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Spectrochimica Acta Part A 77 (2010) 740–748

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journa l homepage: www.e lsev ier .com/ locate /saa

series of transition and non-transition metal complexes from a N4O2

exadentate Schiff base ligand: Synthesis, spectroscopic characterization andfficient antimicrobial activities

aikat Sarkara, Kamalendu Deyb,∗,1

Department of Chemistry, Santipur College, Santipur 741404, West Bengal, IndiaDepartment of Chemistry, University of Kalyani, Kalyani 741235, West Bengal, India

r t i c l e i n f o

rticle history:eceived 9 April 2010eceived in revised form 19 June 2010ccepted 28 June 2010

eywords:chiff baseexadentate

R

a b s t r a c t

Some transition and non-transition metal complexes of the hexadentate N4O2 donor Schiff base ligand1,8-N-bis(3-carboxy)disalicylidene-3,6-diazaoctane-1,8-diamine, abbreviated to H4fsatrien, have beensynthesized. All the 14 metal complexes have been fully characterized with the help of elemental analyses,molecular weights, molar conductance values, magnetic moments and spectroscopic (UV–Vis, IR, NMR,ESR) data. The analytical data helped to elucidate the structures of the metal complexes. The Schiff base,H4fsatrien, is found to act as a dibasic hexadentate ligand using N2N2O2 donor set of atoms (leaving theCOOH group uncoordinated) leading to an octahedral geometry for the complexes around all the metalions except VO2+ and UO2

2+. However, surprisingly the same ligand functions as a neutral hexadentate2+ 2+

lectronic spectrantimicrobial

and neutral tetradentate one towards UO2 and VO , respectively. In case of divalent metal complexesthey have the general formula [M(H2fsatrien)] (where M stands for Cu, Co, Hg and Zn); for trivalent metalcomplexes it is [M(H2fsatrien)]X·nH2O (where M stands for Cr, Mn, Fe, Co and X stands for CH3COO,Cl, NO3, ClO4) and for the complexes of VO2+ and UO2

2+, [M(H4fsatrien)]Y (where M = VO and Y = SO4;M = UO2 and Y = 2 NO3). The Schiff base ligand and most of the complexes have been screened in vitro tojudge their antibacterial (Escherichia coli and Staphylococcus aureus) and antifungal (Aspergillus niger and

activi

Pencillium chrysogenum)

. Introduction

Polydentate Schiff base ligands are of great interest becauseheir binding selectivity can be altered subtly by having the ligandnforce a specific spatial arrangement of donor atoms (i.e. preorga-ization of the donor atoms) or incorporate a set of donor atoms tai-

ored to the metal of interest. In addition, polydentate ligands havehe potential to form complexes of high stability and slow ligandxchange kinetics. This is important in preventing rapid demetalla-ion or complex decomposition if the ligand is to direct the radionu-lide to the target organ or if the ligand is to be used as an effectivehelating agent for the treatment of metal overload [1]. More-

ver, the metal complexes derived from such ligand systems havemmense importance in different fields like biochemistry, cataly-is, magnetochemistry, materials sciences, etc. [2]. These ligandsre generally consisting of N3O3, N4O3, N2O4, etc. donor atoms [2].

∗ Corresponding author. Tel.: +91 9433664151.E-mail addresses: saikat s@rediffmail.com (S. Sarkar),

dey chem@rediffmail.com (K. Dey).1 Former Professor of Chemistry and UGC Emeritus Fellow.

386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2010.06.041

ties.© 2010 Elsevier B.V. All rights reserved.

Besides, very little is known about the coordination behaviourof the trivalent metal ions towards such polydentate compart-mental ligands [3–6]. There is currently considerable interest inthe coordination chemistry of the trivalent group 13 metal ions,the biomedical interest originates from the association of Al(III)with neurological disorders such as Alzheimer’s disease [7] andfrom the incorporation of Ga(III) and In(III) radionuclides (67Ga,68Ga and 111In) into diagnostic radio pharmaceuticals [8]. Besides,siderophores are interesting chelating agents for Fe(III), which areinvolved in the transport of iron in biological systems [9]. Thesecomplexes usually have six oxygens in an octahedral sphere aroundthe Fe(III) with the most important classes containing hydroxamicacid (e.g. ferrichrome, ferrioxamine) or catechol groups (e.g. enter-obactin). These neutral siderophores, and synthetic analogues havebeen extensively studied with respect to their use in iron overloadtreatment [10]. Only a few Fe(III) complexes have been isolated sofar, where the Fe(III) is shown to enter either of the compartments,

by the isolation of pure N2O2/N2N2O2 and O2O2 isomers [3–6].

In few of our recent works, a series of polydentate ligands withwell characterized properties and suitably designed metal com-plexes including organoderivatives have been reported [11–18].In this context and in continuation of our very recent work [19],

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S. Sarkar, K. Dey / Spectrochim

e report here the synthesis and characterization of some transi-ion and non-transition metal complexes of the hexadentate Schiffase ligand, H4fsatrien. Efficient antimicrobial activities have alsoeen excavated with most of these metal complexes which in-turnives the illustration of these series to be environmentally benignaterials.

. Experimental

.1. Materials and general methods

Triethylenetetramine was purchased from Aldrich and used asuch. 3-Formylsalicylic acid was synthesized according to Duff andills [20]. Spectrograde solvents were used for physical measure-ents. All the other reagents and solvents were purchased from

ommercial sources, purified and dried by standard procedures21].

.2. Physical measurements

The elemental analyses were carried out on Elementar VarioL III, Carlo Erba 1108 elemental analyzers at the Sophisticatednalytical Instrument Facility (SAIF), Central Drug Research Insti-

ute, Lucknow. The electronic spectra (in DMSO and other medias indicated in the text) were recorded on a Shimadzu UV-2401PCpectrophotometer and infrared spectra (KBr) on a Perkin-Elmer120-000A spectrophotometer. The ESR spectrum was measuredn an ESR spectrometer at SAIF, Kolkata. The molar conductance10−3 M in DMSO) was measured using an Elico conductivity bridgend the magnetic data were measured on a Guoy Balance.

