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Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=gsch20 Download by: [CSMCRI Central Salt & Marine Chemicals Res. Inst.] Date: 02 November 2015, At: 02:53 Supramolecular Chemistry ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20 Synthesis, crystal structures and competitive complexation property of a family of calix- crown hybrid molecules and their application in extraction of potassium from bittern Vallu Ramakrishna, E. Suresh, Vinod P. Boricha, Anjani K. Bhatt & Parimal Paul To cite this article: Vallu Ramakrishna, E. Suresh, Vinod P. Boricha, Anjani K. Bhatt & Parimal Paul (2015) Synthesis, crystal structures and competitive complexation property of a family of calix-crown hybrid molecules and their application in extraction of potassium from bittern, Supramolecular Chemistry, 27:10, 706-718, DOI: 10.1080/10610278.2015.1080367 To link to this article: http://dx.doi.org/10.1080/10610278.2015.1080367 View supplementary material Published online: 15 Oct 2015. Submit your article to this journal Article views: 14 View related articles View Crossmark data

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Full Terms & Conditions of access and use can be found athttp://www.tandfonline.com/action/journalInformation?journalCode=gsch20

Download by: [CSMCRI Central Salt & Marine Chemicals Res. Inst.] Date: 02 November 2015, At: 02:53

Supramolecular Chemistry

ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20

Synthesis, crystal structures and competitivecomplexation property of a family of calix-crown hybrid molecules and their application inextraction of potassium from bittern

Vallu Ramakrishna, E. Suresh, Vinod P. Boricha, Anjani K. Bhatt & ParimalPaul

To cite this article: Vallu Ramakrishna, E. Suresh, Vinod P. Boricha, Anjani K. Bhatt & ParimalPaul (2015) Synthesis, crystal structures and competitive complexation property of a familyof calix-crown hybrid molecules and their application in extraction of potassium from bittern,Supramolecular Chemistry, 27:10, 706-718, DOI: 10.1080/10610278.2015.1080367

To link to this article: http://dx.doi.org/10.1080/10610278.2015.1080367

View supplementary material Published online: 15 Oct 2015.

Submit your article to this journal Article views: 14

View related articles View Crossmark data

Page 2: supramolecular chemistry pub. V.R

Synthesis, crystal structures and competitive complexation property of a family of calix-crownhybrid molecules and their application in extraction of potassium from bittern

Vallu Ramakrishnaa, E. Suresha,b, Vinod P. Borichaa, Anjani K. Bhatta and Parimal Paula,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, Bhavnagar364002, India

(Received 1 April 2015; accepted 2 August 2015)

A family of calix-crown hybrid molecules containing calix[4]arene and crown-5/6, either at lower rim or at both upper and

lower rims, have been synthesised, characterised and their competitive complexation property towards alkali and alkaline

earth metal ions in aqueous media have been investigated. The competitive metal ion extraction study, carried out with

equimolar mixture of Liþ, Naþ, Kþ, Mg2þ, Ca2þ and Sr2þ in aqueous media, revealed that the amount of Kþ extracted is

remarkably high compared to other metal ions. Complexation with Kþ has been investigated by 1H NMR, association

constants and thermodynamic parameters have been determined by isothermal calorimetric study. The molecular structures

of one of the receptors and two of the Kþ complexes have been established by single crystal X-ray study. One of the

receptors formed bimetallic complex and it exhibited interesting polymeric network structure with bridged picrate anion.

These receptors have been applied for extraction of metal ions from bittern.

Keywords: Calixarene; crystal structures; two-phase extraction; isothermal calorimetric study; alkali and alkaline earthmetal ions

Introduction

Calixarenes are receiving increasing attention because of

their applications in various areas such as supramolecular

chemistry, coordination chemistry, molecular sensor etc.

(1–15). The chemistry of calixarene has become more

versatile because they can be easily modified according to

the requirement and these modified calixarenes provide a

highly organised architecture for the assembling of

converging binding sites (16–20). Among various sizes

of calixarenes, calix[4]arenes have been mainly used for

the application in supramolecular chemistry and molecular

sensing because of their rigid structure and binding ability

towards various ions and molecules (21–26). Calix[4]

arenes exist in four conformations such as cone, partial

cone, 1,2 alternate and 1,3-alternate cone, among which

cone and 1,3 alternate cone conformations have been

extensively used for the study related to interaction with

metal ions (27–31). Another class of calixarene

derivatives has been developed incorporating crown ethers

as ionophore and these calix-crown hybrid molecules are

suitable to use as molecular sensor, particularly for the

interaction with alkali and alkaline earth metal ions (32–

38). This class of compounds has been extended to calix[4]

arene bis-crown ethers, in which two crown rings

incorporated at both sides of the calix moiety in its 1,3-

alternate conformation and it can accommodate two metal

ions in the two crown moieties (33, 34, 39–42).

The aim of the present study is to design and develop

calix-crown hybrid molecules, which can be used for

extraction of alkali metal ions, preferably potassium, from

aqueous solution/natural sources such as sea bittern.

Bittern is a solution, which left out after separation of

common salt (NaCl) from sea water by solar evaporation,

it mainly contains Naþ, Kþ, Mg2þ and Ca2þ and small

amount of Sr2þ and Liþ(43). If an extractant can be

developed, which can extract Kþ selectively in presence of

other metal ions mentioned above, then it may find

potential application in recovery of Kþ from the most

abundant natural source. It is commercially important as

India imports its entire requirement of potash.

In this paper we report synthesis, characterisation and

competitive complexation property of a family of calix-

crown hybrid receptors, in which crown-6 is incorporated

and the calixarene moiety is in cone and 1,3-alternate

conformations. In addition to the crown-6 moiety at the

lower rim, another crown-5/6 ring has also been

incorporated at the upper rim forming calix[4]arene

biscrown-5/6 receptors, which have the possibility of

complex formation with two metal ions. All of these

molecules have been characterised on the basis of

analytical and spectroscopic data and molecular structure

of one of the compound was established by single crystal

X-ray study. Competitive complexation property of all of

these molecules has been investigated using equimolar

q 2015 Taylor & Francis

*Corresponding author. Email: [email protected]

Supramolecular Chemistry, 2015

http://dx.doi.org/10.1080/10610278.2015.1080367Vol. 27, No. 10, 706–718,

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mixture of Liþ, Naþ, Kþ, Mg2þ, Ca2þ and Sr2þ in aqueous

media by two-phase extraction method followed by

analysis of metal ions in the extract by ICP. Kþ complexes

were also characterised in solid state and molecular

structures of two of the complexes were established by

single crystal X-ray study. Binding constant and other

thermodynamic parameters for the formation of two of the

Kþ complexes were evaluated by isothermal calorimetric

titration. These receptor molecules were applied for

extraction of metal ion(s) from bittern.

