Two silver(I) complexes with bis(benzimidazole)-2-oxopropane ligands: Syntheses, crystal structures and DNA binding studies

Article abstract of DOI:10.1002/aoc.3747

Two Ag(I) complexes, [Ag₂(bobb)₂]?(NO₃)₂ (1) and [Ag₂(crotonate)₂(aobb)]_n (2) (bobb =1,3-bis(1-benzylbenzimidazol-2-yl)-2-oxapropane; aobb =1,3-bis(1-allylbenzimidazol-2-yl)-2-oxopropane), have been synthesized and characterized using elemental analysis, electrical conductivities, infrared and UV–visible spectral measurements and single-crystal X-ray diffraction. Complex 1 is binuclear and three-coordinated by two N atoms from two bobb ligands, while complex 2 is a unique metal organic compound with diamond-like multinuclear Ag centers with each Ag bridged by two aobb ligands and two crotonate ions to form one-dimensional single polymer chain structures and extended into two-dimensional frameworks through π–π and intermolecular C─H???O hydrogen bonds. The adjacent Ag(I) centers are bridged by allyl from aobb which is not only a σ-bonding ligand, but also a π-acid ligand. The DNA binding modes of complexes 1 and 2 were investigated using electronic absorption titration, fluorescence spectra and viscosity measurements. The results suggest that the two complexes bind to DNA via an intercalative mode, and their binding affinity for DNA follows the order 2?>?1. This is due to the chelating effects which can enhance the planar functionality of the metal complexes.

Full text of DOI:10.1002/aoc.3747

Received: 8 December 2016
DOI 10.1002/aoc.3747
Revised: 30 December 2016
Accepted: 31 December 2016
F U L L PA P E R
Two silver(I) complexes with bis(benzimidazole)‐2‐oxopropane
ligands: Syntheses, crystal structures and DNA binding studies
Shanshan Mao | Kesheng Shen | Xinkui Shi | Yuling Xu | Huilu Wu
School of Chemical and Biological
Engineering, Lanzhou Jiaotong University,
Two Ag(I) complexes, [Ag2(bobb)2]⋅(NO3)2 (1) and [Ag2(crotonate)2(aobb)]n
Lanzhou, Gansu, 730070, People's Republic
of China
(2) (bobb =1,3‐bis(1‐benzylbenzimidazol‐2‐yl)‐2‐oxapropane; aobb =1,3‐bis(1‐
allylbenzimidazol‐2‐yl)‐2‐oxopropane), have been synthesized and characterized
using elemental analysis, electrical conductivities, infrared and UV–visible spectral
measurements and single‐crystal X‐ray diffraction. Complex 1 is binuclear and
three‐coordinated by two N atoms from two bobb ligands, while complex 2 is a
Correspondence
Huilu Wu, School of Chemical and
Biological Engineering, Lanzhou Jiaotong
University, Lanzhou, Gansu, 730070,
People's Republic of China.
Email: wuhuilu@163.com
unique metal organic compound with diamond‐like multinuclear Ag centers with
each Ag bridged by two aobb ligands and two crotonate ions to form one‐
dimensional single polymer chain structures and extended into two‐dimensional
frameworks through π–π and intermolecular C─H⋅⋅⋅O hydrogen bonds. The
adjacent Ag(I) centers are bridged by allyl from aobb which is not only a σ‐bonding
ligand, but also a π‐acid ligand. The DNA binding modes of complexes 1 and 2 were
investigated using electronic absorption titration, fluorescence spectra and viscosity
measurements. The results suggest that the two complexes bind to DNA via an
intercalative mode, and their binding affinity for DNA follows the order 2 > 1. This
is due to the chelating effects which can enhance the planar functionality of the
metal complexes.
Funding information
National Natural Science Foundation of
China, Grant/Award Number: 21367017.
