One- and two-dimensional cobalt(II) coordination polymers derived from flexible bis(benzimidazole) and aromatic carboxylate co-ligands

Article abstract of DOI:10.1007/s11243-014-9820-2

Two new coordination polymers, formulated as [Co(L1)(btec)_0.5] _n (1) and {[Co(L2)(bdc)]·H₂O} _n (2) (L1 = 1,3-bis(2-methylbenzimidazol-1-ylmethyl)benzene, H₂bdc = 1,3-benzenedicarboxylic acid, H₄btec = 1,2,4,5-benzenetetracarboxylic acid, L2 = 1,3-bis(benzimidazol-1-ylmethyl)benzene), have been hydrothermally synthesized and characterized by physicochemical and spectroscopic methods as well as single-crystal X-ray diffraction. The cobalt atoms present different environments, with a trigonal pyramidal geometry in 1 and a distorted octahedral configuration in 2. Complex 1 shows a 2D (4,4) network linked by L1 and btec^4- anions, giving an uninodal 4-connected sql topology with a point symbol of {4²·6²}, while complex 2 displays a 1D ladder-like chain structure, which is further assembled into a 3D supramolecular architecture via C-H···π hydrogen bonding interactions. The fluorescence properties of both complexes have been investigated in the solid state.

Full text of DOI:10.1007/s11243-014-9820-2

Transition Met Chem (2014) 39:461–467
DOI 10.1007/s11243-014-9820-2
One- and two-dimensional cobalt(II) coordination polymers
derived from flexible bis(benzimidazole) and aromatic carboxylate
co-ligands
•
Xiao Xiao Wang Yong Guang Liu
•
•
Yue Hua Li Guang Hua Cui
Received: 16 January 2014 / Accepted: 17 March 2014 / Published online: 5 April 2014
Ó Springer International Publishing Switzerland 2014
Abstract Two new coordination polymers, formulated as
[Co(L1)(btec)0.5]n (1) and {[Co(L2)(bdc)]ÁH2O}n (2) (L1 =
1,3-bis(2-methylbenzimidazol-1-ylmethyl)benzene, H2bdc =
1,3-benzenedicarboxylic acid, H4btec = 1,2,4,5-benzenetet-
racarboxylic acid, L2 = 1,3-bis(benzimidazol-1-ylmethyl)
benzene), have been hydrothermally synthesized and charac-
terized by physicochemical and spectroscopic methods as well
as single-crystal X-ray diffraction. The cobalt atoms present
different environments, with a trigonal pyramidal geometry in 1
and a distorted octahedral configuration in 2. Complex 1 shows
a 2D (4,4) network linked by L1 and btec4- anions, giving an
uninodal 4-connected sql topology with a point symbol of
{42Á62}, while complex 2 displays a 1D ladder-like chain
structure, which is further assembled into a 3D supramolecular
architecture via C–HÁÁÁp hydrogen bonding interactions. The
fluorescence properties of both complexes have been investi-
gated in the solid state.
ligands used, the metal–ligand ratio, solvent, and so on [5–7].
Moreover, the architectures of the resulting coordination net-
works and their supramolecular organization can also be
changed by noncovalent interactions, including hydrogen
bonds, pÁÁÁp, C–HÁÁÁp and/or van der Waals interactions [8–
11]. Hence, systematic research on this topic is crucial in
understanding how these factors affect the metal coordination
framework for the rational design of crystalline materials. In
recent decades, there has been increasing interest in coordi-
nation architectures constructed from flexible ligands. In par-
ticular, flexible bis(benzimidazole) ligands have been widely
used as classic N-containing ligands, which can satisfy the
coordination needs of the metal centers and consequently
create robust and intricate networks [12–16]. Moreover, this
type of ligand contains both the imidazole ring and a larger
conjugated p-system, capable of participating in hydrogen
bondingandp–pstacking interactions. Usingthe mixed-ligand
synthetic strategy [17–19], we selected two related flexible
bis(benzimidazole) ligands (L1 and L2 only differ by a methyl
group) and rigid aromatic polycarboxylate (see Scheme 1) co-
ligands to construct two new MOCPs, namely [Co(L1)
(btec)0.5]n (1) and {[Co(L2)(bdc)]ÁH2O}n (2), where L1 =
1,3-bis(2-methylbenzimidazol-1-ylmethyl)benzene, H2bdc
= 1,3-benzenedicarboxylic acid, H4btec = 1,2,4,5-benzene-
tetracarboxylic acid, L2 = 1,3-bis(benzimidazol-1-ylmethyl)
benzene. The thermogravimetric analyses and solid-state
luminescence properties of the complexes are also reported.
