Encapsulation of fluoride/chloride in the C3v-symmetric cleft of a pentafluorophenyl-functionalized cyanuric acid platform based tripodal amide: Solid and solution-state anion-binding studies

Article abstract of DOI:10.1002/ejic.201200074

A simple electron-deficient cyanuric acid based tripodal amide, 1,3,5-tris[2-(2,3,4,5,6-pentafluorobenzamido)ethyl]-1,3,5-triazinane-2,4, 6-trione (L), was synthesized and characterized by NMR spectroscopy, ESI mass spectrometry, and single-crystal X-ray crystallographic studies. The binding of various anions towards L was thoroughly examined bysingle-crystal X-ray crystallography as well as solution-state isothermal titration calorimetry (ITC). The crystallographic results show that L has an unsymmetrical cleft, where the third arm is perpendicularly disposed to the other two arms. Interestingly, L upon complexation with tetrabutylammonium fluoride/chloride shows encapsulation of monotopic fluoride/chloride in the C_3v- symmetric cleft by means of N-H···X (X = F^-, Cl^-) hydrogen-bonding interactions in complexes 1 and 2. In complex 2, the encapsulated chloride ion shows evidence of anion-π interactions with the pentafluorophenyl moiety. A detailed solution-state ITC study of L with tetrabutylammonium salts of different halides in acetonitrile showed an exothermic binding profile with 1:1 (host/guest) stoichiometry for fluoride (log K_a = 4.86 M^-1), chloride (log K_a = 3.83 M ^-1), and bromide (log K_a = 2.97 M^-1). In the case of iodide, no such binding was observed. Oxyanions like acetate and benzoate also show an exothermic binding profile with a 1:1 (host/guest) binding pattern, whereas other oxyanions like phosphate, sulfate, and nitrate failed to exhibit a 1:1 binding model. The presence of [L(X)]^- (X = F, Cl) species as the base peak in the ESI (negative) mass spectra further confirmed the strong binding of the halide ions in the gaseous phase. A simple tripodal amide based on the cyanuric acid scaffold showed encapsulation of fluoride and chloride by C_3v-symmetric cleft formation. Isothermal calorimetric titration (ITC) studies showed a 1:1 (host/guest) binding with various anions in solution. Copyright

Full text of DOI:10.1002/ejic.201200074

FULL PAPER
DOI: 10.1002/ejic.201200074
Encapsulation of Fluoride/Chloride in the C3v-Symmetric Cleft of a
Pentafluorophenyl-Functionalized Cyanuric Acid Platform Based Tripodal
Amide: Solid and Solution-State Anion-Binding Studies
Ranjan Dutta[a] and Pradyut Ghosh*[a]
Keywords: Anion recognition / Fluorides / Cyanuric Acid / Receptors / Halides / ITC
A simple electron-deficient cyanuric acid based tripodal
amide, 1,3,5-tris[2-(2,3,4,5,6-pentafluorobenzamido)ethyl]-
1,3,5-triazinane-2,4,6-trione (L), was synthesized and charac-
terized by NMR spectroscopy, ESI mass spectrometry, and
single-crystal X-ray crystallographic studies. The binding of
various anions towards L was thoroughly examined by
single-crystal X-ray crystallography as well as solution-state
isothermal titration calorimetry (ITC). The crystallographic
results show that L has an unsymmetrical cleft, where the
third arm is perpendicularly disposed to the other two arms.
