Synthesis, characterization crystal structures and DNA binding studies of zinc complexes with oxygen and nitrogen donor ligands

Article abstract of DOI:10.1016/j.poly.2019.114273

Three zinc metal complexes viz, [Zn(L¹)₂phen] (1), [Zn₂(L²)₄(bipy)₂] (2) and [Zn(L²)₂bipy·H₂O] (3) where HL¹ = 4-(o-toluidino)-4-oxobutanoic acid

Full text of DOI:10.1016/j.poly.2019.114273

Polyhedron 177 (2020) 114273
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization crystal structures and DNA binding studies of
zinc complexes with oxygen and nitrogen donor ligands
a,
c
d,
Mehr-un-Nisa a, Muhammad Sirajuddin b, , Saqib Ali , Muhammad Nawaz Tahir , Muhammad Iqbal
⇑
⇑
⇑
a Department of Chemistry, Quaid-I-Azam University, Islamabad 45320, Pakistan
b Department of Chemistry, University of Science & Technology, Bannu 28100, KP, Pakistan
c Department of Physics, University of Sargodha, Sargodha, Pakistan
d Department of Chemistry, Bacha Khan University, Charsadda, Pakistan
a r t i c l e i n f o
a b s t r a c t
Three zinc metal complexes viz, [Zn(L1)2phen] (1), [Zn2(L2)4(bipy)2] (2) and [Zn(L2)2bipyꢀH2O] (3) where
HL1 = 4-(o-toluidino)-4-oxobutanoic acid and HL2 = 4-(4-nitrophenyl amino)-4-oxobut-2-enoic acid,
bipy = 2,2-bipyridine and phen = 1,10-phenenthroline. To elucidate their structure and geometry, the
synthesized complexes were characterized by different analytical techniques such as FT-IR, 1H NMR,
13C NMR and single crystal X-ray diffraction analysis. Single crystal X-ray analysis confirmed slightly dis-
torted square pyramidal geometry for complex 2 and distorted octahedral geometry for complexes 1 and
3. To explore the applications of synthesized complexes, their DNA binding studies were performed by
viscometry and UV–Visible spectroscopy. By using these techniques, binding constants with DNA and
Gibb’s free energy change was calculated of synthesized complexes. All the complexes showed sponta-
neous binding with DNA with Kb values 7.05 ꢁ 104, 1.06 ꢁ 104, 1.6 ꢁ 104 Mꢂ1 and the Gibb’s free energy
Article history:
Received 29 October 2019
Accepted 27 November 2019
Available online 18 December 2019
Keywords:
Zn(II) complexes
Crystal structure
FR-IR
NMR
DNA binding study
D
G = ꢂ27.6, ꢂ22.9 and ꢂ23.8 kJmolꢂ1, respectively for complexes 1–3.
Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction
applications in different areas of chemistry such as solution, sur-
face and redox chemistry [14–17].
Metal complexes have fascinating coordination chemistry,
remarkable physical and chemical properties, structural diversity
and wide applications as extractants, drugs, dyes, pesticides and
catalysts [1–4]. There isa variety or diversity in structures of
metal–organic coordination complexes due to several factors such
as counter ions [5], reaction temperature [6], coordination mode of
metal ion [7], pH [8] solvent [9] and metal-to-ligand ratio [10]. The
carboxylate ligands have different coordination modes such as
monodentate, bidentate, tridentate, bridging and chelating [11].
These ligands play an important role to form supra-molecular
structures of the complexes [12].
DNA is the primary target of many anticancer drugs due to its
prime role in cell life. It is highly important to know the interaction
mode of the anticancer drug as DNA offers a number of interactive
sites for covalent and non-covalent binders. Zinc complexes are the
favorable candidates for DNA binding as well as for its cleavages
[18]. The synthesis of new compounds with better binding affinity
like cis-platin is the demand of the day for nucleic acid structures
[19]. The study of the interaction of transition metal complexes
with DNA has been the main focus of recent research. Subsequent
structural changes in a DNA strands can easily be examined by a
number of experimental techniques [20].
Zinc has many applications in the field of biology, industries,
medicine and agriculture. Zinc has some antioxidant properties
which may protect the body muscles and skin from aging [13]. Zinc
complexes with a variety of nitrogen donor ligands have many
Among the N-donor ligands, 2,20-bipyridine and 1,10-phenan-
throline are biologically important ligands having chelating nature
as well as planar structure. Such ligands possess good coordinating
ability due to p–p interactions and thus their use for complexation
import enhancement in various chemical and biological properties
[16,17].
