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2 Chromogenic sensing of Cu(II) by imino linked thiacalix[4]arene in mixed

3 aqueous environment

4 Manoj Kumar *, J. Nagendra Babu, Vandana Bhalla, Abhimanew Dhir

5 Department of Chemistry, UGC-Centre for Advanced Studies, Guru Nanak Dev University, Amritsar, Punjab 143005, India

67

9a r t i c l e i n f o

0 Article history:

1 Received 28 September 20082 Accepted 13 February 20093 Available online xxxx

4 Keywords:5 Thiacalix6 Chromogenic7 Colorimetric8 Cu2+ selective9

0

a b s t r a c t

Thiacalix[4]arene based imino receptors 45 carrying azophenol appendage have been synthesized and

studied for their binding abilities towards different metal ions (Li+, Na+, K+, Cd2+, Ni2+, Cu2+, Zn2+, Ag+,Pb2+ and Hg2+). Receptor 4 showed selective chromogenic sensing for Cu2+ ions in mixed aqueous condi-

tions (THF:H2O, 9:1 v/v).

2009 Published by Elsevier B.V.

7

8 The selective sensing of cations [1] and anions [2] by synthetic9 receptors is one of the most important areas of supramolecular0 chemistry. A variety of effective chromogenic chemosensors for al-1 kali and alkaline earth metal ions have already been developed [3],2 however, a few reports on the development of chromogenic sen-3 sors for soft metal ions, in particular Cu(II) ions. The selective sig-4 naling of copper ion is a very important topic for the detection and5 treatment of these ions in various chemical systems including liv-6 ing systems [4,5]. It plays an important role in fundamental phys-7 iological processes in organisms ranging from bacteria to8 mammals [4]. Copper is also capable of displacing other metals9 which act as co-factors in enzyme-catalysed reactions [4b]. Alter-0 ation in copper cellular homeostasis is related to serious neurode-1 generative diseases like Menkes and Wilson diseases [6], familial2 amyotropic lateral sclerosis [7], Alzheimers disease [8], and prion3 diseases [9]. Thus, designing sensors for copper has recently drawn4 worldwide attention [10]. A few chromoionophores based on azo-5 benzene have been reported for copper (II) ions [11].6 Calixarenes are one of the most important molecular scaffolds7 in the hostguest supramolecular chemistry [12]. There are few8 examples of chromogenic receptors based on calixarenes [139 18], but most of these chromogenic receptors are selective for alka-0 lialkaline metal ions [14] or alkylamines [15]. There are only a1 few reports on chromogenic sensing of soft metal ions like Hg2+

2 [16], Pb2+ [17] and In3+ [18] by calix[4]arene based receptors.3 Our research work involves the design, synthesis and evaluation4 of calix[4]arene and thiacalix[4]arene based receptors selective for5 soft metal ions [19] and anions [20]. Recently, we reported fluoro-

genic receptor for Cu2+ and Hg2+ ions based on thiacalix[4]arenes

[21] and calix[4]arene [22], respectively. Herein, we have synthe-

sized new azophenol linked imino derivatized thiacalix[4]arenes

45 in cone conformation as chromogenic sensor for Cu(II) ions.

To the best of our knowledge this is the first report where chromo-

genic moiety introduced at the lower rim of thiacalix[4]arenes

have been studied for sensing of copper ions.

Thus, [1 + 2] condensation of diamine 1 [23] with 2.0 mol equiv.

of aldehyde 23 [24] in refluxing ethanol furnished compounds 4

5 [25] in 88% and 85% yields, respectively (Scheme 1). The 1H NMR

spectra of these compounds 45 showed two singlets (18H each)

corresponding to tert-butyl group, two triplets (4H each) corre-

sponding to NCH2 and OCH2 protons, three singlets (4H each, 2H

each) corresponding to aromatic and imino protons. In the IR spec-

tra, compounds 45 show absorption bands at 1642 and

1632 cm1, respectively, due to the imine groups, and there is no

band corresponding to free amino and aldehyde group which indi-

cates that the condensation has taken place. The FAB mass spectra

of receptors 45 showed parent ion peaks at 1313 and 1291

(M+ + 1), respectively, corresponding to a 1:2 condensation prod-

ucts. These spectroscopic data corroborate the structures 45 for

these receptors.

Receptors 45 contain iminophenol unit supplemented with sul-

phur rich environment of thiacalix[4]arene for cation binding and

two p-nitrophenylazo groups for monitoring the cation-binding

event. To evaluate the binding abilities of receptors 45 toward dif-

ferent cations, we carried out UVVis experiments in tetrahydrofu-

ran/H2O (9:1, v/v) solution at pH 7.0 maintained using N-2-

hydroxyethylpiperazine-N0-2-ethane sulphonicacid (HEPES) buffer.

The cation binding properties of receptors 45 were investi-

gated by UVVis spectroscopy. All the titration experiments were

1387-7003/$ - see front matter 2009 Published by Elsevier B.V.doi:10.1016/j.inoche.2009.02.012

* Corresponding author. Tel.: +91 183 2258802 09x3205; fax: +91 183 2258820.

