Synthesis, crystal structure, and DNA-binding studies of different coordinate binuclear silver(I) complexes with benzimidazole open-chain ether ligands

Article abstract of DOI:10.1039/c5nj01684c

Three new binuclear silver(I) complexes, namely, [Ag₂(Meobb)₂](pic)₂·2H₂O (1), [Ag₂(Etobb)₂(pic)](pic)·(CH₃CN) (2) and [Ag₂(Bobb)₂(pic)₂] (3) (Meobb = 1,3-bis(1-methylbenzimidazol-2-yl)-2-oxapropane, Etobb = 1,3-bis(1-ethylbenzimidazol-2-yl)-2-oxapropane, Bobb = 1,3-bis(1-benzylbenzimidazol-2-yl)-2-oxapropane, pic = picrate), have been synthesized and characterized by elemental analyses, IR spectroscopy and X-ray single crystal diffraction. Complex 1 displays an Ag₂(Etobb)₂ dimeric structure. Each silver(I) ion is coordinated to two nitrogen atoms that adopt a distorted linear configuration. In complex 2, the coordination environment of silver(I) atoms is different. The Ag1 ion is coordinated in a T-shaped tri-coordinated geometry and the Ag2 is best described to be in a distorted tetrahedron. Complex 3 exhibits a discrete di-silver metallacyclic framework. One silver atom (Ag1) is present in a distorted tetrahedral geometry, whereas the other silver atom (Ag2) is five-coordinated with two nitrogen atoms, oxygen atoms and the Ag1 to form a distorted square-based pyramidal configuration. The interactions of the three complexes with calf thymus DNA (CT-DNA) has been investigated by electronic absorption titration, fluorescence spectroscopy and viscosity measurements, and the modes of CT-DNA binding to the complexes have been proposed. Experimental results suggest that the silver(I) complexes bind to DNA in an intercalation mode, and their binding affinity for DNA follows the order 1 > 2 > 3. The DNA-binding studies demonstrate that steric hindrance has a large influence on the binding ability to DNA. The complex that has smaller steric hindrance binds more strongly to DNA.

Full text of DOI:10.1039/c5nj01684c

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Synthesis, crystal structure, and DNA-binding
studies of different coordinate binuclear silver(I)
complexes with benzimidazole open-chain ether
ligands†
Cite this:DOI: 10.1039/c5nj01684c
Huilu Wu,* Jiawen Zhang, Chengyong Chen, Han Zhang, Hongping Peng,
Fei Wang and Zaihui Yang
Three new binuclear silver(I) complexes, namely, [Ag2(Meobb)2](pic)2Á2H2O (1), [Ag2(Etobb)2(pic)](pic)Á(CH3CN)
(2) and [Ag2(Bobb)2(pic)2] (3) (Meobb = 1,3-bis(1-methylbenzimidazol-2-yl)-2-oxapropane, Etobb =
1,3-bis(1-ethylbenzimidazol-2-yl)-2-oxapropane, Bobb = 1,3-bis(1-benzylbenzimidazol-2-yl)-2-oxapropane,
pic = picrate), have been synthesized and characterized by elemental analyses, IR spectroscopy and
X-ray single crystal diffraction. Complex 1 displays an Ag2(Etobb)2 dimeric structure. Each silver(I) ion is
coordinated to two nitrogen atoms that adopt a distorted linear configuration. In complex 2, the coordination
environment of silver(I) atoms is different. The Ag1 ion is coordinated in a T-shaped tri-coordinated geometry
and the Ag2 is best described to be in a distorted tetrahedron. Complex 3 exhibits a discrete di-silver
metallacyclic framework. One silver atom (Ag1) is present in a distorted tetrahedral geometry, whereas the
other silver atom (Ag2) is five-coordinated with two nitrogen atoms, oxygen atoms and the Ag1 to form a
distorted square-based pyramidal configuration. The interactions of the three complexes with calf thymus
DNA (CT-DNA) has been investigated by electronic absorption titration, fluorescence spectroscopy and
viscosity measurements, and the modes of CT-DNA binding to the complexes have been proposed.
