Amide-based tripodal receptors for selective anion recognition

Article abstract of DOI:10.1007/s10847-009-9731-6

In order to improve efficiency and reduce waste production; the microwaves offer mild methods to prepare amides based neutral tripodals receptors directly from non activated carboxylic acids and amines in the absences of coupling reagents and solvents, wi

Full text of DOI:10.1007/s10847-009-9731-6

J Incl Phenom Macrocycl Chem (2010) 68:49–54
DOI 10.1007/s10847-009-9731-6
ORIGINAL ARTICLE
Amide-based tripodal receptors for selective anion recognition
¨
•
•
˘
Gu¨ls¸en Oztu¨rk Mehmet C¸ olak Mahmut Togrul
Received: 24 August 2009 / Accepted: 14 December 2009 / Published online: 7 January 2010
ꢀ Springer Science+Business Media B.V. 2010
Abstract In order to improve efficiency and reduce waste
production; the microwaves offer mild methods to prepare
amides based neutral tripodals receptors directly from non
activated carboxylic acids and amines in the absences of
coupling reagents and solvents, with high yield and very
short time. The preliminary 1H NMR titration experiments
revealed that tripodals receptor 1 and 2 can recognize
The selectivity of a synthetic receptor towards a specific
analyte is determined by multiple interactions between host
and guest in a complementary fashion. The receptor may
contain a variety of functionalities, which must be organ-
ised to complement the size and shape of the analyte. The
topology of the receptor is of importance in determining
the overall receptor-anion interactions. The tripodal syn-
thetic receptors constitute a special class of acyclic iono-
phores, which consist of multi-armed ligands that can
coordinate with the target anion. The tripodal molecular
platform provides three arms to which ligating groups are
tailored or attached. The molecular design allows the
rational control of binding properties such as complex
stability and selectivity. The selectivity of tripodal syn-
thetic receptors relates greatly to the rigidity of its arms and
its cavity size [7–9].
-
-
H2PO4 and C6H5CO2 through a 1:1 binding-stoichiom-
-
etry in preference over other anions (PF6-, ClO4-, HSO4
and Br-). The tripodal receptor 2 showed higher binding to
the all examined anions than the tripodal receptor 1.
Keywords Anion recognition ꢀ Tripodal receptor ꢀ
Amides ꢀ Microwave ꢀ NMR titration
Introduction
The main aims of designing tripodal anion receptors are:
(i) there are a sufficient number of positively charged or
neutral electron deficient groups in the ligand to serve as
interaction sites. (ii) Receptors with a flexible tripodal
structure have a strong affinity for trigonal oxoanions, such
as phosphate, carbonate and chlorate, because the geometry
and orientation of the host molecules favour the formation
of stable host guest complexes [10]. The host molecules can
be ensured interactions with guest based on non-covalent
interactions. The non-covalent interactions include elec-
trostatic interactions, hydrogen bonding, hydrophobicity,
p–p-stacking and a combination of these interactions. In
addition, for the anions itselves, the size, shape, H-bonding
capability, acid/base properties and the number of interac-
tion sites are also important.
A great deal of attention has recently been focused on the
selective recognition of anions by means of synthetic
receptors due to their potential applications in many fields,
ranging from environmental monitoring, industrial pur-
poses to clinical diagnostics [1, 2]. More specifically anions
play important roles in medicinal chemistry (maintenance
of phosphate and sulphate concentration during dialysis [3])
and catalysis (such as anion templated synthesis [4]). Same
anions have been linked to waterway pollution (from runoff
of nitrate- and phosphate-containing fertilizer [5]) and
carcinogenesis (metabolites of nitrate [6]).