Results and discussion

Synthesis of compounds 1–4

The route followed for the synthesis of the compounds

1–4 is shown in Scheme 1 and details of the

experimental procedures are given in the ‘Experimental’

section. The starting compounds (A–D) were syn-

thesised following the literature procedure (44). The

compounds 1–4 were synthesised from the starting

compounds A–D, respectively by the reaction with one

equivalent amount of 1,2-catechol modified pentaethy-

lene glycol ditosylate in presence of two equivalent of

K2CO3/CsCO3 (for 2–4) as a base in refluxing

acetonitrile. The compounds were purified by column

chromatography using 1:2 ethyl acetate-hexane as eluent.

The C, H and N analysis of these compounds

(‘Experimental’ section) are in excellent agreement

with the proposed composition of the compounds. The

ES-MS spectra of all the four compounds are given as

supporting information (ESI, Figures S1–S4), the m/z

values are matched well with that of calculated values;

713.42 for 1 (calculated for [1 þ Kþ]þ ¼ 713.83),

797.56 (calculated for [2 þ Kþ]þ ¼ 797.99), 855.67

(calculated for [3 þ Naþ]þ ¼ 855.90) and 871.66

(calculated for [3 þ Kþ]þ ¼ 872.01), 1009.49 (calcu-

lated for [4 þ Csþ]þ ¼ 1009.86). The m/z values for all

of these compounds are in excellent agreement with the

metal containing cation (Mþ) instead of Hþ, indicatingthat these receptors can readily form complexes with

alkali metal ions. These compounds were further

characterised with the aid of 1H NMR spectroscopy.

The 1H NMR spectra of 1–4 have been submitted as ESI

(Figures S5–S8) and the data with assignment of peaks

are given in the ‘Experimental’ section. For compound 1,

the appearance of two doublets at d 3.41 and 4.34 for

Ar-CH2-Ar methylene protons confirmed its cone

conformation and the singlet appeared at d 7.73 is due

Scheme 1. Synthetic route for the synthesis of compounds (1–4); reagents/solvents: (i) iodo propane/K2CO3/acetonitrile, reflux, (ii)Tetra/pentaethyleneglycol ditosylate/K2CO3/acetonitrile, reflux, (iii) 1,2-catechol modified pentaethyleneglycol ditosylate /Cs2CO3/acetonitrile, reflux, (iv) 1,2-catechol modified pentaethyleneglycol ditosylate/K2CO3/acetonitrile, reflux and (v) 1,2-catechol modifiedpentaethyleneglycol ditosylate /Cs2CO3/acetonitrile, reflux.

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to OH protons (37). For the compounds 2–4, the signals

at d 3.86, 3.76 and 3.85, respectively are due to

ArCH2Ar methylene protons, suggesting 1,3-alternate

cone conformation for these compounds (33, 45). The

aromatic protons for the calix moiety, as expected,

appeared as two triplets and two doublets in the region d6.6–7.2 and the protons due to catechol moiety appeared

as multiplet at d 7.1. The methylene protons due to

crown moieties appeared in the region d 3.3–4.2. On the

basis of elemental analysis, ES-MS and 1H NMR data

the molecular structure assigned for 1–4 are shown in

Scheme 1. The molecular structure of 2 has been

established from single-crystal X-ray study (discussed

later).

Competitive complexation study of the compounds 1–4

The compounds 1–4 were used to investigate their

competitive complexation ability towards Liþ, Naþ, Kþ,Ca2þ, Mg2þ and Sr2þ (constituents of bittern) in aqueous

media containing equimolar amount of these metal ions

and picrate as counter anion. This study was carried out by

two-phase extraction in water-dichloromethane using

mixture of the metal ions and picrate, which facilitates

transport of metal complex formed from aqueous to non-

aqueous media by strong ion-pair interaction. The

concentrations of the metal ions in the extract were

determined by inductively coupled plasma (ICP) spec-

trometer, detail of the procedure has given in the

‘Experimental’ section and the data are given in Table 1.

The data indicate that the amount of metal ions extracted is

in the order Kþ . Naþ q Mg2þ, Ca2þ and no Liþ and

Sr2þ was detected in the extract. As far as the receptors are

concerned, the amount of Kþ in the extract increases in the

order 1 , 2 , 3 , 4, the maximum Kþ/Naþ ratio

observed for 4 is 4.31 (80.7 molar percentage). A bar

diagram showing the fraction of metal ions extracted using

all the four receptors is shown in Figure 1. Significantly

high amount of Kþ extracted for 3 and 4 is due to

incorporation of two crown rings at the opposite sides of

the calix moiety, the second crown ring incorporated is

without benzene ring and is flexible and can adjust for

making effective interaction with metal ion. Presence of

two rings can also encapsulate two metal ions enhancing

capability for extraction of metal ions. Lower extracting

capability of 1 and 2 for Kþ compared to 3 and 4 is due to

presence of one crown rings compared to two rings in 3

and 4, rigid nature of the 1,2-catechol containing crown-6

ring and the cavity size of the crown ring which is such that

it can accommodate Kþ as well as Naþ. For 1, probablyintramolecular H-bonding played important role for the

extraction of lowest amount of Kþ (discussed later).

Crystal structure of the Kþ complex of 2 (discussed in the

‘Crystal structures’ section) exhibited that the metal ion

bounded with four oxygen atoms of the crown ring and the

two oxygen atoms of the catechol moiety remained

uncoordinated, though Kþ in this case is co-ordinately

unsaturated and a water molecule occupied the fifth

coordination site. This observation clearly suggests the

rigid nature of the catechol containing crown-6 moiety and

the cavity size is such that it is difficult to discriminate Naþ

in the solution containing mixture of metal ions. For detail

solution study, we were interested to investigate

interaction of these receptors with Kþ with the aid of 1H

NMR spectroscopy, isothermal calorimetric titration and

solid state characterisation including structure determi-

nation by X-ray study.