Interestingly, L upon complexation with tetrabutylammo-
nium fluoride/chloride shows encapsulation of monotopic
fluoride/chloride in the C3v-symmetric cleft by means of N–
H···X (X = F–, Cl–) hydrogen-bonding interactions in com-
plexes 1 and 2. In complex 2, the encapsulated chloride ion
shows evidence of anion–π interactions with the pentafluoro-
phenyl moiety. A detailed solution-state ITC study of L with
tetrabutylammonium salts of different halides in acetonitrile
showed an exothermic binding profile with 1:1 (host/guest)
stoichiometry for fluoride (logKa = 4.86
M
–1), chloride (logKa
= 3.83
M
–1), and bromide (logKa = 2.97
M
–1). In the case of
iodide, no such binding was observed. Oxyanions like acet-
ate and benzoate also show an exothermic binding profile
with a 1:1 (host/guest) binding pattern, whereas other oxy-
anions like phosphate, sulfate, and nitrate failed to exhibit a
1:1 binding model. The presence of [L(X)]– (X = F, Cl) species
as the base peak in the ESI (negative) mass spectra further
confirmed the strong binding of the halide ions in the
gaseous phase.
form.[4g] The cyanuric acid platform has potential for the
design of different anion receptors, which has not been ex-
plored to the same extent as the other platforms.[9] An ear-
lier report of anion binding by a cyanuric acid platform was
observed in solution where a chloride selectivity pattern was
achieved by a sulfonamide host.[9a] Mascal et al. reported
the synthesis of a cyanuric acid platform based cylindro-
phan, which shows selectivity towards fluoride in its tripro-
tonated state.[10]
Recently, our group showed recognition of tetrabutylam-
monium sulfate as an ion pair by a cyanuric acid based
pentafluorophenyl-substituted tris(urea) receptor.[11] Herein
we report on the synthesis of a new cyanuric acid based
tris(amide) receptor (L) decorated with pentafluorophenyl
(C6F5) moieties, which structurally demonstrates the encap-
sulation of fluoride/chloride in the C3v-symmetric cavity of
L. We also show proof for 1:1 (host/guest) solution-state
binding with the halides and different oxyanions by an iso-
thermal calorimetric titration (ITC) study.
Introduction
The recognition of fluoride is of special interest due to
its role in health, medicine, environmental sciences, and also
in the purification of drinking water.[1] Amide-based ligands
are important in the recognition of anions since anion re-
ceptors in nature often involve amide linkages.[2] Pascal et
al. were the first to report an amide-based macrobicyclic
cage for fluoride.[3] Since then various amide receptors have
been developed for anion binding with versatile utility.
These amide receptors were designed on tris(2-aminoethyl)-
amine (tren)[4] and benzene platforms.[3,5] Some of these re-
ceptors are selective towards fluoride.[4g,4h,5b] In addition to
these amide receptors, different tripodal and macrobicyclic
polyammonium receptors,[6] calixpyrrole,[7] and a silses-
quioxane cage[8] have shown fluoride/chloride encapsul-
ation. Recently, our group showed hydrated fluoride and
monotopic chloride encapsulation in the C3v-symmetric
cleft of a tripodal amide receptor based on the tren plat-
[a] Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science,
Results and Discussion
2A and 2B Raja S. C. Mullick Road, Kolkata 700032, India
Fax: +91-33-2473-2805
The tripodal amide host L was synthesized by reaction
of the precursor tripodal amine with pentafluorobenzoyl
chloride as shown in Scheme 1. The precursor tripodal
E-mail: icpg@iacs.res.in
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200074.
3456
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F–/Cl– Encapsulation in a Cyanuric Acid Platform Based Tripodal Amide
amine was synthesized according to our previous report.[11]
Triethylamine was added to a suspension of the precursor
amine in dry dichloromethane followed by slow addition of
pentafluorobenzoyl chloride. After evaporation of the sol-
vent and washing with plenty of water, the receptor L was
obtained as a white powder in good yield.
Figure 1. ORTEP diagram of the solid-state structure of L showing
the intramolecular N–H···O interaction.
