Synthesis and Spectral Characterization of 14- and 16-membered tetraazamacrocyclic Schiff base ligands and their transition metal complexes and a comparative study of interaction of calf thymus DNA with copper(II) complexes

Article abstract of DOI:10.1016/j.saa.2009.03.022

14 and 16 membered Schiff base macrocyclic ligands, 7,14-dimethyl-5,12-di(N-amino)-2-methylphenyl-1,4,8,11-tetraaza-cyclotetradecane-4,7,11,14-tetraene (L¹) and 8,16-dimethyl-6,14-di(N-amino)-2-methylphenyl-1,5,9,13-tetraaza-cyclohexadecane-5,8

Full text of DOI:10.1016/j.saa.2009.03.022

Spectrochimica Acta Part A 73 (2009) 622–629
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and Spectral Characterization of 14- and 16-membered
tetraazamacrocyclic Schiff base ligands and their transition metal
complexes and a comparative study of interaction of calf thymus
DNA with copper(II) complexes
Tahir Ali Khana,∗, Sultana Naseema, Shahper N. Khanb, Asad U. Khanb, Mohammad Shakira,1
a Division of Inorganic Chemistry, Department of Chemistry, Aligarh Muslim University, Aligarh-202002, India
b Interdisciplinary Biotechnology Unit, Aligarh Muslim University, Aligarh-202002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
14 and 16 membered Schiff base macrocyclic ligands, 7,14-dimethyl-5,12-di(N-amino)-2-methyl-
phenyl-1,4,8,11-tetraaza-cyclotetradecane-4,7,11,14-tetraene (L1) and 8,16-dimethyl-6,14-di(N-amino)-2-
methylphenyl-1,5,9,13-tetraaza-cyclohexadecane-5,8,13,16-tetraene (L2) were synthesized by condensa-
tion reaction between 2ꢀ-methyleacetoacetanilide and aliphatic diamines. The metal complexes of the
types, [ML1](NO3)2 and [ML2(NO3)2] [M = Co(II), Ni(II), Cu(II) and Zn(II)] were prepared by interaction of
ligands, L1 or L2 with hydrated metal(II) nitrates. The ligands and their complexes were characterized by
elemental analysis, IR, 1H and 13 C NMR, EPR, UV–Vis spectroscopy, magnetic susceptibility, conductivity
measurements and ESI-mass spectral studies. The results of elemental analyses, ESI-mass and conductiv-
ity measurements confirmed the stoichiometry of ligands and their complexes while the characteristic
absorption bands and resonance peaks in IR and NMR spectra confirmed the formation of ligand frame-
works around the metal ions. The square planar geometry for complexes derived from ligand L1 and
octahedral environment for complexes derived from ligand L2 with distortion in Cu(II) complex have
been confirmed on the basis of results of electronic and electron spin resonance spectral studies and
magnetic moment measurements. Absorption and fluorescence spectral studies revealed different bind-
ing mode for complex, [CuL1](NO3)2 as compared with [CuL2(NO3)2] on interaction with calf thymus DNA.
Received 25 September 2008
Received in revised form 28 February 2009
Accepted 16 March 2009
Keywords:
Schiff base macrocyclic ligands
Spectral studies
Square planar complexes
Octahedral complexes
DNA binding studies
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
plexes containing macrocyclic ligands for supramolecular science,
bioinorganic chemistry, biomedical applications, separation and
Schiff base macrocycles were among the first artificial metal
macrocyclic complexes to be synthesized [1]. Studies on com-
plexes of Schiff base macrocyclic ligands have received considerable
attention in view of their applications [2–6] development of coor-
dination and bioinorganic chemistry [7,8] models for biologically
occurring metelloenzymes [9,10] and biomedical significance [11].
A large variety of [1 + 1] and [2 + 2] macrocyclic ligands have been
reported defining the role of donor atoms, their relative posi-
tions, the number and size of the chelating rings, the flexibility
and the shapes of the coordinating moiety on the selective bind-
ing of charged or neutral species and the properties arising from
these aggregations [12,13]. Recognition of the importance of com-
encapsulation processes as well as formation of compounds with
unusual properties and structures tempted the chemists all over
world to develop methods for the synthesis of these compounds.
