Synthesis, crystal structures and competitive binding property of a family of calix[4]arene-biscrown-5/6 and their application in extraction of alkali metal ions from sea bittern

Article abstract of DOI:10.1016/j.poly.2015.07.001

Abstract A family of calix[4]arene compounds containing crown moieties at both the upper and lower rims of the calix unit with different substituents attached to the nitrogen atom of the aza-crown moiety have been synthesized, characterized and their competitive binding property towards Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺ and Sr²⁺ in aqueous media has been investigated. These ionophores exhibit high selectivity towards K⁺, assessed by two-phase extraction method followed by ion chromatographic assay of the metal ions in the extract. Isothermal calorimetric titration for complex formation with K⁺ was carried out to evaluate stoichiometry, binding constant and other thermodynamic parameters (ΔH, ΔS and ΔG). For structural elucidation in solution, detail ¹H NMR study was carried out and K⁺ complexes were isolated for solid state characterization. Molecular structures of two of the ionophores and K⁺-complex of one of the ionophores has been established by single crystal X-ray study. ¹H NMR, ITC and single crystal XRD studies suggest 1:1 complex formation with encapsulation of K⁺ in the crown-6 ring at the upper rim. These ionophores have been applied for extraction of metal ions from bittern.

Full text of DOI:10.1016/j.poly.2015.07.001

Accepted Manuscript
Synthesis, crystal structures and competitive binding property of a family of
calix[4]arene-biscrown-5/6 and their application in extraction of alkali metal
ions from sea bittern
Vallu Ramakrishna, E. Suresh, Vinod P. Boricha, Anjani K. Bhatt, Parimal Paul
PII:
S0277-5387(15)00356-3
DOI:
http://dx.doi.org/10.1016/j.poly.2015.07.001
POLY 11387
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
27 April 2015
2 July 2015
Please cite this article as: V. Ramakrishna, E. Suresh, V.P. Boricha, A.K. Bhatt, P. Paul, Synthesis, crystal structures
and competitive binding property of a family of calix[4]arene-biscrown-5/6 and their application in extraction of
alkali metal ions from sea bittern, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.07.001
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Synthesis, crystal structures and competitive binding property
of a family of calix[4]arene-biscrown-5/6 and their application
in extraction of alkali metal ions from sea bittern
Vallu Ramakrishnaa, Suresh Ea,b, Vinod P Borichaa, Anjani K Bhatta and
Parimal Paul*,a,b
aAnalytical Division and Centralized Instrument Facility, CSIR-Central Salt and
Marine Chemicals Research Institute, G. B. Marg, Bhavnagar 364002, India.
bAcademy of Scientific and Innovative Research (AcSIR), CSIR-CSMCRI, G. B. Marg,
Bhavnagar 364002, India.
_____________________________________________________________________
Abstract
A family of calix[4]arene compounds containing crown moieties at both the upper
and lower rims of the calix unit with different substituents attached to the nitrogen
atom of the aza-crown moiety have been synthesized, characterized and their
competitive binding property towards Li+, Na+, K+, Mg2+, Ca2+ and Sr2+ in aqueous
media has been investigated. These ionophores exhibit high selectivity towards K+,
assessed by two-phase extraction method followed by ion chromatographic assay of
the metal ions in the extract. Isothermal calorimetric titration for complex formation
with K+ was carried out to evaluate stoichiometry, binding constant and other
thermodynamic parameters (∆H, ∆S and ∆G). For structural elucidation in solution,
1
detail H NMR study was carried out and K+ complexes were isolated for solid state
characterization. Molecular structures of two of the ionophores and K+-complex of
1

1
one of the ionophores has been established by single crystal X-ray study. H NMR,
ITC and single crystal XRD studies suggest 1:1 complex formation with
encapsulation of K+ in the crown-6 ring at the upper rim. These ionophores have been
applied for extraction of metal ions from bittern.
Keywords: Calixarenes; Crystal structures; Selectivity; Isothermal calorimetry; Two-phase
extraction; Alkali metal ions.
*Corresponding author. E-mail: ppaul@csmcri.org (P. Paul); Tel.: +91 278 2565180.
1. Introduction
Calixarenes are versatile class of macrocyclic compounds, which have been
extensively used in the areas of host-guest chemistry, supramolecular chemistry,
molecular sensors, coordination chemistry etc. [1-7]. This chemistry has become more
versatile because of the ease with which calixarene can be modified depending on the
requirement. These modified calixarenes provide a highly preorganized architecture
for the assembling of converging binding sites [8-14]. By the appropriate choice of the
substituents, they can be made selective binders for metal ions, anions and neutral
molecules [15-21]. Among the calixarene derivatives, a new class of hybrid molecules
has been developed by incorporating crown ethers moiety into calix[4]arene and it has
attracted intense interest because of their complexation ability with alkali and alkaline
earth metal ions [22-31]. The ion selectivity of this class of compounds depends
mainly on the conformation of the calixarene moiety, the size of the crown ether ring
and substituents attached with the calixarene or crown moiety. In this calix-crown
2

hybrid family, another type of molecules has been developed by incorporating aza-
crown ring, the advantage of this class of compound is that any desire substituents can
be easily attached with the nitrogen atom of the ring and it can act like lariat crown
ether making three dimensional cavities for metal ions and also it can impose control
steric crowding to tune selectivity [32-35]. This chemistry has been further extended
incorporating two crown moieties at the opposite sides of the calixarene unit with 1,3-
alternate conformation [25,32,36]. It provides the possibility of complex formation
with two metal ions at the two crown rings and it can also facilitates complexation
with polymeric network structures [16,37].
One of the important applications of these calix-crown hybrid ionophores is to use
them as sensor/complexing reagent for alkali and alkaline earth metal ions [38-43]. If
good selectivity for a particular metal ion is found, these ionophores can be used for
recovery/extraction of that specific metal ion. In this regard, K+ is most important as it
uses as fertiliser and has demand worldwide and India imports its entire requirement
[44-46]. The present work is aimed to develop K+ selective ionophore, which may find
potential application in recovery of this metal ion from natural sources such as sea
bittern. As the ionic size of K+ matches very well with the ring size of crown-5/6, so
the ionophores reported in the present study is designed incorporating crown -5/6 to
make them selective towards K+ among the other metal ions present in bittern with
significant concentration (Na+, K+, Mg2+, Ca2+). Substituents such as 2-hydroxy-5-
nitrobenzyl, ethyl acetate and acetyl groups, which contain donor atom and can act as
lariat moiety, are attached with the nitrogen atom of the aza-crown ring. For the
determination of selectivity, most of the cases in the literature, the interaction of the
ionophore with individual metal ion and comparison of their binding constants has
been reported to evaluate the selectivity [25,27]. However, for practical use it is better
3