Structural Description of the Fluoride Complex 1
In order to explore the anion complexation and confor-
mational flexibility of this newly synthesized tripodal
amide, L was treated with tetrabutylammonium (TBA)
fluoride. A single crystal of complex 1 (L·TBAF) was ob-
tained by slow concentration of an acetonitrile solution that
contained L and tetrabutylammonium fluoride. The OR-
TEP diagram and space-filling model show complete encap-
sulation of the fluoride ion in the C3v-symmetric cleft of L
by three N–H···F– interactions (Figure 2). The encapsulated
fluoride F16 strongly interacts with the N4, N5, and N6
atoms with N···F bond lengths and N–H···F bond angles
ranging from 2.65 to 2.70 Å and 153.6 to 158.5°, respec-
tively. The detailed hydrogen-bonding parameters are listed
in Table 1. The fluoride-encapsulated cleft possesses a dis-
torted C3v-symmetric cavity, which is evident from the slight
differences in the N···N distances (N4···N5 4.563, N5···N6
4.550, N6···N4 4.577 Å) of the basal plane (plane consisting
of N4, N5, and N6 atoms).
Scheme 1. Synthesis of the receptor L.
Single-Crystal X-ray Diffraction Studies
Structural Description of Receptor L
A single-crystal of L was isolated upon slow crystalli-
zation of L from its acetonitrile solution. The receptor L
crystallizes in the orthorhombic crystal system with a
P212121 space group. Figure 1 depicts the ORTEP diagram
of L, where the arms are devoid of structural preorganiza-
tion. An oxygen atom (O6) of one arm is in an intramolecu-
lar hydrogen-bonding interaction with the NH (N5–H5)
group of another arm (dN5···O6 = 2.856 Å, ЄN5–H5···O6
=
148.29 Å), thus preventing the C3v-symmetric cleft forma-
tion (Table S4, Supporting Information). This strong intra-
molecular hydrogen bonding brings the two arms composed
of the amide nitrogen atoms N5 and N6 into closer proxim-
ity where the torsion angles for N2CCN5 and N3CCN6 are
–57.62° and 57.81°, respectively, whereas the two arms are
further apart from the third arm that contains the N4
amide nitrogen, which has an N1CCN4 torsion angle of
–178.21° and imposes an unsymmetrical cleft (amide nitro-
gen distances between two arms are: N4···N5 7.391,
N5···N6 5.084 Å, N6···N4 7.471 Å). The intramolecular hy-
drogen bonding and unfolded orientation of electron-de-
ficient aromatic rings favors an open structure for L, which
might favor the binding of a guest in the tripodal cavity.
Low-temperature 1H NMR spectra of L at various tem-
peratures, namely –55, –40, –20, and 0 °C, were carried out
in order to verify its conformation in solution (Figure S4,
Supporting Information). A continuous downfield shift and
a broadening of the amide (NH) proton signal were ob-
served upon lowering the temperature. This supports intra-
molecular/intermolecular hydrogen-bonding interactions
even in the solution state.
Figure 2. (a) ORTEP diagram showing the encapsulation of F– in
the cavity of L. (b) Space-filling model depicting the complete en-
capsulation of F– inside the C3v-symmetric cleft of L (tetrabutyl-
ammonium cation omitted for clarity).
Table 1. Hydrogen-bonding parameters of complex 1.
D–H···A
H···A [Å]
D···A [Å]
ЄD–H···A [°]
N4–H4···F16
N5–H5···F16
N6–H6···F16
1.90
1.88
1.83
2.70
2.69
2.65
153.6
156.5
158.5
The encapsulated fluoride ion is in a distorted trigonal-
pyramidal geometry where it is located above the basal
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R. Dutta, P. Ghosh
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plane at a distance of 0.501 Å. The torsion angles involving encapsulated chloride ion to cyanuric centroid distance of
N1CCN4amide, N2CCN5amide, and N3CCN6amide are in a 3.512 Å (Figure 5), which supports anion–π interaction.
folded conformation with angles 51.60, 53.80, and 53.05°,
Table 2. Hydrogen-bonding parameters for complex 2.
