Aerial CO2 Trapped as CO32- ions in a dimeric capsule that efficiently extracts chromate, sulfate, and thiosulfate from water by anion-exchange metathesis

Article abstract of DOI:10.1002/ejic.201402139

The tris(2-aminoethyl)amine-based (tren-based) 3-cyanophenyl-substituted tripodal urea L1, one of the familiar urea-based anion receptors, has shown encapsulation of CO₃^2- ions as the carbonate capsule [(L1)₂·(CO₃

Full text of DOI:10.1002/ejic.201402139

FULL PAPER
DOI:10.1002/ejic.201402139
2
–
Aerial CO Trapped as CO Ions in a Dimeric Capsule
2
3
That Efficiently Extracts Chromate, Sulfate, and
Thiosulfate from Water by Anion-Exchange Metathesis
[
a]
[a]
[a][‡]
Ranjan Dutta, Sourav Chakraborty, Purnandhu Bose,
Pradyut Ghosh*[
and
a]
Keywords: Environmental chemistry / Carbon dioxide fixation / Ion exchange / Anions / Liquid–liquid extraction /
Sulfates / Chromates
2–, S
4
2 3
O
2–,
The tris(2-aminoethyl)amine-based (tren-based) 3-cyano-
peak at δ = –99.98 ppm. The complexes of CrO
2
–
phenyl-substituted tripodal urea L1, one of the familiar urea-
and SO
4
ions with L1 (i.e., 2–4, respectively) were obtained
2
–
based anion receptors, has shown encapsulation of CO
ions as the carbonate capsule [(L1) ·(CO )·(TBA) ] (1, TBA =
tetrabutylammonium) by the fixation of aerial carbon dioxide
3
from crystallization of the bulk extracts and show anion-as-
sisted dimeric capsular assemblies of L1 through multiple N–
H···X (X = O, S) interactions. The dimensions of the anion-
2
3
2
from basic dimethyl sulfoxide (DMSO) solution. Single-crys- encapsulated capsular assemblies are quite similar to that of
tal X-ray structural analysis confirmed the encapsulation of the carbonate capsule and are 9.70 Å for [(L1) ·(CrO )·(TBA)
(2), 9.61 Å for [(L1) ·(S )·(TBA) ] (3), and 9.71 Å for [(L1)
(SO )·(TBA) ] (4). Quantification by weighing the bulk ex-
2
4
2
]
2
·
2
–
CO
L1 (9.62 Å) through the formation of twelve strong N–H···O
hydrogen-bonding interactions. The excellent CHCl and
CH Cl solubility of 1 has been exploited for the liquid–liquid
L–L) extraction of CrO
by anion-exchange metathesis. The extraction of these
anions from water was unambiguously confirmed by 1
3
ions in the cavity of a dimeric capsular assembly of
2
2
O
3
2
4
2
3
tract shows that 1 can separately extract ca. 90% of the
above three anions from water by anion-exchange metathe-
sis. The quantitative estimations of the extractions of SO
ions were further verified by gravimetric analysis
4 4
and BaCrO precipitation techniques, respectively.
ions from water was also demon-
strated under alkaline conditions (pH 12.5) and in the pres-
2
2
2
–
2–
2–
2–
(
4
, SO
4
, and S
2
O
3
ions from water
4
2
–
and CrO
by BaSO
The extraction of SO
4
H
2–
NMR spectroscopy, IR spectroscopy, powder XRD (PXRD),
and single-crystal X-ray diffraction analysis. The 1H NMR
spectroscopic analysis of the bulk extracts supports the for-
mation of 2:1 (host–guest) complexes. For the CrO
Cr NMR spectrum of the bulk extract shows a characteristic
4
ence of an excess of nitrate ions. Further, the quantification
2
–
2–
4
ion, the
of CrO
Vis studies.
4
extraction was established by solution-state UV/
5
3
vitrification of the waste, and excess SO 2–
ions are also
Introduction
4
[
3]
responsible for permanent hardness of water. On the
Anions in groundwater, specifically a few inorganic oxy-
anions, can be toxic to human health even at submicromo-
lar concentrations. The contamination of drinking water by
2–
other hand, S O3 ions are beneficial in various biological
2
and human health aspects. In addition to the therapeutic
use of sodium thiosulfate, thiosulfate ions have recently
been used as an antidote for the treatment of cyanide poi-
2
–
CrO4 ions can induce respiratory cancer and can affect
the gastrointestinal tract, immune system, liver, and kid-
[4]
soning. Thus, the liquid–liquid separation of the environ-
[
1]
neys. The current recommended by the World Health Or-
2–
2–
mentally and biologically relevant CrO4 , SO4 , and
VI
ganization (WHO) for Cr ions in drinking water is a
maximum level of 0.05 mgL . The removal of SO4 ions
from radioactive nuclear waste is essential for the improved
2–
S2O3 anions, which have similar H-bonding properties, is
–
1 [2]
2–
very important.
Metal–organic frameworks (MOFs), organic cages, zeo-
lites, and amines have been widely explored for the removal
[
a] Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science,
[5]
and storage of CO2. A few synthetic anion receptors are
useful for aerial CO fixation through its conversion to
2
A & 2B Raja S.C. Mullick Road, Kolkata 700032, India
2
CO32– anions. Gale et al. have shown the fixation of aerial
E-mail: icpg@iacs.res.in
http://www.iacs.res.in/inorg/icpg
2–
CO as CO3 anions by amidourea macrocycles and acyclic
2
[
‡] Current address: Division of Chemistry and Biological
Chemistry, School of Physical and Mathematical Sciences,
Nanyang Technological University,
amine anion receptors from basic dimethyl sulfoxide
[6]
(
DMSO) solutions. Three other synthetic anion receptors
SPMS-CBC-02-01, 21 Nanyang Link, 637371 Singapore
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402139.
2–
for the fixation of aerial CO as CO3 anions under similar
2
reaction condition have also been described.[7] On the other
Eur. J. Inorg. Chem. 2014, 4134–4143
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FULL PAPER
hand, the design of anion receptors for the extraction of of L1 through multiple N–H···X (X = O/S) interactions for
2
–
2–
anions such as CrO4 and SO4 from water is highly all anion complexes. To the best of our knowledge, this rep-
challenging owing to the large hydration energies of resents the first report on the efficient L–L extraction of
2
–
–1
2–
2–
2–
CrO4 (ΔGh = –950 kJmol ) and SO
ions (ΔGh
=
CrO4 and S O3 ions by an anion-exchange strategy.
