Fluoride selectivity induced transformation of charged anion complexes into unimolecular capsule of a π-acidic triamide receptor stabilized by strong N-H???F- and C-H???F- hydrogen bonds

Article abstract of DOI:10.1021/cg200679x

The structural aspects of binding of anions such as halides (chloride, 1, and bromide, 2) and oxyanions (perchlorate, 3, and hydrogen sulfate, 4) with the protonated tris(amide) receptor L were carried out in detail. In all of these complexes, the hydroge

Full text of DOI:10.1021/cg200679x

ARTICLE
pubs.acs.org/crystal
Fluoride Selectivity Induced Transformation of Charged Anion
Complexes into Unimolecular Capsule of a π-Acidic Triamide Receptor
Stabilized by Strong NꢀH Fꢀ and CꢀH Fꢀ Hydrogen Bonds
3 3 3
3 3 3
Sandeep Kumar Dey and Gopal Das*
Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India
S Supporting Information
b
ABSTRACT: The structural aspects of binding of anions
such as halides (chloride, 1, and bromide, 2) and oxyanions
(perchlorate, 3, and hydrogen sulfate, 4) with the protonated
tris(amide) receptor L were carried out in detail. In all of these
complexes, the hydrogen of protonated bridgehead nitrogen of
the ligand is endo-oriented, forming a strong NꢀH
O
3 3 3
hydrogen bond with an amide oxygen of a tripodal side arm,
and binding of anions is primarily governed by NꢀH anion
3 3 3
and CꢀH anion interactions involving multiple receptor
3 3 3
cations. Furthermore, transformation of the charged complexes
into a unimolecular capsule of L has also been accomplished in the presence of excess fluoride ion in dimethyl sulfoxide (DMSO)
solution. Crystallographic analysis of fluoride complex [TBA(L F)]2DMSO 4H2O (5; TBA = tetrabutylammonium) shows that
3
3
Fꢀ is fully encapsulated within the tripodal cleft governed by six strong hydrogen bonds from the amide -NH and aryl -CH protons
of the π-acidic receptor. 1H NMR titration experiments further provide evidence for the formation of the Fꢀ encapsulated receptor
capsule from the charged complexes.
reorient or rupture the self-assembled architectures. Although
numerous synthetic molecular capsules have been achieved, the
challenges still exist to control the capsular assembly formation in
the presence of a guest anion that acts as a template in the
process. Template-induced association of molecular species
represents one of the main approaches in the control of
supramolecular assembly formation.11 Template-directed pro-
cesses that are anion specific can lead us to the challenging
development of new selective systems with industrial, ecological,
and biomedical applications.12 Acyclic podand receptors with
multiarmed functionality have been shown to be effective
systems for binding of a variety of anions; however, their uses
as selective anion encapsulating hosts involving equal and strong
participation from both -NH and -CH protons are rarely known.
Although not typically considered to be significant donors, there
is increasing evidence that -CH groups can actively take part in
H-bonding and lead to enhanced anion-binding affinity.13 Anion
receptors in nature often involve amide linkages in association
with -OH and -CH groups as hydrogen bond donors,14 and
therefore, amide-based receptors with polarized -CH donors are
important from the perspective of anion binding study.
1. INTRODUCTION
The field of anion receptor chemistry continues to expand its
horizon with new synthetic hosts capable of recognizing anions
with environmental and biomedical relevance.1 The observations
in natural systems have inspired researchers to develop numer-
ous neutral receptors that employ hydrogen bonds offered by
specific binding sites from amide,2 urea/thiourea,3 pyrrole4 and
indole5 functionalities for the recognition and binding of anionic
guests on suitable frameworks. In contrast, cationic hosts with
guanidinium6 and polyammonium7 functionalities ensure an
adequate electrostatic attraction reinforced by H-bond contacts
with the coordinated anions, and the selectivity can be attributed
to the charge and basicity factors, rather than true selectivity of
the host for anions. Anions generally have very high solvation
energies that must be compensated by the host for effective anion
recognition.8 Tripodal scaffolds offer a flexible and structurally
preorganized cavity, which has previously been explored in the
area of anion coordination chemistry and anion induced forma-
tion of capsular assemblies.9 One of the most fascinating features
of molecular capsules is their ability to create a distinct micro-
environment that isolates the encapsulated guest from the bulk of
the solvent media and, thereby, leads to phenomena such as
molecular sorting when formation is possible for different
capsules present in the same solution.10
In our recent communication, we have shown that the
dinitrophenyl-functionalized tris(amide) receptor L (Scheme 1)
Furthermore, when anions are an integral part of supramole-
cular aggregates, it is expected that if the templating anion is
exchanged with other anions; it should in principle be possible to
Received: May 30, 2011
Revised:
August 9, 2011
Published: August 11, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/cg200679x Cryst. Growth Des. 2011, 11, 4463–4473
|

Crystal Growth & Design
ARTICLE
Scheme 1. Schematic Representation Depicting the Formation of Fꢀ Encapsulated Unimolecular Capsule of L from Charged
Anion Complexes in the Presence of Excess Tetrabutylammonium Fluoride ((TBA)F)
behaves as a selective chemosensor for fluoride ion by encapsula-
tion within the tripodal scaffold in polar aprotic solvents exhibiting
solvatochromism and solvatomorphism.15 Herein, we structurally
demonstrate the anion binding properties of receptor L in iꢀts
stir at room temperature overnight. The precipitate formed was then
filtered through a filter paper and washed several times with (3 ꢁ 50 mL)
of water, two times with (2 ꢁ 10 mL) of methanol, and finally with
diethyl ether. Yield of L: 75%; mp 252 ꢀC. 1H NMR (DMSO-d6, 400
MHz; ppm): δ 2.84 (s, 6H, NCH2), 3.48 (d, 6H, CONHꢀCH2), 8.88
(d, 3H, p-ArCH), 8.91 (s, 6H, o-ArCH), 9.15 (s, 3H, amideꢀNH). 13C
NMR (100 MHz, DMSO-d6; ppm): δ 45.73 (ꢁ3C, -NCH2), 53.08
(ꢁ3C, CONHꢀCH2), 120.58 (ꢁ3C, ArH), 127.35 (ꢁ6C, ArH),
137.06 (ꢁ3C, ArH), 148.01 (ꢁ6C, ArH), 162.20 (ꢁ3C, CdO).
