Synthesis, structure, DNA-binding properties and antioxidant activity of silver(i) complexes containing V-shaped bis-benzimidazole ligands

Article abstract of DOI:10.1039/c2dt30512g

A V-shaped ligand bis(2-benzimidazol-2-ylmethyl)benzylamine L¹ with its two derivatives bis(N-methylbenzimidazol-2-ylmethyl)benzylamine L ² and bis(N-benzylbenzimidazol-2-ylmethyl)benzylamine L³ have been prepared. Reaction of these shape-specific designed ligands with Ag(pic) (pic = picrate) afforded three novel complexes, namely, [Ag ₂L¹₂](pic)₂1, [Ag₂L ²₂](pic)₂·2DMF 2 and [AgL ³(pic)] 3. The ligands and complexes were characterized on the basis of elemental analysis, UV-Vis, IR, NMR spectroscopy and X-ray crystallography. Complex 1 is a dinuclear metallacycle with a 2-fold rotational symmetry in which two syn-conformational L¹ ligands are connected by two linearly coordinated Ag(i) atoms. Due to the strong interaction between two adjacent Ag(i) atoms, the coordination mode of the central Ag(i) atom can be described as T-shaped. Complex 2 consists of a centrosymmetric dinuclear pore canal structure assembled from two nearly linearly coordinated Ag(i) atoms and two L² ligands. The structure of complex 3 adopts a four-coordinate environment for AgN₂O₂, with the counterion participating in an eight-shaped geometry. In order to explore the relationship between the structure and biological properties, the DNA-binding properties have been investigated by viscosity measurements, electronic absorption, and fluorescence. The results suggest that the ligands and complexes bind to DNA in an intercalation mode, and their binding affinities for DNA are also different. Moreover, the three Ag(i) complexes also exhibited potential antioxidant properties in vitro studies. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt30512g

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PAPER
Synthesis, structure, DNA-binding properties and antioxidant activity of
silver(I) complexes containing V-shaped bis-benzimidazole ligands†
Huilu Wu,* Jingkun Yuan, Ying Bai, Guolong Pan, Hua Wang, Jin Kong, Xuyang Fan and Hongmei Liu
Received 3rd March 2012, Accepted 11th May 2012
DOI: 10.1039/c2dt30512g
AV-shaped ligand bis(2-benzimidazol-2-ylmethyl)benzylamine L1 with its two derivatives bis(N-methyl-
benzimidazol-2-ylmethyl)benzylamine L2 and bis(N-benzylbenzimidazol-2-ylmethyl)benzylamine
L3 have been prepared. Reaction of these shape-specific designed ligands with Ag(pic) (pic = picrate)
afforded three novel complexes, namely, [Ag2L1 ](pic)2 1, [Ag2L2 ](pic)2·2DMF 2 and [AgL3(pic)] 3.
2
2
The ligands and complexes were characterized on the basis of elemental analysis, UV-Vis, IR, NMR
spectroscopy and X-ray crystallography. Complex 1 is a dinuclear metallacycle with a 2-fold rotational
symmetry in which two syn-conformational L1 ligands are connected by two linearly coordinated Ag(I)
atoms. Due to the strong interaction between two adjacent Ag(I) atoms, the coordination mode of the
central Ag(I) atom can be described as T-shaped. Complex 2 consists of a centrosymmetric dinuclear pore
canal structure assembled from two nearly linearly coordinated Ag(I) atoms and two L2 ligands. The
structure of complex 3 adopts a four-coordinate environment for AgN2O2, with the counterion
participating in an eight-shaped geometry. In order to explore the relationship between the structure and
biological properties, the DNA-binding properties have been investigated by viscosity measurements,
electronic absorption, and fluorescence. The results suggest that the ligands and complexes bind to DNA
in an intercalation mode, and their binding affinities for DNA are also different. Moreover, the three Ag(I)
complexes also exhibited potential antioxidant properties in vitro studies.
structural, spectroscopic and catalytic features of active sites of
1. Introduction
metallo-enzymes.5 As a typical heterocyclic ligand, interest in
exploring benzimidazole derivatives and their metal complexes
has continually increased since the recognition that many of
these materials may serve as models which mimic both the struc-
ture and reactivity of metal ion sites in complex biological
systems and possess a broad spectrum of biological activity.4,6
Due to their intriguing varying architectures and their important
properties that span from luminescence to biological activities,7
benzimidazoles and their derivatives, including the designed
ligands which are benzimidazole-based, also exhibit wide-
ranging antiviral activities,8 photochemical and photophysical
properties,9 versatile coordination modes, and the potential to
form supramolecular aggregates through π⋯π stacking inter-
actions and hydrogen bonding.10
In the framework of our research project, mainly focusing on
studying transition metal complexes containing benzimidazole
derivatives, we have investigated the DNA binding ability of
such complexes in our previous publications.11 In this paper, the
synthesis, characterization and DNA-binding activities of the
silver complexes with three different V-shape ligands are
presented. According to relevant reports in the literature,12
some similar transition metal complexes can exhibit
antioxidant activity. We therefore also conducted an investigation
into the hydroxyl radical scavenging properties of these
complexes.
