Syntheses, structures, and properties of transition metal complexes with 2-(n-pyridyl)benzimidazole (n=2, 3, and 4)

Article abstract of DOI:10.1080/00958972.2011.635375

Three pyridylbenzimidazoles (2-PBIM, 3-PBIM, and 4-PBIM) have been prepared (2-PBIM: 2-(2-pyridyl)-benzimidazole, 3-PBIM: 2-(3-pyridyl)-benzimidazole, 4-PBIM: 2-(4-pyridyl)-benzimidazole). Reactions of several transition metals (Cd²⁺, Cu²⁺, Fe²⁺) with the three ligands gave four new coordination complexes, [(Cd)₂(2-PBIM)₂(CH ₃COO)₄] (1), [Cu(3-PBIM)₂ (CH ₃COO)₂] · 2H₂O (2), [Cu(4-PBIM) ₂(CH₃COO)₂(H₂O)] · H ₂O (3), and [Fe(4-PBIM)₂ (Cl)₂(H ₂O)₂] (4), respectively. These four complexes have been characterized by X-ray crystallography, IR spectroscopy, and UV absorption spectroscopy. Thermogravimetric properties of 2 and 4 were also measured. X-ray crystallographic studies reveal that these four complexes are very different, although the ligands are similar in structure. The role of hydrogen-bonding and π-π interactions in extending dimensionality of simple complexes has been discussed.

Full text of DOI:10.1080/00958972.2011.635375

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Syntheses, structures, and properties
of transition metal complexes with 2-(
n -pyridyl)benzimidazole ( n ꢀ=ꢀ2, 3, and
4)
Ying-Xia Zhou a , Xia Li b , Hai-Yan Zhang a , Cai-Ling Fan a , Hong-
Yun Zhang b & Ben-Lai Wu a b
a College of Sciences, Henan Agricultural University , Zhengzhou
450002 , People's Republic of China
b Department of Chemistry , Zhengzhou University , Zhengzhou
450052 , People's Republic of China
Published online: 15 Nov 2011.
To cite this article: Ying-Xia Zhou , Xia Li , Hai-Yan Zhang , Cai-Ling Fan , Hong-Yun Zhang & Ben-
Lai Wu (2011) Syntheses, structures, and properties of transition metal complexes with 2-( n -
pyridyl)benzimidazole ( n ꢀ=ꢀ2, 3, and 4), Journal of Coordination Chemistry, 64:23, 4066-4078, DOI:
10.1080/00958972.2011.635375
To link to this article: http://dx.doi.org/10.1080/00958972.2011.635375
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Journal of Coordination Chemistry
Vol. 64, No. 23, 10 December 2011, 4066–4078
Syntheses, structures, and properties of transition metal
complexes with 2-(n-pyridyl)benzimidazole (n ^ 2, 3, and 4)
YING-XIA ZHOUy, XIA LIz, HAI-YAN ZHANGy, CAI-LING FANy,
HONG-YUN ZHANG*z and BEN-LAI WU*z
yCollege of Sciences, Henan Agricultural University, Zhengzhou 450002,
People’s Republic of China
zDepartment of Chemistry, Zhengzhou University, Zhengzhou 450052,
People’s Republic of China
(Received 29 September 2010; in final form 14 October 2011)
Three pyridylbenzimidazoles (2-PBIM, 3-PBIM, and 4-PBIM) have been prepared (2-PBIM: 2-
(2-pyridyl)-benzimidazole, 3-PBIM: 2-(3-pyridyl)-benzimidazole, 4-PBIM: 2-(4-pyridyl)-benz-
imidazole). Reactions of several transition metals (Cd2þ, Cu2þ, Fe2þ) with the three ligands
gave four new coordination complexes, [(Cd)2(2-PBIM)2(CH3COO)4] (1), [Cu(3-PBIM)2
(CH3COO)2] ꢀ 2H2O (2), [Cu(4-PBIM)2(CH3COO)2(H2O)] ꢀ H2O (3), and [Fe(4-PBIM)2
(Cl)2(H2O)2] (4), respectively. These four complexes have been characterized by X-ray
crystallography, IR spectroscopy, and UV absorption spectroscopy. Thermogravimetric
properties of 2 and 4 were also measured. X-ray crystallographic studies reveal that these
four complexes are very different, although the ligands are similar in structure. The role of
hydrogen-bonding and ꢀ–ꢀ interactions in extending dimensionality of simple complexes has
been discussed.
Keywords: Pyridylbenzimidazole; Complex; Crystal structure; Hydrogen bond; ꢀ–ꢀ Interaction
1. Introduction
Pyridylbenzimidazole has three N-donors and can act as a neutral mono- or bidentate
ligand and as an anionic tridentate ligand. Pyridylbenzimidazole has tunable coordi-
nation modes and supramolecular interactions because it is a multidentate ligand and
includes hydrogen-bonding donor (benzimidazole group) and acceptors (aromatic N
atoms) [1]. Therefore, chelate complexes or polymers can be obtained with
pyridylbenzimidazoles, and the stability of chelate complexes is much better than that
of monodentate complexes. Thus, this type of N-heterocyclic ligand is a good choice in
constructing metal-organic frameworks with interesting structures and properties.
Many pyridylbenzimidazole complexes have been studied, but most are simple
complexes and few polymers are observed [1]. Coordination architectures with
pyridylbenzimidazoles as ligands are terminal [2–4] with few examples bridging
(scheme 1) [5, 6]. Moreover, this kind of ligand has attracted considerable interest for its
*Corresponding authors. Email: wzhy917@zzu.edu.cn; wbl@zzu.edu.cn
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2011 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2011.635375