Experimental results suggest that the silver(I) complexes bind to DNA in an intercalation mode, and their
binding affinity for DNA follows the order 1 4 2 4 3. The DNA-binding studies demonstrate that steric
hindrance has a large influence on the binding ability to DNA. The complex that has smaller steric
hindrance binds more strongly to DNA.
Received (in Montpellier, France)
30th June 2015,
Accepted 1st July 2015
DOI: 10.1039/c5nj01684c
www.rsc.org/njc
coordinated polymers as well as non-covalent supramolecular
Introduction
interactions with aromatic clouds such as pÁ Á Áp, CHÁ Á Áp and
In recent decades, the chemistry of silver(I) complexes is an anionÁ Á Áp.12 Moreover, silver(I) has a high affinity towards
active, thriving field that has attracted a great deal of research heteroatoms, like N, O and S, present on the ligands.13 Thus,
interest.1–4 As more and more silver(I) complexes have been reported, more and more ligands containing N and O atoms have been
many of them have been used in the fields of luminescence, synthesized such as benzimidazoles and their derivatives.
catalysis, conduction and anticancer agents.5–8 As a representative
Benzimidazoles and their derivatives have drawn significant
d10 electronic configurational metal, silver(I) has a very flexible attention as an important class of heterocyclic compounds in
coordination sphere, which enables it to adopt coordination chemistry and pharmacology.14–17 Benzimidazole is an impor-
numbers ranging from 2 to 6, even 7 and 8, resulting in tant fragment in medicinal chemistry and plays very important
coordination geometries varying from linear to T-shaped, tetra- roles in numerous pharmaceutically important molecules with a
gonal, square pyramidal and octahedral.9–11 Another interesting wide range of biological properties. Plenty of compounds con-
feature of silver(I) is its feasibility to form the short AgÁ Á ÁAg taining the benzimidazole group have been reported to exhibit
(argentophilicity), AgÁ Á Áp and Ag–C interactions in silver(I) anticancer,18 antiviral,19 anti-inflammatory,20 antimicrobial,21
antioxidant22 and anticoagulant properties.23
Therefore, the investigation of silver(I) complexes with benz-
imidazole is becoming a very popular and interesting field.
Nowadays, plenty of silver(I) complexes with benzimidazole have
School of Chemical and Biological Engineering, Lanzhou Jiaotong University,
Lanzhou, Gansu, 730070, P. R. China. E-mail: wuhuilu@163.com;
Fax: +86 13893117544; Tel: +86 13893117544
been developed and many of them exhibit wide-ranging antimicro-
bial activity,24 photoluminescence and catalytic properties25,26
† CCDC 721072, 721073 and 1408501. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5nj01684c
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as well as the potential to form supramolecular aggregates
through pÁ Á Áp stacking interaction and hydrogen bonding.27
Based on our previous investigations on silver(I) complexes
containing benzimidazole derivatives,28,29 in this study, we
wish to report the synthesis, characterization and DNA-binding
activities of silver(I) complexes 1–3 containing three different
V-shaped ligands.
Results and discussion
Synthetic routes towards binuclear silver(I) complexes are
exhibited in Scheme 1. The binuclear silver(I) complexes were
obtained by the reaction of Ag(pic) with three ligands, namely,
Meobb, Etobb and Bobb, in hot methanol. In contrast to silver(I)
complexes, the three ligands are stable under atmospheric
conditions. The silver(I) complexes are remarkably soluble in
polar aprotic solvents such as DMF, DMSO and MeCN; slightly
soluble in water, ethanol, methanol, ethyl acetate and chloro-
form; and insoluble in Et2O and petroleum ether.
Fig. 1 The molecular structure of 1; hydrogen atoms and picrate anions
are omitted for clarity.
