Selective ammonium nitrate recognition by a heteroditopic macrotricyclic ion-pair receptor

Article abstract of DOI:10.1021/jo4003322

The heteroditopic macrotricyclic molecular receptor 1, which bears a tripodal anion binding domain and 4,10,16-triaza-18-crown-6 cation recognition domain, proves to be an effective ion-pair receptor. In the absence of the cobound cation (TBA⁺ salts) receptor 1 preferably binds nitrate and nitrite over other anions, including basic anions such as acetate or dihydrogenphosphate. Ammonium cation binding by the 4,10,16-triaza-18-crown-6 subunit significantly enhances the strength of the nitrate and nitrite complexation at the triamide recognition site of the receptor. In the presence of ammonium cations, the association constants of nitrate binding reach an impressive value of 1050 M^-1 in highly polar DMSO-d₆. Interestingly, the binding of other anions such as chloride and bromide is not enhanced in the presence of a cobound NH₄⁺ cation. The increased affinity of [1·NH₄⁺]PF₆ ^- for anionic species is attributed to a strong cooperative effect that arises from the properly positioned binding sites in the receptor 1 cavity, thus allowing for the formation of the ion pair. Under liquid/liquid conditions, receptor 1 is able to extract NH₄NO₃ from an aqueous to an organic phase, as inferred from ¹H NMR spectroscopic and nitrite/nitrate colorimetric analyses.

Full text of DOI:10.1021/jo4003322

Article
pubs.acs.org/joc
Selective Ammonium Nitrate Recognition by a Heteroditopic
Macrotricyclic Ion-Pair Receptor
Jan Romanski and Piotr Piątek*
́
Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
S
* Supporting Information
ABSTRACT: The heteroditopic macrotricyclic molecular
receptor 1, which bears a tripodal anion binding domain and
4,10,16-triaza-18-crown-6 cation recognition domain, proves
to be an effective ion-pair receptor. In the absence of the
cobound cation (TBA+ salts) receptor 1 preferably binds
nitrate and nitrite over other anions, including basic anions
such as acetate or dihydrogenphosphate. Ammonium cation
binding by the 4,10,16-triaza-18-crown-6 subunit significantly
enhances the strength of the nitrate and nitrite complexation at
the triamide recognition site of the receptor. In the presence of ammonium cations, the association constants of nitrate binding
reach an impressive value of 1050 M−1 in highly polar DMSO-d6. Interestingly, the binding of other anions such as chloride and
+
bromide is not enhanced in the presence of a cobound NH4+ cation. The increased affinity of [1·NH4 ]PF6− for anionic species is
attributed to a strong cooperative effect that arises from the properly positioned binding sites in the receptor 1 cavity, thus
allowing for the formation of the ion pair. Under liquid/liquid conditions, receptor 1 is able to extract NH4NO3 from an aqueous
1
to an organic phase, as inferred from H NMR spectroscopic and nitrite/nitrate colorimetric analyses.
biphenylcarboxylic acid.7 This molecular receptor strongly
binds iodide, p-toluenosulfonate, and nitrate anions in weakly
INTRODUCTION
■
Nitrate contamination of atmospheric liquids and surface water
polar solvent systems. However, in polar solvents such as
is a global problem. The main source of nitrate contamination
DMSO-d6, the association constants of halide and nitrate
appears to be from agricultural operations, farm runoff, and
anions decreased dramatically. Guided by DFT calculations,
fertilizer usage.1 There is also some nitrate formed in the
Herges, Konig et al. prepared a macrocyclic tris(thiourea)
̈
atmosphere through the oxidation of nitrogen oxides that are
emitted from power plants and internal combustion engines.2
An excess of nutrients, particularly nitrates and phosphates, in
aquatic systems results in its eutrophication, i.e. excessive plant
growth.3 This phenomenon is one of the most visible examples
of human changes to the biosphere, affecting aquatic
ecosystems from the Arctic to the Antarctic.