In the present study, three Zn(II) mixed ligand complexes have
been synthesized and characterized by various spectroscopic tech-
niques. Both shows a remarkable DNA interactions and the results
of DNA binding in UV Visible spectroscopy have been correlated
with viscometry.
⇑
Corresponding authors.
E-mail addresses: m.siraj09@gmail.com (M. Sirajuddin), drsa54@hotmail.com
(S. Ali), iqbal@bkuc.edu.pk (M. Iqbal).
https://doi.org/10.1016/j.poly.2019.114273
0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.

2
Mehr-un-Nisa et al. / Polyhedron 177 (2020) 114273
with distilled water and then air dried. Complex 2 was crystallized
2. Experimental
in DMSO while complex 3 in methanol.
2.1. Materials
2.4.3. HL1
1H NMR (DMSO d6, 300 MHz) d (ppm): 12.12 (s, 1H), 2.58 (t, 2H,
H2, 3J[1H, 1H] = 6.9 Hz); 2.54 (t, 2H, H3, 3J[1H, 1H] = 6.9 Hz); 9.29 (s,
1H NH); 7.38 (d, 3J[1H, 1H] = 7.5 Hz, 1H, H6); 7.20 (m, 2H, H7 and
H8); 7.34 (d, 1H, H9, 3J[1H, 1H] = 7.2 Hz); 2.18 (s, 3H, H11): 13C
NMR (DMSO d6, 75 MHz) d (ppm): 174.4 (C1); 31.0 (C2); 29.67
(C3); 170.5 (C4); 137.5 (C5); 126.3 (C6); 125.4 (C7); 130.7 (C8);
132.1 (C9), 136.8 (C10), 18.3 (C11).
4-Nitroaniline, o-toludine, maleic anhydride, succinic anhy-
dride, acetic acid, Zn(CH3COO)2ꢀ2H2O, NaHCO3, 1,10-phenanthro-
line and 2,20-bipyridine were acquired from Aldrich and used as
such. Solvents like methanol, DMSO were purchased from Merck,
Germany and used as such without further purification. Distilled
water was used for washing of precipitates and salt solutions
preparation.
2.4.4. Complex 1
2.2. Physical measurements
1H NMR (DMSO d6, 300 MHz) d (ppm): 2.58 (t, 2H, H2, 3J[1H,
1H] = 6.9 Hz); 2.54 (t, 2H, H3, 3J[1H, 1H] = 6.9 Hz); 9.29 (s, 1H
NH); 7.38 (d, 3J[1H, 1H] = 7.5 Hz, 1H, H6); 7.20 (m, 2H, H7 and
H8); 7.34 (d, 1H, H9, 3J[1H, 1H] = 7.2 Hz); 2.18 (s, 3H, H11); {8.85
(d, 1H, 3J[1H, 1H] = 7.8 Hz); 8.00 (m, 1H); 8.48 (m, 1H); 9.07 (s,
1H) (1,10-phenanthroline H)}: 13C NMR (DMSO d6, 75 MHz) d
(ppm): 179.0 (C1); 33.1 (C2); 31.8 (C3); 171.4 (C4); 140.1 (C5);
126.0 (C6); 125.0 (C7); 126.2 (C8); 127.4 (C9), 136.8 (C10), 18.2
(C11); {150.0, 125.0, 128.8, 127.4, 123.0, 140.4 (1,10-phenanthro-
line C)}.
Capillary tubes were used for determining melting points in an
electro thermal melting point apparatus model MP-D Mitamura
Riken Kogyo (Japan). FT-IR spectra were obtained in the range of
4000–400 cmꢂ1 using Nicolet-6700 FT-IR spectrophotometer. 1H
and 13C NMR were determined at room temperature using DMSO
as an internal reference on a Bruker Advance Digital 300 MHz
NMR spectrometer (Switzerland). Analyses of X-ray single crystal
were performed using Bruker Kappa APEXII CCD diffractometer
using graphite-monochromated Mo-K
a radiation (k = 0.71073 Å).
