Highly efficient extraction of sulfate ions with a tripodal hexaurea receptor

Article abstract of DOI:10.1002/anie.201004461

Fatal extraction: The Hofmeister bias for sulfate ions in aqueous environments can be overcome by using a tripodal hexaurea receptor (L) that completely encapsulates the sulfate ion in a complementary cavity protected by hydrophobic aromatic rings (see pi

Full text of DOI:10.1002/anie.201004461

Communications
DOI: 10.1002/anie.201004461
Anion Recognition
Highly Efficient Extraction of Sulfate Ions with a Tripodal Hexaurea
Receptor**
Chuandong Jia, Biao Wu,* Shaoguang Li, Xiaojuan Huang, Qilong Zhao, Qian-Shu Li,* and
Xiao-Juan Yang
The design of artificial receptors for sulfate ions is of great
interest because of the importance of sulfate ions in environ-
mental and biological systems.[1] One of the applications of
sulfate ion receptors is extraction of the sulfate ion from
nitrate-rich mixtures in the remediation of nuclear waste.[2]
Based on liquid–liquid anion exchange technology, extraction
of sulfate ions from an aqueous to an organic phase was
realized by using macrocyclic receptors.[2b] In particular, the
distribution ratio (Dsulfate = [SO42À)]org/[SO42À)]aq) can reach
technologically useful values (> 1) when a fluorinated cal-
ixpyrrol is used.[2c] However, high concentrations (about 1000
times SO42À) of the receptor were needed in this case to
ensure applicable extraction. Hence, the extraction efficiency
has yet to be improved for sulfate ion extractants. This aim is
quite challenging because of the extremely large hydration
ethyl)amine) has also been found to encapsulate the sulfate
ion in a 2:1 (host/guest) ratio.[8] Although saturated coordi-
nation (12 hydrogen bonds) for sulfate and phosphate ions has
been achieved by these receptors, the complementarity for
the ions is not optimal in most cases. Calculations have
demonstrated that the optimal saturated coordination mode
for sulfate ions is binding in a tetrahedral cavity with 12
hydrogen bonds along the edges.[9] In this regard, the ideal
sulfate ion receptor would possess a complementary tetrahe-
dral cavity surrounded by 12 optimally arranged binding sites.
The chelate effect may also play an important role in the
host–guest binding affinity because of the favorable contri-
butions from both entropy and enthalpy. As a typical example
of the chelate effect, the Co2+ complex of the bidentate ligand
1,2-diaminoethane is 108 times more stable than that of the
unidentate ligand ammonia.[10] Moreover, the hexadentate
ligand ethylenediaminetetraacetic acid (EDTA) displays
extremely high binding affinities toward most metal ions
(for example, 1014.3 mÀ1 for Fe2+ and 1016.3 mÀ1 for Co2+).[11]
Given the similarities between anion coordination and
classical transition-metal coordination chemistry,[12] increas-
ing the number of binding sites to achieve high chelate effects
should be an effective way to improve the extraction
efficiency of sulfate ion extractants.
energy of the sulfate ion (DGh = À1080 kJmolÀ1 for SO4
2À
compared to À300 kJmolÀ1 for NO3 )[3] according to the
À
Hofmeister series,[4] as well as the high nitrate/sulfate ratios
present in the crude waste. To overcome the Hofmeister bias,
which disfavors the separation of the extremely hydrophilic
sulfate ion from water, the receptor must have both excellent
affinity and selectivity for sulfate ions.