The primary health risks associated with elevated nitrate
levels are methemoglobinemia, which causes the “blue baby”
syndrome in infants, and the potential formation of
carcinogenic nitrosamines.4
The trigonal-planar geometry of the nitrate anion, its high
hydration energy (ΔGhyd = −300 kJ/mol), large ionic radii
(1.79 Å), and low basicity (pKa = −1.44) result in the low
affinity for hydrogen-bonding interactions.5 Therefore it is
particularly challenging to design molecular receptors with
appropriately matched complementary recognition motifs for
selective and effective nitrate anion recognition, particularly
with neutral receptors.
In recent years, some elegant, neutral receptors for nitrate
ions have been synthesized. For example, Ansyln and co-
workers reported an amide-linked C3-symmetric macrobicyclic
receptor where six converging amide hydrogens were used for
selective binding of nitrate anions.6 In 2001, Hamilton et al.
synthesized a rigid macrocyclic triamide of 3′-amino-3-
receptor that binds bromide and nitrate in DMSO-d6.8 Very
recently Singh and Sun reported a charged, zwitterionic
fluorescent chemosensor that displays one of the highest
binding affinities for nitrate anions so far reported in polar
media (550 M−1, DMSO).9,10
The molecular receptors mentioned above bind the nitrate
anion in the presence of the soft, noncompeting tetra-n-
butylammonium countercation (TBA). However, in real-life
applications, the anion is accompanied by hard cations such as
+
Na+, K+, or NH4 , and ion pairing of the target anion with its
countercation can lead to diminished receptor-anion affinity.11
Therefore, to overcome this problem, receptors capable of
binding both the anion and cation of an ion pair are needed.12
Properly designed salt receptors have the potential, due to
positive cooperativity, to bind anions more strongly than the
monotopic receptors. Moreover, complexation of both the
cation and the anion enhances salt lipophilicity, thus facilitating
its solubilization, extraction, and membrane transport.13
Among the heteroditopic ion-pair receptors the most
effective, but unfortunately the most difficult to design, are
salt receptors that recognize contact ion pairs. An elegant
Received: February 15, 2013
© XXXX American Chemical Society
A
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a
example of such a heteroditopic receptor consists of 1,10-diaza-
18-crown-6 and 1,3-phenylenedicarboxamide subunits, as
reported by Smith’s group.14 A single crystal X-ray structural
analysis revealed that this receptor is able to bind, as contact ion
pairs, Na+ and K+ salts of halides, as well as trigonal oxyanions,
Scheme 1. Synthesis of 1
−
such as NO3 and AcO−. In solution, in the presence of either
Na+ or K+ and Cl−, the association constants for complexation
of the corresponding counterion increased significantly. The
solution studies of nitrate salts have been limited, althought it
has been confirmed that this receptor is able to slowly extract
solid LiNO3, NaNO3, and KNO3 into chloroform.15,16
RESULTS AND DISCUSSION
■
Design and Synthesis. Inspired by Smith’s receptor, we
designed a macrotricyclic heteroditopic receptor that displays a
three-dimensional molecular architecture (Figure 1). The
a
Reagents and conditions: (i) 3-nitrobenzyl chloride, K2CO3, KI,
THF, reflux, overnight, 72%; (ii) NaBH4, Pd/C, THF/MeOH, 1h,
91%; (iii) Et3N, CH2Cl2, slow addition over 6 h, 42%; (iv) Ac2O,
CH2Cl2, 25%.
−
salts of AcO−, Cl−, Br−, and NO3 into CDCl3 solution.
However, after 1 h of stirring a solution of 1 in the presence of
sodium and potassium salts, the 1H NMR spectra of the
receptor had changed only slightly, which suggests that these
salts are extracted rather moderately into the organic phase (see
Supporting Information). In contrast, in the presence of
ammonium salts, the amide-NH signal shifted downfield
considerably by ∼1.2 ppm, which indicates hydrogen-bonding
interactions of 1 with anions. Additionally, the complex-
induced changes in the chemical shifts of the N-benzylic and
crown ether-CH2 were consistent with the ammonium cation
being encapsulated by the triaza-18-crown-6 moiety.
Figure 1. Structure of receptor 1.