KEYWORDS
Ag(I) complexes, bis(benzimidazole)‐2‐oxopropane, crystal structure, DNA binding property, syntheses
1 | INTRODUCTION
agents and potential anticancer drugs.[8] A large number of
complexes have been synthesized and explored for their
Benzimidazole derivatives are important constituents in
many pharmacologically, catalytically and biologically
active compounds, and therefore correspond to significant
synthetic targets.[1–3] These benzimidazoles are known for
their potential to poison DNA topoisomerases or to stabilize
complexes of DNA topoisomerases that ultimately result in
strand cleavage.[4] Study of the interaction of transition
metal complexes with DNA has been of particular interest
to researchers in the field of bioinorganic chemistry.[5–7]
Metal complexes binding to nucleic acids are currently
investigated because of their utility as DNA structural
probes, DNA foot‐printing and sequence‐specific cleavage
biological activities.[9]
It is well known that Ag(I) is a favorable and well‐studied
ion for the construction of coordination clusters because of its
coordination diversity as well as its positive coordination
tendency with various donor atoms.[10–12] The compounds
formed by Ag(I) atoms and multi‐topic N‐donor ligands have
attracted great interest for a number of reasons. The soft Ag+
(d10) ion has been commonly used as a metal center in the
construction of coordination polymers.[13] The Ag(I) center
has a closed‐shell electronic configuration and can adopt
diverse coordination numbers from 2 to 8, with no strong
energetic preference for any particular geometry,[14,15]
Appl Organometal Chem. 2017;e3747.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3747

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MAO ET AL.
varying from linear to trigonal, tetragonal, square pyramidal,
octahedral, and so on. In recent years, researchers have
explored silver‐, gold‐ and platinum‐based N‐heterocyclic
carbenes having benzimidazole cores as excellent antitumor
and anticancer agents in the form of complexes. Their various
biological applications with fewer side effects are now
attracting global attention.[16]
In the study reported here, two new complexes, namely
[Ag2(bobb)2]⋅(NO3)2 (1) and [Ag2(crotonate)2(aobb)]n (2)
(bobb =1,3‐bis(1‐benzylbenzimidazol‐2‐yl)‐2‐oxapropane; aobb
=1,3‐bis(1‐allylbenzimidazol‐2‐yl)‐2‐oxopropane), were synthe-
sized and characterized. The DNA binding behaviors of the two
complexes were investigated.
SCHEME 1 Synthetic route for ligands bobb and aobb
2.2 | Synthesis of complexes
2.2.1 | Preparation of [Ag2(bobb)2] (NO3)2 (1)
To a stirred solution of bobb (0.0916 g, 0.20 mmol) in hot
MeOH (5 ml) was added AgNO3 (0.0340 g, 0.20 mmol) in
MeOH (5 ml) at room temperature for 4 h. This was then
filtered and a few drops of acetonitrile added to the
colorless filtrate avoiding light. Slow evaporation at room
temperature led to block‐structure crystals after about
3–5 weeks. Yield 69%; m.p. 283–285 °C (decomp.). Anal.
Calcd for C60H52Ag2N10O8 (%): C, 61.4; H, 4.7; N, 8.9.
Found (%): C, 61.5; H, 4.6; N, 9.0. IR (KBr, ν, cm−1):
746 ν(O─Ar), 1043 ν(C─O), 1382 ν(NO3−), 1548
ν(C═N). UV–visible (DMF, λ, nm): 273, 288 nm. ΛM
2 | EXPERIMENTAL
2.1 | Materials and methods
C, H and N elemental analyses were conducted using a Carlo
Erba 1106 elemental analyzer. Electrolytic conductance
measurements were made with a DDS‐11A type conductivity
bridge using 10−3 M solutions in dimethylformamide (DMF)
at room temperature. Infrared (IR) spectra were recorded in
the region 400–4000 cm−1 with a Nicolet FT‐VERTEX 70
spectrometer using KBr pellets. Electronic spectra were
recorded with a Lab‐Tech UV Bluestar spectrophotometer.
1H NMR spectra were recorded using a Varian VR
300 MHz spectrometer with teramethylsilane as an internal
standard. Fluorescence spectra were recorded with a
PerkinElmer LS‐45 spectrofluorophotometer. Melting points
were determined with an X‐4 digital micro melting point
apparatus.