Introduction
In recent years, the design and synthesis of metal–organic
coordination polymers (MOCPs) have been the subject of
extensive research, not only due to their variety of structures
and interesting properties, but also their potential applications
in catalysis, luminescence, gas adsorption, molecular sieves,
ion exchange, corrosioninhibition, and antitumoragents [1–4].
The self-assembly ofsuch polymers by hydrothermal reactions
is heavily influenced by factors such as the selection of metal
ions with different coordination geometries, the nature of the
Experimental
Materials and measurements
X. X. Wang Á Y. G. Liu Á Y. H. Li Á G. H. Cui (&)
College of Chemical Engineering, Hebei United University,
Tangshan 063009, Hebei, People’s Republic of China
e-mail: tscghua@126.com
Reagents and solvents were commercially available and
used without further purification. The ligands L1 and L2
were prepared by the literature method [20]. Elemental
123

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Transition Met Chem (2014) 39:461–467
analyses were obtained on a Perkin-Elmer 240C automatic
analyzer. IR spectra within the range of 4,000–400 cm-1
were obtained on a Nicolet FTIR Avatar 360 spectropho-
tometer with KBr pellets. Thermogravimetric analysis
(TGA) was obtained on a NETZSCH TG 209 thermal
analyzer from room temperature to 800 °C under N2
atmosphere with a heating rate of 10 °C/min. The lumi-
nescence spectra for the powdered solid samples were
measured at room temperature on a Hitachi F-4500 fluo-
rescence spectrophotometer. X-ray powder diffraction
(XRPD) investigations were carried out with a Rigaku
D/Max-2500 diffractometer at 40 kV, 100 mA with Cu Ka
Pink crystals suitable for single-crystal X-ray diffraction
were obtained in 53 % yield (based on Co(OAc)2Á4H2O).
Calcd. for C30H24CoN4O5 (Fw = 579.46): C 62.2, H 4.2, N
9.7 %; found: C 61.5, H 4.1, N 9.4 %. FTIR (KBr pellet,
cm-1): 3,462 w, 1,605 m, 1,542 m, 1,430 m, 1,390 s,
1,259 m, 1,190 m, 836 m, 753 s, 421 s.
X-ray crystallography
X-ray diffraction data for complexes 1 and 2 were obtained
at room temperature on a Bruker Smart 1000 CCD dif-
fractometer with graphite-monochromated Mo–Ka radia-
˚
˚
(k = 1.5406 A) radiation.
tion (k = 0.71073 A) and x-2h scan mode at 293 K.
Semi-empirical absorption corrections were applied using
the SADABS program [21]. Both the structures were
solved by direct methods, and all non-hydrogen atoms were
refined anisotropically with the SHELXTL package using a
full-matrix least-squares procedure based on F2 values
Synthesis of complex 1
A mixture of Co(OAc)2Á4H2O (0.1 mmol, 25 mg), L1
(0.1 mmol, 36.7 mg), H4btec (0.1 mmol, 25.4 mg), and
H2O (12 mL) was stirred for 1 h, and then, the pH was
adjusted to 6.5 by the addition of 0.1 mol/dm3 NaOH
solution. The resulting mixture was sealed in a 25-mL
Teflon-lined stainless steel vessel, which was heated at
140 °C for 72 h, and then cooled to room temperature at a
rate of 5 °C/h. Purple block-shape crystals of complex 1
suitable for single-crystal X-ray diffraction were obtained
in 45 % yield (based on Co(OAc)2Á4H2O). Calcd. for
C29H23CoN4O4 (Fw = 550.46): C 63.3, H 4.2, N 10.2 %;
found: C 63.0, H 4.0, N 9.9 %. FTIR (KBr pellet, cm-1):
3,426 m, 1,654 m, 1,515 m, 1,403 s, 1,284 m, 1,157 m,
989 m, 855 m, 764 s, 433 m.
