Three coordination polymers of 5-aminoisophthalic acid with similar benzimidazole derivative ligands: Synthesis, structure and DNA-binding studies

Article abstract of DOI:10.1080/10610278.2012.720021

Three novel coordination polymers, [Co(NH₂-Aip)(H2Bibim)] _n(1), [Co(NH₂-Aip)(HBibimop)]_n nH₂O (2) and [Mn(NH₂-Aip)H₂O]_n 2nH₂O (3) with NH₂-Aip an

Full text of DOI:10.1080/10610278.2012.720021

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Supramolecular Chemistry
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Three coordination polymers of 5-aminoisophthalic
acid with similar benzimidazole derivative ligands:
synthesis, structure and DNA-binding studies
Yan Yang a b , Liu-Ting Yan b , Xu-Jian Luo a , Rong-Huan Qin a & Wen-Gui Duan b
a School of Chemistry and Material, Yulin Normal University , Yulin , 537000 , P.R. China
b School of Chemistry and Chemical Engineering, Guangxi University , Nanning , 530004 ,
P.R. China
Published online: 25 Sep 2012.
To cite this article: Yan Yang , Liu-Ting Yan , Xu-Jian Luo , Rong-Huan Qin & Wen-Gui Duan (2012) Three coordination
polymers of 5-aminoisophthalic acid with similar benzimidazole derivative ligands: synthesis, structure and DNA-binding
studies, Supramolecular Chemistry, 24:11, 810-818, DOI: 10.1080/10610278.2012.720021
To link to this article: http://dx.doi.org/10.1080/10610278.2012.720021
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Supramolecular Chemistry
Vol. 24, No. 11, November 2012, 810–818
Three coordination polymers of 5-aminoisophthalic acid with similar benzimidazole derivative
ligands: synthesis, structure and DNA-binding studies
Yan Yanga,b*, Liu-Ting Yanb, Xu-Jian Luoa, Rong-Huan Qina and Wen-Gui Duanb
b
aSchool of Chemistry and Material, Yulin Normal University, Yulin 537000, P.R. China; School of Chemistry and Chemical
Engineering, Guangxi University, Nanning 530004, P.R. China
(Received 9 July 2012; final version received 7 August 2012)
Three novel coordination polymers, [Co(NH2-Aip)(H2Bibim)]n (1), [Co(NH2-Aip)(HBibimop)]n z nH2O (2) and [Mn(NH2-
Aip) z H2O]n z 2nH2O (3) with NH2-Aip and similar benzimidazole derivative ligands (NH2-Aip ¼5-aminoisophthalic acid,
H2Bibim ¼ 2,20-bibenzimidazole and HBibimop ¼1,3-bis(benzimidazol-2-yl)-2-oxapropane), have been synthesised under
hydrothermal conditions and structurally characterised by single-crystal X-ray diffraction analysis, elemental analysis and
IR spectroscopy. 5-Aminoisophthalic acid ligand adopts m2-, m3- and m4-bridge coordination fashion and benzimidazole
derivatives exist as terminal- and bridge-mode in complexes 1–3. The intricate hydrogen bonds and p–p stacking
interactions in supramolecular framework are discussed. Using the combination of ultraviolet–visible absorption titration
and fluorescence spectra, the experimental results show that complexes 1 and 2 bind to DNA in an intercalative mode and
their DNA-binding constants (Kb) are also found.
Keywords: crystal structure; mixed ligand; fluorescence; DNA-binding studies
Introduction
to transition metals (Chart 1) (13–17). We have reported
some complexes (18–20) in the last several years.