to prepare an artificial solution containing mixture of constituent metal ions and to
study the competitive complexing property of the ionophore using this solution. With
this idea, the competitive binding ability towards Li+, Na+, K+, Mg2+, Ca2+ and Sr2+ has
been investigated using equimolar mixture of these metal ions by two-phase extraction
method using picrate as counter anion. Picrate anion makes strong ion-pair interaction,
which facilitates transport of metal complex from aqueous to non-aqueous media
[24,35].
In this paper, we report synthesis and characterization of four calix-bis-crown
ethers, their competitive complexation ability from the equimolar mixture of the
above-mentioned metal ions in aqueous media and also from sea bittern as real
sample. Metal extraction was carried out by two-phase extraction procedure followed
by estimation of the concentration of metal ions in the extract (organic phase) by ion
chromatographic assay. The binding constants and other thermodynamic parameters
were determined by the isothermal calorimetric titration, stoichiometry of the
1
complexes formed was determined from ESMS data and H NMR spectral change was
used to investigate binding sites of the metal ions. For solid state structural
characterization, the molecular structures of two of the ionophores and K+-complex of
one of them have been determined by single crystal X-ray study and reported herein.
2. Experimental
2.1. Materials
The compound 2-hydroxy-5-nitrobenzyl bromide was purchased from Sigma-Aldrich
and other chemicals were purchased from S.D Fine chemicals. All the solvents were of
analytical grade and purified by standard procedures before use [47]. Milli-Q
(Millipore Corporation) water was used for extraction and ion chromatography. Metal
4

picrate salts were prepared by the reaction of picric acid and metal hydroxide in
aqueous media. The reagents 2-(2-chloroethoxy)ethyl, 4-methylbezene sulfonate and
pentaethyleneglycolditosylate were prepared following the literature procedures [48].
The starting materials such as p-tert-butyl calix[4]arene [49], dealkylated
calix[4]arenes [50], calix[4]arene-bis-crown ether were synthesized following the
literature procedures [32,39].
2.2. Physical measurements
Elemental analyses (C, H & N) were performed on a model Vario Micro CUBE
elemental analyzer. Mass spectra were recorded on a Q-TOF MicroTM LC-Ms
instrument. Infrared spectra were recorded on a Perkin-Elmer spectrum GX FT-system
as KBr pellets. NMR spectra were recorded on models DPX200 and Avance II 500
Brucker FT-NMR instruments. The UV/vis spectra were recorded on a CARY 500
scan Varian spectrophotometer. The cation concentration was measured with a
Dionex 500 ion chromatography. Single crystal X-ray structures were determined
using a Bruker SMART1000 (CCD) diffractometer. Thermodynamic parameters were
determined using isothermal titration calorimetry (ITC200) provided by Microcal
Company.
2.3. Synthesis of compounds
2.3.1. Synthesis of calix[4]arene-crown-6-aza-crown-5 (1)
This compound was synthesised following a modified literature procedure [32]. In a
typical procedure, N-tosylated calix[4]arene-crown-6-aza-crown-5 (2.72 g) dissolved
in 1,4-dioxane (100 mL) and methanol (20 mL) was taken and Na2HPO4 (0.91 g) and
6% Na/Hg amalgam (10.0 g) were added into this solution and the reaction mixture
5

was heated to reflux at 80 C for 48 h under nitrogen atmosphere. The solution was
°
evaporated under reduced pressure and the residue was extracted with
dichloromethane and saturated aqueous solution of NaHCO3, the organic layer thus
separated was dried with anhydrous MgSO4. The compound thus obtained was
purified by column chromatography using neutral silica gel as column material and
chloroform as eluent. The compound was recrystallized from ethyl acetate by slow
evaporation, which gave colourless crystals suitable for single crystal X-ray study.
1
Yield: 60 % (1.35 g). H NMR (CD3CN): δ 7.14 (d,
J
= 6.0 Hz, 8H, Ar-Hm-calix),
= 7.5 Hz, 2H, Ar-Hp-calix), 3.70 (s,
12H, Ar-CH2-Ar, OCH2CH2O-azacrown-5), 3.66 (s, 8H, OCH2CH2O), 3.62 (s, 4H,
6.92 (t,
J = 7.5 Hz, 2H, Ar-Hp-calix), 6.76 (t, J
O
O
CH2CH2O-crown-6), 3.59 (tt, 4H, Ar-OCH2CH2O-azacrown-5), 3.5 (br, 4H, Ar-
CH2CH2O-azacrown-5), 3.51 (t, = 9.5 Hz, 4H, ArOCH2CH2O-crown-6), 3.47 (t,
J
J
= 5.25 Hz, 4H, Ar-OCH2CH2O-crown-6), 2.70 (t,
(selected bands): 3210 sh cm-1,
+H+]+ 784.89. Anal. Calcd. for C46H57NO10: C, 70.48; H, 7.32; N, 1.78. Found: C,
J = 5.2 Hz, 4H, -CH2CH2NH). IR
ν
(N-H). ES-MS: m/z = 784.53 (100%), calcd. for
[
1
69.83; H, 7.09; N, 1.60.
2.3.2. Synthesis of calix[4]arene-crown-6-N-(2-hydroxy-5-nitrobenzyl)aza-crown-5
(2)
To a solution of freshly distilled 100 mL tetrahydrofuran (THF), 0.6 g (0.765 mmol)
of compound 1, 0.4 mL (0.153 mmol) of triethylamine was added and stirred for 1 h at
°
0 C and then 0.354 g (0.153mmol) of N-(2-hydroxy-5-nitrobenzyl) bromide was
°
o
added and stirring was continued for 4 h at 0 C and then it was slowly heated to 65 C
and the stirring was continued for 48 h maintaining the same temperature. The solution
was then evaporated under reduced pressure, which gave yellow oil. The crude
6