.3. Preparation of the ligand,,8-N-bis(3-carboxy)disalicylidene-3,6-diazaoctane-1,8-diamine,4fsatrien

The hexadentate Schiff base ligand was prepared followingur earlier method [19] as follows. 3-Formylsalicylic acid (H2fsa)1.66 g, 10 mmol) was dissolved in methanol (25 mL) to which tri-thylenetetramine (trien) (0.73 g, 5 mmol) taken in methanol wasdded (20 mL) dropwise with stirring. Immediately a yellow solu-ion was obtained. Glacial acetic acid (3/4 drops) was then added tot with stirring to facilitate completion of the condensation reactionnd stirring continued for 6 h at −10 ◦C. Then the temperature wasllowed to rise to room temperature with the separation of a yellowowdery compound. It was filtered off, washed with methanol andther and dried in vacuo over fused CaCl2. The compound is solublen DMSO and DMF. Yield 25%.

Due to poor yield of the ligand, we used the in situ reactionrocedure for the synthesis of the metal complexes.

.4. Preparation of the metal complexes

.4.1. Preparation of mononuclear complexes of H4fsatrien withome divalent, VO2+ and UO2

2+ metal ionsThe mononuclear divalent metal complexes [Cu(H2fsatrien)]

1), [Co(H2fsatrien)] (2), [Hg(H2fsatrien)] (3) and [Zn(H2fsatrien)]4) were isolated as follows:

3-Formylsalicylic acid (H2fsa) (1.66 g, 10 mmol) was dissolved inethanol (25 mL) to which was added triethylenetetramine (trien)

0.73 g, 5 mmol) in methanol (20 mL) dropwise with stirring and

yellow solution was obtained. Few drops glacial acetic acid was

hen added to it. The mixture was then heated under gentle refluxor 1/2 h on water bath. Metal(II) acetate (5 mmol) dissolved in

inimum volume of methanol (15 mL) was added dropwise to thechiff base (H4fsatrien) solution with stirring and the mixture was

ta Part A 77 (2010) 740–748 741

kept under reflux for 1–2 h. Instantaneous precipitation occurredin the case of Co(II), Hg(II) and Zn(II).

The Cu(II) acetate gave a clear greenish-blue solution fromwhich the complex separated out only after evaporation to halfof the initial volume and on cooling to ice temperature. Yield 50%.

The precipitated complexes were filtered, washed thoroughlywith methanol and diethyl ether and dried in vacuum over anhy-drous CaCl2. The reaction of Co(II) acetate was carried out under anatmosphere of dinitrogen to avoid aerial oxidation. All the com-pounds are soluble in DMF and DMSO. The yields were around70–85%.

2.4.1.1. [VO(H4fsatrien)]SO4 (5). Similarly a methanolic solution ofH4fsatrien was prepared using H2fsa (1.66 g, 10 mmol) and trien(0.73 g, 5 mmol) in which few drops of glacial acetic acid wereadded. To this ligand solution was then added a methanolic solu-tion of VOSO4·2H2O (0.995 g, 5 mmol) followed by a methanolicsolution of NaOAc (0.82 g, 10 mmol) and the whole mixture washeated under gentle reflux on a water bath for 1 h. During reflux agreenish-yellow compound was separated out, which was filteredoff, washed with ethanol and dried in vacuo. It is soluble in DMFand DMSO. Yield 85%.

2.4.1.2. [UO(H4fsatrien)](NO3)2 (6). This yellow compound wasprepared by a method similar to that employed for the prepara-tion of vanadyl complex except using of UO2(NO3)2·6H2O(2.51 g,5 mmol) in place of vanadyl sulphate. It was filtered off, washedwith ethanol and dried in vacuo. It is soluble in DMF and DMSO.Yield 80%.

2.4.2. Preparation of mononuclear complexes of H4fsatrien withtrivalent metal ions2.4.2.1. [Cr(H2fsatrien)]CH3COO·2H2O (7). Similarly the reactionof the solution of H4fsatrien (5 mmol) with the solution ofCr(CH3COO)3·6H2O (1.69 g, 5 mmol) in methanol–water mixture(20 mL, 50:50, v/v) afforded this greenish powdery compound. Itwas filtered off, washed with ethanol and dried in vacuo. The com-pound is soluble in hot methanol, DMF and DMSO. Yield 70%.

2.4.2.2. [Cr(H2fsatrien)]Cl·2H2O (8). This greenish-yellow com-pound was prepared by following the same method used for itsacetate analogue using CrCl3·6H2O (1.33 g, 5 mmol) instead ofCr(CH3COO)3·2H2O. It was filtered off, washed with ethanol anddried in vacuo. It is also soluble in DMF and DMSO. Yield 60%.

2.4.2.3. [Mn(H2fsatrien)]CH3COO (9). A solution of H4fsatrien(5 mmol) was prepared as described above. To this was addedMn(CH3COO)3·2H2O (1.34 g, 5 mmol) dissolved in 15 mL ofmethanol with stirring. The mixture was heated under reflux for1/2 h, while air was drawn through it. The brown solution, thusproduced, gave yellowish-brown powdery compound on concen-tration and cooling. It was filtered off, washed with methanol anddried in vacuo. This compound is soluble in DMF and DMSO. Yield60%.

2.4.2.4. [Fe(H2fsatrien)]NO3·H2O (10). The solution ofFe(NO)3·9H2O (2.02 g, 5 mmol) in dry methanol was added tothe preformed Schiff base (H4fsatrien) solution (5 mmol) in drymethanol dropwise with stirring. The solution, which immediately

turned dark brown, was refluxed gently for 1/2 h and the methanolwas evaporated slowly on a water bath. On cooling, the deepchocolate coloured compound, which separated out, was filtered,washed with diethyl ether and dried in air. This complex is solublein water, hot methanol, DMF and DMSO. Yield 60%.

742 S. Sarkar, K. Dey / Spectrochimica Acta Part A 77 (2010) 740–748

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.4.2.5. [Fe(H2fsatrien)]Cl·2H2O (11). This reddish-brown com-ound was synthesized by a method similar to that used for (10)sing FeCl3·6H2O (1.35 g, 5 mmol) in place of Fe(NO)3·9H2O. Solu-ility is same as for compound (10). Yield 60%.

.4.2.6. [Co(H2fsatrien)]Cl (12). To a solution of H4fsatrien5 mmol), CoCl2·6H2O (1.19 g, 5 mmol) in methanol was addedropwise and air was drawn through the solution for 4 h. Theolution was filtered and evaporated on water bath till the solidtarted separated out from the solution. The mixture was filtered,ashed with methanol, acetone and finally with diethyl ether and

ir-dried. This Co(II) complex is soluble in DMF and DMSO. Yield0%.