2-(n-Pyridyl)benzimidazole
4067
(b)
(a)
M
H
N
N
N
N
N
N
M
M
M
H
N
N
N
N
N
N
M
M
M
H
N
N
N
M
M
N
N
N
Scheme 1. Coordination mode of pyridylbenzimidazole ligands: (a) chelating-terminal-mode and terminal-
mode; (b) chelating-bridging-mode and bridging-mode.
wide-ranging antiviral activities, photochemical and photophysical properties, versatile
coordination modes, and potential to form supramolecular aggregates through ꢀ–ꢀ
stacking and hydrogen-bonding [7–16]. To extend the relevant structural types and
establish synthetic strategies which could control dimensionality of the complexes,
leading to desired organic–inorganic frameworks, we report four transition metal
complexes assembled from pyridylbenzimidazoles, 2-(2-pyridyl)-benzimidazole,
2-(3-pyridyl)-benzimidazole, and 2-(4-pyridyl)-benzimidazole. These three ligands
coordinate differently and provide low-dimensional building units which further
propagate into 3-D supramolecular structures via hydrogen bonds, ꢀ–ꢀ interaction, and
other weak intermolecular interactions. In these four complexes, three pyridylbenzimi-
dazoles are terminal [17, 18].
2. Experimental
2.1. Materials and physical measurements
All reagents and solvents were reagent grade, purchased from Zhengzhou Huafeng
Chemical Reagent Company, and used without purification. Elemental analysis was
carried out on a PE 1700 CHN auto elemental analyzer and IR spectra were recorded
with an FTS-40 infrared spectrophotometer as KBr pellets from 4000 to 400 cmꢁ1
.
Ultraviolet absorption spectra were measured using an Agilent 8453 spectrophot-
ometer. Thermal decomposition experiments were carried out by using a Netzsch STA
409 PC/PꢁG1 instrument from room temperature to about 700ꢂC at a heating rate of
10ꢂCꢀmin in air.