protonatedꢀform with different anions such as Clꢀ, Brꢀ, ClO4
,
and HSO4 in complexes 1ꢀ4, respectively. Anion binding by
protonated L is attributable entirely to the NꢀH anion and
3 3 3
CꢀH anion interactions that presented a means of participat-
3 3 3
ing in the reexamination of the role of CꢀH hydrogen bonding.
Furthermore, in the proof-of-concept experiments described here,
the high selectivity of L toward recognition of Fꢀ has been
employed in the transformation of charged complexes (1ꢀ4)
into unimolecular capsules of L, wherein Fꢀ is encapsulated within
the tripodal scaffold governed by six strong hydrogen bonds from
the polarized -NH and -CH functions as evident in the crystal
structure of Fꢀ encapsulated complex 5. The transformation
phenomenon and strong participation of -NH and -CH protons
in the Fꢀ binding event has been manifested in solution state
as well.
Synthesis of Complex [HL Cl], 1. Complex 1 was obtained by adding
3
0.4 mL of 37% hydrochloric acid (HCl) to 5 mL of dimethyl sulfoxide
(DMSO) solution of L (364 mg, 0.5 mmol). After the addition of acid,
the solution was stirred at room temperature for 30 min and filtered in a
test tube. The filtrate was allowed to evaporate at room temperature,
which yielded colorless crystals suitable for X-ray crystallography
analysis within 6ꢀ7 days. Yield of 1: 68% based on L. 1H NMR
(DMSO-d6, 400 MHz; ppm): δ 3.61 (s, 6H, NCH2), 3.82 (s, 6H,
CONHꢀCH2), 8.91 (d, 3H, p-ArCH), 8.94 (d, 6H, o-ArCH), 9.66 (s,
3H, amideꢀNH), 10.57 (s, 1H, apical-NH). 13C NMR (DMSO-d6, 100
MHz; ppm): δ 38.88, 52.00, 120.96, 127.44, 136.28, 147.98, 163.26.
Synthesis of Complex [HL Br]H2O, 2. Complex 2 was obtained by
3
adding 0.3 mL of 49% hydrobromic acid (HBr) to 5 mL of dimethyl
sulfoxide solution of L (364 mg, 0.5 mmol). After the addition of acid,
the solution was stirred at room temperature, then filtered, and kept for
crystallization at room temperature. Colorless crystals suitable for X-ray
crystallography analysis were obtained after 6ꢀ7 days. Yield of 2: 62%
2. EXPERIMENTAL SECTION
2.1. 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.
Triethylamine, 37% hydrochloric acid, 49% hydrobromic acid, 70%
perchloric acid, and concentrated sulfuric acid were purchased from
Merck and used as received. NMR spectra were recorded on a Varian
FT-400 MHz instrument, and chemical shifts were recorded in parts per
million (ppm) on the scale using tetramethylsilane (TMS) or 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 450ꢀ4000 cmꢀ1. Thermal analysis was performed by using an
SDTA 851 e TGA thermal analyzer (Mettler Toledo) with a heating rate
of 5 ꢀC/min in a N2 atmosphere.
2.2. Syntheses and Characterization. Tripodal receptor L was
synthesized following our recent report where reaction of tren with 3,5-
dinitrobenzoyl chloride in a 1:3 molar ratio at room temperature yielded
L in high yield. Tren (0.292 g, 2 mmol) was dissolved in 30 mL of dry
chloroform (CHCl3) in a 100 mL round bottomed flask, and 0.708 g
(7.0 mmol) of dry triethylamine (Et3N) was added to the reaction
mixture. Then, 1.380 g of 3,5-dinitrobenzoyl chloride (6 mmol) was
added in portions to the reaction mixture over a period of 1 h with
constant stirring at room temperature. After the addition was complete,
a pale brown precipitate formed and the reaction mixture was allowed to
1
based on L. H NMR (DMSO-d6, 400 MHz; ppm): δ 3.64 (s, 6H,
NCH2), 3.82 (s, 6H, CONHꢀCH2), 8.90 (s, 6H, o-ArCH), 8.92 (s, 3H,
p-ArCH), 9.57 (s, 3H, amideꢀNH), 9.62 (s, 1H, apical-NH). 13C NMR
(DMSO-d6, 100 MHz; ppm): δ 37.14, 51.69, 121.66, 128.14, 136.98,
148.67, 163.96.
Synthesis of Complex [HL ClO4]H2O DMSO, 3. Complex 3 was
3
3
obtained by adding 0.2 mL of 70% perchloric acid (HClO4) to 5 mL of
dimethyl sulfoxide solution of L (364 mg, 0.5 mmol). After stirring for
about 30 min, the solution was filtered and kept for crystallization at
room temperature. Colorless crystals suitable for X-ray crystallography
analysis were obtained within 10ꢀ12 days. Yield of 3: 66% based on L.