4,10,16-triaza-18-crown-6 is the key structural element that
allows the construction of receptor 1.17 It is well recognized
that N-alkyl derivatives of this macrocycle are able to selectively
bind primary ammonium cations.18 The structural unit that
closes the macrotricyclic architecture of receptor 1 and creates a
convergent, trigonal anion binding site is 2,4,6-triethylbenzene-
1,3,5-tris(acetic acid).
Solution Binding Studies. The association constant
between 1 and the nitrate anion accompanied by the bulky,
1
The synthesis of the target receptor was accomplished by a
three-step synthetic procedure. As shown in Scheme 1, the
reaction of the tri-HBr salt of 4,10,16-triaza-18-crown-6 with 3-
nitrobenzyl chloride in the presence of excess K2CO3 in THF
under reflux gave the tris-nitrobenzyl derivative 2 in 72% yield.
Subsequent reduction of nitro groups with sodium borohydride
in the presence of palladium on carbon then gave the tris-amine
3 in 91% yield. Finally, this compound was condensed with
2,4,6-triethylbenzene-1,3,5-tris(acetyl chloride) under high
dilution conditions in CH2Cl2, to give macrotricyclic receptor
1 in 42% yield. The relatively high yield of the macrocyclization
step could be rationalized in terms of the preorganization of
2,4,6-triethylbenzene-1,3,5-tris(acetyl chloride). The alternating
steric interactions in 2,4,6-triethylbenzene-1,3,5-tris(acetic acid)
ensure that the carboxylic groups are oriented toward the same
face of the aromatic ring.19 Compound 4, the open counterpart
of 1, was prepared as a reference compound by acylation of 3
with acetyl anhydride.
noncoordinating TBA cation was determined, by H NMR
titration experiments in CDCl3, to be 115 M−1. The limited
+
−
−
solubility of NH4 salts of PF6 or ClO4 precluded the
preparation of a solution of this cation in sufficient
concentration. Therefore, an unspecified amount of NH4PF6
was introduced into the solution of receptor 1 via solid−liquid
extractions. In the presence of the ammonium cation the
association constant for nitrate was higher than 5 × 104 and
1
therefore could not be accurately determined by the H NMR
titration method. This remarkable positive cooperativity effect
enhances the efficacy of nitrate recognition by nearly 3 orders
of magnitude in this low polar solvent.
Quantitative information about the binding ability of
receptor 1 toward anions, cations, and ion pairs was obtained
1
by H NMR titration experiments in highly polar DMSO-d6.
First, the affinity of the receptor for anions in the presence of
the noncoordinating TBA cation was established. The addition
of anions to a 2.6 mM solution of 1 caused nonlinear downfield
shifts of the amide NHs and aromatic protons (H2) signals
directed into the center of the cavity (Figure 1). The former
Solid/Liquid Extraction. Receptor 1 was initially examined
1
+
by H NMR for its ability to extract solid Na+, K+, and NH4
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signal was used to determine the association constant for
complexes of 1 with a variety of anionic guests. The association
constants calculated by the nonlinear regression analysis of the
binding isotherms are presented in Table 1.
the association constants increases more than 3-fold. The
recognition of nitrate is significantly enhanced by the presence
of K+, yet the largest positive cooperativity factor (KNH /KTBA
=
+
4
3.8) is observed for simultaneous binding of nitrate and NH4
−
cations. In this case the association constants of NO3 binding
reach an impressive value of 1050 M−1 in DMSO-d6.
Table 1. Association Constants (Ka) Values for Interactions
a
+
of 1 with Various Anions (TBA Salts)
Interestingly, the presence of NH4 cations not only enhances
the affinity for nitrate but also increases the selectivity of
receptor 1 toward this anion. This is due to the fact that the
binding of halogen anions is not enhanced by ammonium
AcO−
H2PO4
110
Cl−
Br−
155
NO2
290
NO3
280
−
−
−
Ka (M−1
)
120
30
a
1H NMR, solvent: DMSO-d6, temperature 293 K, (1) = 2.6 mM,
anions added as TBA salts (TBAX) ∼20 mM, Errors < 10%. No
appreciable change in the H NMR spectrum of 1 was seen upon
cations and the association of AcO− and H2PO4 with 1 is
−
+
actually suppressed in the presence of NH4 (see Supporting
1
Information). The ability of 1 to strongly associate with
NH4NO2 is also worthy of note because it is a highly toxic
anion that is an intermediate product of both the nitrification
and denitrification processes, and therefore it is present in
treatment with excess TBAPF6.