In recent years, many anticancer drugs have been found to
exert their biological activities by interacting with DNA
(1). Therefore, the drug–DNA interaction has been
extensively studied for their potential application in
understanding the molecular mechanisms of drug actions
and designing specific DNA-targeted drugs. Many drugs
bind to DNA mainly through intercalation, groove-binding
and covalent linkages (2). These interactions may make
changes in the structure of DNA to inhibit DNA
duplication and cancer cell cleavage. As the functional
groups of the active sites of a number of enzymes,
benzimidazoles can participate in many important
biochemical reactions (3), benzimidazole derivatives
contain multiple nitrogen donor sites with the possibility
of reversible protonation and deprotonation properties and
can be coordinated to transition metals by elaborating
tunable coordination modes (Chart 1), which have been
utilised in coordination polymer for the preparation of
different networks (4) through intermolecular hydrogen
bonds (5–8) and p–p stacking interactions (9–12). On
the other hand, as a rigid and versatile bridging ligand, 5-
aminoisophthalic acid has been extensively studied for
designing new coordination polymer because its two
carboxylic groups can bond to metal centres and the amino
group coexisting in isophthalic acid with lone pair
electrons situated in sp3 hybrid orbital can not only act
as a hydrogen bond acceptor, but can also be coordinated
In this context, we synthesised three novel coordi-
nation polymers of [Co(NH2-Aip)(H2Bibim)]n (1),
[Co(NH2-Aip)(HBibimop)]n z nH2O (2) and [Mn(NH2-
Aip) z H2O]n z 2nH2O (3) with NH2-Aip and similar
benzimidazole derivative ligands (NH2-Aip ¼5-aminoi-
sophthalic acid, H2Bibim ¼2,20-bibenzimidazole and
HBibimop ¼1,3-bis(benzimidazol-2-yl)-2-oxapropane),
in which both ligands can be coordinated to cobalt or
manganese ions with terminal- and bridge-mode (Chart 1).
Ultraviolet–visible (UV–vis) absorption titration and
fluorescence spectra methods have been used for the
study of the interaction of complex with DNA.
Experimental
Materials and methods
Calf thymus DNA (ct-DNA) was purchased from the Sino-
American Biotechnology Company. Other reagents were
bought from commercial sources and used without further
purification. IR spectra were recorded in the range of
4000–400 cm21 on Perkin-Elmer Spectrum One FT/IR
spectrometer using a KBr pellet. Elemental analysis (C, H
and N) was performed on a Perkin-Elemer 2400II CHN
elemental analyser. UV–vis absorption titration was
performed on a Cary 100 Conc. UV–vis spectropho-
tometer. Fluorescence emission titration was performed on
*Corresponding author. Email: yy135175@163.com
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2012 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2012.720021
http://www.tandfonline.com

Supramolecular Chemistry
811
Teflon reactor, which was heated at 1108C for 4 days and
then cooled to room temperature. Red needle-shaped
crystals were obtained (yield 48% based on Co), filtered
off, washed with distilled water and dried in air. Elemental
analysis (%): Anal. Calcd. C, 54.06; H, 4.08; N, 14.18;
found C, 54.03; H, 4.09; N, 14.17. IR (KBr pellets, cm21):
n ¼ 3058, 1559, 1427, 1375, 1329, 1268, 1121, 1049, 999,
958, 920, 850, 783, 757, 733.
Synthesis of [Mn(NH2-Aip) z H2O]n z 2nH2O (3)
For the hydrothermal reaction of MnSO4 (1 mmol), NH2-
Aip (0.5 mmol) and water (15 ml), the mixture was stirred
for 30 min in air, then transferred and sealed into a 23 ml
Teflon reactor, which was heated at 1608C for 5 days and
then cooled to room temperature. Colourless crystals were
obtained (yield 63% based on NH2-Aip), filtered off,
washed with distilled water and dried in air. Elemental
analysis (%): Anal. Calcd. C, 33.45; H, 3.48; N, 4.88;
found C, 33.51; H, 3.53; N, 4.83. IR (KBr pellets, cm21):
n ¼ 3093, 1690, 1624, 1588, 1550, 1451, 1402, 1291,
1256, 1223, 1111, 908, 751, 683, 592.
Chart 1. Coordination mode of the three ligands.
a Shimadzu RF-5301/PC spectrofluorometer. The crystal
structure was determined by a Bruker APEX area-detector
diffractometer and employing the SHELXTL crystal-
lographic software. In DNA-binding studies, all com-
plexes were dissolved in dimethyl sulfoxide (DMSO) for
preparation of stock solution at 1.0 £ 1026 M. Buffer
(5 mM tris(hydroxymethyl)aminomethane (Tris)-hydro-
chloride, 50 mM NaCl, pH 7.35) was used for UV
absorption and emission titration experiments. The
concentration of ct-DNA was determined spectrophoto-
metrically assuming that molar absorption is
6600 M21 cm21 (260 nm) (21, 22). The UV absorption
and emission titrations of complexes 1 and 2 in water/
DMSO 10% in the presence of a buffer were performed by
using a fixed complex concentration to which increments
of the DNA stock solution were added ([DNA]/
[compound] ranged from 0 to 10). Complex–DNA
solutions were allowed to incubate for 15 min before the
UV absorption and emission spectra were recorded.