In recent years, some receptors for sulfate ions have been
synthesized by employing different binding groups (mostly
NH moieties), such as protonated Schiff base macrocycles,[5]
diindolylureas,[6] and an M4L6 cage containing a bipyridine-
functionalized monourea;[7] these receptors bind the anion in
the 1:1, 3:1 and 6:1 (host/guest) mode, respectively. The tren-
based tripodal trisurea backbone (L1; tren = tris(2-amino-
We have devoted our efforts to the synthesis of selective
anion receptors based on the urea functionality.[8a,13] In recent
work, we designed a trisurea receptor (L2, Scheme 1a) for
sulfate and phosphate ions by mimicking the terpyridine
scaffold, which displays a fully complementary conformation
with the tetrahedral anions and achieves saturated coordina-
tion with PO4 ions in a 2:1 (host/guest) binding mode.[13a]
3À
[*] Prof. B. Wu
Compared to the 2:1 sulfate capsule of the tripodal receptor
L1, in which one of the ligands shows a complementary
conformation with the three axial edges but the other ligand
contacts with the vertices of the bottom triangular face
(Scheme 1b) of the sulfate ion, both molecules of L2 adopt
favorable conformations in the 2:1 phosphate ion complex.
The results indicate that the ortho-substituted phenyl bridge
may serve as a suitable “corner” (or vertex) in constructing
tetrahedral cages for sulfate and phosphate ions. Based on the
sulfate ion binding properties of L1 and L2, it is reasonable to
believe that a combination of the properties, that is, incorpo-
ration of both the excellent complementarity and chelate
effect in one molecule, may lead to an excellent extraction
efficiency for sulfate ions.
College of Chemistry and Materials Science
Northwest University, Xi’an 710069 (China)
E-mail: wubiao@nwu.edu.cn
C. Jia, S. Li, X. Huang, Q. Zhao, Prof. X.-J. Yang
State Key Laboratory for Oxo Synthesis & Selective Oxidation
Lanzhou Institute of Chemical Physics, CAS
Lanzhou 730000 (China)
Prof. Q.-S. Li
Center for Computational Quantum Chemistry
South China Normal University, Guangzhou 510631 (China)
E-mail: qsli@scnu.edu.cn
C. Jia, S. Li, Q. Zhao
Graduate University of Chinese Academy of Sciences
Beijing 100049 (China)
[**] This work was supported by the National Natural Science
Foundation of China (20872149) and the “Bairen Jihua” project of
the Chinese Academy of Sciences.
With this approach in mind, we extended each of the three
monourea arms of L1 to an ortho-phenyl bridged bisurea
(Scheme 1) to produce a hexaurea ligand (L3). The receptor
L3 was readily synthesized by reaction of p-nitro-phenyl-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004461.
486
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Angew. Chem. Int. Ed. 2011, 50, 486 –490

Scheme 1. Design strategy for the tripodal hexaurea receptor L3.
a) Structures of L1 (R=3-pyridyl), L2 (R=p-nitrophenyl), and L3;
b) schematic illustration of the construction of the tetrahedral cage.
Figure 1. Crystal structure of the sulfate complex 1: a) side view
showing the hydrogen bonds and CH···p interactions (only one of the
three pairs is shown); b,c) top and bottom views showing the tight
binding of the sulfate ion (non-acidic hydrogen atoms, solvents, and
countercations omitted for clarity); d) arrangement of the 6 urea
groups that donate 12 hydrogen bonds to the sulfate ion.
isocyanate with tris(2-aminophenyl)urea, which was reduced
from tris(2-nitrophenyl)urea (L1a).[14] X-ray diffraction anal-
ysis[15] of the free receptor L3 indicates that the three outer
urea arms are tilted up from the phenyl bridges in a folding-
umbrella fashion, and the urea groups are involved in either
intra- or intermolecular urea···urea self-association to lead to
a hydrogen-bonded 1D chain. There are also additional intra-
and intermolecular p–p stacking interactions between the
terminal p-nitrophenyl groups (see the Supporting Informa-
tion for details).
only receptor reported to date that shows saturated coordi-
nation (12 hydrogen bonds) of a sulfate ion by a single organic
molecule.