Crystal structures were solved by using direct method followed
by final refinement carried on F2 with full-matrix least-squares
using the program SHELXL-97 [21]. The Hydrogen atoms were
treated as riding atoms and included in calculated positions;
CAH = 0.93, 0.96 and 0.97 Å for CH, CH3 and CH2 H-atoms, respec-
tively, with Uiso(H) = kUeq(C), where k = 1.5 for CH3 and 1.2 for all
other H-atoms. DNA interactions studies were done using UV–Vis-
ible spectroscopy on a Beckman U-2020 spectrophotometer and
Ubbelohde viscometer.
2.4.5. HL2
1H NMR (DMSO d6, 300 MHz) d (ppm): 12.79 (s, 1H), 6.51 (d, 1H,
H2, 3J[1H, 1H] = 12 Hz); 6.37 (d, 1H, H3, 3J[1H, 1H] = 12 Hz); 10.86 (s,
1H NH); 7.87 (d, 2H, H6,60, 3J[1H, H] = 9 Hz); 8.25 (d, 2H, H7,70, 3J
1
[1H, 1H] = 9 Hz): 13C NMR (DMSO d6, 75 MHz) d (ppm): 169.6 (C1);
138.6 (C2); 130.3 (C3); 163.2 (C4); 142.0 (C5); 121.3 (C6,60); 122.5
(C7,70); 144.0 (C8).
2.4.6. Complex 2
1H NMR (DMSO d6, 300 MHz) d (ppm): 6.26 (d, 1H, H2, 3J[1H,
1H] = 12 Hz); 6.12 (d, 1H, H3, 3J[1H, 1H] = 12 Hz); 12.2 (s, 1H
2.3. Synthesis of the ligands HL1 and HL2
NH); 7.85 (d, 2H, H6,60, 3J[1H, H] = 7.2 Hz); 8.18 (d, 2H, H7,70, 3J
1
[1H, 1H] = 7.2 Hz); {8.66 (d, 1H, 3J[1H, 1H] = 5.2 Hz); 7.93 (m,
The ligand HL1 (4-(o-toluidino)-4-oxobutanoic acid) was pre-
pared by treating the equimolar quantities o-toluidine with suc-
cinic anhydride in glacial acetic acid at room temperature.
Similarly HL2 (4-(4-nitrophenyl amino)-4-oxobut-2-enoic acid)
was prepared from the reaction of equimolar quantities p-nitroani-
line with maleic anhydride in glacial acetic acid at room tempera-
ture [22].
1H); 7.70 (m, 1H); 9.2 (d, 1H, 3J[1H, H] = 5.2 Hz) (2,20-Bipyridine
1
H)}: 13C NMR (DMSO d6, 75 MHz) d (ppm): 170.5 (C1); 135.3
(C2); 130.1 (C3); 163.8 (C4); 141.4 (C5); 119.1 (C6,60); 125.4
(C7,70); 142.4 (C8); {145.8, 127.0, 136.9, 122.4, 149.3 (2,20-Bipyri-
dine C)}.
2.4.7. Complex 3
1H NMR (DMSO d6, 300 MHz) d (ppm): 6.32 (d, 1H, H2, 3J[1H,
1H] = 12.6 Hz); 6.06 (d, 1H, H3, 3J[1H, 1H] = 12.9 Hz); 13.12 (s, 1H
NH); 7.75 (d, 2H, H6,60, 3J[1H, 1H] = 7.0 Hz); 8.19 (d, 2H, H7,70, 3J[1H,
1H] = 7.0 Hz); {8.75 (d, 1H, 3J[1H, 1H] = 6.3 Hz); 8.28 (m, 1H); 7.71
(m, 1H); 8.64 (d, 1H, 3J[1H, 1H] = 6.3 Hz) (2,20-Bipy H)}; 3.32 (s, H of
water molecule): 13C NMR (DMSO d6, 75 MHz) d (ppm): 171.3 (C1);
136.9 (C2); 130.4 (C3); 164.8 (C4); 141.3 (C5); 119.1 (C6,60); 125.5
(C7,70); 142.4 (C8); {145.9, 127.0, 136.9, 124.7, 149.3 (2,20-Bipyri-
dine C)}.
2.4. Synthesis of the complexes 1–3
2.4.1. [Zn(L1)2phen] (Complex 1)
The sodium salt of HL1 was prepared by the reaction of HL1
(4 mmol) suspended in distilled water with the aqueous solution
of sodium bicarbonate (4 mmol) with constant stirring at room
temperature [22]. The reaction mixture was stirred to get the clear
solution. Then aqueous solution of zinc nitrate hexahydrate
(2 mmol) was added dropwise to the reaction mixture followed
by the addition of 1,10-phenanthroline (2 mmol dissolved in little
methanol) and stirred at 40 °C for 3–4 h. The resulting desired
white precipitates of complex 1 were filtered, washed with dis-
tilled water and then air dried.
