Synthesis, spectral, and biological studies of transition metal chelates of N-[1-(3-aminopropyl)imidazole]salicylaldimine

Article abstract of DOI:10.1080/00958972.2011.557432

Tridentate chelate complexes M[LX·2H₂O], where M = Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) have been synthesized from the Schiff base L = N-[1-(3-aminopropyl)imidazole]salicylaldimine and X = Cl. Microanalytical data, UV-Vis, magnetic suscep

Full text of DOI:10.1080/00958972.2011.557432

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Synthesis, spectral, and
biological studies of transition
metal chelates of N-[1-(3-
aminopropyl)imidazole]salicylaldimine
M. Kalanithi a , M. Rajarajan b & P. Tharmaraj c
a Department of Chemistry , Jayaraj Annapackiam College for
Women , Periyakulam – 625601 , India
b Department of Chemistry , Cardamom Planters Association
College , Bodinayakkanur , India
c Department of Chemistry , Thiagarajar College , Madurai –
625009 , India
Published online: 18 Feb 2011.
To cite this article: M. Kalanithi , M. Rajarajan & P. Tharmaraj (2011) Synthesis, spectral, and
biological studies of transition metal chelates of N-[1-(3-aminopropyl)imidazole]salicylaldimine,
Journal of Coordination Chemistry, 64:5, 842-850, DOI: 10.1080/00958972.2011.557432
To link to this article: http://dx.doi.org/10.1080/00958972.2011.557432
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Journal of Coordination Chemistry
Vol. 64, No. 5, 10 March 2011, 842–850
Synthesis, spectral, and biological studies of transition metal
chelates of N-[1-(3-aminopropyl)imidazole]salicylaldimine
M. KALANITHIy, M. RAJARAJAN*z and P. THARMARAJ*x
yDepartment of Chemistry, Jayaraj Annapackiam College for Women,
Periyakulam – 625601, India
zDepartment of Chemistry, Cardamom Planters Association College,
Bodinayakkanur, India
xDepartment of Chemistry, Thiagarajar College, Madurai – 625009, India
(Received 26 August 2010; in final form 3 December 2010)
Tridentate chelate complexes M[LX ꢀ 2H2O], where M ¼ Co(II), Ni(II), Cu(II), Zn(II), and
Cd(II) have been synthesized from the Schiff base L ¼ N-[1-(3-aminopropyl)imidaz-
ole]salicylaldimine and X ¼ Cl. Microanalytical data, UV-Vis, magnetic susceptibility, IR,
1H-NMR, mass, and EPR techniques were used to confirm the structures. Electronic
absorption spectra and magnetic susceptibility measurements suggest square-planar geometry
for copper complex and octahedral for other metal complexes. EPR spectra of copper(II)
complex recorded at 300 K confirm the distorted square-planar geometry of the copper(II)
complex. Biological activities of the ligand and metal complexes have been studied on
Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Candida albicans by the
well diffusion method. The activity data show the metal complexes to be more potent than the
parent ligand against two bacterial species and one fungus. The electrochemical behavior of the
copper complex was studied by cyclic voltammetry.
Keywords: Tridentate ligand; N-(3-Aminopropyl)imidazole; Metal complexes; Spectral studies;
Biological activities
1. Introduction
Schiff bases have been used as ligands in coordination chemistry for their interesting
and important properties, such as the ability to reversibly bind oxygen, catalytic activity
in the hydrogenation of olefins, and proton transfer from the hydroxyl to the imine [1].
Complexes of Schiff bases showed promising applications in biological activity and
biological modeling applications [2–5].
Salicylaldimines and their metal complexes possess important biological applications
[6, 7]. Synthesis, characterization, and antimicrobial studies of transition metal
complexes of 1-(3-aminopropyl)imidazole and imidazole derivative were reported
[8–12]. Much research has been carried out on Schiff bases of salicylaldehyde and
derivatives [13–16]. Tas et al. reported the synthesis and characterization of copper(II)
*Corresponding authors. Email: rajarajan_1962@yahoo.com; ptharma@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2011 Taylor & Francis
DOI: 10.1080/00958972.2011.557432

