Binding discrepancy of fluoride in quaternary ammonium and alkali salts by a tris(amide) receptor in solid and solution states

Article abstract of DOI:10.1039/c2ce25592h

A dinitrophenyl functionalized tris(amide) receptor, L, showed distinct complexation behaviour towards the F^- anion when tetrabutylammonium fluoride (TBAF) and potassium fluoride (KF) salts were individually employed for the recognition of F

Full text of DOI:10.1039/c2ce25592h

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PAPER
Binding discrepancy of fluoride in quaternary ammonium and alkali salts by a
tris(amide) receptor in solid and solution states{{
Sandeep Kumar Dey, Barun Kumar Datta and Gopal Das*
Received 18th April 2012, Accepted 3rd June 2012
DOI: 10.1039/c2ce25592h
A dinitrophenyl functionalized tris(amide) receptor, L, showed distinct complexation behaviour
towards the F2 anion when tetrabutylammonium fluoride (TBAF) and potassium fluoride (KF) salts
were individually employed for the recognition of F2 with L. X-ray crystallography analyses revealed
2
2
…
the formation of a F –encapsulated complex (1 : 1 host–guest) stabilized by three N–H
2
three C–H
F and
F
hydrogen bonds when TBAF was employed as the F2 source, whereas in the KF
…
complex of L (1 : 1 host–guest), the receptor is involved in side-cleft binding of a hydrated KF
2
…
contact ion-pair governed by amide N–H F , aryl C–H
2
F and lp(F ) p interactions. The
2
…
…
binding of hydrated KF is identical to the side-cleft binding of solvents such as DMSO and DMF via
…
N–H O, aryl C–H O and lp(O) p interactions, which has been exemplified by X-ray
…
…
crystallography and a detailed Hirshfeld surface analyses of the crystals. The binding discrepancy of
1
F2 in the TBAF and KF complexes of L has also been manifested in the solution state by H NMR
1
and 2D NOESY NMR experiments. In the H NMR analyses, a huge downfield shift of the
coordinating –NH and ortho–CH protons was observed in the TBAF complex in comparison to the
KF complex whereas, in the 2D NOESY NMR experiments, a disappearance of the signals
corresponding to the through-space NOE coupling between the –NH and ortho–CH protons was
observed in the former when compared to the latter and free receptor, L.
functionalities in abiotic receptors.1 This has led to an
Introduction
exponential growth of studies in the area of molecular
recognition with varying binding motifs appended on a suitable
platform. The field of anion receptor chemistry continues to
expand with new synthetic hosts capable of recognizing anions
with environmental and biomedical relevance.2 It has already
been established that, apart from the charge density of the
anionic species, the spatial arrangement of the binding motif(s)
in the receptor and geometry of the anions is also crucial in
influencing the receptor–anion binding efficiency and specificity.
Among anionic analytes, the recognition and sensing of fluoride
is an area of immense interest to the chemical society due to its
diverse role in industry, food and toxicity.3 Due to its high
electronegativity and high hydration enthalpy, fluoride exists as
various types of fluoride–water clusters (the hydrated form) in
water or in presence of moisture and thus, its recognition by
abiotic receptors becomes a challenging task.4 Furthermore,
receptors for anions such as fluoride should target the hydrated
alkali metal salts rather than the quaternary ammonium salts
because in nature, fluoride exists mostly as its Na/K salts
(minerals such as villiaumite and carobbiite). On the other hand,
self-assembled supramolecular capsules that provide an isolated
nanocavity have attracted much attention in recent years for
unusual guest encapsulation.5 Within the area of self-assembly
driven by hydrogen bonding, numerous molecular capsules have
been constructed by different laboratories via the self-assembly
The importance of supramolecular interactions in nature has
been increasingly recognized in recent years and thus, enormous
efforts have been put forward by researchers to gain a better
insight and understand the ion-specific interaction of certain
Department of Chemistry, Indian Institute of Technology Guwahati,
Assam, 781 03, India. E-mail: gdas@iitg.ernet.in; Fax: +91-361-258-2349;
Tel: +91-361-258-2313
{ Electronic supplementary information (ESI) available: X-ray crystal-
lographic file of the structures in CIF format, selected non-covalent
interactions, crystal packing networks, 1H and 13C spectra. CCDC
reference numbers 761161–76113. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2ce25592h
{ Crystal data for 1d: Fw = C46H60FN11O16, M = 1042.05, CCDC =
¯
761161, T = 298(2) K, triclinic, space group P1, a = 10.1372(3), b =
˚
16.4160(5), c = 17.3515(8) A, a = 109.413(2)u, b = 97.059(2)u, c =
3
106.906(1)u, V = 2527.35(17) A , Z = 2, m = 0.107 mm21, 9814 unique
˚
reflections, 7233 observed , R(F) = 0.0645 (I . 2s(I), wR(F2) = 0.2429 (all
data), GOF (F2) = 1.005. Crystal data for 2: Fw = C27H24FKN10O17, M
¯
= 1858.21, CCDC = 761162, T = 298(2) K, triclinic, space group P1, a =
˚
11.2710(12), b = 12.4543(13), c = 12.8718(14) A , a = 84.825(6)u, b =
3
21
˚
79.744(6)u, c = 88.329(6)u, V = 1770.6(3) A , Z = 2, m = 0.246 mm
,
7784 unique reflections, 6870 observed, R(F) = 0.0838 (I . 2s(I), wR(F2)
= 0.2728 (all data), GOF (F2) = 1.035. Crystal data for L?DMF: Fw =
C30H31N11O16, M = 801.66, CCDC = 761163, T = 298(2) K, monoclinic,
˚
space group P21/c, a = 17.2139(6), b = 11.1890(3), c = 25.1968(7) A ,a =
˚
3
90.00u b = 131.890(2)u c = 90.00u, V = 3612.8(2) A , Z = 4, m = 0.122
mm21, 8991 unique reflections, 4777 observed, R(F) = 0.0580 (I . 2s(I),
wR(F2) = 0.2115 (all data), GOF (F2) = 1.023.