There has been extensive effort devoted to the devel-
opment of synthetic anion receptors with a multitude of
review articles written on the subject [1, 11–16]. Among
these anion receptor compounds, bearing as binding sites
¨
˘
G. Oztu¨rk (&) ꢀ M. C¸ olak ꢀ M. Togrul
Faculty of Science, Departmen of Chemistry, University of
Dicle, 21280 Diyarbakır, Turkey
e-mail: gozturk@dicle.edu.tr
123

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J Incl Phenom Macrocycl Chem (2010) 68:49–54
amides [17, 18], ureas [19, 20] and thiureas [21, 22] groups
are the dominant structures.
with benzyl amine and picolyl amine under microwave
condition to give amide-based tripodal receptors 1 and 2,
respectively as shown in Scheme 1. Despite the simplicity
of the reaction procedure, few examples have been reported
on the direct synthesis of amides-based tripodals from
carboxylic acids under microwave irradiation in the
absences of the solvent. The structures proposed for these
amide-based tripodal receptors are consistent with data
Generally, many observable signal such as redox
potential changes, NMR and UV-vis spectral changes,
colour changes, and emission fluorescence changes have
been used as indication of host guest interactions. In par-
ticular, NMR most widely used techniques for measuring
interactions between host and guest [23, 24].
1
The application of microwave technology in organic
chemistry has been explored extensively within the last
decade and a large number of publication and reviews have
clearly shown that many types of chemical transformation
can be carried out successfully under microwave condition.
Most importantly, microwave irradiation processing fre-
quently leads to dramatically reduced reaction times,
higher yields, less formation of by products, easier work up
matching with the goal green chemistry, solvent-free
organic transformations, atom economy and selective of
reactions [25–30].
obtained from H, 13C NMR, IR and elemental analyses.
All 1H and 13C NMR signals were assigned on the basis of
1
DEPT and H-13C correlation experiment.
Anion Binding in Solution
NMR has become a routine tool in the study of host–guest
supramolecular chemistry and there are now hundreds of
studies where 1H NMR titration was used to measure
intermolecular association. A major advantage of the
NMR method over other techniques is that the results are
not greatly affected by the presence of minor impurities
and valuable structural information can be obtained [23].
This study reports the synthesis of tripodal amides
receptors 1 and 2 by microwave and evaluated their bind-
-
ing properties of anions such as H2PO4-, C6H5CO2
,
In this paper, the abilities of tripodal receptors 1 and 2 to
bind anions such as H2PO4-, C6H5CO2-, PF6-, ClO4
PF6-, ClO4-, HSO4- and Br- by 1H NMR titration. It has
been found that especially both tripodals receptor 1 and 2
show high affinity to dihydrogenphosphate and benzoate
anions. However, tripodal receptor 2 show higher recog-
nition towards dihydrogenphosphate and benzoate than
tripodal receptor 1.
,
-
HSO4- and Br- were investigated using 1H NMR titration
method.
The differences in chemical shifts in 1H NMR spectra of
the free and the complexed state of either host receptor or
guest may suffice to estimated thermodynamics of molec-
ular recognition. The technique is based on addition of
varying concentration of guest anions in the prescience of
Result and Discussion
Synthesis of Tripodal Receptors
In this study, in order to improve efficiency and reduce
waste production; the microwaves offer mild methods to
prepare amides directly from non activated carboxylic
acids and amines in the absences of coupling reagent and
solvents. The application of microwave technology in the
amide solvent-free synthesis is not frequently described in
literature [31–36]. The most popular methods for the syn-
thesis of carboxyamides involve the conversation of car-
boxylic acid to more reactive functional group or in situ
activation by using coupling reagents. Although good
results are obtained with both approaches, they need often
expensive coupling reagents, which lead to the formation
of by products requiring further separation [37].
N
O
O
N
H
H
N
O
NH2
O
O
OH
O
N
N
MW
HO
H
HO
1
MW
N
N
NH2
N
O
O
N
H
H
N
N
O
In the present study, amides-based tripodal receptors 1
and 2 were synthesised by microwave with high yield and
in very short time. The synthesis of amide-based tripodal
receptor 1 was previously reported in the literature [38].