N-[1-(3-Aminopropyl)imidazole]salicylaldimine
843
complexes with bidentate salicylaldimines derived from 3,5-di-t-butyl-2-hydroxyben-
zaldehyde [17]. Taking into account the importance of salicylaldimines in various fields
we attempted to synthesize and characterize transition metal complexes with NNO
tridentate Schiff base N-[1-(3-aminopropyl)imidazole]salicylaldimine. The biological
activities of the ligand and its complexes were examined.
2. Experimental
All chemicals were obtained from Aldrich Chemical & Co. and used without
purification. The UV-Vis spectra of the ligand and metal complexes were recorded in
DMF using a JASCO V-530 spectrophotometer. IR spectra in KBr discs were recorded
on a JASCO FT-IR 460plus spectrophotometer at Thiagarajar College, Madurai.
Cyclic voltammetry measurements were carried out at room temperature in DMSO
(CH Instruments, USA, voltammograph) using a three-electrode cell containing a
reference Ag/AgCl electrode, Pt wire auxiliary electrode, and glassy carbon working
electrode with tetrabutylammonium perchlorate (TBAP) as supporting electrolyte.
1
Elemental analyses were performed at SAIF, CDRI, Lucknow. H-NMR spectra were
recorded in CDCl3 using a Bruker DRX-300, 300 MHz NMR spectrometer. EI mass
spectra were recorded at IIT, Madras. EPR spectrum was recorded at SAIF, IIT,
Bombay. Magnetic moments of the complexes were measured on a Magnetic
Susceptibility Balance Mark 1 Sherwood UK at Thiagarajar College, Madurai.
Effective magnetic moments were calculated using the formula ꢀeff ¼ 2.828 (ꢁMT)1/2
,
where ꢁM is the corrected molar susceptibility. Molar conductances of the complexes
(10ꢁ3 mol Lꢁ1) were measured in DMF at room temperature using a Systronic
conductivity bridge.
2.1. Synthesis of N-[1-(3-aminopropyl)imidazole]salicylaldimine (L)
1.25 g (10 mmol) of 1-(3-aminopropyl)imidazole in 10 mL ethanol was added to 1.22 g
(10 mmol) of salicylaldehyde in 20 mL of ethanol. The mixture was refluxed for 6 h and
the ethanol was removed by distillation. The yellow liquid was reduced to one-half
of the volume and refrigerated for 1 day giving yellow crystals, which were filtered
and dried in vacuum over P4O10, m.p. ¼ 78ꢂC, IR in (KBr) ꢂmax cmꢁ1 1631 (C¼N), 1618
(C¼N, imidazole ring). 1H-NMR ꢃ (300 MHz; CDCl3), 8.32 (H, s, N¼CH), 13.25 (H, s,
OH), 2.23–2.15 (2H, m, CH2–CH2–CH2), 3.576–3.55 (2H, t, CH2–CH2–CH2), 4.09–4.05
(2H, t, CH2–CH2–CH2–N), 7.471–7.244 (7H, m, aromatic protons). The Schiff base
1
has been characterized by H-NMR spectroscopy. The spectrum shows proton signals
for the proton attached to the imine group and the OH proton.
2.2. Synthesis of complexes
1.34 g (10 mmol) of CuCl2 ꢀ 2H2O in 10 mL of ethanol was added to 30 mL solution of
10 mmol (2.3 g) of L in ethanol. The mixture was stirred for 3 h and the green precipitate
obtained was filtered and washed with water and then with ethanol and dried. The yield
was 60%. Other complexes of CoCl2 ꢀ 6H2O, ZnCl2, CdCl2 ꢀ 2.5H2O were prepared by
the same procedure, taking 10 mmol of the ligand and 10 mmol of the metal salt in
ethanol (scheme 1). For the preparation of nickel(II) complex 10 mmol of L was added