X-ray crystallography
Diffraction data were collected on a Bruker Smart Apex
CZN diffractometer with graphite-monochromated MoKa
˚
radiation (l ¼ 0.71073 A) at 296 K. Absorption correction
was applied by SADABS (23). The structure was solved by
Direct Methods and refined with full-matrix least-squares
technique using SHELXTL (24). All non-hydrogen atoms
were refined with anisotropic displacement parameters.
Crystal data, details of the data collection and refinement
are summarised in Table 1, and selected bond lengths and
angles are presented in Table 2; the hydrogen bond lengths
Synthesis of [Co(NH2-Aip)(H2Bibim)]n (1)
˚
(A) and bond angles (deg) are listed in Table 3.
For the hydrothermal reaction of CoCl2 (0.5 mmol),
H2Bibim (0.5 mmol), NH2-Aip (0.5 mmol) and water
(18 ml), the mixture was stirred for 10 min in air, then
transferred and sealed into a 23-ml Teflon reactor, which
was heated at 1608C for 5 days and then cooled to room
temperature. Red block crystals were obtained (yield 23%
based on NH2-Aip), filtered off, washed with distilled
water and dried in air. Elemental analysis (%): Anal.
Calcd. C, 55.93; H, 3.18; N, 14.83; found C, 55.78; H,
3.24; N, 14.79. IR (KBr pellets, cm21): n ¼ 3022, 2955,
1622, 1569, 1531, 1454, 1428, 1390, 1319, 1266, 1246,
1179, 1159, 1080, 1043, 1005, 931, 918, 788, 695.
Crystal structures of complexes 1–3
Single-crystal X-ray diffraction analysis reveals that
compound 1 has a supramolecular framework structure
formed by one-dimensional (1D) bi-chain and crystallises
in the monoclinic P21/n space group. Asymmetric unit
contains one Co(II) atom, one H2Bibim ligand and one Aip
ligand (Aip ¼ 5-aminoisophthalic acid) (Figure 1). The
distorted octahedral Co(II) atom geometry (Scheme 1(a)) is
established by three oxygen atoms (O1a, O2a and O4b) and
one nitrogen atom (N5c) from three different Aip ligands
and two nitrogen atoms (N1a and N3a) from one H2Bibim
ligand. The basal plane consists of four almost coplanar
atoms (O1a, O2a, O4b and N3a), and the N1a and N5c
atoms occupy the apical positions (Figure 1(a)). The CoZO
Synthesis of [Co(NH2-Aip)(HBibimop)]n z nH2O (2)
For the hydrothermal reaction of Co(NO3)216H2O
(0.5 mmol),
HBibimop
(0.25 mmol),
NH2-Aip
˚
distances are in the range of 2.012(3)–2.209(4) A, and the
˚
CoZN distances are in the range of 2.105(4)–2.270(4) A
(0.25 mmol) and water (15 ml), the mixture was stirred
for 20 min in air, then transferred and sealed into a 23 ml

812
Y. Yang et al.
Table 1. Crystal data and structure refinement for complexes 1–3.