Calf thymus DNA (CT‐DNA) and ethidium bromide
(EB) were obtained from Sigma‐Aldrich. All chemicals used
for experiments were of analytical grade. Other reagents and
solvents were of reagent grade obtained from commercial
sources and used without further purification. Tris–HCl
buffer, Na2HPO4–NaH2PO4 buffer and EDTA–Fe(II)
solution were prepared using doubly distilled water. Stock
solutions of the complexes were prepared in DMF at
3 × 10−3 M. The experiments involving interaction of
ligands and complexes with CT‐DNA were carried out in
doubly distilled buffered water containing 5 mM Tris and
50 mM NaCl and adjusted to pH = 7.2 with hydrochloric
acid. A solution of CT‐DNA gave a ratio of UV absorbance
at 260 and 280 nm of about 1.8–1.9, indicating that the
CT‐DNA was sufficiently free of protein.[17] The CT‐DNA
concentration per nucleotide was determined spectrophoto-
metrically by employing an extinction coefficient of
6600 M−1 cm−1 at 260 nm.[18] Ligands bobb and aobb were
synthesized according to a literature method,[19,20] as shown
in Scheme 1.
(DMF, 297 K): 127 S cm2 mol−1
.
2.2.2 | Preparation of [Ag2(crotonate)2(aobb)]
n (2)
An ethanol aqueous solution (5 ml) of sodium crotonate
(0.0217 g, 0.20 mmol) was added to a ethanol solution
(3 ml) of AgNO3 (0.0340 g, 0.20 mmol) and stirred for
several minutes until a precipitate was generated. Then aobb
(0.0716 g, 0.20 mmol) in ethanol (5 ml) was added and
stirred at room temperature for 4 h until the white precipitate
disappeared. This was then filtered and a few drops of
acetonitrile added to the colorless filtrate avoiding light. Slow
evaporation at room temperature led to block‐structure
crystals after about 3–5 weeks. Yield 71%; m.p.
259–262 °C (decomp.). Anal. Calcd for C30H32Ag2N4O5
(%): C, 48.4; H, 4.3; N, 7.5. Found (%): C, 48.3; H, 4.4; N,
7.5. IR (KBr, ν, cm−1): 738 ν(O─Ar), 1053 ν(C─O), 1498
ν(C═N), 1549 νas(COO), 1402 νs(COO). UV–visible
(DMF, λ, nm): 280, 288. ΛM (DMF, 297 K): 8 S cm2 mol−1
.
2.3 | X‐ray crystallography
For each complex, a suitable single crystal was mounted on a
glass fiber, and the intensity data were collected using a Bruker
APEX‐II CCD diffractometer with graphite‐monochromatized
Mo Kα radiation (λ = 0.71073 Å) at 296(2) K. Data reduction
and cell refinement were performed using the SAINT suite of

MAO ET AL.
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programs.[21] The absorption corrections were made using
empirical methods. The structures were solved by direct
methods and refined by full‐matrix least‐squares against F2
using SHELXTL software.[22] All H atoms were found in dif-
ference electron maps and subsequently refined in a riding
model approximation with C─H distances ranging from 0.95
to 0.99 Å and Uiso(H) = 1.2Ueq(C) or 1.5Ueq(Cmethyl). The
crystal data and experimental parameters relevant to the struc-
ture determination are listed in Table 1. Selected bond dis-
tances and angles are given in Table 2.
½DNAꢀ ½DNAꢀ
1
¼
þ
εa−εf
εb−εf
Kbðεb−εfÞ
where [DNA] is the concentration of CT‐DNA in base pairs,
εa corresponds to the observed extinction coefficient (Aobs
/
[M]), εf corresponds to the extinction coefficient of the free
compound, εb is the extinction coefficient of the complex
when fully bound to CT‐DNA and Kb is the intrinsic binding
constant. The ratio of slope to intercept of a plot of [DNA]/
(εa − εf) versus [DNA] gave the value of Kb. When measur-
ing the absorption spectra, an equal amount of CT‐DNA
was added to both the test solution (2.5 ml of Tris +25 μl
of stock solution of test compound) and the reference solution
(2.5 ml of Tris +25 μl of DMF) to eliminate the absorption of
CT‐DNA itself.