844
M. Kalanithi et al.
O
N
N
H2N
N
N
N
+
OH
OH
(L)
N-[1-(3-aminopropyl)imidazole]-salicylaldimine
N
N
N
N
MCl2 xH2O
Ethanol
M
Cl
N
N
O
OH
M=Cu(II)
N
N
N
N
OH2
MCl2 xH2O
M
Ethanol
N
N
O
Cl
H2O
OH
M=Co(II), Ni(II), and Zn(II)
N
N
N
N
H2O
OH2
MCl2 xH2O
M
Ethanol
N
N
O
Cl
H2O
OH
M=Cd(II)
Scheme 1. Synthesis of ligand and metal(II) complexes.
to 10 mmol of NiCl2 ꢀ 6H2O in 15 mL of hot ethanol and refluxed with stirring at
50–60ꢂC for 2 h. The precipitated green nickel complex was filtered and washed with
water and then with ethanol and dried.
3. Results and discussion
The tridentate ligand N-[1-(3-aminopropyl) imidazole]salicylaldimine forms stable
complexes with Co(II), Ni(II), Cu(II), Zn(II), and Cd(II). The complexes have been

N-[1-(3-Aminopropyl)imidazole]salicylaldimine
845
characterized by IR, UV-Vis, 1H-NMR, and EPR spectra, magnetic susceptibility
measurements, and conductance. The analytical data of the ligand and the complexes
together with their physical properties are given in table 1. Elemental analyses of the
complexes were in agreement with calculated data for 1 : 1 (M : L) ratio. Based on the
elemental analyses and mass spectra, [MLCl] was suggested for the copper(II) complex
and [MLCl ꢀ 2H2O] for other complexes. Arish and Sivasankaran Nair [18] reported
coordination by N2O ligand with L-histidine which is a 4-substituted imidazole nucleus.
Gao et al. [19] reported 2 : 1 complexes with N2O4 metal centers. Sang and Lin [20]
reported complexes of chloride-bridged centrosymmetric Cu(II) complexes. Budagumpi
et al. [21] synthesized complexes of M2L composition. This study describes the
coordination of N2O of N-(1)substituted imidazole salicylaldimine with metal ions, and
the complexes were found to be mononuclear. The purity of the Schiff-base complexes
was tested by TLC and elemental analysis.
3.1. Electronic spectra and magnetic moment
The electronic spectra of the complexes were recorded in DMF (table 2). Electronic
spectra of the ligand in ethanol showed bands at 30,769 and 35,335 cmꢁ1 attributed to
benzene ꢄ–ꢄ* transition. The band at 39,215 cmꢁ1 is assigned to imino ꢄ–ꢄ* or n–ꢄ*
transition [22]. Absorption at 14,903 cmꢁ1 for copper(II) complex is tentatively assigned
2
2
to B1g ! A1g for square-planar geometry [23]. Co(II) complexes exhibit three
4
4
absorption maxima at 11,160, 15,873, and 16,806 cmꢁ1 attributed to A2g ! T2g,
4
4
4A2g ! T1g(F), and 4A2g ! T1g(P), respectively, characteristic of octahedral geometry
[24]. The Ni(II) complex shows absorptions at 15,627 and 24,154 cmꢁ1, tentatively
3
3
3
3
assigned as A2g ! T1g(F) and A2g(F) ! T1g(P), suggesting an octahedral structure
[25]. Octahedral geometry has been proposed for zinc and cadmium complexes [19, 26].
Magnetic moment data (table 2) show paramagnetic behavior of Cu(II), Co(II), and
Ni(II) complexes. The value of 2.06 BM for copper complex falls within the range
normally observed for square-planar Cu(II) complexes [23]. The magnetic moment
measurement for cobalt(II) complex of 4.68 BM is indicative of octahedral geometry
[27]. The Ni(II) complex reported herein has a room temperature magnetic moment
value of 3.01 BM in the normal range observed for octahedral Ni(II) complexes
(ꢀeff ¼ 2.9–3.3 BM) [27]. As expected, the Zn(II) and Cd(II) complexes are diamagnetic
having a d10 system.
3.2. IR spectra
Spectral data for N-[1-(3-aminopropyl)imidazole]salicylaldimine and the complexes are
presented in table 3. A strong band of the free ligand at 1631 cmꢁ1, which is attributed
to the C¼N stretch shifted to ca 1618–1641 cmꢁ1 in spectra of all complexes, indicates
coordination of the azomethine nitrogen to copper [28, 29]. The C¼N (ring) stretching
frequency in the Schiff base at 1618 cmꢁ1 shifts to 1618–1634 cmꢁ1 in the complexes,
showing coordination of imidazole nitrogen [12]. The absence of O–H stretching
frequency in the complexes indicates the coordination of deprotonated hydroxyl [17].
The broad band at 3419–3456 cmꢁ1 is due to O–H stretching in coordinated water [30].

