Inorganica Chimica Acta 357 (2004) 4286–4290

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Note

Zinc mediated synthesis of a new heteroditopic ligandwith hard and soft sites q

Narinder Singh, Manoj Kumar, Geeta Hundal *

Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India

Received 29 December 2003; accepted 15 May 2004

Available online 15 June 2004

Abstract

A new heteroditopic, Schiff’s base tripodal ligand has been synthesized by employing a thermodynamic template effect using Zn2þ

as a template which leaves the pseudo cavity after the Schiff’s base condensation.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Schiff’s base; Template effect; Crystal structure

1. Introduction

Schiff’s base condensation reactions are extensively

used in the preparation of an enormous range of mac-

rocyclic, macrobicyclic and open chain podand com-

pounds in high yields [1]. These compounds aregenerally formed by thermodynamic template effects

because unless water is removed during the course of the

reaction, the condensation is reversible. Templated

synthesis allows complexation to sequester the most

stable metal-product compound. However, one disad-

vantage of the template procedure is that if a metal free

ligand is required for other experiments then the tem-

plating cation must be removed [2]. If the metal ion isweakly coordinating then the ligand can be extracted

into an organic solvent, otherwise the metal ion may be

extracted by a stronger coordinating ligand such as a

cyanide ion, a sulfide ion or EDTA [3]. Here, we wish to

report a Zn2þ mediated thermodynamic templated syn-

thesis of a heteroditopic tripodal ligand 3, where after

the synthesis of the ligand 3 the template cation does not

need to be removed as it leaves the tripod on its own

qSupplementary data associated with this article can be found, in the

online version, at doi:10.1016/j.ica.2004.05.032.* Corresponding author. Tel.: +91-0183-2258803x3196; fax: +91-

0183-2258820.

E-mail addresses: geetahundal@yahoo.com, geeta.hundal@angel-

fire.com (G. Hundal).

0020-1693/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2004.05.032

owing to changes in the conformation of the ligand thus

formed. In continuation of our ongoing studies on

complexes of tripodal and tetrapodal ligands [4], we

have synthesised this new heteroditopic ligand with hard

and soft ligating sites to form complexes with bistable

metal ions which could find an application in the for-mation of molecular switches [5].

2. Results

The synthetic strategy is outlined in Scheme 1. Re-ceptor 3 was obtained by a templated synthesis involv-

ing condensation of 1 with salicylaldehyde in the

presence of traces of zinc perchlorate. This provided

the product 3 in quantitative yield with high purity and

the product separates out within 15 min from the reac-

tion mixture. This separation did not occur in the ab-

sence of Zn2þ even after stirring for three hours, instead

a crude product which had to be recrystallised repeat-edly to remove impurities was obtained in very small

yield. The absence of Zn2þ from the final product 3 was

confirmed by elemental analysis, mass spectra and the

X-ray crystal analysis of 3. The synthetic route sug-

gested in Scheme 1 was confirmed by reacting 1 with

Zn(ClO4)2 in acetonitrile when Zn2þ underwent com-

plexation immediately and the reaction mixture became

colorless. Evaporation of the solvent gave a viscouscompound, which on addition of chloroform gave a

NH2

NS

S

S

NH2

NH2

HO

OHC

CH3CN

HO

OHC

CH3CNZn(ClO4)2

NS

S

NCH

HOS

NCH

OH

N

S

S

Zn2+S

NH2

NH2

NH2

N CH OH

OHO

OHO

OHO

1

3

2

Scheme 1.

N. Singh et al. / Inorganica Chimica Acta 357 (2004) 4286–4290 4287

white colored solid complex 1 �Zn(ClO4)2. A compari-

son of the 1H NMR of this complex with that of 1 (both

taken in deuterated DMSO) showed significant shifts.