E-mail address: [email protected] (M. Kumar).

Inorganic Chemistry Communications xxx (2009) xxxxxx

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Inorganic Chemistry Communications

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carried out in THF:H2O (9:1, v/v) at pH 7.0 in HEPES, by adding ali-

quots of different metal ions (Li+, Na+, K+, Cd2+, Ni2+, Cu2+, Zn2+, Ag+,

Pb2+ and Hg2+) as their perchlorate salts. The absorption spectra of

compounds 45 (10 lM) exhibit typical absorption bands at 387

and 351 nm, respectively. This absorption maxima is attributedto the internal charge transfer (ICT) transition of the azo-chromo-

phore from OH towards the electron withdrawing group [26]. Fig. 1

shows the UVVis changes upon addition of 2 mol. equiv. of differ-

ent metal ions (Li+, Na+, K+, Cd2+, Ni2+, Cu2+, Zn2+, Ag+, Pb2+ and

Hg2+) to the solution of receptor 4 (10lM) in THF/H2O (9:1, v/v)

at pH 7.0. It is found that the absorption band of receptor 4 under-

goes a red shift (Dd 64 nm) as 2.0 mol equiv. of Cu2+ ions (20 lM)

are added, whereas no significant changes were observed upon

addition of metal ions such as Li+, Na+, K+, Cd2+, Ni2+, Zn2+, Ag+,

Pb2+ and Hg2+. Under similar conditions receptor 5 showed no

change in absorption in presence of any of these ions. Thus, it is in-

ferred that there is an interaction of receptor 4 with Cu2+ ions.

Further, the binding abilities of receptor 4 towards Cu2+ ions

were investigated by carrying out UVVis titrations. Upon addition

of increasing amount of Cu2+ ions (020 lM) to the solution of

receptor 4, the absorption peak at 387 nm decreases, while a

new peak gradually moving to longer wavelength finally reaching

maximum value at 451 nm appeared (Fig. 2). The formation of

the new absorption band at 451 nm upon addition of Cu2+ ions

to the solution of receptor 4, is due to the selective coordination in-

duced deprotonation of the azo-phenol group upon addition of

Cu2+ ions. This deprotonation of azophenolic group causes a change

in electron density shift towards the nitro substituent of the chro-

mogenic receptor. This increases the dipole moment, which ulti-

mately leads to a bathochromic shift due to the stabilization of

the photoexcited state more than the ground state. The relative

1absorbance of the complexed form to uncomplexed form (A451/

1A387) of free ligand was 0.52 and it increases 3.0 fold to 1.56 (Inset1ofFig. 2) with addition of 20 lM (2.0 equiv) of Cu2+ ion. Thus, UV1Vis spectra of receptor 4 show ratiometric behaviour upon com-

1plexation with Cu2+

ions.1A noticeable colour change could be observed by naked eye by1mixing the receptor 4 (10 lM) with Cu2+ ion. The free ligand 41showed a light yellow color in THF/H2O (9:1, v/v) at pH 7.0 main-

1tained using HEPES buffer. Upon addition of 20 lM of Cu2+ ion to

1the solution of receptor 4 (Inset in Fig. 1), the receptor solution1turns dark-red. Thus, there is a selective sensing visible to the1naked eye.13The stoichiometry of 4-Cu2+ complex was determined by the

13method of continuous variation. The total concentration of recep-

S

OHOH OOSS S

NH2 H2N

O

OH

N

N

S

OHOH OOSS S

N N

OH

N

N

HO

N

N

+

Ethanol, Reflux

2

1 2-3

4 R=NO25 R=Cl

2 R=NO23 R=Cl

4-5R R

R

Scheme 1. Synthesis of calix[4]arene derivatives 45.

0

0.1

0.2

0.3

0.4

0.5

300 350 400 450 500 550 600 650

Wavelength (in nm)

Absorbance

Receptor 1

Li+

Na+

K+

Ni2+

Cd2+

Cu2+

Zn2+

Ag+

Pb2+

Hg2+

4 4-Cu2+

Li+

Na+

K+

Ni2+

Cd2+

Cu2+

Zn2+

Ag+

Pb2+

Hg2+

Receptor 4

Fig. 1. Absorption spectra of receptor 4 (10 lM) upon addition of various metal ions

(20 lM) in THF:H2O (9:1, v/v) at pH 7.0 maintained by HEPES buffer. Inset showingthe change in colour of solution upon addition of Cu2+ ions (20 lM).

0

0.1

0.2

0.3

0.4

0.5

300 350 400 450 500 550 600

Wavelength (in nm)

Absorbance

0.5

0.7

0.9

1.1

1.3

1.5

0 0.5 1 1.5 2 2.5 3

Mol Equiv. of Cu2+

ions

A451/A387

Fig. 2. UVVis spectra of receptor 4 (10 lM) upon addition of Cu2+ ions (020 lM)

in THF:H2O (9:1, v/v) at pH 7.0 maintained by HEPES buffer. Inset: showing theratiometric increase in absorption as function of [Cu2+].