The interactions between L3 and the sulfate ion (added as
1
the TBA salt) in solution were studied by H NMR experi-
ments. Two slow exchange processes were observed during
the titration of two equivalents of sulfate ions in
a
[D6]DMSO/0.5% water solution. This observation is ration-
alized by a two-step binding mechanism involving the change
of the binding mode from 1:1 to 1:2 (host/guest; Figure 2). In
the first step, when 0.5 equivalents of sulfate ions were added,
all the NH signals experienced considerable downfield shifts,
Single crystals of the sulfate complex of L3, (TBA)2-
[L3SO4]·DMSO (1)[15] were obtained by slow evaporation of a
1:1 (v/v) water/DMSO solution of the receptor in the presence
of excessive (TBA)2SO4 (TBA = tetrabutylammonium).
Notably, when other anions such as (TBA)H2PO4,
(TBA)AcO, and (TBA)Cl were used, only the free receptor
was crystallized, which may reveal a specific strong binding
with sulfate in aqueous environments. In contrast to the
divergent conformation of the free receptor, in complex 1
(Figure 1), each of the three terminal arms is folded along an
edge of the bottom triangular face (while the three inner urea
groups occupy the axial edges), thus completing a tetrahedral
cage to encapsulate the sulfate ion. The complementarity
achieved by complex 1 fulfils our expectation that all six urea
groups are involved in binding with the anion; each urea
group chelates an edge of the tetrahedron to form a total of
twelve hydrogen bonds (N–O distances ranging from 2.903 to
3.157 ꢀ, average distance 2.992 ꢀ; N-H-O angles ranging
from 142.68 to 172.68, average angle 160.58; Table S3 in the
Supporting Information) between the receptor and the sulfate
ion. In addition, there are three pairs of T-shaped CH···p
interactions between the terminal p-nitrophenyl and adjacent
1,2-substituted phenyl groups, both of which are located near
the vertices of the bottom triangle (C–p distances ranging
from 3.51 to 3.61 ꢀ; Figure 1a and Figure S2 in the Support-
ing Information).[16] To the best of our knowledge, L3 is the
Figure 2. 1H NMR ([D6]DMSO/0.5% D2O, 400 MHz) spectra of L3
2À
~
(5 mm) in the presence of SO4 ions. The symbol denotes the new
NH signals, overlapped peaks are not marked.
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487

Communications
thus indicating the cooperative binding of the sulfate ion by
all the NH groups, most likely in a similar fashion to the solid-
state structure of complex 1. Nevertheless, the shifts (Dd,
ppm) of the “inner” urea groups NHa (Dd = 0.98) and NHb
(Dd = 1.17) are remarkably larger than the “outer” NHc
(Dd = 0.36) and NHd (Dd = 0.19), thus implying that the
bound sulfate ion is located in the “inner” position of the
tetrahedral cavity and is closer to NHa and NHb than to NHc
and NHd. The first step was finished after one equivalent of
sulfate ions was added, at which point the receptor formed the
1:1 complex with the sulfate ion, and the signals of the free
receptor disappeared completely. In the second step, a new set
of signals appeared as more sulfate ions were added; this
result may be attributed to the formation of the 1:2 (host/
guest) species. In contrast to the first step, the downfield shifts
(Dd, ppm) of the outer NHc (Dd = 1.63) and NHd (Dd = 1.32)
groups were greater than those of the inner groups, which
showed only slight (NHa, Dd < 0.2) or no changes (NHb) in
this step, thus indicating that the outer arms with NHc and
NHd were opened to bind the second sulfate ion. Although
the structure of the 1:2 complex is unclear, the NMR signals
are broadened rather than sharp as in the 1:1 complex, and
may be average signals that arise from rapid equilibration of
multiple binding modes (Figure 2). The changes were com-
pleted after addition of two equivalents of sulfate ions.