product was purified by column chromatography using silica gel as column material
1
and chloroform-methanol (1:0.5) as eluent. Yield: 63.0 % (0.45 g). H NMR
(CD3CN): δ 8.08 (d,
Ar -chromogenic part), 7.98 (d,
4Hz, 8H, ArHm-calix), 6.88 (t,
3.82 (s, 8H, Ar-CH2Ar), 3.63 (s, 4H, Ar-OCH2CH2O-crown-6), 3.60 (t,
4H, Ar-OCH2CH2O-crown-5), 3.56-3.52 (m, 12H, OCH2CH2O-crown-6), 3.32 (t, 4H,
= 4.75 Hz, Ar-OCH2CH2O-crown-6), 3.25 (t, = 4.75 Hz, 4H, Ar-OCH2CH2O-
J
= 2.5 Hz, 1H, Ar
= 2.5 Hz, 1H, Ar
= 7.5 Hz, 4H, ArHp-calix), 3.94 (s, 2H, N-CH2Ar),
= 4.5 Hz,
H
-chromogenic part), 8.06 (d,
J = 2.5 Hz, 1H,
H
J
H
-chromogenic part),7.11(dd,
J =
J
J
J
J
crown-5), 3.16 (t,
J
= 4.75 Hz, 4H, Ar-OCH2CH2O-crown-5), 2.68 (t,
(O-H); 1514 and 1334 cm-1, NO2 (2-
+H+]+ 936.00. Anal.
J = 4.7Hz, 4H,
O
CH2CH2N-). IR (selected band): 3433 cm-1,
ν
hydroxy-5-nitrobenzyl). ES-MS: m/z: found 935.93, calcd. for [
2
Calcd. for C53H62N2O13: C, 68.08; H, 6.68; N, 2.99. Found: C, 67.06; H, 6.68; N, 3.14.
2.3.3. Synthesis of calix[4]arene-crown-6-N-ethylester-aza-crown-5 (3)
This compound was synthesized from
1 following the same procedure as described for
2
, except thylbromoester was used instead of N-(2-hydroxy-5-nitrobenzyl) bromide.
e
The crude product was purified by column chromatography using silica gel and
hexane-chloroform (1:2) as eluent. The product was recrystallized from acetonitrile to
obtain colourless crystals suitable for single crystal X-ray study. Yield: 60.0 % (0.4 g).
1H NMR (CD3CN): δ 7.10 (dd,
ArHp-calix), 4.14 (q, = 7.33 Hz, 2H, -CH2-ester part), 3.78 (s, 8H, Ar-CH2-Ar), 3.64
(s, 4H, Ar-OCH2CH2O), 3.57 (overlapped, 8H, OCH2CH2O-crown-6 and OCH2CH2O-
aza-crown-5), 3.53 (t, = 4.5 Hz, 4H, Ar-OCH2CH2O-crown-6), 3.50 (t, = 5.0 Hz,
4H, Ar-OCH2CH2O-), 3.40 (s , 2H, -CH2CH3), 3.31 (q, 8H, Ar-OCH2CH2O-crown-6
J = 5.0 Hz, 8H, Ar-Hm-calix), 6.87 (tt, J = 7.5 Hz, 4H,
J
J
J
and Ar-OCH2CH2O-azacrown-5), 3.26 (t,
J
= 6.0 Hz, 4H, Ar-OCH2CH2O-azacrown-
7

5), 2.74 (s, 4H, OCH2CH2NH), 1.26 (t,
J
= 7.25 Hz, -CH3). IR (selected band): 1737
cm-1,
ν
(C=O, ester). ES-MS: m/z: found 870.72, calcd for [3
+H+] 870.97. Anal. Calcd.
for C50H63NO12: C, 69.03; H, 7.30; N, 1.61. Found: C, 68.30; H, 7.41; N, 1.57.
2.3.4. Synthesis of calix[4]arene-crown-6-N-acetyl aza-crown-5 (4)
This compound was synthesized from
1 following similar procedure as described for 2
except, acetyl chloride was used instead of N-(2-hydroxy-5-nitrobenzyl) bromide. The
crude product was purified by column chromatography using silica gel and hexane-
1
chloroform (1:2) as eluent. Yield: 63.0 % (0.2 g). H NMR (CD3CN): δ 7.13-7.10 (m,
8H, ArHm-calix), 6.91-6.88 (tt, 2 H, ArHp-calix), 6.82 (t,
3.77 (s, 8H, Ar-CH2-Ar), 3.64 (s, 4H, OCH2CH2O-crown-6), 3.63-3.59 (m, 12H,
CH2CH2NH, OCH2CH2O-crown-6), 3.56-3.51 (m, 4H, Ar-OCH2CH2O-azacrown-5),
3.41-3.38 (m, 8H, -OCH2CH2O-, ArOCH2CH2O-azacrown-5), 3.35-3.29 (m ,6H,
J = 7.5 Hz, 2H, ArHp-calix),
O
overlapped, -CH2CH2O-, -OCH2CH2O-azacrown-5), 3.24 (t,
J = 6.25 Hz, 4H,
O
CH2CH2O-), 2.07 (s, 3H, -NCH3). IR (selected band): 1677 cm-1,
ν
(C=O, acetate).
+H+]+ 826.92,
4
+H++K+]+ 866.02. Anal. Calcd. for C48H59NO11:
ES-MS: m/z: found 826.67, 848.66, 865.63, and 866.63; calcd for [
+Na+]+ 848.91, [ +K+]+ 865.01, [
4
[
4
4
C, 69.80; H, 7.19; N, 1.69; found: C, 70.10; H, 7.30; N, 1.70.
2.4. Selectivity study by two-phase extraction method
Selectivity of the ionophores towards Li+, Na+, K+, Mg2+, Ca2+ and Sr2+ were
determined by two-phase extraction method using aqueous solution of the equimolar
mixture of these metal ions with picrate as counter anion. In a typical experiment,
equal volume (10 mL) of aqueous solution of equimolar mixture of alkali metal picrate
(Li+, Na+, K+, Mg2+, Ca2+ and Sr2+, 0.1 M each) and CH2Cl2 solution (10 mL) of the
8

