A series of amino acid functionalized tripodal hexaamide anion receptors: Ion-pair-assisted capped-cleft formation by a pentafluorophenyl-functionalized amide

Article abstract of DOI:10.1002/asia.201200428

A new series of tris(2-aminoethyl)amine (tren)-based L-alanine amino acid backboned tripodal hexaamide receptors (L1-L5) with various attached moieties based on electron-withdrawing fluoro groups and lipophilicity have been synthesized and characterized.

Full text of DOI:10.1002/asia.201200428

DOI: 10.1002/asia.201200428
A Series of Amino Acid Functionalized Tripodal Hexaamide Anion
Receptors: Ion-Pair-Assisted Capped-Cleft Formation by a
Pentafluorophenyl-Functionalized Amide
Purnandhu Bose, Iyyamperumal Ravikumar, Bidyut Akhuli, and Pradyut Ghosh*[a]
Abstract: A new series of tris(2-amino-
ethyl)amine (tren)-based l-alanine
amino acid backboned tripodal hexaa-
mide receptors (L1–L5) with various
attached moieties based on electron-
withdrawing fluoro groups and lipophi-
licity have been synthesized and char-
acterized. Detailed binding studies of
L1–L5 with different anions, such as
halides (FÀ, ClÀ, BrÀ, and IÀ) and oxy-
anions (AcOÀ, BzOÀ (Bz=benzoyl),
owing to weak interactions between
these anions and receptors; this is fur-
amide receptor (L1) encapsulated ClÀ
in its cavity by hydrogen bonds from
amides, and the cavity of L1 was
capped with a TBA cation through hy-
drogen bonding and ion-pair interac-
tions to form a capped-cleft orienta-
tion. To understand the role of the cat-
ionic counterpart in solution-state ClÀ
binding processes with this series of re-
ceptors (L1–L4), a detailed ClÀ binding
study was carried out with three differ-
1
ther confirmed by H NMR spectrosco-
py. The ITC binding studies of FÀ and
À
H2PO4 do not fit well for a 1:1 binding
model. Furthermore, ITC binding stud-
ies also revealed slightly higher selec-
tivity of this series of receptors towards
AcOÀ over ClÀ, BzOÀ, and HSO4 .
À
Solid-state structural evidence for the
recognition of ClÀ by this new category
of receptor was confirmed by single-
crystal X-ray structural analysis of the
complex of tetrabutylammonium chlo-
ride (TBACl) and L1. Single-crystal X-
ray diffraction clearly showed that the
pentafluorophenyl-functionalized
À
À
À
NO3 , H2PO4 , and HSO4 ), have been
carried out by isothermal titration calo-
rimetric (ITC) experiments in acetoni-
trile/dimethylsulfoxide (99.5:0.5 v/v) at
298 K. ITC titration experiments have
clearly shown that receptors L1–L4 in-
variably form 1:1 complexes with ClÀ,
ent
tetraalkylammonium
(Me4N+,
Et4N+, and Bu4N+) salts of ClÀ. The
binding affinities of these receptors
with different tetralkylammonium salts
of ClÀ gave binding constants with the
TBA cation in the following order:
butyl>ethyl>methyl. This study fur-
ther supports the role of the TBA
countercation in ion-pair recognition
by this series of receptors.
À
AcOÀ, BzOÀ, and HSO4 , whereas L5
Keywords: host–guest systems · ion
pairs · receptors · thermodynamics ·
X-ray diffraction
forms a 1:1 complex only with AcOÀ.
À
In the case of BrÀ, IÀ, and NO3 , no ap-
preciable heat change is observed
Introduction
ation. In particular, the active site of the narrowest pore
region of the ClÀ ion channel consists of an amino acid con-
taining backbone, in which ClÀ interacts through hydrogen
In recent years, one of the most interesting topics in host–
guest chemistry is the design and synthesis of receptors able
to recognize anions and ion pairs efficiently and selective-
ly.[1] Selective anions and ion-pair receptors are important
because of their decisive role in biological, environmental,
and industrial applications.[2–32] Specifically, ClÀ plays a pivo-
tal role in biological processes, for example, in signal trans-
duction, pH regulation, transportation of organic solutes
through the cell membrane, cell migration, and cell prolifer-
À
bonds to two amide NH protons of a phenylalanine residue
and two hydroxy groups of serine and tyrosine residues in
the protein chains.[33] During our search for a new type of
synthetic receptor of biological relevance, we have chosen
an amino acid as the backbone to design a tris(2-aminoe-
thyl)amine (tren)-based tripodal amide[5a,23–25] scaffold deco-
rated with terminal aromatic moieties (Scheme 1). Herein,
we have designed and synthesized a new series of tren-
based tripodal hexaamide receptors with l-alanine amino
acid backbones functionalized with pentafluorophenyl (L1),
tetrafluorophenyl (L2), trifluorophenyl (L3), 3,5-bis(trifluor-
omethyl)phenyl (L4), and phenyl (L5) terminal groups for
anion binding and selectivity studies. We have also structur-
ally demonstrated the formation of a capped-cleft arrange-
ment by receptor L1 through ion-pair recognition, in which
the receptor and tetrabutylammonium (TBA) group are in
contact through hydrogen-bonding interactions and ClÀ is
buried in the cavity of the ternary complex. To the best of
our knowledge, this represents a new example of a tripodal
[a] P. Bose, Dr. I. Ravikumar,+ B. Akhuli, Prof. P. Ghosh
Department of Inorganic Chemistry
Indian Association for the Cultivation of Science
2A & 2B Raja S. C. Mullick Road, Kolkata 700 032 (India)
E-mail: icpg@iacs.res.in
[+] Current address: Department of Chemistry and Biochemistry
The University of Texas at Austin,
A5300, Austin, TX, 78712-0165 (USA)
1
University Station
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200428.