1H NMR study

The interaction of Kþ ion with the receptors 1–4 were

investigated in acetonitrile by 1H NMR study using

Kþpic2. The 1H NMR spectral changes upon addition of

incremental amount of Kþpic2 were recorded, details of

the experimental procedure is given in the ‘Experimental’

Table 1. Ratio of the amount of metal ions extracted fromequimolar mixture of ions by two-phase extraction.a

Ratio of metal ions extractedb

Compounds Kþ/Naþ Kþ/Mg2þ Kþ/Ca2þ

1 1.15 10.97 2.752 1.66 16.79 6.753 4.19 9.53 23.554 4.31 11.97 35.94

a Concentration (%) of metal ion in the original solution (beforeextraction), Liþ ¼ 3.18, Naþ ¼ 10.4, Kþ ¼ 17.7, Mg2þ ¼ 10.9,Ca2þ ¼ 18.1 and Sr2þ ¼ 39.5.b The ratio is calculated by [% of Kþ in the extract][% of Mþ in theoriginal solution]/ [% of Mþ in the extract][% of Kþ in the originalsolution].

Figure 1. (Colour online) Bar diagram showing fraction ofmetalions extracted from equimolar mixture of metal ions using 1–4.

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section and the spectral changes for receptors 2 and 3 are

shown in Figures 2 and 3, respectively and that of 1 and 4

are submitted as ESI (Figures S9 and S10). It may be noted

that spectral changes upon incremental addition of Kþ ion

are two types, for 1 and 2 some of the peaks have shifted;

(Figures 2 and S9) on the other hand, for 3 and 4 new peaks

generated at the expense of some of the original peaks

(Figures 3 and S10). The continuous shift of peaks and

formation of new peaks with the progress of complex

formation is related to stability of complexes formed in

solution. If the donor atoms in the complexation unit can’t

make strong interaction due to improper size matching of

Figure 2. 1H NMR spectra for compound 2 upon addition of 0.24 (a), 0.48 (b), 1.20 (c), 3.61 (d) and 4.40 (e) molar equivalent amountsof Kþpic; shifting of some of the signals were noted upon addition of K-picrate.

Figure 3. 1H NMR spectra for compound 3 upon addition of 0.24 (a), 0.48 (b), 1.20 (c), 4.40 (d) and 6.62 (e), molar equivalent amountsof Kpic; new peaks are growing with the disappearance of the peaks of the original complex.

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the metal ion and the cavity size or for any other reason(s),

then simultaneous bond formation and dissociation

between metal ion and donors goes on in solution and in

such a situation the chemical shifts of the signals of the

complex formed and that of free receptor averaged out

resulting in a single peak for both the species showing

continuous shift of the signal with the progress of the

reaction (33, 34, 46). On the other hand, if the size of the

metal ion fits well in the calix-crown cavity, then it can

make stable complex with strong interaction with the

donor atoms and in such a situation new peaks grow due to

formation of the stable complex and the original signals of

the receptor disappear with the progress of complex

formation (33, 34). In the present case, the NMR data

indicate that for 1 and 2, the Kþ ion did not coordinate with

all the oxygen atoms of the crown moiety and the co-

ordinately unsaturated metal ion is involved in bond

formation and dissociation in solution involving all the

donor oxygen atoms of the crown moiety. For 3 and 4, the

NMR data indicates formation of stable complex, the

second crown ether moiety, which is more flexible, and

also involvement of picrate anion in coordination might

have played a major role in the formation of strong

complexes with Kþ. The crystal structures of 2 and 3 have

provided more information about it (discussed in the

crystallography section).

Isolated K1 complexes of the compounds 1–4

For solid state characterisation, Kþ complexes of 1–4

were synthesised. These complexes were obtained by the

reaction of the receptors with Kþ-pic2 in chloroform at

room temperature, as described in the ‘Experimental’

section. These complexes were characterised on the basis

of elemental analysis, ES-MS, IR and 1H NMR spectral

data, detail data of which are given in the ‘Experimental’

section. The C, H and N analysis data suggested 1:1

stoichiometry for the complexes derived from 1–3 and

1:2 stoichiometry for the complex of 4 with picrate as

counter anion. The ES-MS spectra of the Kþ complexes

of 1, 2, 3 and 4 have submitted as ESI (Figures S11–

S14). The m/z values of these complexes are in excellent

agreement with the calculated values. The values are

713.75 for [1 þ Kþ]þ (calculated 713.25), 797.72 for [2þ Kþ]þ (calculated 797.38), 871.57 for [3 þ Kþ]þ

(calculated 871.36), 915.83 and 477.38 for [4 þ Kþ]þ

and [4 þ K2þ]2þ, respectively (calculated 915.38 and

477.16). 1H NMR data with assignment of peaks are

given in the ‘Experimental’ section. The IR bands for

picrate anion are also given in the ‘Experimental’

section. Molecular structures of the Kþ-complexes of 2

and 4 have been established by single crystal X-ray study

and described below.

Table 2. Crystal data and refinement parameters for the compound 2 and the Kþ complexes, [2·Kþ·H2O]pic2 and [4·Kþ

2 ·1.5pic2]

0.5pic2·C6H5CH3.

Identification code cbc6prxm cb6prkm kcbc6prf

Chemical formula C48H54O8 C54H58KN3O16 C71H72K2N6O26

Formula weight 758.91 1044.13 1503.55Crystal Colour Colourless Yellow YellowCrystal Size (mm) 0.58 £ 0.14 £ 0.09 0.34 £ 0.29 £ 0.25 0.43 £ 0.20 £ 0.09Temperature (K) 150(2) 293(2) 150(2)Crystal System Monoclinic Monoclinic OrthorhombicSpace group P21 P21/n Pna21a(A) 11.6016(15) 19.0640(2) 15.760(2)b(A) 10.4070(13) 15.3264(16) 13.2369(17)c(A) 17.2280(2) 20.2810(2) 33.381(4)a (8) 90 90 90b (8) 107.377(2) 117.417(2) 90g (8) 90 90 90Z 2 4 4V(A3) 1985.1(4) 5260.3(10) 6963.8(15)Density (Mg/m3) 1.270 1.318 1.434m (mm21) 0.085 0.174 0.225F(000) 812 2200 3144Reflections collected 11,384 25,963 33,066Independent reflections 7204 9251 6255Rint 0.0221 0.0332 0.0521Number of parameters 507 759 947GOF on F 2 1.125 1.022 0.947Final R1/wR2 (I) $ 2s(I) 0.0451/ 0.1033 0.0625/0.1536 0.0697/0.2007Weighted R1/wR2 (all data) 0.0481/0.1050 0.1003/0.1775 0.0783/0.2118CCDC number CCDC 1043882 CCDC 1043883 CCDC 1043884

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Crystal structures

Single crystal X-ray structures of the compound 2 and that

of the Kþ complexes, [2·K1·H2O]pic2 and [4·K12 -

K12 ·1.5pic

2]0.5pic2·C6H5CH3 were determined and

details of crystallographic parameters are given in

Table 2. Compound 2 was crystallised in monoclinic

system with P21 space group and the calix moiety adopted

1,3-alternate conformation as depicted in Figure 4. The

packing diagram viewed down a-axis (Figure S15) shows

the layered orientation of the molecules along bc-plane.