Identification code
Empirical formula
1
C22H15CoN5O4
2
C40H36Co2N9O8
3
C8H10MnN1O7
Formula weight
Temp (K)
Crystal system
472.32
296(2)
Monoclinic
P21/n
12.605(5)
8.774(3)
16.706(6)
90.00
98.571(5)
90.00
888.64
296(2)
Monoclinic
C2/c
26.209(3)
9.6927(12)
19.143(4)
90.00
130.19(10)
90.00
288.12
293(2)
Triclinic
P-1
Space group
˚
a (A)
˚
7.7732(16)
8.6392(17)
8.7331(17)
85.45(3)
76.06(3)
66.56(3)
522.14(18)
0.26 £ 0.20 £ 0.18
7
b (A)
˚
c (A)
a (deg)
b (deg)
g (deg)
3
˚
V (A )
1827.0(12)
0.36 £ 0.30 £ 0.20
17
3714.9(10)
0.30 £ 0.26 £ 0.22
4
Cryst size (mm)
Z
Dc (g cm23
)
1.710
0.985
956
1.589
0.962
1828
1.826
1.289
292
m (mm21
F(000)
Limiting indices
)
214 # h # 14
210 # k # 10
216 # l # 19
9304
231 # h # 31
210 # k # 11
222 # l # 22
10132
210 # h # 10
211 # k # 11
211 # l # 11
5492
Reflections
Independent
Observed data
Npar
3204
2398
289
0.1276
1.067
0.0742
3279
2490
268
0.0401
1.021
0.0428
2373
2178
178
0.0187
1.166
0.0290
Rint
GOF
R1a (I . 2s(I))
wR2 (all data)
b
0.2180
1.046/20.990
0.993, 25.01
0.1137
0.657/20.606
0.997, 25.01
0.0716
0.380/20.293
0.994, 27.44
23
˚
Max/min electron density (eA
Theta range (o)
)
P
P
a R1 ¼ kFoj 2 jFck= jFoj.
P
P
b wR2 ¼ ½ wðFo2 2 Fc2Þ = wðF2oÞ ꢀ
.
2
2
1=2
˚
Table 2. Selected bond lengths (A) and bond angles (8) for complexes 1–3.
Complex 1
Complex 2
Complex 3
Co1ZO1
Co1ZO2
Co1ZO4a
Co1ZN1
Co1ZN3
Co1ZN5b
O1ZCo1ZO2
O1ZCo1ZN1
O1ZCo1ZN5b
O2ZCo1ZN5b
O4aZCo1ZO1
O4aZCo1ZO2
O4aZCo1ZN1
O4aZCo1ZN3
O4aZCo1ZN5b
N1ZCo1ZO2
N1ZCo1ZN5b
N3ZCo1ZO1
N3ZCo1ZO2
N3ZCo1ZN1
N3ZCo1ZN5b
2.127(4)
2.209(4)
2.012(3)
2.179(4)
2.105(4)
2.270(4)
Co1ZO1
Co1ZN2
Co1ZN4
Co1ZN5
2.022(2)
Mn1ZO1
Mn1ZO2
Mn1ZO3
Mn1ZO4
Mn1ZO5
Mn1ZN1
2.2459(15)
2.3519(16)
2.1189(17)
2.1438(16)
2.1574(16)
2.3816(19)
57.12(5)
94.33(6)
85.91(6)
90.59(6)
145.81(5)
100.30(6)
120.90(7)
85.74(6)
89.97(6)
91.34(6)
89.51(7)
2.045(3)
2.030(3)
2.041(3)
60.40(12)
92.60(15)
84.87(15)
90.03(14)
95.33(14)
155.67(14)
98.09(15)
99.71(16)
85.72(14)
85.62(15)
175.62(16)
163.51(15)
104.58(15)
78.70(16)
102.86(16)
O1ZCo1ZN2
O1ZCo1ZN4
O1ZCo1ZN5
N4ZCo1ZN2
N4ZCo1ZN5
N5ZCo1ZN2
101.38(11)
109.19(11)
96.97(10)
124.37(11)
110.82(11)
110.13(11)
O1ZMn1ZO2
O1ZMn1ZN1
O2ZMn1ZN1
O3ZMn1ZO1
O3ZMn1ZO2
O3ZMn1ZO4
O3ZMn1ZO5
O3ZMn1ZN1
O4ZMn1ZO1
O4ZMn1ZO2
O4ZMn1ZO5
O4ZMn1ZN1
O5ZMn1ZO1
O5ZMn1ZO2
O5ZMn1ZN1
172.56(5)
148.04(6)
90.95(7)
83.63(7)
Note: Symmetry transformations used to generate equivalent atoms: Complex 1, a ¼ 2x þ 2, 2y þ 2, 2z þ 2; b ¼ 2x þ 2, 2y þ 1, 2z þ 2.