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FULL PAPER
Anion Binding and Selectivity Studies
Attempts have been made to study the binding properties of
L1 with various anions in the solution state by 1H NMR
spectroscopy experiments in [D6]DMSO (DMSO=dime-
thylsulfoxide) at 298 K. Qualitative 1H NMR spectroscopy
À
studies show that there is a shift in the signal for the NH
protons of L1 with FÀ, AcOÀ, and H2PO4 , whereas other
À
À
À
anions ClÀ, BrÀ, IÀ, NO3 , and HSO4 do not show any such
appreciable shift in [D6]DMSO (see Figures S30–S38 in the
Supporting Information). On the other hand, titration
À
curves of FÀ, AcOÀ, and H2PO4 are inconclusive because
they do not show the saturation point of the chemical shift
À
of the NH protons, even after the addition of several
equivalents of the anions with respect to L1 (see Figure S67
in the Supporting Information). Thus, we have undertaken
isothermal titration calorimetry (ITC) experiments in aceto-
nitrile (ACN)/DMSO (99.5:0.5 v/v) to evaluate the detailed
thermodynamic and kinetic signatures of different anions
with this receptor. Our choice of solvent system is based on
the fact that L1 is insoluble in ACN and negligible heat
changes for L1 are observed with various anions in DMSO.
The detailed energetic signature of receptors L1–L5 with
À
various anions (FÀ, ClÀ, BrÀ, IÀ, AcOÀ, BzOÀ, NO3 ,
À
À
H2PO4 , and HSO4 ) is explored by using ITC experiments
in ACN/DMSO (99.5:0.5 v/v) at 298 K (Figure 1 and Figur-
es S40–S66 in the Supporting Information). For BrÀ, IÀ, and
Scheme 1. Schematic representation of receptors L1–L5. Reagents and
conditions: a) DCC, HOBT, THF; 0–58C, 3 h; RT, 3 days; b) TFA,
DCM; RT, 12 h; c) RCOCl, NEt3, DCM; 0–58C, 1 h; RT, 24 h.
À
NO3 , no appreciable heat change is observed during ITC
experiments, that is, these anions do not show 1:1 complexa-
tion in the solution state (see Figure S40 in the Supporting
Information). The 1H NMR spectroscopy experiments for
hexaamide receptor with an amino acid backbone capable
of recognizing an ion pair through the capping of a TBA
À
countercation over
a
chloride-encapusulated hexaamide
L1 in the presence of excess BrÀ/IÀ/NO3 do not show any
À
complex, as evidenced by single-crystal X-ray crystallogra-
phy. Furthermore, we have also demonstrated the effect of
the tetraalkylammonium countercation in anion-binding
processes.
shift of amide NH protons with respect to L1; this clearly
supports the nonbinding nature of these anions towards L1
in ITC experiments (see Figures S32, S33, and S36 in the
Supporting Information). The ITC profile of FÀ binding
with L1 does not show a good fit for the 1:1 binding model
(see Figure S40 in the Supporting Information); this is prob-
À
Results and Discussion
ably as a result of deprotonation of amide NH protons in
the presence of FÀ, which is supported by the disappearance
Syntheses
À
of the signal for L1 amide NH protons in the presence of
1
Receptors L1–L5 were synthesized in good yield following
a three-step synthetic process (Scheme 1). To develop tripo-
dal anion receptors with biologically relevant components
and higher flexibility, the l-alanine moiety was chosen and
incorporated in the design of a series of tren-based hexaa-
mide receptors. To investigate the anion-binding properties
of these tripodal receptors with different types of spherical
anions (FÀ, ClÀ, BrÀ and IÀ), planar oxyanions (AcOÀ, BzOÀ
excess FÀ in the H NMR spectroscopy experiment (see Fig-
À
ure S30 in the Supporting Information). H2PO4 also does
not exhibit a good fit to the 1:1 binding model in ITC ex-
periments, although we observed a shift in the amide NH
protons in the H NMR spectroscopy experiment (see Fig-
À
1
ure S38 in the Supporting Information). The upper panels of
the ITC titration profiles for L1 with the ClÀ, AcOÀ, BzOÀ
À
and HSO4 in Figure 1 show the heat pulses experimentally
À
(Bz=benzoyl), and NO3 ), and tetrahedral oxyanions
observed in each titration step. The lower panels report the
respective time integrals, which translates to the heat ab-
sorbed for each aliquot and its coherence to a 1:1 binding
model. The ITC titration profiles of receptor L1 with the in-
teracting anions indicates 1:1 binding (n=1.01 to 1.12) of
the host–guest complexes (Table 1). The binding of ClÀ is in-
fluenced by both enthalpic and entropic contributions; this
is evident from the comparable values of these two thermo-
À
À
(H2PO4 and HSO4 ), the tetralkylammonium salts were
used. Several attempts have been made to obtain single
crystals of complexes, but we were only able to isolate single
crystals for L1 with TBACl. Complex 1 [L1ACHTNUTRGENNUG HCATUNGTRENNGUN
(ClÀ)(Bu4N+)]
was isolated in moderate yield. Detailed crystallographic in-
formation for complex 1 is given in Table S1 in the Support-
ing Information.