2.5. DNA interaction study by UV–Visible spectroscopy
The commercially available salmon sperm DNAwas dissolved in
distilled water and kept at 4 °C for 4 days. During the interaction
study of DNA with complex molecules, the changes or shifting in
the absorption peaks were monitored. DNA shows maximum
absorption peak at 260 nm. The nucleotide to protein ratio in the
2.4.2. [Zn2(L2)4(bipy)2] (Complex 2) and [Zn(L2)2 (bipy) (H2O)]
(Complex 3)
Sodium salt of HL2 was prepared by similar way as that of HL1
explained in the synthesis procedure of complex 1. 2,20-bipyridine
(2 mmol) was used instead of 1,10-phenanthroline and resulting
desired light yellow and half white precipitates of complexes 2
and 3, respectively were appeared which were filtered, washed
range of 1.8–1.9 was calculated by the absorbance ratios (A260/
A280 and A260/A230) which showed the DNA was free from protein
molecules [22]. The concentration of DNA was determined through
absorption spectroscopy by using the value of molar absorption
coefficient, i.e., 6600 Mꢂ1 cmꢂ1 (at 260 nm) and calculated to be

Mehr-un-Nisa et al. / Polyhedron 177 (2020) 114273
3
5.5 ꢁ 10ꢂ5 Mꢂ1. Working solutions were prepared from this stock
solution by dilution method. The complex solutions of concentra-
tions 0.5 mM were prepared in DMSO. During UV absorption mea-
surements concentration of complexes were kept constant while
DNA concentration was varied. Equivalent concentration of DNA
was added to the reference and sample cells to cancel the effect
of DNA absorbance so that DNA itself does not show any absor-
bance. Quartz cells having 1 cm path length were used for record-
ing absorption spectra at room temperature. Before recording the
next spectrum, quartz cells were rinsed with acetone and air dried.
Benesi-Hildebrand’s equation [23] was used for calculating the
D
m
=
{
m
OCOasym
ꢂ
m
OCOsym
}
calculated for complex
1
are
114 cmꢂ1 indicating a bidentate coordination mode of carboxylate
moiety while for complex 3, the value of
Dm
is 206 cmꢂ1, respec-
tively indicating a monodentate coordination mode of carboxylate
groups [27]. In complex 2 two types of bands regarding asymmet-
ric and symmetric stretching vibrations of carboxylate ligands
were observed which are
m
OCOasym (1550, 1720 cmꢂ1
)
and
m
OCOsym (1374, 1365 cmꢂ1). The
Dm values are (176 and
355 cmꢂ1), suggesting both bidentate bridging and monodentate
type of bonding modes of ligands co-existing in solid state
[28,29]. The broad band at 3451 cmꢂ1 is attributed to OH of water
molecules in complex 3. The NH- group showed absorption bands
at 3322, 3340 and 3343 cmꢂ1 for complexes 1–3. The absorption
bands observed in the range 417–428 cmꢂ1 correspond to Zn–O
cm-1of the complexes. The bands at 513, 531 and 527 cmꢂ1 are
attributed to the Zn–N vibrations [30] of the complexes 1–3.
value of drug-DNA binding constant and Gibb’s free energy (DG)
was calculated to check the spontaneity of drug-DNA binding
processes.
2.6. Viscosity measurements
Ubbelohde viscometer was used for measuring viscosity at
room temperature. Flow time was measured by using digital stop-
watch. At least three readings were taken and average flow time
was measured. After each measurement viscometer was rinsed
3.2. NMR data
1H and 13C NMR chemical shifts of ligands and their complexes
are presented in experimental section while the numbering
scheme of the ligands has been given in Scheme 1 while the repre-
sentative 1H and 13C NMR spectra are given in supplementary data
(Figs. S1–S4). A comparative NMR analysis gives evidence for the
complex formation and the ligand coordination with metal ion.
