Synthesis and characterization of binuclear manganese(IV,IV) and mononuclear cobalt(II) complexes based on 2-(2-hydroxyphenyl)-1H-benzimidazole

Article abstract of DOI:10.1080/00958970903302707

The bidentate benzimidazolic hpbm (1), [2-(2-hydroxyphenyl)-1H-benzimidazole], was obtained under mild conditions, and its corresponding metal complexes, di-μ-oxo dimanganese(IV,IV) [Mn₂O₂(hpbm)₄ · 2Py · 5H₂O] (

Full text of DOI:10.1080/00958970903302707

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Synthesis and characterization of
binuclear manganese(IV,IV) and
mononuclear cobalt(II) complexes
based on 2-(2-hydroxyphenyl)-1H-
benzimidazole
Ming-Yue Duan a , Jun Li a , Yun Xi a , Xiang-Fei Lü a , Jing-Zhou
Liu a , Giuseppe Mele b & Feng-Xing Zhang a
a Department of Chemistry , Key Laboratory of Synthetic and
Natural Functional Molecule Chemistry of Ministry of Education,
Northwest University , Xian, Shaanxi 710069, P.R. China
b Dipartimento di Ingegneria dell’Innovazione , Università del
Salento , Via Arnesano, 73100 Lecce, Italy
Published online: 02 Oct 2009.
To cite this article: Ming-Yue Duan , Jun Li , Yun Xi , Xiang-Fei Lü , Jing-Zhou Liu , Giuseppe
Mele & Feng-Xing Zhang (2010) Synthesis and characterization of binuclear manganese(IV,IV) and
mononuclear cobalt(II) complexes based on 2-(2-hydroxyphenyl)-1H-benzimidazole, Journal of
Coordination Chemistry, 63:1, 90-98, DOI: 10.1080/00958970903302707
To link to this article: http://dx.doi.org/10.1080/00958970903302707
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Journal of Coordination Chemistry
Vol. 63, No. 1, 10 January 2010, 90–98
Synthesis and characterization of binuclear
manganese(IV,IV) and mononuclear cobalt(II)
complexes based on 2-(2-hydroxyphenyl)-1H-benzimidazole
¨
MING-YUE DUANy, JUN LI*y, YUN XIy, XIANG-FEI LUy, JING-ZHOU LIUy,
GIUSEPPE MELEz and FENG-XING ZHANGy
yDepartment of Chemistry, Key Laboratory of Synthetic and Natural
Functional Molecule Chemistry of Ministry of Education,
Northwest University, Xian, Shaanxi 710069, P.R. China
zDipartimento di Ingegneria dell’Innovazione, Universita del Salento,
Via Arnesano 73100 Lecce, Italy
(Received 21 February 2009; in final form 10 July 2009)
The bidentate benzimidazolic hpbm (1), [2-(2-hydroxyphenyl)-1H-benzimidazole], was obtained
under mild conditions, and its corresponding metal complexes, di-m-oxo dimanganese(IV,IV)
[Mn2O2(hpbm)4 ꢀ 2Py ꢀ 5H2O] (2), and mononuclear complex [Co(hpbm)2] (3), were prepared
and characterized. The crystal structures of 2 and 3 have been established by X-ray diffraction.
Complex 2 consists of two six-coordinate manganese(IV) coordinated to two hpbm ligands and
bridged by two O2ꢁ with a Mn–Mn distance of 2.777 A. For 3, a Co(II) is coordinated to two
˚
deprotonated hpbm in a nearly tetrahedral environment. Hydrogen bonds play pivotal
roles in constructing the dimensional structures of both the compounds.
Keywords: 2-(2-Hydroxyphenyl)-1H-benzimidazole; Di-m-oxo dimanganese(IV,IV) complex;
Cobalt(II) complex; Crystal structure
1. Introduction
Metal complexes containing imidazole-based ligands have rich coordination chemistry
and a number of established applications [1–4]. The 2-(2-hydroxyphenyl)benzimidazole
(hpbm) is an N,O-bidentate ligand with two donor groups of relevance to metal centers
in biological systems, phenolate (tyrosine), and imidazole (histidine). Hpbm has been
studied theoretically as laser dyes and fluorescent probes [5, 6], but its metal complexes
are limited, especially crystal structures [5, 7, 8].
We obtained trinuclear Ni(II) and mononuclear Cu(II) complexes with hpbm [9, 10]
and a mononuclear Mn(III) complex with 2-(5-bromine-2-hydroxyphenyl) benzimida-
zole (Br-hpbm) [11]. Hpbm exhibited variable coordination with Ni(II), Cu(II), and
Mn(III). Continuing, we report here the structures of mononuclear Co(II) and
dinuclear Mn(IV,IV) complexes.
*Corresponding author. Email: junli@nwu.edu.cn
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958970903302707