Supramolecular chemistry of silver(I) coordination assemblies is
a dynamic, thriving field which has drawn ever increasing
research interest in recent decades.1 Due to its flexible coordi-
nation sphere, the Ag(I) ion exhibits versatile coordination geo-
metries, varying from liner to trigonal, tetragonal, square
pyramidal, and octahedral, corresponding to coordination
numbers 2 to 6, respectively. Such coordination flexibility con-
tributes greatly to the structural diversity of Ag(I) polymers. So
far, a large number of Ag(I) coordination polymers with diverse
topologies and dimensionalities [one-, two- or three-dimensions
(1D, 2D, or 3D)2] have been constructed. As more and more
silver complexes with novel topological structures have been
synthesized, the investigation into the relationship between the
structure and properties, especially biological properties, is
becoming another popular and promising field.3
Nitrogen ligands have been extensively used in coordination
chemistry,4 especially to obtain derivatives able to mimic
School of Chemical and Biological Engineering, Lanzhou Jiaotong
University, Lanzhou 730070, People’s Republic of China.
E-mail: wuhuilu@163.com; Fax: +(86)931-4956512;
Tel: +(86)13893117544
†Electronic supplementary information (ESI) available. CCDC 831425,
831426 and 831427. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2dt30512g
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Fig. 1 Ball and stick representation of the [Ag2L1 2+ cation.
]
2
(imidazole) transitions.11 Analogously, the UV bands of L2
(279, 287 nm), L3 (279, 287 nm) are also marginally redshifted
about 2–3 nm in complexes 2 and 3. This phenomenon also
shows that CvN is involved in coordination to the metal center.
The picrate bands (observed at 381 nm in the complexes) are
assigned to n → π* transitions.11
Scheme 1 Synthesis of ligands and complexes 1–3.
2. Results and discussion
Synthetic routes of ligands and Ag(I) complexes are shown in
Scheme 1. L1 has been reported in previous literature. Ligands
L2 and L3 were synthesized on the basis of ligand L1. By con-
trast with the Ag(I) complexes, the three ligands are very stable
in air. Both ligands and complexes are remarkably soluble in
polar aprotic solvents such as DMF, DMSO and MeCN; slightly
soluble in ethanol, methanol, ethyl acetate and chloroform; inso-
luble in water, Et2O, petroleum ether. The molar conductivities
in DMF solution indicate that the ligands are nonelectrolyte
compounds, while the electrolytic conductivity of complexes 1
and 2 show that they are 1 : 1 electrolytes in DMF.13 In theory,
complex 3 should be a neutral compound, but the conductivity
shows that there may be partial ionization of the discrete
[AgL3(pic)] in DMF.
2.2. X-ray structures of the complexes
Crystal structure of complex 1. The crystal structure of
complex 1 consists of a binuclear [Ag2L1 ]2+ motif and two
2
picrate anions per formula unit. Two ligands are arranged in a
face-to-face syn-conformation to coordinate with two Ag(I)
atoms from opposite directions, generating a locally linear geo-
metry around the metal atoms. However, the coordination geo-
metry around Ag(I) should be best described as T-shaped owing
to the strong interaction between the two Ag(I) atoms themselves
(dAg⋯Ag = 3.080(0) Å),16 as shown in Fig. 1. The [Ag2L1 ]2+
2
cation has high symmetry. A mirror plane m, containing two
nitrogen atoms of the benzylamine, divides the ligand into two
equivalent parts, while a C2 axis passes through the two Ag(I)
atoms. Thus, a crystallographically imposed 2/m symmetry is
located at the center of the cluster and generates the entire mole-
cule from the asymmetric unit, containing only half of a ligand
and an Ag(I) atom. The two benzimidazole rings belonging to
the same ligands are nearly parallel to each other (end-to-end,
dihedral angle τ = 15.13°), resulting in intramolecular π⋯π inter-
actions (centroid-to-centroid distance, d = 3.584 Å).17
2.1. IR and electronic spectra
The IR spectra of complex 1 are closely related to that of the free
ligand L1. One of the most diagnostic changes occurs in the
region between 1650 and 1250 cm−1. The spectrum of L1 shows
a strong band at 1438 cm−1 and weak bands at 1622 cm−1. By
analogy with the assigned bands of imidazole, the two bands are
attributed to the ν(CvN) and ν(CvC) frequencies of the benzi-
midazole group, respectively.14 The location of the two bands
was slightly shifted for complex 1; the band at 1438 cm−1 is
shifted to 1454 cm−1, which can be attributed to the coordination
of the benzimidazole nitrogen to the metal center atom.15 Similar
shifts also appear in complexes 2 and 3, which gives the same
conclusion. Moreover, information regarding the possible
bonding modes of the picrate and benzimidazole rings may also
be obtained from the IR spectra.11
As shown in Fig. 2, two benzene rings from two contiguous
[Ag2L1 ]2+ located in the same line are in a parallel position, so
2
the geometry of the complex ion is propagated into an infinite
1-D chain via inter-ligand π⋯π interactions (i, d = 3.864(0) Å).
Two adjacent picrate anions are inlayed in the coordination
cations as a sandwich and are concatenated by the π⋯π inter-
actions (ii, d = 3.462(0) Å). Such arrangement allows NH and
other groups to contribute to formation of hydrogen bonds,
which make the crystal structure more stable. Neighboring
chains are connected by N–H⋯O and C–H⋯O hydrogen bonds,
thus generating an infinite 2-D layer.
DMF solutions of ligands and Ag(I) complexes show, as
expected, almost identical UV spectra. The UV bands of L1
(278, 284 nm) are only marginally redshifted about 3 nm for
complex 1, which is evidence of CvN coordination to the metal
center. These bands are assigned to n → π* and π → π*
Crystal structure of 2. For complex 2, the structure consists of
a dinuclear [Ag2L2 ]2+, two picrate anions and two dimethyl
2
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Fig. 2 2-D layer formed via π⋯π interactions and H bonds in 1 (differ-
ent interactions are distinguished by different colors).