Details of the intermolecular CZH· · ·O contacts with

symmetry codes are given in Table 3.

The Kþ complex of 2, [2 þ Kþ·H2O]þ, was

crystallised in monoclinic system with P21/n space

group. The crystal structure of this complex is shown in

Figure 5, it may be noted that Kþ in this complex is

penta-coordinated with four oxygen atoms from crown

moiety and one from coordinated water molecule

encapsulating the metal ion in the calix-crown cavity.

The K-O distances ranges from 2.804(2) to 3.344(3) A

and the K1-O16 distance is 2.604(4) A, respectively. The

packing diagram viewed down a-axis (Figure S16) shows

pairs of [2 þ Kþ·H2O]þ cations are tuned with opposite

orientations in which the coordinated water molecules

are positioned up and down fashion with the picrate

anions oriented between the pairs of the cations. Details

of pertinent H-bonding interactions with symmetry codes

are given in Table. 3.

Crystal structure analysis of the Kþ-complex of 4

revealed that it consists of two independent Kþ, two

picrate anions and a molecule of toluene as solvent of

Table 3. H-bonding parameters for the compounds 2 and the Kþ complexes, [2·Kþ·H2O] pic2 and [4·K2·1.5pic]0.5pic·C6H5CH3.

Compound DZH· · ·A d(H· · ·A)(A) d(D· · ·A)(A) ,DZH· · ·A(8)

Ionophore 2 C(7)ZH(7A)· · ·O(5)1 2.49(1) 3.435(3) 165.5(2)C(34)ZH(34)· · ·O(4)2 2.57 (2) 3.432(3) 153.6(3)

Symmetry code: (1) x, 2 1 þ y,z; (2) 2 2 x, 2 1/2 þ y,1 2 z[2·Kþ·H2O]pic

2 O(16)ZH(16C)· · ·O(9)1 1.84(2) 2.714(4) 157.2(3)O(16)ZH(16D)· · ·O(3) 1 2.40(2) 3.128(4) 135.3 (2)C(31)ZH(31A)· · ·O(14)2 2.54(3) 3.340(5) 139.7(3)C(41)ZH(41B)· · ·O(13)3 2.54(2) 3.293(3) 134.1(4)C(42)ZH(42B)· · ·O(13)3 2.55(2) 3.317(5) 136.2(3)C(10)ZH(10)· · ·O(9)1 2.56(4) 3.430(7) 158.2(3)

Symmetry code: (1) x,y,z; (2) 1/2 þ x,1/2 2 y, 2 1/2 þ z; (3) 1 2 x, 2 y,1 2 z[4·K2·1.5pic]0.5 pic·C6H5CH3 C(11)ZH(11)· · ·O(20)1 2.43(3) 3.312(8) 158.9(2)

C(30)ZH(30B)· · ·O(23)2 2.43(4) 3.324(7) 152.8(4)C(31)ZH(31B)· · ·O(17)3 2.54(5) 3.310(6) 136.6(6)C(32)ZH(32B)· · ·O(23)2 2.58(6) 3.480(8) 154.7(4)C(35)ZH(35)· · ·O(19)4 2.50(6) 3.390(5) 160.6(3)C(39)ZH(39B)· · ·O(19)3 2.53(4) 3.225(6) 128.7(3)C(44)ZH(44B)· · ·O(18)5 2.55(6) 3.288(7) 132.7(4)C(65)ZH(65A)· · ·O(15)5 2.49(5) 3.280(8) 139.7(3)C(65)ZH(65B)· · ·O(18)6 2.51(6) 3.448(8) 166.2(3)C(65)ZH(65B)· · ·N(3)6 2.48(4) 3.411(7) 164.4(4)

Symmetry code: (1)21/2 þ x,3/2 2 y,z; (2) 1/2 þ x,3/2 2 y,z; (3) 1 2 x,1 2 y, 2 1/2 þ z; (4) 3/2 2 x, 2 1/2 þ y, 2 1/2 þ z; (5) x,y,z;(6) 1/2 þ x,1/2 2 y,z

Figure 4. (Colour online) Ball and stick model of 2 depictingthe structure of the calix-crown ligand moiety with atomnumbering scheme.

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crystallisation. This complex was crystallised in orthor-

hombic system with Pna21 space group. The crystal

structure, shown in Figure 6, exhibits those two

independent potassium ions K1 and K2, which are

encapsulated in the crown cavity by coordinating with

the oxygen atoms of the crown moieties and picrate

anions. The K1–O distances involving oxygen atoms of

the crown moiety are in the range 2.828(5) to 3.307(6) A.

The other metal ion (K2) is hexa-coordinated with four

oxygen atoms from crown ether moiety with K2-O

distances ranging from 2.876(8) to 3.119(10) A and the

phenolate and nitro oxygen atoms of the picrate anion.

Packing and various hydrogen bonding interactions

between the K2-calix-crown monocationic strands with

the uncoordinated picrate and lattice toluene molecule

viewed down b-axis is shown in Figure S17. The zigzag

monocationic strands are oriented along c-axis and the

uncoordinated picrate anions and toluene molecules are

aligned and oriented along a-axis. Details of all these

hydrogen bonding interactions and the relevant symmetry

code are given in Table 3.

Powder XRD study

To confirm that the crystal structures of the Kþ-complexes are truly represent the bulk materials, the

powder XRD pattern of the bulk materials of the Kþ-complexes of 1 and 3 were recorded and the same were

simulated from the single crystal X-ray data. The

simulation was carried out following the method of Spek

(47). Excellent matching between the experimental and

simulated diffractograms (Figures S18 and S19) con-

firmed that the crystal structures actually represent the

bulk material.

Isothermal calorimetric titration

Isothermal calorimetric titration (ITC) for the reaction of

the ionophores 1, 3 and 4 with potassium picrate was

carried out in dry acetonitrile at 298 K for the

determination of association constant (Ka), stoichiometry

of the complexes formed and other thermodynamic

parameters. Detail experimental procedure has given in

the ‘Experimental’ section. The ITC titration profiles for

the receptors 1 and 3 are presented in Figure 7 and the

data such as association constant (Ka), entropy change

(TDS), enthalpy change (DH) and free energy change

(DG) are summarised in Table 4. The ITC titration

profiles indicate that the binding process is exothermic

and the curves for 1 and 3 exhibited complex formation

with a typical 1:1 stoichiometry. The ITC titration profile

for 4 didn’t fit well either for 2:1 or for 1:1 metal-ligand

stoichiometry, which probably due to the formation of

the polymeric complex and therefore the data for this

compound has not reported here. The log values of the

association constants (logKa) for 1 and 3 are 3.43 and

Figure 5. (Colour online) Ball and stick representation of[2·Kþ

. H2O]þ with atom numbering scheme (picrate anion is

omitted for clarity).