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Tripodal Hexaamide Anion Receptors
Figure 1. ITC titration profiles in ACN/DMSO (99.5:0.5 v/v) at 298 K of a) TBACl (2.82 mm); b) TBAOAc (2.26 mm); c) TBAOBz (2.21 mm), and
d) TBAHSO4 (2.20 mm) added to a 0.10 mm solution of host L1.
anion binding, it can be clearly stated that AcOÀ
Table 1. ITC titrations of various TBA anion salts with L1–L5 carried out in ACN/
DMSO (99.5:0.5 v/v) at 298 K.
has the highest selectivity of the anions investigat-
Host Guest
n
TDS [kcalmolÀ1
1.12 +3.24
TBAOAc 1.03 +0.54
TBAOBz
1.03 À1.26
TBAHSO4 1.01 À12.22
TBACl 1.18 +2.61
TBAOAc 1.17 +1.26
TBAOBz 1.13 +0.83
TBAHSO4 1.00 À8.43
TBACl 0.94 +1.97
TBAOAc 1.11 +1.45
TBAOBz 1.01 +0.09
TBAHSO4 1.13 À7.84
TBACl 1.04 +1.87
TBAOAc 1.01 À1.38
TBAOBz
]
DH [kcalmolÀ1
]
DG [kcalmolÀ1
]
log Ka
ed. The anion binding order for receptor L1 can be
written as AcOÀ >BzOÀ >HSO4À ꢀClÀ. To general-
ize this new series of tripodal hexaamide anion re-
ceptors with amino acid backbones, we synthesized
five receptors (L1–L5) with various aryl group at-
tachments with a variety of electron withdrawing
and lipophilic natures. Table 1 clearly shows that
the binding processes for AcOÀ and ClÀ with recep-
tors L2–L5 are entropy and enthalpy driven; this is
evident from the positive entropy and negative en-
thalpy values, similar to those observed for L1. Fa-
vorable entropy-driven association probably results
from their willingness to enter and fit into the host
cavity with suitable hydrogen-bonding interactions.
On the other hand, the binding of BzOÀ is almost
entirely enthalpy driven throughout the series of re-
ceptors; no appreciable contributions come from
the entropy value. This phenomenon can be attrib-
L1
L2
L3
L4
L5
TBACl
À2.30
À5.70
À7.21
À17.91
À2.78
À4.99
À4.63
À12.96
À4.12
À4.77
À5.28
À13.62
À4.00
À6.96
À5.37
À15.52
À4.39
À5.55
À6.24
À5.95
À5.69
À5.50
À6.25
À5.46
À4.53
À6.09
À6.22
À5.19
À5.78
À5.87
À5.58
À5.23
À4.98
À5.18
4.05
4.57
4.36
4.16
3.95
4.58
4.00
3.32
4.22
4.56
3.94
4.23
4.09
4.08
3.83
3.63
3.79
1.00 À0.14
TBAHSO4 0.94 À10.54
TBAOAc 1.05 +0.79
dynamic parameters, that is, the ClÀ complex is formed with
a DH contribution of about 41% of the interaction energy.
In combination with a comparable entropic term, this gives
rise to a favorable (negative) DG and a log Ka value of 4.05
in this solvent system. Interestingly, the interaction of L1
with AcOÀ is almost driven by enthalpy. The AcOÀ complex
is formed with a DH contribution of >91% to the overall
interaction energy. It is also observed that, for the combina-
tion of entropy and enthalpy, this gives rise to the highest fa-
vorable DG value of all investigated anions. The highest
binding constant value, log Ka of 4.57 for the AcOÀ–L1
adduct also provides justification for the highest negative
uted to the larger size of BzOÀ relative to that of AcOÀ and
ClÀ, which restricts the guest from entering or fitting into
À
the tripodal receptor cavity. For HSO4 , high negative en-
thalpy values for the receptors in the series easily compen-
sate for the moderate unfavorable negative entropy values
in the binding processes; this probably arises from the larger
À
size and higher solvation of HSO4 under the experimental
conditions. For receptor L5, we were only able to evaluate
the binding constant with AcOÀ (Table 1). All of the anions
under investigation in our experimental conditions were
unable to produce a one-site binding model with L5, except
À
À
for AcO . The amide NH in L5 was the least acidic of the
À
DG value (Table 1). For BzOÀ and HSO4 , the energetic sig-
receptors in the series. Thus, in general, L5 should show
minimum interactions with the anions. For AcOÀ, the overall
shape, size, and basicity of the ion might play a role in 1:1
complex formation with L5 under our experimental condi-
tions. It has been shown that, for low binding constants (<
natures of these interactions are characterized by high exo-
thermicities (large negative DH values) with opposing entro-
pies (negative TDS values). After considering all of the ther-
modynamic and kinetic parameters for receptor L1 towards
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Pradyut Ghosh et al.