Fig. 4 2-D layer generated by the π⋯π interactions and weak O⋯H–C
hydrogen bonding in the bc plane in 2 (for clarity, some atoms were
omitted).
2+
Fig. 5 A view of the waves configuration of the [Ag2L2
]
2
cation in
the ab plane in 2 (for clarity, only two picrate molecules are shown).
Fig. 3 Ball and stick representation of the [Ag2L2 2+ cation.
]
2
formamide (DMF) molecules in a unit. As Fig. 3 shows, two
Ag(I) atoms are bridged by two L2 ligands, arranged in an end-to-
end to fashion, forming an [Ag2L2 ]2+ metallacycle, in which
2
every Ag(I) atom is almost linearly coordinated by two N atoms
from two benzimidazole rings belonging to the different ligands,
and the distance between the two Ag(I) centres is 5.968(1) Å,
precluding any bonding interaction. The line connected between
two Ag(I) atoms has a role of 2-fold axis in the dinuclear
Fig. 6 3-D network of the [Ag2L2 2+ cation.
]
2
stacking interactions, an infinite 3-D network was created in
complex 2.
[Ag2L2 ]2+.
2
In complex 2, adjacent benzimidazole rings and picrates are
concatenated by the π⋯π interactions (d = 3.480(0) Å), as
shown in Fig. 4. In addition, hydrogen bonds (C–H⋯O) also
contribute to the stability of the structure. The formation of the
2-D infinite layer in the bc plane undoubtedly owes to such
arrangement.
Crystal structure of 3. Compared with complexes 1 and 2, the
coordination geometry of Ag(I) adopts a 4-coordinating configur-
ation. For complex 3 (Fig. 7a), the structure consists of a central
metal Ag(I) atom, a L3 and a picrate. The Ag(I) center is a four-
coordinated distorted tetrahedron, in which two oxygen atoms
from picrate and two nitrogen atoms from L3 participate in
coordination (Fig. 7c). Owing to this coordination geometry, an
8-membered ring and a 6-membered ring were constructed,
which are connected through the Ag(I) center and displaying an
eight-shaped geometry (Fig. 7b).
In complexes 1 and 2, the interactions, including π⋯π inter-
actions and hydrogen bonds, contribute to formation of a 2-D
infinite sheet. Similarly, as depicted in Fig. 8, there are two
kinds of π⋯π interactions in complex 3: (i) Between two adja-
cent picrates from different units, d = 3.800(4) Å. (ii) Between
two benzimidazole rings located in the face-to-face position,
d = 3.842(2) Å. It is precisely such layout that makes the
formation of an infinite 1-D zigzag chain.
Due to the unique spatial distribution and interactions, the
central metal atoms present a wavy arrangement along the c-axis,
as depicted in Fig. 5, which has been reported rarely in previous
studies.
Additionally, the [Ag2L2 ]2+ cation makes up a centrosym-
2
metric dinuclear pore canal structure via two ligands which are
coordinated to two Ag(I) atoms. The maximum and minimum
distances of the pore are 5.968(1) and 4.514(5) Å, respectively,
as shown in Fig. 6. From the stacking of complex 2, many parallel
channels were constructed, which afforded an opportunity to
investigate the potential in molecular recognition and gas absorp-
tion. Due to the existence of the hydrogen bonds and the π⋯π
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By comparing the ligands and complexes above-mentioned,
the reason that the three Ag(I) complexes showed different
spatial geometric structure can be attributed to the different steric
hindrance and distortion of the ligands, which can be explained
by the introduction of the different substituent groups.
sedimentation) are regarded as the least ambiguous and the most
critical tests of the interaction model in solution in the absence
of crystallographic structural data.18 Viscosity titration measure-
ments were carried out to clarify the interaction modes between
the investigated compounds and CT-DNA. Intercalation involves
the insertion of a planar molecule between DNA base pairs,
which results in a decrease in the DNA helical twist and
lengthening of the DNA, therefore intercalators cause the
unwinding and lengthening of DNA helix as base pairs become
separated to accommodate the binding compound.19 Whereas,
agents bound to DNA through groove binding do not alter the
relative viscosity of DNA, and agents electrostatically bound to
DNA will bend or kink the DNA helix, reducing its effective
length and its viscosity, concomitantly.18a,e,20 The effects of
ligands and Ag(I) complexes on the viscosities of CT-DNA are
shown in Fig. 9. With the ratios of the investigated ligands to
DNA increasing, the relative viscosities of DNA also show an
upward trend, and almost at the same magnitude of change, indi-
cating that there exist intercalations between all the ligands with
DNA helix. However, for the three complexes, though the results
indicated they all bind to DNA via an intercalation binding
mode, the difference in magnitude of change suggested the
extents of the unwinding and lengthening of DNA helix and
DNA binding affinities follow the order: 2 > 3 > 1.