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…
of various hydrogen bonding motifs such as calixarenes,
resorcinarenes, glycoluril and tripodal derivatives, mostly in
the presence of a neutral or ionic guest species.6
lp(O) p interactions involving the oxygen atom of the
respective lattice solvents. The structural similarities between
complex 2 and the solvatomorphs of L have also been
accounted for in terms of Hirshfeld surface analyses of the
crystals which show the presence of identical supramolecular
interactions, as observed in the single-crystal X-ray crystal-
lography analyses. The binding discrepancy of F2 in complexes
Furthermore, as anions display a wide range of geometries, the
directionality of the hydrogen bonds is frequently utilized to
achieve complementarity between the designed receptors and
…
target anion(s). Artificial receptors mostly utilize N–H anion
1
…
…
1d and 2 has also been demonstrated in the solution state by H
hydrogen bonds whereas C–H anion and anion p interactions
are rarely utilized for the recognition/sensing of anions even
though they play an important role in nature.7 More recently,
NMR and 2D NOESY NMR experiments.
…
…
C–H anion and anion p interactions have been independently
employed by Das and Saha et al. for the selective sensing of the
F2 ion, based on an appropriate receptor–chromophore format
that showed an enhanced F2 binding affinity.8 Increasing
evidence of these interactions comes in the form of the direct
observation of close contacts in crystallographic structures,
anion-induced chemical shifts of C–H protons in NMR spectra
and theoretical calculations.9 Although the binding of anionic
guests within pre-organized macrocyclic systems is relatively
straightforward to understand, the binding processes of flexible
podand receptors remain more elusive. In this context, a recent
theoretical investigation by Hay et al. and several structural
reports on anion complexes of multi-armed receptors showed
that the effect of electron withdrawing substituents on the aryl
terminals significantly enhances the stability of anion com-
plexes.10 Although, acyclic podand receptors with multi-armed
functionality have been shown to be effective systems for the
binding of a variety of anions, their uses as selective fluoride
binding hosts are barely known.
Experimental section
Materials and methods
All reagents and solvents were obtained from commercial
sources and used as received without further purification.
Tris(2-aminoethyl)amine, (tren) and 3,5-dinitrobenzoyl chloride
were purchased from Sigma-Aldrich and used as received.
Tetrabutylammonium fluoride and potassium fluoride salts were
purchased from Merck-India and used as received. The 1H NMR
and 2D NOESY NMR spectra were recorded on a Varian FT-
400 MHz instrument and the chemical shifts were recorded in
parts per million (ppm) using tetramethylsilane (TMS) or a
residual solvent peak as a reference. The FT-IR spectra were
recorded on a Perkin-Elmer-Spectrum One FT-IR spectrometer
with KBr disks in the range of 4000–450 cm21
.
X-ray crystallography
In each case, a crystal of suitable size was selected from the
mother liquor and immersed in silicone oil. It was then mounted
onto the tip of a glass fiber and cemented using epoxy resin. The
intensity data was collected using a Bruker SMART APEX-II
CCD diffractometer, equipped with a fine focus 1.75 kW sealed
In our recent communication, we have shown that the
dinitrophenyl functionalized tris(amide) receptor, L, (Scheme 1)
behaves as a selective chemosensor for the fluoride ion via
encapsulation of the anion within the tripodal scaffold in a library
of polar aprotic solvents, exhibiting solvatochromism and
solvatomorphism (TBA[L(F)]?H2O (1a), TBA[L(F)]?DMF (1b)
and TBA[L(F)]?THF (1c) solvates).11 In a continuation of our
previous effort, herein we report the acetone solvate of the
F2–bound receptor capsule (TBA[L(F)]?(CH3)2CO, 1d), prepared
in an acetone–ethyl acetate media and the solvent-specific
crystallization of the KF complex of L ([L?KF(H2O)2], 2) from
an acetonitrile media, where a hydrated KF contact ion-pair is
˚
tube using Mo–Ka radiation (l = 0.71073 A) at 180 or 298 K,
with increasing v (width of 0.3u per frame) at a scan speed of 5 s
frame21. SMART software was used for the data acquisition.
Data integration and reduction was undertaken with SAINT and
XPREP12 software. Multi-scan empirical absorption corrections
were applied to the data using the SADABS program.13 The
structures were solved by direct methods using SHELXS-9714
and refined with full-matrix least-squares on F2 using SHELXL-
97.15 All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms attached to all carbon atoms were geome-
trically fixed and the positional and temperature factors were
refined isotropically. Structural illustrations have been drawn
with MERCURY-2.316 for Windows.
2
2
…
F
…
coordinated to the receptor molecule(s) via N–H F , C–H
2
…
and lp(F ) p interactions. The attempted crystallization of the
1 : 1 KF complex (2) in solvents such as DMSO and DMF yielded
the respective solvates of the ligand, where the lattice solvent
… …
interacts with the receptor molecules via N–H O, C–H O and
Synthesis and characterization
Receptor L. The tripodal amide receptor, L, was synthesized
following our recent report where the reaction of tris(2-
aminoethyl amine) with 3,5-dinitrobenzoyl chloride in a 1 : 3
molar ratio at room temperature yielded L in high yield (75%)
and the characterization data matched with the recently
published data.11 m.p: 252 uC; 1H NMR (DMSO-d6, 400
MHz) d (ppm) 2.84 (s, 6H, NCH2), 3.48 (d, 6H, CONH–CH2),
8.88 (d, 3H, para-ArCH), 8.91 (s, 6H, ortho-ArCH), 9.15 (s, 3H,
amide–NH); 13C NMR (100 MHz, DMSO-d6): d 45.73 (63C,
–NCH2), 53.08 (63C, CONH–CH2), 120.58 (63C, ArH),
Scheme 1 Molecular structure of the tris(amide) receptor, L.