Supramolecular Chemistry
Table 3. Distances (A) and angles ( ) of hydrogen bonds for complexes 1–3.
813
o
˚
DZH···A
d(DZH)
d(H···A)
d(D···A)
,(DZH···A)
Complex 1
N2ZH2···O3
N4ZH4···O3
N5ZH5A···O3
Complex 2
0.8600
0.8600
0.9000
2.0500
2.0520
2.3820
2.853
2.860
3.173
155.03 (x 2 1, y, z)
156.30 (x 2 1, y, z)
146.60 (2x þ 5/2, y 2 1/2, 2z þ 3/2)
N1ZH1B···O4
N1ZH1C···O4
N3ZH3···O2
Complex 3
0.8900
0.8900
0.8600
2.3900
2.3200
1.9800
3.029(8)
3.029(8)
2.696(4)
128.40 (x, y 2 1, z)
136.40 (2x, y 21, 2z þ 1/2)
140.60 (2x, 2y þ 1, 2z)
O5ZH5A···O2
O5ZH5B···O6
O6ZH6A···O1
O6ZH6B···O6
O7ZH7A···O6
O7ZH7B···O4
N1ZH1B···O7
0.7970
0.8360
0.8500
0.8500
0.8500
0.8500
0.9000
1.9330
1.9390
2.0440
2.1850
2.0470
2.0760
2.2030
2.721
2.758
2.834
2.894
2.867
2.863
3.033
169.66 (2x þ 1, 2y, 2z)
165.77 (2x þ 1, 2y, 2z þ 1)
154.32 (2x þ 1, 2y þ 1, 2z þ 1)
140.78 (2x þ 1, 2y þ 1, 2z þ 1)
161.90
153.71
152.90 (x 2 1, y, z)
Figure 1. (a) Coordination environment of complex 1; (b) 1D bi-chain of complex 1 with two different subrings A and B, hydrogen
atoms are omitted for clarity. (c) p–p Stacking and hydrogen-bonding interactions of complex 1; (d) simplified (4, 4) topology structure
of complex 1, hydrogen atoms are omitted for clarity.
Scheme 1. (a) Coordination mode of Aip and the coordination sphere of the Co(II) ion in complex 1. (b) Coordination mode of Aip and
coordination sphere of the Co(II) ion in complex 2. (c) Coordination mode of Aip and coordination sphere of the Mn(II) ion in complex 3.

814
Y. Yang et al.
(Table 2), respectively. In the structure, the Aip ligand
adopts (m3 2 ((h 2 2 O1, O2), O3, N1)) bridging coordi-
nation fashion (Chart 1(c)): one carboxylate group adopts
mono-bisdentate coordination mode to chelate two Co(II)
centres, and another carboxylate group and amino group act
as monodentate ligands. The adjacent Co(II) atoms are
connected by m3-bridged Aip into 1D bi-chain structure
along the b direction, while the H2Bibim ligand acts as a
chelating ligand (Chart 1(a)) and alternately arranges at the
sides of the chains in an outward fashion (Figure 1(b)). The
most interesting feature for polymer 1 is that the Aip ligands
link Co(II) centres in different ways to produce two
different subrings A and B, which are 16-membered (A
rings) and 14-membered rings (B rings) (Figure 1(b)). It
should be noted that there are three types of hydrogen bonds
in this structure: the uncoordinated O atoms of Aip ligands
act as hydrogen-bonding acceptors to form hydrogen bonds
with the uncoordinated imidazole N atoms of H2Bibim
ligands and amino-group nitrogen atoms of Aip ligands,
N2ZH2···O3, N4ZH4···O3 and N5-H5A···O3 with the
bridging mode (Chart 1(d)) to connect Co(II) atoms to
form 1D undulate chain along the a axis (Figure 2(b)).