required ionophore (0.002 M) were mixed and vigorously shaken in a vortex mixture
for 30 min and the solution was transferred to a separating funnel and allowed to stand
for 4 h. The metal ions added with stoichiometric amount and also with 25, 50, 75 and
100 times excess compared to the concentration of ionophores. After settling down,
the dichloromethane layer was separated and transferred to a crucible, the solvent was
evaporated by gentle heating on a water bath, and then heated in a furnace at 550 ºC
for 5 h. The residue was dissolved in deionized water (~ 5 mL) and filtered on 0.2 µm
filter paper. The relative concentrations of the cations in the filtrate were determined
by ion chromatography using Ion Pac CS12 (2 mm) analytical column and 20 mM
methanesulphonic acid as eluent with a flow rate of 0.25 mL/min. Quantification was
made using a standard solution containing a mixture of LiCl, NaCl, KCl, MgCl2,
CaCl2 and SrCl2 (10 ppm each).
2.5. Application for extraction of metal ions from bittern
The ionophores were applied for extraction of metal ions from bittern using the similar
procedure as described for selectivity study, except bittern and picric acid were used
instead of mixture of the picrate salts of different metal ions. The concentration of
metal ions in the organic phase were analysed by ion chromatography as described
above.
2.6. Synthesis of K+ complexes of the ionophores 1-4
Metal (K+) complexes of all the ionophores were synthesised following the similar
procedure. In a typical experiment, a mixture of 0.05 mmol of the ionophores (1/2/3/4)
and K+ picrate (0.5mmol, ten-fold excess) was stirred in chloroform at room
temperature for 24 h. The reaction mixture was then filtered to remove unreacted
picrate salt and the complex was obtained by removing the solvent from the filtrate
9

under vacuum. The yellow complex thus obtained was dissolved in a minimum
volume (ca. 3 mL) of dichloromethane (in which K+ picrate is almost insoluble)
followed by filtration to remove trace quantities of unreacted K+ picrate. The solution
1
was then evaporated to dryness to get yellow product. The H NMR spectra of the
complexes did not show any signal corresponding to free ionophore or excess picrate
anion, which confirmed the complete complexation. Yield: 90–95%. For the K+
complex of the ionophore 1, crystals suitable for the single crystal X-ray study were
grown from ethyl acetate by slow evaporation.
2.7. Characterization data
[1.K+.Pic-].HPic.EtAc.H2O: 1H-NMR (CD3CN): δ 8.82 (br,1H, -NH), 8.63 (s,4H,
picrate), 7.31 (d,
7.03 (t, = 7.5 Hz, 2H, ArHp-calix), 6.95 (t,
(m, 4H, OCH2CH2O-crown-6), 3.83 (br, 4H, Ar-OCH2CH2O-crown-6), 3.80 (d,
J
= 7.5 Hz, 4H, ArHm-calix), 7.20 (d,
= 7.5 Hz, 2H, ArHp-calix), 3.92-3.91
=4.5
J =7.5 Hz, 4H, ArHm-calix),
J
J
J
Hz, 8H, Ar-CH2-Ar), 3.72 (overlapped s, 16H, OCH2CH2O, Ar-OCH2CH2O-crown-5),
3.68 (br, 8H, Ar-OCH2CH2NH), 3.10 (s, 4H, OCH2CH2NH). IR (selected band): 1559
and 1362 cm-1 (NO2, picrate). ES-MS: m/z: found 822.55, calcd for [ +K+]+ 822.99.
1
Anal. calcd. for C64H72N7O25K: C, 55.70; H, 5.26; N, 7.11. Found: C, 55.51; H, 5.47;
N, 6.94.
[2.K+.Pic-].HPic: 1H NMR (CD3CN): δ 8.63 (s, 2H, picrate), 8.12 (d,
Ar -chromogenic part), 8.10 (d, = 2.5 Hz, 1H, Ar -chromogenic part), 8.02 (d,
2.5 Hz, 1H, Ar -chromogenic part), 7.42 (d, = 7.5Hz, 4H, ArHm-calix), 7.35 (d,
7.5Hz, 4H, ArHm-calix), 7.10 (t,
CH2Ar), 4.02 (s, 4H, Ar-OCH2CH2O-crown-6), 3.88 (overlapped d,
J = 2.5 Hz, 1H,
H
J
H
J
J
=
=
H
J
J
= 13.0 Hz, 4H, ArHp-calix), 4.12 (s, 2H, N-
J
= 13.5 Hz, 8H,
Ar-CH2-Ar, 8H Ar-OCH2CH2O-azacrown-5), 3.74 (overlapped d,
J = 12.0Hz, 16H,
10