.4.2.7. [Co(H2fsatrien)]NO3 (13). This deep-brown compound wasrepared by the same method used for the preparation ofCo(H2fsatrien)]Cl using Co(NO3)2·6H2O (1.45 g, 5 mmol). The mix-ure was filtered, washed with methanol, acetone and finally withiethyl ether and air-dried. Solubility remains same as (12). Yield0%.

.4.2.8. [Co(H2fsatrien)]ClO4 (14). The above-mentioned Co(III)omplex, [Co(H2fsatrien)]NO3 (13), was dissolved in minimummount of water and aqueous NaClO4 was added in excess. Thisark reddish-brown perchlorate complex separated out of the solu-ion immediately. The product was digested for 15 m on water bath,ltered and washed thoroughly with acetone and diethyl ether andried in air. It is partially soluble in water but fully soluble in DMFnd DMSO. Yield 80%.

[For all the above-mentioned preparative reactions the pH ofhe mother liquor was found to lie between 4 and 6.]

. Results and discussion

.1. Syntheses

The ligand 1,8-N-bis(3-carboxy)disalicylidene-3,6-iazaoctane-1,8-diamine, H4fsatrien, was prepared by the reactionf 3-formylsalicylic acid with triethylenetetramine (2:1 molaratio) in methanol at room temperature. The results of the ele-ental analysis, molecular weight, infrared, 1H and 13C NMR data

re consistent with the formula shown in Scheme 1.In the case of the present ligand, H4fsatrien, the characteris-

ics of mononuclear complexes where the metal is occupying inhe inner coordination site (N2N2O2) may be compared to thatf the simple ligand derived form salicylaldehyde and the respec-ive diamines. In the case of mononuclear complexes derived fromimilar type hexadentate ligands, if the metal occupies the innerompartment, it yields one of the positional isomeric forms MN2N2,N2O2 or MN2N2O2. Simple ligands derived from salicylaldehyde

nd tetradentate �,�-polyamines react with metal ions to form

eutral complexes where the phenolic protons are displaced byhe metal ion. It is presumed that similar situation exists with theresent ligand, H4fsatrien and the possibility of the positional iso-er MN2N2 might be excluded. Then we will be left with the MN2O2

nd MN2N2O2 isomers, where the extra donor sites incorporated

.

into long methylene chain may or may not coordinate to the metal.This can be confirmed by comparing the spectral and magneticproperties of the complexes with that of the simple complexesderived from salicylaldimines.

So, the Schiff base ligand H4fsatrien, in principle, can function inthe following fashions to afford mononuclear complexes with di-and trivalent metal ions:

(i) dibasic hexadentate, utilizing two phenolic oxygen (afterdeprotonation), two azomethine nitrogens and two secondaryamine nitrogens, and

(ii) dibasic tetradentate, utilizing two phenolic oxygen (afterdeprotonation) and two azomethine nitrogens.

However, spectral and magnetic data obtained on all the mononuclear complexes of the hexadentate ligand H4fsatrien indicatethat the metal ion enters the inner compartment occupying theN2N2O2 sites. But, H4fsatrien is being observed to act as a neutraltetradentate and neutral hexadentate ligand towards UO2

2+ andVO2+, respectively.

The present paper reports the results of the interaction ofH4fsatrien with CoCl2·6H2O, Co(NO3)2·6H2O, Co(CH3COO)2·4H2O,Cu(CH3COO)2·H2O, Zn(CH3COO)2·2H2O, VOSO4·2H2O,Cr(CH3COO)3·6H2O, CrCl3·6H2O, Mn(CH3COO)3·2H2O,Fe(NO3)3·9H2O, FeCl3·6H2O and UO2(NO3)2·6H2O under variedreaction conditions leading to the isolation of new mononuclearcomplexes of these metal ions. The chemical composition of theisolated complexes depends to some extent on the metal templateeffect, solvents, temperature and pH of the solution. Elementalanalyses and other characterization data are shown in Table 1,which support their formulations. The molecular weights are alsoin good agreement with the theoretical values. The spectra ofsome representative complexes are given in Supplementary files.Unfortunately no single crystal could be grown in spite of our bestefforts.

The formation of the metal complexes may be expressed by thechemical equations (Scheme 2):

3.2. The 1H and 13C NMR spectra

The 1H NMR spectra of the ligand H4fsatrien has been measuredin DMSO-d6 with TMS as internal reference. The data is depicted inTable 2. A sharp singlet at 8.94 ppm and a broad one at 6.2 ppm areassigned to the azomethine and N–H protons, respectively. Bridg-ing methylene protons give rise to broad signals at 2.50–2.79 and3.00–3.83 ppm assignable to –X–(CH2)n–X– and N–CH2, respec-tively [18,22,23]. The broad signal at 10.8 and at 12.28 ppm may beconsidered as phenolic OH, carboxylic COOH protons, respectively.Multiplets of aromatic protons appear in the region 7.98–6.60 ppm.The 13C NMR spectrum of the ligand exhibits 11 signals. The sig-

nals at 167.1, 139.1, 136.6, 130.3, 116.93 and 114.09 ppm are due toaromatic carbons. The carboxylic carbon and imine carbon appearat 172 and 161.5 ppm, respectively. The methylene carbons areobserved at 51.2, 50.2 and 49.15 ppm. These data support the for-mulation of the isolated Schiff base, H4fsatrien.

S. Sarkar, K. Dey / Spectrochimica Ac

Tab

le1

Som

ech

arac

teri

zati

ond

ata

ofth

eli

gan

dan

dco

mp

lexe

s.

Com

ple

xC

olou

rM

.p.a

(◦ C)

Mol

ecu

lar

wei

ght

fou

nd

(Cal

c.)

An

alys

esfo

un

d(C

alc.

)%

�ef

fdB

.M.

�M

f(�

−1cm

2m

ol−1

)

CH

NM

H4fs

atri

enC

22H

26N

4O

6Y

ello

w23

0–23

340

1b(4

42.4

7)59

.63

(59.

72)

5.89

(5.9

2)12

.71

(12.