Table 1 Crystallographic data and structure refinement for 1 and 2
Complex
1
2
Empirical formula
Formula weight
Temperature
C29H23CoN4O4
550.44
C30H24CoN4O5
579.46
293(2) K
0.71073
293(2) K
0.71073
˚
Wavelength/A
Crystal system
Space group
Monoclinic
P21/c
Orthorhombic
Pnna
Unit cell dimensions
˚
a/A
12.020(3)
9.571(3)
21.090(6)
92.244(4)
2,424.4(11)
4
17.2366(18)
12.5117(13)
11.8458(13)
Synthesis of complex 2
˚
b/A
˚
c/A
Complex 2 was synthesized by a similar procedure to that for
complex 1, replacing L1 and H4btec with L2 (0.1 mmol,
33.8 mg) and H2bdc (0.1 mmol, 16.6 mg), respectively.
b/°
3
˚
V/A
2,554.7(5)
4
Z
Dc/g cm-3
1.508
1.507
1,236
F(000)
1,136
Crystal size/mm
h Range/°
Index range h, k, l
0.28 9 0.22 9 0.22 0.26 9 0.22 9 0.21
1.70–27.50
2.09–25.02
-9/15, -12/12,
-27/26
-15/20, -14/13,
-14/12
Reflections collected 14,198
Rint 0.0688
Goodness of fit on F2 0.884
12,160
0.0455
0.864
Final R indices
[I [ 2r(I)][a,b]
R (all data)[a,b]
R1 = 0.0599,
wR2 = 0.1715
R1 = 0.0358,
wR2 = 0.1078
R1 = 0.1171,
wR2 = 0.2112
R1 = 0.0505,
wR2 = 0.1234
Largest diff. peak
and hole
0.607, -0.492
0.230, -0.381
a
b
2
R1 = R||Fo|–|Fc||/R|Fo|; wR2 = {R [w (F2o–Fc2) ]/R [w (Fo2)2]}1/2
Scheme 1 All ligands in this paper
123

Transition Met Chem (2014) 39:461–467
463
˚
Table 2 The selected bond lengths (A) and angles (°) for 1 and 2
Parameter
Value
Parameter
Value
[Co(L1)(btec)0.5]n (1)
Co1–O2
1.942(3)
1.951(3)
Co1–N1
2.052(4)
Co1–O3
Co1–N4
2.048(4)
117.28(14)
101.13(14)
102.28(15)
112.1(4)
O2–Co1–O3
O2–Co1–N4
O3–Co1–N4
N2–C24–C28
103.20(15)
110.64(14)
122.85(15)
111.8(4)
O2–Co1–N1
O3–Co1–N1
N4–Co1–N1
N3–C17–C15
{[Co(L2)(bdc)]ÁH2O}n (2)
Co1–N1
2.083(2)
Co1–O2
2.1228(19)
83.97(11)
60.35(7)
Co1–O1
2.215(2)
100.63(12)
101.32(8)
157.98(8)
O1–Co1–O1C
O2–Co1–O1
O2–Co1–O1C
O2C–Co1–O2
N1–Co1–N1C
N1–Co1–O2
N1C–Co1–O1
94.98(7)
148.15(10)
Symmetry transformation used to generate equivalent atoms: C =
-x ? 1, y, -z ? 3/2
[22]. All non-hydrogen atoms were refined anisotropically,
and hydrogen atoms bonded to carbon atoms were placed
in geometrically calculated positions. H atoms of one water
molecule for 1 were not located due to their disorder and
high thermal parameters of O1W, whose occupation factor
is 0.5. Crystallographic data and experimental details for
structural analysis of the complexes are summarized in
Table 1, and selected bond lengths and angles are listed in
Table 2.