Additionally, the upfield shift (Dd = 0.42 ppm) of the hydro-
gen atom H8 on the terminal aryl ring (see Scheme 1 for atom
numbering) in the first step followed by a slight downfield
shift (Dd = 0.06 ppm) in the second step also implies that the
change of conformation and binding mode occurred in the
two-step binding process.[13a]
Figure 3. 1H NMR (400 MHz) spectra of L3 (1 mm) in the presence of
various equivalents of SO4 ions (added as TBA salt) in a) [D6]DMSO/
10% D2O and b) [D6]DMSO/25% D2O solution.
2À
Interestingly, in the presence of 10% or 25% D2O (more
water will cause precipitation), the 1:1 binding mode became
the only stable form, as supported by the fact that addition of
one equivalent of sulfate ions resulted in saturation and no
further changes were observed upon further addition of the
anion (Figure 3). Since the NH signals did not appear in the
presence of D2O, the binding affinity was evaluated by the
upfield shift of H8. As mentioned above, in the 1:1 complex,
the proximity of the terminal p-nitrophenyl and the bridging
1,2-substituted phenyl groups can lead to significant shielding
(and an upfield shift) of H8. Thus the upfield shift of this
proton, which can be monitored in NMR titrations, may serve
as evidence for the structure of 1. Such a shielding effect has
also been reported in foldamer formation[17] and in our
previous work on the 2:1 phosphate complex of L2.[13a] In
general, a dramatic decrease of the binding affinity is
expected as the percentage of water in the solution increases
because of the highly competitive effect of water. Biological
proteins, such as the sulfate-binding protein (SBP), overcome
this problem by fully encapsulating the anion in a pocket with
a hydrophobic surface, which can avoid the strong competi-
tion with water.[18] Encouragingly, the binding behavior of L3
displays a similar water-tolerant property as almost identical
upfield shifts of H8 were induced by one equivalent of sulfate
ions (Dd = 0.39–0.42 ppm), regardless of the amount of water
in the titration system (from 0.5% to 25%). In accordance
with the SBP, the main competitive effect from water may
have been excluded by the complete encapsulation of the
anion in the tetrahedral cavity that is protected by hydro-
phobic aromatic phenyl rings (Figure 1c).
Competitive experiments with other anions were carried
out in the presence of 25% D2O. The results demonstrated
that L3 selectively binds sulfate ions over equal amounts of
various competitive anions, and the selectivity follows the
sequence SO42À > H2PO4À @ other anions (Figure S3 in the
Supporting Information). Although the association constant
for sulfate ions could not be accurately determined by UV/Vis
titration because of the irregular (in DMSO) or too weak (in
DMSO/10% or 25% H2O) colorimetric changes induced by
the anion, the association constant can be estimated from the
1H NMR titration data to be larger than 104 mÀ1, even in the
presence of 25% D2O, because one equivalent of sulfate ions
resulted in a completely saturated spectrum.[19]
As L3 has a water-tolerant nature and selectivity for
sulfate ions, we then studied the extraction behavior of this
ligand by 1H NMR spectroscopy. A solution containing
NaNO3 (blank, Figure 4a) or Na2SO4/NaNO3 (1:10; Fig-
ure 4b) in deionized water (0.5 mL) was layered onto a
solution of L3 and (TBA)Cl in CDCl3 (0.5 mL) in an NMR
tube ((TBA)Cl was used to aid the dissolution of L3 in CDCl3
and exchange with sulfate ions). The two layers were
thoroughly mixed for 10 seconds and allowed to settle for
10 seconds, during which time the two layers separated and
the organic phase was immediately analyzed. The resulting
aqueous layers in the two experiments were shown by UV/Vis
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(Figure S6 in the Supporting Information). From these data,
together with the solubility product constant Ksp of BaSO4
and the concentrations of Ba2+ thus deduced, the apparent
sulfate binding constant Kapp (K(LSO4)/K(LCl2)) under the
extraction conditions was calculated as 1.2 ꢁ 103 m (see the
Supporting Information).[7] This process also enabled a direct
evaluation of the extraction efficiency by the gravimetric
method which indicated an extraction yield higher than 95%
(Table S6 in the Supporting Information).