respectively. Interestingly, the distance of the encapsulated
fluoride ion to the centroid of the cyanuric platform is
found to be 3.03 Å and the fluoride ion is positioned exactly
above the plane of the cyanuric ring with a doffset value cal-
D–H···A
H···A [Å]
D···A [Å]
ЄD–H···A [°]
N4–H4···Cl1
N5–H5···Cl1
2.36
2.43
2.35
3.15
3.18
3.15
153.7
146.7
156.4
culated as zero (Figure 5). Furthermore, the distances of the N6–H6···Cl2
encapsulated fluoride ion to the individual carbon/nitrogen
atoms of the cyanuric platform are found to be in the range
of 3.31–3.36 Å, which is within the upper limit of anion–π
interactions between F– and C/N.[12] Thus, besides three N–
H···F– hydrogen bonds, anion–π interaction does exist be-
tween the fluoride ion and the cyanuric acid platform. At
this juncture, it is important to mention that in case of the
cyanuric acid platform based tris(ammonium) macrobicy-
clic cage, Mascal et al. have shown encapsulation of fluoride
by means of N–H···F– interactions along with anion–π con-
tacts with fluoride and the cyanuric acid platform.[10,13]
Figure 3. (a) ORTEP diagram showing the encapsulation of Cl– in
the cavity of L. (b) Space-filling model depicting the encapsulation
of Cl– inside L (tetrabutylammonium cation omitted for clarity).
Structural Description of the Chloride Complex 2
Crystals of complex 2 (L·TBACl) that were suitable for
single-crystal X-ray diffraction were obtained upon adding
excess tetrabutylammonium chloride to an acetonitrile solu-
tion of L. Complex 2 crystallizes in the triclinic crystal sys-
¯
tem with the P1 space group. Similar to the fluoride com-
plex, chloride is also encapsulated in the C3v-symmetric cav-
ity of the receptor L by three N–H···Cl– interactions in the
solid state. The encapsulated chloride Cl1 strongly interacts
with the N4, N5, N6 amide groups resulting in three N–
H···Cl– interactions with the N···Cl bond lengths ranging
from 3.150 to 3.187 Å. The N–H···Cl bond angles were
found to be in the 146.7–156.4° region. The detailed hydro-
gen-bonding parameters are listed in Table 2. Monotopic
recognition of Cl– inside the C3v-symmetric cleft of L
(amide nitrogen distances between two arms are: N4···N5
5.192, N5···N6 5.105, N6···N4 5.137 Å) is shown in Fig-
ure 3. The encapsulated chloride ion is also in a distorted
trigonal-pyramidal geometry where it is located above the
basal plane (plane consisting of N4, N5, and N6 atoms) at
a distance of 1.088 Å. The torsion angles involving
N1CCN4amide, N2CCN5amide, and N3CCN6amide are in a
folded conformation with angles 52.94, 48.84, and 52.46°,
respectively. The distances of the encapsulated chloride ion
to the individual carbon/nitrogen atoms of the cyanuric
platform were found to be in the range of 3.73–3.82 Å,
which is within the upper limit of anion–π interactions be-
tween Cl– and C/N.[9b,12a] In addition to the three N–
H···Cl– interactions, the encapsulated chloride ion is further
involved in two weak anion···π interactions with the elec-
tron-deficient C6F5 units (C1g···Cl1 3.712 Å with a shortest
distance of C23···Cl1 3.358 Å, and C2g···Cl1 3.485 Å with
a shortest distance of C14···Cl1 3.291 Å, where C1g and
C2g are the centroids of the C6F5 rings of C22–C27 and
C13–C18, respectively) (Figure 4). Furthermore, similar to
the fluoride complex, the encapsulated chloride ion is posi-
Figure 4. ORTEP diagram depicting the encapsulation of Cl– inside
the tripodal cavity, where the dotted line presents Cl–···C6F5 inter-
actions.
Figure 5. (a) Distance of the centroid of cyanuric ring to the encap-
sulated fluoride ion in complex 1. (b) Distance of the centroid of
the cyanuric ring to the encapsulated chloride ion in complex 2.