66)

[Cu

(H2fs

atri

en)]

(1)

C22

H24

N4O

6C

uG

reen

ish

-blu

e26

3–26

559

8c(5

04.0

0)52

.35

(52.

42)

4.74

(4.8

0)11

.04

(11.

12)

12.5

7(1

2.61

)1.

9617

.9[C

o(H

2fs

atri

en)]

(2)

C22

H24

N4O

6C

oB

iscu

it24

3–24

551

0c(4

99.3

9)52

.86

(52.

91)

4.90

(4.8

4)11

.27

(11.

22)

11.7

5(1

1.80

)4.

389.

6[H

g(H

2fs

atri

en)]

(3)

C22

H24

N4O

6H

gW

hit

ish

-yel

low

278–

281

(641

.05)

41.1

7(4

1.22

)3.

83(3

.77)

8.81

(8.7

4)D

iae

16.4

[Zn

(H2fs

atri

en)]

(4)

C22

H24

N4O

6Zn

Pale

yell

ow25

1–25

451

2c(5

05.8

5)52

.27

(52.

24)

4.83

(4.7

8)11

.16

(11.

08)

12.8

6(1

2.93

)D

iae

10.3

[VO

(H4fs

atri

en)]

SO4

(5)

C22

H26

N4O

11SV

Gre

enis

h-y

ello

w27

1–27

3(6

05.4

7)43

.69

(43.

64)

4.38

(4.3

3)9.

31(9

.25)

8.48

(8.4

1)1.

7353

.2[U

O2(H

4fs

atri

en)]

(NO

3) 2

(6)

C22

H26

N6O

13U

Yel

low

>300

(652

.42)

40.5

7(4

0.50

)4.

11(4

.02)

12.9

3(1

2.88

)D

iae

62.1

[Cr(

H2fs

atri

en)]

CH

3C

OO

·2H

2O

(7)

C24

H31

N4O

10C

rG

reen

257–

260

598c

(587

.53)

49.1

1(4

9.06

)5.

38(5

.32)

9.57

(9.5

4)8.

91(8

.85)

3.82

47.6

[Cr(

H2fs

atri

en)]

Cl·2

H2O

(8)

C22

H28

N4O

8C

lCr

Gre

enis

h-y

ello

w26

6–26

9(5

37.7

3)49

.21

(49.

14)

5.31

(5.2

5)10

.45

(10.

42)

9.73

(9.6

7)3.

8851

.2[M

n(H

2fs

atri

en)]

CH

3C

OO

(9)

C24

H27

N4O

8M

nY

ello

wis

h-b

row

n23

1–23

357

1c(5

54.4

4)51

.93

(52.

00)

4.95

(4.9

1)10

.16

(10.

10)

9.96

(9.9

1)4.

7848

.9[F

e(H

2fs

atri

en)]

NO

3·H

2O

(10)

C22

H26

N5O

10Fe

Ch

ocol

ate

286–

288

(576

.32)

45.9

1(4

5.85

)4.

62(4

.55)

12.2

1(1

2.15

)9.

74(9

.69)

4.90

52.1

[Fe(

H2fs

atri

en)]

Cl·2

H2O

(11)

C22

H28

N4O

8C

lFe

Red

dis

h-b

row

n29

2–29

5(5

67.7

8)46

.49

(46.

54)

5.03

(4.9

7)9.

93(9

.87)

9.80

(9.8

3)5.

2349

.8[C

o(H

2fs

atri

en)]

Cl(

12)

C22

H24

N4O

6C

lCo

Bro

wn

273–

274

(534

.84)

49.4

8(4

9.41

)4.

55(4

.52)

10.4

9(1

0.47

)11

.10

(11.

02)

Dia

e48

.1[C

o(H

2fs

atri

en)]

NO

3(1

3)C

22H

24N

5O

9C

oD

eep

-bro

wn

269–

272

(561

.39)

47.1

3(4

7.06

)4.

39(4

.31)

12.5

4(1

2.47

)11

.02

(10.

49)

Dia

e49

.6[C

o(H

2fs

atri

en)]

ClO

4(1

4)C

22H

24N

4O

10C

lCo

Red

dis

h-b

row

n28

6–28

8(5

98.8

4)44

.16

(44.

12)

4.10

(4.0

4)9.

40(9

.36)

9.91

(9.8

4)D

iae

48.6

aD

ecom

pos

itio

nte

mp

erat

ure

.b

Osm

omet

ry.

cR

ast

met

hod

.d

Soli

dst

ate

atro

omte

mp

erat

ure

.e

Dia

mag

net

ic.

f10

−3M

solu

tion

inD

MSO

atro

omte

mp

erat

ure

.

ta Part A 77 (2010) 740–748 743

The 1H NMR spectra of the diamagnetic complexes[Hg(H2fsatrien)] (3), [Zn(H2fsatrien)] (4), [Co(H2fsatrien)]Cl(12), [Co(H2fsatrien)]NO3 (13) and [Co(H2fsatrien)]ClO4 (14) weremeasured in TFA and DMSO-d6, respectively with TMS as internalstandard and the data are recorded in Table 2. The 1H NMR spectraof the complexes are almost similar to that of the ligand H4fsatrien,with slight shift to higher fields. The broad signal for the hydrogenfor phenolic OH at about ı 10.8 ppm is absent in all the complexes.This indicates deprotonation and coordination of the phenolic OH.The imino protons appear as doublets in this complex indicatingthe non-equivalence of the two imino groups of the moleculewhich would result from distorted octahedral geometry aroundthe metal ion. All the observed resonances are assigned based onsimilar assignments for simple salicylaldehyde or acetylacetonederived ligands and their similar complexes. The broad signal forthe secondary NH appeared at almost the same location as foundfor the free ligand position. Slightly lower frequency shifts arealso observed for the aromatic protons when compared with thefree ligand. All these suggest coordination of the ligand H4fsatrienas a dibasic hexadentate N2N2O2 donor in the complexes. It maybe inferred that the ligand H4fsatrien behaves similarly in othercomplexes reported in Table 1.

3.3. Infrared spectra

The inferences drawn from the 1H and 13C NMR spectral dataof the ligand are substantiated by the appearance of infrared bandsat ∼3412 cm−1 (br) for OH, at 3290 cm−1 (br) for NH, at 2930 cm−1

for CH, at 1690 cm−1 for COOH and 1657 cm−1 for >C N, respec-tively.