1H NMR (DMSO-d6, 400 MHz; ppm): δ 3.64 (s, 6H, -NCH2), 3.82 (s,
6H, CONHꢀCH2), 8.84 (s, 6H, o-ArCH), 8.91 (s, 3H, p-ArCH), 9.51
(s, 3H, amideꢀNH), 9.36 (s, 1H, apical-NH). 13C NMR (DMSO-d6,
100 MHz; ppm): δ 34.79, 52.59, 121.03, 127.39, 136.26, 148.03, 163.53.
Synthesis of Complex [HL HSO4]DMSO, 4. Complex 4 was obtained
3
by adding 0.2 mL of concentrated sulfuric acid (H2SO4) to 5 mL of
dimethyl sulfoxide solution of L (364 mg, 0.5 mmol). After stirring for
about 30 min, the solution was filtered and kept for crystallization at
room temperature. Colorless crystals suitable for X-ray crystallography
analysis were obtained after 10ꢀ15 days. Yield of 4: 56% based on L.
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Crystal Growth & Design
ARTICLE
Table 1. Crystallographic Parameters and Refinement Details of Complexes 1ꢀ5
parameters
1
2
3
4
5
formula
C
27H25ClN10O15
C54H52Br2N20O32
827 380
1652.96
monoclinic
P21/c
C29H33ClN10O22
827 381
1201.21
triclinic
P1
S
C29H31N10O21S2
827 382
919.78
C47H72FN11O21S2
835 049
1210.30
triclinic
P1
CCDC
FW
827 379
765.02
triclinic
P1
cryst syst
space group
a/Å
monoclinic
Cc
12.3254(4)
13.5063(5)
20.0938(7)
93.146(3)
95.493(2)
92.688(2)
3320.1(2)
4
15.1066(6)
20.5781(8)
22.9600(8)
90.00
11.2376(9)
11.5873(10)
16.4312(12)
100.305(4)
106.260(4)
94.657(5)
2001.2(3)
2
22.249(2)
11.9035(11)
15.5067(13)
90.00
9.8296(6)
19.1561(13)
19.1973(13)
61.201(3)
83.109(4)
89.652(4)
3139.3(4)
2
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
102.365(2)
90.00
112.327(5)
90.00
6971.9(5)
4
3799.0(6)
4
Z
Dc/(g cmꢀ3
)
1.531
1.575
1.562
1.608
1.280
μ (Mo Kα)/mmꢀ1
0.203
1.267
0.247
0.242
0.166
T/K
298(2)
28.530
30 898
16 876
13 623
987
298(2)
28.36
298(2)
298(2)
28.28
298(2)
24.92
θmax
.
28.39
total reflecns
independent reflecns
obsd reflecns
params
90 986
25 763
22 691
40 060
17 344
9 585
4 706
10 638
14 087
6 198
3 954
9 471
1 013
594
576
748
R1, I > 2σ(I)
Rw2 (all data)
GOF (F2)
0.0565
0.1584
0.951
0.0555
0.0919
0.0605
0.0914
0.1666
0.2428
0.1578
0.1859
0.901
0.919
0.998
1.011
1H NMR (DMSO-d6, 400 MHz; ppm): δ 3.63 (s, 6H, NCH2), 3.82 (s,
6H, CONHꢀCH2), 8.88 (s, 6H, o-ArCH), 8.90 (s, 3H, p-ArCH), 9.58
(s, 3H, amideꢀNH), 9.82 (s, 1H, apical-NH). 13C NMR (DMSO-d6,
100 MHz; ppm): δ 34.45, 51.91, 120.77, 127.28, 136.18, 147.82, 163.15.
Synthesis of Complex [TBA(L F)]2DMSO 4H2O, 5. Complex 5 was
0.3ꢀ per frame) at a scan speed of 5 s/frame. The crystal-to-detector
distance was about 51 mm. The SMART software was used for data
acquisition. Data integration and reduction were undertaken with
SAINT and XPREP16 software. Multiscan empirical absorption correc-
tions were applied to the data using the program SADABS.17 Structures
were solved by direct methods using SHELXS-9718 and refined with full-
matrix least-squares on F2 using SHELXL-97.19 All non-hydrogen atoms
were refined anisotropically, and hydrogen atoms attached to all carbon
atoms were geometrically fixed and the positional and temperature
factors are refined isotropically. Hydrogen atoms attached with the
amide nitrogen atoms were located from electron Fourier map and
refined isotropically. However, H-atoms attached to the lattice water
molecules in complex 5 could not be located by Fourier map due to
disorder. Usually, temperature factors of H-atoms attached to carbon
atoms are refined by restraints ꢀ1.2 or ꢀ1.5 Uiso (C), although the
isotropic free refinement is also acceptable. Structural illustrations have
been drawn with MERCURY 2.320 for Windows. Parameters for data
collection and crystallographic refinement of complexes 1ꢀ5 are
summarized in Table 1.
3
3
obtained by adding an excess (5 equiv) of tetrabutylammonium fluoride
(1.0 M solution in tetrahydrofuran (THF)) solution either to a 3 mL of
DMSO solution of 1 (382 mg, 0.5 mmol) or a 3 mL of DMSO solution
of 4 (413 mg, 0.5 mmol). After the addition of Fꢀ ions, the solution was
stirred at room temperature and filtered in a test tube for crystallization.