An upfield shift of Dd ¼ 0:91 for the amine protons, 0.18

and 0.25 for the –CH2 protons and 0.08 for the aromatic

protons was seen in the 1H NMR on addition of Zn2þ to

1, thus confirming the formation of 1 �Zn(ClO4)2. To a

solution of 1 �Zn(ClO4)2 in acetonitrile, salicylaldehydewas added in the calculated amount (see Section 4) and

stirred. The product obtained in the form of yellow

crystals was then separated and found to be 3 by using

Fig. 1. Showing one molecule of the asymmetric unit and intra-molecular H-

S(2)–C(17) 1.802(8), S(3)–C(33) 1.776, S(3)–C(32) 1.798, N(1)–C(1) 1.464(9),

chromatography, 1H NMR and X-ray diffraction

methods. It confirmed the initial participation and

subsequent removal of Zn2þ during the synthesis.

2.1. Spectroscopic data

The ES mass spectrum of 3 shows a molecular ion

peak (Mþ) at 783 with 20% relative abundance. The bandat 1610 cm�1 corresponds to the imine bond, the presence

of which was also confirmed by 1H and 13C NMR

showing peaks at d 8.60 and d 161.88, respectively.

bonding. S(1)–C(3) 1.763(10), S(1)–C(2) 1.788(9), S(2)–C(18) 1.754(9),

N(1)–C(31) 1.472(9), N(1)–C(16) 1.478(9) �A.

Fig. 2. UV–Vis (acetonitrile) (A) spectrum of 0.5� 10�3 M 1 �Zn2þ in

acetonitrile, (B) time variable spectrum of 0.5� 10�3 M 1 �Zn2þ and

1.5� 10�3 M salicylaldehyde in acetonitrile.

4288 N. Singh et al. / Inorganica Chimica Acta 357 (2004) 4286–4290

Receptor 3 shows a band at kmax 389 nm (e 7200) in

acetonitrile in the absorption spectra. This band was

designated as an intraligand O(phenolato)!CN(imino)charge transfer transition in the molecule [6]. The desig-

nation is also based on the fact that this band was absent

in amine 1 and is formed on Schiff’s base condensation. A

variable time UV–Vis spectrum of the compound, taken

at a regular interval of 1 min, after addition of salicylal-

dehyde to the mixture of 1 and Zn(ClO4)2 in CH3CN,

showed the appearance of a new band at kmax 389 nm,

whose intensity increased gradually with time (Fig. 2).This indicated that this band was due to the imine

chromophore in the molecule.

2.2. X-ray crystallographic studies

The final structure of the molecule is shown in Fig. 1.

The compound contains two crystallographically inde-

pendent molecules with slightly different geometricalparameters. The angles around the tripodal nitrogen N1

in the first molecule are almost equal to a tetrahedral

angle (average being equal to 110.6� (6)), whereas thosearound N1A in the second molecule are closer to being

pyramidal (average 116.6� (8)). This is because of the

disorder found in one of the legs attached to N1A. Out

of the three N–C–C–S torsion angles on each molecule,

two are anti and one is in a gauche conformation,whereas half of the C–C–N–C angles are anti with the

other half being gauche. Thus, the tripod is not in a fully

extended conformation as found in the case of analo-

gous triethanolamine structure [7]. The most important

feature of the structure is intramolecular H-bonding

interactions between the hydroxyl oxygens and the im-

ine nitrogens. An average O� � �N distance of 2.622(10) �Aindicates a fairly strong bonding in the solid state. Avery broad O–H stretching band in the IR of the solid

compound also indicated this. This H-bonding exists in

solution as indicated by a very low field shifted OH in

the 1H NMR, which showed no change in chemical shift

value on dilution. The H-bonding gives rise to the for-

mation of stable six membered rings involving imine

nitrogens and hydroxyl groups. Apart from these, there

are some weak C–H� � �O interactions as well. There is

an edge to edge intramolecular p–p interaction between

two molecules of the asymmetric unit (C8A� � �C383.305(9) �A).