In ligands, a sharp peak appears in the region 10–12 ppm due to
OH group of acids and after complexation this OH peak disap-
peared. In 1H NMR spectra aromatic protons appeared in the region
6.5–8.75 ppm showing multiplets. At 6.3–6.55 ppm olefenic pro-
tons appeared. Their value of coupling constant was calculated to
be 12 which indicate that these protons are trans to each other.
with acetone and oven dried, using this method
culated and graph was plotted between (
1/3 and [compound]/
[DNA] concentration ratio, where is the viscosity of DNA in the
g and go was cal-
g/go)
g
presence of drug whereas go is the viscosity of free DNA without
any drug [24]. Viscosity was calculated from the observed flow
time of DNA containing solution (to): go = t ꢂ to [25].
3. Results and discussion
The physical data including color, physical state, melting point,
molecular weight, solubility and percentage yield of the synthe-
sized compounds are given in Table 1.
´
The peaks of 1,10-phenanthroline and 2,2-bipyridine were
appeared in their respective regions. In complex 3, the peak
observed at 3.32 ppm confirms the coordination of water molecule
[30].
3.1. FT–IR data
In the 13C NMR of the ligands spectra, the signals at 174.3 ppm
(HL1) and 169.6 ppm (HL2) were assigned to the carboxylate C1
carbons. The signal of C1 is more sensitive to environmental
changes than other carbon atoms so after complexation it became
deshielded and shifted to downfield region. The acquired data
showed shifting of the C1 signal upon complexation to 179 ppm
in complex 1 and 170.5–171.3 ppm in complexes 2 and 3. This
The characteristics FT-IR peaks for the synthesized compounds
are shown in Table 2. There are two possibilities for carboxylate
ligand to bind to a central metal ion, either in monodentate or
bidentate mode. In Zn-carboxylates it can adopt both types of
bonding modes [26]. The carboxylate moiety showed
mOCOasym
bands at 1540, 1587 and 1580 cmꢂ1 while
mOCOsym bands at
1432, 1382 and 1374 cmꢂ1 for complexes 1–3, respectively. The
Table 1
Physical data of HL1, HL2 and their Zinc(II) complexes.
Compounds
Color
Physical Melting Mol. Solubility
%
State
Point
Wt.
Yield
(°C)
HL1
White
White
Yellow
Solid
Solid
Solid
Solid
165–167 207 DMSO
208–210 657 DMSO
156–158 236 DMSO
204–206 1496 DMSO
82
77
79
79
Zn(L1)2phen (1)
HL2
Zn(L2)4(bipy)2 (2) Light
yellow
Half white Solid
Zn(L2)2bipyH2O
189–191 644 DMSO
75
(3)
Scheme 1. Numbering scheme of ligands for 1H NMR and 13C NMR data.
Table 2
FT-IR data (cmꢂ1) of HL1, HL2 and their Zn(II) complexes.
Comp.#
m
OH
m
NH
m
CO
m
COOasym
m
COOsym
D
m
m
C@C
m
Zn-O
m
Zn-N
HL1
1
3442
–
3425
–
3326
3322
3345
3340
3343
1693
1661
1702
1679
1667
I657
1546
1623
1550, 1720
1560
1530
1432
1431
1374, 1365
1384
–
114
–
174,355
206
–
–
–
428
–
417
420
–
513
–
531
527
HL2
2
1645
1632
1627
3
3451

4
Mehr-un-Nisa et al. / Polyhedron 177 (2020) 114273
Table 4
downfield shifting is due to the transfer of electron density from
the ligands toward to electropositive Zn metal [30,31]. Additional
signals were observed for the carbon atoms of 1,10-phenanthroline
Selected bond lengths (Å) for complexes 1–3.
Complex 1
´
Zn-O1
Zn-O2
Zn-O4
2.024
2.434
2.148
Zn-O5
Zn-N1
Zn-N2
2.197
2.118
2.076
and 2,2-bipyridine in their respective regions [30,31].