Slow evaporation of the filtrate at room temperature yielded yellow
colored crystals suitable for X-ray crystallography analysis within 2ꢀ3
weeks, in both cases. It should be noted that, during the process of
crystallization, the solution mixtures do not evaporate completely and
the mother liquor that remained contains the excess (TBA)F as well as a
complex equilibrium of other salts. It is worth mentioning that, crystal-
lization of complex 5 can also be accomplished from a solution mixture
of L and excess (TBA)F in DMSO and can be confirmed by FT-IR,
thermogravimetric (TGA), and 1H NMR analyses of the isolated
1
crystals; mp 198 ꢀC. H NMR (DMSO-d6, 400 MHz; ppm): δ 0.91
(t, 12H, TBAꢀCH3), 1.30 (q, 8H, TBAꢀCH2), 1.57 (t, 8H,
TBAꢀCH2), 2.60 (s, 6H, -NCH2), 3.16 (t, 8H, TBAꢀN+CH2), 3.19
(s, 6H, CONHꢀCH2), 8.79 (s, 3H, p-ArCH), 9.64 (s, 6H, o-ArCH),
12.69 (s, 3H, amideꢀNH). 13C NMR (DMSO-d6, 100 MHz; ppm): δ
13.76, 19.51, 23.40, 38.55, 53.89, 57.91, 120.63, 127.97, 137.65, 148.23,
162.57.
2.3. X-ray Crystallography. In each case, a crystal of suitable size
was selected from the mother liquor and immersed in silicone oil, and it
was mounted on the tip of a glass fiber and cemented using epoxy resin.
The intensity data were collected using a Bruker SMART APEX-II CCD
diffractometer, equipped with a fine focus 1.75 kW sealed tube, with Mo
Kα radiation (λ = 0.710 73 Å) at 298(3) K, with increasing ω (width of
3. RESULTS AND DISCUSSION
For a receptor to bind with the anionic guests, it should in
principle possess preorganized anion binding elements deco-
rated on the suitable platform/framework. Receptor L possesses
a preorganized tripodal cleft with amide functionality suitable for
anion recognition/encapsulation. In addition, functionalization
of L with π-acidic dinitrophenyl moiety as aryl terminals
significantly enhances the binding ability of the receptor toward
anionic guests. It has been well established that the electron-
withdrawing substituents on the benzene ring assist the active
participation of the aryl -CH protons toward anion binding via
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Crystal Growth & Design
ARTICLE
Figure 1. (a) Structural representation depicting the H-bonding contacts on Clꢀ ions with two symmetry-independent receptor cations in complex 1,
and (b) structural representation showing the H-bonding interactions of Brꢀ with two symmetry identical receptor cations in complex 2. H-bonds have
been shown with blue dotted lines.
CꢀH anion interactions. Moreover, protonation at the apical
interactions is responsible for the binding of halides with two
adjacent receptor cations of identical or different symmetry
outside the tripodal cavity, whereas binding of oxoanions is
primarily governed by NꢀH O and CꢀH O interactions
3 3 3
nitrogen could considerably enhance the acidity of the methylene
-CH2 protons and, thereby, could possibly form moderate to
weak CꢀH anion hydrogen bonds perhaps of similar strength
3 3 3
3 3 3
3 3 3
to aryl CꢀH anion hydrogen bond. Thus, the attached
in association with a CꢀH O contact from the lattice DMSO
3 3 3
3 3 3
electron-withdrawing nitro substituents on the aryl terminals
and protonation at the bridgehead nitrogen in the ligand
architecture can presumably play a variable role in changing
the binding nature of L toward anions in its protonated and
neutral structure.
molecule. The detailed structural analyses of these complexes are
described as follows.
Binding of Clꢀ to [HL]+ in Complex 1. Complex 1 [(HL) Cl]
3
crystallizes in triclinic space group P1, and the asymmetric unit
contains two symmetry-independent receptor cations (Z0 = 2)
and two chloride anions. The two conformers of [HL]+ (con-
formers C1 and C2) differ considerably in their torsion angles
involving the amide functionality of each tripodal side arm
(Table S2 in the Supporting Information). The endo-oriented
proton of the apical amine is in strong intramolecular Nꢀ
3.1. Anion Binding with Protonated Receptor [HL]+.
Structural information obtained from single-crystal X-ray anal-
ysis of anion complexes 1ꢀ4 can provide insight into the proper
binding topology of halides and oxyanions with the protonated
receptor molecule and anion templated supramolecular assembly
formation. We have attempted to isolate the protonated salt of L
with anions of various geometries under identical crystallization
conditions in DMSO. However, we were able to isolate only four
complexes (1ꢀ4) as single crystals suitable for X-ray crystal-
lography analysis. Structural analyses revealed that complexation
of anions is primarily governed by NꢀH anion and CꢀH
H
O hydrogen bonding with one of the amide oxygens of
3 3 3
the receptor cation in both conformers C1 and C2 (N1 O11 =
3 3 3
2.718(2), —N1ꢀH O11 = 151(2)ꢀ; N11 O21 = 2.774(4);
3 3 3
3 3 3 ꢀ
—N11ꢀH O21 = 156(2)ꢀ). Binding of Cl with adjacent
3 3 3
receptor cations clearly reveals that both of the chloride ions
Clꢀ(1) and Clꢀ(2) are in interaction with two receptor cations
of dissimilar conformations with a five-point attachment each via
three NꢀH Clꢀ and two CꢀH Clꢀ interactions having an
3 3 3
3 3 3
anion interactions involving multiple receptor cations. In all
four complexes, the tripodal cavity of L is closed by strong
3 3 3
3 3 3
average donor-to-acceptor H-bond distance of 3.268 (N Clꢀ)
intramolecular NꢀH O hydrogen bonding of the endo-
3 3 3
3 3 3
oriented apical NꢀH proton with one of the amide oxygens of
and 3.589 Å (C
Clꢀ), respectively. It is evident from
3 3 3
the receptor side arms. In addition, the complexes are further
Figure 1a, that the amide hydrogen H8N and aryl proton H23 of
conformer C1 are involved in coordination with Clꢀ(1), whereas
the hydrogen H2N, H5N, and H5 are in interaction with Clꢀ(2).