Solution-State Studies
Isothermal Calorimetric Titration Studies
The solution-state binding affinity of the receptor L with
tioned exactly above the plane of the cyanuric ring with an various anions was performed by ITC experiments. In a
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F–/Cl– Encapsulation in a Cyanuric Acid Platform Based Tripodal Amide
typical ITC experiment a solution of the respective anion –5.428 kcalmol–1 (Table 3). Titration experiments were also
as its tetrabutylammonium salt in freshly dried acetonitrile carried out with oxyanions like acetate, benzoate, sulfate,
was titrated into a solution of receptor L at 298 K. A clear phosphate, and nitrate under similar experimental condi-
exothermic titration profile was obtained for L upon ti- tions in acetonitrile. Exothermic titration profiles and 1:1
tration with F–, Cl–, Br–, AcO–, and BzO–, and subsequent (host/guest) binding modes were observed for both benzo-
fitting to a 1:1 binding profile provided access to the associ- ate (Figure 7) and acetate (Figure 8) as evident from the
ation constant (Ka), enthalpy change (ΔH), entropy change stoichiometry (n = 0.95 for TBABzO and n = 1.02 for
(TΔS), and free energy change (ΔG) of the binding pro- TBAAcO) (Table 3) of the titration data. Estimated associa-
cesses. The ITC profiles of the fluoride and chloride ti- tion constants of the binding processes are also comparable
trations show the presence of a single equilibrium in solu- (logK = 4.19 for TBABzO and logK = 4.29 for TBAAcO).
tion that corresponds to the formation of a 1:1 (host/guest) The nature of the association and thermodynamic param-
adduct, which is evident from the stoichiometry (n = 1.12 eters for acetate and benzoate were further verified by ti-
for TBAF, n = 0.95 for TBACl) (Figure 6 and Figure S6, tration experiments with propionate and p-methylbenzoate
Supporting Information). The titration experiment with tet- (Figures S9 and S10, Supporting Information). Indeed, a
rabutylammonium bromide also fits to a 1:1 binding model similar trend in enthalpy and entropy changes was observed
with a stoichiometry of n = 1.07 (Figure S7, Supporting for the acetate/propionate and benzoate/p-methylbenzoate
Information). The titration curve for fluoride does not fit pairs (Table S1, Supporting Information). The ITC profiles
well to the ideal 1:1 (host/guest) binding pattern; this may of the other anions do not fit to the 1:1 (host/guest) binding
be attributed to the possible deprotonation of the receptor pattern. Comparative thermodynamic parameters for the ti-
by the basic fluoride ion. However, the kinetic and thermo- tration experiments of the anions are listed in Table 3. The
dynamic parameters estimated from the above experimental binding of bromide and acetate is equally facilitated by en-
data show logKa = 4.86, TΔS = –0.098 kcalmol–1, ΔH = tropy and enthalpy factors, whereas chloride and benzoate
–5.330 kcalmol–1, and ΔG = –6.609 kcalmol–1. On the binding is strongly enthalpy-driven. The estimated free en-
other hand, the host–guest interaction of L and tetrabut-
ylammonium chloride appears as a clean, exothermic, and
Table 3. Comparative thermodynamic parameters for ITC experi-
1:1 (n = 0.95) stoichiometric binding process with the
thermodynamic parameters estimated to be logKa = 3.83,
TΔS = 3.903 kcalmol–1, ΔH = –2.706 kcalmol–1, and ΔG =
ments in acetonitrile at 298 K.
Anion
n
logK ΔH
TΔS
ΔG
[kcalmol–1] [kcalmol–1] [kcalmol–1]
TBACl
TBABr
TBABz
0.95 3.83
1.07 2.97
0.95 4.19
–5.330
–1.675
–4.939
–2.649
–0.098
2.375
0.778
3.218
–5.428
–4.05
–5.717
–5.867
TBAAcO 1.02 4.29
Figure 6. Isothermal calorimetric titration in acetonitrile at 298 K
for the addition of a solution of tetrabutylammonium chloride
(2.996 mm) to a solution of L at 0.1087 mm. The upper panel shows
the heat pulses experimentally observed in each titration. The lower
panel reports the respective time integrals translating as the heat
evolved for each aliquot and its coherence to the 1:1 binding model.