FULL PAPER
102), ITC does not provide any reliable values for 1:1 bind-
ing; this is probably the case for L5.[32] Detailed analysis of
the results given in Table 1 shows that all of the receptors
(L1–L5) have slightly higher selectivity toward AcOÀ than
the other investigated anions, although for receptor L4,
AcOÀ and ClÀ have comparable binding constant values.
Figure S41 in the Supporting Information shows ITC titra-
tion profiles for AcOÀ with receptors L1–L5. The ITC titra-
tion profiles of receptors L1–L5 with AcOÀ show a good fit
for the 1:1 binding model with stoichiometry (n) ranging
from 1.03 to 1.17 for the host–guest complex (Table 1 and
Figure S41 in the Supporting Information).
Figure 2. a) Ball and stick model depicting ion-pair recognition in com-
Effect of Countercations on Anion Recognition and Ion-
Pair-Assisted Capped-Cleft Formation
plex 1 (non-acidic hydrogen atoms are omitted for clarity).
After several attempts to obtain solid-state evidence of
anion complexes with the receptors, we were able to isolate
a single crystal for L1 with TBACl. Complex 1 crystallized
in a monoclinic crystal system with the P2(1) space group.
The asymmetric unit of 1 has a distorted C3v symmetric
cavity, which is evident from the difference in basal N···N
distances (N2···N4=4.188, N4···N6=4.516, and N6···N2=
Figure 3. Space-filling model depicting a) the ClÀ complex of tren-based
pentafluoroammonium salt; b) the ClÀ complex of tren-based pentafluor-
oamide; c) ClÀ–L1 recognition in complex 1 (the TBA group is omitted
to show the space available after chloride recognition); and d) a space-
filling model of 1.
4.208 ꢁ). Single crystals of complex 1 as [L1ACTHNUGRTNENUG ACHUTNGTRENNUGN
(ClÀ)(Bu4N+)]
were grown from the ACN/diisopropyl ether binary solvent
system in moderate yield upon very slow evaporation of a so-
lution of L1 in the presence of TBACl. Along with receptor
L1, the asymmetric unit of complex 1 contains one encapsu-
lated ClÀ and one TBA countercation to balance the overall
charge. Evidence for the recognition of TBACl by L1 as an
ion pair was obtained from single-crystal X-ray structural
analysis of 1. In the solid-state structure of 1, receptor L1
completely engulfed ClÀ in its three extended arms through
L5 have extended amino acid based arms, and thus, in case
of L1, upon recognition of ClÀ in complex 1, there is enough
space to accommodate another guest (Figure 3c). In fact, in
complex 1 this space is occupied by the TBA countercation,
which directly interacts with L1 through multiple hydrogen-
bonding interactions (Figure 3d). Thus, this category of tri-
podal amide receptor recognizes an ion pair and results in
a pseudo-closed assembly of a ternary complex formed by
L1 and TBACl. Similar tetramethylammonium (TMA)
versus TBA ion-pair recognition has been demonstrated by
uranyl–salophen-based ditopic receptors. Ion-pair recogni-
tion of TMA salts by halogenated resorcinarenes have also
been reported very recently.[34] Solid-state X-ray structural
evidence of TBA-capped ClÀ encapsulation led us to investi-
gate the effect of the countercation in the capping process
for ClÀ binding with receptors L1–L4. Detailed ClÀ binding
studies of the receptors with different tetraalkylammonium
salts (TBA, tetraethylammonium (TEA), TMA) were per-
formed by ITC. From these studies it becomes clear that, in
the above-mentioned solvent system, the values of kinetic
and thermodynamic parameters obtained by ITC experi-
ments depend on the size of the alkyl group of the counter-
cation (Table 2). The ClÀ binding constant, log Ka, for L1
was 4.05 (Figure 4 and Table 2) when TBA was used as the
countercation. Surprisingly, when the alkyl group is methyl
(i.e., TMACl) rather than butyl, the ClÀ binding affinity is
reduced to a value of 3.59 (Figure 4), whereas for TEACl,
which is a moderate volume countercation, the ClÀ binding
À
three strong N H···Cl hydrogen-bonding interactions; two
À
À
from tren amide NH protons and one from the NH
proton of the amide linkage of the l-alanine moiety
(Figure 2 and Table S1 in the Supporting Information). In-
terestingly, the ClÀ-encapsulated expanded tripodal receptor
L1 has adequate space to interact with the TBA countercat-
ion, thus resulting in partial engulfment of the respective
À
À
cation through two strong C H···O interactions and one C
À
H···F interaction with the acidic CH2 group adjacent to the
cationic bridgehead nitrogen (Figure 2 and Table S1 in the
Supporting Information). The distance measured between
the bridgehead nitrogen atoms of L1 and the TBA counter-
cation is 8.140 ꢁ. This hydrogen-bonded attachment of the
TBA group to the extended arms of L1 makes a close mo-
lecular arrangement that can be regarded as a capped-cleft
obtained by ion-pair interactions, in which the bridgehead
nitrogen of receptor L1 and the quaternary nitrogen of the
TBA cation form two terminals of the ternary complex. It is
important to note that our previous studies on tren-based
pentafluorophenyl-substituted
tripodal
amine[22]
and
amide[23] systems show that the ClÀ guest is encapsulated
inside the C3v symmetric cavity of the respective receptor
without any interactions with the countercation (Figure 3a
and b). The present design of tripodal amide receptors L1–
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Tripodal Hexaamide Anion Receptors
Figure 4. ITC titration profiles in ACN/DMSO (99.5:0.5 v/v) at 298 K of a) TBACl (2.82 mm), b) TEACl (2.32 mm), and c) TMACl (3.71 mm) added to
a 0.10 mm solution of host L1.