As can be clearly seen in Table 1, receptor 1 selectively
associates with nitrate and nitrite anions with fairly high binding
constants in polar solvents such as DMSO-d6. Because the ionic
volumes of bromide and nitrate anions are comparable,20
bromide could interact strongly with receptors that bind
nitrate.8 However, this is not the case with receptor 1, which
binds bromide more weakly than nitrate. The chloride ion is
apparently too small to be effectively recognized by 1.
Interestingly, a moderate affinity to basic anions, such as
dihydrogenphosphate and even acetate, which has the same
trigonal geometry as nitrate, is observed. Such a high selectivity
toward nitrate and nitrite ions is, to our knowledge,
unprecedented in abiotic chemical systems.
21
−
biological systems along with the NO3 anion.
Interestingly, when the reference compound 4 was titrated
with the nitrate anion in the presence and absence of the
ammonium cation, no binding was detected. This supports the
notation that the observed binding affinity for receptor 1 and
nitrates mainly originates from its tricyclic structure and the
consequent proper orientation of the binding domains.
Taking advantage of the sufficient solubility of ammonium
nitrate in DMSO-d6, ion-pair NMR titrations were conducted.
Specifically, a 2.5 mM solution of receptor 1 was titrated with a
50 mM solution of NH4NO3 in DMSO-d6. The association
constant value was determined to be 75 M−1. That surprisingly
low value, in comparison to titration with TBANO3, can be
explained in terms of the ion pairing of NH4NO3 in organic
solvent, which must be much greater than that for TBANO3.22
In the case of anion titration, in the presence of 1 equiv of
cation, the TBANO3 salt was added to the solution of 1
pretreated with NH4PF6 (Figure 2). The ion-pair titration was
therefore conducted in an analogous manner, and the binding
constant was calculated to be 340 M−1. That binding constant
enhancement can be rationalized in terms of the formation of
The cation binding properties of receptor 1 were probed by
monitoring the aromatic protons H2 using H NMR spectros-
1
copy in DMSO-d6. These studies revealed that receptor 1 binds
Na+, K+, and NH4+ cations (as hexafluorophosphate salts) with
similar strengths (Ka ∼35 M−1). Analogous titrations conducted
−
in the presence of 1 equiv of NO3 anions showed significant
enhancement of association constants for K+ (Ka = 80 M−1)
and NH4 (Ka = 50 M−1) cations.
+
To gain more insight into the cooperative enhancement of
anion recognition by the presence of the cation, H NMR
1
+
−
the [1·NH4 ]PF6 complex. The binding of the ammonium
cation by receptor 1 creates a positively charged complex,
which holds an unoccupied anion binding domain (no binding
titrations of receptor 1 were conducted with anions in the
presence of 1 equiv of hard cations such as Na+, K+, and NH4 .
+
The calculated association constants are listed in Table 2.