4
2
–
1 [8]
–
1080 kJmol ). Absorbents, resins, and membranes are
VI
popular for the removal of toxic Cr ions from water, but
the drawbacks of such systems are their poor selectivity and
slow process kinetics.[ A few calixarene-based anion recep-
Results and Discussion
9]
The 3-cyanophenyl-substituted urea receptor L1 was syn-
thesized by the literature procedure.
isolated in high yield (85%) by the addition of TBAOH to
a DMSO solution of L1. We have previously reported the
synthesis of L2 and its CO3 complex.
were isolated from the crystallization of the extracted so-
lid in DMSO and also by the reaction of L1, TBAI, and
the corresponding sodium salt in DMSO/H O (19:1). The
2–
tors have been employed for the extraction of CrO and
[17b]
4
Crystals of 1 were
2
–
[10]
Cr O anions from water. Custelcean et al. have shown
2
7
2
–
the selective removal of CrO4 anions from water by a self-
assembled metal–organic framework by a crystallization
technique. However, the industrial use of L–L extraction
processes for the removal of CrO4 anions from water is
challenging. Since the first report on the transfer of sulfate
2–
[7b,17d]
Crystals of 2–
[
11]
4
2
–
2
anions across the interface between two immiscible solvents
structures of 2–4 are discussed in the last section.
by Teramae and co-workers,[
12]
different strategies have
2
–
been employed for the extraction of SO
anions from
4
aqueous media over the years.[
13]
Sessler, Moyer et al.
Fixation of Aerial CO2
widely utilized a dual-host and ion-exchange strategy for
2
–
[14]
In our previous communication, we have reported the
SO4 anion extraction. A neutral hexakis(urea) receptor
and a tristhiourea receptor for the extraction of sulfate
anions from aqueous environments with tetrabutylammo-
nium chloride and iodide, respectively, as phase-transfer
agents have been demonstrated by Wu et al. and our
2–
efficient capture of aerial CO in the form of CO anions
2
3
[
7b]
by L2.
In our continuing search for new and alternate
systems, we have found the sequestration of aerial CO as
2
CO32– ions by TBAOH and subsequent encapsulation of
2–
[
15]
the CO
3
ions in the dimeric capsular assembly of L1 in
group. Our group has also shown the efficient and quan-
2
–
DMSO. When TBAOH was added to a DMSO solution of
L1 in an open container, diamond-shaped crystals of [(L1)2·
titative extraction of SO4 ions by employing L–L extrac-
tion techniques that utilize the carbonate capsule of a
pentafluorophenyl-attached tripodal tris(urea) receptor as
(
CO )·(TBA) ] (1) were obtained within 2–3 d in high yield
3 2
[
16]
(85%). The crystallization of 1 occurs at the air–solvent
DMSO) interface at which the CO concentration is higher
Figure S1). The high-yield generation of carbonate cap-
an extractant.
(
2
Tris(2-aminoethyl)amine-based (tren-based) urea and
(
thiourea compounds are an intriguing class of anion recep-
tors, particularly for tetrahedral oxyanions.[17] In 2005, Cus-
sules indicates the high encapsulating affinity of L1 towards
in situ generated CO32– ions. The structural analysis of 1
telcean et al. reported a tren-based 3-cyanophenyl-function-
alized urea receptor (L1) for the encapsulation of SO42–
ions in a metal–organic framework, and we have recently
shown the encapsulation of staggered oxalate ions by L1
2–
shows the encapsulation of a CO
dimeric capsular assembly of L1 with a dimension of 9.62 Å
ion in the cavity of a
3
(
Figures 1 and S26). Each oxygen atom of the encapsulated
2
–
[
17b,17p]
CO
3
ion (i.e., O7, O8, and O9) is involved in four N–
(
Scheme 1).
Herein, we report the utilization of L1
2
–
for the encapsulation of CO3 ions in its dimeric capsular
assembly by the tetrabutylammonium hydroxide (TBAOH)
induced fixation of aerial CO . Further, we have shown the
2
2
–
2–
2–
efficient extraction of CrO4 , SO4 , and S O ions from
2
3
water by the above carbonate capsule by an L–L extraction
2
–
process. The SO4 anion extraction properties of the
carbonate capsules of L1 and L2 (Scheme S1, Supporting
Information) under alkaline conditions are also compared.
Structural analysis reveals anion-assisted dimeric assemblies
Figure 1. Molecular structure of 1 showing the complete encapsu-
2
–
lation of a CO
3
ion inside the dimeric assembly of L1 through
twelve N–H···O interactions. The diamond-shaped crystals of 1 are
shown in the inset (nonacidic hydrogen atoms and countercations
are omitted for clarity).
Scheme 1. Chemical structure of the receptor L1.
Eur. J. Inorg. Chem. 2014, 4134–4143
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1
Figure 2. (1) The H NMR spectra of L1, 1, and recycled L1 formed by methanol/water (1:4 v/v) treatment. (2) (i) Simulated and (ii)
experimental PXRD patterns of 1.
H···O hydrogen-bonding interactions with the –NH group carbonate complex of L2 is only capable of the extraction
of L1, which results in a total of twelve strong H-bonding of SO42– ions among all of the above anions. The details of
interactions in 1 (Figure 1). In L2, thirteen such strong H- the L–L extraction studies and characterization techniques
2
–
bonding interactions were observed upon CO3 ion encap- are described below.
[
7b]
sulation, and the capsular dimension is smaller (9.17 Å).