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127.35 (66C, ArH), 137.06 (63C, ArH), 148.01 (66C, ArH),
162.20 (63C, CLO); FT-IR (v, cm21) 1346 (asym., NO2), 1540
(sym., NO2), 1670 (CLO), 3088 (C–H), 3428 (N–H).
the binding ability of the receptor towards anionic guests. The
chelate-effect may also play an important role in the anion
binding affinity because of the favourable contributions from
both entropy and enthalpy. From the perspective of the anion
receptor chemistry, crystallization has traditionally been a route
to understand the structural insights of the anion complexes
formed, primarily by single-crystal X-ray diffraction (XRD)
analysis which are then related to the observed binding in
solution using mostly NMR spectroscopy.
Complex TBA[L(F)]?(CH3)2CO, (1d). Complex 1d was pre-
pared by adding an excess (10 equiv.) of tetrabutylammonium
fluoride (TBAF) into a suspension of L (370 mg, 0.5 mmol) in
15 mL of an acetone–ethyl acetate (2 : 1) binary solvent mixture.
After the addition of the TBAF salt, the mixture was stirred for
approximately 15 min at RT during which the suspension turned
clear. Finally, the resulting purple coloured solution was filtered
in a test-tube and allowed to slowly evaporate below 10 uC in a
refrigerator. Light pink crystals of 1d suitable for single crystal
X-ray analysis were obtained within 7 days. Yield: 42–45% based
on L. It is worth mentioning that our previous efforts to obtain
single crystals of 1d at RT were unsuccessful. However, RT
crystallization of the receptor–fluoride complex can reproducibly
be accomplished from other aprotic solvents viz, MeCN, THF
and DMF, which yielded the respective solvates of TBA[L(F)].11
1H NMR (DMSO-d6, 400 MHz) d (ppm) 0.92 (t, 12H, TBA-
CH3), 1.30 (q, 8H, TBA-CH2), 1.56 (t, 8H, TBA-CH2), 1.98 (s,
6H, (CH3)2CO) 2.61 (s, 6H, NCH2), 3.16 (t, 8H, TBA-N+CH2),
8.80 (s, 3H, para-ArCH), 9.60 (s, 6H, ortho-ArCH), 12.49 (s, 3H,
amide-NH); 13C NMR (DMSO-d6, 100 MHz) d (ppm) 13.48
(64C, TBA-CH3), 19.22 (64C, TBA-CH2), 23.08 (64C, TBA-
CH2), 38.30 (63C, –NCH2), 57.56 (63C, CONH-CH2), 79.19
(64C, TBA-N+CH2), 120.21 (63C, ArH), 128.17 (66C, ArH),
137.59 (63C, ArH), 147.95 (66C, ArH), 162.23 (63C, CLO);
FT-IR (v, cm21) 1343 (asym., NO2), 1538 (sym., NO2), 1645
(CLO), 2962(C–H), 3421 (N–H).
Complex TBA[L(F)]?(CH3)2CO, 1d crystallizes in the triclinic
¯
space group P1 with Z = 2 and an acetone molecule from the
solvent of crystallization. The crystal structure analysis shows
that a fluoride ion is bound within the receptor scaffold and
governed by six hydrogen bonds from the amide-NH functions
and three aryl-CH protons (ortho) of the p-acidic receptor,
resulting in an anion entrapped unimolecular capsule (Fig. 1a).
2
The encapsulated F2 ion is N–H
F
hydrogen bonded to the
˚
…
three amide protons with an average contact distance of 2.685 A
whereas the coordinating ortho-aryl protons interact through
2
F hydrogen bonds with an average distance of 2.965 A,
…
˚
C–H
demonstrating the strong binding of F2 by L in the solid state
2
…
F
(Table 1). A correlation of the D–H
2
distance vs. the
…
D–H
F
angle reveals that hydrogen bonds formed between
the encapsulated F2 ion with the six coordinating donor atoms
are in the strong hydrogen bonding interaction region with
…
donor-to-acceptor (D F) distances ranging from 2.662 to
…
˚
3.022 A and D–H F angles ranging from 143(2)u to 179(3)u
(Table 1). Intermolecular short contact analysis shows the
dimeric association of two F2-encapsulated receptor units with
…
opposite orientation by aryl C–H O and nitro–nitro interactions.
Complex [L?KF(H2O)2], 2. Complex 2 was prepared by adding
an equivalent amount of a potassium fluoride solution in water
(1 ml) into a suspension of L (370 mg, 0.5 mmol) in 20 mL of
acetonitrile. After the addition of the KF salt, the suspension
was stirred for approximately an hour at 60 uC under reflux. The
resulting solution which was thus obtained was filtered into a
test-tube and allowed to slowly evaporate at RT for crystal-
lization. Red crystals of 2, suitable for single crystal X-ray
analysis, were obtained within 10–12 days. Yield: 28–30% based
on L. It is worth mentioning that room temperature crystal-
lization of L in the presence of an equivalent amount of KF in
other aprotic solvents such as DMSO and DMF were
unsuccessful in obtaining the complex and yielded the respective
1
solvates of the ligand. H NMR (DMSO-d6, 400 MHz) d (ppm)
2.70 (s, 6H, NCH2), 8.83 (s, 3H, para-ArCH), 9.19 (s, 6H, ortho-
ArCH). Due to the poor solubility of the isolated crystals of 2 in
deuterated solvents such as CD3CN and DMSO-d6, the 13C
NMR spectrum could not be recorded. FT-IR (v, cm21) 1342
(asym., NO2), 1537 (sym., NO2), 1646 (CLO), 2921 (C–H), 3254
and 3354 (N–H).