Figure 6. (Colour online) Ball and stick model for the complex [4·K12 ·1.5pic

2]0.5pic2 depicting the coordination of Kþ and formationof 1D coordination network (hydrogen atoms and lattice toluene molecule are omitted for clarity).

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5.67, respectively, which indicate that binding of Kþ

with 3 is much stronger than that of 1. For 1, the

catechol containing crown moiety is the only site for the

metal ion to interact; however, for 3 another crown ring

at the opposite side of the catechol containing crown

moiety is available for complex formation. Though the

cavity size of the crown-5 moiety does not fit well for

Kþ, yet the metal ion can interact strongly from above of

the plane of the crown moiety and also can interact with

the oxygen atom of the picrate anion to satisfy its

coordination number. The low association constant for 1

compared to 3 is probably due to strong intramolecular

H-bonding interaction between OH proton and adjacent

oxygen atom (OZH· · ·O) of the crown moiety, which

might have prevented easy entry of the metal ion into the

crown cavity to form complex. Similar situation was also

noted earlier for complexation of a calix-crown receptor

with Kþ and in that case the H-bonding interaction,

which prevented entry of metal ion into the cavity, has

been demonstrated with the help of crystallographic

study (32). The thermodynamic parameters obtained

from ITC study (Table 4) indicate that it is an enthalpy

driven process, which partially compensated by

unfavourable entropy change. The values of free energy

change (DG) are in agreement with the observed

association constants.

Application

All of these receptors were applied to extract metal ions

from a natural source such as sea bittern (the solution

obtained after removal of common salt from sea water by

solar evaporation). This sea bittern mainly contains

Naþ ¼ 6.8%, Kþ ¼ 1.4%, Mg2þ ¼ 3.6%, Ca2þ ¼ 0.02%

and trace amount of other metal ions such as Liþ, Sr2þ etc.,

however concentration of these metal ions may vary

slightly depending on the conditions under which bittern is

collected. The metal ions were extracted following the

0.0 0.5 1.0 1.5

–6.0

–4.0

–2.0

0.0

–15.00

–10.00

–5.00

0.00

0 10 20 30 40 50 60

Time (min)

µcal

/sec

Molar Ratio

kcal

mol

–1 o

f inj

ecta

nt

0.0 0.5 1.0 1.5 2.0

–35.0

–30.0

–25.0

–20.0

–15.0

–10.0

–5.0

0.0

–25.00

–20.00

–15.00

–10.00

–5.00

0.00

0 10 20 30 40 50 60

Time (min)

µcal

/sec

Molar Ratio

kcal

mol

–1 o

f inj

ecta

ntFigure 7. Isothermal calorimetric titration profiles of 1 and 3 with Kþ·pic2 in acetonitrile at 298K.

Table 4. Association constant (Ka), entropy change (TDS),enthalpy change (DH) and free energy change (DG) obtainedfrom isothermal calorimetric titration.

Isothermal calorimetric titrations’ data for Kþ ion

Ionophore logKa

DH(kcalmol21)

TDS(kcalmol21)

DG(kcalmol21)

1 3.43 28.95 24.26 24.693 5.67 233.40 225.63 27.77

Table 5. Ratio of the amount of metal ions extracted frombittern by two-phase extraction using all the four receptors.a

Ratio of metal ions extractedb

Compounds Kþ/Naþ Kþ/Mg2þ Kþ/Ca2þ

1 8.18 30.59 0.022 18.11 140.61 0.103 27.59 131.52 0.114 32.65 205.69 0.16

aConcentration (%) of metal ions in the bittern (before extraction)Naþ ¼ 57.65, Kþ ¼ 11.56, Mg2þ ¼ 30.64, and Ca2þ ¼ 0.13 %.b The ration is calculated by [% of Kþ in the extract] [% of Mnþ inbittern]/[% of Mnþ in the extract][% of Kþ in bittern].

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procedure similar to that used for extraction of metal ions

from their equimolar mixture (two-phase extraction). The

only difference is that bittern is used instead of solution

containing equimolar mixture of metal ions. The data are

presented in Table 5 and the bar diagram showing the

fraction of metal ions in bittern and that in the extract using

all the receptors is shown in Figure S20. It may be noted

that the amount of Kþ extracted is increased in the order

1 , 2 , 3 , 4, however considerable amount of Ca2þ

was also extracted.

Conclusion

A family of calix[4]arene derivatives incorporating crown-

6 moiety at the lower rim and crown-5/6 at the upper rim

for two of them have been synthesised, characterised and

their competitive complexation property towards alkali

and alkaline earth metal ions in aqueous media has been

investigated. The competitive complexation ability has

been studied by two-phase extraction method with

equimolar mixture of Liþ, Naþ, Kþ, Mg2þ, Ca2þ and

Sr2þ in aqueous media using picrate as counter anion. The

metal ions in the extract (organic phase) were analysed by

inductively coupled plasma (ICP) spectrometer and the

data suggests that the amount of Kþ extracted increases in

the order 1 , 2 , 3 , 4 and the maximum value noted

for 4 is 80.7 molar percentage. It also showed small

amount of Naþ, Ca2þ and Mg2þ, however no Liþ and Sr2þ

was detected in the extract. Binding constants, stoichi-

ometry of the complexes formed and thermodynamic

parameters such as entropy change, enthalpy change and

free energy change for two of the complexes have been

evaluated by isothermal calorimetric titration. The

complex formation in solution was investigated by 1H

NMR study. Molecular structures of one of the receptors

(1) and two of the Kþ complexes with 3 and 4 have been

established by single crystal X-ray study. These molecules

were applied to extract metal ions from the sea bittern,

where similar trend as observed for equimolar mixture of

metal ions, was noted. The larger ring size of the crown

moiety at upper rim, which fits well with the ionic size of

Kþ, and formation of bimetallic complexes, probably

helped to achieve high molar percentage of Kþ recovery

with the receptor 4.