In a typical extraction experiment, 1 (1 mmol) was
The N···O bond lengths ranges from 2.78 to 3.21 Å in 1, dissolved in CH Cl (5 mL), followed by the addition of
2
2
whereas the N–H···O bond angles are 145 to 171°. Details deionized water (5 mL) containing K CrO (10 equiv.,
2
4
of the hydrogen-bonding parameters and a scatter plot are ca. 10–2 m). The aqueous–organic biphasic solution was
presented in the Supporting Information (Figure S27 and then stirred for 6 h at room temp. Upon L–L extraction,
1
3
Table S2). The C NMR spectrum of 1 shows a signal at δ the organic phase appeared yellow. The mixture was then
169.72 ppm, which corresponds to the carbon atom of allowed to settle for half an hour, and the organic layer
=
2
–
the encapsulated CO3 ion. The downfield shift of 11 ppm was filtered through silicone-treated Whatmann 1PS filter
compared with the resonance of the free CO3 ion of tetra- paper. The evaporation of the organic layer yielded a bright
2
–
ethylammonium hydrogen carbonate indicates that the yellow solid, which crystallized from DMSO as [(L1) ·
2
2
–
1
CO3 ion is strongly bound in the dimeric capsular as- (CrO )·(TBA) ] (2). The H NMR spectroscopic analysis of
4
2
sembly of 1 (Figure S12). The purity of the bulk material the crude solid extract shows upfield shifts of 0.85 ppm for
of the carbonate capsules was verified by powder X-ray dif- NH and 0.63 ppm for NH with respect to the values for
a
b
fraction; the simulated and experimental diffraction pat- 1; the shifts are the same as those for crystals of 2 (Figure 3,
terns are similar (Figure 2). Similarly to L2, L1 can also be a). Further, the integration of the signals of the CH proton
e
easily recycled by treating 1 with a methanol/water (1:4) of L1 and the CH proton of the TBA group suggest a
δ
mixture (Figures 2, S2, and S11).[ First, 1 was dissolved 2:1 (host–guest) stoichiometry in the extracted solid (Figure
7b]
2–
in MeOH in a test tube to give a clear solution, which be- S13). Thus, the pure extraction of CrO4 ions is clearly
1
53
came turbid after the addition of water. After the solution evident from the H NMR spectroscopic study. The Cr
had settled, a colorless precipitate deposited. The compara- NMR spectrum of the extracted solid in [D ]DMSO shows
6
2
–
tive IR data of the solid shows the absence of CO3 stretch- a peak at δ = –99.98 ppm, which is characteristic of bound
ing frequencies, and the spectral pattern is similar to that CrO42– ion, compared to the standard Cr NMR signal of
53
1
of pure L1. Further, the H NMR spectra of the recycled K CrO in D O at δ = 0 ppm (Figure S15). The UV/Vis
2
4
2
2–
solid, 1, and L1 show the recovery of pure L1 from 1 upon spectra of crystals of 2 and the bulk CrO extract in aceto-
4
MeOH/H O treatment. Almost quantitative (95%) recovery nitrile show a characteristic CrO42– peak at 372 nm. Similar
2
of L1 from 1 is estimated by this technique. This recycled absorbance values [optical density (OD) = 0.53] were ob-
L1 was further reacted with TBAOH in DMSO to obtain served for both 2 and the bulk extract at the same experi-
crystals of 1 in a yield of more than 80%.
mental concentrations; this further supports the purity of
the extraction from water (Figure 3, b). The amount of
CrO42– ions was estimated both by weighing the bulk ex-
tract and by gravimetric analysis. Simple weighing of the
bulk extract shows ca. 94% extraction of CrO42– ions (with
respect to 1) from water (Table S6). Gravimetric analysis of
Liquid–Liquid Extraction Studies
The quantitative formation of 1 and its solubility in
water-immiscible solvents such as CHCl and CH Cl en-
3
2
2
the CrO42– ion extraction was performed by BaCrO pre-
4
couraged us to investigate the L–L extraction by 1 of anions
2
–
2–
2–
–
2–
–
cipitation upon the addition of aqueous BaCl
CH Cl solution of the bulk extract; the amount of CrO
4
2
to the
such as CrO4 , S O , SO4 , H PO , HAsO , and F
2
3
2
4
4
2
–
2
–
2–
2
2
from water. We found that the extraction of CrO , S O3
,
4
2
2
–
ions extracted by 1 was calculated to be Ͼ90% (Table S6).
Moreover, the similarity of the powder XRD (PXRD)
pattern of the bulk extract and the simulated pattern of
and SO4 ions by 1 by anion-exchange metathesis was ef-
–
2–
ficient. However, the extraction of H PO , HAsO
and
2
4
4
–
F
ions by 1 was unsuccessful. On the other hand, the
Eur. J. Inorg. Chem. 2014, 4134–4143
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1
Figure 3. (a) H NMR spectra of (i) 1, (ii) 2, and (iii) bulk extract. (b) UV/Vis spectra of 2 and bulk extract.
1
Figure 4. (a) H NMR spectra of (i) 1, (ii) 3, and (iii) bulk extract. (b) PXRD patterns of (i) 3 (simulated) and (ii) bulk extract (experimen-
tal).
crystals of 2 supports the purity of the bulk extract (Figure spectroscopy, IR spectroscopy, and single-crystal X-ray dif-
S14). In a similar fashion, water (5 mL) containing fraction analysis. A comparison of the IR spectra of 1 and
2
–
Na S O (10 equiv.) was added to a solution of 1 (1 mmol) the bulk SO4 extract shows the disappearance of the peak
2
2
3
–
1
in dichloromethane (DCM, 5 mL). After the usual extrac- at 1380 cm and the appearance of a new peak at
1
–1
tion and workup, the bulk extract was characterized by H 1121 cm , which corresponds to the stretching frequency
NMR spectroscopy, PXRD analysis, and single-crystal X- of SO42– ions (Figure S19). The H NMR spectra of 1, the
1
1
2–
1
ray structure analysis. The H NMR spectrum of the bulk bulk SO4 extract, and 4 are presented in Figure 5. The H
extract shows signals for NH and NH at δ = 9.34 and NMR spectra of 1 and the bulk extract show chemical shift
a
b
6.88 ppm, respectively, compared with δ = 10.27 and changes of the parent NHa signal from δ = 10.27 to
7
.80 ppm for 1. The single crystal of [(L1) ·(S O )·(TBA) ] 9.46 ppm and the NH signal from δ = 7.80 to 7.24 ppm.