X-ray structure characterization
The structures of 1, 2 and 3 are confirmed by X-ray diffraction. two different Meobb ligands, forming a slightly distorted linear
The crystal structures are shown in Fig. 1, 3 and 4, respectively. AgN2 motif with an N1–Ag–N3 angle of 169.941. It is noteworthy
Crystal structure of 1. Crystallographic analysis reveals that that this N1–Ag–N3 interaction is the ligand bridge. The
complex 1 crystallizes in the triclinic space group P1, and the [Ag2(Meobb)2]2+ cation comprises a centrosymmetric binuclear
%
structure of complex 1 displays an M2L2 dimeric structure pore canal structure through the N1–Ag–N3 interaction (Fig. 1).
composed of a dinuclear [Ag2(Meobb)2]2+ cation, two picrate The distance of the pore ranges from 5.992 Å (dAg–Ag) to 6.7 Å
anions and two aqua molecules. As seen from Fig. 1, two ligands (dO1–O1A). The distance between the two silver(I) centres is
are arranged in an end-to-end fashion to coordinate with two Ag(I) 5.992 Å, and is considerably longer than the limit (2.91–3.24 Å)
atoms. Each silver(I) ion is coordinated to two nitrogen atoms from of those reported for weak AgÁ Á ÁAg interactions in other silver(I)
Scheme 1 Synthesis of complexes 1–3.
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Fig. 2 A view of the packing in complex 1, showing pÁ Á Áp stacking and hydrogen bonds between neighboring units.
Fig. 4 The neutral unit and atom-labeling scheme of complex 3 (left);
hydrogen atoms and solvent molecules are omitted for clarity. The 8-shaped
geometric structure of complex 3 (right).
Fig. 3 The cation complex 2 with H-atoms and the uncoordinated
picrate anion omitted for clarity (left) and the 8-shaped geometric struc-
ture built by two nine-membered rings (right).
complexes,30–32 excluding any bonding interactions. It is obvious [Ag2(Meobb)2](pic)2ÁH2O in Fig. 2. Owing to the existence of
that the two picrate anions do not participate in the coordination hydrogen bonds and pÁ Á Áp stacking interaction, an infinite 2-D
and only act as counter anions for charge equilibrium.
layer was created.
As depicted in Fig. 2, the [Ag2(Meobb)2]2+ cation in crystalline
Crystal structure of 2. Single-crystal X-ray diffraction analysis
[Ag2(Meobb)2](pic)2Á2H2O stacks through strong pÁ Á Áp inter- reveals that complex 2 crystallizes in the triclinic space group
%
action between pairs of benzimidazole rings containing N3 P1. The asymmetric unit of 2 is composed of two silver(I) ions,
and N4 (and symmetry related atoms), extending into a 1-D two Etobb ligands and one picrate anion. The average N–Ag bond
chain. The distance of the centroid-to-centroid between the two distance of 2.127 Å is similar to that in {[Ag(MeIm)2](DNB)}2ÁH2O
benzimidazole rings is 3.438 Å. This type of interaction plays (2.1196 Å, MeIm = 2-methylimidazole, DNB = 3,5-dinitrobenzoate),34
an important role in stabilization of the framework. It is greatly but longer than that in complex 1 (2.106 Å) and shorter than that
fascinating that two adjacent picrate anions and two aqua mole- in complex 3 (2.155 Å). Careful inspection of the crystal structure
cules are inlayed in the coordination cations as a sandwich and shows that the two silver(I) ions are coordinated differently; they
are connected by hydrogen bonds.
are three- and four-coordinated (Fig. 3). The coordination geo-
The oxygen atoms of picrate anion and Meobb ligand are the metry around Ag1 ion is best described as T-shaped with
strongest hydrogen bond acceptors, while the O–H groups of the angle of N1–Ag1–N7 as 177.901. The N3–Ag2–N5 angle
hydrone are the strongest hydrogen bond donors in the system.33 (164.911) is slightly deviated from linearity, but the coordination
O–HÁ Á ÁO hydrogen bonds from hydrogen of O–H to picrate, geometry around Ag2 should be best described to be a distorted
water and Meobb ligand are shown in the structure of tetrahedron considering the O3–Ag2 (2.526 Å) and Ag1–Ag2
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Fig. 5 1-D chain of complex 3 by benzimidazole ring pÁ Á Áp stacking.