The general trend of the infrared spectra of the present metalcomplexes is comparable with those published previously withsimilar type of ligands [18,19]. All the metal complexes showbroad to medium bands in the region 3600–3000 cm−1, which areassignable to OH stretching vibrations due to the presence of freeCOOH groups and/or due to the presence of water molecule(s)(see Table 1). The NH stretching frequencies of the complexesare observed at around 20–50 cm−1 lower compared to the freeligand value. These lower values compared to the free ligand sug-gest coordination of the nitrogen atom of the secondary amine(NH) to the metal. The �(C N) frequency in the complexes isobserved around 1648–1621 cm−1 (which is 9–36 cm−1 lowerthan that observed in the free ligand) and lends support to thecoordination of the imine nitrogen. The infrared spectra of thecomplexes (1)–(14) showed the vibration of the free carboxylgroup �as(COOH) around 1700 cm−1, suggesting a mononuclearformulation of the complexes. The phenolic �(C–O) frequency gen-erally appeared at around 1500 cm−1 in the free Schiff base andat 1540 cm−1 when attached to a single metal (non-bridging). Onthe other hand, the bridging phenolic �(C–O) frequency appearsat 1560 cm−1. Hence the higher-energy shift of the band hasbeen used for a diagnosis of the formation of a phenolic oxy-gen bridge. On the basis of this and also on the basis of the dataavailable [24,25], we can safely say that the complexes (1)–(14)are mononuclear in nature with free COOH groups. Characteristicbands for water of crystallization in the complexes (7), (8), (10)and (11) are observed at 3120–3450 cm−1 (br) assignable to �(OH)[26].

Band assignments in the region 3100–3400 cm−1 and around

1600 cm−1 are complicated by the presence of secondary amineand C N chromophoric groups in the present complexes. Besides,absorption of water molecule also appears in the 3100–3500 cm−1

range. All these complicate the proper interpretation of the bandsin the region.

744 S. Sarkar, K. Dey / Spectrochimica Acta Part A 77 (2010) 740–748

eme 2

ceTaaannTp[

a[bApiTtt8n(m

T1

Sch

Infrared spectrum of the VO2+ complex shows bands almostomparable to that of the free ligand H4fsatrien excepting the low-ring of azomethine (C N) band by about 34 cm−1 in the complex.he �3 and �4 modes of vibration of sulphate are observed at 1015nd 622 cm−1, respectively, which confirms that sulphate remainss free ion. These observations suggest that the ligand functionss a neutral tetradentate ligand bonding through two-azomethineitrogen and two phenolic OH groups. Unfortunately, we couldot grow any suitable crystal for X-ray crystal structure analysis.he �(V O) of the complex observed at 960 cm−1, supporting in allrobability the presence of monomeric V O unit in the complex27,28].

The infrared spectrum of the complex (6) shows an intensebsorption bands at 890 cm−1 attributable to �as(UO2) mode27,28]. However, the �sym(UO2) vibration in the complex could note detected unequivocally since the ligand absorbs in this region.ppearance of a strong sharp band at 1380 cm−1 demonstrates theresence of ionic nitrate (NO3

−). It is thus plausible that the UO22+

on in the complex (6) may attain eight coordination [27,28] (Fig. 1).he other infrared bands of the complex may be discussed as inhe case of [VO(H4fsatrien)]SO4 (5), supporting neutral hexaden-ate nature of the ligand. The infrared bands observed at about

20–830(m), 1380–1390(s) and 750–760(s) cm−1 regions in theitrate salts, [UO2(H4fsatrien)](NO3)2 (6), [Fe(H2fsatrien)]NO3·H2O10) and [Co(H2fsatrien)]NO3 (14) may be assigned to the �1, �2, �3

odes of vibrations (in D3h symmetry) [28]. On the other hand, the

able 2H NMR spectral data of ligand and some complexes.

Ligand/complex NH COOH OH

H4fsatrien 6.2 (2H, br) 12.28 (2H, br) 10.8[Hg(H2fsatrien)] (3) 6.11 (2H, br) 12.36 (2H, br) –[Zn(H2fsatrien)] (4) 6.04 (2H, br) 12.3 (2H, br) –[Co(H2fsatrien)]Cl (12) 6.12 (2H, br) 12.27 (2H, br) –[Co(H2fsatrien)]NO3 (13) 6.08 (2H, br) 12.31 (2H, br) –[Co(H2fsatrien)]ClO4 (14) 6.18 (2H, br) 12.39 (2H, br) –

.

appearance of a very strong band at 1170 cm−1 in the perchloratecomplex, [Co(H2fsatrien)]ClO4 (14), can be assigned to the �3 modeof vibrations (in Td symmetry) [28]. The �(M–N) and �(M–O) fre-quencies for all the complexes appear around 496–596 (w/m) cm−1

and 382–486 (w/m) cm−1, respectively.Based on the above discussion, the following structures

(Figs. 1 and 2) may be proposed for the isolated complexes (1)–(14).

3.4. Molar conductance values

The molar conductance values of the complexes (1)–(4) in DMSOsolution are found in the range �M = 9.6–17.9 ohm−1 cm2 mol−1,indicating non-electrolyte nature of the complexes in the solu-tion of DMSO [29]. On the other hand, (5) and (7)–(14) show �Mvalues in the range 48.1–53.2 ohm−1 cm2 mol−1, suggesting 1:1electrolyte nature [29]. The molar conductance value of the com-plex (6) is 62.1 ohm−1 cm2 mol−1 indicating that the complex is a1:2 electrolyte [29].

3.5. Magnetic moments, electronic and ESR spectra

The magnetic susceptibility of all the newly synthesized com-plexes were measured in the solid state at room temperatureand the calculated magnetic moment values are recorded inTable 1. The complexes [Cu(H2fsatrien)] (1), [Co(H2fsatrien)](2), [VO(H4fsatrien)]SO4 (5), [Cr(H2fsatrien)](CH3COO)·2H2O

H–C N C–H –X–(CH2)n–X N–CH2

(2H, br) 8.86 (2H, d) 7.98–6.6 (6H, m)3.51 (8H, br) 4.3 (4H, br)8.53 (2H, d) 6.89–6.65 (6H, m)3.47 (8H, br) 4.2 (4H, br)8.45 (2H, d) 6.85–6.65 (6H, m)3.40 (8H, br) 4.1 (4H, br)8.49 (2H, d) 6.80–6.56 (6H, m)3.48 (8H, br) 4.21 (4H, br)8.58 (2H, d) 6.92–6.73 (6H, m)3.5 (8H, br) 4.2 (4H, br)8.50 (2H, d) 6.82–6.61 (6H, m)3.43 (8H, br) 4.16 (4H, br)

S. Sarkar, K. Dey / Spectrochimica Acta Part A 77 (2010) 740–748 745

valent, VO2+ and UO22+ metal complexes.