Results and discussion
Fig. 1 (a) X-ray structure of 1d showing the encapsulation of F2 inside
2
…
the receptor scaffold, where the dotted lines represent (D–H)
F
Crystal structure analyses
interactions; (b) Spacefill representation depicting the formation of an
F2-encapsulated neutral molecule capsule in 1d; (c) Dimeric association
Receptor L possesses a highly organized tripodal scaffold with
hydrogen bond-donating amide functions suitable for anion
binding and encapsulation. In addition, functionalizations of L
with p-acidic dinitrophenyl aryl terminals significantly enhance
2
…
of two F -encapsulated receptor units by aryl C–H O and nitro–nitro
nAp*(k) interactions. The TBA cation and the solvent are omitted for
clarity.
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Table 1 Hydrogen bond interactions involved in the crystal structures of L?DMF, 1d and 2
…
…
d (H A)
…
d (D A)
…
, (DH A)
Crystals
D–H
A
[L?DMF]
Intraligand interactions
…
N2–H O11
2.12(2)
2.46(2)
—
2.04(2)
2.42(2)
2.64(2)
—
2.17(2)
2.59(2)
2.63(2)
2.68(2)
2.69(2)
—
2.973(3)
3.308(3)
3.830
2.846(3)
3.337(3)
3.504(6)
3.429
2.912(3)
3.076(5)
3.298(4)
3.530(3)
3.420(3)
3.218(6)
3.182(3)
171(1)
151(2)
—
154(2)
165(2)
154(2)
—
144(1)
111(2)
126(2)
146(2)
132(2)
—
…
C9–H O11
…
C2g C3g
…
Interactions with DMF
Interligand interactions
N8–H O16
…
C23–H O16
…
C30–H O6
…
O16 C3g
…
…
N5–H O1
C2–H O10
…
…
C11–H O8
C10–H O9
…
C19–H O15
…
O12 C21
…
O13 C6
—
—
[L?KF(H2O)2] (2)
Intraligand interactions
…
…
N2–H O6
C9–H O6
2.04(2)
2.46(2)
—
2.00(3)
2.34(2)
—
2.09(2)
2.61(4)
2.65(4)
2.67(4)
2.71(2)
—
2.904(3)
3.188(4)
3.750
2.841(4)
3.240(4)
3.527
2.860(3)
3.274(5)
3.522(6)
3.134(6)
3.432(4)
2.983(4)
3.068(5)
3.153(5)
3.509(4)
174(2)
134(2)
—
163(2)
160(2)
—
147(2)
125(2)
149(2)
110(2)
131(2)
—
…
C2g C3g
…
Interactions of KF(H2O)2
Interligand interactions
N5–H F1
…
C14–H F1
…
F1 C2g
…
N8–H O1
…
C1–HA O3
…
C1–HB O3
…
C2–H O13
…
C19–H O1
…
O2 N9
…
O14 N11
—
—
—
—
—
—
…
…
O8 C6
O8 C12
TBA[L(F)]?(CH3)2CO (1d)
Interactions with F2
…
…
N2–H F1
N5–H F1
1.77(3)
1.78(2)
1.82(2)
2.12(1)
2.15(1)
2.09(2)
2.39(2)
2.51(3)
2.79(3)
—
2.662(3)
2.680(2)
2.713(2)
3.022(3)
2.949(2)
2.925(4)
3.252(4)
3.427(4)
3.495(4)
2.939(4)
3.350(4)
3.058(3)
3.068(4)
2.974(3)
3.425(4)
3.530(3)
3.555(3)
3.534(4)
3.343(4)
3.528(3)
3.558(3)
3.269(4)
3.317(3)
3.254(4)
3.512(4)
170(3)
179(2)
173(2)
162(2)
143(2)
164(2)
154(2)
169(2)
129(2)
—
…
…
N8–H F1
C5–H F1
…
C14–H F1
…
C23–H F1
…
C16–H O1
Interligand interactions
…
C9–H O9
…
C20–HA C27
…
O5 N7
…
…
C21 C21
—
—
—
—
—
—
—
—
O15 C3
…
O11 C6
…
O11 N3
Interactions with TBA+
C30–HB O1
C32–HA O4
C34–HA O4
C37–HB O4
C28–HA O6
C41–HA O6
C31–HB O8
C28–HB O9
2.46(2)
2.61(2)
2.65(2)
2.67(2)
2.44(2)
2.57(1)
2.69(2)
2.40(2)
2.44(2)
2.47(2)
2.67(2)
172(2)
157(2)
154(2)
148(2)
154(2)
166(2)
150(2)
148(2)
150(2)
137(2)
144(2)
…
…
…
…
…
…
…
…
…
…
…
C36–HA O9
C42–HA O10
C34–HB O14
The C9H and C16H aryl protons from each receptor unit of the
dimer are hydrogen bonded to the O9 (nitro oxygen) and O1
(amide oxygen) oxygen atoms, respectively of the other unit
whereas the nitro groups are involved in nAp*(k) interactions
(Fig. 1c). Two such dimers are interlinked with one another via
units (ESI{). Similar nAp*/nAp*(k) and pAp* electron donor–
acceptor interactions have recently been revealed by Gilli et al.