HBibimop ligand acts as chelating-bidentate and bridging-
monodentate mode (Chart 1(b)) to link Co(II) atoms of
neighbouring chains to produce an undulate 2D layer
parallel to the ab plane through interchain CoZN bonds
˚
(Co1ZN5 ¼ 2.041(3) A) (Figure 2(b)). So, Auxiliary
HBibimop ligands in this 2D structure act actually as
chelating-bidentate and bridging-monodentate mode in 2,
as shown in Chart 1(b). Up to date, the terminal-mode
(m1-(h2-N, N)) of HBibimop has been mostly reported
(25–27); however, rare examples with bridge-mode
(m2 2 (h2 2 N, N), N) have been observed. In addition,
there exist N1ZH1B···O4 and N1ZH1C···O4 hydrogen-
bond interactions with their distances (d(D···A)) being
o
˚
3.029(8) A, and NZH···O angles being 128.40 and
136.40o (Figure 2(b) and Table 3) in the 2D structure of
2 which further stabilise the layer structure. Furthermore,
strong interlayer NZH···O hydrogen-bond interactions
˚
with the separation (d(D···A)) being 2.696(4) A and
˚
˚
NZH···O angles being 140.60 also contribute to this 3D
supramolecular structure (Figure 2(c)).
distances (d(D···A)) being 2.853, 2.860 and 3.173 A, and
OZH···N angles being 155.03, 156.30 and 146.60o (Table
3). At the same time, the face-to-face p–p stacking
Single-crystal X-ray diffraction analysis indicates that
compound 3 is a 2D supramolecular network structure in
which asymmetric unit contains one Mn(II) atom, one Aip
ligand, one coordinated water and two lattice water
molecules (Figure 3(a)). Mn(II) is six-coordinated by four
oxygen atoms from three distinct Aip ligands, one amino
group nitrogen atom from one Aip ligand and one oxygen
atom of aqueous molecule to form distorted octahedral
geometry (Scheme 1(c)), as shown in Figure 3(a).
Compared with complexes 1 and 2, the Aip ligand adopts
(m4 2 (h 2 2 O1, O2), O3, O4, N) mode (Chart 1(e)): one
carboxylate group adopts mono-bidentate (syn-syn)
coordination mode to bridge two Mn(II) atoms, another
carboxylate group acts as a chelating bidentate ligand and
its amino group uses bridging monodentate mode to join
Mn(II) centres. The binuclear Mn2(CO2) unit in compound
3 is bridged by four Aip ligands to result in the 2D layers
parallel to the bc plane (Figure 3(b)) with four kinds of
interlocking rings which are the 14-membered
rings (A rings), the 16-membered rings (B rings), the
8-membered rings (C rings) and the 14-membered rings
˚
interactions (the centroid···centroid separation is 3.696 A)
are alsoobserved, as shown in Figure 1(c). The 1D bi-chains
mentioned above are interconnected through these hydro-
gen bonds and p–p stacking interactions between benzene
rings of the adjacent H2Bibim groups to result in a layer
structure in the ab plane (Figure 1(c)) and further stabilise
the supramolecular network. This layer can be reduced to a
(4, 4) topology (Figure 1(d)).
Single-crystal X-ray diffraction analysis shows that
compound 2 is a two-dimensional (2D) structure in which
there are one Co(II) atom, moiety bis-monodentate Aip
ligand, one HBibimop ligand and one lattice water
molecule in each independent crystallographic unit
(Figure 2(a)). Each Co(II) atom in 2 is primarily four-
coordinated by one carboxylate oxygen atom of one Aip
˚
ligand (Co1ZO1 ¼ 2.022(2) A) and three nitrogen atoms
˚
from HBibimop ligand (CoZN ¼ 2.030(3)–2.045(3) A,
˚
Table 2, the average value is 2.039 A) to furnish a distorted
trigonal pyramid geometry (Figure 2(a) and Scheme 1(b)).
Different from 1, Aip ligand adopts bis-monodentate
Figure 2. (a) Coordination environment of complex 2, (b) the 2D layers of complex 2 and (c) the three-dimensional supramolecular
network of complex 2, hydrogen atoms are omitted for clarity.

Supramolecular Chemistry
815
Figure 3. (a) Coordination environment of complex 3 and (b) the 2D layers of complex 3, hydrogen atoms are omitted for clarity.