((w([d

iantiasmp

3

iysckl8icet

Table 3ESR spectral data of [Cu(H2fsatrien)] (2) at room temperature in the solid state.

g|| g⊥ go G

Fig. 1. Proposed structures of the di

7), [Mn(H2fsatrien)](CH3COO) (9), [Fe(H2fsatrien)]NO3·H2O10), [Fe(H2fsatrien)]Cl·2H2O (11) are paramagnetic,hile the complexes [Hg(H2fsatrien)] (3), [Zn(H2fsatrien)]

4), [UO2(H4fsatrien)](NO3)2 (6), [Co(H2fsatrien)]Cl (12),Co(H2fsatrien)]NO3 (13) and [Co(H2fsatrien)]ClO4 (14) are alliamagnetic.

The UV–Vis band observed for H4fsatrien around 265 nms assigned to � → �* transition. Another band around 360 nmppears as a shoulder may be assigned to n → �* transition of phe-olic moiety. A prominent band in the region around 400 nm mayentatively be assigned to the enaminone form of the ligand presentn equilibrium with the phenol-imino tautomer [30,31]. However,ll these bands have appreciably changed in the metal complexesuggesting involvement of the ligand in complex formation. Theagnetic moment, electronic and ESR spectra of the metal com-

lexes are discussed individually.

.5.1.1. Copper(II) complex, [Cu(H2fsatrien)] (1)The mononuclear Cu(II) complex (1) exhibits very broad band

n the region 690–610 nm in DMF solution. The solid complex alsoields similar spectrum indicating similarity in the geometry inolid and in the solution. The observed magnetic moment of theomplex (1.96 B.M.) corresponds to one unpaired electron. It isnown that Cu(II) square planar complexes derived from Schiff baseigands absorb above 590 nm and tetrahedral complexes below

30 nm. The observed absorption of the present Cu(II) complex

n the region 690–610 nm can, therefore, be assigned to the six-oordinate geometry around Cu(II) ion. It has been demonstratedarlier [32,33] that the Schiff bases derived by the condensation ofriethylenetetramine with salicylaldehyde or acetylacetone form

Fig. 2. Proposed structures of the

2.201 2.056 2.104 3.6

six-coordinate complexes with Ni(II), Co(III) and Fe(III). It is there-fore plausible that in the present Cu(II) complex, [Cu(H2fsatrien)](1), the ligand attains pseudo-octahedral geometry around Cu(II)as found in the Ni(II) complex [19].

The powder ESR spectrum at room temperature (Table 3)exhibits characteristic axial spectrum with g|| > g⊥ for the Cu(II)complex, [Cu(H2fsatrien)] (1) indicating tetragonal symmetry withdx2−y2 ground state. The G value is less than four, which indicatesthat the tetragonal axes are misaligned [34–36].

3.5.1.2. Cobalt(II) complex, [Co(H2fsatrien)] (2)The Co(II) complex exhibits low intensity bands at around 875

and 650 nm, characteristic of octahedral Co(II) species. The sim-ilarity of the spectra in the solid state and in solution indicatesthe similarity of the species in both the states. The details of themagnetic moment values of Co(II) complexes in different geome-tries have been discussed elsewhere [18]. The observed magneticmoment value of 4.38 B.M. for the present Co(II) complex corre-sponds to the presence of three unpaired electrons. As found in

Cu(II) complex, described above, this Co(II) complex is also six-coordinated occupying the inner N2N2O2 compartment with all thedonor atoms coordinating to the metal ion.

trivalent metal complexes.

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3c

deTiaioSd

3

1emb

od6

dhselra

baCtt[tacapeslotf

3(

t(e

t(Tctttoe

46 S. Sarkar, K. Dey / Spectrochim

.5.1.3. Mercury(II) complex, [Hg(H2fsatrien)] (3), zinc(II)omplex, [Zn(H2fsatrien)] (4)

The electronic spectra of Hg(II) and Zn(II) complexes do notiffer much from those of ligand spectrum. No d–d transition isxpected for Hg(II) and Zn(II) complexes due to d10 configuration.hey can show both L → M and M → L charge-transfer bands. Butn this case bands at 270 nm and 350 nm are observed and may bessigned to the � → �* and n → �* transitions of the ligand systemn the complexes. The physico-chemical data also support pseudo-ctahedral geometry for these two metal complexes where thechiff base, H4fsatrien functions as a dibasic hexadentate N2N2O2onor ligand.

.5.1.4. Oxovanadium(IV) complex, [VO(H4fsatrien)]SO4 (5)The VO2+ complex (5) is paramagnetic, the �eff value being

.73 B.M., which is quite close to spin-only value for one unpairedlectron as is expected for VO2+ chelates. This normal magneticoment value indicates that there is no significant interaction

etween neighbouring V(IV) ions [27].The electronic spectrum of the complex (5) in nujol mull shows

ne prominent maximum at about 670 nm. In addition; a few shoul-ers (or broad/weak bands) appeared in the spectrum at about 770,35 and 550 nm.

Although the electronic spectra of VO2+ complexes have beeniscussed in many places [27], yet assignments of absorption bandsave been a matter of controversy. However, most of the discus-ions are made in terms of C4v symmetry. In this symmetry threelectronic transitions are expected for the d1 VO2+ ion [37]. Theowering of the symmetry from C4v to C2v or Cs has the effect ofemoving the degeneracy in the d orbitals and thus four transitionsre predicted.