846
M. Kalanithi et al.

N-[1-(3-Aminopropyl)imidazole]salicylaldimine
847
Table 2. Electronic spectral data of ligand and metal(II) complexes.
Compound
Ligand (L)
ꢅmax (cmꢁ1
)
Transition
Geometry
–
ꢀeff (BM)
30,769
35,335
39,215
14,556
11,160
15,873
16,806
15,267
24,154
INCT
INCT
INCT
–
2
[CuLCl]
[CoLCl ꢀ 2H2O]
2B1g ! A1g
Distorted square planar
Distorted octahedral
2.06
4.68
4
4T1g(F) ! T2g(F)
4
4T1g(F) ! A2g(F)
4
4T1g(F) ! T2g(P)
3
[NiLCl ꢀ 2H2O]
3A2g ! T1g(F)
Distorted octahedral
3.04
3
3A2g(F) ! T1g(P)
Table 3. IR spectral data of the ligand and metal(II) complexes (cmꢁ1).
Compound
ꢂ(O–H)
ꢂ(C¼N)
ꢂ(C¼N) ring
ꢂ(C–O)
Ligand
3383
–
3419
3446
3420
3456
1631
1618
1618
1626
1617
1641
1618
1609
1605
1597
1600
1634
1275
1226
1232
1228
1240
1234
Cu complex
Co complex
Ni complex
Zn complex
Cd complex
3.3. 1H-NMR spectra
1
The H-NMR spectrum of L recorded in CDCl3 (Supplementary material) is assigned
as follows: azomethine proton as singlet (s, 8.32 ꢃ), hydroxyl group proton as singlet
(s, 13.25 ꢃ), aromatic protons as multiplet at 7.471–7.244 ꢃ, CH2–CH2–CH2 at (2H,
m, 2.23–2.15 ꢃ) and 2H, t, CH2–CH2–CH2 (3.576–3.55 ꢃ), 2H, t, CH2–CH2–CH2–N
1
(4.09–4.05 ꢃ). The H-NMR spectra of zinc(II) and cadmium(II) complexes have the
azomethine proton signal shifted downfield (8.45 ꢃ for Zn(II) and 8.47 ꢃ for Cd(II))
compared to the free ligand, suggesting coordination of azomethine nitrogen. The
absence of OH proton signal in the zinc(II) and cadmium(II) complexes indicates
deprotonation of the ligand and coordination to the metal.
3.4. Mass spectra
The mass spectra of L, Zn(II), and Cd(II) complexes were recorded (Supplementary
material). The molecular ion peak for L was observed at 229 m/z, whereas molecular ion
peaks of Zn(II) and Cd(II) complexes were observed, respectively, at 364 and 429 m/z,
confirming [ZnLCl ꢀ 2H2O], [CdLCl ꢀ 2H2O]H2O. The presence of lattice water in the
Cd(II) complex was confirmed by loss of mass (4.19%) while heating the complex in the
temperature range 80–105ꢂC [31]. Moreover, complexation of Zn(II) and Cd(II) were
confirmed by good observed and calculated isotopic distributions [32].
3.5. EPR spectra
A powder ESR spectrum of copper(II) complex was recorded at room temperature
using 2,2-diphenyl-1-picrylhydrazyl (DPPH) as reference. The spectrum exhibits