3.3. Single crystal X-ray analysis
Complex 2
Zn1-O1
Zn1-O12
Zn1-O6
Zn1-N3
Zn1-N4
1.980(2)
2.049(2)
2.080(2)
2.084(2)
2.112(2)
Zn2-O16
Zn2-O11
Zn2-O7
Zn2-N10
Zn2-N9
1.979(2)
2.030(2)
2.040(2)
2.119(2)
2.135(2)
3.3.1. Crystal structure of complex 1
The crystal structure of complex 1 is shown in Fig. 1 and struc-
tural parameters in Table 3. The selected bond angles and lengths
are presented in Tables 4 and 5. In this complex the central metal
atom zinc is coordinated with oxygens of two carboxylate ligands
and with two nitrogen atoms of phenanthroline ligand. Both car-
boxylates and phenanthroline has a bidentate mode. The crystal
system of this complex is triclinic with space group P-1. The geom-
etry around the zinc is distorted octahedral due to the bulkiness or
steric hindrance of the ligands and electron pair repulsion. The
bond distance between nitrogens of 1,10-phenanthroline and zinc
metal is asymmetric, Zn-N bond lengths are 2.07 and 2.11 Å. The
bond angles between N1ZnO2 and N2ZnO4 are 146.11° and
138.2°, respectively which also confirms the distortion in octahe-
dral geometry and the other bond angles and bond lengths are
Complex 3
Zn1-O6
Zn1-O11
Zn1-O1
Zn1-N1
Zn1-N2
2.008(2)
2.030(2)
2.038(2)
2.106(2)
2.114(2)
Zn2-O17
Zn2-O12
Zn2-O22
Zn2-N7
Zn2-N8
2.016(2)
2.023(2)
2.037(2)
2.114(3)
2.117(3)
given in Table 5. The structural parameters are found typical of
the other structurally related zinc complex [30–33]. The two car-
boxylate ligands chelate to the zinc metal in an asymmetric man-
ner as depicted by four different bond distances of Zn-O. Both
Fig. 1. Molecular diagram of complex 1.
Table 3
Crystal data and structure refinement parameters for complexes 1–3.
Refinement parameters
1
2
3
Chemical Formula
Formula weight
T(K)
C
26H20N4O8 Zn
C62H53N12O23SZn2
1496.96
296(2)
C30H24N6O11Zn
644.48
296(2)
657.02
296(2)
Wavelength (nm)
Crystal system
Space group
a/b/c(Å)
0.71073
triclinic
P-1
0.71073
Orthorhombic
P b c a
26.1556(10), 15.0546(4), 32.9176(13)
90, 90, 90
12961.7(8)/8
0.863
6152
0.71073
monoclinic
P21/c
13.4790(4), 17.2382(6), 28.5319(9)
90, 101.932(2), 90
6486.3(4)/4
0.834
3104
8.2179(3), 14.0016(6), 14.1707(6)
80.129(2), 80.231(3), 88.664(2)
1583.08(11)/2
0.831
a
/b/
V(Å3)/Z
(mmꢂ1
F(0 0 0)
c
(°)
l
)
704
Crystal size (mm)
h-range for data calculation
Reflection collected
0.40 ꢁ 0.30 ꢁ 0.18
1.48 to 25.99
6198
0.40 ꢁ 0.32 ꢁ 0.24
1.462 to 25.5
11,677
0.38 ꢁ 0.30 ꢁ 0.24
1.38 to 27.172
14,361
Independent reflection
5121
1.010
4025
1.020
8750
1.013
Goodness-of-fit on f2
Final R indices [I > 2
R indices (all data)
Data/restrains/parameters
CCDC #
r
(I)]
R1 = 0.0436, wR2 = 0.0883
R1 = 0.0332, wR2 = 0.0817
6198/0/423
R1 = 0.0406, wR2 = 0.0901
R1 = 0.0713, wR2 = 0.1058
11677/0 /925
1959883
R1 = 0.0953, wR2 = 0.1208
R1 = 0.0483 wR2 = 0.1208
14361/2/937
1951633
1951634

Mehr-un-Nisa et al. / Polyhedron 177 (2020) 114273
5
Table 5
and two bipyridine molecules in the structure. The distance
between two zinc ions of the molecule is 3.707 Å which is longer
than 2.95 Å found in the tetra-bridged carboxylate zinc complex
[27–30]. The bridged oxygen atom is slightly asymmetrically coor-
dinated to the two zinc ions with 2.08 and 2.04 Å. The bipyridine
moieties are coordinated with different bond lengths to both Zn
atoms.
Selected bond angles (°) for complexes 1–3.