Many rational synthetic routes to macrocyclic ligands involve the
use of a metal ion as template to orient the reacting groups to lin-
ear substrates in the desired conformation prior to ring closure
[1,14]. The Schiff base have also been employed as ligands for com-
plexation of metal ions [15]. Macrocyclic complexes of Cu(II) have
been reported to interact with DNA by different binding modes
and exhibited efficient nuclease activity [16,17] but unfortunately
have received little attention. However, the exact mode and extent
of binding and cleavage mechanism still remain unknown leav-
ing behind a scope for an extensive studies involving macrocycles
with different structures to evaluate and understand the feature
that determine the mode of the binding interaction with DNA
and mechanism. The metal complexes of 2ꢀ-methylacetoacetanilide
and its derivatives have been reported to show interesting bio-
chemical properties such as antitumour, antioxidant, antifungal
∗
Corresponding author. Tel.: +91 9456033667.
E-mail address: takhan213@yahoo.co.in (T.A. Khan).
Present address: Chemistry Department, College of Science, King Khaled
1
University-Abha, Kingdom of Saudi Arabia.
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.03.022

T.A. Khan et al. / Spectrochimica Acta Part A 73 (2009) 622–629
623
and antimicrobial activities [18] and laboratory uses and many
industrial applications [19]. N. Raman and C. Thangaraju have
reported a fully conjugated tetraazamacrocyclic Schiff base ligand
by condensation reaction 3ꢀ-cinnamalideneacetoacetanilide and o-
phenylenediamine and its bivalent metal complexes [20].
Therefore, we thought it worth to synthesize macrocyclic lig-
ands involving 2ꢀ-methylacetoacetanilide and their complexes and
to investigate their spectro-chemical and DNA binding properties.
Herein two novel 14- and 16-membered Schiff base tetraaza-
macrocyclic ligands are reported by [2 + 2] condensation reaction
of 2ꢀ-methylacetoacetanilide with 1,2-diaminoethane and 1,3-
diaminopropane, respectively and their complexes with Co(II),
Ni(II), Cu(II) and Zn(II) ions.
2. Experimental
2.1. Materials and methods
Fig. 1. Stern–Volmer plot for the binding of complexes 1d and 2d with DNA at 310 K.
The chemicals, 2ꢀ-methylacetoacetanilide, 1,2-diaminoethane,
1,3-diaminopropane (Acros, India) were commercially pure com-
pounds and used as such. Metal salts (all Merck) were used as
received. Methanol used as a solvent was dried before use [21].
Highly polymerized calf thymus DNA sodium salt (7% Na content)
was purchased from Sigma Chemical Co. Other chemicals were
of reagent grade and used without further purification. Deionized
water was used through out the study. Calf thymus DNA was dis-
solved to 0.5% w/w, (12.5 mM DNA/phosphate) in 0.1 M sodium
phosphate buffer (pH 7.40) at 310 K for 24 h with occasional stir-
ring to ensure formation of homogeneous solution. The purity of
2.5. Preparation of the complexes, [ML2(NO3)2] [M = Co(II), Ni(II),
Cu(II) and Zn(II)]
A similar procedure was adopted for the synthesis of complexes
of L2 (0.001 mol, 0.458 g) as for complexes derived from L1.
2.6. Binding analysis of complexes 1d and 2d
To elaborate the fluorescence quenching mechanism the
Stern–Volmer Eq. (1) was used for data analysis [22]:
the DNA solution was checked from the absorbance ratio A260/A280
.
Since the absorption ratio lies in the range 1.8 < A260/A280 < 1.9,
therefore no further deproteinization of DNA was needed. The stock
solution of complexes 1d and 2d with 5 mg/ml concentration was
also prepared.
F0
= 1 + Ksv[Q]
(1)
F
where F0 and F are the steady-state fluorescence intensities
in the absence and presence of quencher, respectively. Ksv the
Stern–Volmer quenching constant and [Q] is the concentration
2.2. Synthesis of 7,14-dimethyl-5,12-di(N-amino)-
2-methylphenyl-1,4,8,11-tetraaza-cyclotetradecane-
4,7,11,14-tetraene (L1)
of quencher (DNA). The Ksv for the complex 1d (2.7 × 104 LM−1
)
was found to be onefold higher than the Ksv of complex 2d
(2.4 × 104 LM−1). A higher Ksv value of 1d as compared to 2d sug-
gests its stronger quenching ability. Further implicates its higher
binding affinity toward the DNA than 2d. The linearity of the F0/F
versus [Q] (Stern–Volmer) plots for DNA-1d and 2d complexes
(Fig. 1) depicts that the quenching may be static or dynamic, since
the characteristic Stern–Volmer plot of combined quenching (both
static and dynamic) is an upward curvature. When ligand molecules
A solution of 1,2-diaminoethane (0.002 mol, 0.13 ml) taken in
20 cm3 of methanol was slowly added to a methanolic solution
(∼20 cm3) of 2ꢀ-methylacetoacetanilide (0.002 mol, 0.382 g) placed
in round bottom flask. The reaction mixture was stirred overnight
followed by refluxing for 6 h. The reaction mixture was allowed to
stand at room temperature resulting in the isolation of microcrys-
talline solid product after couple of days. The product was washed
several times with methanol and dried in vacuo.