(D rings) with the Mn···Mn distances of 8.0008, 8.0693,
˚
4.1435 and 8.0794 A, respectively (Figure 3(b)). In the
DNA-binding studies
Absorption titrations
structure, there exist five types of hydrogen-bonding
interactions: the oxygen atoms (O5) of coordinated water
act as hydrogen bond donors to form hydrogen bonds with
the coordinated carboxylate oxygen atoms (O2) of Aip
ligands and the oxygen atoms (O6) of lattice water,
O5ZH5A···O2 and O5ZH5B···O6 with the separations
Figure 4 shows the UV–vis absorption spectra of
complexes 1 and 2 in the presence of increasing
concentrations of DNA. Complexes 1 and 2 represent the
two absorption peaks at 241, 275 nm (Figure 4(a)) and at
277, 313 nm (Figure 4(b)), respectively, which can be
assigned to the p ! p (at 241 nm) and n ! p (at 275,
*
*
˚
(d(D···A)) being 2.721 and 2.758 A, and OZH···O angles
277 and 313 nm) transitions of the benzimidazole rings.
Such an observation is the result of the so-called ‘additive
effect’ of absorption spectra which the metal atoms are
simultaneously coordinated to two ligands, which are Aip
ligands and H2Bibim (for 1) or HBibimop ligands (for 2).
With increasing concentrations of DNA added, significant
hyperchromism, red-shift and blue-shift are observed.
There are 93% hyperchromism with a 9-nm red-shift (from
241 to 250 nm) at 241 nm and 400% hyperchromism with
an 8-nm blue-shift (from 275 to 267 nm) at 275 nm for 1.
Different from the peak absorption intensities of
compound 1, the peak absorption intensities of compound
2 only increase about 8% at 277 nm and do hardly change
at 313 nm, but the observation of the red-shift or blue-shift
being 169.66o and 165.77o (Table 3); at the same time, the
oxygen atoms (O6 or O7) of lattice water act as hydrogen
bond donors to form hydrogen bonds with the coordinated
carboxylate oxygen atoms (O1 or O4) and amino group N
atoms of Aip ligands, O6ZH6A···O1, O7ZH7B···O4 and
N1ZH1B···O7 with the distances (d(D···A)) being 2.834,
˚
2.863 and 3.033 A, respectively, OZH···O angles being
154.32o and 153.71o, as well as OZH···N angles being
152.90o (Table 3). In addition, hydrogen bonds have been
found between lattice waters, O6ZH6B···O6 and
O7ZH7A···O6 with the separations (d(D···A)) being
o
˚
2.894 and 2.867 A, and OZH···O angles being 140.78
and 161.90o (Table 3).
Figure 4. UV absorption spectra of complexes 1 and 2 with increasing concentration of DNA (concentration of complex is
1.0 £ 1026 M, [DNA]/[compound] ranged from 0 to 10).

816
Y. Yang et al.
2.99 £ 105 and 4.05 £ 105. In the plot of [DNA]/(1f21a)
versus [DNA], the intrinsic binding constants, Kb, can be
obtained by the ratio of the slope to intercept. Complex 2
displays larger binding constant than does complex 1 since
the groove binding of complex 2 is much more intense than
the intercalation of complex 1.
has not been found. When the ratio of [DNA]/[Complex]
increases from 0 to 10, the compound can insert the
adenine base stacks of double-helical DNA, which causes
hydrophobic interaction to be composed of base stacking
strength and van der Waals force to vary correspondingly,
affecting the stability of the conformation of DNA and
making the double-helical structure of DNA cause damage
which lead to unwinding of the double-helical structure
and increasing in UV–vis absorption of purine and
pyrimidine bases (28). However, hyperchromism is likely
the result of the damaged hydrogen-bonding interactions
between the complex aggregates and complex molecules
(29, 30) brought about by the penetration of complexes
into the DNA base stacks. According to the literature (31–
34), hyperchromism (or hypochromism) and bathochro-
mism can be observed as the small molecule binds to
double-strand DNA by classical intercalation mode.
However, there is no obvious hypochromism or bath-
ochromism when small molecule can interact with DNA
via groove binding or electrostatic interaction. In brief,
complex 1 can interact into the base pairs of double-helical
DNA via an intercalative mode and complex 2 may bind to
DNA via the combined mode of intercalation and groove
binding.