O
O
CH2CH2O-crown-6, 4H, OCH2CH2O-azacrown-5), 2.64 (t,
J = 4.7Hz, 4H,
CH2CH2N-). IR (selected band): 1560 and 1362 cm-1 (NO2, picrate). ES-MS: m/z
:
found 973.55, calcd for [2
+K+]+ 974.09. Anal.calcd. for C65H67N8O27K: C, 54.54; H,
4.72; N, 7.82. Found: C, 53.97; H, 4.57; N, 7.63.
[3.K+.Pic-].Hpic: 1H NMR (CD3CN): δ 8.61 (s, 2H, picrate), 7.31 (d,
J
= 7.5 Hz, 4H,
Ar-Hm-calix), 7.23 (d,
calix), 6.98 (t, = 7.5 Hz, 2H, ArHp-calix), 4.21 (q,
(br, 4H, Ar-OCH2CH2O-azacrown-5), 3.80 (s, 8H, Ar-CH2-Ar), 3.75 (br, 4H, Ar-
CH2CH2O-azacrown-5), 3.70 (br s, 4H, OCH2CH2O-crown-6 and OCH2CH2O-
J
= 7.5 Hz, 4H, ArHm-calix), 7.03 (t,
J = 7.5 Hz, 2H, ArHp-
J
J
= 7.16 Hz, 2H, -CH2-ester), 3.91
O
azacrown-5), 3.67 (overlapped s, 8H, OCH2CH2O-crown-6 and OCH2CH2O-aza-
crown-5), 3.61 (br, 8H, Ar-OCH2CH2O-crown-6), 3.55 (br, 4H, Ar-OCH2CH2O-),
3.50 (s , 2H, -CH2CH3), 2.93 (br, 4H, OCH2CH2N-), 1.3 (t,
(selected band): 1557 and 1361 cm-1 (NO2, picrate). ES-MS: m/z: found 908.63, calcd
+K+]+ 909.06. Anal.calcd. for C62H68N7O26K: C, 54.51; H, 5.01; N, 7.17. Found:
J = 7.0 Hz, 3H, -CH3). IR
for [
3
C, 54.21; H, 4.96; N, 6.95.
[4.K+.Pic-].Hpic.H2O: 1H NMR (CD3CN): δ 8.61(s,2H, picrate),7.30 (d,
J = 7.5 Hz,
4H, ArHm-calix), 7.21 (t,
J
= 6.75,4H, ArHm-calix) 6.93 (tt,4H, ArHp-calix), 3.87 (br,
= 3Hz, 8H, Ar-CH2-Ar), 3.72 (d, = 5, 20H,
CH2CH2O-crown-6, ArOCH2CH2O-azacrown-5), 3.60 (broad s, 4H, Ar-
4H, OCH2CH2O-crown-6), 3.80 (d,
J
J
O
O
CH2CH2O-azacrown-5), 3.40 (d,
J = 5.5 Hz, 2H, azacrown-5), 3.35 (t, 6H, Ar-
O
CH2CH2O-azacrown-5), 2.10 (s, 3H, -NCH3). IR (selected band): 1553 and 1361 cm-
1
(NO2, picrate). ES-MS: m/z: found 864.38, calcd for [4
+K+]+ 865.019. Anal. calcd.
for C60H66N7O26K: C, 53.75; H, 4.96; N, 7.31. Found: C, 53.45; H, 5.14; N, 7.13.
2.8. Isothermal calorimetric titration
11

The stoichiometry of the complexes formed, binding constant, thermodynamic
parameters such as enthalpy change (∆H), entropy change (∆S) and free energy change
(
∆G) for the reaction of the ionophores with K+Pic- in acetonitrile were determined by
isothermal calorimetric titration (ITC). In a typical procedure, first a blank experiment
was carried out using solute and solvent (without taking ionophore) and this data was
subtracted from the titration data for complex formation for curve fitting. For
complexation study, 1 mM solution of the respective ionophore in dry acetonitrile was
taken in the cell and 7 mM solution of Kpic in the same solvent was taken in the
syringe. The solution of the Kpic was then added maintaining the successive additions
of 2 µL, spacing 180 second intervals to allow the exothermic peak resulting from the
interaction to return to the baseline. All these calorimetric studies were performed at
298 K. The blank data was then subtracted from the ITC data for complex formation
and the resultant data was fitted with the aid of Origin 7 provided by MicroCal by
using one set of sites curve fitting model. This computational plot gave the values of
stoichiometry (n), binding constant (Ks), enthalpy change (∆H), entropy change (∆S)
and free energy change (∆G) was calculated using the equation ∆G = ∆H – T∆S.
2.9. Single Crystal X-Ray Study
Crystals of suitable size of the compounds 1, 3 and the complex [1
.K+.Pic-] were
selected, immersed in partone oil and then mounted on the tip of a glass fibre using
epoxy resin. Intensity data for all three crystals were collected at 100 K using graphite
monochromatised MoKα (
λ
= 0.71073Å) radiation on a Bruker SMART APEX
diffractometer equipped with CCD area detector. The data integration and reduction
were processed with SAINT software [51]. An empirical absorption correction was
applied to the collected reflections with SADABS [52]. The structures were solved by
12

direct methods using SHELXTL [53] and refined on F2 by the full-matrix least-
squares technique using the SHELXL-97 [54] package. Graphics are generated using
PLATON [55] and MERCURY 1.3 [56]. Even though data was collected under 150 K,
in the case of compound
1 some of the atoms in the crown moiety showed dynamic
disorder and the occupancy factor and positions for the disordered atoms were
determined using FAVR and PART command. In all the compounds non-hydrogen
atoms were refined anisotropically till convergence is reached and the hydrogen atoms
attached to the ligand moieties were stereochemically fixed. Crystallographic
parameters for both the compounds are given in Table 2.
3. Results and discussion
3.1. Synthesis of ionophores (1-4)
The compound calix[4]arene crown-6 aza-crown-5 (1) was synthesized following the
modified literature procedure [32,39], details of the steps involved in this reaction is
given in Scheme S1 (ESI) and the characterization data are given in the Experimental
Section. The proton of the N-H group of
), ethyl acetate ( ) and acetyl ( ) groups, as shown in Scheme 1 and details of
synthetic procedure and characterization data are given in the Experimental Section.
The compounds were obtained from by the reaction with 2-hydroxy-5-
1 was substituted by 2-hydroxy-5-nitrobenzyl
(
2
3
4
2
-
4
1
nitrobenzyl bromide, ethylbromoester and acetyl chloride, respectively in presence of
a base (Et3N) in dry THF and were purified by column chromatography using silica
gel as column material and chloroform/hexane/methanol of different compositions as
eluent. The C, H and N analysis of these compounds are in excellent agreement with
the composition of the compounds. The ES-MS spectra of all these four compounds
are submitted as ESI (Figs. S1-S4) and the m/z values are matched well with that of
13

