ in the presence of a methyl quaternary
ammonium cation, such as 2a.
Solid-state evidence in support of a putative co-complexation
of the methyl-bearing exchanger 2a came from the crystal
structure of 1 with tetramethylammonium (TMA) sulfate.z
As shown in the structure in Fig. 2, 1 adopts the cone
conformation in binding one O-atom of sulfate, with one of
Fig. 2 Crystal structure showing insertion of the methyl group of the
tetramethylammonium (TMA) cation into the cavity of C4P (1) and
pyrrolic H-bonding to sulfate. For clarity, the second TMA cation and
ethanol solvent are not shown.
7
612 Chem. Commun., 2011, 47, 7611–7613
This journal is c The Royal Society of Chemistry 2011

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approach by studying other simple-to-prepare anion receptor
systems as sulfate anion extractants.
This research was sponsored by the Division of Chemical
Sciences, Geosciences, and Biosciences, Office of Basic Energy
Sciences, U.S. Department of Energy. Support from the
Robert A. Welch Foundation (grant F-1018 to J.L.S.) and
the Korean WCU program (grant R32-2010-10217-0 to J.L.S.)
is also acknowledged.
Notes and references
z Single crystals were obtained by recrystallization from dichloro-
Fig. 3 Calculated global minima for adducts formed by insertion of
methane/ethanol. Data for C4Pꢁ(TMA) ꢁ(CH ꢁ(EtOH)
SO
Cl
)
2
4
2
2
2
2
the tributylmethylammonium (left) and tetrabutylammonium (right)
crystals: C42
P2 /m (No. 11), a = 10.6011(7), b = 20.9725(15), c = 11.2587(8) A,
b = 91.6120(10)1, V = 2502.2(3) A , Z = 2, Dc = 1.241 g cm
H76Cl N O S, M = 934.95, monoclinic, space group
4 6 6
2ꢀ
˚
cations into the cavity of the C4P-sulfate complex, 1ꢁSO
4
. In these
1
3
ꢀ3
˚
,
views, the second TMA cation is omitted for clarity.
˚
Bruker APEX, Mo-Ka radiation, l = 0.71073 A, T = 173(2)K,
2
0
y
max = 56.61, 15435 reflections collected, 6353 unique (Rint
=
.0331). Final GooF = 1.665, R = 0.1450, wR = 0.4056, R indices
1
2
groups, forming four H-bonds with NHꢁ ꢁ ꢁO contact distances
2
based on 4292 reflections with I 4 2s(I) (refinement on F ),
282 parameters, 0 restraints. Lp and absorption corrections applied,
m = 0.326 mm
˚
of 2.074, 2.082, 2.123, and 2.123 A. The remaining three
ꢀ1
.
O atoms of sulfate interact with TMA cations and included
ethanol solvent in the crystal through C–Hꢁ ꢁ ꢁO and O–Hꢁ ꢁ ꢁO
H-bonding, respectively. For the sake of clarity, these inter-
actions are not shown in Fig. 2 (cf. ESIw).
Using the tributylmethylammonium cation (TBMA) as a
model for 2a and tetrabutylammonium cation (TBA) as a
model for 2b and 2c, conformational analysesy were performed
to locate the most stable forms that incorporate the cation
within the cup of the sulfate complex with 1. The predicted
y Calculations were performed with the default MMFF94 force field as
implemented in PCModel, Version 9.0, Serena Software, Bloomington,
Indiana, USA.