towards controlling the crystal packing of picric acid and it’s
adducts with nitrogen bases.17 Expansion through intermolecular
hydrogen bonds generates a 1D chain of F2-entrapped receptor
capsules along the crystallographic b-axis.
aliphatic C–H p interactions, nAp* (O:AN C21) interactions,
…
¯
Complex [L?KF(H2O)2], 2, crystallizes in the P1 triclinic space
where the lone pair on the oxygen is added to the NLO or CLO
double or partially double bonds, and C:AC contacts occurring as
pAp* interactions between the CLO functions of adjacent dimeric
group with Z = 2. Structural elucidation reveals that the receptor
unit is conformationally locked due to intramolecular hydrogen
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…
bonding between the receptor side arms involving N–H O,
… …
˚ ˚ ˚
2.860(3) A, C19 O1 = 3.432(4) A, C19 O2 = 3.622(6) A and
…
…
…
˚
C–H O and p p interactions, where the amide oxygen O6 is
involved in bifurcated hydrogen bonding with the amide proton
N2H and aryl hydrogen C9H from the same receptor side arm
C2 O13 = 3.134(6) A), resulting in a 1D chain of inversely
oriented receptor molecules in association with hydrogen bonded
KF(H2O)2 moieties diagonally along the a-axis (Fig. 2b).
Furthermore, two such 1D chains are interlinked among
…
…
˚
(N2 O6 = 2.904(3) A; C9 O6 = 3.188(4) A). Complementary
˚
21
…
…
…
N–H O and C–H O hydrogen bonding between two arms of
the receptor presumably assists one of the aryl functions of the
intramolecularly hydrogen bonded tripodal arms to be in closer
proximity with the aryl ring of the third side arm, resulting in a
themselves by multiple C–H O and nAp* (O:AN C
interactions which eventually govern the overall packing of the
)
crystal (ESI{).
The DMF solvate of the receptor L (L?DMF) crystallizes in
the P21/c triclinic space group with Z = 4. Structural elucidation
of L?DMF reveals that the combined effect of intramolecular
…
˚
significant face-to-face interaction (C2g C3g = 3.750 A).
Structural elucidation further reveals that KF is bound as a
contact ion-pair (K+–F2 = 1.494(3) A) by L where K is
+
…
hydrogen bonding and aromatic p p stacking resists the open
˚
+
˚
coordinated to two water molecules (K –O18 = 1.788(5) A and
conformation of the receptor, as observed in 2. The amide
hydrogen N2H and an aryl proton C9H from the same receptor
side arm (involving C1–C9) is intramolecularly hydrogen bonded
to the amide oxygen O11 of another arm (involving C10–C18)
K –O19 = 1.778(6) A). The strong K –F2 ionic interaction that
results from the much higher charge density of K+ (as compared
to TBA+) is certainly of high importance in disabling the
expected ‘‘encapsulating mode’’ of the fluoride recognition.
Notably, the K+–F2 bond length in 2 is distinctly shorter when
compared to those of anhydrous KF,18 which exhibits a KF6
+
+
˚
…
via N–H O and C–H O interactions, respectively (N2 O11 =
…
…
…
˚
˚
2.973(3) A and C9 O11 = 3.308(3) A), whereas the aryl
function C3g of the third arm (involving C19–C27) is involved in
an aromatic face-to-face interaction with the ring C2g, attached
octahedron with a K+–F2 bond length of 2.674 A, and
KF?2H2O19 having a K(H2O)4(F)2 octahedron with a K+–F2
˚
bond length of 2.716 A. Furthermore, each hydrated KF contact
˚
…
to the amide function involving oxygen O11 (C3g C2g =
…
3.830(3) A). Short contact analyses (D A , 4.0 A) of the
˚
˚
ion-pair is hydrogen bonded to two adjacent receptor molecules
2
crystal structure further shows that the DMF oxygen (O16) is
…
2
via amide N–H F , aryl C–H
2
and lp(F ) p interactions
…
…
…
F
hydrogen bonded to the amide proton N8H (N8 O16 =
…
…
…
…
˚
(N5 F1 = 2.841(4) A; C14 F1 = 3.240(4) A and F1 C2g =
˚
˚
˚
2.846(3) A) and aryl hydrogen C23H (C23 O16 = 3.337(3) A)
from the same flexible side arm (involving C19–C27) of the
receptor and also interacts with the p-acidic aryl function C3g,
involving the identical side arm of an adjacent receptor unit via
˚
3.527 A), resulting in the dimeric association of the receptor
molecules bridged by two coordinated KF(H2O)2 moieties
(Fig. 2a). Expansion through hydrogen bonds shows that two
such dimeric units are further associated with each other by
…
…
˚
an lp(O) p interaction (O16 C3g = 3.429 A). Additionally,
…
…
…
amide N–H O and aliphatic C–H O interactions (N8 O1 =
the carbonyl hydrogen (C30H) of the lattice DMF interacts with
Fig. 2 (a) X-ray structure of complex 2 showing the intramolecular
hydrogen bonding and interactions with the KF(H2O)2 adduct; (b)
Crystal packing of 2, as viewed down the a-axis, showing the H-bonding
and electron donor–acceptor interactions between two adjacent 1D
chains of the receptor molecules.
Fig. 3 X-ray structures of the solvatomorphs of L showing the identical
modes of intramolecular H-bonding and interactions with the lattice
solvent molecule in (a) L-DMF and (b) L-DMSO.