4068
Y.-X. Zhou et al.
Table 1. Crystal data and structure refinement for 1–4.
1
2
3
4
Empirical formula
Formula weight
Temperature (K)
C16H15N3O4Cd C28H28N6O6Cu C28H28N6O6Cu C24H22Cl2N6O2Fe
425.71
293(2)
608.14
291(2)
608.10
291(2)
553.23
291(2)
˚
Wavelength (A)
0.71073
Monoclinic
P2(1)/c
0.71073
Monoclinic
P2(1)/c
0.71073
Triclinic
ꢀ
P1
0.71073
Triclinic
ꢀ
P1
Crystal system
Space group
Unit cell dimensions (A, )
ꢂ
˚
a
b
c
ꢂ
ꢃ
ꢄ
9.8337(7)
21.4801(15)
7.92 57(6)
90.0
108.0690(10)
90.0
8.879(16)
9.2105(16)
18.354(3)
90.0
99.969(2)
90.0
1478.4(4), 2
1.447
0.798
7.787(16)
12.77(3)
14.46(3)
83.62(3)
77.33(3)
82.85(3)
1387(5), 2
1.456
7.019(6)
9.215(7)
9.360(8)
76.127(9)
74.557(9)
86.709(9)
566.5(8), 1
1.622
3
˚
Volume (A ), Z
1591.6(2), 4
1.777
Calculated density (Mg mꢁ3
)
Absorption coefficient (mmꢁ1) 1.398
0.841
630
0.939
284
F(000)
Crystal size (mm3)
ꢅ range for data collection (ꢂ) 2.38–27.50
848
670
0.18 ꢃ 0.11 ꢃ 0.09 0.18 ꢃ 0.17 ꢃ 0.14 0.26 ꢃ 0.19 ꢃ 0.11 0.19 ꢃ 0.09 ꢃ 0.08
2.33–25.50
2.70–25.50
2.32–27.49
ꢁ9 ꢄ h ꢄ 9;
ꢁ11 ꢄ k ꢄ 11;
ꢁ12 ꢄ l ꢄ 12
4927/2522
Limiting indices
ꢁ12 ꢄ h ꢄ 5;
ꢁ27 ꢄ k ꢄ 27;
ꢁ10 ꢄ l ꢄ 9
9591/3645
ꢁ10 ꢄ h ꢄ 10;
ꢁ11 ꢄ k ꢄ 7;
ꢁ22 ꢄ l ꢄ 21
8062/2689
ꢁ9 ꢄ h ꢄ 9;
ꢁ15 ꢄ k ꢄ 15;
ꢁ17 ꢄ l ꢄ 17
10,250/5114
Reflections collected/unique
[R(int) ¼ 0.0251] [R(int) ¼ 0.0391] [R(int) ¼ 0.0333] [R(int) ¼ 0.0275]
Data/restraints/parameters
3645/0/219
1.021
0.0253
2689/6/197
1.036
0.0548
5114/0/380
1.059
0.0499
2522/3/160
1.016
0.0444
Goodness-of-fit on F 2
R1
wR2
0.0578
0.1723
0.1341
0.1039
ꢁ3
˚
Dꢆmin and Dꢆmax (e A
)
0.414 and ꢁ0.475 0.382 and ꢁ0.304 0.324 and ꢁ0.737 0.463 and ꢁ0.426
2.2. X-ray crystallography
Suitable single crystals of 1–4 were chosen for data collection. Crystal data collection
was performed using graphite monochromated Mo-Ka (ꢁ ¼ 0.071073 nm) radiation on
a Rigaku Raxis-IV X-ray diffractometer at 293(2) K for 1 and 291(2) K for 2, 3, and 4.
The structures were solved by direct methods with SHELXS-97 and refined on F2 by
full-matrix least-squares using SHELXL-97 program package [19]. All non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were located and included at
their calculated positions. Details of the crystal structure determinations and structure
refinement for 1–4 are summarized in table 1, and selected bond lengths and angles are
listed in tables 2 and 3, respectively. Full atomic data are available as a file in
crystallographic information file (CIF) format.
2.3. Syntheses of 2-PBIM, 3-PBIM, and 4-PBIM
2-PBIM was synthesized following a modified literature method [20, 21] and the
synthesis procedure was modified. 2-Pyridinecarboxylic acid (60 mmol), PPA (polypho-
sphoric acid) (25 g), and glycol (70 mL) were added to a four-necked round-bottomed
Fask (250 mL) with a reflux condenser. Then, the mixture was heated and
o-phenylenediamine (60 mmol) was added to the Fask with vigorous stirring at 80ꢂC.