2.4 | DNA binding experiments
2.4.1 | Electronic absorption titration
Absorption titration experiments were performed with fixed
concentrations of the complexes, while gradually increasing
the concentration of CT‐DNA. To obtain the absorption
spectra, the required amount of CT‐DNA was added to both
compound and reference solutions to eliminate the absorption
of CT‐DNA itself. From the absorption titration data, the
binding constant (Kb) was determined using the following
2.4.2 | Competitive binding with EB
The extent of fluorescence quenching of EB bound to
CT‐DNA can be used to determine the extent of binding
between an additional molecule and CT‐DNA.[24] The
competitive binding experiments were carried out in buffer
equation[23]
:
TABLE 1 Crystal data and structure refinement for complexes 1 and 2
1
2
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
60H52Ag2N10O8
C30H32Ag2N4O5
744.34
1256.86
Monoclinic
P2(1)/c
11.532(2)
15.374(3)
35.824(7)
90
Monoclinic
C2/c
19.913(11)
8.807(5)
16.437(10)
90
b (Å)
c (Å)
α (°)
β (°)
92.300(3)
90
92.084(7)
90
γ (°)
V (Å3)
6346(2)
4
2881(3)
4
Z
ρcalcd (mg m−3
)
1.315
1.716
μ (mm−1
)
0.674
1.406
F[000]
2560
1496
Crystal size (mm3)
0.40 × 0.38 × 0.30
0.40 × 0.38 × 0.30
h/k/l (max., min.)
−13 ≤ h ≤ 12, −18 ≤ k ≤ 18, −43 ≤ l ≤ 41
1.44 to 25.50
−23 ≤ h ≤ 24, −10 ≤ k ≤ 10, −19 ≤ l ≤ 16
2.05 to 25.50
θ range for data collection (°)
Goodness‐of‐fit on F2
1.036
1.069
Final R1, wR2 indices [I > 2σ(I)]
R1, wR2 indices (all data)
Largest differences, peak and hole (e Å−3
R1 = 0.0451, wR2 = 0.1163
R1 = 0.0669, wR2 = 0.1235
0.432 and −0.432
R1 = 0.0516, wR2 = 0.1418
R1 = 0.0627, wR2 = 0.156
1.693 and −0.959
)

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MAO ET AL.
TABLE 2 Selected bond lengths (Å) and bond angles (°) of Ag(I) complexes 1 and 2
Complex 1
Bond distances
Bond angles
Ag(1)–N(5)
Ag(1)–Ag(2)
Ag(2)–N(3)
2.115(3)
3.0450(6)
2.157(3)
Ag(1)–N(2)
Ag(2)–N(7)
2.121(3)
2.153(3)
N(5)–Ag(1)–N(2)
N(2)–Ag(1)–Ag(2)
N(3)–Ag(2)–Ag(1)
C(7)–N(5)–Ag(1)
C(32)–N(3)–Ag(2)
C(22)–N(7)–Ag(2)
C(51)–N(2)–Ag(1)
172.36(12)
86.20(9)
84.42(8)
128.8(3)
130.2(2)
125.4(3)
125.6(3)
N(5)–Ag(1)–Ag(2)
N(7)–Ag(2)–N(3)
N(7)–Ag(2)–Ag(1)
C(1)–N(5)–Ag(1)
C(38)–N(3)–Ag(2)
C(21)–N(7)–Ag(2)
C(52)–N(2)–Ag(1)
94.13(9)
156.33(12)
78.65(9)
124.5(3)
123.8(3)
128.0(3)
127.8(2)
Complex 2
Bond distances
Ag(1)–N(1)
Ag(1)–O(2)#1
Ag(1)–O(2)
O(2)–Ag(1)#3
C(9)–Ag(1)#4
2.274(5)
2.330(5)
2.330(5)
2.540(6)
2.353(6)
Ag(1)–C(9)#2
Ag(1)–C(8)#2
Ag(1)–O(2)#3
C(8)–Ag(1)#4
2.353(6)
2.426(5)
2.540(6)
2.426(5)
Bond angles
N(1)–Ag(1)–O(2)#1
N(1)–Ag(1)–C(9)#2
O(2)–Ag(1)–C(9)#2
O(2)#1–Ag(1)–C(8)#2
C(9)#2–Ag(1)–C(8)#2
O(2)#1–Ag(1)–O(2)#3
C(9)#2–Ag(1)–O(2)#3
C(10)–N(1)–Ag(1)
104.01(18)
111.7(2)
144.2(2)
111.9(2)
32.6(2)
78.7(2)
93.8(2)
128.1(4)
103.2(4)
100.7(2)
108.5(4)
N(1)–Ag(1)–O(2)
104.01(18)
144.2(2)
143.8(2)
111.9(2)
101.30(18)
78.7(2)
90.55(19)
126.0(4)
137.4(4)
70.7(3)
O(2)#1–Ag(1)–C(9)#2
N(1)–Ag(1)–C(8)#2
O(2)–Ag(1)–C(8)#2
N(1)–Ag(1)–O(2)#3
O(2)–Ag(1)–O(2)#3
C(8)#2–Ag(1)–O(2)#3
C(1)–N(1)–Ag(1)
C(12)–O(2)–Ag(1)#3
C(9)–C(8)–Ag(1)#4
C(8)–C(9)–Ag(1)#4
C(12)–O(2)–Ag(1)
Ag(1)–O(2)–Ag(1)#3
C(7)–C(8)–Ag(1)#4
76.7(4)
by keeping [DNA]/[EB] = 1.13 and varying the concentra-
tions of the compounds. The fluorescence spectra of EB were