1
2
F. P. Schmidtchen, Chem. Rev., 1997, 97, 1609–1646.
P. Gale, Chem. Soc. Rev., 2010, 39, 3746–3771.
3 P. Gale, Chem. Commun., 2011, 47, 82–86.
4
5
S. K. Kim and J. L. Sessler, Chem. Soc. Rev., 2010, 39, 3784–3809.
A. L. Sisson, J. P. Clare, L. H. Taylor, J. P. H. Charmant and
A. P. Davis, Chem. Commun., 2003, 2246–2257.
6
7
C. J. Fowler, T. J. Haverlock, B. A. Moyer, J. A. Shriver,
D. E. Gross, M. Marquez, J. L. Sessler, M. A. Hossain and
K. Bowman-James, J. Am. Chem. Soc., 2008, 130, 14386–14387.
B. A. Moyer, J. Frederick, V. Sloop, C. J. Fowler, T. J. Haverlock,
H.-A. Kang, L. H. Delmau, D. M. Bau, M. A. Hossain,
K. Bowman-James, J. A. Shriver, N. L. Bill, D. E. Gross,
M. Marquez, V. M. Lynch and J. L. Sessler, Supramol. Chem.,
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˚
structures show that TBMA, dN4–N = 3.90 A, is able to
˚
penetrate further into the cavity than TBA, dN4–N = 4.35 A
(
see Fig. 3). These values are in good agreement with crystal-
1
structure data of related chloride complexes with 1.
2,13
Comparison of the DE values for formation of these adducts
ꢀ1
reveals that the TBMA form is 15.2 kcal mol more stable
than the TBA form in the gas phase.
8
B. A. Moyer, L. H. Delmau, C. J. Fowler, A. Ruas, D. A. Bostick,
J. L. Sessler, E. Katayev, G. D. Pantos, J. M. Llinares,
M. A. Hossain, S. O. Kang and K. Bowman-James, in Advances
in Inorganic Chemistry, ed. R. van Eldik and K. Bowman-James,
Elsevier, Amsterdam, 2007, vol. 59: Template Effects and
Molecular Organization, pp. 175–204.
The results presented here indicate that the structure of the
cation can significantly influence the selectivity of anion
binding by a neutral anion receptor, even effecting a reversal
in direction of enhancement in liquid–liquid anion exchange
expected based on the ubiquitous Hofmeister bias. The
observed enhancement in sulfate selectivity is attributed to
the insertion of the methyl group in 2a into the cup of C4P, 1,
the receptor in this case functioning as an ion-pair receptor.
Additional thermodynamic stability thereby arises, as inferred
9
F. Hofmeister, Archiv Fur Experimentelle Pathologie und Pharma-
kologie, 1888, 24, 247–260.
1
0 B. A. Moyer and P. V. Bonnesen, in Supramolecular Chemistry of
Anions, ed. A. Bianchi, K. Bowman-James and E. Garcia-Espana,
Wiley-VCH, New York, 1997, pp. 1–44.
11 R. Custelcean and B. A. Moyer, Eur. J. Inorg. Chem., 2007, 10,
321–1340.
1
1
2 R. Custelcean, L. H. Delmau, B. A. Moyer, J. L. Sessler,
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and P. A. Gale, Angew. Chem., Int. Ed., 2005, 44, 2537–2542.
from calculations involving the model systems (TBMA)
2
ꢁ1ꢁSO
4
and (TBA) . The present findings illustrate a rational
2
ꢁ1ꢁSO
4
approach to ion-pair recognition that involves consideration
of both anion and cation binding. In a broader sense, the
present results serve to underscore the importance of supra-
13 J. L. Sessler, D. E. Gross, W.-S. Cho, V. M. Lynch,
F. P. Schmidtchen, G. W. Bates, M. E. Light and P. A. Gale,
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1
7
molecular structure in the design of ion receptors.
Based on such generalized design considerations, we
propose that the selection of an appropriate counter cation
in mixed anion receptor/exchanger combinations could
provide a convenient alternative to more classic strategies that
involve incorporating complementary anion and cation
binding motifs within elaborate receptor structures. We are
thus seeking to generalize the present anion exchanger-based
14 J. L. Sessler, W.-S. Cho, D. E. Gross, J. A. Shriver, V. M. Lynch
and M. Marquez, J. Org. Chem., 2005, 70, 5982–5986.
1
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1
6 D. E. Gross, F. P. Schmidtchen, W. Antonius, P. A. Gale,
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1
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7611–7613 7613