Table 2. ITC titrations of various tetraalkylammonium chloride salts with L1–L4 car-
ried out in ACN/DMSO (99.5:0.5 v/v) at 298 K.
through ITC experiments. From the solution-state
binding of the receptors, it was found that AcOÀ
Host Guest
n
TDS [kcalmolÀ1
]
DH [kcalmolÀ1
]
DG [kcalmolÀ1
]
log Ka
had the highest selectivity of the anions investigat-
ed. The anion selectivity was systematically justified
from the thermodynamic and kinetic parameters.
Single-crystal X-ray structural evidence showed that
one of the receptors could bind an ion pair in its
cavity thanks to the extended tripodal arms, which
result in the formation of a capped-cleft-like molec-
ular assembly. We have also shown the effect of the
countercation on anion binding by using a solution-
state study. ITC studies indicated that TBA would
be a better choice of cap to favor anion-recognition
processes.
L1
L2
L3
L4
TBACl 1.12 +3.24
TEACl 1.00 +1.75
TMACl 1.00 +1.16
TBACl 1.18 +2.61
TEACl 0.90 +1.33
TMACl 0.95 +1.22
TBACl 0.94 +1.97
TEACl 0.74 À0.78
TMACl 0.73 À0.08
TBACl 1.04 +1.87
TEACl 1.18 +2.15
TMACl 1.05 +1.35
À2.30
À3.67
À3.75
À2.78
À4.04
À3.92
À4.12
À6.31
À5.56
À4.00
À3.36
À4.10
À5.55
À5.42
À4.91
À5.50
À5.37
À5.14
À6.09
À5.53
À5.48
À5.87
À5.51
À5.45
4.05
3.97
3.59
3.95
3.94
3.76
4.22
4.05
4.02
4.09
4.04
3.99
affinity of receptor L1 lies between these values (3.97). Fur-
ther evidence was obtained by solid-state single-crystal X-
ray structural investigations into complex 1, for which both
ClÀ and the TBA countercation were strongly hydrogen
bonded to receptor L1. Therefore, the effect of the counter-
cation in the capping process follows the order TBAꢀ
TEA>TMA; this is evident from the decreasing trends in
free energy change (DG) for this particular process
(Table 2). The other receptors (L2–L4) follow almost the
same binding trend as that observed for L1.
Experimental Section
Materials and Methods
Boc-Ala-OH (Boc=tert-butyloxycarbonyl); tren; pentafluorobenzoyl
chloride; 2,3,4,5-tetrafluorobenzoyl chloride; 3,4,5-trifluorobenzoyl chlo-
ride; 3,5-bis(trifluoromethyl)benzoyl chloride; benzoyl chloride; and
À
TBA, TEA, and TMA salts of FÀ, ClÀ, BrÀ, IÀ, AcOÀ, BzOÀ, NO3
,
H2PO4À, and HSO4 were purchased and used without further purifica-
tions. N,N’-Dicyclohexylcarbodiimide (DCC), 1-hydroxybenztriazole
(HOBT), and trifluoroacetic acid (TFA) were purchased from Sigma–Al-
drich and Cyno-chem, India. All NMR spectra were recorded on 300 and
500 MHz FT-NMR spectrometers in CDCl3, [D7]DMF (DMF=N,N-di-
methylformamide), and [D6]DMSO at 258C. Chemical shifts for 1H and
13C NMR spectra were reported in ppm, calibrated to the residual solvent
peak set, with coupling constants reported in Hz.
À
Conclusion
Synthesis of Compound I
We have designed and synthesized a new series of hexaa-
mide tripodal receptors with amino acid backbones and de-
termined their solution-state anion-binding capabilities
Boc-Ala-OH (2.0 g, 10.57 mmol) was added to a 250 mL two-necked
round-bottomed flask, dissolved in dry THF (50 mL), and cooled to 08C
in an ice bath. DCC (2.18 g, 10.57 mmol) was added to the reaction mix-
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FULL PAPER
ture. The reaction mixture was allowed to stir at 08C for 15 min. Then,
HOBT (1.65 g, 10.57 mmol) was added to the stirring mixture. The sus-
pension formed was stirred for another 30 min at same temperature.