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…
…
˚
and lp(O) p interactions from the same tripodal side arm of
two adjacent receptor molecules, which has also been observed in
complex 2 involving the F2 atom of the KF(H2O)2 moiety in
replacement of the DMSO or DMF oxygen. It is important to
mention here that complex 2 and the previously reported crystal
of L?DMSO11 are isostructural, which is evident from the
identical unit cell parameters and simulated PXRD patterns.
The binding discrepancy of F2 in complexes 1d and 2 has also
been established by FT-IR analysis (Fig. 4). In the case of the
free receptor, the amido carbonyl (NH–CLO) stretching
frequency is observed at 1670 cm21 and the N–H stretching
frequency is observed at 3428 cm21. However, in complex 1d the
–CLO stretching vibration is observed at 1645 cm21, showing a
considerable shift of 25 cm21 relative to the free L and the N–H
peak is significantly broadened due to the formation of strong
the amide oxygen O6 (C30 O6 = 3.504(4) A), resulting in a
hydrogen bonded dimer formation with two DMF molecules
and two molecules of L (Fig. 3a). Each solvent bridged dimer is
in association with the adjacent dimeric unit via a strong
…
N–H O hydrogen bond that involves an interaction between
the amide proton N5H and the amide oxygen O1 of two
…
neighboring dimers (N5 O1 = 2.912(3) A). The hydrogen
˚
bonded association of solvent bridged dimers is further stabilized
…
by two aliphatic C–H O interactions in which the methylene
protons C2H(B) and C10H(B) from two different arms of a
receptor dimer make contacts with the nitro oxygen O10 and
…
amide oxygen O1 of another dimeric unit (C2 O10 = 3.076(5) A
˚
…
˚
and C10 O1 = 3.501(4) A), resulting in a 1D chain of
oppositely oriented receptor units along the c-axis (ESI{).
However, the overall packing of the crystal is additionally
2
…
…
N–H
F
hydrogen bonds with the encapsulated F2 anion, as
governed by three aliphatic C–H O (nitro) interactions and
evident from the crystal structure of 1d. However, in complex 2,
the –CLO stretching vibration experiences an observable shift of
24 cm21 relative to the free L and the peak for the N–H
nAp* (O:AC) interactions, where the lone pair on the oxygen
from the nitro group is added to the CLC or CLO double or
partially double bonds,17 resulting in the formation of a
hydrogen bonded sheet-like structure when viewed down the
crystallographic b-axis (ESI{). The details of the hydrogen bonds
involved in the crystal structure of L?DMF are provided in
Table 1.
stretching is split into two distinct peaks at 3254 and 3354 cm21
,
indicating the existence of two different types of hydrogen
2
bonded N–H protons (N–H O and N–H F ), as observed in
…
…
the crystal structure of 2.
Similar types of intra- and intermolecular hydrogen bonding
has previously been observed in the DMSO solvate of L, where
the DMSO oxygen atom is hydrogen bonded to an amide-NH
function and an aryl-CH proton (ortho) from the same flexible
side arm of the receptor and interacts significantly with the
p-acidic aryl function involving the identical side arm of an
Hirshfeld surface analyses. The structural similarities between
complex 2 and the DMSO or DMF solvates of L have also been
visualised by Hirshfeld surface analysis, which is a useful tool to
describe the surface characteristics of molecules.20 Hirshfeld
surfaces offer a novel way of visualizing intermolecular interac-
tions by colour-coding short or long contacts and two-
dimensional fingerprint plots complement these surfaces, quan-
titatively summarizing the nature and type of intermolecular
interactions experienced by the molecules in the crystal as
‘‘contact contribution’’.
…
adjacent receptor unit via an lp(O) p interaction (Fig. 3b and
ESI{). The details of the hydrogen bonds involved in the crystal
structure of L?DMSO are provided in Table S2 in the ESI.{
Thus, from the experimental conditions and structural
elucidation, it is obvious that, irrespective of the solvent of
crystallization, F2 is encapsulated and bound within the tripodal
scaffold by six strong hydrogen bonds from the amide-NH and
three aryl-CH functions when the TBAF salt was employed as
the primary source of the F2 ion. However, when KF was used
as the source for the complexation of the F2 ion by L, it has been
Fig. 5 displays the Hirshfeld surfaces of the receptor unit
mapped with dnorm for solvated L and complex 2 and highlights
…
the intermolecular N–H O interactions as bright red spots and
crystallographically observed that
a receptor molecule is
involved in the side-cleft binding of the hydrated KF contact
2
2
…
…
F
ion-pair by an amide N–H
F
and an aryl C–H
interaction from the same tripodal side arm of the receptor
and interacts significantly with the neighbouring KF(H2O)2
2
…
moiety via an lp(F ) p interaction. Furthermore, the crystal-
lization of the KF complex (2) is solvent specific and formed
only in an acetonitrile (MeCN) media. Complexation of KF in
other aprotic solvents such as DMSO and DMF were not fruitful
and resulted in the exclusive crystallization of the respective
solvates of L, as confirmed by the 1H NMR and FT-IR analyses
of the isolated crystals. This can be rationalized in terms of the
hydrogen bonding interactions where solvents such as DMSO
and DMF are highly capable of competing for the amide-NH
functions and p-acidic arenes and thereby disfavour the
formation of complex 2. This scenario is, however, clear from
the X-ray crystallographic observation where similar modes of
supramolecular interactions have been observed in the crystals of
2 and the DMSO/DMF solvates of L. Notably, the DMSO/
Fig. 4 Overlay FT-IR spectra of the free receptor, L, and complexes 1d
and 2.