2-(n-Pyridyl)benzimidazole
4069
˚
Table 2. Selected bond distances (A) for 1–4.
1
2
3
4
Cd(1)–N(2)
Cd(1)–O(1A) 2.3611(17) Cu(1)–N(1)
2.3116(19) Cu(1)–O(4)
1.937(3) Cu(1)–O(3)
2.041(4) Cu(1)–N(4)
Cu(1)–O(4A) 1.937(3) Cu(1)–O(2)
2.5497(18) Cu(1)–N(1A) 2.041(4) Cu(1)–N(3)
1.937(4) Fe(1)–O(1A) 2.159(2)
2.024(5) Fe(1)–N(1)
1.936(4) Fe(1)–Cl(1)
2.035(5) Fe(1)–O(1)
2.216(3)
2.5503(15)
2.159(2)
Cd(1)–O(3)
Cd(1)–O(2)
Cd(1)–O(4)
Cd(1)–N(1)
Cd(1)–O(1)
2.463(2)
2.293(2) O(1)–H(2W) 0.8621
2.403(2) O(2)–H(3W) 0.8381
2.4063(19) O(3)–C(1)
Cu(1)–O(5)
O(1)–C(26)
1.221(7) O(2)–C(26)
2.262(4) Fe(1)–N(1A) 2.216(3)
1.240(4) Fe(1)–Cl(1A) 2.5502(15)
1.277(5) O(1)–H(1W) 0.8261
O(1)–C(13)
O(2)–C(13)
O(3)–C(15)
O(4)–C(15)
1.264(3)
1.245(3)
1.244(4)
1.260(3)
N(1)–C(7)
N(1)–C(3)
N(2)–C(8)
N(2)–C(9)
1.328(5) O(4)–C(28)
1.247(5) N(1)–C(11)
1.345(4)
1.334(3)
1.398(4)
1.370(4)
1.341(6) O(5)–H(2W) 0.8291
1.362(5) O(6)–H(3W) 0.8875
1.385(6) O(6)–H(4W) 0.8408
N(2)–C(7)
N(2)–C(1)
N(3)–C(7)
N(1)–C(1)
N(1)–C(5)
N(2)–C(6)
N(2)–C(7)
N(3)–C(6)
N(3)–C(12)
1.337(3)
1.350(3)
1.319(3)
1.385(3)
1.358(3)
1.385(3)
N(2)–H(2)
N(3)–C(8)
N(3)–C(10)
0.8600 N(1)–C(6)
1.299(6) N(1)–C(7)
1.409(5) N(1)–H(1N) 0.890(4)
N(2)–C(6)
N(2)–C(8)
1.356(5) N(3)–C(6)
1.367(5) N(3)–H(3N)
1.377(3)
0.8600
1.323(5)
1.387(5)
The mixed solution was kept stirring at 180–200ꢂC for 4 h, turning deep green. The
mixture was cooled, 100 mL of H2O added to the reaction solution at 100ꢂC, and the
pH of the solution adjusted to about 7.0 with aqueous solution of Na2CO3. Gray-green
precipitate appeared, was collected by filtration, and recrystallized with 95% ethanol,
giving gray needle-like crystals of 2-PBIM with 36.2% yield based on o-phenylenedia-
mine. M.p.: 222–224ꢂC. Elemental Anal. (%): C, 73.75; H, 4.55; N, 21.36 (Calcd (%):
C, 73.83; H, 4.65; N, 21.52). IR (KBr, cmꢁ1): 3065(m), 2872(m), 1587(s), 1457(m),
1385(s), 1144(m), 750(s), 696(s).
The synthesis procedure of 3-PBIM and 4-PBIM is similar to that of 2-PBIM. Yellow
needle-like crystals of 3-PBIM (1.5 g) were obtained. Yield based on o-phenylenedia-
mine was 39.5%. M.p.: 216–218ꢂC. Elemental Anal. (%): C, 73.72; H, 4.56; N, 21.38
(Calcd (%): C, 73.83; H, 4.65; N, 21.52). IR (KBr, cmꢁ1): 3051(m), 1625(m), 1581(m),
1442(s), 1317(m), 1278(m), 1117(m), 815(m), 744(s), 705(m). Yellow needle-like crystals
of 4-PBIM (1.6 g) were obtained. Yield based on o-phenylenediamine was 53.0%. M.p.:
219–222ꢂC. Elemental Anal. (%): C, 74.03; H, 4.52; N, 21.29 (Calcd (%): C, 73.83; H,
4.65; N, 21.52). IR (KBr, cmꢁ1): 3055(w), 1609(s), 1536(w), 1434(s), 1317(m), 1282(m),
1234(m), 1001(s), 839(m), 745(s), 693(m).
2.4. Syntheses of 1–4
2.4.1. [(Cd)2(2-PBIM)2(CH3COO)4] (1). Solution of Cd(CH3COO)2 ꢀ 2H2O (0.27 g,
1.0 mmol) in DMF (10 mL) was added dropwise to a solution of 2-PBIM (0.2 g,
1.0 mmol) dissolved in water (10 mL) while vigorously stirring, and the mixed solution
was continuously stirred for 20 min, then sealed in a 15 mL TeFon-lined steel reactor.
The steel reactor was heated in an oven at 140ꢂC for 72 h, then cooled to room
temperature at a rate of 10ꢂCꢀhꢁ1. The mixed solution was filtered and the Eltrate was
allowed to evaporate at room temperature. Colorless block crystals of 1 suitable for