Thereafter, tren (0.53 mL, 3.54 mmol) in dry THF (50 mL) was added to
a pressure-equalizing funnel and added slowly over a period of 1–2 h to
the reaction mixture at 08C. The reaction mixture was allowed to stir at
0–58C for 3 h and at RT for 3 days under nitrogen atmosphere. The pre-
cipitate formed was filtered off and washed with cold THF (5ꢂ5 mL).
The filtrate was evaporated to dryness under reduced pressure. The
crude semisolid product obtained was dissolved in CH2Cl2 (100 mL). The
organic layer was washed with water (3ꢂ50 mL), 5% NaHCO3 (1ꢂ
100 mL), and brine solution (2ꢂ50 mL). The organic layer was collected
and dried over anhydrous sodium sulfate. The solvent was removed
under vacuum to yield a crude off-white solid. Compound I was isolated
as a white solid (1.75 g, 75%) by column chromatography (60–120 mesh
Ligand L2
1
Brown solid; yield: 72%; H NMR (500 MHz, [D7]DMF): d=1.59 (d, J=
7.5 Hz, 9H), 2.70–2.75 (m, 6H), 3.34–3.42 (m, 6H), 4.75–4.78 (m, 3H),
8.04–8.09 (m, 3H), 8.12–8.14 (t, J=5.5 Hz, 3H), 8.93 ppm (d, J=7 Hz,
1
3H); H NMR (500 MHz, [D6]DMSO): d=1.29 (d, J=6.5 Hz, 9H), 3.11–
3.17 (m, 6H), 4.42–4.45 (m, 3H), 7.59–7.62 (m, 3H), 7.92 (br, 3H),
8.59 ppm (br, 3H); 13C NMR (125 MHz, [D6]DMSO): d=18.7, 37.7, 49.7,
53.8, 112.4, 139.5, 142.4, 144.6, 145.5, 146.6, 147.5, 161.0, 172.1 ppm;
19F NMR (500 MHz, [D6]DMSO): d=À157.93, À155.50, À141.70 ppm;
HRMS (ESI+): m/z calcd for C36H33F12N7O6: 888.2301 [M+]; found:
888.3749; elemental analysis calcd (%) for C36H33F12N7O6: C 48.71, H
3.75, N 11.05; found: C 48.70, H 3.72, N 11.01.
Ligand L3
1
silica gel: methanol/CHCl3 (0.03:1 v/v)). H NMR (300 MHz, CDCl3): d=
White solid, yield: 75%; 1H NMR (500 MHz, [D7]DMF): d=1.59 (d, J=
7.0 Hz, 9H), 2.76 (s, 6H), 3.07–3.09 (m, 6H), 4.77–4.80 (m, 3H), 7.82–
7.86 (m, 3H), 8.22 (s, 6H), 8.69 ppm (d, J=5.5 Hz, 3H); 1H NMR
(500 MHz, [D6]DMSO): d=1.31 (d, J=7.0 Hz, 9H), 3.10 (s, 6H), 4.39–
4.422 (m, 3H), 7.80–7.86 (m, 9H), 8.68 ppm (s, 3H); 13C NMR (125 MHz,
[D6]DMSO): d=17.7, 37.1, 49.4, 53.2, 112.6, 130.3, 139.7, 148.9, 150.9,
162.8, 171.9 ppm; 19F NMR (500 MHz, [D6]DMSO): d=À159.62,
À136.83 ppm; HRMS (ESI+): m/z calcd for C36H36F9N7O6Na: 856.2481
[M++Na]; found: 856.0809; elemental analysis calcd (%) for
C36H36F9N7O6: C 51.86, H 4.35, N 11.76; found: C 51.85, H 4.28, N 11.73.
1.41 (s, 27H), 2.55 (t, 6H), 2.88–2.94 (m, 3H), 3.35–3.41 (m, 3H), 4.31 (s,
3H), 5.63 (d, 3H), 7.69 ppm (br, 3H); 13C NMR (75 MHz, CDCl3): d=
15.3, 18.8, 28.4, 38.7, 50.1, 54.8, 79.8, 156.0, 174.2 ppm; HRMS (ESI+): m/
z calcd for C30H57N7O9: 659.4218 [M+]; found: 660.2113; elemental analy-
sis calcd (%) for C30H57N7O9: C 54.6, H 8.71, N 14.86; found: C 54.52, H
8.65, N 14.80.
Synthesis of Compound II
Compound I (500 mg, 0.76 mmol) was dissolved in dry CH2Cl2 (30 mL)
in a 100 mL round-bottomed flask and allowed to stir at room tempera-
ture under a nitrogen atmosphere for 30 min. Trifluoroacetic acid (TFA;
ꢀ5 mL) was dissolved in dry CH2Cl2 (20 mL) and added to a 50 mL pres-
sure-equalizing funnel. This acid solution was added dropwise over
a period of 1 h with constant stirring at room temperature. The reaction
mixture was then stirred overnight at RT under an inert atmosphere. The
solution turned brown at the end of the reaction. The solvent was evapo-
rated and dried under high vacuum to give a highly viscous sticky mass.