Results and discussion
IR spectra
In the IR spectra of the complexes, bands at 1,515 cm-1 for
1 and 1,542 cm-1 for 2 are assigned to the mC=N absorption
of the benzimidazole rings of the ligands L1 and L2,
respectively. A broadband around 3,426 and 3,462 cm-1
for 1 and 2, respectively, can be attributed to the stretching
vibrations of lattice water molecules or the existence of
hydrogen bonds. The asymmetric and symmetric stretching
vibrations of the carboxyl groups are observed at 1,654 and
1,403 cm-1 for complex 1, and at 1,605 and 1,430 cm-1
for complex 2. The separations (Dm[mas(COO)–ms(COO)])
between these bands indicate the presence of monodentate
(251 cm-1 for 1) and chelating (175 cm-1 for 2) carbox-
ylate groups [23]. There is no characteristic band at around
1,700 cm-1, indicating that all carboxyl groups in both
complexes are completely deprotonated.
Fig. 1 a Coordination environment of the Co atom in 1, and
hydrogen atoms were omitted for clarity; b 2D (4,4) supramolecular
structure in 1; c The uninodal 4-connected 2D net with a point symbol
of {42Á62} topology network in 1
Structural analysis of the complexes
Single-crystal X-ray diffraction analysis reveals that com-
plex 1 crystallizes in the monoclinic space group P21/
c. The asymmetric unit contains one Co(II) center, one L1
ligand, and half a btec4- ligand. As shown in Fig. 1a, each
Co atom is four-coordinated by two nitrogen atoms (N1,
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Transition Met Chem (2014) 39:461–467
N4) from two different L1 ligands and two oxygen atoms
(O2, O3) belonging to two different btec4- ligands to give
a trigonal pyramidal geometry, with a s4 value of 0.85 [24].
The angle of O3–N1–N4 is 67.93(11)°. The bond lengths of
Co–N1, Co–N4, Co–O2, and Co–O3 are 2.052(4),
˚
2.048(4), 1.942(3), and 1.951(3) A, respectively. The bond
angles range from 101.13(14) to 122.85(15)°, which are
comparable to values observed for similar cobalt(II) com-
plexes [25].
In the structure of complex 1, the L1 ligand adopts cis-
conformation, with the dihedral angle between the mean
planes of the two benzimidazole rings being 89.64(2)°.
These L1 ligands act in a bis-monodentate mode, bridging
neighboring Co atoms into infinite 1D [CoL]n zigzag
chains with a distance between adjacent Co atoms of
˚
11.290(1) A. Furthermore, four carboxyl groups of the
btec4- ligands assume monodentate coordination modes to
extend the [CoL]n chains into a 2D (4,4) layer (Fig. 1b). In
order to understand the architecture of complex 1 more
fully, the topological method has been used to simplify and
analyze the 2D structure by TOPOS 4.0 software [26]. The
L1 ligands can be simplified to be linear connectors; the
cobalt center should be considered as a 4-connected node,
connected by two L2 and btec4- ligands. The btec4-
ligands linking four Co atoms should also be viewed as
4-connected nodes. The network of 1 shows a uninodal 2D
4-connected sql topology with point symbol of {44.62}
(Fig. 1c). As is shown in this figure, the grid-like layer is
made of rhomboids and irregular squares with a through-
4-
˚
ligand Co–Co distance of 5.5438(6) A for anionic btec
˚
ligands, and 11.2859(5) A for L1 ligand, providing a dis-
torted (4,4) structure (the red nodes represent btec4-
ligands, and the blue nodes are Co atoms). In addition, the
crystal packing is enhanced by two kinds of weak C–HÁÁÁO
˚
hydrogen bonds (C19–H19ÁÁÁO4A: 3.244(6) A, 154°; C27–
Fig. 2 a Coordination environment of the Co atom in 2, hydrogen
atoms were omitted for clarity, and symmetry transformation was
used to generating equivalent atoms: C = -x ? 1, y, -z ? 3/2;
D = 2.5-x, 1-y, z; E = x, 1 ? y, z; b 1D zigzag chain connected by
Co atoms and bdc2- ligands in 2; c 1D ladder-like chain connected by
L2 and bdc2- ligands in 2
˚
H27ÁÁÁO4B: 3.296(6) A, 164°, symmetry codes: A = 2-x,
2-y, -z; B = x, 5/2-y, 1/2 ? z) between L1 and atom O4
of the btec4- ligands.
Complex 2 crystallizes in the orthorhombic space group
Pnna. X-ray crystal structural determination reveals that
complex 2 has an infinite 1D ladder-like chain structure.