A similar procedure was followed for the synthesis of tri-
podal receptor 2. Nitrilotriacetic acid was directly treated
N
H
2
N
Scheme 1 Microwave assisted synthesis of amide-based tripodal
receptors
123

J Incl Phenom Macrocycl Chem (2010) 68:49–54
51
constant receptor concentration resulted in shifting of some
1H NMR signals up field or downfield. The complexation
of anions [G] with receptors [H] is expressed by Eq. 1
1
1
1
À
Á
¼
þ
Dd
Ddmax
Ka ꢀ Ddmax ꢀ ½Gꢁo
where
K
H þ GꢀH ꢀ G
ð1Þ
Dd ¼ ðdH ꢂ dobsÞ and Ddmax ¼ ðdH ꢂ dHG
The binding properties of tripodal receptors 1 and 2
Þ
ð2Þ
-
Tetrabutylammonium form of anions such as H2PO4
,
C6H5CO2-, PF6-, ClO4-, HSO4 and Br- were used as
guest molecules. The association constant of the
supramolecular system formed was calculated according
to the modified Benesi–Hildebrant equation of (Eq. 2)
derived from (Eq. 1), where [H]o and [G]o refer to the total
concentration of tripodal receptors and tetrabutyl-
ammonium salts, respectively. The original Benesi–
Hildebrant experiment was an optical spectroscopy study
of the association of iodine with aromatic hydrocarbons
[39]. The key feature of this method is that by working
with a large excess of component H, the concentration
of uncomplexed H can be set equal to the initial
concentration, [H] = [H]o. Relationships between known
quantities (initial concentrations) and experimental
observations can now be derived, Marthur [40] and
Ashbaugh [41] have independently derived the NMR
version of the Benesi–Hildebrant equation. For all the
anions examined, plots of calculated 1/d values as a
function 1/[G]o values give a linear relationship with a slop
of calculated 1/KaꢀDdmax and intercept 1/Ddmax supporting
1:1 complex formation.
-
were examined toward tetrabuthylammonium form
-
of anions such as H2PO4-, PF6-, HSO4-, ClO4
,
C6H5CO2-, Br- by using 1H NMR titration. The
concentration of tripodal receptors was kept constant at
1 9 10-3 M in DMSOd6. The binding constants were
1
calculated from chemical schifts in the H NMR peak of
NH in the tripodal receptors against addition of various
concentrations (0–6 9 10-3 M) of tetrabuthylammonium
form of anions. The addition of tetrabuthylammonium salts
-
-
of H2PO4 and C6H5CO2 to the solution of tripodal
receptors 1 and 2 caused significant downfield shifts
in amide NH signals and upfields shifts in N–CH2–C=O
(see Fig. 1) signals indicating fast equilibrium between
complexed and free receptor and also suggesting that
the encapsulation of anions inside the receptor through
N–H…A- hydrogen bonding interactions. But the addition
-
of HSO4 to receptor 1 shows small changes in chemical
shifts and PF6-, ClO4-and Br- basically leads to no
changes in chemical shifts. For the receptor 2, the addition
-
-
of H2PO4-, C6H5CO2 and HSO4 leads to significant
chemical shifts, but the addition of PF6-, HSO4-, and
1
Fig. 1 Typical H NMR
spectral changes upon addition
of various concentrations (0 -
6 9 10-3 M) of Bu4NH2PO4 to
the constant concentration
(1 9 10-3 M) of tripodal
receptor 1 in [D6] DMSO
123

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J Incl Phenom Macrocycl Chem (2010) 68:49–54
ClO4 shows small chemical shifts and the addition Br-
showed basically no shift, indicating that the association
-
Table 1 Binding constants (Ka) and binding free energies (DGꢁ) for
1:1 complexes of tripodal receptors 1 and 2 with anions in [D6]
DMSO at 298 K
1
constant cannot be determined using the data. Typical H
Receptor
Anion
Ka (dm3 mol-1 a
)
-DGꢁ (kJ mol-1
)
NMR spectral changes upon addition of tetrabuthyl-
-