2
2
3
1
2
b
(3) obtained from DMSO shows a very similar H NMR Similar chemical shifts of the signals of the urea NH and
a
1
spectrum to that of the bulk extract (Figure 4, a). Again, NH protons were observed in the H NMR spectra of 4,
b
the integration of the signal of the CH proton of L1 and which suggests that the extraction of SO42– ions by anion-
e
the CHδ proton of TBA indicates a 2:1 (host–guest) exchange metathesis proceeds cleanly (Figure 5, a). Import-
2
–
stoichiometry in the bulk S O3 extraction product (Figure antly, the integration of the signals of CH of L1 and CH
2
e
δ
S16). The purity of the bulk extract was confirmed by of the tetrabutylammonium cation indicate a 2:1 L1/SO42–
PXRD analysis (Figure 4, b). An estimation of the amount ratio in the extracted solid (Figure S17). Further, the purity
2
–
of S O
ions extracted by weighing the bulk extract re- of the bulk extract was confirmed by the resemblance of its
2
3
veals ca. 94% extraction (Table S7). PXRD pattern with the simulated pattern of the single crys-
2
–
2–
2–
Like that of CrO and S O ions, the L–L extraction tals of 4 (Figure 5, b). Finally, the quantification of SO4
4
2
3
2
–
of SO4 ions was performed with 1 (1 mmol) in DCM and ion extraction was performed by simple weighing of the ex-
K SO (10 equiv.) in water. The expected carbonate–sulfate tracted mass after L–L extraction. This suggested that ca.
2
4
exchange was verified by the pink coloration of the aqueous 95% of the SO42– ions were extracted (Table S8). Further,
phase in the presence of phenolphthalein indicator, which the extent of the SO42– ion extraction was verified by pre-
confirmed the basic nature of aqueous phase (Figure S17). cipitation of BaSO from the bulk extract by the addition
4
The bulk extracted solid was then crystallized from DMSO of an aqueous BaCl solution to a CH Cl solution of the
2
2
2
1
as [(L1) ·(SO )·(TBA) ] (4) and characterized by H NMR bulk extract. The gravimetric analysis of the isolated BaSO4
2
4
2
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1
Figure 5. (a) H NMR spectra of (i) 1, (ii) 4, and (iii) bulk extract. (b) PXRD patterns of (i) 4 (simulated) and (ii) bulk extract (experimen-
tal).
also shows the high efficiency of the extraction. To verify CO 2– complex of L2 and found that the CO complex of
2–
3
3
2–
the extraction ability of 1 in the presence of 1 equiv. of L2 can extract SO4 ions from water only up to pH 10.5.
anions, the above extraction method was repeated in the The bulk extract for L2 was also characterized by IR spec-
presence of a 1:1 stoichiometric ratio of 1 and the respective troscopy, H NMR spectroscopy, and single-crystal X-ray
1
anions. The average extraction efficiency of 1 towards all crystallography. The comparative IR study shows the disap-
three anions was ca. 90% (Table S9–11S). In all cases, a pearance of the stretching frequency at 1370 cm–1 for the
1
slightly lower extraction percentage was observed by this bulk extract, as observed previously (Figure S21). The H
process. Moreover, we tested the extraction ability of 1 NMR spectrum of the bulk extract in [D ]DMSO reveals
6
2
–
towards CrO4 ions at lower concentration and found that upfield shifts of both NH (Δδ = 0.97 ppm) and NH (Δδ
can extract CrO ions at 10 m in water; however, the = 0.73 ppm) compared with the resonances of the parent
CrO4 ion concentration in drinking water is much lower CO3 complex (Figure 6, b). Very similar chemical shifts
a
b
2–
–5
1
4
2
–
2–
2–
than this.
for the NH and NH protons were observed for the SO
a b 4
Additionally, we have studied the extraction of SO 2– complex of L2 in [D ]DMSO.
[17g]
A drawback of the carb-
ions by 1 in highly alkaline media, which is required for the onate complex of L2 towards the extraction of SO42– ions
practical application of the separation of SO4 ions from is its relatively narrow pH window of extraction activity
4
6
2
–
2–
nuclear waste for improved vitrification. We maintained the (Figure S22). On the other hand, in addition to SO ions,
4
2–
2–
pH of an aqueous solution of K SO by adding NaOH complex 1 can also extract S O3 and CrO ions. Interest-
2
4
2
4
2–
solution and then performed the usual L–L extraction ex- ingly, complex 1 can extract SO4 ions more effectively
periment under increasingly alkaline condition. Interest- than the CO32– complex of L2 under highly alkaline condi-
2
–
ingly, the clean and efficient extraction of SO4 ions from tions (pH 12.5), which are more relevant for practical appli-
water by 1 to pH 12.5 was successfully observed by compar- cations of SO42– separation. Similar to those with 1, recy-
1
ative H NMR spectral analysis (Figure 6, a). In this con- cling experiments were performed with 2–4 to recover L1.
text, Custelcean et al. have employed a crystallization ap- In all three cases, we recovered pure L1 upon treatment
2
–
proach for the removal of SO4 ions with high efficiency with MeOH/H O (1:4 v/v; Figure S23–S25). A comparison
2
[17n]
1
under highly alkaline conditions.
the clean extraction of SO ions in the presence of 5 equiv. tion of pure L1 from the complexes in good yield. A 75%
Complex 1 also shows of the H NMR spectroscopic data supports the regenera-
2
–
4
–
of NO3 ions (Figure S20). We also extended our study recovery of pure L1 is estimated for 4, whereas ca. 80%
2
–
towards the extraction of SO4 ions at alkaline pH to the recovery of L1 is calculated for 2 and 3.
1
2–
1
Figure 6. (a) H NMR spectra of (i) 1, (ii) 4, and (iii) the bulk SO
4
extract at pH 12.5. (b) H NMR spectra of the carbonate capsule
2–
of L2 and the bulk SO
4
6
extract at pH 10.5 in [D ]DMSO.