2.3. Synthesis of 8,16-dimethyl-6,14-di(N-amino)-
2-methylphenyl-1,5,9,13-tetraaza-cyclohexadecane-
5,8,13,16-tetraene (L2)
The same procedure was adopted for the synthesis of L2 as
for L1 except that 1,3-diaminopropane was used instead of 1,2-
diaminoethane.
2.4. Preparation of the complexes, [ML1](NO3)2 [M = Co(II), Ni(II),
Cu(II) and Zn(II)]
To a methanolic solution (∼25 cm3) of the hydrated metal
nitrate (0.001 mol) taken in round bottom flask was slowly added a
methanolic solution (∼20 cm3) of ligand, L1 (0.001 mol, 0.430 gm).
The reaction mixture was stirred for 6 h followed by refluxing for
4 h. Microcrystalline solid product was isolated on evaporation over
a period of a few days. The product was washed several times with
methanol and dried in vacuo.
Fig. 2. Comparative plot of log ((F0 − F)/F) versus log [Q] for determining the binding
constant and number of binding sites, of complexes 1d and 2d on DNA.

624
T.A. Khan et al. / Spectrochimica Acta Part A 73 (2009) 622–629
bind independently to a set of equivalent sites on a macromolecule,
the equilibrium between free and bound molecules is given by Eq.
(2) [23]:
out using a Faraday balance at 25 ◦C. The UV–visible spectrophoto-
metric studies of the freshly prepared 10−3 M DMSO solutions of the
complexes in the range 200–1100 nm were conducted using a Cintra
5 GBC scientific Spectrophotometer at room temperature. EPR spec-
tra were recorded at room temperature on a varian E-4 X-band spec-
trometer using TCNE as the g-marker. The electrical conductivities
(10−3 M solution in DMSO) were obtained on a Systronic type 302
conductivity bridge thermostated at 25.00 0.05 ◦C. Fluorescence
measurements were performed on a spectrofluorimeter Model RF-
5301PC (Shimadzu, Japan) equipped with a 150 W Xenon lamp and
a slit width of 3 nm. A 1.00 cm quartz cell was used for measure-
ments. For the determination of binding parameters, 50 ␮M of com-
plex solution was taken in a quartz cell and increasing amounts of
ctDNA solution were titrated. Fluorescence spectra were recorded
at temperature 310 K in the range of 300–400 nm upon excitation
at 266 (ꢀem was 336 nm). The UV measurements of calf thymus
DNA were recorded on a Shimadzu double beam spectrophotome-
ter model-UV 1700 using a cuvette of 1 cm path length. Absorbance
values of DNA in the absence and presence of complex were made in
the range of 225–290 nm. DNA concentration was fixed at 0.2 mM,
while the compound was added in increasing concentrations.
ꢀ
ꢁ
F0 − F
log
= log K + n log [Q]
(2)
F
where K and n are the binding constant and the number of binding
sites, respectively. Thus, a plot of log (F0 − F)/F versus log [Q] (Fig. 2),
is used to determine K as well as n. The binding parameters for
complexes 1d and 2d were found to be K = (27.5 0.81) × 103 M−1
;
n = 1.02 and K = (1.39 0.41) × 103 M−1; n = 0.72, respectively. The
results suggest that the compound have different degrees of affin-
ity toward the DNA molecule. This differential binding of 1d and 2d
is attributed in terms of different molecular structures around the
Cu(II) ion.