calculated values; 784.53 for
+H+]+ is 784.89), 935.93 for
3 (calculated for [3
+H+]+ is 935.99), 870.72 for +H+]+ is 870.97) and
1
(calculated for [
1
2
(calculated for [
2
826.67 for
4
(calculated for [
4
+H+]+ is 826.92). The structures of the compounds
1-4
1
in solution have been assigned on the basis of H NMR spectra (Figs. S5-S8, ESI), the
data of which with assignment of peaks are given in the Experimental Section. The
1,3-alternate conformation of the calix moiety is confirmed from the appearance of Ar-
CH2-Ar methylene protons (8H) as a singlet at δ = 3.70, 3.82, 3.79 and 3.78 for
respectively [32]. For the attached 2-hydroxy-5-nitrobenzyl moiety of , the methylene
protons appeared at δ = 3.94 (2H) as singlet and the aromatic protons appeared as
singlet at 7.98 (1H) and two doublets at δ = 8.07 (1H) and 8.08 (1H) ppm. For , the
peaks at δ = 2.74 (t, 4H), 3.40 (s, 2H) and 4.14 (q, 2H) are due to ester group attached
with the nitrogen atom. The methyl protons of the acetyl moiety of appeared as a
1-4,
2
3
4
singlet at δ = 2.07 (3H) ppm. The signals assigned for the aromatic protons of the calix
moiety and for ethylene groups of the crown rings are given in the Experimental
Section, overlap of different signals in a few cases have also been noted. In the solid
state, structural characterization of two compounds,
1 and 3, were carried out on the
basis of single crystal X-ray study, which discussed in the latter section.
3.2. Competitive Complexation Study of the Ionophores (1-4)
These ionophores were synthesised with the aim to use them for selective extraction of
industrially important metal ion like K+ from natural sources such as brine and bittern,
which mainly contain Na+, K+, Ca2+ and Mg2+ and small amount of Li+ and Sr2+ [57].
It is therefore necessary to investigate competitive complexing ability of these
ionophores using these metal ions in aqueous media. It has been carried out by two-
phase extraction in water-dichloromethane using equimolar mixture of these metal
14

ions and picrate as counter anion, which facilitates transfer of metal complex from
aqueous to non-aqueous media by strong ion-pair interaction [24]. The concentration
of the metal ions in the extract was determined by ion chromatographic assay, a detail
of the procedure has given in the Experimental Section. The ion chromatography of
the solution containing equimolar amount of Li+, Na+, K+, Ca2+, Mg2+ and Sr2+ and
that in the extract after two-phase extraction using the ionophore
Ion chromatographs for the metal ions in the extracts obtained using the ionophores
and have given as ESI (Figs. S9-S11). The ion chromatographs suggest that all the
3 are shown in Fig. 1.
1
,
2
4
four ionophores exhibited very high selectivity towards K+ (83.5, 74.4, 85.6, 81.9%), it
also showed some amount of Na+ (12.1, 13.2, 10.3, 10.3.%) and trace amount of Ca2+
and Mg2+, however no Li+ and Sr2+ was detected in the extract. The observed
selectivity ratios of the metal ions in the extract are shown in Table 1. The fraction of
metal ions extracted from the solution containing equimolar (0.1 M) amount of Li+,
Na+, K+, Ca2+, Mg2+ and Sr2+ using all the four ionophores (0.002 M) expressed in the
form of a bar diagram is shown in Fig. 2. The data in Table 1 and also the bar diagram
clearly show that these ionophores prefer to bind K+ ion over the other metal ions used
in this study. All of these ionophores exhibited very high selectivity towards K+ and
therefore we were interested to investigate interaction of these ionophores with K+ in
1
solution with the aid of H NMR spectroscopy and solid state structural study of the
K+ complex.
3.3. NMR study
1
The H NMR spectra of
1
-
4
were recorded with incremental addition of K+-picrate (0
to 8.95 molar equivalents) in CD3CN. The spectra for all the four ionophores exhibited
significant changes in chemical shifts for almost all the peaks. The spectral change for
15

3
is shown in Fig.3 and that of 1, 2 and 4 are submitted as ESI (Figs. S12-S14). It may
be noted that substantial changes in chemical shifts were observed in both aliphatic
and aromatic regions and also a new peak appeared around δ = 8.6, which is due to
picrate anion [15,24]. It is interesting to note that upon addition of metal ion, a few
new peaks generated at the expense of the original signals, indicating that the size of
the metal ion fits well in the calix-crown cavity making strong interaction with the
donor atoms forming stable complex [25,27,58]. The NMR data therefore, clearly
supports the high selectivity for these ionophores towards K+. The change in chemical
shifts for the signals in the entire spectra is because of the distribution of positive
charge of K+ throughout the molecule by
-interaction and also due to conformational
π
change of the ionophores due to complex formation. These ionophores form 1:1
complex (discussed in the later sections), therefore it is important to know where the
metal ion has encapsulated out of the two crown rings. From the present NMR data it
is difficult to make an unambiguous assignment for the location of K+, as the
methylene protons of the two crown moieties overlapped in many places and also most
of the peaks have shifted. The crystal structures of the K+ complex of
1 (discussed in
the later section) showed that the metal ion has encapsulated in the crown ring at the
upper rim (not aza-crown). On the basis of crystal structure, it is therefore, proposed
that the metal ion has encapsulated in the crown ring at the upper rim.
3.4. Isolated K+ complexes of the ionophores 1-4
For solid state characterization, K+ complexes of
1-4 were synthesised by the reaction
of appropriate ionophore and K+ picrate in chloroform at room temperature, as
described in the Experimental Section. These complexes were characterized on the
1
basis of elemental analysis, mass, IR and H NMR spectral data, details of which are
16