2.3. DNA binding properties
Viscosity titration measurements. Hydrodynamic measure-
ments that are sensitive to the length change (i.e., viscosity and
It is well known that varying the substituting group or substi-
tuent position in the intercalative ligand can create some interest-
ing differences in the space configuration and the electron
density distribution of transition metal complexes, which will
result in some differences in spectral properties and the DNA-
binding behaviors of the complexes and will be helpful to more
clearly understand the binding mechanism of transition metal
complexes to DNA.21 Based on the above results, we found that
the affinity for DNA is stronger in case of Ag(I) complexes when
compared with the ligands. For this difference, we attributed
three possible reasons. (i) By comparison of the molecular struc-
ture of the ligands and Ag(I) complexes, we find the greater
number of coplanar aromatic rings, which facilitate intercalation
to the base pairs of double helical DNA, may lead to higher
affinity for DNA.11f,g (ii) The charge transfer of coordinated
ligands caused by the coordination of the central atom lead to
the decrease of the charge density of the plane conjugate system,
which is conducive to insertion. (iii) This difference in their
DNA binding ability also could be attributed to the presence of
an electron deficient center in the charged Ag(I) complexes
Fig. 7 Ball and stick representation of complex 3.
Fig. 8 1-D zigzag chain formed via the π⋯π interactions in 3.
Fig. 9 Effect of increasing amounts of (a) ligands and (b) Ag(I) complexes on the relative viscosity of CT-DNA at 25.0 0.1 °C.
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where an additional interaction between the complex and phos-
phate-rich DNA backbone may occur.22
In addition, the reason for the difference in the binding
strength for three Ag(I) complexes can attributed to the differ-
ence in steric hindrance and electron density, which are both
caused by the introduction of substituents and geometric
structure.
Electronic absorption titration. Absorption titration can
monitor the interaction of a compound with DNA. The obvious
hypochromism and bathochromism shift are usually character-
ized by the noncovalent intercalative binding of a compound to
the DNA helix, due to the strong stacking interaction between
the aromatic chromophore of the compound and base pairs of
DNA.18,23 However, the intercalation between a compound and
DNA helix cannot be excluded only by no or small redshift of
UV-vis absorption bands. In fact, some groove binders of the
Hoechst 33258 family can also present redshifts or even blue-
shifts of absorption bands when they bind to the DNA helix by
groove binding modes, especially for multiple binders.24
The absorption spectra of ligands and complexes in the
absence and presence of CT-DNA (at a constant concentration of
compounds) are given in Fig. 10a–c. As can be seen from
Fig. 10a–c, the ligands and complexes exhibit intense absorption
bands at 274–277 nm assigned to π → π* transition of the benzi-
midazole, and addition of increasing amounts of CT-DNA
results in hypochromism and bathocromic shift in the UV-vis
spectra of the compounds. In the present case, with addition of
DNA, three ligands exhibit hypochromism of about 28.5%,
17.3% and 25.6% accompanied by bathochromism of about a
1–2 nm shift in the absorption maxima. Corresponding com-
plexes exhibit hypochromism of about 38.5%, 46.8% and
57.0%, and also accompanied by bathochromism of about a
1–2 nm shift in the absorption maxima. The hypochromism
suggested that the compounds interact with CT-DNA.25
To compare quantitatively the affinity of ligands and Ag(I)
complexes toward DNA, the intrinsic binding constants Kb were
calculated by plotting the changes in the absorbance of the
complex upon incremental addition of increasing concentration
of DNA. The Kb values of ligands L1–L3 were 1.07 × 103 M−1
(R = 0.97 for 16 points), 7.41 × 103 M−1 (R = 0.99 for 13
points) and 1.78 × 103 M−1 (R = 0.99 for 16 points), respect-
ively. That of complexes 1–3 were 4.85 × 104 M−1 (R = 0.97 for
8 points), 6.25 × 105 M−1 (R = 0.99 for 16 points) and 3.24 ×
105 M−1 (R = 0.99 for 16 points), respectively. Therefore, com-
pared with the classic DNA-intercalative reagents, such as ethi-
dium bromide (EB), acridine orange (AO) and methylene blue
(MB),26 the binding constants (Kb) of the Ag(I) complexes can
suggest that three Ag(I) complexes most probably bind to DNA
in an intercalation mode. The magnitude of the Kb value is parallel
to the intercalative strength and the affinity of a compound
binding to DNA.27 Hence, the magnitude of Kb values can prove
that the three ligands have similar DNA-binding ability. Besides,
it also indicates that the binding affinities of the Ag(I) complexes
follow the order: 2 > 3 >1, which are in good agreement with
the orders of viscosity titration results.
Fig. 10 Absorption spectra of compounds in the presence of CT-DNA
(the DNA absorption was subtracted). The concentration of complexes
(a) 1, (b) 2, and (c) 3 was kept constant at 3 × 10−5 M−1. Arrows show
the absorbance changes upon increasing DNA concentration. Inset: plots
of [DNA]/(εa − εf) versus for the titration of DNA with compound; ■,
experimental data points; solid line, linear fitting of the data.
bromide (EB) in its EB–DNA adduct has been reported as a
standard intercalating agent of DNA and it is a reliable tool to
measure the affinity of the complex for DNA, irrespective of the
binding modes. Therefore, a solution of EB has been used as a
spectral probe since it does not emit in the buffer solution due to
probable quenching of its emission by the solvent.28 However,
intense emission is observed when EB strongly intercalates with
the adjacent DNA base pairs. But a decrease in emission inten-
sity results from the displacement of EB by a quencher molecule.