4070
Y.-X. Zhou et al.
Table 3. Selected bond angles (ꢂ) for 1–4.
1
2
O(4)–Cd(1)–N(2)
O(4)–Cd(1)–O(1A)
N(2)–Cd(1)–O(1A)
O(4)–Cd(1)–N(1)
N(2)–Cd(1)–N(1)
O(1A)–Cd(1)–N(1)
O(4)–Cd(1)–O(1)
N(2)–Cd(1)–O(1)
O(1A)–Cd(1)–O(1)
N(1)–Cd(1)–O(1)
O(4)–Cd(1)–O(3)
N(2)–Cd(1)–O(3)
O(1A)–Cd(1)–O(3)
N(1)–Cd(1)–O(3)
O(4)–Cd(1)–O(2)
N(2)–Cd(1)–O(2)
O(1A)–Cd(1)–O(2)
N(1)–Cd(1)–O(2)
O(1)–Cd(1)–O(2)
O(3)–Cd(1)–O(2)
98.75(7)
104.29(7)
143.32(7)
169.81(7)
71.16(7)
83.94(7)
90.49(7)
135.76(7)
72.65(8)
97.80(7)
54.87(7)
84.57(7)
85.81(6)
120.96(7)
99.70(7)
84.11(6)
118.88(6)
81.06(6)
51.66(6)
149.95(6)
O(4A)–Cu(1)–O(4)
O(4A)–Cu(1)–N(1)
O(4)–Cu(1)–N(1)
N(1)–Cu(1)–N(1A)
H(1W)–O(1)–H(2W)
H(3W)–O(2)–H(4W)
C(1)–O(4)–Cu(1)
C(7)–N(1)–C(3)
C(7)–N(1)–Cu(1)
C(3)–N(1)–Cu(1)
C(8)–N(2)–C(9)
C(8)–N(2)–H(2)
C(9)–N(2)–H(2)
C(8)–N(3)–C(10)
180.0(3)
89.55(14)
90.45(14)
180.000(1)
119.2
110.4
124.3(4)
117.8(4)
120.2(3)
122.0(3)
105.7(4)
127.1
127.1
105.1(4)
3
4
O(2)–Cu(1)–O(3)
O(2)–Cu(1)–N(4)
O(3)–Cu(1)–N(4)
O(2)–Cu(1)–N(3)
O(3)–Cu(1)–N(3)
N(4)–Cu(1)–N(3)
O(2)–Cu(1)–O(5)
O(3)–Cu(1)–O(5)
N(4)–Cu(1)–O(5)
N(3)–Cu(1)–O(5)
C(26)–O(2)–Cu(1)
C(28)–O(3)–Cu(1)
Cu(1)–O(5)–H(1W)
Cu(1)–O(5)–H(2W)
H(1W)–O(5)–H(2W)
H(3W)–O(6)–H(4W)
179.28(10)
88.04(12)
91.55(12)
88.64(13)
91.64(13)
168.03(12)
89.92(19)
90.72(19)
97.18(11)
94.31(12)
123.1(3)
125.6(3)
116.9
O(1)–Fe(1)–O(1A)
O(1)–Fe(1)–N(1)
O(1)–Fe(1)–N(1A)
N(1)–Fe(1)–N(1A)
N(1)–Fe(1)–Cl(1A)
O(1)–Fe(1)–Cl(1)
O(1A)–Fe(1)–Cl(1)
N(1)–Fe(1)–Cl(1)
Cl(1A)–Fe(1)–Cl(1)
179.999(2)
91.60(9)
88.40(9)
180.00(12)
89.63(8)
86.00(8)
94.00(8)
90.37(8)
180.0
130.3
110.2
110.1
Symmetry transformations used to generate equivalent atoms: A ꢁx þ 1, ꢁy þ 1, ꢁz þ 1 for 1 and 2;
A ꢁx, ꢁy þ 2, ꢁz þ 2 for 4.
single-crystal X-ray diffraction analysis were obtained after 3 days with 23% yield
based on 2-PBIM. IR (cmꢁ1, KBr): 3002(w), 2872(m), 1557(s), 1529(m), 1436(s),
1420(s), 1157(m), 1013(m), 750(s), 670(s).
.
2.4.2. [Cu(3-PBIM)2(CH3COO)2] 2H2O (2). A solution of Cu(CH3COO)2 ꢀ H2O
(0.06 g, 0.3 mmol) in methanol (10 mL) was added dropwise to a solution of 3-PBIM
(0.06 g, 0.3 mmol) dissolved in ethanol (10 mL) with stirring at room temperature for
60 min; a little blue precipitate appeared after 30 min. The mixture was filtered and the
Eltrate was allowed to evaporate at room temperature. Blue block single crystals of 2
were obtained after about 30 days, filtered, washed with a small amount of methanol,

2-(n-Pyridyl)benzimidazole
4071
and air-dried. Yield 0.13 g (46%) based on 3-PBIM. IR (KBr, cmꢁ1): 2926(w), 1565(s),
1446(s), 1407(s), 1319(w), 1195(m), 1011(w), 818(m), 749(m), 697(m).
.
2.4.3. [Cu(4-PBIM)2(CH3COO)2(H2O)] H2O (3) and [Fe(4-PBIM)2(Cl)2(H2O)2]
(4). The syntheses of 3 and 4 are similar to that of 2. Blue block crystals for 3 and
yellow acerose crystals for 4 were obtained with 42% yield for 3 and 51% yield for 4
based on 4-PBIM. IR for 3 (KBr, cmꢁ1): 3060(w), 1618(s), 1562(s), 1533(m), 1433(s),
1412(m), 1318(s), 1116(w), 1025(m), 814(m), 743(s). IR for 4 (KBr, cmꢁ1): 3097(w),
1614(s), 1542(w), 1498(w), 1430(m), 1317(m), 1234(m), 1114(w), 1014(m), 835(m),
745(s), 700(w).
3. Results and discussion
3.1. Crystal structures of 1–4
3.1.1. [(Cd)2(2-PBIM)2(CH3COO)4] (1). Crystal structure parameters, selected bond
lengths, and angles of 1 are listed in tables 1–3, respectively. The coordination geometry
and atom labeling in the crystal structure of 1 are shown in figure 1. Complex 1 is
binuclear, [(Cd)2(2-PBIM)2(CH3COO)4], with two equivalent structural units corre-
sponding to one-half of the dimer. In 1, the coordination environment of two
symmetrical Cd2þ is the same, and two Cd2þ are bridged by two CH3COOꢁ with each
Cd2þ seven-coordinate with five oxygen atoms from three CH3COOꢁ and two nitrogen
atoms from one 2-PBIM. The coordination sphere around Cd2þ can be described as a
distorted pentagonal bipyramid. Acetate is flexible and can possess many coordination
modes, roughly classified into five types: monodentate, bidentate-chelating, bidentate-
bridging, tridentate-bridging-chelating, and tetradentate-bridging-chelating [22, 23]. In
1, CH3COOꢁ is tridentate-bridging-chelating with two Cd2þ ions connected into a
dimeric structure by two tridentate carboxylates and bidentate-chelating with two
oxygen atoms of a carboxylate coordinated to the same Cd2þ. In 1, the dimeric
structure is constrained by a crystallographic centre of inversion that lies at the centre of
the Cd2O2 core and the quadrilateral Cd(1)O(1)Cd(1A)O(1A) is a parallelogram. In 1,
two 2-PBIM are chelating terminal, forming two five-membered chelate rings. The
chelating mode of 2-PBIM results from suitable location of two nitrogen atoms, in
contrast to 3-PBIM (in 2) and 4-PBIM (in 3 and 4) where the chelate ring formation is
impossible.
In the crystal structure of 1, there is one hydrogen bond (table 4), the intermolecular
hydrogen bond of the non-coordinating N–H group from imidazole ring and oxygen of
0
˚
chelated carboxyl from adjacent structural units, N(3)–H(1 ) ꢀ ꢀ ꢀ O(2B) ¼ 2.724(3) A.
The structural units are linked into a 1-D chain by this intermolecular hydrogen, and
the 1-D chains are stacked in parallel.
In 1, 2-PBIM is almost in a plane with face-to-face distances between pyridine
˚
rings and benzene rings of neighboring units of 3.7087(2) A, indicating ꢀ–ꢀ
stacking interactions. Another kind of ꢀ–ꢀ stacking interactions exist inter-chain; the
face-to-face distances between pyridine rings of 2-PBIM in adjacent chains are