2.7. Physical measurements
The elemental analyses were obtained from the Micro-analytical
laboratory of the Central Drug Research Institute (CDRI) Lucknow,
India. The IR spectra (4000–200 cm−1) were recorded as KBr/CsI
discs on the Perkin Elmer—2400 spectrometer. Metal was deter-
mined volumetrically [24]. 1H and 13C-NMR spectra were recorded
in DMSO-d6 using a Bruker Avance II 400 NMR spectrometer with
Me4Si as an internal standard from SAIF, Punjab University, Chandi-
garh. Electrospray mass spectra of the ligands and its complexes
were recorded on a micromass quattro II triple quadruple mass
spectrometer. Magnetic susceptibility measurements were carried
3. Results and discussion
Schiff base macrocyclic ligands were prepared by [2 + 2]
condensation reaction between 2ꢀ-methylacetoacetanilide and
aliphatic diamines, 1,2-diaminoethane (L1) or 1,3-diaminopropane
Scheme 1. Formation and Suggested structure of macrocyclic ligands.

T.A. Khan et al. / Spectrochimica Acta Part A 73 (2009) 622–629
625
Scheme 2. Suggested structure of [M(L1)](NO3)2 and [M(L2)(NO3)2], where M = Co(II), Ni(II), Cu(II) and Zn(II).
(L2) (Scheme 1). The complexes of the types, [ML1](NO3)2 and
[ML2(NO3)2] [M = Co(II), Ni(II), Cu(II) and Zn(II)] were synthe-
sized by reacting with the ligands, L1 and L2 respectively in
the presence of appropriate metal nitrate in 1:1 molar ratio
(Scheme 2). The purity of the ligands and the complexes was
checked by TLC run in 3:1 chloroform to methanol ratio. The for-
mation of ligand frameworks and the complexes, was deduced
on the basis of results of elemental analyses, molecular ion peak
Table 1
Elemental analyses, m/z values, colors, yields, molar conductances, and melting points of the ligands and their complexes.
Complexes
m/z found (calc.)
Color
Yield (%)
M
Found (calc.) (%)
N
Molar conductivity
(ohm−1 cm2 mol−1)/m.p. (◦C)
C
H
Ligand L1 1a
C26H34N6
430.68
(430.59)
Cream
62
61
70
72
71
70
63
65
68
69
–
–
72.12
(72.52)
7.46
(7.96)
19.35
(19.52)
–/166
Ligand L2 2a
C28H38N6
458.72
(458.65)
Light yellow
Brown
73.25
(73.33)
8.11
(8.35)
18.20
(18.32)
–/164
[Co(L1)](NO3)2 1b
C26H34CoN8O6
613.59
(613.54)
9.63
(9.61)
50.90
(50.89)
5.60
(5.58)
18.25
(18.26)
120/280
18/246
110/253
19/250
115/278
17/264
112/256
20/260
[Co(L2)(NO3)2] 2b
C28H38CoN8O6
641.60
(641.59)
Brown
9.20
(9.18)
52.39
(52.42)
5.92
(5.97)
17.42
(17.46)
[Ni(L1)](NO3)2 1c
C26H34NiN8O6
613.34
(613.29)
Blue
9.53
(9.57)
50.90
(50.92)
5.60
(5.58)
18.25
(18.27)
[Ni(L2)(NO3)2] 2c
C28H38NiN8O6
641.37
(641.35)
Grey
9.20
(9.15)
52.48
(52.44)
5.90
(5.97)
17.50
(17.47)
[Cu(L1)](NO3)2 1d
C26H34CuN8O6
618.16
(618.15)
Light purple
Blue
10.22
(10.28)
50.55
(50.52)
5.59
(5.54)
18.10
(18.13)
[Cu(L2)(NO3)2] 2d
C28H38CuN8O6
646.23
(646.19)
9.83
(9.83)
52.10
(52.04)
5.95
(5.93)
17.38
(17.34)
[Zn(L1)](NO3)2 1e
C26H34ZnN8O6
619.92
(619.98)
Light yellow
Light yellow
10.55
(10.55)
50.37
(50.37)
5.50
(5.53)
18.12
(18.07)
[Zn(L2)(NO3)2] 2e
648.18
10.10
51.90
5.85
17.25
C28H38ZnN8O6
(648.04)
(10.09)
(51.89)
(5.91)
(17.29)

626
T.A. Khan et al. / Spectrochimica Acta Part A 73 (2009) 622–629
Table 2
IR spectral data of the ligands and their complexes (cm−1).