The extent of emission quenching could be used to determine
the extent of binding between the metal complex with DNA.28
For all the ligands and Ag(I) complexes, no emission was
observed either alone or in the presence of CT-DNA in the
buffer. So the binding of complexes with CT-DNA cannot be
Competitive binding with ethidium bromide. The ability of a
complex to change the fluorescence intensity of ethidium
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Fig. 12 The inhibitory effect of the three Ag(I) complexes on OH˙ rad-
icals; the suppression ratio increases with increasing concentration of the
test compound.
for 16 points), 3.06 × 104 M−1 (R = 0.99 for 16 points) and 2.85
× 104 M−1 (R = 0.98 for 11 points), respectively. The Stern–
Volmer dynamic quenching constants can also be interpreted as
binding affinities of the complexation reactions.29 Consequently,
for three ligands, the values of Ksv are very close, which means
that they have proximate binding affinity with DNA. However,
the values of Ksv present the order 2 > 3 > 1 for complexes,
which indicate the abilities of displacement of EtBr from EtBr–
DNA systems by compounds and the binding affinities between
compounds and DNA, which is consistent with the previous
conclusions.
2.4. Hydroxyl radical scavenging activity
Since the ligands and Ag(I) complexes exhibit reasonable DNA-
binding affinity, it was considered worthwhile to study other
potential aspects, such as antioxidant and antiradical activity. It
is a well-documented fact that some transition metal complexes
display significant antioxidant activity12 and therefore we under-
took a systematic investigation on the antioxidant potential of
free ligands and Ag(I) complexes against OH˙ radicals with
respect to different concentrations of the test compounds.
We compared the abilities of several present compounds to
scavenge hydroxyl radicals with those of the well-known natural
antioxidants mannitol and vitamin C, using the same method as
reported in a previous paper.30 The 50% inhibitory concentration
(IC50) value of mannitol and vitamin C are about 9.6 × 10−3 and
8.7 × 10−3 M−1, respectively. According to the antioxidant
experiments, the IC50 values of complexes 1, 2 and 3 are 7.52 ×
10−6 M−1, 8.34 × 10−6 M−1 and 6.25 × 10−6 M−1, respectively
(Fig. 12), which implies that the three Ag(I) complexes exhibit
better scavenging activity than mannitol and vitamin C. It can be
concluded that a much less or no scavenging activity was exhib-
ited by the ligands when compared to that of Ag(I) complexes
which is due to the chelation of ligand with the central metal
atom.30 The lower IC50 values observed in antioxidant assays
did demonstrate that the three Ag(I) complexes have a strong
potential to be applied as scavengers to eliminate radicals.
Fig. 11 Fluorescence spectra of the DMF solution of complexes (a) 1,
(b) 2, and (c) 3 in Tris-HCl buffer upon addition of CT-DNA.
[Complex] = 3 × 10−5 M−1. Arrow shows the intensity changing upon
increasing CT-DNA concentrations. A Stern–Volmer quenching plot of
the Ag(I) complexes inserting in their own fluorescence spectra with
increasing concentrations of CT-DNA.
directly presented in the emission spectra. Therefore, competitive
EB binding studies could be undertaken in order to examine the
binding of each complex with DNA. The fluorescence quenching
of EB bound to CT-DNA by complexes 1–3 is shown in
Fig. 11a–c. The quenching of EB bound to CT-DNA by three
Ag(I) complexes are in good agreement with the linear Stern–
Volmer equation, which provides further evidence that the Ag(I)
complexes bind to DNA and only one type of quenching process
occurs. The Ksv values of ligands L1–L3 were 1.05 × 104 M−1
(R = 0.99 for 16 points), 1.67 × 104 M−1 (R = 0.99 for 8 points)
and 1.40 × 104 M−1 (R = 0.98 for 12 points), respectively. The
Ksv values for complexes 1–3 are 2.36 × 104 M−1 (R = 0.99
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4.2. Synthesis of the ligands
3. Conclusions
Synthesis of L1. L1 was synthesized following a slight modifi-
cation of the procedure in ref. 32. The conventional solid-phase
reaction reported in ref. 32 was replaced by the ethylene glycol
solution method. Refluxing for 24 h, the resulting solution was
poured into ice water, a pale yellow precipitate was obtained, left
to rest for 30 min then filtered, and recrystallized from ethanol to
give L1. (Yield: 74%); m.p.: 104–106 °C. Found (%): C, 75.31;
H, 5.53; N, 19.15. Calcd (%) for C23H21N5: C, 75.18; H, 5.76;
Three V-shaped bis-benzimidazole ligands and their silver(I)
complexes have been synthesized and characterized. In complex
1, the coordination geometry around the central Ag(I) atom is
T-shaped owing to the strong interactions between the two Ag(I)
atoms themselves. The cation of complex 2 makes up a centro-
symmetric dinuclear pore canal structure via two ligands coordi-
nated to two Ag(I) atoms. The geometric structure of complex 3
displays an eight-shaped geometry. The experimental results of
DNA-binding of the Ag(I) complexes suggest that the three
ligands and Ag(I) complexes bind to DNA in an intercalation
mode, which is due to the large planar aromatic rings, hydrogen
bonds and π⋯π stacking interactions that facilitate them interca-
lating into the base pairs of double helical DNA. The DNA-
binding affinities of these three complexes follow the order 2 > 3
> 1. Moreover, the three Ag(I) complexes also exhibit the effec-
tive scavenging of hydroxyl radicals. Information obtained from
this study will be helpful to the understanding of the mechanism
of interactions with DNA, and should be useful in the develop-
ment of potential probes of DNA structure and conformation and
new therapeutic regents for some diseases.