The deprotected amine (as a triflate salt) was used for subsequent steps
without further purification.
Ligand L4
White solid; yield: 60%; 1H NMR (500 MHz, [D6]DMSO): d=1.35 (d,
J=6.5 Hz, 9H), 3.13 (s, 6H), 4.47–4.49 (m, 3H), 7.93 (s, 3H), 8.23 (s,
3H), 8.49 (s, 6H), 7.93 ppm (d, J=7.0 Hz, 3H); 13C NMR (125 MHz,
[D6]DMSO): d=18.3, 37.8, 50.0, 53.9, 122.6, 124.8, 128.9, 130.1, 131.0,
136.7, 163.7, 172.4 ppm; 19F NMR (500 MHz, [D6]DMSO): d=
À62.80 ppm; HRMS (ESI+): m/z calcd for C42H39F18N7O6: 1079.2674 [M+
]; found: 1080.0648; elemental analysis calcd (%) for C42H39F18N7O6: C
46.72, H 3.64, N 9.08; found: C 46.72, H 3.62, N 9.01.
Ligand L5
General Synthetic Procedure of Compound L1–L5
Off-white solid; yield: 70%; 1H NMR (300 MHz, [D6]DMSO): d=1.32
(d, J=7.2 Hz, 9H), 3.10 (d, J=5.5 Hz, 6H), 4.45–4.49 (m, 3H), 7.41–7.48
(m, 6H), 7.50 (s, 3H), 7.84–7.90 (m, 9H), 8.46 ppm (s, 3H); 13C NMR
(75 MHz, [D6]DMSO): d=18.0, 37.1, 48.9, 53.2, 64.9, 127.4, 127.4, 127.5,
128.1, 134.0, 166.0, 172.3 ppm; HRMS (ESI+): m/z calcd for C36H45N7O6:
671.3431 [M+]; found: 672.4031; elemental analysis calcd (%) for
C36H45N7O6: C 64.36, H 6.75, N 14.59; found: C 64.32, H 6.68, N 14.72.
Triamine (500 mg, 0.71 mmol) was added to a 100 mL round-bottomed
flask and dissolved in dry CH2Cl2 (50 mL). Excess TEA (1.0 mL,
10 mmol) was added to this solution and the reaction mixture was al-
lowed to stir at RT under a nitrogen atmosphere for 15 min for complete
dissolution. Thereafter, the corresponding acid chloride (0.31 mL,
2.13 mmol) was dissolved in dry CH2Cl2 (25 mL) and added to a 100 mL
pressure-equalizing funnel. This solution was added dropwise over
a period of 1 h with constant stirring at room temperature. After com-
plete addition, the reaction mixture was allowed to stir at room tempera-
ture in an inert atmosphere for 1 day. The white precipitate formed was
filtered off and washed several times with CH2Cl2 and water to remove
excess TEA and TEACl. The colorless solid was washed finally with di-
ethyl ether and dried in air to yield the desired product as a white solid
for L1. For ligands L2–L5, the organic layer was washed with a saturated
solution of NaHCO3 (2ꢂ50 mL) and brine (2ꢂ50 mL). The resulting or-
ganic layer was dried over Na2SO4 and the solvent was removed under
reduced pressure to give the desired product as a solid.
Synthesis of Complex 1
L1 (40 mg, 0.042 mmol) was suspended in ACN (4 mL) and then TBACl
(23 mg, 0.02 mmol) was added to the solution. The solution became clear
after sonication and the mixture was stirred at room temperature for
10 min. A large amount of diisopropyl ether was then added and the so-
lution was allowed to slowly evaporate to allow crystallization. After
a week, colorless crystals of complex 1 suitable for X-ray diffraction stud-
1
ies were obtained (40%). H NMR (300 MHz, [D6]DMSO): d=0.91–0.96
(m, 12H), 1.27–1.35 (m, 8H), 1.54–1.57 (m, 8H), 3.19–3.07 (m, 14H),
4.49–4.54 (m, 3H), 8.07 (s, 3H), 9.13 ppm (d, J=12.0 Hz, 3H); HRMS
(ESIÀ): m/z calcd for C52H66ClF15N8O6: 976.1707 [M+]; found: 976.8221;
elemental analysis calcd (%) for C52H66ClF15N8O6: C 51.21, H 5.45, N
9.19; found: C 51.14, H 5.42, N 9.20.