4072
Y.-X. Zhou et al.
Figure 1. The molecular structure of 1.
˚
Table 4. Hydrogen-bond distances for 1–4 (A)
D–H ꢀ ꢀ ꢀ A
d(D ꢀ ꢀ ꢀ A)
D–H ꢀ ꢀ ꢀ A
d(D ꢀ ꢀ ꢀ A)
Complex 1
N(3)–H(10) ꢀ ꢀ ꢀ O(2B)
Complex 2
N(2)–H(2) ꢀ ꢀ ꢀ O(1B)
O(2)–H(4W) ꢀ ꢀ ꢀ O(1A)
O(2)–H(3W) ꢀ ꢀ ꢀ N(3A)
O(1)–H(2W) ꢀ ꢀ ꢀ O(3)
O(1)–H(1W) ꢀ ꢀ ꢀ N(2C)
Complex 3
O(6)–H(4W) ꢀ ꢀ ꢀ O(4)
O(5)–H(2W) ꢀ ꢀ ꢀ N(2A)
O(5)–H(1W) ꢀ ꢀ ꢀ O(1B)
N(6)–H(6N) ꢀ ꢀ ꢀ O(1C)
N(1)–H(1N) ꢀ ꢀ ꢀ O(4D)
Complex 4
O(1)–H(2W) ꢀ ꢀ ꢀ Cl(1B)
O(1)–H(1W) ꢀ ꢀ ꢀ N(2C)
N(3)–H(3N) ꢀ ꢀ ꢀ Cl(1D)
2.724(3)
2.889(7)
2.864(7)
2.766(7)
2.789(7)
2.836(7)
2.793(5)
2.763(6)
2.735(6)
2.542(7)
2.793(5)
3.571(4)
2.812(3)
3.196(3)
Symmetry transformations used to generate equivalent atoms: A ꢁx þ 1, ꢁy þ 1, ꢁz þ 1; B ꢁx,
ꢁy þ 1, ꢁz (for 1), A ꢁx þ 1, ꢁy þ 1, ꢁz þ 1; B x þ 1, y, z; C xꢁ1, y, z (for 2), A ꢁx þ 1, ꢁy þ 2,
ꢁz þ 2; B xþ1, y, z; C ꢁx þ 1, ꢁy þ 2, ꢁz þ 1; D ꢁx þ 1, ꢁy þ 1, ꢁz þ 2 (for 3), A ꢁx, ꢁy þ 2,
ꢁz þ 2; B ꢁx þ 1, ꢁy þ 2, ꢁz þ 2; C x, y þ 1, z; D x, y, z ꢁ 1 (for 4).
˚
3.5071(2) A. These ꢀ–ꢀ stacking interactions and the intermolecular hydrogen
bond mentioned above play an important role in the 3-D supramolecular structures of 1.
.
3.1.2. [Cu(3-PBIM)2(CH3COO)2] 2H2O (2). Complex 2 is obtained by reaction of 3-
PBIM and Cu(CH3COO)2 ꢀ H2O in methanol; 2 is a discrete monomer. The structure
consists of one Cu(II) cation, two 3-PBIM, two CH3COOꢁ, and two lattice water
molecules, as shown in figure 2. Complex 2 has Cu(II)N2O2 conEguration with
approximate diamond-shaped quadrilateral geometry with Cu(1)–O(4A) ¼ Cu(1)–
˚
˚
O(4) ¼ 1.937(3) A and Cu(1)–N(1) ¼ Cu(1)–N(1A) ¼ 2.041(4) A (listed in table 2), two
nitrogen atoms from 3-PBIM and two oxygen atoms from CH3COOꢁ located at the