…
…
DMF oxygen atom is involved in amide N–H O, aryl C–H
O
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…
O contacts. A
merged with the upper spike of the
H
noteworthy feature that differentiates the fingerprint plots of
solvated L with complex 2 is the substantial decrease in the
…
characteristic H O contact contribution in 2 by 12.33% and
20.52% w.r.t. L?DMSO and L?DMF. To compensate for this
decrease, the contributions from other close contacts increase
2
…
with an additional contribution from the H
(Table 2).
F
contacts
Fig. 6 represents the Hirshfeld surface of the receptor molecule
mapped with dnorm for complex 1d and the corresponding 2D
2
…
…
fingerprint plots for the H O and H
F
close contacts with a
contact contribution of 44.2% and 2.5% respectively (Table 2).
Several bright red to faint red spots on the dnorm surface of the
receptor can be attributed to the donor and/or acceptor atoms
…
involving mostly intermolecular C–H O interactions between
adjacent receptor molecules or between a receptor molecule and
aliphatic-CH donors of neighbouring TBA cations. However, it
2
Fig. 5 Hirshfeld surface analysis of L?DMSO, L?DMF and complex 2,
showing the dnorm surfaces of the respective receptor unit and the
…
…
is important to mention that the interaction spots for the H
F
contacts were not observed on the receptor surface due to the full
encapsulation of F2 within the receptor scaffold where the donor
corresponding 2D fingerprint plots with the H O interactions high-
lighted in blue colour.
2
…
atoms are directed towards the cavity forming strong D–H
F
…
C–H O interactions as bright to faint red spots that exist
(D = donor atoms, N C21) hydrogen bonds. The fingerprint
…
plots for the H O close contacts in 1d display the characteristic
between the adjacent receptor molecules and between a tripodal
side arm of the receptor with the lattice solvent in the
solvatomorphs of L (L?DMF and L?DMSO) or KF(H2O)2
adduct in 2. The corresponding fingerprint plots for the
‘‘spikes’’ in the upper left and lower right of the plot and show
pseudosymmetry on either side of the diagonal where de = di
(Fig. 6b), identical to the plots for solvated L and complex 2.
2
…
However, the strong H
F contacts appear as a sharp ‘‘spike’’
…
Hirshfeld surfaces show the characteristic ‘‘spikes’’ in the upper
…
between the two symmetric ‘‘spikes’’ of the H O close contacts
with a contact contribution of 2.5% (Fig. 6c).
left and lower right of the plot that represent the H
O
interactions, with a contact contribution of 54.6%, 49.5% and
43.4% for L?DMF, L?DMSO and 2 respectively (Table 2). Two
closely spaced bright spots on the left edge of the dnorm surface of
1
Solution state H and 2D NOESY NMR studies
…
L?DMF can be attributed to the amide N–H(8) O(16) and aryl
…
The free receptor molecule, L, shows the amide-NH resonance
at d 9.15 ppm whereas the aromatic-CH protons resonate at
8.91 (s, ortho-CH) and 8.88 (s, para-CH) ppm in DMSO-d6.
C–H(23) O(16) interactions with the lattice solvent whereas a
bright spot on the middle and another towards the right edge of
the surface corresponds to the donor and acceptor atoms
…
involving interligand N–H(5) O(1) interaction. Identical sur-
face behaviour has also been observed in complex 2 where two
closely spaced intense red spots towards the left edge of the
2
surface can be attributed to the amide N–H(5) F (1) and aryl
…
2
…
C–H(14) F (1) interactions with the KF(H2O)2 adduct. The
bright spot on the middle and another towards the right edge of
the surface corresponds to the donor and acceptor atoms
…
involving an interligand N–H(8) O(1) interaction. However,
it is important to mention that several faint red spots over the
dnorm surface of L?DMF/L?DMSO and complex 2 can be
…
2
assigned to interligand C–H O(nitro) interactions. In the
…
fingerprint plot of complex 2, the H
F
contacts occur as a
sharp spike in the upper left of the plot adjacent and partially
Table 2 Contact contributions from the dnorm surface area of the
receptor molecule in solvated L and complexes 1d and 2
Contacts
L?DMSO
L?DMF
1d
2
…
…
…
H
H
H
O
H
C
N
C
O
49.5
16.7
5.5
54.6
17.5
07.1
3.6
7.3
7.1
44.2
20.0
14.1
2.9
5.5
5.3
43.4
18.6
2.5
…
…
…
O
O
O
H
4.2
3.7
Fig. 6 (a) dnorm surface of the receptor unit in complex 1d; (b) 2D
…
2
16.9
10.5
—
10.6
12.5
2.5
fingerprint plot highlighting the H O interactions in 1d; (c) 2D
…
fingerprint plot highlighting the H
…
F
interactions in 1d; (d) 2D
…
F
—
2.5
2
F interactions in 2.
fingerprint plot highlighting the H
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ortho-CH resonances are significantly broadened, which can be
attributed to the binding-induced broadening of the 1H-NMR
signals and is often observed in the 1H NMR experiments of
anions with –NH-containing receptors. However, in the 1H
NMR spectrum of the isolated crystals of 2 (DMSO-d6), the
amide-NH resonance could not be observed21 and the ortho-CH
resonance shows a comparatively minor downfield shift of
0.28 ppm with concomitant broadening of the signal (Fig. 7).