3. Discussion

The templated synthesis is proposed to involve com-

plexation of Zn2þ by 1 through the N3S3 unit, which

changes the conformation of 1 from an extended to a

convergent conformation 2 (Scheme 1). A similarchange in conformation of tripodal ligands based on

Tren (tris(2-aminoethyl)amine) in the presence of Zn2þ

has already been suggested in the templated synthesis of

peptides containing helicogenic aminoisobutyric acid

connected to Tren [8]. Here, this change in the confor-

mation of the tripodal amine by the Zn2þ helps in pre-

organising it so that there are more chances of

salicylaldehyde molecules undergoing condensation withall three arms of the same molecule of the amine from

one direction. The process should be faster and cleaner

than the condensation taking place in the extended

conformation of the amine, where aldehyde molecules

are required to approach randomly from all directions

and will require more time to undergo condensation on

all three arms of a particular amine molecule. Once the

condensation is complete, nitrogens in the newly formedligand 3 will change from sp3 hybridised primary amine

to sp2 hybridised planar imine nitrogens. The sp2 ni-

trogens have smaller radii and more directional lone

pairs and in the present case are attached to heavy end

groups by rigid double bonds. Hence, these nitrogens

become less flexible and are unable to coordinate the

small Zn2þ, which is expelled. The free ligand 3moves to

an extended conformation, which is more stable in theabsence of a suitable anchor. Thus, Zn2þ picks out a

ligand complementary to it from an equilibrating mix-

ture of products and drives the equilibrium over to the

product. The template effect is thermodynamic in nature

[2] as the reactivity depends on the disassociation of the

amino group from the metal ion and the ligand may be

produced in the absence of the metal ion as well, though

at the cost of purity and yield.

4. Experimental

4.1. Synthesis of tris(2-mercaptoanilinethyl)amine 1

Compound 1 can be prepared by the method given in

the literature [9] or by the following method which gives

a better yield. 1.035 g of freshly cut sodium metal was

dissolved in absolute alcohol and 3.75 g/3.2 ml of 2-

mercaptoaniline. The reaction mixture was allowed to

N. Singh et al. / Inorganica Chimica Acta 357 (2004) 4286–4290 4289

reflux for 15 min. 2.04 g of trischloroethylamine [10] was

added to the reaction mixture and refluxed for 8 h. On

completion of the reaction, most of the alcohol was

evaporated and the contents were poured onto crushed

ice. The organic material was extracted with chloroform,the organic layer was dried on sodium sulfate and

treated with activated charcoal. Upon evaporation of

chloroform, the material was crystallised at 10 �C to give

light yellow colored tripodal amine 1. Yield 80%,

M.p.¼ 55 �C, 1H NMR (CDCl3, 200 MHz): d 2.57

(t, 6H, –CH2, J ¼ 3:6 Hz), 2.67 (t, 6H, –CH2, J ¼ 3:6Hz), 4.31 (s, 6H, –NH2), 6.64 (m, 6H, Ar), 7.09 (t, 3H,

Ar, J ¼ 7:6 Hz), 7.30 (d, 3H, Ar, J ¼ 7:6 Hz), 1H NMR(DMSO:CDCl3(1:2), 200 MHz): d 2.60 (t, 6H, –CH2,

J ¼ 3:8 Hz), 2.67 (t, 6H, –CH2, J ¼ 3:6 Hz), 4.85 (s, 6H,

–NH2), 6.57 (t, 3H, Ar, J ¼ 7:6 Hz), 6.72 (d, 3H, Ar,

J ¼ 8:0 Hz), 7.06 (t, 3H, Ar, J ¼ 7:6 Hz), 7.24 (d, 3H,

Ar, J ¼ 7:8 Hz), 13C NMR (CDCl3, 50 MHz): d 32.12

(–CH2), 53.01 (–CH2), 114.67 (Ar), 117.19 (Ar), 118.19

(Ar), 129.47 (Ar), 135.53 (Ar), 146.22 (Ar).

4.2. Synthesis of 3

Ligand 3 was prepared by taking 1 (470 mg 1.0 mmol)

and traces of zinc perchlorate in acetonitrile along with

salicylaldehyde (0.25 ml, 4.0 mmol) with stirring. The

color of the solution changed immediately from reddish

brown to yellow and after ten minutes the yellow pre-

cipitate separated in quantitative yield. This was filteredand dried at room temperature. An impure sample of 3

was formed on mixing the above amounts of amine and

salicylaldehyde in acetonitrile, in the absence of zinc

perchlorate. The reaction mixture was stirred for three

hours. TLC checked the completion of reaction for

consumption of the amine. At the end of the reaction,

the solvent was evaporated to give a crude product.