Complexes
ꢁ(C N)
ꢁ(N–H)
ꢁ(C–H)
ı(C–H)
ꢁ(M–N)
ꢁ(M–O)
Ring Vibration
Ligand L1
1630 s
1635 s
1600 s
1595 s
1590 s
1600 s
1605 s
1610 s
1595 s
1590 s
3250 s
3250 s
3245 s
3246 s
3240 s
3245 s
3248 s
3244 s
3242 s
3246 s
2930 s
2925 s
2910 s
2915 s
2925 s
2930 s
2885 s
2880 s
2920 s
2915 s
1460 s
1465 s
1450 s
1445 s
1455 s
1462 s
1470 s
1480 s
1475 s
1470 s
–
–
–
–
–
1440 s
1450 s
1430 s
1420 s
1450 s
1430 s
1440 s
1460 s
1480 s
1470 s
1060 s
1050 s
1070 s
1080 s
1080 s
1070 s
1030 s
1040 s
1040 s
1050 s
745 s
740 s
740 s
745 s
760 s
750 s
750 s
735 s
755 s
750 s
Ligand L2
[Co(L1)](NO3)2
[Co(L2)(NO3)2]
[Ni(L1)] (NO3)2
[Ni(L2)(NO3)2]
[Cu(L1)](NO3)2
[Cu(L2)(NO3)2]
[Zn(L1)](NO3)2
[Zn(L2)(NO3)2]
485 m
490 m
480 m
485 m
495 m
500 m
500 m
495 m
235 m
–
240 m
–
230 m
–
235 m
*s = strong; **m = medium.
Table 3
1H NMR spectra of the ligands and their Zn(II) complexes ı (ppm).
Complexes
–CH3 (12H)
N–CH2–C (8H)
C–CH2–C (4H)
NH (2H)
Aromatic (8H)
ı (s)
ı (t)
ı (m)
ı (s)
ı (m)
L1
2.35
2.37
2.46
2.57
2.83
2.88
2.96
2.98
–
1.56
–
8.55
8.52
8.57
8.56
7.67
7.72
7.80
7.86
L2
[Zn(L1)](NO3)2
[Zn(L2)(NO3)2]
1.72
Table 4
3.1. Infrared spectra
13 C NMR spectral data (ppm) of the ligands.
The IR spectra (4000–200 cm−1) of the ligands and their metal
complexes feature absorption bands characteristics of various
functional groups of macrocyclic moiety providing information
regarding the formation of macrocyclic ligands and their coor-
dination mode in the complexes (Table 2). The i.r. spectra of
both the ligands show a new strong intensity bands at 1630 and
1635 cm−1, which may reasonably be assigned [25] to the imine
function ꢁ(C N) in the macrocyclic system. However, no band
has been observed characteristic of either free primary amine or
carbonyl functions supportive of the formation of proposed macro-
cyclic skeleton. A significant negative shift (25–40 cm−1) in ꢁ(C N)
stretching mode has been observed for the complexes as compared
to free ligands suggesting [26] the involvement of imine nitrogen
of the (C N) group in coordination with metal ions. This is fur-
ther supported by the appearance of a new medium intensity band
in the region 480–500 cm−1 assignable [27] to ꢁ(M–N) vibration.
A broad strong intensity band in the i.r. spectra of ligands and
its complexes in the region (3240–3250 cm−1) may reasonably be
assigned to secondary amine group. A weak absorption band in the
Carbon position
L1
L2
C1 and C3
158
85.26
44.26
–
20.12
18.12
169
86.12
43.82
32.82
20.9
C2
C4
C5
C6
C7
18.75
Aromatic carbon 130.1, 137.6, 138.6 142.5, 141.3 129.0, 135.6, 136.5, 143.7, 142.7
in ESI-mass spectra (Table 1), characteristic bands in the FT-IR
(Table 2), and resonance signals in the 1H and 13C NMR spec-
tra (Tables 3 and 4). The overall geometry of the complexes
was inferred from the observed values of magnetic moments
and the position of bands in the EPR and electronic spectra
(Table 5). The molar conductance measurements of all the com-
plexes in DMSO corresponding to ligand L1 show 1:2 electrolytic
nature while complexes of ligand L2 exhibit non electrolyte
nature.
Table 5
Magnetic moments, electronic spectral bands (cm−1) with their assignments and EPR data of the complexes of L1 and L2 ligands.