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2
–
Figure 7. UV/Vis spectral analysis of (a) CrO
4
ion extraction by 1 [absorbance (A) = 0.8868 and 0.3942 at 372 nm before and after
–4 2–
2–
extraction, respectively. Conc. of CrO
4
before extraction = 2.49ϫ10 m] and (b) CrO
and K SO [A = 1.0354 and 0.5553 at 372 nm before and after extraction, respectively. Conc. of CrO
4
2 4
ion extraction from a 1:1 mixture of K CrO
2
–
–4
2
4
4
before extraction = 2.48ϫ10 m].
CrO42– Ion Extraction and Selectivity Study by UV/Vis
Spectroscopic Analysis
against a solution of tetraethylammonium hydrogen carb-
onate (TEAHCO ) in [D ]DMSO. On the other hand, for
3
6
CrO42–, S O3 , and SO4 ions, a solution of L1 in [D6]-
2–
2–
2
Additionally, solution-state UV/Vis studies were per-
formed to allow estimation of the amount of CrO42– ions
extracted by 1. The details of this study are described in
the Supporting Information. In this case, ca. 1.2 equiv. of
K CrO (relative to 1) was dissolved in water during the
DMSO/D O (9:1 v/v) was titrated against solutions of the
2
anions as their sodium salts in a [D ]DMSO/D O (1:1.1
6
2
v/v) solvent mixture. For the titration of L1 with CO32–
ions, we observed downfield shifts of the NH and NHb
a
2
4
signals and an upfield shift of the signal of CH (Figure 8,
d
extraction. First, we measured the absorbance of the aque-
2–
a). Upon the gradual addition of CO3 ions, a broadening
of the NH and NH signals was observed with the disap-
2
–
ous CrO4 solution before the L–L extraction study. Then,
upon completion of the extraction (ca. 6 h), the absorbance
of the aqueous layer was measured again. By this method,
we estimated the amount of CrO42– ions removed during
the L–L extraction, and our calculation shows ca. 70% ex-
a
b
pearance of the NHa signal. Upfield shifts of 2.61 to
2
1
.5 ppm were observed for CH upon the addition of ca.
equiv. of CO3 ions. A Job plot analysis of the upfield
d
2–
shift of CH reveals that there is 1:1 (host–guest) binding
2
–
d
traction of CrO4 ions from water with respect to 1 (Fig-
2–
between L1 and CO3 (Figure 8, b). This discrepancy of
ure 7, a). The same experiment was performed to estimate
1:1 (host–guest) binding in solution and 2:1 (host–guest)
2
–
the amount of CrO ions removed by 1 from a mixture of
4
binding in the solid state is common for tripodal urea and
2
–
2–
CrO4 and SO4 ions. The UV/Vis spectral calculations
[17l,17p]
thiourea receptors.
2
–
revealed ca. 50% extraction of CrO4 ions from the mix-
ture (Figure 7, b).
2–
The binding constant (logK) for L1 with CO3 ions was
calculated as 3.12 by using the WINEQNMR software.[
18]
In this context, L2 shows a logK value of 4.04 with CO32–
1
Solution-State H NMR Titration Study for L1
1
ions in [D ]DMSO as determined by H NMR titration.
6
The solution-state binding of L1 with CO32–, CrO42–, For CrO4 , S O3 , and SO4 ions, upfield shifts of the
2–
2–
2–
2
2
–
2–
1
S2O3 , and SO ions was studied by H NMR titration. signals of the CH and CH protons and the disappearance
4
c
d
2
–
For CO3 ions, a solution of L1 in [D ]DMSO was titrated of the signal of the NH and NH protons were observed
6
a
b
1
Figure 8. (a) Partial H NMR spectral changes for the titration of L1 (5.18 mm) in [D
]DMSO (298 K). [TEAHCO ]/[L1]: (i) 0, (ii) 0.08, (iii) 0.16, (iv) 0.24, (v) 0.32, (vi) 0.40, (vii) 0.48, (viii) 0.56, (ix) 0.64, (x) 0.72, (xi)
.12, (xii) 1.20. (b) Job plot for L1 with TEAHCO in [D ]DMSO.
6 3
]DMSO and standard TEAHCO (25.99 mm) in
[
1
D
6
3
3
6
Eur. J. Inorg. Chem. 2014, 4134–4143
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Figure 9. (a) Partial 1H NMR spectral changes for the titration of L1 (3.66 mm) in [D
6
2 2 4
]DMSO/D O (9:1 v/v) and standard K CrO
2–
(25.4 mm) in [D
6
]DMSO/D
2
O (1:1.1) at 298 K. [CrO
4
]/[L1]: (i) 0, (ii) 0.11, (iii) 0.22, (iv) 0.33, (v) 0.44, (vi) 0.55, (vii) 0.67, (viii) 0.78,
(
ix) 0.89, (x) 0.99, (xi) 1.11, (xii) 1.22. (b) Job plot for L1 with K
2
CrO in [D ]DMSO/D O (1:1.1).
4
6
2
upon the gradual addition of anions. Chemical shift metathesis feasible across water–dichloromethane biphases.
2
–
changes (Δδ) of 0.17 and 0.13 ppm for CH were observed This anion-exchange metathesis process results in CO
c
3
2
–
2–
+
+
for the addition of ca. 1 equiv. of CrO4 and S O anions, anions and Na and K cations in the water layer, whereas
2
3
respectively (see part a of Figure 9 and Figure S29a). An L1, TBA, and CrO42–, S O3 , and SO4 ions are found in
2–
2–
2
upfield shift of 0.17 ppm (Δδ) was found for CH upon the the DCM layer. A CH Cl solution of the carbonate cap-
d
2
2
2
–
addition of ca. 1 equiv. of SO4 anions (Figure S30a). A sule can exist as an equilibrium mixture of carbonate-
Job plot analysis shows that there is 1:1 (host–guest) bound dimer and monomer; the monomeric species pre-
stoichiometry between L1 and all three anions (see part b dominates in solution, as H NMR spectroscopy supports
1
of Figure 9 and Figures S29b and S30b). One such repre- stoichiometry of ca. 1:1 in solution. During liquid–liquid
sentative titration profile along with the Job plot for the extraction, the exchange between CO32– and SO4 ions oc-
2–
2
–
CrO4 ion titration is provided in Figure 9. The binding curs because the relatively higher association constant of
4
2–
2–
constant (logK) values were calculated to be ca. 10 for L1 SO4 ions and the higher hydration energy of CO ions
with all three anions.