In a similar fashion, the amide hydrogens H12N, H18N and aryl
proton H36 of the other conformer C2 provides a three-point
coordination to Clꢀ(1) while H15N and H41 make interactions
with Clꢀ(2) completing the five-point attachment on chloride
anions. The details of these H-bonding interactions are provided
in Table 2. The packing diagram of complex 1 as viewed down
the crystallographic b-axis shows the bilayer assembly formation
of the cationic receptor moieties along the a-axis with chloride
being entrapped between the adjacent bilayers (Supporting
Information). The receptor moieties are further organized via
stabilized by multiple weak intermolecular CꢀH O hydrogen
3 3 3
bonds, which induce rigidity in the formed cationic podand and
serve as the foundation for crystallization of the desired com-
plexes. In supramolecular chemistry, the existence of more than
one molecular conformer in the same crystal structure has been
described by the term conformational isomorphism, as exhibited by
the halide complexes 1 and 2. According to Desiraju, structures
which contain more than one molecule in the crystallographic
asymmetric unit (Z0 > 1) are a useful tool for close inspection of
the reaction coordinates of a supramolecular reaction and their
occurrence enlightens concepts such as kinetic and thermody-
namic crystal stability as they are considered to be consequences
of interrupted crystallization.21 A five-point coordination mostly
via amide NꢀH X and aryl CꢀH X (X = Clꢀ and Brꢀ)
several intermolecular CꢀH
Onitro interactions between
3 3 3
the alkyl/aryl hydrogen of tripodal side arms with oxygen
3 3 3
3 3 3
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Crystal Growth & Design
ARTICLE
Table 2. Characteristic Hydrogen Bonds with Anions Observed in Complexes 1ꢀ4
charged complex
DꢀH
A
d(H A)/Å
d(D A)/Å
—DꢀH A/deg
3 3 3
3 3 3
3 3 3
3 3 3
complex 1
N8ꢀH8N Cl1
2.53(3)
2.66(3)
2.28(3)
2.75(7)
2.79(7)
2.64(3)
2.42(4)
2.43(3)
2.70(7)
2.87(7)
2.69(4)
2.95(3)
2.65(3)
2.80(4)
2.98(5)
2.48(4)
2.72(4)
2.90(6)
2.98(5)
2.42(2)
2.01(4)
2.54(5)
2.38(5)
2.48(9)
2.69(1)
2.69(7)
2.63(2)
2.59(6)
2.43(6)
1.86(5)
2.48(6)
2.64(8)
2.49(6)
2.51(5)
3.228(2)
3.389(2)
3.136(2)
3.673(3)
3.671(3)
3.408(3)
3.197(3)
3.252(2)
3.630(3)
3.384(3)
3.375(4)
3.663(4)
3.463(4)
3.734(4)
3.386(4)
3.335(4)
3.437(4)
3.807(4)
3.449(4)
3.314(8)
2.952(7)
3.122(1)
3.183(1)
3.340(1)
3.495(1)
3.354(9)
3.450(2)
3.264(9)
3.077(9)
2.732(7)
3.318(8)
2.990(1)
3.361(7)
3.391(1)
154(2)
155(2)
153(2)
168(2)
157(2)
162(2)
158(3)
170(2)
173(2)
115(2)
155(4)
178(3)
165(3)
173(3)
108(2)
165(4)
167(4)
164(3)
112(3)
172(2)
171(4)
125(4)
153(4)
154(3)
140(3)
126(3)
144(7)
155(6)
149(6)
156(4)
143(3)
103(3)
155(3)
151(5)
3 3 3
N12ꢀH12N Cl1
3 3 3
N18ꢀH18N Cl1
3 3 3
C23ꢀH23 Cl1
3 3 3
C36ꢀH36 Cl1
3 3 3
N2ꢀH2N Cl2
3 3 3
N5ꢀH5N Cl2
3 3 3
N15ꢀH15N Cl2
3 3 3
C5ꢀH5 Cl2
3 3 3
C41ꢀH41 Cl2
3 3 3
complex 2
N12ꢀH12N Br1
3 3 3
N15ꢀH15N Br1
3 3 3
N18ꢀH18N Br1
3 3 3
C32ꢀH32 Br1
3 3 3
C41ꢀH41 Br1
3 3 3
N2ꢀH2N Br2
3 3 3
N5ꢀH5N Br2
3 3 3
C5ꢀH5 Br2
3 3 3
C18ꢀH18 Br2
3 3 3
O31ꢀH2O Br2
3 3 3
N8ꢀH8N O31
3 3 3
complex 3
complex 4
N2ꢀH2N O17
3 3 3
N2ꢀH2N O18
3 3 3
C5ꢀH5 O18
3 3 3
C1ꢀH1B O17
3 3 3
C2ꢀH2A O17
3 3 3
C28ꢀH28A O16
3 3 3
N2ꢀH2N O16
3 3 3
N2ꢀH2N O18
3 3 3
N5ꢀH5N O17
3 3 3
C1ꢀH1B O18
3 3 3
C9ꢀH9 O16
3 3 3
C23ꢀH23 O16
3 3 3
C28ꢀH28C O17
3 3 3
atoms from each nitro group (Table S1 in the Supporting
Information).