1
N, 19.06. H NMR (DMSO-d6 400 MHz) δ/ppm: 3.47 (m, 2H,
–CH2–Ar), 3.85 (s, 4H, –CH2–benzimidazol), 7.20 (m, 5H,
H–benzene ring), 7.35–7.62 (m, 8H, H–benzimidazol ring). Λm
(DMF, 297 K): 1.45 S cm2 mol−1. UV-vis (λ, nm): 278, 284.
FT-IR (KBr ν/cm−1): 743, ν(o-Ar); 1271, ν(C–N); 1438,
ν(CvN), 1622, ν(CvC).
Synthesis of L2. 7.34 g (20 mmol) L1 and 1.56 g (40 mmol)
potassium were added in tetrahydrofuran (150 mL) and the sol-
ution was refluxed on a water bath for 4 h with stirring. Then,
5.68 g (40 mmol) iodomethane was added to this solution. With
the dropping of iodomethane, the solution gradually becomes
cream yellow. After that, the resulting solution was concentrated
and cooled until a pale yellow solid separated out, then the pale
yellow precipitate was filtered, washed with excess water, and
recrystallized from ethanol to give L2. Yield: 4.85 g (58%);
m.p.: 185–186 °C. Found (%): C, 75.69; H, 6.54; N, 17.77.
4. Experimental
Caution: Although no problems were encountered in this work,
transition metal picrate salts are potentially explosive and should
thus be prepared in small quantities and handled with care.
1
Calcd (%) for C25H25N5: C, 75.92; H, 6.37; N, 17.71. H NMR
(DMSO-d6 400 MHz) δ/ppm: 3.45–3.62 (s, 6H, –CH3), 3.70 (m,
4H, –CH2–Ar), 3.90 (m, 4H, –CH2–benzimidazol), 7.23 (m, 5H,
H–benzene ring), 7.27–7.61 (m, 8H, H–benzimidazol ring). UV-
vis (λ, nm): 279, 287. IR (KBr ν/cm−1): 750, ν(o-Ar); 1230,
ν(C–N); 1475, ν(CvN); 1616, ν(CvC).
4.1. General methods
All chemicals and solvents were reagent grade and were used
without further purification. The C, H and N elemental analyses
were determined using a Carlo Erba 1106 elemental analyzer.
Electrolytic conductance measurements were made with a
DDS-307 type conductivity bridge using 3 × 10−3 mol L−1 sol-
utions in DMF at room temperature. The IR spectra were
recorded in the 4000–400 cm−1 region with a Nicolet FT-VER-
TEX 70 spectrometer using KBr pellets. Electronic spectra were
taken on a Lab-Tech UV Bluestar spectrophotometer. The fluor-
escence spectra were recorded on a LS-45 spectrofluorophoto-
meter. The absorbance was measured with a Spectrumlab 722sp
Synthesis of L3. L3 was prepared by a similar procedure as for
L2, except using 6.84 g (40 mmol) benzyl bromide instead of
iodomethane. (Yield: 5.56 g, 61%); m.p.: 156–158 °C. Found
(%): C, 81.02; H, 6.26; N, 12.69. Calcd (%) for C37H33N5:
1
C, 81.14; H, 6.07; N, 12,79. H NMR (DMSO-d6 400 MHz)
δ/ppm: 3.77 (m, 2H, –CH2–Ar), 3.85 (s, 4H, –CH2–benzimida-
zol), 5.27 (m, 4H, –CH2–Ar), 6.89 (m, 5H, H–benzene ring),
7.19–7.26 (s, 10H, H–benzene ring), 7.42–7.64 (m, 8H, H–ben-
zimidazol ring). Λm (DMF, 297 K): 4.16 S cm2 mol−1. UV-vis
(λ, nm): 279, 287. FT-IR (KBr ν/cm−1): 740, ν(o-Ar); 1286,
ν(C–N); 1464, ν(CvN), 1612, ν(CvC).
1
spectrophotometer at room temperature. H NMR spectra were
recorded on a Varian VR300-MHz spectrometer with TMS as an
internal standard.
4.3. Preparation of complexes
Calf thymus DNA (CT-DNA) and ethidium bromide (EB)
were purchased from Sigma. All chemicals used were of analyti-
cal grade. The solution of CT-DNA gave a ratio of UV absor-
bance at 260 and 280 nm, A260/A280, of 1.8–1.9, indicating that
the DNA was sufficiently free of protein.18b,e The stock solution
of DNA (2.5 × 10−3 M) was prepared in 5 mM Tris-HCl/50 mM
NaCl buffer solution (pH = 7.2, stored at 4 °C and used within 4
days). The CT-DNA concentration per nucleotide was deter-
mined spectrophotometrically by employing an extinction coeffi-
cient of 6600 M−1 cm−1 at 260 nm.31 The stock solution of
ligands and Ag(I) complexes were dissolved in DMF at the con-
centration 3 × 10−3 M.