(2.997 Å) interactions. The geometry for the four-coordinate substrate recognition or for the arrangement of complicated
transition metal complex can be determined using the t4 architectures. In complex 3, the self-assembly of silver(I) ions and
geometry index, t4 = [360 À (a + b)]/141, where a and b are Bobb ligands generate infinite 1-D chains, and the asymmetric
the largest angles around the metal center.35 The four coordi- molecules are connected by pÁ Á Áp stacking interactions from the
nate t4 values range from one for a perfect tetrahedral geometry benzimidazole rings (Fig. 5). The average centroid-to-centroid
to zero for a perfect square planar geometry. The t4 parameter distance is 3.632 (1) Å.
for Ag2 is 0.44 and it can be considered to have a slightly
In summary, three new binuclear silver(I) complexes were
distorted seesaw geometry between tetrahedral and square synthesized using three ligands Meobb, Etobb and Bobb,
planar. The Ag–Ag contacts lead to dinuclear silver units that respectively, which exhibited diverse intriguing structures. For
are further bridged by the ligand for constructing two nine- complex 1, two Meobb ligands were coordinated with two silver(I)
membered rings, thus exhibiting an 8-shaped geometry (Fig. 3). atoms with two N1–Ag–N2 interactions, and each silver(I) ion is two-
Crystal structure of 3. The X-ray structural characterization coordinated. The major structures of complexes 2 and complex 3
shows that complex 3 belongs to the triclinic crystal system, are similar and exhibit an 8-shaped geometric structure, but the
%
crystallizing in the space group P1. As illustrated in Fig. 4, in coordination environment of the silver(I) atoms is different. In
complex 3, the self-assembly of silver(I) ions and ligands gives complex 2, the coordination numbers of the two silver(I) atoms
a dinuclear [Ag2(Bobb)2(pic)2] motif. There are two silver(I) are three and four, while the coordination numbers of two
ions that have different coordination environments. The Ag1 silver(I) atoms are four and five in complex 3. By comparing the
ion adopts a distorted tetrahedron coordination geometry (t4 = molecular structures of the above-mentioned complexes, the
[360 À (a + b)]/141 = 0.21; herein, a = N(3)–Ag(1)–N(7) = 163.51 reason that the silver(I) complexes exhibit different structural
and b = O(12)–Ag(1)–Ag(2) = 166.261) with an oxygen atom (O12) conformation could be ascribed to different distortions and
from the picrate group and two nitrogen atoms (N3, N7) from steric hindrance of the ligands.
the two chelating Bobb ligands. However, the Ag2 center is five-
coordinated with two nitrogen atoms (N5, N1), oxygen atoms
DNA-binding studies
(O3, O4) and Ag1 and forms a rarely distorted square-based
Electronic absorption titration. DNA-binding is the critical
pyramidal configuration. The parameter (t5) of the five-coordinate step for DNA cleavage in most cases. The application of electronic
geometry from a perfect trigonal bipyramidal geometry (t5 = 1) absorption spectroscopy in DNA-binding studies is one of the
towards a regular square-based pyramidal (t5 = 0) has been most useful techniques.38 The binding of metal complexes to
calculated according to the method of Addison et al., t5
=
DNA helix is often characterized through absorption spectral
(b À a)/60, where a and b are the largest angles around the titration by following the changes in the absorbance and shifts
metal center.36 The t5 parameter for Ag2 is 0.036 (herein, b = in the wavelength. To clarify the interaction between the complex
O(4)–Ag(2)–Ag(1) = 169.241 and a = N(1)–Ag(2)–N(5) = 167.081). and DNA, the absorption spectra of 1–3 in the absence and
The Ag1–Ag2 distance is 3.137 Å, which is significantly shorter presence of CT-DNA are illustrated in Fig. 6a–c, respectively.