given in the Experimental Section. The C, H and N analysis suggested 1:1 complex
formation with picrate as counter anion and a picric acid as an inclusion. Selected
portion of the ES-MS spectrum of the K+ complex of
3 is shown in Fig. 4 and that of
the ionophores and are submitted as ESI (Figs S15-17). The m/z values for all of
1,
2
4
the four complexes are in excellent agreement with the calculated values for 1:1
+K+]+ (calculated
(calculated 974.09), 908.63 for [3+K+ +
909.06) and 864.38 for [4+K+ +
complex with picrate counter anion. The values are 822.55 for [
822.98), 973.55 for [2+K+ +
1
]
]
(calculated
]
(calculated 865.02). Selected IR bands with
assignment are given in the Experimental Section, the notable new bands appeared
around 1560 and 1361cm-1 are due to picrate anion [42]. Molecular structure of the
K+-complex of
1, [1
+K]+pic-, has been established by single crystal X-ray study and
described below.
3.5. Crystal structures
Single crystal X-ray structures of two of the four ionophores (
complex of have been determined and details of crystallographic parameters are
given in Table 2. The crystal of was grown with the aid of picric acid and obtained
as a co-crystal composed of one molecule each of and picric acid, it also contains a
water molecule as solvent of crystallisation. The ionophore with picric acid
1
and 3
) and the K+
1
1
1
1
crystallised in orthorhombic system with P212121 space group. The crystal structure
clearly shows that the calix-crown moiety has adopted the 1,3-alternate conformation
(Fig. 5). The water molecule is held via intermolecular H-bonding interaction
involving N-H...O and O-H...O contacts and encapsulated in the tethered aza-crown
moiety, as depicted in Fig. 5. The Packing diagram viewed down a-axis shows
alternate layers of calix crown moiety and picric acid oriented along b-axis, as shown
17

in Fig. 6. It is interesting to note that one picric acid moiety is involved in six H-
bonding interactions with four different calix-crown moieties, as depicted in Fig. S18
(ESI). Thus nitro oxygen atoms O12 and O16 act as acceptors and involved in three
and two C-H...O contacts, respectively and the acidic proton H11c from the picric acid
makes O-H...N contact with the amino nitrogen of the calix moiety bridging the
adjacent layers in generating a two dimensional hydrogen bonded network. Details of
all these pertinent H-bonding interactions with symmetry code are given in Table 3.
The ionophore
3 crystallized in orthorhombic system (Pna21 space group) with 1,3-
alternate conformation of the calix moiety with two crown rings at the opposite side of
the calix moiety, as depicted in Fig.7. The intramolecular C-H...O interaction between
oxygen atoms and phenyl/methylene hydrogen atoms play important role in adopting
1,3-alternate conformation of the calix moiety. Details of these C-H...O contacts with
symmetry code is given in Table 3. The packing viewed down b-axis shows the
layered orientation of the molecules along ac-plane (Fig. 8). Detail analysis of the
packing revealed intermolecular C-H... interaction between the phenyl ring (C1 to C6
π
with C1g centroid) with methylene hydrogens H39A and H39B, which stabilised the
molecule in the crystal lattice. Different parameters of these interactions with
symmetry
codes are: (C(39)-H(39A)....Cg(1): H(39A)....Cg(1) = 3.288 Å;
C(39)....Cg(1) = 3.652Å; <C(39)-H(39A)....Cg(1) = 104.40°; symmetry code: -1/2+x,-
1/2-y,z and C(39)-H(39B)....Cg(1): H(39B)....Cg(1) = 3.153Å; C(39)....Cg(1) =
3.652Å; <C(39)-H(39B)....Cg(1) = 113.62°; symmetry code=-1/2+x,-1/2-y,z).
For the K+ complex of 1, ([ +K]+pic-), suitable crystals for X-ray study were grown
1
from dichloromethane/ethyl acetate and the compound crystalized in monoclinic
system with P21/c space group. As depicted in Fig. 9, the K+ ion is encapsulated in the
crown ring at the upper rim making interaction with four oxygen atoms of the crown
18

moiety and with the deprotonated phenolic oxygen atom from the picrate anion. The
aza-crown moiety encapsulated a water molecule making O-H...O interaction with the
oxygen atoms of the crown ring. In addition to the complex formed, one molecule
each of picric acid and ethyl acetate are found in the crystal lattice. The K...O distance
ranges from 2.14 to 2.56 Å, which is well within the range for previously reported
complexes [24,42]. The packing diagram viewed down a-axis with various inter
molecular H-bonding interactions are shown in Fig.10. It shows that the complex
molecules with opposite orientations are aligned along the layers along c-axis and the
picric acid and ethyl acetate are oriented between the adjacent layers of the complex
generating ABAB... motifs as depicted in Fig.10. The picric acids in the lattice are
associated by π...π stacking interactions with the centroid...centroid distance between
the phenyl rings 3.65 Å. The picric acid and ethyl acetate (B layers) are involved in C-
H...O contacts linking the AA motifs via C-H...O and N-H...O contacts with the
complex generating hydrogen bonded 2D supramolecular network. Details of all these
H-bonding interactions and symmetry code are given in Table 3.
3.6. Isothermal calorimetric titration study
For determination of association constant (Ka), stoichiometry of the complexes formed
(n) and other thermodynamic parameters, isothermal calorimetric titration (ITC) for
the reaction of all of the four ionophores with potassium picrate was carried out in dry
acetonitrile at 298 K. Detail experimental procedure has given in the Experimental
Section. The ITC titration profiles for all the four ionophores are presented in Fig. 11
and the data such as association constant (Ka), stoichiometry (n), entropy change
(T∆S), enthalpy change (∆H) and free energy change (∆G) are summarized in Table 4.
The ‘n’ values in the table are in the range 0.71 to 1.04, which suggests 1:1 K+-
19