The three complexes were prepared by a similar procedure. To a
stirred solution of ligand (0.50 mmol; L1, 183.5 mg; L2,
197.5 mg; L3, 273.5 mg) in hot EtOH (10 mL) was added Ag
(pic) (170.0 mg, 0.50 mmol) in mixed MeCN–EtOH solution
(2 mL; MeCN–EtOH = 3 : 1). A yellow crystalline product
formed rapidly. The precipitate was filtered off, washed with
EtOH and absolute Et2O, and dried in vacuo. The dried precipi-
tate was dissolved in acetonitrile to form a yellow solution into
which Et2O was allowed to diffuse at room temperature. Crystals
suitable for X-ray measurement were obtained after several
weeks.11
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

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Table 1 Crystal and structure refinement data for complexes 1, 2 and 3
Complex
1
2
3
Empirical formula
Molecular weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C58H46Ag2N16O14
1406.85
C68H68Ag2N18O16
1609.14
Monoclinic
P21/c
12.4814(17)
18.698(3)
16.038(2)
90
111.682(2)
90
C43H35AgN8O7
883.66
Triclinic
Triclinic
ˉ
ˉ
P1
P1
8.6538(12)
13.3823(19)
13.6460(19)
67.8070(10)
79.521(2)
88.5650(10)
1437.2(3)
1, 1.625
9.725(9)
14.739(14)
16.516(16)
65.574(9)
81.098(9)
77.451(9)
2098(3)
2, 1.399
0.540
904
α (°)
β (°)
γ (°)
V (Å3)
3477.9(8)
2, 1.537
0.645
Z, Dc (mg m−3
)
μ (mm−1
F(000)
)
0.764
712
1648
θ range for data collection (°)
Crystal size (mm)
Limiting indices, h k l
2.64–26.00
0.32 × 0.28 × 0.24
−10 to 10
−16 to 16
−16 to 16
10 975
2.09–25.00
0.34 × 0.30 × 0.24
−12 to 14
−22 to 21
−19 to 18
17 486
2.15–25.50
0.34 × 0.31 × 0.26
−11 to 11
−17 to 17
−19 to 19
15 185
Reflections collected
Unique reflections
Rint
5573
0.0197
6107
0.0301
7677
0.0234
Data/restraints/parameters
Goodness-of-fit on F2
R1/wR2 [I > 2σ(I)]
R1/wR2 (all data)
5573/0/406
1.015
0.0384/0.0786
0.0543/0.0858
0.577, −0.526
6107/0/473
1.045
0.0374/0.0876
0.0544/0.0954
0.703, −0.482
7677/14/532
1.098
0.0527/0.1492
0.0781/0.1737
1.705, −0.463
Largest diff. peak, hole (e Å−3
)
Table 2 Selected bond distances (Å) and angles (°) for complexes 1–3
Complex 1
1, (Yield: 235.5 mg, 67%). Found (%): C, 40.29; H, 2.56;
N, 14.74. Calcd (%) for C58H46Ag2N16O14: C, 40.45; H, 2.42;
N, 14.82. Λm (DMF, 297 K): 90.54 S cm2 mol−1. UV-vis
(λ, nm): 275, 282, 381. FT-IR (KBr ν/cm−1): 736, ν(o-Ar);
1269, ν(C–N); 1363, ν(O–N–O); 1454, ν(CvN), 1616,
ν(CvC).
Ag–N(1)
2.1088(16) N(3)–Ag#1a
2.1038(15)
2.1038(15)
Ag–N(3)#1a
C(9)–O(1)–C(8)
112.12(14) C(1)–N(1)–Ag
123.11(13)
129.50(13)
126.57(13)
N(3)#1–Ag–N(1)a
169.94(6)
C(7)–N(1)–Ag
C(10)–N(3)–Ag#1a 126.54(13) C(16)–N(3)–Ag#1a
2, (Yield: 257.4 mg, 64%). Found (%): C, 42.32; H, 3.04;
N, 14.81 Calcd (%) for C68H68Ag2N18O16: C, 42.13 H, 3.18;
N, 14.74. Λm (DMF, 297 K): 77.87 S cm2 mol−1. UV-vis
(λ, nm): 277, 284, 381. FT-IR (KBr ν/cm−1): 748, ν(o-Ar); 1259,
ν(C–N); 1365, ν(O–N–O); 1442, ν(CvN), 1629, ν(CvC).
3, (Yield: 265.7 mg, 75%). Found (%): C, 58.21; H, 3.92;
N, 12.83 Calcd (%) for C43H35AgN8O7: C, 58.45 H, 3.99;
N, 12.68. Λm (DMF, 297 K): 55.98 S cm2 mol−1. UV-vis
(λ, nm): 276, 284, 381. FT-IR (KBr ν/cm−1): 742, ν(o-Ar); 1264,
ν(C–N); 1365, ν(O–N–O); 1449, ν(CvN), 1622, ν(CvC).