cations with conformation C2 provides a three-point coordination
to Brꢀ(1) whereas hydrogens H15N and H41 of another receptor
cation with identicalconformation provides the other two contacts
on Brꢀ(1). However, in the case of Brꢀ(2), two adjacent receptor
cations of conformation C1 provide a four-point contact via one
NꢀH Brꢀ and one CꢀH Brꢀ interactions each, while the
Binding of Brꢀ to [HL]+ in Complex 2. Complex 2
[(HL) Br]H2O, crystallizes in monoclinic space group
3
P21/c, and the asymmetric unit contains two symmetry-
independent receptor cations (Z0 = 2) and two bromide
anions with two lattice water molecules (O31 and O32) as
the solvent of crystallization. Identical to complex 1, intra-
3 3 3
3 3 3
fifth coordination contact is provided by a lattice water molecule
(O31) which is in strong H-bond interaction with the amide hy-
drogen H8N. The details of these H-bonding interactions are
provided in Table 2. The lattice diagram viewed normal to the ab-
plane shows the hexagonal channels formed by intermolecular
molecular NꢀH
O hydrogen bonding involving the
3 3 3
endo-oriented apical proton and one of the amide oxygen
is also prevalent in both of the conformers of complex 2
(N1 O6=2.903(4), —N1ꢀH O6=152(4)ꢀ;N11 O21 =
CꢀH Onitro interactions among conformers C1 are filled with
3 3 3
3 3 3
3 3 3
3 3 3
2.868(4), —N11ꢀH O21 = 147(3)ꢀ), and the conformers C1
Brꢀ ions in association with the lattice water molecules from one
end to the other endꢀof the crystals (Supporting Information).
Binding of ClO4 to [HL]+ in Complex 3. Complex 3
[(HL) ClO4]H2O DMSO crystallizes in triclinic space group
3 3 3
and C2 of receptor cation differ appreciably in their torsions
involving each tripodal side arm (Table S2 in the Supporting
Information). Binding of bromide with adjacent receptor cations
clearly shows that bromide ions Brꢀ(1) and Brꢀ(2) are in
interaction with two receptor units of identical symmetry with a
five-point attachment each (Figure 1b). The amide hydrogens
H12N, H18N and aryl proton H32 from one of the receptor
3
3
P1 with one disordered DMSO and a water molecule as the
solvent of crystallization. The solid-state structure of complex 3 shows
intramolecular NꢀH O hydrogen bonding between the apical
3 3 3
proton H1N with the amide oxygen O1 (N1 O1 = 2.742(5),
3 3 3
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Figure 2. (a) Structural representation depicts the H-bonding interactions of ClO4ꢀ anion and lattice DMSO with two protonated receptor units in
complex 3, and (b) coordination environment of ClO4ꢀ showing the formation of six H-bonds with two receptor cations and lattice DMSO molecule.
—N1ꢀH O1 = 168(3)ꢀ) and between amide hydrogen H5N
sulfate anion, in order to unambiguously determine the degree
of protonation and charge upon the anion. However, structural
elucidation reveals the 1:1 stoichiometric salt formation con-
firming the hydrogen sulfate complex of L. Similar to the halide
complexes, there exists intramolecular H-bonding between the
apical proton H1N and amide oxygen O6 of a receptor side arm
(N1 O6 = 2.816(5), —N1ꢀH O6 = 154(4)ꢀ). The
3 3 3
with O11 (N5 O11 = 2.984(4), —N5ꢀH O11 = 167(4)ꢀ)
3 3 3
3 3 3
of the other receptor arm restricting the size of the tripodal cavity
toward encapsulation of perchlorate ion.ꢀThe magnified view
of the H-bonding interactions on ClO4 (Figure 2) clearly
shows that each perchlorate anion is involved in a six-point
coordination provided by two adjacent receptor cations and
lattice DMSO. Perchlorate oxygen O17 is engaged in a tri-
furcated H-bonding contact with the amide hydrogen H2N
and two methylene protons H2A and H1B from the two
coordinating receptor units, whereas O18 is in bifurcated
interaction with the same amide hydrogen H2N and aryl
proton H5 of a receptor cation. The hexacoordination on
3 3 3
3 3 3
H-bonding contacts on HSO4ꢀ (Figure 3) clearly demonstrate
that each bisulfate anion is involved in a seven-point attachment
via NꢀH O and CꢀH O interactions provided by three
3 3 3
3 3 3
adjacent receptor cations and lattice DMSO. In nature, a seven-
coordinate sulfate structure was observed in a sulfate binding
protein, where three of four oxygen atoms were linked with two
hydrogen bonds. However in 4, sulfate oxygen O16 behaves as a
trifurcated H-bond acceptor by making interactions with the amide
hydrogen H2N and two aryl protons H9 and H23 from two
coordinating receptor units whereas O18 is involved in bifurcated
interaction with the amide hydrogen H2N and methylene proton
H1B of the same receptor cation. Finally, O17 interacts with the
amide hydrogen H5N of a third coordinating receptor cation and
methyl hydrogen H28C of lattice DMSO, completing the seventh
coordination contacts on HSO4ꢀ. The details of these H-bonding
interactions are provided in Table 2.
ClO4ꢀ is finally satisfied by the weak CꢀH
O interaction
3 3 3
between O16 and methyl hydrogen H28A of lattice DMSO.
The details of these H-bonding interactions are provided in
Table 2. The complex is further stabilized by several moderate
to weak H-bonds formed between the lattice DMSO with the
receptor cation and lattice water molecule (Table S1 in the
Supporting Information).