Complexes
ꢂeff (BM)
Band position (cm−1
)
Assignments
EPR Parameters
g
ꢁ
g
⊥
G
[Co(L1)](NO3)2
[Co(L2)(NO3)2]
[Ni(L1)](NO3)2
3.12
17,055
14,960
4A2g
4A2g
→
→
4T1g(P)
4T1g(F)
–
–
–
–
–
–
–
4
4.55
19,841
14,750
4T1g(F) → T1g(p)
–
–
4T1g(F) → A2g(F)
4
1
Diamagnetic
13,720
21,207
25,070
1A1g(D) → A2g(G)
1A1g(D) → B2g(G)
1
1A1g(D) → 1Eg(G)
3
[Ni(L2)(NO3)2]
3.14
10,235
16,070
24,517
3A2g(F) → T2g(F)
–
–
–
3
3A2g(F) → T1g(F)
3A2g(F) → T1g(P)
3
[Cu(L1)](NO3)2
[Cu(L2)(NO3)2]
1.72
1.85
17,827
22,275
2B1g
→
→
2A1g
2.1792
2.0916
2.0882
2.0507
2.0317
1.8067
2B1g
2Eg
16,500
19,127
2B1g
2B1g
→
→
2B2g
2Eg

T.A. Khan et al. / Spectrochimica Acta Part A 73 (2009) 622–629
627
region 2920–2925 cm−1 may be assigned [28] to CH3 stretching
vibration. All the complexes show sharp bands corresponding to
ꢁ(C–H), ı(C–H) and phenyl ring vibrations which appear at their
expected positions (Table 2). The coordination of nitrato groups
has been ascertained by appearance of bands in 230–240 cm−1
regions, which may reasonably be assigned [29] to ꢁ(M–O) of
the O-NO2 group in [M(L2)(NO3)2] complexes. The IR spectra of
these complexes show further bands in 1230–1270, 1040–1070
and 860–890 cm−1 region, which are consistent with monodentate
coordination of the nitrato group [29]. However, the bands corre-
sponding to free nitrate group in [ML1](NO3)2 complexes appear in
the region 1384–1390 cm−1 [30].
3.2. 1H NMR spectra
The 1H NMR data for macrocyclic ligands and their Zn(II) com-
plexes (Table 3) recorded in DMSO-d6 show a sharp signal in
the region ı2.35–2.57 which may be assigned to the methyl pro-
tons (–CH3; 12H). However, a singlet at ∼ı8.55 may be attributed
to the pendant secondary amino protons (C–NH–C; 2H) [20]. A
triplet observed in the region ı2.83–ı2.88 for L1 and L2 corre-
sponds [31] to the methylene protons (N–CH2–C; 8H) adjacent to
the nitrogen atom of the imine function of macrocyclic frame-
work. Another multiplet at ı1.56 may reasonably be assigned
[31] to the middle methylene protons (C–CH2–C; 4H) of the 1,3-
diaminopropane moiety in ligand L2. A multiplet observed in the
range ı7.67–ı7.72 for L1 and L2 may be attributed to the phenyl
protons (C6H4; 8H) [20]. However, the resonance signals obtained
for Zn(II) complexes show downfield shift as compared to the corre-
sponding ligands (Table 3) indicating the coordination of ligands to
Zn(II) ion.
3.3. 13 C NMR
The 13C NMR spectra of the ligands and their Zn(II) complexes
recorded in DMSO-d6 at room temperature gave 13C NMR signals
characteristic of carbon atoms of imine functions and all other
carbons in the macrocyclic skeleton at their appropriate positions
corresponding to the proposed structure. However, the position of
peaks in complexes compared with the free Schiff base macrocyclic
ligands (Table 4) show slight shifts invoking coordination of the
ligands to Zn(II) ion [20].
Fig. 3. (a) EPR spectra of the complex 1d at room temperature. (b) EPR spectra of
the complex 2d at room temperature.
3.4. EPR spectra
The EPR spectra of Cu(II) complexes were recorded as polycrys-
talline sample on X-Band at frequency 9.1 GHz under the magnetic
field strength 3100 G scan rate 1000, recorded at room temperature
which measure the exchange interaction between the copper cen-
ters in the polycrystalline solid. If G > 4, the exchange interaction
is negligible and if G < 4 considerable exchange interaction occurs
in solid complexes [33]. The G values in the range 1.8067–2.0317,
indicate for these complexes considerable interaction between two
copper centers.