−
1
of PF6 was evidenced by H NMR analysis). Moreover this
“positively charged anion receptor” possesses an additional
hydrogen bonding donor which can interact with anions (see
Table 2. Association Constant (Ka) Values for Interactions
of 1 with Various Anions in the Presence of 1 equiv of
+
−
a
Figure 3). Therefore the ability of the [1·NH4 ]PF6 complex
for anion binding is stronger than that of free receptor 1. These
data suggest that receptor 1 binds salts in a sequential
manner.23
Cation
TBA+
Na+
K+
NH4
+
Cl−
30
35
50
30
Br−
155
290
275
280
220
280
290
250
765
180
980
1050
Molecular Modeling. The remarkable affinity of receptor 1
toward nitrate in the presence of ammonium was also
investigated by means of Density Functional Theory (DFT)
calculations. The structures of the 1 + NH4NO3 complex were
optimized by DFT using the accurate M06-2X functional with
the 6-31+G* basis set in DMSO solution described by a
polarizable continuum solvent model (see Experimental
Section for full details). These calculations lead to two low
energy structures that have almost the same energy (ΔE = 0.6
kcal/mol, Figure 3). Both structures have C3 symmetry, with
the principal axis passing through the nitrogen atom of the
nitrate anion, the nitrogen of the ammonium cation, and the
center of the hexasubstituted aromatic ring of the receptor. The
nitrate anion is bound inside the cavity by H-bonds between O-
atoms of the nitrate anion and three amide N−H protons
−
−
NO2
NO3
a
1H NMR, solvent DMSO-d6, temperature 293 K, (1) = 2.6 mM,
−
cations added as PF6 salts (MPF6) = 2.6 mM, anions added as TBA
salts (TBAX) ∼20 mM; M−1, Errors < 10%.
Examination of these data reveals a number of trends. First,
from the perspective of halogen anions binding, only the
presence of K+ cations results in a notable increase of anion
association constants. Other cations generally have little
influence on the halogen anion binding abilities of 1, with the
exception of NaBr. In contrast, nitrite anion recognition is
actually slightly reduced in the presence of both K+ and Na+
cations. However, in the presence of NH4+ cations the value of
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Figure 2. Top: 1H NMR (200 MHz) partial spectra of receptor 1 in the presence of NH4PF6 upon progressive addition of TBANO3 (0, 0.6, 1.1, 2.2,
and 7.5 equiv from bottom to top). Bottom: Chemical shift of NH signals of 1 as a function of increasing amounts of TBANO3 in the absence and
presence of NH4PF6.
the calculated structures, the nitrate anion is located parallel to
the plane defined by three amide N-atoms. This observation is
in agreement with NMR titration experiments. According to
the calculations of the nitrate shielding surface, a deshielding
effect is observed in the range of 0° to 60° above/below the
plane of the NO3− anion, whereas a shielding effect is observed
directly above/below the N-atom.13 Therefore the downfield
shift of the NH and H2 protons of receptor 1 upon addition of
the nitrate anion indicates that the NO3− anion is located in the
deshielding range. This observation rules out a perpendicular
−
Figure 3. DFT optimized structures of 1·NH4NO3.
location of the NO3 anion inside the receptor cavity.
The ammonium cation resides in proximity to the
triazacrown ether binding domain. However, in one structure
(Figure 3, structure I) the ammonium NHs are directed toward
the central N-atom of the nitrate anion (dN−H···N 3.97 Å,
θN−H···N 177.1°) and the oxygen atoms of crown ether (dN−
H···O from 2.871 Å to 2.90 Å, θN−H···O from 176.6° to
(dN−H···O from 3.01 to 3.07 Å, θN−H···O from 158.1° to
159.2°). The N−H···O interactions observed here are directed
−
primarily toward the lone pairs on the NO3 oxygens. The
distances and angles between amide N−H and nitrate anion O-
atoms are almost identical in the second structure. Moreover, in
D
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Figure 4. Partial 1H NMR (200 MHz) spectra of receptor 1: (a) 11.8 mM solution in wet CDCl3; (b) after NH4NO3 extraction from water phase;
(c) after back-extraction to distilled water.
179.0°). In contrast, in the second structure (Figure 3, structure
II) ammonium NHs form three H-bonds to nitrogen rather
than the oxygen atoms of the triazacrown moiety (dN−H···N
from 3.23 Å to 3.26 Å, θN−H···O from 170.4° to 172.0°).
DMSO-d6. However, remarkable enhancement of both nitrate
association strength and selectivity in the presence of a
cobound ammonium cation has been observed. The increased
+
−
affinity of [1·NH4 ]PF6 for anionic species is attributed to a
strong cooperative effect that arises from the properly
positioned binding sites in the receptor 1 cavity, thus allowing
the formation of the ion pair. With receptor 1 as a liquid−liquid
extractant, extraction of the NH4NO3 ion pair from an aqueous
to an organic phase in a recyclable manner has been achieved.