Binding of HSO4 to [HL]+ in Complex 4. Complex 4
ꢀ
[(HL) HSO4]DMSO crystallizes in monoclinic non-centro-
3
symmetric space group Cc with one disordered DMSO
molecule as the solvent of crystallization. Due to disorder, it
was not possible to locate the hydrogen for the monovalent
3.2. Binding of Fꢀ within Unimolecular Capsule of L in
Complex 5. The fluoride encapsulated neutral complex
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Crystal Growth & Design
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Figure 3. (a) Structural representation shows the H-bonding interactions of HSO4ꢀ with three protonated receptor units in complex 4, and (b) Cclose-up
view of the coordination environment of HSO4ꢀ showing the formation of seven H-bonds with three receptor cations and lattice DMSO molecule.
Figure 4. (a) Crystal structure of 5 showing encapsulation of Fꢀ within the tripodal cleft of L where blue dotted lines represent DꢀH Fꢀ
3 3 3
interactions (D = donor atoms, N or C), (b) spacefill representation depicting the formation of a Fꢀ encapsulated receptor capsule in 5, (c) self-assembly
of the unimolecular capsule into hexagonal channels when viewed down the a-axis, and (d) cyclohexane type of chair conformation adopted by the Fꢀ
encapsulated receptor units in the crystal of 5 (dinitrophenyl rings are omitted from the receptor skeleton for clarity of presentation).
[TBA(L F)] 2DMSO 4H2O, 5, crystallizes in triclinic space
bonds from the amide -NH and three aryl -CH protons of the
π-acidic receptor (Figure 4a,b). The encapsulated Fꢀ ion is
hydrogen-bonded to the amide protons with an average
3
3
3
group P1 with two disordered DMSO and four water molecules in
the crystal lattice. To gain a better insight into the nature of the
solvents included in the crystal lattice, we have carried out the
TGA of the crystals that shows a weight loss of 15.72% (ꢀ1.51
mg) for the solvent molecules close to the calculated value of
16.19% (Supporting Information). Structural analysis of complex
5 shows that the fluoride anion is completely encapsulated within
the tripodal cavity governed by six intramolecular hydrogen
N
Fꢀ distance of 2.708 Å, whereas the coordinating o-CH
3 3 3
protons interact with an average C Fꢀ distance of 2.993 Å,
3 3ꢀ3
demonstrating the strong binding of F with L in the solid state
(Table 3). Furthermore, the encapsulated Fꢀ interacts with one
of the π-acidic ringS (C3g) of L via weak Fꢀ π interactions with
3 3 3
a contact distance of 4.039 Å. A correlation of the DꢀH Fꢀ
3 3 3
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Crystal Growth & Design
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angle vs DH Fꢀ distance shows that all contacts are in the strong
dD ꢀ e 3.02 Å (Table 3). The active participation of the aryl -
F
3 3 3
3 3 3
hydrogen bonding interaction region with dH ꢀ e 2.35 Å and
CH protons toward Fꢀ binding is primarily due to the presence of
electron-withdrawing nitro functions in the aryl terminals of the
receptor which render these protons considerably acidic toward
F
3 3 3
Table 3. Characteristic Hydrogen Bonds with Fꢀ in Unim-
olecular Capsule 5
forming strong CꢀH Fꢀ hydrogen bonds. Additionally, each
3 3 3
arm of the Fꢀ encapsulated receptor unit is in interaction with the
DꢀH
F
d(H F)/Å
d(D F)/Å
—DꢀH F/deg
identical arm of an adjacent unit via intermolecular CꢀH O-
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
(nitro) hydrogen bonds involving the p-CH proton and one of the
nitro oxygen atoms of neighboring receptor molecules (Figure 5a).
A schematic diagram depicting the participation of aryl -CH protons
toward Fꢀ binding event and H-bonded synthon formation
has been shown in Figure 5b. Expansion through hydrogen
bonds resulted in hexagonal arrangement of Fꢀ encapsulated
L units with opposite orientation in a cyclohexane type of
N2ꢀH F1
1.85(2)
1.85(3)
1.90(2)
2.12(2)
2.13(3)
2.35(2)
2.703(5)
2.696(5)
2.726(3)
3.027(5)
3.025(5)
2.929(4)
170(3)
167(2)
158(3)
163(3)
160(2)
120(3)
3 3 3
N5ꢀH F1
3 3 3
N8ꢀH F1
3 3 3
C5ꢀH5 F1
3 3 3
C14ꢀH14 F1
3 3 3
C23ꢀH23 F1
3 3 3
Figure 5. (a) Structural representation showing the H-bonded synthon formation between the identical arms of adjacent receptor units involving the
p-aryl CH proton and nitro oxygen atoms in 5, (b) schematic representation depicting the involvement of the aryl CH protons toward Fꢀ (green)
binding and H-bonded synthon formation in 5, (c) honeycomb-like structure formed as a result of intermolecular short interactions between the Fꢀ
bound receptor units (TBA cations and solvent molecules are omitted for clarity), and (d) structural representation showing the H-bond formation
between lattice solvent molecules along crystallographic b-axis.