(Fig. 3a and b). The g , g and G values obtained from these spectra
ꢁ
⊥
are presented in Table 5. The g and g values were computed from
ꢁ
⊥
the spectrum using TCNE free radical as ‘g’ marker. In square pla-
nar and distorted octahedral geometry, the unpaired electron lies
2
2
in the dx − dy orbital giving 2B1g as the ground state with g > g .
ꢁ
⊥
3.5. Electronic spectra and magnetic moments
The observed g values (Table 5) are characteristic of a square planar
and a distorted octahedral geometry in complexes [Cu(L1)](NO3)2
and [Cu(L2)(NO3)2], respectively. Kivelson and Neiman [32] have
The electronic spectra of the complexes were recorded in
DMSO (Table 5). The electronic spectrum of [Co(L1)](NO3)2 shows
two absorption bands at 17,055 and 14,960 cm−1 assignable to
reported the g < 2.3 for covalent character of the metal ligand band
ꢁ
and g > 2.3 for ionic character. Appling this criterion the covalent
ꢁ
4A2g → T1g(P) and A2g → T1g(F) transitions, respectively and a
band at 22,730 cm−1 assignable to ␲ → ␲* transition. These tran-
sition are in support of a square-planar geometry [20]. This is
further supported by observed magnetic moment of 3.12 B.M. The
observed magnetic moment of 4.55 B.M. and absorption bands in
the electronic spectrum at 14,750 and 19,841 cm−1 assigned to the
4
4
4
character of the metal ligand bond in the complexes under study
can be predicted. The trend g > g > ge (2.0023) observed for these
ꢁ
⊥
2
2
complexes show that the unpaired electron is localized in dx − dy
orbital of the Cu(II) ions. The g values are related by the expression
4
4
4T1g(F) → A2g(F) and 4T1g(F) → T1g(P) transitions, respectively for
g − 2
ꢁ
G =
the [Co(L2)(NO3)2] are consistent with an octahedral environment
g
⊥ − 2

628
T.A. Khan et al. / Spectrochimica Acta Part A 73 (2009) 622–629
around the Co(II) ion in the complex similar to that reported earlier
[34].
The electronic spectrum of [Ni(L1)](NO3)2 exhibits three absorp-
tion bands at 13,720, 21,207 and 25,070 cm−1 which may be
1
1
assigned to the three spin allowed transitions, A1g(D) → A2g(G),
1A1g(D) → B2g(G) and A1g(D) → Eg(G), respectively character-
istic of square planar geometry around Ni(II) ion [31]. The
diamagnetic nature revealed by magnetic moment studies fur-
ther confirm the square planar environment around the Ni(II) ion
[31]. While the observed magnetic moment value of 3.14 B.M. of
[Ni(L2)(NO3)2] and three electronic spectral bands at 10,235, 16,070
and 24,517 cm−1 corresponding to three spin allowed transitions
1
1
1
3A2g(F) → T2g(F), 3A2g(F) → T1g(F) and 3A2g(F) → T1g(P), respec-
3
3
3
tively indicate an octahedral geometry [35].
The spectrum of [Cu(L1)](NO3)2 shows two bands in the vis-
ible region, at 17,827 and 22,275 cm−1 assignable to B1g → A1g
2
2
and 2B1g → Eg transitions, respectively and the observed magnetic
2
moment of the complex is 1.72 B.M. similar to the square planar
complexes of Cu(II) ion [20].
The electronic spectra of [Cu(L2)(NO3)2] show a shoulder at
16,500 cm−1 with a main broad band at 19,127 cm−1 which may
2
2
2
2
unambiguously be assigned to B1g → B2g and B1g → Eg transi-
tions, respectively consistent with those reported for a distorted
octahedral geometry [35] around the Cu(II) ion. The observed mag-
netic moments of 1.85 B.M. further complement the electronic
spectral results [35].
Fig. 4. Fluorescence emission spectra of complexes 1d (a) and 2d (b) in the absence
and presence of increasing amount of DNA (a) 0 (b) 5 (c) 10 (d) 15 (e) 20 (f) 25 (g)
30 (h) 35 (i) 40 and (j) 45 ␮M; pH 7.4; T = 298K.