ammonium form of H2PO4 anion to tripodal receptor 1
-
Tripodal 1
H2PO4
-
PF6
241
ND
43
13.6
–
are shown in Fig. 1 while typical plots are shown for the
complexation of the same anion and tripodal receptor 2 in
Fig. 2. In the Fig. 3, the Job’s Plot of receptor 1 and
H2PO4- anion in DMSOd6 shows the maximum at a molar
fraction of 0.5. This result indicated that the receptor 1
-
HSO4
9.3
–
-
ClO4
ND
110
ND
810
73
-
C6H5CO2
Br-
11.7
–
-
binds H2PO4 anion guest with a 1:1 ratio. Moreover,
-
Tripodal 2
H2PO4
-
PF6
16.6
10.6
9.1
10.1
14.0
–
similar results were obtained for tripodal receptor 2 and for
both tripodal receptors 1 and 2 with other anions. It can be
thought that the occurrence of protonation equilibria
-
HSO4
39
-
ClO4
-
-
59
between the protic anions, such as H2PO4 and HSO4
,
-
C6H5CO2
Br-
286
ND
and the pyridine substituents of 2 is possible. However, if
this was the case the NMR spectrum of 2 was going to be
changed. As it can be seen that just some signals has
shifted, on the other hand the main peaks still remains
same. This proposed that this type of interactions can not
occure. Therefore, this possibility has been ignored.
The guest anions were used as the salts of Bu4NH2PO4, Bu4NPF6,
Bu4NHSO4, Bu4NClO4, Bu4NBr, Bu4NC6H5O2
ND indicated that the spectra showed little or no change with the
addition of anion so that the binding constants cannot be determined
using the spectra
The binding constant, Ka, of the complexes of the tri-
podal receptors 1 and 2 with tetrabuthylammonium form of
anions (H2PO4-, PF6-, HSO4-, ClO4-, C6H5CO4-, Br-)
were determined by the Benessi–Hildebrant equation on
a
Binding constant were represented as the average of three experi-
ments, errors were 10%
1
the basis of the H NMR spectrum of the complexes in
and the Gibbs free energy changes (-DGꢁ) of these tri-
podal receptors with anions obtained from usual curve-
fitting analyses (R2 [ 0.993) of the observed chemical shift
changes are summarized in Table 1. As shown in Table 1,
the selectivity trends of binding affinity of anions for tri-
podal receptor 1 was in order of H2PO4- [ C6H5CO2
[ HSO4 and for tripodal 2 the order was H2PO4
DMSOd6 collected at 25 0.1 ꢁC. Binding constant (Ka)
-
Tripodal2_H 2PO4
-
y = -2,9042x - 2,3526
-
-
0
R2 = 0,993
[
-
C6H5CO2- [ ClO4- [ PF6- [ HSO4 in DMSOd6. The
study clearly demonstrated that the spacer of tripodal-type
receptor influences the recognition for anions. Replacing
the spacer of phenyl with pyridine moiety, the binding
ability for desired anions strongly improve. As it can be
seen from Table 1, the tripodal receptor 2 showed much
better binding for all examined anions than the tripodal
receptor 1. This showed the benefit of a multidentate
binding site. Although a full understanding of the princi-
ples that govern anion recognition has not yet been
achieved, it already becomes clear that the selectivity of
receptors for special anions can be rationalized on the basis
of the anion basicity and shape complementarity between
the receptor and the anionic guests [42]. In particular,
multiple hydrogen bonding interaction are necessary in
high-affinity anion binding sites. As expected from the
-5
-10
-15
-20
0
1
2
3
4
5
6
1/[G]o (mM-1
)
Fig. 2 Typical plot of 1/Dd versus 1/[G]o for host–guest complex-
ation of tripodal receptor 2 and Bu4NH2PO4
0,3
0,2
0,1
-
-
basicity of anions, H2PO4 and C6H5CO2 gave stronger
complexation than other anions. Basicity of C6H5CO2- and
its capable p-p interaction with aromatic moiety of tripodal
receptors may also play important role as a cooperatively
and an additional binding force for recognition.