Fluorescence studies
The fluorescence emission spectra of complexes 1 and 2 in
the presence of increasing concentrations of DNA are
shown in Figure 5. As shown in Figure 5(a), upon excitation
using a wavelength of 326 nm, complex 1 displays
luminescence with three emission maxima at 346, 364
and 382 nm, which can be assigned to the p ! p
*
transitions of the benzimidazole rings. Upon addition of
DNA, the luminescence intensity is reduced compared with
that of the parent [Co(NH2-Aip)(H2Bibim)]n in the absence
of DNA under similar conditions, reaching a minimum at
the ratio of [DNA]/[Complex 1] ¼ 10, at which there is a
0.88-, 0.91- and 0.92-fold decrease in the fluorescence
intensity of complex 1 compared with that in the absence of
DNA. The reduced luminescence observed for complex 1 in
0.1 M Tris buffer (pH 7.35) containing 10% DMSO can be
rationalised, primarily, in terms of an intramolecular
photoinduced electron-transfer quenching of its metal-to-
ligand charge transfer (MLCT) state. The decreases in
emission intensities of compound 1 most probably result
from the interaction betweenthe small molecules and DNA;
since there are enrichment domains of A-T and C-G
sequence in the base pairs of DNA sequence, fluorescence
emission will give rise to change if the compound is
intercalating the different orientation of the DNA bases.
When the compound is intercalating into the C-G sequence,
the easier electron transfer between the excited state and
guanosine G can result in effective quenching of
fluorescence because the distance between them is very
close, that is 0.34 nm or so, and guanosine G has better
electron donating ability (36, 37). Furthermore, the
fluorescence emission of the excited state in complex 1 is
To compare quantitatively the binding strength of the
compounds, the intrinsic binding constants Kb with DNA
are determined from the increase in the absorbance with
increasing concentrations of DNA using the equation (35):
DNA
DNA
1
¼
þ
;
1a 2 1f 1b 2 1f ½Kbð1b 2 1f Þꢀ
where 1a, 1f and 1b are the extinction coefficients of the
complex at a given DNA concentration, the complex free
in solution and the complex fully bound to DNA,
respectively, Kb is the equilibrium binding constant,
[DNA] is the DNA concentration in 0.1 M Tris buffer (pH
7.35) containing 10% DMSO. The intrinsic binding
constants, Kb, of 1 and 2 for DNA are determined to be
Figure 5. Emission spectra of complexes 1 and 2 in 0.1 M Tris buffer (pH 7.35) containing 10% DMSO in the absence ( . . . ) and
presence (—) of DNA.

Supramolecular Chemistry
817
probably quenched due to proton transfer from the solvents
to the benzimidazole rings (38). The decreases in
fluorescence emission also suggest that compound 1
interacts with DNA strongly. As shown in Figure 5(b),
complex 2 exhibits three fluorescent emission bands at 346,
365 and 381 nm upon being excited at 312 nm, which is
obtained via the Cambridge Crystallographic Data Centre
(deposit@ccdc.cam.ac.uk; http://www.ccdc.cam.ac.uk/
deposit).
predominantly p ! p transition fluorescence. As
*
Acknowledgements
opposed to the fluorescence emission of complex 1, the
fluorescence emission for complex 2 increases with
increasing amounts of DNA. When the ratio [DNA]/
[Complex 2] ¼ 10, the 1.35-, 1.33- and 1.42-fold increase
in the fluorescence intensity of complex 2 has been found
compared with that in the absence of double-helical DNA
for 2. This observation suggests that complex 2 binds to
DNA by an intercalative mode. The strong binding of
complex 2 to DNA can give rise to the so-called ‘molecular
light switch effect’, in which the nearly undetectable
emission from the MLCT excited state in water becomes
strongly enhanced upon binding (34). This is mostly
because of the intercalation of complex 2 into the base pairs
of DNA, which is hydrophobic inside the helix, limits the
collision and energy dissipation with the environmental
water and oxygen (39–41). As far as compound 2 is
concerned, the weak fluorescence is the result of increasing
the loss of energy via non-radiative decay of the intraligand
excited state, which was suggested to be highly quenched
by lattice aqueous in compound 2 due to proton transfer
from the solvent to the benzimidazole ligand (42–45).
We acknowledge financial support by the National Natural
Science Foundation of China (No. 51062016) and the Project of
Science Research & Technology Exploitation of Guangxi
Province (No. 11107013-8).
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