3.6. Fluorescence measurements
3.6.1. Binding property of the DNA to complexes 1d and 2d
The fluorescence spectroscopy provides insight of the changes
taken place in the microenvironment of DNA molecule on ligand
binding. The binding of these compounds with calf thymus DNA,
were studied by monitoring the changes in the intrinsic fluores-
cence of these compounds at varying DNA concentration. Fig. 4a
and b, show the representative fluorescence emission spectra of
the synthesized compound upon excitation at 266 nm. The addi-
tion of DNA caused a gradual decrease in the fluorescence emission
intensity of both the compounds with a conspicuous change in
the emission spectra. The spectra illustrate that an excess of DNA
led to more effective quenching of the fluorophore molecule flu-
orescence. The quenching of the compound fluorescence clearly
indicated that the binding of the DNA to complexes 1d and 2d
changed the microenvironment of fluorophore residue. The shift in
emission peak of the synthesized molecule further depicts effective
interaction at higher DNA concentration, which is more prominent
in case of 1d. The reduction in the intrinsic fluorescence of synthe-
sized molecules upon interaction with DNA could be due to masking
or burial of compound fluorophore upon interaction between the
stacked bases with in the helix and/or surface binding at the reac-
tive nucleophilic sites on the heterocyclic nitrogenous bases of DNA
molecule.
mode of complex is electrostatic effect or intercalation which can
stabilize the DNA duplex [36,37], and hyperchromism means the
breakage of the secondary structure of DNA. So we primarily spec-
ulate that the complex interacting with the secondary structure
3.7. Absorption spectroscopy
UV–Vis absorption studies were performed to further ascertain
the DNA-complexes 1d and 2d interaction. The UV absorbance
showed an increase with the increase in the complex 1d/2d con-
centrations (Fig. 5). Since the complexes 1d and 2d do not show any
peak in this region (Fig. 5), hence the rise in the DNA absorbance is
indicative of the interaction between DNA and the complexes 1d/2d
molecules. Both complexes exhibited hyperchromism but of varied
degree. Slight bathochromic shift was observed with complex 1d,
which was negligible with complex 2d. As hypochromism and
hyperchromism are both the spectral features of DNA concerning of
its double helix structure. Hypochromism means the DNA binding
Fig. 5. Absorbance spectra of DNA and DNA-1d (A)/2d (B) system. DNA concen-
tration was 0.20 mM (a). 1d/2d concentration for DNA-compound system was at
12.5 ␮M (b), 25 ␮M (c), 50 ␮M (d) and 100 ␮M (e).

T.A. Khan et al. / Spectrochimica Acta Part A 73 (2009) 622–629
[2] S. Chandra, X. Sangeetika, Spectrochim. Acta 60 A (2004) 147.
629
with calf thymus DNA resulting in its breakage and perturbation.
After interaction with the base pairs of DNA, the ␲–␲* orbital
of the bound ligand can couple with the ␲ orbital of the base
pairs, due to the decrease ␲–␲* transition energy, which results in
bathochromic shift [38]. The shift in the spectra of 1d suggests the
more interference of orbital by this molecule, which also corrobo-
rates with the high binding affinity of this 1d. The above changes are
indicative of the conformational alteration of DNA on but of varied
extent.
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4. Conclusion
The novel 14 and 16 membered Schiff base macrocyclic lig-
ands (L1) and (L2) have been synthesized by condensation reaction
between 2ꢀ-methylacetoacetanilide and aliphatic diamines and
their macrocyclic complexes, prepared by interaction of ligands,
L1 or L2 with hydrated metal(II) nitrates have been character-
ized by various physico-chemical studies. The ligand to metal
stoichiometry and the nature of bonding was ascertained on the
basis elemental analyses, position of molecular ion peaks in the
mass spectra and conductivity data. The formation of the proposed
macrocyclic framework has been inferred by the appearance of
imine bands in the IR and corresponding proton resonance sig-
nals in the 1H and 13C NMR spectra. A square planar geometry
was deduced for complexes derived from ligand L1 while an octa-
hedral geometry with slight distortion in [Cu(L2)(NO3)2] complex
was inferred for complexes resulted from Ligand L2 on the basis of
UV–visible and EPR spectral studies and magnetic moment data.
The fluorescence and absorption studies demonstrated a consider-
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Acknowledgments
The Chairman, Department of Chemistry, Aligarh Muslim Uni-
versity, Aligarh, India is acknowledged for providing the necessary
research facilities. The authors thank the Regional Sophisticated
Instrumentation Center, CDRI Lucknow for providing mass spectral
and analytical data for the compounds.
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