measured using an excitation wavelength of 520 nm and the
emission range was set between 550 and 750 nm. The spectra
were analyzed according to the classical Stern–Volmer
(3 μM), and each was introduced into CT‐DNA solution
(50 μM) present in the viscometer. Data were analyzed as
(η/η0)1/3 versus the ratio of the concentration of the com-
pound to CT‐DNA, where η is the viscosity of CT‐DNA in
the presence of the compound and η0 is the viscosity of
CT‐DNA alone. Viscosity values were calculated from the
observed flow times of CT‐DNA‐containing solutions
corrected for the flow time of buffer alone (t0): η = (t − t0).[26]
equation[25]
:
I0
I
¼ 1 þ KSV½Qꢀ ¼ 1 þ Kqτ0½Qꢀ
3 | RESULTS AND DISCUSSION
3.1 | Characterization of complexes
where I0 and I are the fluorescence intensities at 599 nm in
the absence and presence of the quencher, respectively, KSV
is the linear Stern–Volmer quenching constant, [Q] is the
concentration of the quencher, Kq is the quenching rate con-
stant and τ0 is luminescence decay time in the absence of
The free ligands bobb and aobb and the Ag(I) complexes are
stable in both air and solution environments. The syntheses
of complexes 1 and 2 were carried out in the dark to avoid
photodecomposition. The Ag(I) complexes are soluble in
DMF and dimethylsulfoxide, and partially soluble in water
and other organic solvents such as methanol, ethanol,
acetone, petroleum ether, trichloromethane, etc. The results
of elemental analyses show that the composition of complex
1 is [Ag2(bobb)2]⋅(NO3)2 and 2 is [Ag2(crotonate)2(aobb)]n.
The molar conductance values show that complex 1 is a 1:2
electrolyte, while complex 2 is a nonelectrolyte in DMF.[27]
the
quencher.
In
these
experiments,
[CT‐DNA] = 2.5 × 10−3 M and [EB] = 2.2 × 10−3 M.
2.4.3 | Viscosity titration measurements
Viscosity experiments were conducted with an Ubbelohde
viscometer, immersed in a water bath maintained at
25.0 ꢁ 0.1 °C. Titrations were performed for each complex

MAO ET AL.
5 of 9
The IR spectra of the free ligand bobb and its Ag(I)
complex 1 were compared. Free bobb exhibits a characteris-
tic C═N stretching frequency at 1496 cm−1, while the C═N
stretching band of the complex is observed at 1548 cm−1
.
Hence, the C═N stretching frequencies are shifted upon
complexation,[28] indicating that the nitrogen atoms of the
ligand are coordinated to the Ag(I) center. Similar shifts also
appear in the spectrum of complex 2, which lead to the same
conclusion. A strong, fairly broad absorption at 1382 cm−1
indicates that ionic nitrate groups (D3h) are present in
complex 1.[29,30] Since the carboxylate group can coordinate
to the metal ion in a bidentate bridging, a bidentate chelating
or a monodentate fashion, the ‘Δ criterion’, which is based on
the difference between the νas(COO) and νs(COO) values,
compared to the corresponding value in sodium carboxylate,
is currently employed to determine the coordinating mode of
carboxylate group.[31,32] The νas(COO) is assigned to the
strong bands at 1549 (2) and 1540 cm−1 (sodium crotonate)
whereas the νs(COO) is attributed to the 1402 (2) and
1389 cm−1 (sodium crotonate) peaks. The data suggest that
crotonate groups in complex 2 behave as bidentate bridging
ligands.[33,34] This conclusion is confirmed by the result of
the crystal structure analysis.