848
M. Kalanithi et al.
anisotropic signals with g|| ¼ 2.254 and g ¼ 2.210, characteristic for axial symmetry
?
[33]. Since g|| and g are close to 2 and g|| 4 g suggesting a tetragonal distortion
?
around Cu(II) corresponding to elongation along the four fold z-axis [34]. The trend
?
g|| 4 g 4 ge (2.0023) shows the unpaired electron localized in the dx2ꢁy2 orbital
?
characteristic of distorted square-planar geometry in Cu(II) complexes [18]. In addition,
exchange coupling interaction between two Cu(II) ions is explained by Hathaway
expression G ¼ (g|| ꢁ 2)/(g ꢁ 2). When the value G 5 4.0, a considerable exchange
?
coupling is present in the solid complex (G ¼ 1.2476) [35]. Kivelson and Neiman
showed that for an ionic environment, g|| is normally 2.3 or larger, but for a covalent
environment, g|| is less than 2.3. The g|| value for the Cu(II) complex is 2.254, conse-
quently the environment is covalent.
3.6. Electrochemical studies
3.6.1. Cyclic voltammetry. The redox behavior of Cu(II) complexes has been
investigated by cyclic voltammetry using glassy carbon electrode as the working
electrode, platinum wire as counter electrode, and Ag/AgCl electrode as the reference
and TBAP as supporting electrolyte (Supplementary material). The free ligand does not
show oxidation or reduction. The cathodic current values of copper complex were
independent of the scan rate. The repeated scan as well as different scan rates shows
that dissociation does not take place in these complexes. The cyclic voltammogram of
copper complex shows a well-defined redox process corresponding to the Cu(II)/Cu(I)
couple at Epa ¼ 0.13 V and the associated cathodic peak at Epc ¼ 0.38 V. This couple
is found to be quasireversible with one-electron oxidation/reduction reaction, which is
attributed to Cu(II)/Cu(I) [17].
3.7. Molar conductance
Values of molar conductance of the complexes are given in table 1. The low values in
DMF indicate that the complexes are nonelectrolytes [36].
3.8. Biological activities
The well diffusion method was employed for screening antibacterial and antifungal
activities [37]. The bacteria used are Pseudomonas aeruginosa, Escherichia coli
(Gram-negative), and Staphylococcus aureus (Gram-positive). The fungus used is
Candida albicans. Stock solution (0.001 mol) was prepared by dissolving the compounds
in DMSO and the solutions were serially diluted to find the minimum inhibitory
concentration (MIC) values (mg mLꢁ1). Antimicrobial activity studies were performed
in triplicate and the average was taken as the final reading. Error limits are also
indicated in the respective table. The MIC values of the compounds are summarized
in table 4. A comparative study of the MIC values for ligands and their complexes
indicates that the complexes exhibit higher antimicrobial activity than the ligand.
Such an increased activity of the complexes can be explained on the basis of Overtone’s
concept [38] and Tweedy’s chelation theory [39]. Zinc(II) and cadmium(II) complexes
were found to be more potent than the other complexes.

N-[1-(3-Aminopropyl)imidazole]salicylaldimine
849
Table 4. MIC of the synthesized compounds against growth of bacteria and fungus
( mg mLꢁ1).
Compound
S. aureus
E. coli
P. aeruginosa
C. albicans
Ligand
[CuLCl]
85
74
78
73
52
55
20
–
110
98
94
100
75
73
30
–
80
70
71
75
48
42
15
–
76
58
55
54
38
35
–
[CoLCl ꢀ (2H2O)]
[NiLCl ꢀ (2H2O)]
[ZnLCl ꢀ (2H2O)]
[CdLCl ꢀ 2H2O]H2O
Amikacin
Ketokonazole
20
Each value observed is within the error limits of ꢃ2.
4. Conclusion
Complexes of N-[1-(3-aminopropyl)imidazole]salicylaldimine with Cu(II), Co(II),
Ni(II), Zn(II), and Cd(II) were synthesized and characterized by various physical–
chemical methods. The complexes are different from earlier work with Schiff base of
L-histidine (4-substituted imidazole) and cuminaldehyde. The Schiff base is a tridentate
chelating ligand coordinating through azomethine nitrogen, imidazole nitrogen, and
oxygen. The proposed geometry for copper(II) is distorted square planar and distorted
octahedral for other metal ions. Biological activities of the ligand and their metal
complexes show that the activity of the metal complexes is higher than the ligand.
Acknowledgments
The authors thank the Management of Thiagarajar College, Madurai, and one of the
authors (MK) thanks the UGC, SERO, Hyderabad for awarding Teacher Fellowship
and SAIF, IIT, Bombay and CDRI, (SAIF) Lucknow for providing analytical facilities.
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