2-(n-Pyridyl)benzimidazole
4073
Figure 2. The molecular structure of 2.
four corners of quadrilateral, two H2O molecules do not participate in coordination. In
2, CH3COOꢁ is monodentate with trans-conformations and two 3-PBIM are terminal
because it is impossible for 3-PBIM to form a chelate ring.
Lattice water molecules are connected with uncoordinated nitrogen atoms (from
imine and amidogen) of 3-PBIM and oxygen atoms of coordinated CH3COOꢁ to form
a complicated hydrogen-bonding network (table 4). As a result, the structural units of 2
extend into a 2-D supramolecular framework, and the 2-D networks extend to a 3-D
supramolecule by other weaker interactions.
.
3.1.3. [Cu(4-PBIM)2(CH3COO)2(H2O)] H2O (3). The structure unit of 3 is shown in
figure 3. Cu2þ is five-coordinate by three oxygen atoms from two CH3COOꢁ and one
coordinated water and two nitrogen atoms of 4-PBIM, forming a distorted tetragonal
pyramid; another water molecule does not participate in coordination. In 3, pyridine
rings and imidazole rings of 4-PBIM are not coplanar, with dihedral angle of 2.7ꢂ for
rings C(1)C(2)C(3)C(4)C(5)N(3) and N(1)C(6)N(2)C(7)C(8), and 11.3ꢂ for rings
C(13)C(14)C(15)C(16)C(17)N(4) and N(5)C(18)N(6)C(19)C(20). The different angles
between pyridine and imidazole are perhaps because formation of different hydrogen
bonds of N(1), N(2), N(5) and N(6) lead to distortion of 4-PBIM. The two coordinated
CH3COOꢁ anions are monodentate and cis, different from the trans-geometry in 2.
4-PBIM in 3 are terminal, similar to 3-PBIM in 2.
In the crystal structure of 3, there are five types of hydrogen bonds (table 4).
Coordination water and lattice water with the uncoordinated oxygen of the carboxyl and
nitrogen (from imine or amidogen) form a complicated hydrogen-bonding network. The
units of 3 extended into a 3-D supramolecule by this hydrogen bond network.
3.1.4. [Fe(4-PBIM)2(Cl)2(H2O)2] (4). Complex 4 is also a simple complex, similar to 2
and 3. As shown in figure 4, each Fe(II) is six-coordinate by two nitrogen atoms of
pyridine from the two 4-PBIM, two Clꢁ, and two oxygen atoms of coordinated water.
The coordination geometry of Fe(II) can be described as an elongated octahedron. Two
4-PBIM coordinate terminal to one Fe(II) through pyridine as they do in 3. In 4,
pyridine rings and imidazole rings from 4-PBIM are not coplanar, with dihedral angle
of 2.8ꢂ. The molecular structure of 4 is similar to the Zn(II) complex [24].

4074
Y.-X. Zhou et al.
Figure 3. The molecular structure of 3.
Figure 4. Structure unit drawing of 4.
Hydrogen bonds exist in 4, too (table 4). Coordinated water with imine of 4-PBIM
˚
from neighboring units form hydrogen bonds O(1)–H(1W) ꢀ ꢀ ꢀ N(2C) of 2.812(3) A, the
amidogen N of imidazole from 4-PBIM with Clꢁ of neighboring units form weaꢁk
˚
hydrogen bonds, N(3)–H(3N) ꢀ ꢀ ꢀ Cl(1D) of 3.196(3) A, and coordinated water with Cl
of adjacent units form another weaker hydrogen bond, O(1)–H(2W) ꢀ ꢀ ꢀ Cl(1B) of
˚
3.571(4) A. The lattice construction of 4 can be presented in three stages: the shortest
˚
hydrogen bond (2.812 A) links complex molecules into a ribbon expanding along the
˚
b-axis, another hydrogen bond (3.196(3) A) joins the ribbons into a plane parallel to
˚
(1 0 0), and the last hydrogen bond (3.571(4) A) connects the planes into a 3-D structure.
˚
ꢀ–ꢀ stacking interactions are also observed with centroid-to-centroid distance of 3.5 A
and the deviation angle of 2.7ꢂ between pyridyl and benzene rings of neighbouring
units, indicating weak twofold ꢀ–ꢀ interactions, and the weak ꢀ–ꢀ stacking interactions
made the 2-D frameworks more stable.
As described above, all four complexes are discrete simple complexes and the
pyridylbenzimidazole ligands are terminal. Each crystal is a supermolecule [24–29] and
the crystal-packing mode of 1–4 has been analyzed with construction strongly
depending on hydrogen bonds, ꢀ–ꢀ interactions, and other weaker intermolecule
interactions. Hydrogen bonds are the main forces holding subunits together and
sustaining the crystal framework, while ꢀ–ꢀ interactions play important roles to direct
the stacking fashion [24].
3.2. UV spectra analysis of 1–4
UV absorption spectra for 2-PBIM, 3-PBIM, 4-PBIM, and 1–4 (10ꢁ5–10ꢁ6 molꢀLꢁ1 in
methanol) are shown in figures 5 and 6. They all exhibit absorptions from 200 to 400 nm