than the sum of van der Waals radii (3.44 Å), and is rather close These complexes exhibited intense absorption bands at 278–
to the Ag–Ag distance in silver metal (3.048 Å),37 indicating the 282 nm, ascribed to the p - p* transition of benzimidazole
presence of an argentophilic interaction. The average O–Ag and increasing concentration of CT-DNA. The concentration of
bond length of 2.636 Å is longer than that in complex 2 CT-DNA affected the absorption most likely through the inter-
(2.526 Å). The Ag–Ag contacts result in a dinuclear silver unit calative mode because intercalation would lead to hypochromism
and are further bridged by the ligand to construct two nine- and bathochromism in UV absorption spectra. The intercalative
membered rings, similar to complex 2 (Fig. 3).
mode involves a strong stacking interaction between an aromatic
As one of the important types of supramolecular forces, chromophore and the base pairs of DNA.39 In the present case,
pÁ Á Áp stacking shows a specific structural requirement for with the addition of DNA, three silver(I) complexes exhibited
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Fig. 6 Absorption spectral traces of (a) complex 1, (c) complex 2 and (e) complex 3 in Tris-HCl buffer upon the addition of CT-DNA. Arrows show the
emission intensity changes upon increasing the DNA concentration. Plots of [DNA]/(ea À ef) vs. [DNA] for the titration of (b) complex 1, (d) complex 2 and
(f) complex 3 with CT-DNA. [complex 1], [complex 2] and [complex 3] = 2.0 Â 10À5 mol LÀ1, [DNA] = 0.5–9.0 Â 10À5 mol LÀ1
.
hypochromism of about 10.23%, 72.00% and 46.04%, respec- (R2 = 0.99 for 12 points), respectively. The Kb values obtained
tively, accompanied by a bathochromic shift of about 1–2 nm in here are lower than those reported for classical intercalators
the absorption maxima. The extent of the hypochromism is (for example, ethidium bromide), whose binding constants have
consistent with the strength of intercalative interactions.40
been found to be 106 MÀ141 and is three orders of magnitude more
To compare the affinity of the three Ag(I) complexes towards than our previous investigation on silver(I) complexes containing
DNA, the intrinsic binding constants Kb for complexes 1, 2 and 3 benzimidazole derivatives.29 These results indicate that there is
were obtained and are 2.26 Â 105 MÀ1 (R2 = 0.99 for 11 points), an intercalative binding mode between the three complexes
1.09 Â 105 MÀ1 (R2 = 0.99 for 15 points) and 0.92 Â 105 MÀ1 and CT-DNA. As can been seen from Fig. 6, the binding
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Fig. 7 Emission spectra of EB bound to DNA in the presence of (a) complex 1, (c) complex 2 and (e) complex 3 in Tris-HCl buffer. Fluorescence quenching
curves of EB bound to CT-DNA by (b) complex 1, (d) complex 2 and (f) complex 3. (Plots of I0/I vs. [complex]). [DNA] = 2.5 Â 10À3 mol LÀ1; lex = 520 nm.
strengths of the complexes follow the order 1 4 2 4 3. The were monitored by a fluorescent EB displacement assay. EB is a
reason for the difference in the binding strengths of these three planar aromatic heterocyclic dye that intercalates non-specifically
Ag(I) complexes can be attributed to the difference in steric into the DNA and causes it to fluoresce strongly.42,43
hindrance, which is caused by both the introduction of sub-
EB (weak fluorescence) + DNA (no fluorescence) 2
stituents and geometric structure.