Furthermore, the repeatability of the sulfate extraction
was examined by replacing the aqueous suspension of BaSO4
with a fresh aqueous solution of 10 mm Na2SO4/100 mm
NaNO3. As shown in Figure 4d, the extraction of sulfate
ions is repeatable, thus implying that L3 can be recycled when
used as a sulfate ion extractant. Control experiments per-
formed in the presence of an excess of nitrate ions (from 10 to
100 equivalents) demonstrated that the sulfate ion extraction
ability of L3 was not noticeably influenced by the presence of
a large excess of nitrate ions (Figure S7 in the Supporting
Information).
For comparison, parallel extraction experiments with the
trisurea ligands L1a or L2a (Scheme 2) were also carried out
under similar conditions. In contrast to the high efficiency of
Figure 4. 1H NMR spectra (400 MHz, CDCl3, L3 10 mm) of a) blank,
b) sulfate extraction, c,d) recycling, and e) comparison experiments,
indicating the nearly quantitative and recyclable extraction of sulfate
from water to CDCl3 phase (numbers represent the number of ion
equivalents).
spectroscopy to contain only a trace of L3 (< 30 mm; Figure S4
in the Supporting Information). Comparison experiments (in
organic solution only and in the absence of nitrate ions) were
also conducted by directly adding aliquots of sulfate ions (as
the TBA salt) to a CDCl3 solution of L3/(TBA)Cl. The NMR
signals corresponding to the chloride- and sulfate-binding
receptor appeared independently as a result of slow proton
exchange, and the original signals disappeared completely
when the amount of sulfate ions reached one equivalent. No
further changes were observed as more sulfate ions were
added, which is consistent with a 1:1 binding mode (Figure S5
in the Supporting Information). The spectrum of the extrac-
tion experiment in the presence of excess nitrate ions agreed
well with that induced by one equivalent of sulfate ions in the
comparison experiments, thus indicating that nearly all the
sulfate ions (one equivalent) in the aqueous layer were
extracted into the CDCl3 phase.
The sulfate-binding receptor in CDCl3 can be readily
returned to the chloride-binding form by extracting the
sulfate ions with an aqueous solution of BaCl2 (0.5 mL,
50 mm; Figure 4c). In the back-extraction process with 1–
5 equivalents of BaCl2, increasing amounts of sulfate ions
were extracted as a white suspension of BaSO4 in H2O. The
spectrum of the resulting organic layer was analyzed imme-
diately after each extraction. As a result of slow proton
exchange, a new set of signals corresponding to the regen-
erated receptor appeared and gradually increased, and the
concentrations of the chloride-binding and sulfate-binding
receptor were determined based on the proton integral ratios
Scheme 2. Structures of L1a and L2a.
L3, the sulfate ion extraction abilities of the two receptors are
1
much weaker: the H NMR spectra showed that only a very
small amount of sulfate ions were extracted by L2a and almost
no extraction was observed for L1a (Figure S8 in the Support-
ing Information). These results further proved the unique
sulfate ion separation behavior of the hexaurea receptor L3.
In summary, we have developed a tripodal hexaurea
receptor for sulfate ions. The receptor is capable of com-
pletely encapsulating the anion in a complementary cavity
that is protected by aromatic rings, and represents a successful
strategy for overcoming the “Hofmeister bias” by taking
advantage of a combination of complementarity, the chelate
effect, and the hydrophobic effect. With this receptor as a
liquid–liquid extractant, almost quantitative extraction of
sulfate ions from an aqueous to an organic phase in a
recyclable manner has been achieved, and may be promising
in the remediation of nuclear waste.
Received: July 21, 2010
Published online: December 5, 2010
Keywords: anion recognition · chelate effect ·
.
host–guest chemistry · hydrophobic effect · receptors
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Communications
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