FIGURE 1 (a) molecular structure of complex 1 with displacement
ellipsoids drawn at the 30% probability level. The counter anions and
hydrogen atoms are omitted for clarity. (b) distance between the two
Ag(I) centers is 3.045(0) Å. (c) the π⋅⋅⋅π interactions between two
benzimidazole rings from the same units, d = 3.615(2) Å
DMF solutions of ligands and Ag(I) complexes show, as
expected, almost identical UV spectra. In the UV–visible
spectra, the band observed for free bobb is marginally red‐
shifted by 6 nm in the spectrum of complex 1, again showing
coordination of the C═N group to the Ag(I) center. The
absorption bands at 280 nm are assigned to π–π* (imidazole)
transition.[35–37] Analogously, the UV bands of aobb (279,
287 nm) are also marginally red‐shifted by about 1 nm in
the spectrum of complex 2. This phenomenon also shows that
C═N is involved in coordination to the metal center.
FIGURE 2 Structure of the complex 1 linked via π–π stacking
interactions (dashed lines)
stable. These conjugated effects generate an infinite two‐
dimensional layer.
3.2.2 | Crystal structure of complex 2
3.2 | Description of structures
Complex 2 possesses two crystallographically unique Ag(I)
atoms, one aobb ligand and two crotonate anions, where the
component moieties all lie in general positions with no
crystallographically imposed symmetry (Figure 3). Complex
2 is a unique metal organic compound. The free ligand aobb
is a bridging ligand. It is not only a σ‐bonding ligand, but also
a π‐acid ligand. The adjacent Ag(I) centers are bridged by
allyl from aobb. Ag1 and Ag2 have the same coordination
environments, which are five‐coordinated by two oxygen
atoms from two crotonate anions (Ag(1)–O(2)#1 = 2.331(5)
Å, Ag(1)–O(2)#3 = 2.539(6) Å), one nitrogen atom from
one aobb ligand (Ag–N = 2.274(5) Å) and two carbon atoms
from one allyl(aobb) (C(57)–Ag(1)#4 = 2.430(5) Å and
C(58)–Ag(1)#4 = 2.353(6) Å). Moreover, Ag1, Ag2, O(2)
#1 and O(2)#3 are connected and form a parallelogram.
Different from the structure of 1, in complex 2, the adjacent
Ag(I) centers are bridged by allyl from aobb to form
3.2.1 | Crystal structure of complex 1
The crystal structure of complex 1 consists of a binuclear
[Ag2(bobb)2]2+ motif and two NO3− anions per formula unit.
The Ag(I) atom displays a trigonal planar coordination
structure consisting of three nitrogen atoms from two bobb
ligands. In addition, the coordination geometry between two
Ag(I) atoms indicates strong interaction (Ag⋅⋅⋅Ag = 3.080(0)
Å),[38,39] as shown in Figure 1. In complex 1, the two
benzimidazole rings belonging to the different ligands result
in intramolecular π⋅⋅⋅π interactions (centroid‐to‐centroid
distance, d = 3.615 Å).[40]
As shown in Figure 2, there are two kinds of conjugated
effect between the units, which are between imidazole ring
and benzyl π⋅⋅⋅π interactions ((i) d = 3.602(0) Å; (ii)
d = 3.683(0) Å) and make the crystal structure more

6 of 9
MAO ET AL.