It has been observed in the present investigation that all theands are not well resolved into their components. The broad bandt 635 nm is easily identified and may be attributed to 2B2 → 2B1 in4v symmetry [37]. The shoulder at 550 nm may be due to 2B2 → 2A1ransition (in C4v). The high intensity of this band may be dueo influence of some charge-transfer or intra-ligand transitions27]. The low-energy band observed at 770 nm may be assignedo 2B2 → 2E transition. If the present VO2+ complex (5) of the lig-nd H4fsatrien is square pyramidal in structure like those of VO2+

omplexes of �-diketones, they should then readily add a donor lig-nd to the metal at the vacant axial coordination site and therebyroduce a marked shift the d–d transition bands. However, thelectronic spectrum of the present VO2+ complex is found to beolvent independent. This may be due to steric hindrance of theigand. Alternatively, it may so happen that in the complex the sec-ndary amine (–NH) functionality might form a weak bond withhe vanadium atom and thus prevent the incoming donor ligandrom forming a bond.

.5.1.5. Chromium(III) complexes, [Cr(H2fsatrien)]CH3COO·2H2O7) and [Cr(H2fsatrien)]Cl·2H2O (8)

The magnetic moment values, 3.82 and 3.88 B.M. found at roomemperature for the Cr(III) complexes (7) and (8) are slightly lowerTable 1) than the spin-only values for a d3 and has been discussedarlier [38].

The electronic spectra of the Cr(III) complexes (7) and (8) in solu-ion (DMSO) show three absorption bands in the range 555–475 nmsh), 475–435 nm (ε ∼ 3500) and 400–370 nm (ε ∼ 4000–5000).hese chelates when measured in nujol mulls give almost identi-al spectra, especially in the range 555–385 nm. This suggests that

he environment of Cr(III) in these chelates is almost the same inhe solid as well as in the solution state. The bands observed inhe range 555–385 nm may be considered as the split componentsf the 4T2g (Oh) and 4T1g(F) (Oh) terms, despite their high molarxtinction coefficients. Similar high molar extinction coefficients

ta Part A 77 (2010) 740–748

were observed for bands in this region in the Cr(III) complexes ofsimilar Schiff base ligands.

Considering the data available a pseudo-octahedral structuremay be proposed for these complexes in which the ligand func-tions as a dibasic hexadentate fashion utilizing its N2N2O2 innercompartment. The two COOH groups being remained free.

3.5.1.6. Manganese(III) complex, [Mn(H2fsatrien)]CH3COO (9)The Mn(III) complex (9) is paramagnetic and shows magnetic

moment value 4.78 B.M. at room temperature (Table 1) indicat-ing the presence of trivalent manganese. This value suggests theabsence of exchange or super exchange interactions.

High-spin Mn(III) complexes with octahedral geometry areexpected to give one charge-transfer band around 400 nm(log ε = 3.5) and a spin-allowed d–d transition band, 5Eg → 5T2g,around 500 nm (log ε = 2.5). The electronic spectrum of the presentMn(III) complex (9) measured (DMSO) in the region 250–385 nm,considering several intense bands and shoulders. The positions ofthe bands can be roughly divided into two areas, between 710–670and 625–385 nm. The bands at high wave number are shownas shoulder on intense charge-transfer absorptions. The absorp-tion in the visible region at 625–385 nm can be assigned to the5Eg → 5T2g transition, though there is the possibility that the bandsat 400 nm may be charge-transfer in origin. The splitting of thebands is most likely due to the Jahn–Teller distortion as observedin many complexes of Mn(III) [11]. High-spin Mn(III) complexesalways show at least one absorption in the region 800–770 nm.The relationship of this band to the structure of the complexeshas not been clarified. We also observed a band at 675 nm andaccording to Levason and McAuliffe [39] and Lever [40] such bandmay be assigned to low-energy charge-transfer transition in suchchelates.

As discussed above, a pseudo-octahedral structure may beproposed for this Mn(III) complex (9) considering all the physico-chemical data collected. In the chelate, Mn(III) occupies the innerN2N2O2 compartment.

3.5.1.7. Iron(III) complexes [Fe(H2fsatrien)]NO3·H2O (10) and[Fe(H2fsatrien)]Cl·H2O (11)

The complex obtained from Fe(NO3)3·9H2O and H4fsatrien is[Fe(H2fsatrien)]NO3·H2O (10) showing room temperature mag-netic moment of 4.90 B.M. corresponding to high-spin Fe(III). Onthe other hand, the reaction of FeCl3·6H2O and H4fsatrien affordeda complex [Fe(H2fsatrien)]Cl·2H2O (11) having magnetic moment5.23 B.M. This difference in magnetic moment values may bedue to anionic effect [41–44]. These magnetic moment valuesare higher than low-spin (S = 1/2, 2T2) and lower than high-spin(S = 5/2, 6A1) values. Similar observations were made earlier with[Fe(sal)2trien]+ and [Fe(acac)2trien]+ and discussed [41–44]. Theelectronic spectra of the complexes show bands in the regions475–500 and 590–610 nm which may be assigned to an octahedralenvironment around Fe(III). The DMSO solution of the complexesbehaves as a 1:1 electrolyte with a molar conductance of 52.1 and49.8 �−1 cm2 mol−1, respectively for (10) and (11). The X-ray crys-tal structure of a similar Fe(III) complex, [Fe(sal)2trien]+ is known[41–44], which reveals an octahedral environment around Fe(III)with all the N2N2O2 donors coordinating to the Fe(III) ion. Simi-larly we tentatively propose an octahedral structure for the presentFe(III) complexes [Fe(H2fsatrien)]+ (10) and (11) with Fe(III) inthe inner compartment N2N2O2 leaving the O2O2 compartmentvacant. Sinn et al. [41] have shown that the Fe(III) complexes with

hexadentate ligands derived from triethylenetetramine (trien) andsalicylaldehyde/acetylacetone (sal/acac) exhibit spin equilibriumin solution. The band at around 500 nm was assigned to the high-spin isomer and the ∼625 nm band due to low-spin isomer. Theappearance of these bands in the present complexes is an indication

S. Sarkar, K. Dey / Spectrochimica Acta Part A 77 (2010) 740–748 747

Table 4Antimicrobial activity of the ligand and metal complexes.