Fluorescence spectroscopic studies. No luminescence was
observed for complexes 1–3 at room temperature either in DMF
or in the presence of CT-DNA. The binding of the complexes
EB-DNA (strong fluorescence)
Thus, EB can be used to probe the interaction of the com-
cannot be directly followed in the emission spectra. Therefore, plexes with DNA. The relative binding of the complexes 1–3 to
steady state competitive binding studies of the three complexes CT-DNA is studied with an EB-bound CT-DNA solution in 5 mM
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Conclusions
In this study, three benzimidazole open-chain ether ligands and
their silver(I) complexes have been synthesized and character-
ized. In complexes 1–3, all the benzimidazole ligands adopt the
m2-bridging mode to link two silver atoms, forming three differ-
ent binuclear motifs. Complex 2 and 3 display an obvious Ag–Ag
contact. DNA-binding of the Ag(I) complexes suggest that the
three Ag(I) complexes bind to DNA through an intercalation
mode owing to the large planar aromatic rings, hydrogen bonds
and pÁ Á Áp stacking interactions, which facilitate their intercala-
tion into the base pairs of double helical DNA. Furthermore,
the binding affinities of these Ag(I) complexes follow the order
1 4 2 4 3. This result illustrates that steric hindrance plays a
vital role in the DNA-binding of the three Ag(I) complexes. These
studies suggest that Ag(I) complexes have potential practical
applications in the development of probes for DNA structure
and conformations, as well as new therapeutic reagents for
diseases on the molecular level, thus warranting further in vivo
experiments and pharmacological assays.
Fig. 8 Effect of increasing amounts of the complexes on the relative
viscosity of CT-DNA at 25.0 Æ 0.1 1C.
Tris-HCl/50 mM NaCl buffer (pH = 7.2), as shown in Fig. 7.
Fluorescence intensities (520 nm excitation) are measured at
different complex concentrations. The emission intensity of
the DNA–EB system decreases appreciably, which indicated
that the complexes could replace EB from the DNA–EB system.
Such a characteristic change is often observed in intercalative
binding modes.44 As shown in Fig. 7, the quenching plot illustrates
that the quenching of EB bound to DNA by complex is in agreement
with the Stern–Volmer relationship, which also indicates the
complex binding to DNA. The Ksv values for complexes 1–3
are found to be 7.95 Â 104 (R2 = 0.99 for 10 points), 4.36 Â 104
(R2 = 0.99 for 10 points) and 2.64 Â 104 (R2 = 0.98 for 8 points),
respectively. The Stern–Volmer dynamic quenching constants
can also be interpreted as binding affinities of the complexation
reaction.45,46 The Ksv values obtained here are similar to our
previous study on silver(I) complexes containing benzimidazole
derivatives.28 Moreover, the binding ability to CT-DNA follows the
order 1 4 2 4 3. The results we obtained from the fluorescence
spectra are the same as those from absorption spectra.
Experimental
Caution: Although no problems were encountered in this study,
transition-metal picrate salts are potentially explosive and thus
should be prepared in small quantities and handled with care.
Materials and instrumentation
The Meobb, Etobb and Bobb ligands were synthesized accord-
ing to the procedure given in ref. 52. Ethidium bromide (EB)
and calf thymus DNA (CT-DNA) were purchased from Sigma
Chemicals Co. (USA). Tris-HCl buffer solution was prepared
using double-distilled water.
CHN elemental analyses were performed using a Carlo Erba
1106 elemental analyzer. IR spectra were recorded in the 4000–
400 cmÀ1 region with a Nicolet FT-VERTEX 70 spectrophoto-
meter using KBr pellets. The electronic spectra were obtained
on a Lab-Tech UV Bluestar spectrophotometer. Fluorescence
spectral data were obtained on a 970-CRT fluorescence spectro-
photometer at room temperature.
Viscosity measurements. Hydrodynamic measurements that
are sensitive to changes in DNA length are considered as the least
ambiguous and most critical tests for a binding model in solution
in the absence of crystallographic data.47 To further investigate the
interaction properties between the metal complexes and DNA,
the relative specific viscosity of DNA was examined by varying
Synthesis
the concentration of the added metal complexes. Measuring the All three complexes were prepared using a similar procedure.