The asymmetric unit consists of one Co(II) center, one
anionic bdc2- ligand, one L2 ligand, and one lattice water
molecules. The Co atom is coordinated by four oxygens
(O1, O2, O1C, O2C) from two different bdc2- ligands and
two nitrogen atoms (N1, N1C) (symmetry code: C =
-x ? 1, y, -z ? 3/2) from two L2 ligands in a distorted
octahedral configuration (Fig. 2a). The bond lengths of
Co1–O1, Co1–O2, and Co–N are 2.215(2), 2.124(2), and
to form a 1D [Co2(bdc)2] zigzag chain (Fig. 2b). Each L2
ligand adopts a cis-conformation to link [Co2(bdc)2] units,
developing a 1D ladder-like chain (Fig. 2c). The adjacent
non-bonding CoÁÁÁCo distances are 9.391(9) (Co1ÁÁÁCo1D)
˚
and 12.512(8)
A
(Co1ÁÁÁCo1E) (symmetry codes:
D = 2.5-x, 1-y, z; E = x, 1 ? y, z). In addition, these 1D
chains are further extended into a 3D supramolecular
architecture by five kinds of C–HÁÁÁp hydrogen bonds
(Table 3). The distances of HÁÁÁCg are from 2.73(3) to
˚
˚
2.1228(19) A, respectively. The coordination angles range
2.93(3) A, and the angles of C–HÁÁÁCg are in the range of
112(4)–155(4)°. These weak interactions play an important
role in strengthening the stability in such metal–organic
coordination polymers.
from 60.35(7) to 157.98(8)°.
In complex 2, each bdc2- ligand adopts a chelating
coordination mode connecting two neighboring Co atoms
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Transition Met Chem (2014) 39:461–467
465
˚
Table 3 The C–HÁÁÁp hydrogen bond distances (A) and angles (°) in
2
d(X–H)
d(HÁÁÁCg)
d(XÁÁÁCg)
\XHCg
C5–H5ÁÁÁCg4
2.75(3)
2.93(3)
2.73(3)
2.90(2)
2.91(3)
3.509(4)
3.422(4)
3.627(4)
3.648(4)
3.612(4)
139
112
155
138
134
C8–H8AÁÁÁCg1
C8–H8AÁÁÁCg2
C10–H10ÁÁÁCg2
C15–H15ÁÁÁCg3
Symmetry transformation used to generate equivalent atoms: F = x,
-0.5-y, 0.5-z, G = x, 0.5-y, 0.5-z (Cg1: N1–C1–N2–C2–C7,
Cg2: C2–C3–C4–C5–C6–C7, Cg3: C9–C10–C11–C10F–C9F–C12,
Cg4: C13–C14–C13G–C15G–C16–C15)
The diversity of structures obtained in this and other
studies results from factors including the conformation of
the ligands and the choice of anions [27]. Complexes
related to 1 and 2 were reported previously, namely
Fig. 3 TG curves of compounds 1 and 2
[Co(L3)(btec)0.5]n (3), [CoCl2(L2)]n(DMF)n/2(CH3OH)n/4
(4), and [CoCl2(L2)]2(DMF)2 (5) (L3 = 1,4-bis(2-methyl-
benzimidazol-1-ylmethyl)benzene, DMF = N,N-dimethyl-
formamide) [28, 29]. In all five complexes 1–5, the flexible
bis(benzimidazole) ligands adopt cis- or trans-conforma-
tions to connect the Co centers, but exhibit different tor-
sional arrangements, leading to a variety of different non-
bonding CoÁÁÁCo distances and distinct geometries. Table 4
compares the geometries obtained in these complexes.
Complex 2 displays a 1D ladder-like chain structure, while
4 features 1D linear chain, and 5 is a dinuclear complex.
The different structures can be attributed to distinct con-
formations of the L2 in complexes 4 and 5.
L3), these results reveal that the conformations of such
flexible bis(benzimidazole) ligands have an important
effect on the final structures of the complexes.