Complex 2
Ag(1)–N(1)
Ag(1)–Ag(2)
Ag(2)–N(5)
N(1)–Ag(1)–N(7)
N(7)–Ag(1)–Ag(2)
N(3)–Ag(2)–O(3)
N(3)–Ag(2)–Ag(1)
O(3)–Ag(2)–Ag(1)
2.124(3)
2.9973(4)
2.134(3)
177.90(12) N(1)–Ag(1)–Ag(2)
89.32(8) N(3)–Ag(2)–N(5)
104.34(12) N(5)–Ag(2)–O(3)
Ag(1)–N(7)
Ag(2)–N(3)
Ag(2)–O(3)
2.134(3)
2.117(3)
2.526(3)
90.66(8)
164.91(12)
90.63(12)
89.23(8)
141.5(3)
79.32(8)
132.51(7)
N(5)–Ag(2)–Ag(1)
C(41)–O(3)–Ag(2)
Complex 3
Ag(1)–N(3)
Ag(1)–O(12)
Ag(2)–N(1)
N(3)–Ag(1)–N(7)
N(7)–Ag(1)–O(12)
N(7)–Ag(1)–Ag(2)
N(1)–Ag(2)–N(5)
N(5)–Ag(2)–Ag(1)
2.166(5)
2.551(4)
2.138(5)
163.5(2)
85.46(18)
88.14(14)
Ag(1)–N(7)
Ag(1)–Ag(2)
Ag(2)–N(5)
N(3)–Ag(1)–O(12)
N(3)–Ag(1)–Ag(2)
2.174(5)
3.1365(7)
2.145(5)
103.15(17)
86.16(13)
4.4. X-ray crystallography
Suitable single crystals were mounted on a glass fiber, and the
intensity data were collected on a Bruker Smart CCD diffract-
ometer with graphite-monochromated Mo-Kα radiation (λ =
0.71073 Å) at 296 K. Data reduction and cell refinement were
performed using the SMART and SAINT programs. The absorp-
tion corrections are carried out by the empirical method. The
structure was solved by direct methods and refined by full-matrix
least-squares against F2 of data using SHELXTL software.33 All
H atoms were found in difference electron maps and were sub-
sequently refined in a riding-model approximation with C–H dis-
tances ranging from 0.93 to 0.97 Å and Uiso(H) = 1.2 or 1.5
Ueq(Cmethyl). Basic crystal data, descriptions of the diffraction
experiment, and details of the structure refinement are given in
O(12)–Ag(1)–Ag(2) 166.26(10)
167.08(19) N(1)–Ag(2)–Ag(1)
88.90(14) C(68)–O(12)–Ag(1)
83.42(14)
124.3(4)
a Symmetry transformations used to generate equivalent atoms: #1 1 −
x, 1 − y, −z.
Table 1. Selected bond distances and angles are presented in
Table 2.
Crystallographic data for complexes 1–3 have been deposited
with the Cambridge Crystallographic Data Centre as supplemen-
tary publication. CCDC reference numbers are 831425, 831426
and 831427, respectively.†
Dalton Trans.
This journal is © The Royal Society of Chemistry 2012

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4.5. DNA-binding experiments
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, and [Q] is the concen-
tration of the quencher. In these experiments [CT-DNA] = 2.5 ×
Viscosity titration measurements. Viscosity experiments were
conducted on an Ubbelohde viscometer, immersed in a water
bath maintained at 25.0 0.1 °C. The flow time was measured
with a digital stopwatch and each sample was tested three times
to get an average calculated time. Titrations were performed for
the complexes (3–30 μM), and each compound was introduced
into CT-DNA solution (42.5 μM) present in the viscometer. Data
were presented as (η/η0)1/3 versus the ratio of the concentration
of the compound 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 time of CT-DNA-containing solutions corrected
from the flow time of buffer alone (t0), η = (t − t0).34
Electronic absorption titration. All spectrophotometric
measurements were performed in thermostated quartz sample
cells at 25 °C. Solutions for analysis were prepared by dilution
of stock solutions immediately before the experiments. Spectro-
photometer slit widths were kept at 1 nm for absorption spec-
troscopy and 5/5 nm for emission spectroscopy. Electronic
absorption titration experiments were performed by maintaining
the concentration of the test compounds (ligands/complexes) as
constant (30 μM) while gradually increasing the concentration of
CT-DNA. To obtain the absorption spectra, the required amount
of CT-DNA was added to both compound solution and the refer-
ence solution to eliminate the absorbance of CT-DNA itself.35
Each sample solution was scanned in the range of 190–500 nm,
and the mixture was allowed to equilibrate for 5 min before the
spectra were recorded. From the absorption titration data, the
binding constant (Kb) was determined using the equation:24
10−3 mol L−1, [EB] = 2.2 × 10−3 mol L−1
.
4.6. Hydroxyl radical scavenger measurements
Hydroxyl radicals were generated in aqueous media through the
Fenton-type reaction.39 The aliquots of reaction mixture (3 mL)
contained 1.0 mL of 0.10 mmol aqueous safranin, 1 mL of
1.0 mmol aqueous EDTA-Fe(II), 1 mL of 3% aqueous H2O2, and
a series of quantitative microadditions of solutions of the test
compound. A sample without the tested compound was used as
the control. The reaction mixtures were incubated at 37 °C for
30 min in a water bath. The absorbance was then measured at
520 nm. All the tests were run in triplicate and are expressed as
the mean and standard deviation (SD).40 The scavenging effect
for OH˙ was calculated from the following expression:
Scavenging ratio ð%Þ ¼ ½ðAi ꢁ A0Þ=ðAc ꢁ A0Þꢀ ꢂ 100%
where Ai = absorbance in the presence of the test compound;
A0 = absorbance of the blank in the absence of the test com-
pound; Ac = absorbance in the absence of the test compound,
EDTA-Fe(II) and H2O2.
Acknowledgements
The authors acknowledge the financial support and grant from
‘Qing Lan’ Talent Engineering Funds by Lanzhou Jiaotong Uni-
versity. The grant from ‘Long Yuan Qing Nian’ of Gansu Pro-
vince also is acknowledged.
½DNAꢀ=ðεa ꢁ εf Þ ¼ ½DNAꢀ=ðεb ꢁ εf Þ þ 1=Kbðεb ꢁ εf Þ
where [DNA] is the concentration of CT-DNA in base pairs, εa
corresponds to the extinction coefficient observed (Aobsd/[M]), εf
corresponds to the extinction coefficient of the free compound,
εb is the extinction coefficient of the compound when fully
bound to CT-DNA, and Kb is the intrinsic binding constant. The
ratio of slope to intercept in the plot of [DNA]/(εa − εf) versus
[DNA] gave the value of Kb.
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This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

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