3
2–
lead to the formation of SO4 -bound monomeric and di-
meric species of L1 at the water–chloroform interface. Al-
though, monomeric species may be the dominant species in
the equilibrium in solution, the crystallization of the bulk
Mechanism of CO Sequestration and Liquid–Liquid
2
2–
2–
2–
Extraction of CrO4 , S O
and SO4 Ions
2–
2
3
SO4 extract results in the isolation of a dimeric capsular
The formation of CO32– ions in DMSO solvent is due to
assembly of L1 to satisfy the best packing and higher coor-
2–
2
–
dination number of oxyanions such as SO
4
in the solid
the conversion of aerial CO to CO
base (OH ions). The conversion of aerial CO to CO
in the presence of
2
3
–
2–
state. However, in addition to the characterization of the
crude bulk solid of the liquid–liquid extraction experiment,
we have examined the anion-exchange phenomenon in solu-
2
3
ions is evident from a control experiment. Solid TBAOH
was added to [D ]DMSO in the absence of L1, and the
6
2–
1
3
tion. For SO
4
ions as representative case, carbonate cap-
mixture was kept in open air overnight. The C NMR
spectrum of the solution shows the appearance of a signal
at δ = 157.83 ppm (Figure S31), which indicates the forma-
sules (of L1/L2) were dissolved in CDCl in an NMR tube.
3
1
The H NMR spectra of the capsules were then recorded.
2
–
A D O solution of K SO was added to the NMR tube,
2 2 4
tion of free CO3 ions in solution by CO fixation. The
2
–
2
and the mixture was shaken for 5 min; the bilayer was then
good affinity of the receptor towards CO3 ions facilitates
the formation of the carbonate capsule (1) at the liquid–air
interface in high yield. The high solubility of 1 in water-
immiscible solvents makes 1 a successful L–L extractant.
For the L–L extraction of SO4 ions with a phase-transfer
agent, a 1:1 concentration ratio between the receptor and
phase-transfer agent is needed. Often, an excess of phase-
transfer agent has to be used to ensure the CHCl /CH Cl
1
allowed to settle for a few minutes. A comparison of the H
NMR spectra of the settled CDCl layer and 4 in CDCl
3
3
reveals complete exchange between CO32– and SO4 ions
2–
2
–
(Figures S32 and S33).
Structural Descriptions of 2–4
3
2
2
solubility of the receptor and, in turn, this may cause im-
Crystals of 2–4 were obtained from the crystallization
pure extraction. However, 1 is a 2:1 complex of L1 and of the bulk extracts from DMSO. The data collection and
2
–
CO3 ions with an exact ratio of tetrabutylammonium refinement details, hydrogen-bonding tables, and space-fill
countercations. This balanced feature of the carbonate cap- models of all four complexes are provided in the Supporting
sule rules out the possibility of any excess CO32– ions Information (Tables S1–S5, Figure S26). A comprehensive
(
phase-transfer agent) in the organic layer. The combined overview of the structural features of the crystals of 2–4 is
2–
effect of the higher binding affinity of L1 towards CrO
S2O3 , and SO4 ions over CO ions and the higher hy- assemblies upon tetrahedral oxyanion encapsulation (Fig-
dration energy of CO3 ions (ΔG = –1315 kJmol ) over ure 10). Remarkable similarities are found between the
oxyanions such as CrO4 and SO4 makes anion-exchange structural features of 2 and 4. Both 2 and 4 crystallize in
,
provided here. All three complexes form dimeric capsular
4
2
–
2–
2–
3
2
–
–1
h
2
–
2–
Eur. J. Inorg. Chem. 2014, 4134–4143
4140
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FULL PAPER
Figure 10. Perspective views of the crystal structures of (i) 1, (ii) 2, (iii) 3, and (iv) 4. All four complexes show anion-assisted dimeric
assemblies of L1 with multiple N–H···X (X = O, S) interactions (nonacidic hydrogen atoms and countercations are omitted for clarity).
the monoclinic crystal system in the C2/c space group. The S2 atom of the encapsulated S O32– ions forms two N–H···S
2
capsular dimensions of the dimeric assembly are measured contacts, namely, N6–H6···S2 (dN···S = 3.51 Å, ЄN–H···S =
as 9.70 and 9.71 Å for 2 and 4, respectively (Table 1). This 163°) and N13–H13···S2 (dN···S = 3.41 Å, ЄN–H···S = 170°;
suggests that the capsular assembly is rigid despite the dif- Table S4). The capsular dimension of the dimeric assembly
2
–
ferences in size of the encapsulated SO
(S–O = 1.47 Å) (9.61 Å) of 3 is quite similar to those of 2 and 4 (Table 1).
4
2
–
and CrO4 ions (Cr–O = 1.64 Å) in 4 and 2, respectively.
In both cases, the XO4 (X = S, Cr) encapsulation is as-
2
–
Table 1. Capsular dimension of dimeric assemblies and total
number of N–H···X interactions present.
sisted by ten strong N–H···O interactions (dN···O Ͻ3.2 Å
and ЄN–H···OϾ140°). Two oxygen atom of the XO42– ion Complex Composition
N–H···X
interactions
Capsular
dimension [Å]
[
a]
(
X = S, Cr) form three N–H···O bonds, whereas the remain-
ing two oxygen atoms form two N–H···O contacts. The
N···O bond lengths of the ten N–H···O contacts in 2 range
from 2.82 to 3.02 Å, and the N–H···O angles range from
1
2
3
4
[(L1)
[(L1)
[(L1)
[(L1)
[(L1)
2
2
2
2
2
·(CO
·(CrO
·(S
·(SO
·(Ag)
3
)·(TBA)
2
]
2
12
10
9
10
12
9.62
9.70
9.61
9.71
9.84
4
)·(TBA)
)·(TBA)
)·(TBA)
·(SO )]
]
2
O
4
3
2
]
]
2
152 to 171° (Table S3). The N···O bond lengths of the ten
[7b]
Ref.
2
4
N–H···O contacts of 4 vary from 2.83 to 3.04 Å, and the
N–H···O bond angles vary from 152 to 172° (Table S5).