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Mole Fraction of Cu2+

ions

Absorbance

Fig. 3. Jobs plot for receptor 4 and Cu2+ ions [4] + [Cu2+] = 10 lM in THF:H2O (9:1,v/v) at pH 7.0 maintained by HEPES buffer.

2 M. Kumar et al. / Inorganic Chemistry Communications xxx (2009) xxxxxx






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2 tor 4 and Cu2+ ion was kept constant (10lM), with continuously3 varying molar fraction of guest ([Cu2+]/[4] + [Cu2+]). Fig. 3 shows4 the Jobs plot of compound 4 with Cu2+ ions at 451 nm. The absor-

5 bance at kmax of 451 nm approaches a maximum when the molar6 fraction of Cu2+ ions is between 0.6 and 0.7, which means that 47 and Cu2+ ions formed a 1:2 (4:Cu2+) complex.8 Analysis of the UV-titration of compound 4 with Cu2+ ions was9 performed by means of SPECFIT programme (Global analysis sys-0 tem V3.0 for 32-bit Window System), which uses singular value1 decomposition and non-linear regression modeling by the Lever-2 bergMarquardt method [27]. The global analysis showed that3 the titration curves were consistent with the formation of two4 complexes namely 1:1 and 1:2 (4:Cu2+) and the values obtained5 from the titration data analyzed by SPECFIT are log6 b2 = 13.6 0.22 M

2 and log b1 > 6.0 M1. This data is indicative

7 of a strong complexation between the thiacalix[4]arene receptor8 4 with Cu2+ ion, with no allosteric effect on the second Cu2+ ion

9 complexation.0 To test the practical applicability of compound 4 as a Cu2+ ion1 selective chromogenic sensor, competitive experiments were car-2 ried out in presence of variable concentration of Cu2+ ions (03 20 lM), mixed with 100 lM of various background cations such4 as Li+, Na+, K+, Cd2+, Ni2+, Zn2+, Ag+, Pb2+ and Hg2+ ions (Fig. 4). It5 was found that the ratiometric response of receptor 4 for Cu2+ ions6 at variable concentration, was unaffected by the presence of all7 these background cations with the exception of Pb2+ ions, where8 a small but visible deviation from the normal behaviour was ob-9 served upon addition of 100 lM. These results suggest that recep-0 tor 4 could be used as a potential Cu2+-selective colorimetric sensor1 in mixed aqueous environment.2 In conclusion, novel thiacalix[4]arene based chromogenic3 receptors 45 were synthesized and their chromogenic behaviour4 towards soft metal ions was studied. It was observed that the5 receptor 4 showed a chromogenic ratiometric response upon selec-6 tive binding of Cu2+ ions. A distinct colour change from light yellow7 to dark red of receptor 4 was observed in presence of Cu2+ ions.

8 Acknowledgements

9 We are thankful to the CSIR, DST, UGC (New Delhi) for financial0 support. We are also thankful to the Central Drug Research Insti-1 tute (CDRI), Lucknow, for FAB mass spectra.

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[19] V. Bhalla, J.N. Babu, M. Kumar, T. Hattori, S. Miyano, Tetrahedron Lett. 48(2007) 1581 (and reference 10 cited therein).

[20] (a) J.N. Babu, V. Bhalla, M. Kumar, R.K. Mahajan, R.K. Puri, Tetrahedron Lett. 49(2008) 2772;(b) J.N. Babu, V. Bhalla, M. Kumar, R.K. Puri, R.K. Mahajan, New J. Chem. 33(2009) 20, doi:10.1039/b816610bb.

[21] (a) A. Dhir, V. Bhalla, M. Kumar, Tetrahedron Lett. 49 (2008) 4227;(b) R. Kumar, V. Bhalla, M. Kumar, Tetrahedron 64 (2008) 8095;(c) V. Bhalla, R. Kumar, M. Kumar, A. Dhir, Tetrahedron 63 (2007) 11153.

[22] A. Dhir, V. Bhalla, M. Kumar, M. Org. Lett. 10 (2008) 4891.[23] V. Bhalla, M. Kumar, T. Hattori, S. Miyano, Tetrahedron 60 (2004) 5881.[24] R.N. Sen, B.N. Banerji, J. Indian Chem. Soc. 12 (1935) 293.[25] Synthesis of receptor 45: To a solution of diamine 1 (81.6 mg, 0.1 mmol) in

ethanol (20 ml) was added a solution of aldehyde 23 (0.22 mmol) in ethanol(15 ml). The mixture was refluxed for 12 h to separate a solid, which was

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.5 1 1.5 2 2.5

Mol Equiv. of Cu2+

ions

A451/A387

Blank

Li+

Na+

K+

Ni2+

Cd2+

Zn2+

Ag+

Pb2+

Hg2+

Fig. 4. Ratiometric absorption change of receptor 4 (10 lM) to various equivalents

of Cu2+ ions (4, 8, 12, 16, 20lM) in the presence of interfering cations (100 lM) in

THF:H2O (9:1, v/v) at pH 7.0 maintained by HEPES buffer.

M. Kumar et al. / Inorganic Chemistry Communications xxx (2009) xxxxxx 3




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