ionophore complex formation and the thermodynamic parameters indicate that the
binding process is exothermic. The log values of the association constants (log Ka) are
in the range 4.14 to 5.92, which indicates strong binding of the metal ion with the
ionophore. This observation is consistent to the finding noted in NMR spectral
changes, where new peaks appeared at the cost of original peaks due to strong
complex formation. The thermodynamic parameters obtained (Table 4) suggests that
this reaction is enthalpy driven, which partially compensated by negative entropy
changes for
1-3. Since the ITC data suggest 1:1 complex formation, therefore we
propose that the metal ion is encapsulated in the 6-membered crown ring at the upper
rim. Slight variation in association constant and thermodynamic parameters is
probably due to different substituents attached with the aza- nitrogen atom, which
might have induced conformational changes in the calixarene-biscrown skeleton.
3.7. Comparative study
Metal binding property of calix-crown and calix-bis-crown hybrid molecules has been
studied substantially in different context. The aim of the present study is to develop
calix-crown based systems, which can be applied for extraction of K+ from a mixture
of metal ions in aqueous media. To develop such a system, three important factors to
be considered, high selectivity towards K+ in presence of mixture of metal ions, it
must form strong complex with K+ and the system must work in aqueous media. To
the best of our knowledge, there is no report n the literature, except one from us [42],
i
where mixture of metal ions has been used for competitive complexation study and
selectivity has been determined on the basis of analysis of metal ions in the isolated
extract. In our earlier reported calix-crown-6 systems [42], the best selectivity ratio
obtained for K+/Na+ is 1.08. In the present work, the selectivity ratios for K+/Na+
20

obtained are in the range 3.30 – 4.89, which suggest that the performance of the
present systems are much better than the earlier systems. Since there is no other report
on selectivity study with mixture of metal ions, therefore no comparison on this aspect
can be made. It may be noted that discrimination of Na+ is difficult compared to Ca2+
and Mg2+, that is why comparison is made only with selectivity ratio of K+ to Na+.
There are number of reports with calix-bis-crown systems, where association constant
with various individual metal ions have been determined using methods such as two-
phase extraction, NMR titration, fluorescence titration, ITC etc., mostly in non-
aqueous solvents and the log K values were compared to get an idea about selectivity.
The ratios of log K of K+ to Na+ for some of the calix-bis-crown-systems containing
crown-6 moiety are 1.88 and 2.68 [59], 1.17 [60], 1.26 and 1.74 [61], 0.98 – 1.20 [62],
a system where apparently Na+ could not be detected and log K for K+ is 4.6 [63].
There are number of calix-mono-crown systems, where the ratios are comparable to
bis-crown systems, one of such recent example shows the ratios 1.69 to 2.01 [64]. It is
not appropriate to compare these values with that of the present work as it has been
estimated following a completely different procedure, however it looks like the
ionophores reported here are better than many other reported systems. Moreover, the
method applied in the present study can be applied for practical application for
extraction of metal ions from a real system, which is distinctly different from other
reports.
3.8. Application
High selectivity of these ionophores towards K+ motivated us to use these ionophores
for extraction of K+ from a natural source such as sea bittern (concentrated sea water
obtained after removal of common salt). This sea bittern mainly contains Na+ = 6.8 %,
21

K+ = 1.4 %, Mg2+ = 3.6 %, Ca2+ = 0.02 % and trace amount of other metal ions such as
Li+, Sr2+ etc.. However concentration of these metal ions may vary slightly depending
on the conditions under which bittern is collected. The experiments were carried out
following the procedure similar to that described for equimolar mixture of metal ions,
except bittern is used instead of mixture of metal ions. The bar diagram showing the
fraction of metal ions in bittern and that in the extract from bittern using all the four
ionophores is shown in Fig.12 and the ion chromatogram showing the metal ions in
the extract obtained by two-phase extraction using the ionophore
3 is shown in Fig.
S19 and the data are given in Table 5. It may be noted that in the case of bittern, like
artificial mixture of metal ions, the selectivity towards K+ is very high and ionophore
3
exhibited maximum selectivity (86.1 %). These ionophores may therefore find
application in extraction of K+ from aqueous solution, particularly in presence of Na+,
Mg2+ and Ca2+.
4. Conclusions
A series of 1,3-alternate calixarene derivatives with a crown-6 moiety at one side of
the calix unit and a aza-crown moiety with different substituents at the aza nitrogen
atom on the other side have been synthesized, characterized and their competitive
metal binding property with alkali and alkaline earth metal ions in aqueous media has
been investigated. The selectivity has been investigated with equimolar mixture of Li+,
Na+, K+, Mg2+, Ca2+ and Sr2+ in aqueous media using picrate as counter anion
following two-phase extraction method followed by ion chromatographic assay of
metal ions’ concentrations in the extract (organic phase). All the four ionophores
exhibit K+ selectivity >75 % with highest selectivity (85.6 %) for the ionophore having
ethyl ester substituent at the aza nitrogen atom. Binding constants for all the
ionophores with K+, stoichiometry of the complexes formed and thermodynamic
22

parameters such as entropy change, enthalpy change and free energy change have been
evaluated by isothermal calorimetric titration and the data exhibit most favourable
reaction for the ionophore with ethyl ester substituent at the aza nitrogen atom (3). For
1
structural elucidation in solution, detail H NMR study was carried out. Molecular
structures of the two ionophores (1 and 3 1 have
) and the K+-complex of the ionophore
been established by single crystal X-ray study. On the basis of the structural
information, it has been concluded the K+ ion is encapsulated in the crown-6 cavity at
the upper rim. The high selectivity of these ionophores for K+ is basically due to size-
matching factor, the size of the K+ ion matched very well with the cavity size of the
crown-6, it is also evident from NMR study. These ionophores have also exhibited
high selectivity towards K+ when applied for extraction of metal ions from bittern.
Appendix A. Supplementary data
CCDC 1040795, 1040794 and 1040796 contains the supplementary crystallographic
data for the ionophores 1.pic, 3 and the complex [1.K+.Pic-].HPic.EtAc.H2O,
respectively.
These
data
can
be
obtained
or
free
of
the
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
from
Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
CSIR-CSMCRI publication No.: 223/2014. Financial assistance received in the form
of Network Project CSC 0105 from CSIR, New Delhi is gratefully acknowledged. V.
R gratefully acknowledges the CSIR for awarding Senior Research Fellowship (SRF).
23

We thank Dr. Babulal Rebary, Mr. A. K. Das, and Mr. V. Vakani for recording ion
chromatography, mass and FT-IR spectra, respectively.
24

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