χ2/DoF = 82.55 (DoF = degree of freedom), n = 0.954Ϯ0.0288
sites.
Figure 7. Isothermal calorimetric titration in acetonitrile at 298 K
for the addition of a solution of tetrabutylammonium benzoate
(2.548 mm) to a solution of L at 0.1845 mm. χ2/DoF = 3491 (DoF
= degree of freedom), n = 0.953Ϯ0.0374 sites.
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ergy change (ΔG) values are in agreement with the observed
association constants where maximum change in free en-
ergy is obtained in the case of fluoride.
Figure 9. Change in chemical shift with increasing amounts of
[nBu4N+]Br– in CD3CN at 298 K.
Figure 8. Isothermal titration calorimetry profile for the titration
of L (0.1845 mm) with TBAAcO (2.636 mm) in acetonitrile at
298 K. χ2/DoF
= 1644 (DoF = degree of freedom), n =
1.02Ϯ0.0357 sites.
1H NMR Study
The lower association constant of binding for bromide
with the receptor L in the ITC experiment was rein-
vestigated by a 1H NMR spectroscopy experiment in
CD3CN as its tetrabutylammonium salt. Substantial
changes in the chemical shifts were observed for the amide
protons (NH) with bromide, which indicates the participa-
tion of these NH protons in the binding of this anion. To
Figure 10. Job plot of L with (nBu4N+)Br– in CD3CN at 298 K.
1
evaluate the binding of bromide with L, a H NMR spec-
troscopy titration was carried out in CD3CN at 298 K (Fig-
ure 9).
The titration curve gives a best fit for a 1:1 (host/guest)
binding model, in agreement with a Jobs plot that indicates
a maximum Δδ value of 0.5 (= [L]/[L] + [A]) (Figure 10),
and the association constant was calculated by using
WINEQNMR 2.0. The association constant (logK) was
found to be 2.89 for bromide, which is comparable with the
association constant (2.97) observed from the ITC measure-
ment.
The 1:1 (host/guest) binding mode in solution and the
solid state of L towards fluoride and chloride was corrobo-
rated by ESI (negative) mass spectra of complexes 1 and 2.
Mass spectral analysis of the fluoride and the chloride com-
plex in acetonitrile shows the presence of the base peak at
m/z = 859.23 [L + F]– and 874.99 [L + Cl]–, respectively,
which corresponds to the encapsulated halides in the cavity
of L in the gaseous state (Figure 11 and Figure S5, Sup-
porting Information).
Figure 11. ESI-MS (negative) mass spectra of complex 1.
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F–/Cl– Encapsulation in a Cyanuric Acid Platform Based Tripodal Amide
hydrogen atoms were geometrically fixed at idealized positions,
whereas the hydrogen atoms attached to the nitrogen atoms were
located from the difference Fourier map and were refined iso-
Conclusions
An electron-deficient cyanuric acid platform based tris-
(amide) receptor shows complete encapsulation of halides tropically until convergence was attained. The graphics were gener-
ated with PLATON[18] and MERCURY 2.3.[19] CCDC-855942 (for
L), -855940 (for 1) and -855941 (for 2) contain the supplementary
crystallographic data for this paper. These data can be obtained
like fluoride and chloride in the C3v-symmetric cleft. This
represents the first structural example of halide encapsul-
ation by a cyanuric acid scaffold based amide host. The
free of charge from The Cambridge Crystallographic Data Centre
existence of [L(X)]– (X = F, Cl) species as the base peak in
via www.ccdc.cam.ac.uk/data_request/cif.
the ESI (negative) mass spectra is evidence of a strongly
encapsulated anion in the gaseous state. Solution-state
binding affinity measurement also supports the 1:1 (host/
guest) binding mode for halides and other planar oxy-
anions. Varying the degree of fluorination of the substituent
and attachment of different electron-deficient groups on the
cyanuric acid platform may assist in the development of
1
1H NMR Titration Study: The H NMR titration study of L with
tetrabutylammonium bromide was performed in CD3CN at 298 K.