Complex Growth inhibition zone diameter (in mm)

1 (mg/mL) 1 (mg/mL)

Escherichia coli Staphylococcus aureus Aspergillus niger Pencillium chrysogenum

Control DMSO 0.0a 0.0a 0.0a 0.0a

Ligand (H4fsatrien) 5a 7a 5a 5a

[Cu(H2fsatrien)] (1) 11a 10a 8a 8a

[Co(H2fsatrien)] (2) 9a 12b 10a 11a

[Zn(H2fsatrien)] (4) 12b 14b 13b 12b

[VO(H4fsatrien)]SO4 (5) 16b 16b 13b 18c

[Cr(H2fsatrien)]CH3COO·2H2O (7) 8a 10a 7a 7a

[Cr(H2fsatrien)]Cl·2H2O (8) 14b 21c 15b 17c

[Mn(H2fsatrien)]CH3COO (9) 11a 12b 10a 8a

[Fe(H2fsatrien)]NO3·H2O (10) 15b 12b 14b 15b

[Fe(H2fsatrien)]Cl·2H2O (11) 21c 17c 19c 20c

[Co(H2fsatrien)]Cl (12) 16b 17c 12b 17c

[Co(H2fsatrien)]NO3 (13) 12b 10a 11a 11a

[Co(H fsatrien)]ClO (14) 10a 8a 9a 12b

f(

3[

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4

coEi

2 4

a Inactive.b Fairly active.c Decidedly active.

or the presence of spin equilibrium in [Fe(H2fsatrien)]NO3·H2O13) and [Fe(H2fsatrien)]Cl·2H2O (11).

.5.1.8. Cobalt(III) complexes, [Co(H2fsatrien)]Cl (12),Co(H2fsatrien)]NO3 (13) and [Co(H2fsatrien)]ClO4 (14)

All the Co(III) complexes are diamagnetic at room temperaturendicating that Co(III) is in the low-spin state. The diamagnetismf these chelates serves as a check for the absence of any para-agnetic Co(II) ion as impurity, and it is also indicative of their

ctahedral geometry, a preferred stereochemistry for Co(III) ion.lectronic spectral data also support this view.

All the complexes exhibit very strong absorption in the UV-egion and medium intensity absorption in the visible region. Thebsorptions fall in the range 610–575, 500–495, 380–370, 305–300nd 275–270 nm. The two highest energy bands may be due tonternal ligand transitions or the charge-transfer transitions fromhe ligand to metal ion. The two bands in the range 610–575nd 500–495 nm, tentatively, may be assigned as the split com-onents of the 1A1g → 1T1g transitions [40]. The high-energy bandst 380–370 nm is probably due to the 1A1g → 1T2g transition mixedith metal to ligand (t2g → �*) transitions [40]. All these physico-

hemical data suggest a pseudo-octahedral structure for the Co(III)omplexes (12), (13) and (14).

Linear hexadentate ligands are known to form six-coordinatedo(III) complexes with a variety of donor atoms (viz. O/N/S). Dwyert al. [45] have established the six-coordinate behaviour of theseigands towards Co(III) ion. In all the complexes two azomethineitrogen atoms, two secondary amine nitrogen atoms are cis- toach other. Similar structural propositions may be extended forhe trivalent metal complexes [Cr(H2fsatrien)]+, [Mn(H2fsatrien)]+

nd [Co(H2fsatrien)]+ (Fig. 2). The observed physico-chemical dataustify these formulations that the metal(III) is in N2N2O2 compart-

ent with all the donor atoms coordinating. The 1H NMR spectra ofCo(H2fsatrien)]+ exhibited resonances similar to [Zn(H2fsatrien)]omplex, which also support this tentative structural proposi-ion.

. Antimicrobial activities

The antimicrobial activities of the Schiff base ligand and itsomplexes have been tested following previously published meth-ds [46] against Staphylococcus aureus as Gram-positive bacteria,scherichia coli as Gram-negative bacteria and antifungal activ-ty against the fungi Aspergillus niger and Pencillium chrysogenum.

Nutrient agar (NA) (peptone, beef extract, NaCl and Agar–Agar)has been used for both bacterial and fungal media. The ligandand the complexes have been dissolved in DMSO to make a con-centration of 10 mg/mL in order to obtain final concentration of1 mg/mL by proper dilution. Now to determine the antimicrobialactivities holes made on agar medium were incorporated with0.05 mL of test solution of particular concentration and kept for3 h at room temperature for diffusion of the ligand and its com-plexes. Then the bacterial and fungal cultures have been incubatedat suitable temperature (37 ◦C) for 48 h. After the proper incubation,the zones of inhibition were measured and the results are given inTable 4.

The complexes are found more active than the correspondingSchiff base ligand and it may be attributed to chelation theory [47].Within the chelated metal complexes the polarity of the metal ionsare greatly reduced due to overlapping with the ligand orbitals andthe delocalization of the � electrons over the whole complex isalso observed as well. These two factors are responsible for theenhanced penetration of the metal complexes into the lipid mem-branes as well as it blocks the metal binding sites in the enzymesof microorganisms. Thus, by blocking the synthesis of the proteins,the complexes actually inhibit the growth of the microorganisms[48].

5. Conclusion

Some mononuclear complexes of divalent and trivalent metalions with a hexadentate N4O2 Schiff base ligand have been syn-thesized and properly characterized. The ligand shows dibasicnature offering two imine nitrogens, two amine nitrogens andtwo phenolic oxygen towards the metal ions leading to octahe-dral geometry except for UO2

2+ and VO2+ where the same ligandfunctions as neutral hexadentate and neutral tetradentate sys-tem. The complexes are also shown to act as active antimicrobialagents.

Acknowledgements

S.S. is thankful to UGC, ERO, Kolkata for providing financial

assistance (No. F. PSW-107/09-10 (ERO) dt. 08-10-09.) and K.D.is thankful to the UGC, New Delhi for awarding Emeritus Fellow-ship and Financial grants to carry out this work. We are grateful toSophisticated Analytical Instrument Facility, Central Drug ResearchInstitute, Lucknow, SAIF and IICB, Kolkata for elemental analy-

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48 S. Sarkar, K. Dey / Spectrochim

es and some spectroscopic measurements. Facilities provided byhe Department of Science and Technology, Govt. of India, Newelhi under Funds for Improvement in Science and Technology pro-ramme are also gratefully acknowledged. We are also thankful tor. P. Chaudhuri, Department of Environmental Science, Universityf Calcutta for some helpful discussion. We are also grateful to theeviewers for their valuable comments.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.saa.2010.06.041.

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