Complex 1
O1-Zn-O2
O1-Zn-O4
O1-Zn-O5
O2-Zn-O4
O2-Zn-O5
O4-Zn-O5
O1- Zn-N1
O1-Zn-N2
56.59
102.44
131.9
109.9
86.46
59.38
95.87
119.18
O2-Zn-N1
O2-Zn-N2
O4-Zn-N1
O4-Zn-N2
O5-Zn-N1
O5-Zn-N2
N1-Zn-N2
O1-Zn-N2
146.11
96.48
93.96
138.24
127.05
91.97
79.36
119.18
3.3.3. Crystal structure of complex 3
Structural diagram of the complex 3 is shown in Fig. 3 and
structural parameters in Table 3. The selected bond angles and
lengths are presented in Tables 4 and 5. Single crystal X-ray
diffraction shows that complex 3 has monoclinic system with
space group P21/c. the lattice contains two crystallographically
different molecules as indicated by the different bond lengths
and angles round zinc in both molecules. The separation between
the two zinc ions of the two molecules is 7.197 Å. The central
metal zinc is five coordinated as it is bonded with two oxygen
atoms of two carboxylate ligand, two nitrogens of bipyridine
and with one oxygen of water molecule. Both carboxylate ligands
Complex 2
O1-Zn1-O12
O1-Zn1-O6
O12-Zn1-O6
O1-Zn1-N3
O12- Zn1-N3
O6 -Zn1-N3
O1-Zn1-N4
O12-Zn1-N4
O6-Zn1-N4
N3-Zn1-N4
97.49(9)
84.32(9)
88.77(8)
106.55(9)
155.87(10)
91.49(9)
122.68(10)
90.96(9)
O16-Zn2-O11
O16-Zn2-O7
O11-Zn2-O7
O16-Zn2-N10
O11-Zn2-N10
O7-Zn2-N10
O16-Zn2-N9
O11-Zn2-N9
O7-Zn2-N9
110.84(10)
96.83(9)
90.59(9)
101.24(9)
91.17(10)
159.88(10)
95.49(9)
152.80(10)
92.96(9)
152.74(10)
77.98(10)
N10-Zn2-N9
76.67(10)
Complex 3
O6-Zn1-O11
O6-Zn1-O1
O11-Zn1-O1
O6-Zn1-N1
O11-Zn1-N1
O1-Zn1-N1
O6-Zn1-N2
O11-Zn1-N2
O1-Zn1-N2
N1-Zn1-N2
101.82(9)
91.84(9)
94.87(9)
159.62(9)
98.09(10)
90.81(9)
93.29(9)
104.28(9)
158.69(9)
77.47(10)
O17-Zn2-O12
O17-Zn2-O22
O12-Zn2-O22
O17-Zn2-N7
O12-Zn2-N7
O22-Zn2-N7
O17-Zn2-N8
O12-Zn2-N8
O22-Zn2-N8
N7-Zn2-N8
93.93(9)
show a monodentate mode. The structural distortion index tau (
s)
96.88(10)
97.12(10)
163.97(10)
89.90(9)
98.09(11)
92.81(10)
152.79(9)
108.19(10)
77.07(10)
was used to determine the geometry of five coordinated com-
pounds and calculated by
basal angles. The value of
a
s
-b/60 where
in this structure is 0.04 which indi-
a and b are the largest
cates the geometry around zinc is slightly distorted square pyra-
midal because the value of for perfect square pyramidal is
s
zero and for perfect trigonal bipyramidal is 1 [30]. This discrep-
ancy occurs due to bulkiness of ligand or electron pair repulsion.
In this complex again there is asymmetrical bond distance
between zinc and nitrogens of bipyridine, their bond lengths are
2.106 and 2.114 Å. A water molecule is also present in crystal lat-
tice which helps in the formation of more compact and packed
structure by stabilizing the molecules through non-covalent inter-
actions. One of the ligands in the crystallographically independent
molecules of complex 3 is disordered with occupancy ratio of
0.501(10):0.499(10). This has been tackled partially with inclusion
of restraints yielding satisfactory results. Benzene rings were trea-
ted as regular hexagons with C-atoms having equal anisotropic
thermal parameters. Such explanation and resolution method
has been applied for other complexes as well having crystallo-
graphically independent molecules in the unit cell [31].
carboxylates have one shorter Zn-O and one longer Zn-O due to
unequal distribution of electron density. In this complex zinc car-
ries +2 charges which are balanced by carboxylate ligands.
3.3.2. Crystal structure of complex 2
Structural diagram of the complex 2 is shown in Fig. 2 and
structural parameters in Table 3. The selected bond angles and
lengths are presented in Tables 4 and 5. The complex is dinuclear
with doubly bridged zinc ions each with square pyramidal geome-
try. The square base is formed by two carboxylate ligands and a
bidentate bipyridine molecule. There are four carboxylate ligands
Fig. 2. Molecular diagrams of complex 2. H atoms are omitted for clarity. The red dots represent the oxygen atoms of water molecules. DMSO in distorted form is also present
as solvent molecule. (Colour online.)