The initial concentration of L was 20 mm. An aliquot of anion was
added from a stock solution of the anion (100 mm). Tetramethylsil-
ane in CD3CN was used as an internal reference, and the titration
was performed by 20 measurements at room temperature. The asso-
ciation constant K was calculated by fitting the change in the NH
new generations of receptors for the selective recognition of
various anions.
chemical shifts with the 1:1 association model with non-linear least-
squares analysis. A Job plot revealed a best fit for the 1:1 (host/
guest) binding mode. WINEQNMR 2.0 was used for the evaluation
of the binding constant.[20] The equation Δδ = {([A]0 + [L]0 + 1/K)
Ϯ ({[A]0 + [L]0 + 1/K}2 – 4[L]0[A]0)1/2}Δδmax/2[L]0 was used for
evaluation of the association constant.
Experimental Section
Materials and Methods: All of the reagents, tetrabutylammonium
salts, and solvents for the syntheses were purchased from commer-
cial sources and were used as received. The precursor triamine was
synthesized by applying a modified literature procedure.[11] Tetra-
butylammonium salts of propionate and benzoate were prepared
by treating tetrabutylammonium hydroxide with their correspond-
ing acid. 1H NMR spectra were recorded with 300 MHz Bruker
L: The precursor triamine (400 mg) and triethylamine (1.2 mL)
were suspended in dry dichloromethane (100 mL), and the mixture
was stirred at 0 °C under nitrogen for 15 min. Pentafluorobenzoyl
chloride (0.8 mL) was added dropwise under nitrogen with con-
stant stirring. The reaction mixture was gradually brought to room
temperature and stirred for 24 h. The solution was then concen-
trated, and the crude solid was washed several times with distilled
water to remove triethylammonium chloride. Then the residue was
washed with diethyl ether and dried in air to give L (1.05 g) as a
white powder. Colorless crystals of L were obtained by slow con-
centration of its acetonitrile solution. Yield: 80%. 1H NMR
(300 MHz, CD3CN): δ = 7.434 (t, J = 6.9 Hz, 3 H, NH), 4.087 (t,
J=6.9 Hz,6H,CH),3.66(t,3H,CH)ppm.13CNMR(75 MHz,[D6]-
DMSO): δ = 36.99 (NCH2CH2), 41.86 (NCH2CH2), 112.60 (Ar-
C), 136.45 (Ar-C), 136.56 (Ar-C), 138.44 (Ar-C), 142.57 (Ar-C),
144.57 (Ar-C), 149.34 (CO), 157.59 (CO) ppm. 19F NMR
(500 MHz, CD3CN): δ = –162.631 (t, 2 Ar-F), –153.909 (t, 2 Ar-
F), –142.752 (d, Ar-F) ppm. ESI MS: m/z = 841.1559 [M + H]+,
863.1378 [M + Na]+, 879.1488 [M + K]+. C30H15F15N6O6 (840.46):
calcd. C 42.87, H 1.80, N 10.00; found C 42.95, H 1.91, N 9.88.
DPX-300 and 500 MHz Bruker DPX-500 NMR spectrometers. 13
C
NMR spectra were obtained at 75.47 and 125.77 MHz. 19F NMR
spectra were obtained with a 500 MHz Bruker DPX-500 NMR
spectrometer. ESI-MS experiments were carried out with a Waters
QtoF Model YA 263 mass spectrometer in positive/negative ESI
mode. Elemental analyses for the ligand and the complexes were
carried out with a Perkin–Elmer 2500 series II elemental analyzer.