Ligand L1
White solid; yield 80%; 1H NMR (500 MHz, [D7]DMF): d=1.41 (d, J=
7.5 Hz, 9H), 2.62 (t, J=6.5 Hz, 6H), 3.26–3.29 (m, 6H), 4.67–4.69 (m,
3H), 8.03 (s, 3H), 9.05 ppm (d, J=7.5 Hz, 3H); 1H NMR (500 MHz,
[D6]DMSO): d=1.27 (d, J=7.0 Hz, 9H), 2.54 (t, J=6.5 Hz, 6H), 3.13–
3.09 (m, 3H), 3.20–3.17 (m, 3H), 4.48–4.51 (m, 3H), 7.99 (br, 3H),
9.10 ppm (br, 3H); 13C NMR (125 MHz, [D6]DMSO): d=18.9, 37.6, 49.6,
53.7, 113.0, 136.4, 138.4, 142.7, 144.7, 156.7, 171.7, 188.5 ppm; 19F NMR
(500 MHz, [D6]DMSO): d=À164.29, À155.96, À144.27 ppm; HRMS
(ESI À): m/z calcd for C36H30F15N7O6: 941.2018 [M+]; found: 940.9461;
HRMS (ESI+) Calcd for C36H30F15N7O6 941.2018 [M+]; found: 942.1697;
elemental analysis calcd (%) for C36H30F15N7O6: C 45.92, H 3.21, N 10.41;
found: C 45.90, H 3.18, N 10.36.
ITC Studies
The ITC experiments were performed on a MicroCal VP-ITC instru-
ment. All titrations were carried out at 298 K in ACN/DMSO (99.5:0.5 v/
v). Approximately 0.1 mm solutions of receptors L1–L5 were prepared in
ACN/DMSO (99.5:0.5 v/v) and loaded into the measuring cell. This solu-
tion was titrated with 30 injections (10 mL) of approximately 3.0 mm solu-
tions of tetraalkylammonium salts of halides (FÀ, ClÀ, BrÀ and IÀ) and
À
oxyanions (AcOÀ, BzOÀ, NO3À, H2PO4 and HSO4À) prepared in ACN.
In all cases the anion under study was titrated into the host solution. An
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Tripodal Hexaamide Anion Receptors
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 software supplied with the instrument and fitted to a one-site
binding model. The initial study of the effect of the various salts was re-
peated three times with the receptors and showed good reproducibility.
Blank titrations of tetraalkylammonium salts into plain solvent were also
performed and subtracted from the corresponding titration to remove
any effect from the heats of dilution from the titrant. In general, the stoi-
chiometry (n), association constants (Ka), change in entropy (DS), and
change in enthalpy (DH) values determined from these experiments were
within experimental error limits.
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X-ray Crystallographic Analyses
The crystallographic data, details of data collection, and refinement pa-
rameter for complex 1 are summarized in Table S1 in the Supporting In-
formation. A suitably sized single crystal was selected from the mother
liquor, immersed in paratone oil, mounted on the tip of a glass fiber, and
cemented with epoxy resin. Intensity data for these crystals were collect-
ed by using MoKa (l=0.7107 ꢁ) radiation on a Bruker SMART APEX II
diffractometer equipped with charge-coupled device (CCD) area detector
at 100 K.
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Reflections were measured from a hemisphere of data collected with
each frame covering 0.58 in w. Data integration and reduction were pro-
cessed with SAINT[35] software provided with the software package of
SMART APEX II. An empirical absorption correction was applied to
the collected reflections with SADABS.[36] The structures were solved by
direct methods using SHELXTL[37] and were refined on F2 by the full-
matrix least-squares technique using the SHELXL-97[38] program pack-
age. Graphics were generated by using PLATON[39] and MERCU-
RY 2.3.[40] In all cases, the non-hydrogen atoms were refined anisotropi-
cally until convergence. All of the hydrogen atoms were geometrically
positioned and treated as riding atoms.
Crystallographic data for complex 1
C52H64N8O6F15Cl; Mr =1217.56; monoclinic; space group P2(1); a=
11.924(5), b=19.531(8), c=13.402(5) ꢁ; a=90.00, b=108.455(6), g=
90.008; V=2961(2) ꢁ3; 1calcd =1.366 gcmÀ3
; Z=2; l=0.71073 ꢁ; T=
293(2) K; 15625 reflections; 5161 independent (Rint =0.0992) and 6916
observed reflections [Iꢁ2s(I)]; 745 refined parameters; R1 =0.0582;
wR2 =0.1354; GOF=1.068. CCDC 842426 contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgements
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P.G. gratefully acknowledges the Department of Science and Technology
(DST), New Delhi, India, for financial support through a Swarnajayanti
Fellowship. P.B. and B.A. would like to acknowledge the CSIR, New
Delhi, India, for Senior Research Fellowships. X-ray crystallography
studies were performed at the DST-funded National Single-Crystal X-ray
Diffraction Facility at the Department of Inorganic Chemistry, IACS.
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Received: May 11, 2012
Revised: June 11, 2012
Published online: && &&, 0000
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FULL PAPER
In a bind: Ion-pair recognition and
anion binding studies of a series of tri-
podal amide receptors with l-alanine
amino acid backbones, in which tetra-
butylammonium acts as a cap for the
chloride-encapsulated tripodal cleft
(see picture).
Host–Guest Systems
Purnandhu Bose,
Iyyamperumal Ravikumar,
Bidyut Akhuli,
Pradyut Ghosh*
&&&& — &&&&
A Series of Amino Acid Functional-
ized Tripodal Hexaamide Anion
Receptors: Ion-Pair-Assisted Capped-
Cleft Formation by a Pentafluoro-
phenyl-Functionalized Amide
Chem. Asian J. 2012, 00, 0 – 0
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