Thermal properties and XRPD analysis
Thermogravimetric analysis (TGA) experiments were car-
ried out to measure the thermal stabilities of complexes 1
and 2, as shown in Fig. 3. There is only one weight-loss
stage for complex 1, but a two-step weight-loss process
was observed for complex 2. For complex 1, a rapid weight
loss in the range of 390–565 °C corresponds to the
decomposition of all organic ligand, and the remaining
residue is CoO (calcd: 13.6 %; found: 14.3 %). The TGA
curve of complex 2 shows that the compound begins to lose
weight above 20 °C. Water is lost steadily, giving an
overall loss of 2.8 % at 160 °C (calcd: 3.1 %). The weight
loss continues from 351 to 502 °C, and the final residue is
CoO (calcd: 12.9 %; found: 13.2 %).
In complexes 1 and 3, the Co atom displays four-coor-
dinated environment connecting btec4- and different flex-
ible bis(benzimidazole) ligands (L1 or L3) to perform two
kinds of 2D layer structures. While 1 and 3 show different
configurations (trigonal pyramid for 1, tetrahedron for 3),
which may result from the different conformations of the
flexible bis(benzimidazole) ligands (cis for L1, trans for
Table 4 The comparison of the related complexes of complex 1 and 2
The related complexes
Coordination Conformation Crystal
number
CoÁÁÁCo
Geometry
References
˚
separation (A)
structure
[Co(L1)(btec)0.5]n(1)
4
6
cis
cis
2D layer
11.290(1)
Trigonal pyramidal
Octahedral
This work
This work
{[Co(L2)(bdc)]ÁH2O}n(2)
1D ladder-like 12.512(8)
chain
[Co(L3)(btec)0.5]n(3)
4
4
4
trans
trans
cis
2D layer
1D linear chain 13.1252(6)
0D loop 10.4243(7)
12.5255(3)
Tetrahedron
Tetrahedral
Tetrahedral
[28]
[29]
[29]
[CoCl2(L2)]n(DMF)n/2(CH3OH)n/4(4)
[CoCl2(L2)]2(DMF)2(5)
where L1 = 1,3-bis(2-methylbenzimidazol-1-ylmethyl)benzene, L2 = 1,3-bis(benzimidazol-1-ylmethyl)benzene, L3 = 1,4-bis(2-methylbenzi-
midazol-1-ylmethyl)benzene, btec = 1,2,4,5-benzenetetracarboxylate, bdc = 1,3-benzenedicarboxylate, DMF = N,N-dimethylformamide
123

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Transition Met Chem (2014) 39:461–467
Fig. 5 a Solid-state photoluminescence spectra of free L1 ligand and
1. b Solid-state photoluminescence spectra of free L2 ligand and 2
Fig. 4 a X-ray powder diffraction patterns of 1. b X-ray powder
diffraction patterns of 2
Complexes 1 and 2 were also characterized by X-ray
powder diffraction (XRPD) to confirm the phase purity of
the bulk materials. The experimental and computer-simu-
lated XRPD patterns of the complexes are shown in Fig. 4.
The XRPD patterns of both as-synthesized products closely
match the simulated ones from the single-crystal data,
indicating that both complexes are in a pure phase.
300 nm. For complex 1, the red shift of the emission peak, by
55 nm compared to free ligand L1, probably arises from
ligand-centered p–p* electronic transitions within the
benzimidazole rings of the L1 ligand [33]. Compared with
the free ligand L2, the emission maximum of complex 2 is
blue-shifted, which may be assigned to a ligand-to-metal
charge transfer (LMCT) transition.
Photoluminescence properties
Conclusion
The photoluminescence properties of complexes 1 and 2, and
the free ligands L1 and L2 have been recorded in the solid
state at room temperature (Fig. 5). The free ligands L1 and
L2 display emission maxima at 305 and 345 nm, respec-
tively (kex = 290 nm for L1, and kex = 280 nm for L2),
which can be assigned to p ? p* transitions [30–32]. An
emission peak can be observed at about 360 nm for complex
1 and 316 nm for 2, when both complexes are excited at
In summary, two Co(II) coordination polymers have been
hydrothermally synthesized and structurally characterized.
The structural differences between complexes 1 and 2
probably arise from the different conformations (cis or
trans) of the flexible bis(benzimidazole) ligand and the role
played by the different anionic ligands in the assembly of
the resulting frameworks.
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Transition Met Chem (2014) 39:461–467
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