As the most effective strategy for reducing the content of
“nitrogen” in aquatic systems is removal of both ammonium
cations and nitrate anions, the receptor ability to extract
NH4NO3 is of great value. Ongoing efforts are focused on the
incorporation of receptor 1 derivatives into a polymeric matrix
in order to create materials that could effectively separate
nitrate salts from an aqueous solution.24
+
These modeling results indicate that NH4 flips between two
equally occupied positions.
Liquid/Liquid Extractions. As receptor 1 has a high
affinity and selectivity for NH4NO3 salt in polar solvents, the
liquid/liquid extraction behavior of this receptor was examined
by means of 1H NMR spectroscopy and nitrate anion
colorimetric analysis. A 1.7 M solution of NH4NO3 in distilled
water was layered onto a 11.8 mM solution of 1 in CDCl3. The
two layers were thoroughly mixed and then separated, and the
1H NMR spectrum was recorded (Figure 4b). Inspection of this
spectrum revealed that signals corresponding to the amide NHs
and aromatic protons (H2) are broadened. Furthermore, a new
broad signal at 9.60 ppm appeared. These results indicate that
in wet CDCl3 the complexation/decomplexation process is
slow on the NMR time scale, and the new signal can be
attributed to the NH4NO3 complex of 1. The extraction
efficiency, i.e. the fraction of receptor molecules occupied by
the complex in the organic phase, as determined by NMR
integration, is ∼65%. The organic phase was then back-
EXPERIMENTAL SECTION
■
The tri-HBr salt of 4,10,16-triaza-18-crown-6 and 2,4,6-triethylben-
zene-1,3,5-tris(acetyl chloride) were synthesized according to the
literature procedures.25,26 Other reagents and chemicals were of
reagent grade quality and purchased commercially. The anion TBA
and cation PF6 salts were dried under high vacuum at 30−45 °C prior
to use. 1H and 13C NMR spectra as well as titrations experiments were
1
extracted into H2O. The H NNR spectrum of the CDCl3
1
phase is essentially identical to the spectrum of the free
receptor (Figure 4a and 4c). The nitrate content in the aqueous
layer was determined by means of a nitrite/nitrate colorimetric
method to be 20.4 mg/L, which corresponds to a 71%
extraction efficiency. The high preference of 1 toward both
ammonium cations and nitrate anions was confirmed in
extraction experiments, since no extraction of NaNO3 or
NH4Cl was observed.
recorded on a 200 MHz spectrometer. H NMR chemical shifts δ are
reported in ppm referenced to the tetramethylsilane (CDCl3) or
protonated residual solvent signal (DMSO-d6).
N,N′,N″-Tris(3-nitrobenzyl)-4,10,16-triaza-18-crown-6 (2). To
a stirred solution of the tri-HBr salt of 4,10,16-triaza-18-crown-6 (1.5
g, 3 mmol), potassium carbonate (2.48 g, 18 mmol, 6 equiv) and a
catalytic amount of potassium iodide (70 mg) in 150 mL of dry THF
3-nitrobenzyl chloride (0.268 g, 1.56 mmol, 3 equiv) were added. The
solution was refluxed overnight under argon. THF was removed under
vacuum, and the solid residue taken up in CH2Cl2 and washed with
distilled water. The organic phase was dried over Na2SO4, and the
organic solvent was removed under reduced pressure. The residue was
dissolved in a minimal amount of CH2Cl2 and loaded on silica gel. The
silica gel was eluted first with 80/20 AcOEt/hexanes, and then AcOEt
afforded 2 in the form of a thick light yellow oil (1.5 g, 72%). Rf (5%
MeOH/CH2Cl2) = 0.47; 1H NMR (200 MHz, CDCl3) δ = 8.26 (3H,
CONCLUSIONS
■
In summary, a heteroditopic macrotricyclic receptor for the
NH4NO3 ion pair has been developed. The receptor is capable
of effectively and selectively associating with nitrate anions in
the presence of a noncoordinating TBA cation in highly polar
E
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s), 8.07 (3H, d, J = 8.2 Hz), 7.68 (3H, d, J = 7.8 Hz), 7.44 (3H, t, J =
7.8 Hz), 3.78 (6H, s), 3.59 (12H, t, J = 5.6 Hz), 2.82 (12H, t, J = 5.6
Hz); 13C NMR (50 MHz, CDCl3) δ = 148.5, 142.0, 134.8, 129.2,
123.5, 122.1, 69.9, 59.1, 54.3; IR (thin-film) 3078, 2956, 1620, 1582
cm−1; ESI HR calcd for C33H42N6O9Na 689.2911, found 689.2894.