Experimental

Materials

The materials catechol, 2-(2-chloroethoxy) ethanol,

tetraethylene glycol and pentaethlene glycol were

purchased from S.D Fine chemicals. All the solvents

were of analytical grade and purified by standard

procedures before use (48). Milli-Q (Millipore Corpor-

ation) water was used for two-phase extraction exper-

iments and ICP analysis. Metal picrate salts were prepared

by the reaction of picric acid and metal hydroxide in

aqueous media. All the reagents 2-(2-chloroethoxy)ethyl,

4-methylbezene sulfonate and pentaethyleneglycol dito-

sylate were prepared following the published procedure

(49). The starting materials p-tert-butylcalix[4]arene (50),

dealkylated calix[4]arene (51) were synthesised from the

literature procedure.

Measurements

Elemental analyses (C, H and N) were performed on a

model Vario Micro CUBE elemental analyser. Mass

spectra were recorded on a Q-TOF MicroTM LC-MS

instrument. Infrared spectra were recorded on a Perkin-

Elmer spectrum GXFT-system as KBr pellets. NMR

spectra were recorded on models DPX 200 and Avance II

500 Brucker FT-NMR instruments. The cation concen-

tration was measured with inductively coupled plasma

(ICP) spectrometer, model optima 2000DV, provided by

Perkin Elmer instrument. Single crystal X-ray structures

were determined using a Bruker SMART 1000 (CCD)

diffractometer. Thermodynamic parameters were deter-

mined using isothermal calorimeter (ITC200) provided by

Microcal Company.

Synthesis

Dihydroxycalix[4]arene benzocrown-6 (1)

This compound was prepared from calix[4]arene, in a

typical procedure 3.0 g (7mmol) of calix[4]arene, 0.976 g

(7mmol) K2CO3 were taken in dry acetonitrile (150mL)

and the content was stirred udder reflux for 2 h under

nitrogen atmosphere. After that 4.20 g (7mmol) of 1,2-

catechol modified pentaethyleneglycol ditosylate, dis-

solved in 20mL dry acetonitrile was added drop wise with

duration of 2 h and then the reaction mixture was refluxed

for 48 h. The solvent of the reaction mixture was then

evaporated under reduced pressure and the solid mass thus

obtained was dissolved in dichloromethane (100mL). The

solution was then treated with 100mL of 1N HCl, the

organic phase was separated and dried with anhydrous

MgSO4. After removal of solvent, the crude product

obtained was purified by column chromatography using

silica gel and 1:2 mixture of ethylacetate and hexane as

eluents. Yield: 2.8 g (60%). 1H NMR (CD3CN) d: 7.74 (s,

2H, ArOH-calix), 7.11 (d, J ¼ 7.5Hz, 4H, ArH, calix),

6.97–6.94 (overlapped doublet and multiplet, 6H, ArH,

calix and catechol), 6.92 (dd, 2H, ArH, catechol), 6.77 (t,

J ¼ 7.5Hz, 2H, ArH, calix), 6.65 (t, J ¼ 7.5Hz, 2H, ArH,

calix), 4.34 (d, J ¼ 13Hz, 4H, ArCH2Ar), 4.25 (t,

J ¼ 4.5Hz, 4H, Ar-OCH2), 4.11 (t, J ¼ 4.5Hz, 4H,

ArOCH2), 4.07 (t, J ¼ 4.5Hz, 4H, ArOCH2), 4.05 (t,

J ¼ 4.5 Hz, 4H, ArOCH2), 3.41 (d, J ¼ 13Hz, 4H,

ArCH2Ar). Selected IR band (KBr pellet, cm21) 3363 n

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(OH). ES-MS:m/z ¼ 713.42 (100%), calcd for [1þ Kþ]þ,713.83; Anal. Calcd for C42H42O8: C, 74.76; H, 6.27;

found: C, 74.32; H, 6.52;

1,3-Alternate dipropylcalix[4]arene-2,4-benzocrown-6

(2)

This compound was synthesised following the similar

procedure as described for 1 with the exception that 1,3-

dipropylcalix[4]arene was used instead of calix[4]arene

and Cs2CO3 was used as base instead of K2CO3. This

compound was purified by recrystallisation from chloro-

form and acetonitrile (1:1), which gave colourless crystals.

Yield: 3.0 g (62%). 1H NMR CD3CN d: 7.05 (overlapped,

dd, 6H, ArH, calix and catechol), 7.03 (d, J ¼ 4Hz, 2H,

ArH, calix), 6.95 (dd, J ¼ 6Hz, 2H, catechol), 6.75

(t, J ¼ 7.5Hz, 2H, ArH, calix), 6.68 (t, J ¼ 7.5Hz, 2H,

ArH, calix), 4.23 (t, J ¼ 3.75Hz, 4H, ArOCH2), 3.86

(t, J ¼ 3.5Hz, 4H, ArCH2Ar), 3.73 (t, J ¼ 4Hz, 4H,

ArOCH2), 3.67 (s, 4H, ArCH2Ar), 3.65 (s, 4H, ArOCH2),

3.63 (t, J ¼ 3.75Hz, 4H, ArOCH2), 3.49 (t, J ¼ 7.5Hz,

4H, ArOCH2), 1.57 (quartet, J ¼ 7.5Hz, 4H, -CH2-), 0.84

(t, J ¼ 7.5Hz, 6H, -CH3). ES-MS: m/z ¼ 797.56 (100%),

calcd for [2 þ Kþ]þ, 797.99; Anal. calcd for C48H54O8 C,

75.96; H, 7.17; Found: C, 75.46; H, 7.38.

1,3-Alternate calix[4]arenecrown-5-2,4-benzocrown-6 (3)

This compound was synthesised following the similar

procedure as described for 1, the only differences are calix

[4]arene-crown-5 was used instead of calix[4]arene and

Cs2CO3 was used as base instead of K2CO3. The

compound obtained was purified by column chromatog-

raphy using silica gel and 1:2 mixture of ethylacetate and

hexane. Yield: 60%. 1H NMR (CD3CN) d: 7.12 (d,

J ¼ 7.5Hz, 4H, ArH, calix), 7.01 (overlapped signals, 6H,

ArH, calix and catechol), 6.95 (dd, J ¼ 5.75Hz, 2H,

ArH, catechol), 6.84 (t, J ¼ 7.5Hz, 2H, ArH, calix),

6.63 (t, J ¼ 7.5Hz, 2H, ArH, calix), 4.16 (t, J ¼ 3.75Hz,

4H, -OCH2, crown), 3.76 (t, J ¼ 3.75Hz, 4H, ArOCH2,

crown), 3.75 (s, 8H, ArCH2Ar), 3.66 (t, J ¼ 4.5Hz, 4H,

OCH2CH2O, crown), 3.56 (s, 8H, -OCH2CH2O, crown),

3.50 (t, J ¼ 4.5Hz, 4H, -OCH2CH2O, crown), 3.45

(t, J ¼ 6.0 Hz, 4H, -OCH2CH2O, crown), 3.29

(t, J ¼ 6.25 Hz, 4H, -OCH2CH2O, crown). ES-MS:

m/z ¼ 872.42 for [3 þ Kþ]þ (calculated 872.01), 855.67

(50%), calculated for [3 þ Naþ]þ, 855.90. Anal. calcd for

C50H56O11: C, 72.10; H, 6.77; found: C, 71.71; H, 6.64;