Isothermal Titration Calorimetric (ITC) Studies: The isothermal ti-
tration calorimetric experiments were performed with a MicroCal
VP-ITC instrument. The titrations were carried out at 298 K in
freshly distilled acetonitrile. A solution of L in acetonitrile was
placed in the measuring cell. This solution was then titrated with
30 injections of the respective tetrabutylammonium salt solution
(10 μL) that was prepared in acetonitrile. An interval of 220 s was
allowed between each injection, and the stirring speed was set at
329 rpm. The obtained data was processed by using Origin 7.0 soft-
ware that was supplied with the instrument and was fitted to a
one-site binding model. A blank titration of plain solvent was also
performed and subtracted from the corresponding titration to re-
move any effect from the heats of dilution from the titrant.
L·TBAF (1): Fluoride complex 1 was prepared by adding excess
tetrabutylammonium fluoride to an acetonitrile solution of L.
Then the solution was sonicated for 5 min, filtered, and allowed to
slowly concentrate at room temperature. After 1 week, colorless
crystals of complex 1 were obtained upon slow evaporation of the
solvent. 1H NMR (300 MHz, CDCl3): δ = 4.061 (6 H, NCH2CH2),
3.486 (6 H, NCH2), 3.231 (NCH2), 1.667 (NCH2CH2), 1.643
(NCH2CH2CH2), 1.020 (NCH2CH2CH2CH3) ppm. 19F NMR
(500 MHz, CDCl3): δ = –165.039 (2 Ar-F), –156.56 (2 Ar-F),
–144.386 (Ar-F), –126.111 (F–) ppm. ESI MS (negative): m/z =
839.21 [M – H]–, 859.23 [M + F]–.
X-ray Crystallography: A crystal suitable for single-crystal X-ray
diffraction studies was selected from the mother liquor and im-
mersed in paratone oil and then mounted on the tip of a glass fiber
and cemented by using epoxy resin. The intensity data for crystals
of L, 1, and 2 were collected by using Mo-Kα (λ = 0.7107 Å) radia-
tion with a Bruker SMART APEX II diffractometer that was
equipped with a CCD area detector at 120 K. The data integration
and reduction were processed with SAINT[14] software provided
with the software package of SMART APEX II. An empirical ab-
sorption correction was applied to the collected reflections with
SADABS.[15] The structures were solved by direct methods by using
SHELXTL[16] and were refined on F2 by full-matrix least-squares
techniques using the SHELXL-97[17] program package. The non-
hydrogen atoms were refined anisotropically until convergence. The
L·TBACl (2): Chloride complex 2 was obtained similarly to the
1
fluoride complex. H NMR (300 MHz, CDCl3): δ = 9.3402 (NH),
4.1608 (6 H, NCH2CH2), 3.77 (6 H, NCH2), 3.372 (NCH2), 1.715
(NCH2CH2), 1.674 (NCH2CH2CH2), 1.030 (NCH2CH2CH2CH3)
ppm. ESI MS (negative): m/z
= 839.01 [M –
H]–, 874.99
[M + Cl]–.
Supporting Information (see footnote on the first page of this arti-
cle): X-ray crystallographic details, spectroscopic data, and ti-
tration profiles.
Eur. J. Inorg. Chem. 2012, 3456–3462
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3461

R. Dutta, P. Ghosh
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R. D. acknowledges the Council of Scientific and Industrial Re-
search (CSIR) for a fellowship. P. G. gratefully thanks the Depart-
ment of Science and Technology (DST) for financial support
through a Swarnajayanti fellowship. X-ray crystallography of re-
ceptor L and the two complexes were performed in the DST-funded
National Single-Crystal X-ray Facility at the Department of Inor-
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Received: January 24, 2012
Published Online: June 6, 2012
3462
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Eur. J. Inorg. Chem. 2012, 3456–3462