viscosity of DNA is a classical technique used to analyze the DNA The ligand (0.50 mmol: Meobb, 153 mg; Etobb, 167 mg; Bobb,
binding mode in solution.48 A classical intercalative mode causes 229 mg) in hot MeOH (15 mL) was added to silver(I) picrate
significant increases in the viscosity of the DNA solution due to an (0.25 mmol, 84 mg) in MeOH (5 mL). A colorless crystalline
increase in the separation of base pairs at the intercalation sites, product was formed rapidly. The precipitate was filtered-off,
and hence an increase in overall DNA length. In contrast, the washed with MeOH and absolute Et2O, and dried in vacuo. The
complexes those bind in DNA grooves by partial and/or non- product was dissolved in MeCN to obtain a colorless solution
classical intercalation cause less pronounced (positive or negative) that was allowed to evaporate at room temperature. Colorless
or no change in DNA solution viscosity.49 Fig. 8 depicts the effect of crystals suitable for X-ray diffraction studies were obtained after
complexes 1–3 on DNA viscosity. These results reveal that complex several days.
1, 2 and 3 produce a relatively apparent increase in DNA viscosity,
Complex 1. Yield 71%. Anal. calcd for C48H44Ag2N14O18 (%):
which is consistent with DNA intercalative binding modes C, 43.61; H, 3.33; N, 14.84. Found (%): C, 44.02; H, 3.35; N,
suggested above.50 These results support the fact that the 14.89. Selected IR data (KBr, n/cmÀ1), 1084 (nC–O–C), 1330 (nNO ),
2
complexes 1–3 bind to CT-DNA through intercalation.51
1492 (nCQN–CQC).
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Complex 2. Yield 67%. Anal. calcd for C54H51Ag2N15O16 (%):
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Table 2 The relevant bond distances (Å) and angles (1) for complexes
1–3a
C, 46.89; H, 3.69; N, 15.20. Found (%): C, 46.85; H, 3.71; N,
15.18. Selected IR data (KBr, n/cmÀ1), 1078 (nC–O–C), 1330 (nNO ),
Complex 1
2
Ag–N(1)
Ag–N(3)#1
C(9)–O(1)–C(8)
N(3)#1–Ag–N(1)
C(10)–N(3)–Ag#1
2.1088(16)
2.1038(15)
112.12(14)
169.94(6)
N(3)–Ag#1
2.1038(15)
1483 (nCQN–CQC).
Complex 3. Yield 71%. Anal. calcd for C72H56Ag2N14O16 (%):
C, 54.37; H, 3.52; N, 12.33. Found (%): C, 54.35; H, 3.61; N,
C(1)–N(1)–Ag
C(7)–N(1)–Ag
C(16)–N(3)–Ag#1
123.11(13)
129.50(13)
126.57(13)
12.14. Selected IR data (KBr, n/cmÀ1), 1068 (nC–O–C), 1330 (nNO ),
126.54(13)
2
1471 (nCQN–CQC).
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)
89.32(8)
104.34(12)
79.32(8)
Ag(1)–N(7)
Ag(2)–N(3)
Ag(2)–O(3)
N(1)–Ag(1)–Ag(2)
N(3)–Ag(2)–N(5)
N(5)–Ag(2)–O(3)
N(5)–Ag(2)–Ag(1)
C(41)–O(3)–Ag(2)
2.134(3)
2.117(3)
2.526(3)
90.66(8)
164.91(12)
90.63(12)
89.23(8)
141.5(3)
X-ray diffraction
For each complex, a suitable single crystal was mounted on glass
fibers for data collection in a Bruker Smart CCD diffractometer
with graphite-monochromated Mo-Ka radiation (l = 0.71073 Å)
at 153 K. Data reduction and cell refinement were performed
using the SMART and SAINT programs.53 The absorption correc-
tions were carried out by the empirical method. The structure
was solved by direct methods and refined by a full-matrix of
least-squares against F2 for data using the SHELXTL software.54
All H atoms were found in difference electron maps and were
subsequently refined in a riding-model approximation with C–H
distances ranging from 0.95 to 0.99 Å. Information concerning
the crystallographic data collection and structural refinements is
summarized in Table 1. The relevant bond lengths and angles
are listed in Table 2.