1,3-Alternate calix[4]arenecrown-6-2,4-benzocrown-6 (4)

This compound was prepared following the similar

procedure as described for 3 using calix[4]arene-crown-6

as starting material. Yield 2.2 g (63%). 1H NMR (CD3CN)

d: 7.14 (d, J ¼ 7.5Hz, 4H, ArH, calix), 7.08 (m, 6H, ArH,

calix and catechol), 7.03 (dd, J ¼ 6.0Hz, 2H, catechol),

6.96-6.91 (m, 2H, ArH, calix), 6.76-6.69 (m, 2H, ArH,

calix), 4.22 (t, J ¼ 3.75Hz, 4H, –ArOCH2, crown), 3.85

(t, J ¼ 3.75Hz, 4H, –ArOCH2, crown), 3.73-3.69 (m, 4H,

-OCH2CH2O, crown), 3.66 (m, 12H, -OCH2CH2O,

crown; 8H, ArCH2Ar,), 3.64 (m, 4H, -OCH2CH2O,

crown), 3.60-3.59 (m, 4H, -OCH2CH2O, crown), 3.53

(m, 4H, -OCH2CH2O, crown). ES-MS: m/z ¼ 1009.49,

(100%) calcd for [4 þ Csþ]þ, 1009.86; Anal. calcd for

C52H60O12: C, 71.21; H, 6.89; Found: C, 71.59; H, 6.47.

Selectivity determination

Competitive complexation property of the receptors 1–4

towards Liþ, Naþ, Kþ, Mg2þ, Ca2þ and Sr2þ were

determined by two-phase extraction method using

aqueous solution containing equimolar mixture of these

metal ions with picrate as counter anion. In a typical

procedure, equimolar mixture (10mL) of alkali metal

picrate (Liþ, Naþ, Kþ, Mg2þ, Ca2þ and Sr2þ, 0.1M each)

in aqueous media and CH2Cl2 solution (10mL) of the

required receptor (0.002 M) were mixed and vigorously

shaken in a vortex mixture for 30min. The solution was

then transferred to a separating funnel and allowed to

stand for 4 h. 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 5508C for 5 h. The residue

was dissolved in deionised water (, 5mL) and filtered

through filter paper (0.2mm). The concentrations of the

metal ions in the filtrate were estimated by ICP

spectrometer using a standard solution containing a

mixture of LiCl, NaCl, KCl, MgCl2,CaCl2 and SrCl2(10 ppm each) and the selectivity ratios of metal ions in

the extract are given in Table 1.

Synthesis of metal complexes

The Kþ complexes were synthesised following a general

procedure. In a typical experiment, a mixture of 0.05mmol

of the receptors (1/2/3/4) and Kþ picrate (0.5mmol, ten-

fold excess) was stirred in chloroform at room temperature

for 24 h. The unreacted picrate salt was then removed by

filtration and the complex was obtained by removing the

solvent from the filtrate under reduced pressure. The

yellow complex was then purified by dissolving the mass

in minimum volume (ca. 4mL) of dichloromethane (in

which Kþ picrate is almost insoluble) followed by

filtration to remove trace amount of unreacted Kþ picrate.

The solution was then removed, which gave yellow

product. The 1H NMR spectra of the complexes did not

show any signal corresponding to free ionophore or excess

picrate anion, which confirmed the complete complexa-

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tion. Yield: 85–90%. Crystals suitable for the single

crystal X-ray study were grown by diffusion of toluene

into the dichloromethane solution of the complexes.

Characterisation data of the complexes

[1·K1]pic2: 1H NMR (CD3CN) d: 8.61 (s, 4H, picrate

anion) 7.40 (s, 2H, Ar-OH-calix), 7.13 (d, J ¼ 7.5Hz, 4H,

phenylene), 6.91-6.85 (m, 6H, ArHm2calix, 2H over-

lapped ArHp2calix), 6.76 (t, J ¼ 7.5Hz, 2H, ArHm2calix),

6.70 (t, J ¼ 7.5Hz, 2H, ArHp2calix), 4.23 (t, J ¼ 4.25Hz,

4H, ArOCH2), 4.20 (d, J ¼ 4.5Hz, 4H, OCH2CH2O-

benzocrown-6), 4.17 (d, J ¼ 9.0Hz, 4H, ArCH2Ar), 4.07

(t, J ¼ 4.25Hz, 4H, OCH2CH2O-benzocrown-6), 4.02(t,

J ¼ 4.0Hz, 4H, OCH2CH2O-benzocrown-6), 3.47 (d,

J ¼ 13.5 Hz, 4H, ArCH2Ar). Selected IR band (KBr

pellet, cm21) 1592, 1502 cm21 for NO2 and 1333 cm-1 for

picrate; ES-MS: m/z ¼ 713.75 (100%), calcd for

[2 þ Kþ]þ713.83. Anal. calcd For C48H44O15N3K; C,

61.21; H, 4.71; N, 4.46; Found: C, 62.05; H, 4.51; N, 4.32;

[2·K1. H2O]Pic2: 1H NMR CD3CN d: 8.62(s, 2H,

picrate anion), 7.16 (d, J ¼ 7.5Hz, 4H, phenylene),7.04

(d, J ¼ 7.5Hz, 4H, ArHm2calix), 6.93 (d, J ¼ 4Hz, 2H,

ArHm2calix), 6.85 (t, J ¼ 7.5Hz, 2H, ArHp2calix), 6.73

(t, J ¼ 7.5Hz, 2H, ArHp2calix), 4.23 (broad, s, 4H,

ArOCH2), 4.05 (broad, s, 4H, ArCH2Ar), 3.93 (broad, s,

4H, -OCH2CH2O-crown-6), 3.85 (broad, s, 4H, ArCH2Ar),

3.72 (q, J ¼ 15.5Hz, 8H, -OCH2CH2O-crown-6), 3.56 (t,

J ¼ 7.5 Hz, 4H, -OCH2CH2O-crown-6), 1.57 (q,

J ¼ 7.5Hz, 4H, –CH2–), 0.85 (t, J ¼ 7.5Hz, 6H, -CH3).