N,N′,N″-Tris(3-aminobenzyl)-4,10,16-triaza-18-crown-6 (3).
Sodium borohydride (210 mg, 5.1 mmol, 4.5 equiv) was added to a
vigorously stirred suspension of 2 (755 mg, 1.13 mmol) and palladium
on carbon (150 mg) in 50 mL of a 4:1 mixture of THF/methanol.
After 1 h, the mixture was filtered through a pad of Celite and solvents
were evaporated. The residual solid was taken up in 50 mL of CHCl3
and washed twice with distilled water. The organic layer was dried over
Na2SO4. Evaporation of the solvent gave the triamine 3, as a colorless
thick oil (594 mg, 98%). Rf (10% MeOH/CH2Cl2) = 0; 1H NMR (200
MHz, CDCl3) δ = 7.02 (3H, t, J = 7.8 Hz), 6.71 (3H, bs), 6.64 (3H, d,
J = 7.4 Hz), 6.51 (3H, dd, J = 7.4 Hz, J = 2.4 Hz), 3.56−3.51 (18H,
m), 2,74 (12H, t, J = 5.6 Hz); 13C NMR (50 MHz, CDCl3) δ = 146.6,
141.0, 129.0, 119.1, 115.5, 113.7, 69.9, 60.0, 54.1; IR (thin-film) 3321,
3041, 2982, 1659, 1552 cm−1; ESI HR calcd for C33H48N6O3Na
599.3686, found 599.3676.
Receptor 1. The solutions of 2,4,6-triethylbenzene-1,3,5-tris(acetyl
chloride) (400 mg, 1.03 mmol) and the triamine 3 (590 mg, 1.02
mmol) in CH2Cl2 (20 mL each) were simultaneously added via a
syringe pump to a vigorously stirred solution of triethylamine (0.6 mL,
4.4 mmol, 4.4 equiv) in CH2Cl2 (150 mL) over 6 h at room
temperature. After 12 h of additional stirring, solvent was removed
under vacuum and the solid residue was redissolved in dichloro-
methane (60 mL) and washed with distilled water (40 mL). The
organic layer was dried over 4A molecular sieves, and solvent was
removed under reduced pressure. The residue was dissolved in a
minimal amount of CH2Cl2 and loaded on silica gel. The silica gel was
eluted first with 50% acetone/CH2Cl2 and then 70% acetone/CH2Cl2,
affording 1 in the form of a white powder (419 mg, 48%). Rf (10%
MeOH, acetone) = 0.12; 1H NMR (200 MHz, CDCl3) δ = 8.35 (3H,
bs), 7.46 (3H, d, J = 8.2 Hz), 7.36 (3H, s), 7.23 (3H, t, J = 8.2 Hz),
7.92 (3H, d, J = 7.4 Hz), 3.91 (6H, s), 3.53−3.45 (18H, m), 2.74−2.68
(18H, m), 1,22 (H9, t, J = 7.2 Hz); 13C NMR (50 MHz, CDCl3) δ =
170.6, 143.6, 140.9, 138.0, 129.6, 128.6, 125.7, 122.1, 121.0, 70.3, 60.3,
38.2, 24.2, 14.6; IR (thin-film) 3479, 3271, 2965, 2873, 1656, 1598,
1543 cm−1; ESI HR calcd for C51H66N6O6Na 881.4936, found
881.4941.
a spectrum was acquired after each addition. Titration isotherms for
NH protons were fitted to a 1:1 binding model using the HypNMR
2000 program. All measurements were carried out in at least duplicate
using independent samples. The 1:1 binding stoichiometries were
verified by a Job plot analysis.