0
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0
[H]
o/[H]o[G]o
Fig. 3 Job’s Plot for tripodal receptor 1 with Bu4NH2PO4
123

J Incl Phenom Macrocycl Chem (2010) 68:49–54
A plausible structure of the complexes
53
receptors and the nature of anions play essential roles in the
anion recognition.
The proposed a plausible structure of the complexes of the
-
tripodal receptors 1 and 2 with H2PO4 guest anion
(scheme 2) are the simplest one that can account for the
experimental data. Some general conclusions for interac-
tions of tripodal hosts with anions have been proposed and
confirmed [43, 44]. There are three main factors affecting
the stability of the tripodal receptor-anion complexes. First,
there are at least three hydrogen bonding sites in the
complexes of tripodal receptors with oxoanions, namely
the three H-bond donor sites of tripodal receptors (three
NH). These sites enable a strong binding affinity between
tripodal receptors and oxoanions, as reported in literatures
[45]. Second, the flexible, three-dimensional molecular
structure of tripodal receptors allows the shape and preor-
ganization of the receptors to guide the recognition process.
Last, the fit size and shape of anionic species favour for-
mation of a stable receptor-anion complex. In addition,
tripodal receptor 2 pyridine nitrogen may provide addi-
tional donor sites for receptor with acceptors. Tripodal
receptor 2 has at least two additional hydrogen bonding
sites that ensure the receptor 2 stronger binding to the all
examined guest anions than the tripodal receptor 1.
Experimental section
Tripodal receptor 2 (2,20,200 nitrilotris (N-4-picolyl
acetamide))
Nitrilotriacetic acid (0.50 g, 2.62 mmol) and picolyl amine
(0.843 g, 7.80 mmol) were taken in the reaction vessel. The
resultant mixture was subjected to microwave irradiation.
The temperature of mixture was reached to 150 ꢁC within
2 min at 600 W and remained at 150 ꢁC for 40 min at
600 W. The mixture was then cooled to room temperature
and extracted with chloroform (3 9 50 mL). The organic
layer was dried over MgSO4 and concentrated under vac-
uum. The residue was recrystallised from mixture of eth-
ylacetate and diethylether (4:1) to give tripodal receptor 2 as
a white solid (0.99 g, 82%). Mp.96–97 ꢁC; 1H NMR
(400 MHz, CDCl3): d 3.45 (s, 6H), 4.60 (d, 6H, J = 6 Hz),
7.15 (t, 3H, J = 3.4 Hz), 7.27 (t, 3H, J = 6.8 Hz), 7.61–
7.65 (m, 3H), 8.43 (t, 3H, J = 2.2 Hz), 8.81 (t, 3H,
J = 6 Hz); 13C NMR (100 Hz, CDCl3): d 44.10, 59.14,
121.98, 122.35, 136,97, 148.74, 157.26, 170.37; IR (KBr) :
3287(NH), 3030, 3060 (Ar–H), 1645(C=O) 1066 (C–N);
Anal.Calcd for: C24N7H27O3: C, 62.46; H, 5.90; N, 21.24;.
Found: C, 62.43; H, 5.95; N, 21.17.
Conclusion
The synthesis of tripodal receptor 1 (2,20,200nitrilotris
(N-benzyl acetamide)) was previously reported in the lit-
erature [38].
In summary, we have reported the synthesis of tripodal
amides receptors 1 and 2 by microwave and their calculated
-
binding properties with anions such as H2PO4-, C6H5CO2
,
PF6-, ClO4-, HSO4 and Br- by H NMR titration. The
results indicate that tripodals 1 and 2 possess higher affinity
for dihydrogenphosphate and benzoate anions. However,
tripodal receptor 2 shows higher selectivity for dihydro-
genphosphate and benzoate compared with tripodal recep-
tor 1. The results demonstrated that electrostatic interaction,
hydrogen bonding, and preorganization of binding sites of
-
1
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