Selected IR band (KBr pellet, cm21) 1591, 1500 cm21 for

NO2 and 1382 cm21 for picrate; ES-MS: m/z ¼ 797.72

(100%), calcd for [2 þ Kþ]þ797.99; Anal. calcd for C54

H58 O16 N3 K, C, 62.12; H, 5.59; N, 4.02; Found: C, 62.54;

H, 5.86; N, 4.11;

[3·K1]Pic2: 1H NMR (CD3CN) d: 8.60 (s, 2H, picrateanion), 7.30 (d, J ¼ 7.5Hz, 4H, phenylene), 7.18 (d,

J ¼ 7.5Hz, 4H, ArHm2calix), 7.04 (dd, J ¼ 3.16Hz, 2H,

ArHm2calix), 6.94 (dd, J ¼ 2Hz, 2H, ArHp2calix, 2H,

ArHm2calix), 6.84 (t, J ¼ 7.5Hz, 2H, ArHp2calix), 4.24

(t, J ¼ 5.5 Hz, 4H, Ar-OCH2-crown-5-), 3.88 (t,

J ¼ 2.25Hz, 4H, Ar-OCH2-benzocrown-6), 3.86 (t, 4H, -

OCH2CH2O-crown-5), 3.79 (broad, s, 4H, -OCH2CH2O-

crown-5), 3.74 (overlapped, s, 8H, ArCH2Ar, 2H, -

OCH2CH2O-benzocrown-6), 3.68 (t, 4H, J ¼ 1.5Hz -

OCH2CH2O-crown-5, 2H, -OCH2CH2O-benzocrown-6),

3.66 (s, 8H, –OCH2CH2O-benzocrown-6). Selected IR

band (KBr pellet, cm21) 1552, 1503 cm21 for NO2 and

1307 cm21 for picrate; ES-MS: m/z ¼ 871.42 (100%),

872.54 (85%) calcd for [3 þ Kþ]þ872.00; Anal. calcd for

C56 H58 O18 N3 K, C, 61.14; H, 5.31; N, 3.81; Found: C,

60.27; H, 5.24; N, 4.06;

[4·K12 ·1.5pic

2]0.5pic2·C6H5CH3:1H NMR (CD3CN)

d: 8.61 (s, 4H, picrate anion), 7.32 (d, J ¼ 7.5Hz, 4H,

phenylene), 7.23 (d, J ¼ 7.5Hz, 4H, ArHm2calix), 7.11 (t,

J ¼ 3.5Hz, 2H, ArHm2calix), 7.01 (dd, J ¼ 4.62Hz, 2H,

ArHp2calix, 2H, ArHm2calix), 6.74 (t, 2H, ArHp2calix),

4.26 (t, J ¼ 4.0Hz, 4H, Ar-OCH2-benzocrown-6), 3.96

(broad, s, 4H, ArOCH2-crown-6), 3.85 (t, J ¼ 4.25Hz, 8H,

OCH2CH2O-crown-6), 3.77 (s, 4H, OCH2CH2O-crown-6,

8H, ArCH2Ar), 3.73 (d, J ¼ 6.5Hz, 12H, OCH2CH2O-

benzocrown-6, overlapped, 2H, OCH2CH2O-crown-6).

Selected IR band (KBr pellet, cm21) 1594, 1505 cm21 for

NO2 and 1357 cm21 for picrate; ES-MS: m/z ¼ [4

þ K1þ]þ916.30, (100%) [4 þ K2

þ]þ477.65 (30%) calcd

for, [4 þ Kþ]þ916.05, [4 þ K2þ]þ477.57; Anal. calcd For

C71 H71 O26 N6 K2, C, 56.75; H, 4.76; N, 5.59; Found: C,

55.86; H, 4.97; N, 5.63.

Single crystal X-Ray study

Crystals of suitable size of the ionophore 1, and

the complexes [2·Kþ]pic2 and [4·K12 ·1.5pic

2]·0.5pic2·

C6H5CH3 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 100K

using graphite monochromatised MoKa (l ¼ 0.71073 A)

radiation on a Bruker SMART APEX diffractometer

equipped with CCD area detector. The data integration and

reduction were processed with SAINT software (52).

An empirical absorption correction was applied to the

collected reflections with SADABS (53). The structures

were solved by direct methods using SHELXTL (54) and

refined on F 2 by the full-matrix least-squares technique

using the SHELXL-97 (55) package. Graphics are

generated using PLATON (56) and MERCURY 1.3.

(57). For 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.

Isothermal calorimetric titration

The stoichiometry of the complexes formed, binding

constant, and other thermodynamic parameters for the

reaction of the receptors with KþPic2 in acetonitrile were

determined by isothermal calorimetric titration (ITC).

In this experiment, first a blank experiment was carried out

using solute and solvent (without taking receptor) and this

data was subtracted from the titration data for complex

formation. For complexation study, the solution of the

ionophore (2mM for 1 and 0.67mM for 3) in dry

acetonitrile was taken in the cell and the solution of Kpic

in the same solvent (16mM for 1 and 6mM for 3) was

taken in the syringe. The solution of the Kpic was then

added maintaining the successive additions of 2mL,spacing 180 s intervals. The calorimetric study was

performed at 298K. The blank data was then subtracted

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Page 13: supramolecular chemistry pub. V.R

from the ITC data for complex formation and the resultant

data was fitted with the aid of Origin 7 provided by

MicroCal by using (1:1) curve fitting model. This plot gave

the values of stoichiometry, binding constant (Ks),

enthalpy change (DH), entropy change (DS) and free

energy change (DG) was calculated using the equation

DG ¼ DH–TDS.

Application for extraction of metal ions from bittern

The receptors molecules were applied for extraction of

metal ions from sea bittern using the similar procedure

as described for competitive complexation study by two-

phase extraction method, except sea bittern and picric

acid were used instead of mixture of metal-picrate salts.

The concentration of metal ions in the organic phase was

estimated by ICP spectrometer, as described above.

Acknowledgements

CSIR-CSMCRI Registration No.: 041/2015.We gratefully thankone of the reviewers for valuable suggestions for ITCmeasurement. Financial assistance received in the form ofNetwork Project (CSC 0105) from CSIR, New Delhi is gratefullyacknowledged. V. R gratefully acknowledges CSIR for awardingSenior Research Fellowship (SRF). We thank Rajesh Patidar,Arun. K. Das, and V. Vakani for ICP analysis, mass and FT-IRspectra, respectively.

Disclosure statement

No potential conflict of interest was reported by the authors.

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