6
Mehr-un-Nisa et al. / Polyhedron 177 (2020) 114273
Fig. 3. Molecular diagrams of complex 3. One molecule is omitted from the unit cell for clarity.
3.4. UV–Visible spectroscopy
On addition of different concentrations of DNA, an obvious red shift
of about 10 nm occurred in the absorption position showing some
interaction between complex and DNA. Also with increase in con-
centration of DNA, absorption decreases which indicates the hypo-
chromic effect. These two phenomena of red shift and
hypochromic shift are the indication of intercalative mode of inter-
action [35–37].
Binding constant can be calculated by using Benesi-Hildebrand
Eq. (1) [23] and to check the spontaneity of interaction between
complex and DNA Gibb’s free energy was calculated by using equa-
tion in (2).
UV–Visible absorption spectroscopy is used to study the inter-
action of metal complexes with DNA. The study was performed
by adding various concentrations of DNA to Zn complexes in order
to monitor the change in the position of absorption bands as a
result of some interaction between complex and DNA [34]. Absorp-
tion spectra of complexes 1–3 in the absence and presence of DNA
are given in Figs. 4–6. A strong absorption band at 348.5–350 nm
was appeared in the spectra of all the three complexes showing
p–p* transitions due to aromatic ring present in their structures.
Fig. 4. Absorption spectrum of complex 1 in the absence (a) and presence of 1.17 mM (b), 2.2 mM (c), 3.21 mM (d), 4.317 mM (e), 5 mM (f) DNA. The arrow direction indicates
increasing DNA concentration.

Mehr-un-Nisa et al. / Polyhedron 177 (2020) 114273
7
Fig. 5. Absorption spectrum of complex 2 in the absence(a) and presence of 11.17 mM (b), 2.2 mM (c), 3.21 mM (d), 4.317 mM (e), 5 mM (f) and 6 mM (g) DNA. Increase in DNA
concentration is indicated by the direction of arrow.
Fig. 6. Absorption spectrum of 0.5 mM complex 3 in the absence (a) and presence of 16.8 mM (b), 32 m.3 M (c), 46.8 mM (d), 60.2 mM (e), 72.83 mM. (f) and 87.4 mM (g) DNA.
Where R = general gas constant (8.3141 J molꢂ1 Kꢂ1), K = binding
constant and T = absolute temperature (298 K).
A0
2G
2G
1
¼
þ
ꢀ
ð1Þ
A ꢂ A0 2HꢂG ꢂ 2G 2HꢂG ꢂ 2G K½DNAꢃ
The value of K for complexes 1–3 are: 7.05 ꢁ 104, 6.3 ꢁ 104,
Where K = binding constant, A and Ao = Absorbances of drug-DNA
solution and free drug solution, respectively.
ƐH-G and ƐG = molar absorption coefficients of drug-DNA and
free drug, respectively.
1.6 ꢁ 104 Mꢂ1 while the value Gibbs free energy are:
DG = -27.6,
´
ꢂ27.3, ꢂ23.8 kJmolꢂ1
.
K was obtained from slope to intercept ratio by plotting a graph
between Ao/A-Ao and 1/[DNA]
3.5. Viscosity measurements
Viscometric technique is also useful to investigate the binding
modes of DNA with complexes. Change in viscosity determines
D
G ¼ ꢂRTlnK
ð2Þ

8
Mehr-un-Nisa et al. / Polyhedron 177 (2020) 114273
Fig. 7. Effect of increasing concentration of complexes 1–3 on relative viscosity of DNA at 25 0.1 °C. [DNA] = 53 ꢁ 10-6 M.
the mode of binding. The viscosity of DNA is quite sensitive to
chain length variation. In general, increase in viscosity indicates
the intercalative mode of binding due to stacking of molecule
between the nitrogenous bases causing lengthening of double helix
so the viscosity of solution increases [38]. There is no significant
change occurring in groove binding while in electrostatic binding
mode slight decrease in viscosity takes place.
By the addition of different concentrations of complexes in DNA
solution there was an increase in viscosity which indicates an
intercalative mode of binding [39] as shown in Fig. 7.
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