−
−
Liquid/Liquid Extractions. A commercially available NO2 /NO3
colorimetric test was used for quantitative determination of the nitrate
content in the water phase, after the back-extraction of the organic
phase containing a complex of NH4NO3 and receptor 1. First, nitrate
standard solutions were prepared by diluting the 500 mg/L stock
solution of KNO3 with distilled water in the range from 25 to 0.05
mg/L of nitrate. To these solutions, according to the user manual,
appropriate reagents were added. For each solution UV−vis spectra
were acquired, and a calibration curve was generated by plotting an
absorbance at 540 nm as a function of nitrate concentration. As
described in the manuscript a 1.7 M solution of NH4NO3 in distilled
water was layered onto an 11.8 mM solution of 1 in CDCl3. The two
layers were thoroughly mixed and then separated. The organic phase
was then back-extracted into H2O. A 1 mL aliquot of the aqueous
phase was diluted in a volumetric flask to 25 mL, and then, after
treatment with appropriate reagents, UV−vis spectra of that solution
were acquired. Using the calibration curve the nitrate content in the
aqueous layer was determined to be 20.4 mg/L, which corresponds to
a 71% extraction efficiency.
Molecular Modeling. The model of the 1·NH4NO3 complex was
built using the Maestro suite (Schrodinger LLC, 2012) maintaining
the C3 symmetry and minimized initially using Macromodel. NH4+ and
−
NO3 ions were place inside the structure in positions according to
similar structures reported earlier.27,28 The whole structure was
optimized without any constraints using the hybrid M06-2X density
functional method in DMSO (simulated by means of polarizable
continuum model PCM), in a standard 6-31+G* basis set suitable for
treating ionic species, as implemented in the Gaussian 09 software
suite.29,30
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra, details of solid/liquid experiments,
selected titration isotherms, molecular modeling, and Cartesian
coordinates for the optimized structures. This material is
available free of charge via the Internet at http://pubs.acs.org.
N,N′,N″-Tris(3-acetamidobenzyl)-4,10,16-triaza-18-crown-6
(Reference Compound 4). To a stirred solution of the triamine 3
(300 mg, 0.52 mmol) and triethylamine (1.4 mL, 10 mmol) in 20 mL
of dry dichloromethane acetic anhydride (0.8 mL, 7.8 mmol) was
added. The solution was stirred overnight under argon. The organic
phase was then extracted with sat. NaHCO3 and dried over Na2SO4,
and the organic solvent was removed under reduced pressure. The
residue was dissolved in a minimal amount of acetone and loaded on
silica gel. The silica gel was eluted with acetone, affording 4 in the form
of colorless oil (91 mg, 25%). Rf (40% MeOH, chloroform) = 0.15; 1H
NMR (200 MHz, CDCl3) δ = 8.97 (3H, s), 7.72 (3H, d, J = 8.2 Hz),
7.46 (3H, bs), 7.20 (3H, t, J = 9.6 Hz), 6.95 (3H, d, J = 7.6 Hz), 3.65−
3.46 (18H, m), 2.75 (12H, t, J = 4.6 Hz), 2.05 (9H, s). 13C NMR (50
MHz, CDCl3) δ = 169.6, 138.9, 138.4, 128.9, 125.2, 121.1, 119.6, 68.7,
59.5, 54.5, 24.4. IR (thin-film) 3253, 3055, 2955, 1673, 1553 cm−1; ESI
HR calcd. for C39H54N6O6Na 725.4003, found 725.0412.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ppiatek@chem.uw.edu.pl.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Faculty of Chemistry
University of Warsaw (501-64-BST-163220). We thank Dr.
Bartosz Trzaskowski for his invaluable assistance with the DFT
calculations.
Solid/Liquid Extraction. Inorganic salts were used as received;
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■
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