Notably, the dimension of the dimeric assembly of 4
[
a] X = O for 1, 2, and 4; X = O/S for 3.
(
9.71 Å) is slightly lower than that of the Ag SO -assisted
2 4
[7b]
dimeric assembly (9.84 Å) of L1. Complex 3 with an en-
capsulated S O ion crystallizes in the monoclinic crystal
Conclusions
2
–
2
3
system in the P2 /n space group. The dimeric assembly of
We have established L1 for the efficient encapsulation of
1
L1 in 3 is assisted by seven N–H···O interactions and two CO32– ions through the fixation of aerial CO2 in basic
N–H···S interactions (Figure 10). Of the three oxygen atoms DMSO solution. The crystal structure of 1 shows the en-
2
–
2–
of the encapsulated S O3 ion, two (O7 and O9) are in- capsulation of a CO3 ions inside the dimeric capsular as-
2
volved in two N–H···O interactions each, and the remaining sembly of L1 through twelve N–H···O hydrogen-bonding
O8 atoms form three N–H···O contacts. The N···O bond interactions. Complex 1 was successfully employed for the
lengths of the seven N–H···O contacts range from 2.79 to L–L extraction of environmentally relevant anions such as
.08 Å, and the N–H···O angles vary from 158 to 172°. The CrO42–, S O3 , and SO4 from water by an anion-ex-
2–
2–
3
2
Eur. J. Inorg. Chem. 2014, 4134–4143
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FULL PAPER
2 4 2 6
change technique that overcomes their high hydration ener- [(L1) ·(SO )·(TBA) ]-
] (4): Yield: 85%. 1H NMR (300 MHz, [D
2
–
DMSO): δ = 9.46 (3 H, NH ), 7.77 (3 H, Ar-CH), 7.57–7.60 (3 H,
gies. This is the first report on the L–L extraction of CrO
a
4
2
–
Ar-CH), 7.24 (3 H, NH ), 7.13 (3 H, Ar-CH), 3.13–3.32 (14 H,
b
and S O
ions from water by an anion-exchange tech-
nique. In all three cases, ca. 90% extraction of anions is
achieved by L–L extraction with 1. Further, complex 1 was
successfully employed for the extraction of SO4 ions from
water under highly alkaline conditions and in the presence
of excess NO3 ions. The crystal structures of all of the iso-
2
3
CH
d
merged with CH
), 1.24–1.36 (8 H, CH
]DMSO): δ = 154.91 (C=O), 141.61 (Ar-C),
29.24 (Ar-C), 123.74 (Ar-C), 121.87 (Ar-C), 119.90 (Ar-C),
18.91 (Ar-C), 110.98 (CN), 57.50 (NCH CH CH CH ), 54.09
α
), 2.51–2.54 (6 H, CH
c
), 1.51–1.61 (8 H,
1
3
CH
β
γ
), 0.90–0.99 (12 H, CH
δ
) ppm.
C
NMR (75 MHz, [D
1
1
6
2
–
2
2
2
3
–
(NCH CH ), 37.08 (NCH CH ), 23.03 (NCH CH CH CH ),
2
2
2
2
2
2
2
3
lated anion complexes reveal anion-assisted dimeric capsu- 19.18 (NCH CH CH CH ), 13.45 (NCH CH CH CH ) ppm.
2
2
2
3
2
2
2
3
lar assemblies of L1 of similar capsular dimensions.
–
1
Crystallographic Data for 1: C93
monoclinic, space group P2
c = 32.380(6) Å, α = 90°, β = 98.635(5)°, γ = 90°, V = 9499(3) Å ,
H
132
N
22
O
9
r
, M = 1702.21 gmol ,
1
/c, a = 23.599(4) Å, b = 12.574(2) Å,
3
Experimental Section
–3
–1
Z = 4, ρcalcd. = 1.19 gcm , μ = 0.079 mm , T = 150(2) K, 43915
reflections, 7009 independent (Rint = 0.0662), 5188 observed reflec-
tions [IՆ2σ(I)], 1125 refined parameters, R1 = 0.0434, wR2 =
[
(
(
(L1)
2
·(CO
10 mL), and an excess of tetrabutylammonium hydroxide
TBAOH) was added. The mixture was stirred and warmed to
0 °C for 10 min and then kept in open air for crystallization.
Within 48 h, diamond-shaped colorless crystals of 1 were isolated
3 2
)·(TBA) ] (1): L1 (40 mg) was dissolved in DMSO
0.1088, GOF = 1.013.
6
Crystallographic Data for 2:
1
C
92
H
132
N
22
O
10Cr,
758.2 gmol , monoclinic, space group C2/c, a = 23.396(3) Å, b =
2.7927(15) Å, c = 32.578(4) Å, α = 90°, β = 94.871(4)°, γ = 90°, V
M
r
=
–1
1
in high yield (85%). H NMR (300 MHz, [D
3 H, NH ), 7.80 (3 H, NH ), 7.58 (3 H, Ar-CH), 7.51–7.56 (3 H,
Ar-CH), 7.05–7.06 (3 H, Ar-CH), 3.21–3.12 (14 H, CH merged
with CH ), 2.54–2.51 (6 H, CH ), 1.51–1.61 (8 H, CH ), 1.26–1.36
8 H, CH ), 0.90–0.95 (12 H, CH ]-
) ppm. 13C NMR (75 MHz, [D
DMSO): δ = 169.72 (CO ), 155.63 (C=O), 142.18 (Ar-C), 129.6
Ar-C), 124.16 (Ar-C), 122.37 (Ar-C), 120.42 (Ar-C), 119.49
Ar-C), 111.46 (CN), 58.14 (NCH CH CH CH ), 54.15
NCH CH ), 37.49 (NCH CH ), 23.67 (NCH CH CH CH ),
9.82 (NCH CH CH CH ), 14.08 (NCH CH CH CH ) ppm.
6
]DMSO): δ = 10.27
1
=
(
a
b
3
–3
–1
9715(2) Å , Z = 4, ρcalcd. = 1.202 gcm , μ = 0.184 mm , T =
d
1
50(2) K, 25959 reflections, 5520 independent (Rint = 0.1246), 3816
observed reflections [IՆ2σ(I)], 568 refined parameters, R1 =
.0799, wR2 = 0.2648, GOF = 1.127.