Similar spectral changes have also been recorded in CD3CN
(ESI{), suggesting that even KF can compete and form hydrogen
bonds with the receptor in solvents such as DMSO and DMF
which are highly capable of interacting with L. However, in the
attempted crystallization of 2 from DMSO and DMF, the
receptor molecule tends to become solvated with time which can
…
be attributed to an additional C–H O interaction between the
lattice solvent and an amide oxygen of L (Table 1 and Table S2
in ESI{). Overall, from the 1H NMR analyses, it is customary to
Fig. 7 Comparison of the partial 1H NMR spectra (aromatic region) of
L and complexes 1d and 2 in DMSO-d6 at 298 K.
2
claim that the complexation of F2 involving three N–H
2 hydrogen bonds results in larger deshielding
…
F
F
…
and three C–H
1
Interestingly, the H-NMR spectrum of the isolated crystals of
of the coordinating protons in 1d when compared to 2, which
involves side-cleft binding of hydrated KF via the participation
1d (DMSO-d6) shows a significant downfield shift of the amide-
NH and ortho-CH resonances with high Dd values of 3.34 and
0.70 ppm, respectively (Fig. 7), indicative of the strong solution
2
…
of one N–H F , one C–H
2
2
F and lp(F ) p interactions. It
…
…
is important to mention that there is no significant change in the
chemical shift values of the para-CH resonances in complexes 1d
and 2 due to its non-coordinating nature towards the F2 ion, as
observed in the X-ray structures.
2
state binding of F2 with L via amide N–H
F
and ortho-C–
…
2
…
H
F
interactions, as observed in the X-ray structure of 1d.
Moreover, upon the coordination of the F2 anion, the –NH and
Fig. 8 (a) Schematic representation depicting the through-space NOE couplings in L and the observed binding mode of L with F2 in solution; (b) 2D
NOESY NMR spectrum of the free receptor, L in DMSO-d6; (c) 2D NOESY NMR spectrum of the TBAF complex of L (1d) in DMSO-d6.
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The solution state encapsulation of fluoride in 1d and the
side-cleft binding of KF in 2 has further been confirmed by 2D
NOESY NMR experiments of the isolated complexes and free
receptor (L) in DMSO-d6. As depicted in Fig. 8b, the free
receptor molecule shows several strong NOESY signals
Acknowledgements
GD acknowledges DST (SR/S1/IC-01/2008) and CSIR (01-2235/
08/EMR-II), New Delhi India for financial support and DST-
FIST for the single crystal X-ray diffraction facility. SKD and
BD acknowledges IIT Guwahati, India for their fellowship.
…
between the amide-NH and aryl-CH protons (NH CH) and
…
between the identical set of –NH protons (NH NH) and
…
–CH protons (CH CH). However, in complex 1d, the
References
…
NH CH through-space NOE coupling was found to be
1 (a) J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives,
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Chemistry, Oxford University Press, Oxford, U.K., 1999; (d) J. L.
Sessler, P. A. Gale and W.-S. Cho, Anion Receptor Chemistry;
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absent (Fig. 8c), indicating the binding and encapsulation of
fluoride within the tripodal scaffold in a 1 : 1 host–guest
stoichiometry, an observation which is also supported by the
X-ray structure of the F2-encapsulated complex (1d).
Furthermore, the interactions between the identical set of
…
–NH protons (NH NH) become significantly weaker while
…
the CH CH signals remain (although less intense), indicating
a conformational change in the receptor due to the encapsula-
tion of the fluoride anion (Fig. 8c). In contrast to 1d, strong
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…
…
…
NOESY signals involving NH CH, NH NH and CH CH
through-space interactions have been observed in complex 2
(similar to L) which indeed suggests that fluoride encapsula-
tion is not the prevalent mode of binding with KF in solution
as well. However, it is important to mention that, unlike the
solid-state structures, the receptor molecule exhibits C3v
symmetry in solution where the tripodal side arms are
equivalent with each other and interacts significantly among
the different sets of protons. Upon binding and encapsulation
of the fluoride by the –NH and –CH hydrogen bonds, the
interactions between the different sets of protons is hindered,
due to which they are either found to be absent or become
significantly weaker in 1d. Similar 2D NOESY NMR
experiments have recently been performed by our group and
others to demonstrate the encapsulation of anions by urea-
functionalized tripodal scaffolds.22
5 (a) F. Hof, S. L. Craig, C. Nuckolls and J. Rebek, Jr., Angew. Chem.,
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Fujita, Chem. Commun., 2010, 46, 5897; (e) K. Ikemoto, Y. Inokuma
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Conclusion
In summary, we have structurally authenticated the binding
discrepancy of the fluoride ion in quaternary ammonium
(TBAF) and alkali (KF) salts by a p-acidic tris(amide)
receptor, L. The binding discrepancy has also been exemplified
in the solution state by 1H NMR and 2D NOESY NMR
experiments of the isolated crystals of 1d and 2, which were
subsequently compared to the free receptor. The encapsulation
of F2 in complex 1d and the side-cleft binding of KF in
complex 2 can be captured by following the change in chemical
shift of the –NH and ortho-CH resonances in the 1H NMR
experiments and the disappearance of through-space interac-
tions between the –NH and ortho-CH protons of 1d in the 2D
NOESY NMR experiments when compared to the free
receptor and complex 2. Furthermore, a detailed Hirshfeld
surface analysis of the solvated crystals of L and that of
complex 2 provides a better understanding of the identical
types of supramolecular interactions prevalent in the crystal
structures of L?DMF/L?DMSO and
2 which eventually
7 (a) G. R. Desiraju and T. Steiner, The Weak Hydrogen Bond in
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governs the same crystal packing in the structure. Thus,
receptor L provides an ideal example of a flexible F2 binding
host which adapts its conformation to respond to the demands
of the specific countercation.
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5314 | CrystEngComm, 2012, 14, 5305–5314
This journal is ß The Royal Society of Chemistry 2012