Figure 6. As can be seen, there is a band at 275 nm. Upon
addition of increasing CT‐DNA concentrations, these bands
exhibit hypochromism of 38.1 and 68.2%, respectively. From
the electronic absorption spectroscopy experiments, Kb values
of complexes 1 and 2 are 6.28 × 105 and 7.61 × 105 M−1
,
respectively, as shown in Figure 6(c) and (d). Comparing
the molecular structures of the two complexes, we suggest
that their binding affinities follow the order: 2 > 1. The
charge transfer of coordinated ligands resulting from coordi-
nation to the Ag(I) atom should reduce the charge density of
the planar conjugate system, which is conducive to intercala-
tion.[35,41] This difference in the DNA binding ability could
also be attributed to the presence of an electron‐deficient
center in the Ag(I) complexes, allowing an additional inter-
action between the complexes and phosphate‐rich DNA
backbone compared to the free ligand.[42–44]
FIGURE 3 Environment of the Ag(I) cation in complex 2 showing 30%
thermal probability ellipsoids. The hydrogen atoms are omitted for clarity
3.3.2 | Fluorescence spectra
EB does not show any appreciable emission in buffer solution
due to fluorescence quenching by the solvent. Upon addition
of complex to a solution containing EB, no change in the
fluorescence spectra is observed. However, the fluorescence
intensity of EB is greatly enhanced upon addition of
CT‐DNA, due to its strong intercalation with the DNA base
pairs. Addition of a second molecule, which may bind to
DNA more strongly than EB, can the result in a decrease
the DNA‐induced EB emission by displacement of EB.[45]
The addition of the complexes results in a significant
decrease of the intensity of the emission band of the
DNA–EB system at 595 nm, indicating competitive binding
of the compounds to DNA. The Stern–Volmer plots
(Figure 7) show that the fluorescence quenching of EB bound
to DNA by the two complexes follows a linear relationship,
consistent with intercalation of the test compounds. The
KSV values for complexes 1 and 2 are (3.65 ꢁ 0.15) × 103
and (6.40 ꢁ 0.22) × 103 M−1, respectively. Moreover, the
Kq values for complexes 1 and 2 are 3.65 × 1011 and
─Ag─aobb─Ag0─ chains, which are extended into two‐
dimensional frameworks through strong π–π interactions
(d = 3.536(2) Å), as shown in Figures 4 and 5. In addition,
hydrogen bonds (C─H⋅⋅⋅O) also contribute to the stability
of the structure.
3.3 | DNA binding properties
3.3.1 | Electronic absorption titration
The electronic absorption spectra of the Ag(I) complexes in
the absence and presence of CT‐DNA are shown in
6.40
× , which are far larger than
1011 M−1 s−1
2.0 × 1010 M−1 s−1, the maximum diffusion collision
quenching rate constant of various quenchers with the
biopolymer. Consequently, the probable mechanism of
fluorescence quenching of complexes 1 and 2 binding
reactions should follow a static quenching process rather than
a dynamic one.[46] To a certain extent, reduction of the
emission intensity at 595 nm gives a measure of the binding
propensity of the complexes to CT‐DNA. The phenomena
suggest that the complexes can compete for DNA binding
sites with EB and displace EB from the EB–DNA system,[47]
which is usually characteristic of the intercalative interaction
of compounds with DNA.[48] Moreover, the binding
strengths of the complexes follow the same order as from
the UV–visible experiments.
FIGURE 4 Polymer chain structure of complex 2
FIGURE 5 Infinite two‐dimensional supramolecular layer formed
via π–π interactions and C─H⋅⋅⋅Ag hydrogen bonding in complex 2

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FIGURE 6 Electronic spectra of (a) complex 1 and (b) complex 2 in
Tris–HCl buffer upon addition of CT‐DNA. The arrow shows the
emission intensity changes with increasing DNA concentration. [DNA]/
(εa − εf) versus [DNA] for the titration of (c) complex 1 and (d) complex
2 with CT‐DNA
FIGURE 7 Emission spectra of EB bound to CT‐DNA in the
presence of (a) complex 1 and (b) complex 2 (λex = 596 nm). The
arrows show the intensity changes with increasing concentration of the
complexes. Fluorescence quenching curves of EB bound to CT‐DNA by
(c) complex 1 and (d) complex 2. (plots of I0/I versus [complex])

8 of 9
MAO ET AL.
solid structures. DNA binding studies indicate that the
investigated complexes bind to DNA via an intercalation
binding mode. The Ag(I) complexes can insert and stack
between the DNA base pairs more easily; especially, complex
2 can bind to DNA more strongly than complex 1. This is due
to the chelating effects which can enhance the planar
functionality of the metal complexes. These results demon-
strate the usefulness of Ag(I) complexes in the design of
new functional materials with an impressive range of
potential applications, including new antitumor drugs.
ACKNOWLEDGMENTS
The research presented was supported by the National
Natural Science Foundation of China (grant no. 21367017).
FIGURE 8 Effect of increasing amounts of compounds on relative
viscosity of DNA at 25.0 ꢁ 0.1 °C
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How to cite this article: Mao S, Shen K, Shi X, Xu
Y, Wu H. Two silver(I) complexes with
bis(benzimidazole)‐2‐oxopropane ligands: Syntheses,
crystal structures and DNA binding studies. Appl
Organometal Chem. 2017;e3747. https://doi.org/
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