α
c
β
(
γ
δ
6
0
2–
3
Crystallographic Data for 3:
C
92
H
132
N
22
O
9
S
2
,
M
r
=
(
(
(
–
1
1754.34 gmol , monoclinic, space group P2
1
/n, a = 23.9868(16) Å,
2
2
2
3
b = 13.5376(9) Å, c = 30.130(2) Å, α = 90°, β = 94.405(2)°, γ = 90°,
2
2
2
2
2
2
2
3
3
–3
–1
V = 9755.0(11) Å , Z = 4, ρcalcd. = 1.194 gcm , μ = 0.012 mm ,
T = 150(2) K, 42880 reflections, 9051 independent (Rint = 0.1247),
1
2
2
2
3
2
2
2
3
General Procedure for Liquid–Liquid (L-L) extraction: In a typical
L–L extraction experiment, 1 (30 mg) was dissolved in dichloro-
methane (5 mL), and a deionized water (5 mL) solution containing
5
722 observed reflections [IՆ2σ(I)], 1135 refined parameters, R1
=
0.0791, wR2 = 0.2447, GOF = 1.059.
Crystallographic Data for 4:
1
C
92
H
132
N
22
O10S,
M
r
=
10 equiv. of the respective salt was added. The biphasic solution
–
1
738.27 gmol , monoclinic, space group C2/c, a = 23.428(2) Å, b
was then stirred for 6 h and then settled for 15 min. The separated
organic layer was then filtered through a Whatman 1PS filter paper
and evaporated under reduced pressure. Finally, the bulk extract
was recrystallized from DMSO. The obtained crystals were then
=
12.8969(13) Å, c = 32.641(3) Å, α = 90°, β = 95.256(2)°, γ = 90°,
3
–3
–1
V = 9821.0(16) Å , Z = 4, ρcalcd. = 1.176 gcm , μ = 0.099 mm ,
T = 150(2) K, 42171 reflections, 7817 independent (Rint = 0.0681),
5691 observed reflections [IՆ2σ(I)], 568 refined parameters, R1 =
1
13
characterized by IR spectroscopy, H and C NMR spectroscopy,
and single-crystal X-ray diffraction analysis. The characterization
data of 2–4 are provided below. The yields of 2–4 obtained by the
crystallization of the bulk extracts are provided.
1
0
.0455, wR2 = 0.1135, GOF = 1.008.
CCDC-887538 (for 1), -973806 (for 2), -973807 (for 3), and -887539
for 4) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
(
[
(L1)
DMSO): δ = 9.433 (3 H, NH
Ar-CH), 7.18–7.24 (6 H, Ar-CH and NH
and CH merged), 2.49–2.50 (6 H, CH ), 1.54–1.56 (8 H, CH
.27–1.34 (8 H, CH ), 0.90–0.95 (12 H, CH
75 MHz, [D ]DMSO): δ = 154.93 (C=O), 141.68 (Ar-C), 129.34
Ar-C), 123.77 (Ar-C), 122.13 (Ar-C), 120.06 (Ar-C), 118.94
Ar-C), 111.01 (CN), 57.49 (NCH CH CH CH ), 54.01
NCH CH ), 37.38 (NCH CH ), 23.01 (NCH CH
9.15 (NCH CH
2
·(CrO
4
)·(TBA)
2
] (2): Yield: 92%. H NMR (300 MHz, [D
6
]-
a
), 7.89 (3 H, Ar-CH), 7.65 (3 H,
b
), 3.13–3.18 (14 H, CH
α
d
c
γ
),
Supporting Information (see footnote on the first page of this arti-
cle): H and C NMR spectra, IR spectra, space-fill models, details
of hydrogen bonds.
1
β
δ
) ppm. 13C NMR
1
13
(
(
(
(
6
2
2
2
3
CH
), Acknowledgments
2
2
2
2
2
2
2
CH
3
1
2
CH
2
CH
2
CH
3
), 13.41 (NCH
2
CH
2
CH
2
3
) ppm.
1
P. G. gratefully acknowledges the Department of Science and Tech-
[
(L1) ·(S )·(TBA)
2
2
O
3
2
] (3): Yield: 88%. H NMR (300 MHz, [D
6
]-
nology (DST), New Delhi for financial support through a Swarna-
jayanti Fellowship. R. D. and S. C. acknowledge the Indian Associ-
ation for the Cultivation of Science (IACS), Kolkata for research
fellowships. The single-crystal X-ray diffraction data were collected
at the DBT-funded CEIB program (project number BT/01/CEIB/
DMSO): δ = 9.34 (3 H, NH
a
), 7.94 (3 H, Ar-CH), 7.64–7.68 (3 H,
Ar-CH), 7.28–7.33 (3 H, Ar-CH), 7.22–7.24 (3 H, Ar-CH), 6.88 (3
H, NH ), 3.13–3.18 (14 H, CH merged with CH ), 2.51–2.53 (6
H, CH ), 0.90–0.95
]DMSO): δ = 155.07
b
d
α
c
β γ
), 1.51–1.56 (8 H, CH ), 1.24–1.36 (8 H, CH
) ppm. 13C NMR (75 MHz, [D
(
(
(
(
2
(
12 H, CH
δ
6
11/V/13) awarded to the Department of Organic Chemistry, IACS,
C=O), 141.71 (Ar-C), 129.59 (Ar-C), 123.96 (Ar-C), 122.20
Ar-C), 120.07 (Ar-C), 119.01 (Ar-C), 111.14 (CN), 57.49
Kolkata.
NCH
3.02 (NCH
2
CH
2
CH
CH
CH CH
2
CH
CH
) ppm.
3
), 53.91 (NCH
2
CH
2
2 2
), 37.44 (NCH CH ),
2
2
2
CH ), 19.17 (NCH
3
2
CH CH CH
2
2
3
), 13.44 [1] a) A. Zhitkovich, Chem. Res. Toxicol. 2011, 24, 1617–1629; b)
A. D. Dayan, A. J. Paine, Human Experimental Toxicol. 2001,
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CH
2
2
3
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FULL PAPER
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