132.51(7)
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)
167.08(19)
88.90(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)
O(12)–Ag(1)–Ag(2)
N(1)–Ag(2)–Ag(1)
C(68)–O(12)–Ag(1)
2.174(5)
3.1365(7)
2.145(5)
103.15(17)
86.16(13)
166.26(10)
83.42(14)
124.3(4)
a
Symmetry transformations used to generate equivalent atoms: #1 1 À
x, 1 À y, Àz.
CCDC reference numbers are 721073, 721072 and 1408501,
respectively.
NaCl buffer (pH = 7.2). The solution of CT-DNA gave a ratio of UV
absorbance at 260 nm and 280 nm, A260/A280, of 1.8–1.9, indicat-
DNA-binding studies
Electronic absorption spectra. The DNA-binding experiments ing that the DNA is sufficiently free of protein.55 The CT-DNA
were carried out at room temperature. Using the electronic stock solutions were prepared in 5 mM Tris-HCl/50 mM NaCl
absorption spectral method, the relative bindings of the three buffer, pH = 7.2 (CT-DNA stock solutions were stored at 4 1C
complexes to CT-DNA were studied in 5 mM Tris-HCl/50 mM and used within 4 days after preparation). The concentration
Table 1 Crystallographic data and structural refinement parameters for complexes 1–3
Complex
1
2
3
Empirical formula
Mr
C48H44Ag2N14O18
1320.71
C54H51Ag2N15O16
1381.84
C72H56Ag2N14O16
1589.05
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a (1)
b (1)
Triclinic
P1
Triclinic
P1
Triclinic
P1
%
%
%
7.09520(10)
12.8978(3)
15.0574(4)
110.5830(10)
102.7630(10)
97.5890(10)
1224.46(5)
1, 1.791
11.7172(3)
11.9846(3)
20.1196(6)
89.9760(10)
81.0430(10)
79.8740(10)
2746.33(13)
2, 1.671
13.3730(3)
14.8244(4)
17.3282(4)
86.8670(10)
89.7150(10)
87.8750(10)
3427.75(14)
2, 1.540
g (1)
V (Å3)
Z, Dc (mg mÀ3
)
m (mmÀ1
F(000)
)
0.895
668
0.800
1404
0.652
1616
y Range (1)
Limiting indices, hkl
3.02–27.48
À8 to 9
3.00–27.48
À15 to 13
À15 to 15
À26 to 26
26 608
12 603
0.0350
3.04–25.50
À16 to 16
À17 to 17
À20 to 20
28 124
12 762
0.0352
À16 to 16
À19 to 19
12 128
Reflections collected
Unique reflections
Rint
5602
0.0151
1.031
0.0223/0.0731
0.0245/0.0781
0.519, À0.582
Goodness-of-fit on F2
R1/wR2 [I 4 2s(I)]
R1/wR2 (all data)
Largest diff. peak, hole (e ÅÀ3
1.062
1.179
0.0390/0.1120
0.0534/0.1248
0.729, À0.950
0.0487/0.1228
0.0877/0.1900
1.618, À1.531
)
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of CT-DNA was determined from its absorption intensity at Acknowledgements
260 nm with a molar extinction coefficient of 6600 MÀ1 cmÀ1 56
.
The present research was supported by the National Natural
Science Foundation of China (Grant No. 21367017) and the Natural
Science Foundation of Gansu Province (Grant No. 1212RJZA037).
Absorption titration experiments were performed with fixed
concentrations of the complexes (25 mL) while gradually increas-
ing the concentration of DNA. When measuring the absorption
spectra, an equal amount of DNA was added to both complex
solution (2.5 mL Tris-HCl + 25 mL complexes solution) and the
reference solution (2.5 mL Tris-HCl + 25 mL DMF) to eliminate
the absorbance of DNA itself. The solutions were allowed to
incubate for 5 min before the absorption spectra was recorded.
The titration processes were repeated until there was no change
in the spectra, indicating that binding saturation had been
achieved ([complex] = 2.00 Â 10À3 mol LÀ1). From the absorption
titration data, the binding constant (Kb) was calculated using the
following equation:39
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[DNA]/(ea À ef) = [DNA]/(eb À ef) + 1/Kb(eb À ef)
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