Repeated recrystallisations from acetonitrile/diethylether gave rise to compound 3. Yield 30%, M.p.¼ 102

�C, IR: m 1610 cm�1 (CH@N), MS: m=z 783, Anal. Calc.for C45H42N4S3O3: C, 69.02; H, 5.41; N, 7.16. Found:

C, 68.98; H, 5.38; N, 7.23%. 1 H NMR (CDCl3, 200

MHz): d 2.80 (t, 6H, –CH2, J ¼ 3:6 Hz), 2.94 (t, 6H,

–CH2, J ¼ 3:6 Hz), 6.94 (t, 3H, Ar, J ¼ 7:4 Hz), 7.05 (d,

3H, Ar, J ¼ 8:2 Hz), 7.17–7.57 (m, 18H, Ar), 8.60 (s,

3H, CH@N), 13.25 (s, 3H, –OH). 13C NMR (50 MHz)(CDCl3): d 30.27 (–CH2), 53.01 (–CH2), 117.38 (Ar),

117.67 (Ar), 118.97 (Ar), 119.20 (Ar), 126.33 (Ar),

127.42 (Ar), 127.79 (Ar), 132.27 (Ar), 132.49 (Ar),

133.29 (Ar), 146.56 (Ar), 161.13 (Ar), 161.88 (CH@N).

4.3. 1 Zinc perchlorate

1H NMR (DMSO:CDCl3 (1:2), 200 MHz): d 2.78(t, 6H, –CH2, J ¼ 3:6 Hz), 2.92 (t, 6H, –CH2, J ¼ 3:6Hz), 3.94 (s, 6H, –NH2), 6.64 (t, 3H, Ar), 6.79 (d, 3H,

Ar, J ¼ 7:8 Hz), 7.14 (t, 3H, J ¼ 7:6 Hz, Ar), 7.24 (d,

3H, Ar, J ¼ 7:6 Hz).

4.4. Crystal structure of 3

The data were collected on a Siemens P4 single crystal

diffractometer using graphite monochromatised Mo Karadiation (0.71073 �A). The structure was solved by direct

methods and subsequent difference Fourier synthesis and

refined by full-matrix least squares on F 2 with SHELXTLSHELXTL

[11]. No absorption correctionwas applied. There are two

crystallographically independent molecules in the unit

cell. On anisotropic refinement, one of the –CH2CH2

groups in the secondmolecule showeddisorder in termsof

abnormal bond distances and high thermal parameters.

The disorder could be resolved for atoms C31A and

C32A. Each one was split over two atomic positions with

site occupancies 0.40, 0.60 and 0.56, 0.44, respectively.

These were refined isotropically with restraints on their

bond lengths (C–N 1.470(5), C–C 1.510(5), C–S 1.790(2)

and C–C(non-bonding) 2.50(2) �A). All other atoms wererefined anisotropically. All the hydrogens were fixed

geometrically as riding atoms with a displacement pa-

rameter equal to 1.2 (–CH, CH2) or 1.5 (OH) times that of

the parent atom. R1 ¼ 0.0683, wR2 ¼ 0.1740 for 3720 ob-

served reflections ðI > 2rðIÞÞ and R1 ¼ 0.1356,

wR2 ¼ 0.2309 for 6530 unique reflections (Rint ¼ 0.0676)

for 989 parameters and eight restraints. Crystal data have

been deposited with the Cambridge CrystallographicData Center, under CCDC reference No. 218908.

Crystal data for 3: C45H42N4O3S3, M ¼ 783, mono-

clinic, space group P21=n, a ¼ 23:525(6), b ¼ 12:487(6),c ¼ 29:467(8) �A, b ¼ 110:35(1)�, V ¼ 8116(5) �A3, Z ¼ 8,

Dcalc ¼ 1.282 g cm�3, l ¼ 0:228 mm�1, T ¼ 293 K.

Acknowledgements

GH and Narinder Singh thank the CSIR (New Delhi,

India) for the research Grant and fellowship, respec-

tively. We also thank the CDRI (Lucknow, India) for

performing mass spectral and C, H, N analyses.

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