2-(n-Pyridyl)benzimidazole
4075
Figure 5. UV absorption spectra of 2-PBIM, 3-PBIM, 1 and 2 in methanol (10ꢁ5–10ꢁ6 mol ꢀ Lꢁ1; dashed
line, 2-PBIM; dash-dotted line, 1; solid line, 3-PBIM; dotted line, 2).
Figure 6. UV absorption spectra of 4-PBIM, 3 and 4 in methanol (10ꢁ5–10ꢁ6 mol ꢀ Lꢁ1; solid line, 4-PBIM;
dashed line, 3; dotted line, 4).
with maximum absorptions at 225, 250, and 310 nm, respectively, attributed to n ! ꢀ*
and ꢀ ! ꢀ* transitions of pyridine and benzene. Compared with the free ligands, the
absorption bands of 1–4 are red-shifted (figures 5 and 6). The conjugated system is
expanded due to coordination of metal, leading to energy reduction for n ! ꢀ* and
ꢀ ! ꢀ* transitions [30]. Thus, displacement of electron clouds from pyridine or benzene
rings to metal ions occur, causing red shift of n ! ꢀ* and ꢀ ! ꢀ* transition.
3.3. Thermogravimetric analysis of 2 and 4
Thermogravimetric (TG) analysis in air with a heating rate of 10ꢂCꢀminꢁ1 was
performed on polycrystalline samples for 2 and 4 (figures 7 and 8).

4076
Y.-X. Zhou et al.
Figure 7. TG-DSC curves of 2.
Figure 8. TG-DSC curves of 4.
For 2, the weight loss of 7.2% before 130ꢂC is equivalent to loss of two lattice water
molecules per formula unit (Calcd: 5.9%). Two major weight losses occur after 200ꢂC.
The first from 130ꢂC to 250ꢂC with DSC peak at 209.8ꢂC is due to loss of two
coordinated CH3COOꢁ (calculated mass loss of 19.4% observed, 18.1%). The next
transition from 250ꢂC to 530ꢂC is a consecutive complicated process containing two
contiguous DSC peaks at 445.3ꢂC and 471.0ꢂC. This thermal event is due to
decomposition of two 3-PBIM molecules and is accompanied by the formation of CuO.
The process at 400–530ꢂC is related to gradual elimination of carbon resulting from
decomposition in air [31]. The total mass loss to 600ꢂC is 86.25%, which agrees with the
theoretical value (87.0%) calculated by taking CuO as the final products.

2-(n-Pyridyl)benzimidazole
4077
As shown in figures 7 and 8, 4 is more stable than 2. Four transitions appear in the
decomposition process of 4. The first transition ranging from 200ꢂC to 240ꢂC is loss of
6.9%, related to loss of two coordinated water molecules (Calcd: 6.5%). The seconꢁd
transition ranging from 280ꢂC to 340ꢂC is a loss of 6.1%, related to loss of one Cl
(Calcd: 6.4%). The third transition ranging from 340ꢂC to 420ꢂC is loss of 6.2% from
another Clꢁ (Calcd: 6.4%). The last transition of 420ꢂC to 600ꢂC is two 4-PBIM
molecules. The calculated mass loss of 70.6% for the fourth thermal event agrees with
that on TG curve (68.5%). The decomposition residue of 4 is 13.8% revealed by the TG
curve, which should be FeO (Calcd: 13.0%).
4. Conclusions
2-PBIM coordinates terminal-chelating to Cd(II) and 3-PBIM and 4-PBIM coordinate
terminal. The three isomeric ligands (2-PBIM, 3-PBIM and 4-PBIM) coordinate
terminal with metal ions.
Different coordination environments around the metal ions offer distinctive
supramolecular interaction sites for each structural unit. The complex units were
connected by hydrogen-bonding, ꢀ–ꢀ interactions, and other weaker intermolecule
interactions, with hydrogen bonds the main forces holding the subunits together, while
ꢀ–ꢀ interactions normally play important roles to direct the stacking.
Supplementary material
The supplementary crystallographic data for the structural analyses of this article have
been deposited via the Cambridge Crystallographic Data Centre, CCDC Reference
Nos.: 607529 (for 1), 635053 (for 2), 635051 (for 3) and 635052 (for 4). Copies of these
can be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (Fax: þ44-1223-336-033; Email: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk/data_request/cif).
Acknowledgments
We gratefully acknowledge financial support from the National Natural Science
Foundation of China (20771094, 20671083) and the Science and Technology Key Task
of Henan Province (0524270061).
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