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Figure 6. Comparison of the partial 1H NMR spectra of free L and complexes 1 and 5a in DMSO-d6 at 298 K. The shift of the proton resonances have
been shown with orange dotted lines and identical color codes.
chair conformation, whereas the corresponding Fꢀ ions occupy the
vertices of a hexagon as depicted in Figure 4c,d, respectively. The
packing diagram of the complex as viewed down the crystal-
lographic a-axis shows formation of a honeycomb-like structure
from the extended ligand architecture (Figure 5c), entrapping
the tetrabutylammonium countercations and lattice solvents in
the crystal voids (Supporting Information). The tetrabutylam-
monium cations are held within the hexagonal voids diagonally
To further explore the mechanistic details involved in the
transformation of charged complexes into the unimolecular
capsule of L in the presence of excess fluoride ion, we have
performed 1H NMR titration of complexes 1 and 4 with aliquots
of standard (TBA)F solution in DMSO-d6 at 298 K. It has been
observed that, upon addition of 1 equiv of Fꢀ to a solution of 1 or
4, the aliphatic -CH2 proton signals undergo a considerable
upfield shift with Δδ (NꢀCH2) 0.85 and 0.87 ppm for 1 and 4,
respectively, and noticeable upfield shift of the amide -NH signal
has also been observed which indeed indicates the formation of
the charge neutral receptor L in solution assisted by Fꢀ ions
(Supporting Information). In addition, the signal for the proton
at the apical nitrogen (protonated) which resonates at 10.52 and
9.80 ppm in 1 and 4, respectively, disappears upon addition of
Fꢀ, further confirming the occurrence of the neutral form of L.
With a second equivalent of Fꢀ addition, the amide -NH signal
disappears whereas the signal for aryl o-CH proton gets broa-
dened and experiences a slight downfield shift, which could be
due to binding-induced broadening of signals upon recognition
of Fꢀ. Further addition of Fꢀ ions (up to 5 equiv) results in
gradual and appreciable downfield shift of o-CH proton reso-
nance (Δδ = 0.68 ppm in the case of 1 and Δδ = 0.70 ppm in the
case of 4) with reappearance of the -NH signal at 4 equiv of Fꢀ
(Δδ = 2.95 ppm in the case of 1 and Δδ = 2.84 ppm in the case
of 4), indicative of a structural alteration of the receptor unit in
complexes 1 and 4 in the presence of excess Fꢀ, which could
influence both the -NH and -CH protons for its encapsulation in
solution as well (Supporting Information).
along the bc-plane involving four CꢀH
O and one Cꢀ
3 3 3
H
π interactions (Table S1) that presumably provide addi-
3 3 3
tional stability to the capsular assembly formation whereas the
H-bonded lattice solvents (Figure 5d) are sandwiched between
the tetrabutylammonium cations within the voids that run
from one end to the other end of the crystal along the a-axis
(Supporting Information).
3.3. 1H NMR Study. The free receptor molecule (L) shows the
amide -NH resonance at δ 9.15 ppm whereas the aromatic -CH
protons resonate at 8.91 (s, o-CH) and 8.88 (s, p-CH) ppm when
recorded in DMSO-d6 at 298 K. Appreciable downfield shift of
the aliphatic -CH2 protons (Δδ (ppm; NꢀCH2): 0.77 in 1, 0.80
in 2, 0.81 in 3, and 0.79 in 4 wrt L) in the 1H NMR spectra of the
charged complexes 1ꢀ4 indicate the influence of protonation at
the apical nitrogen on the neighboring methylene protons.
Furthermore, observable downfield shift of the amide -NH
protons (Δδ (ppm): 0.51 in 1, 0.42 in 2, 0.37 in 3, and 0.43 in
4 wrt L) indicate the active interactions of anions with the amide
functions as established in the solid state (Supporting In-
formation). However, no notable changes in chemical shift values
of the aryl -CH resonances are observed, suggesting weak
interactions of anions with the aryl -CH protons of the receptor
1
4. CONCLUSION
cation in solution. In contrast, the H NMR spectra of the
isolated crystals of 5 show a significant downfield shift of the
amide -NH and aryl o-CH resonances with high Δδ values of
∼3.54 and ∼0.73 ppm, respectively, indicative of a strong
In summary, we have shown the structural insights of co-
ordination of halides and oxyanions (tetrahedral) with the
protonated form of the π-acidic tris(amide) receptor (L), and
transformation of these charged complexes into a unimolecular
capsule of L has been accomplished in the presence of excess
fluoride ion. 1H NMR titration experiments reveal that fluoride
ion encourages deprotonation of the apical nitrogen in com-
plexes 1 and 4 with 1 equiv of (TBA)F, followed by formation of
the capsular assembly with excess addition of fluoride, which is in
solution state binding of Fꢀ with L via amide NꢀH Fꢀ and
3 3 3
o-CꢀH Fꢀ interactions as observed in the solid state
3 3 3
(Figure 6), whereas only a slight upfield shift (Δδ = 0.09 ppm)
of the aryl p-CH protons were observed which could be the result
of p-CꢀH O(nitro) H-bonded synthon formation in solution
3 3 3
as well.
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Crystal Growth & Design
ARTICLE
agreement with the results observed in the crystal structure of 5.
Structural elucidation of the charged complexes clearly demon-
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3 3 3
and CꢀH anion interactions involving multiple receptor
3 3 3
cations wherein the tripodal cavity gets locked by intramolecular
NꢀH
O(amide) hydrogen bonding between the endo-
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3 3 3
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic files in CIF
b
format, additional crystallographic data, characterization data,
and 1H NMR titration spectra. This information is available free
of charge via the Internet at http://pubs.acs.org/.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: gdas@iitg.ernet.in.
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’ ACKNOWLEDGMENT
G.D. gratefully acknowledges DST (Grant SR/S1/IC-01/
2008) and CSIR (Grant 01-2235/08/EMR-II), New Delhi, India,
for the financial support, DST-FIST for the single-crystal X-ray
diffraction facility, and Central Instrument Facility at IIT Guwahati.
S.K.D. acknowledges IIT Guwahati, India, for a senior research
fellowship.
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