Bidentate Benzimidazole
91
2. Experimental
2.1. Material and physical measurements
The following commercially available chemicals were of analytical grade and used
without puriEcation: MnCl2 ꢀ 4H2O, Co(OAc)2 ꢀ 6H2O, NaOH, CH3CH2OH, pyridine,
salicylic acid, and o-phenylendediamine. Elemental analysis was performed on a Vario
EL-III CHNOS instrument. FT-IR spectra were recorded using KBr from 4000 to
400 cmꢁ1 employing a BEQ UZNDX-550.
2.2. Synthesis of 2-(2-hydroxyphenyl)benzimidazole (hpbm) (1)
Hpbm was synthesized in correspondingly high yield by one-step condensation of
o-phenylenediamine (2.05 g, 19 mmol in 25 mL EtOH) with salicylaldehyde (2.32 g,
19 mmol in 15 mL EtOH) at 80ꢂC under stirring without using any catalyst. The
resulting orange solution was refluxed for 2 h and then cooled to room temperature.
After 12 h of refrigeration (ꢁ4ꢂC), the orange solution was filtered and 15 mL ethyl
ether was added. Needle-shaped orange crystals appeared after 2 days in the open air
and were filtered, washed with ethanol, and recrystallized from methanol. Yield
65%. m.p. ¼ 240–241ꢂC. IR spectrum (KBr; cmꢁ1): 1593[ꢀ(C¼C)], 1488[ꢀ(C¼N)],
1265[ꢀ(C–O)], 2800–2500[H bond]. Anal. Calcd for C13H10N2O (%): C, 74.29; H, 4.76;
N, 13.33. Found (%): C, 74.27; H, 4.59; N, 13.18.
.
2.3. Synthesis of [Mn2O2(hpbm)4 2Py 5H2O] (2)
.
Hpbm (0.210 g, 1 mmol) and NaOH (0.04 g, 1 mmol) were dissolved in 10 mL methanol
and then a solution of MnCl2 ꢀ 4H2O (0.1 g, 0.5 mmol) in methanol was added slowly
under stirring. After 1 h brown precipitate was obtained by filtration, presumed to be
Mn(III)[(hpbm)2Cl]CH3OH according to the elemental analysis. The brownish solid
was dissolved in pyridine and black crystals were formed through ethyl ether vapor
diffusion after 20 days, during which air oxidized the Mn3þ and formed a rare
oxo-bridged binuclear Mn(IV) complex. This reaction indicates that Mn(II) or Mn(III)
can be easily oxidized to Mn(IV) in air when coordinated by hpbm. Yield 15%.
IR spectrum (KBr; cmꢁ1): 1621[ꢀ(C¼N)], 1260[ꢀ(C–O)], 635[ꢀ(Mn–O–Mn)]. Anal.
Calcd for C62H56N10Mn2O11 (%): C, 57.77; H, 7.86; N, 10.82. Found (%): C, 57.81;
H, 7.59; N, 11.01.
2.4. Synthesis of [Co(hpbm)2] (3)
A solution of Co(OAc)2 ꢀ 6H2O (0.5 mmol) in water was added to the solution of
hpbm in ethanol. After the pH was adjusted to 6–7, the mixture was transferred into
a 25 mL PTFE pot after 1 h reaction under stirring. Dark purple crystals were obtained
after 4 days of heating at 160ꢂC under hydrothermal condition with cooling at 10ꢂC hꢁ1
to room temperature. Yield 63%. IR spectrum (KBr; cmꢁ1): 3093[ꢀ(N–H, O–H)],
1603[ꢀ(C¼C)], 1528[ꢀ(C¼N)], 1249[ꢀ(C–O)]. Anal. Calcd for C26H18N4O2Co (%):
C, 65.41; H, 3.77; N, 11.7. Found (%): C, 65.11; H, 3.95; N, 11.94.

92
M.-Y. Duan et al.
2.5. X-ray crystallography
The single crystals of 2 and 3 of suitable size were mounted in a SMART APEX CCD
diffractometer equipped with Mo-Kꢁ radiation. Absorption corrections were applied
using SADABS [12]. The structures were solved by direct methods and reEned by
full-matrix least-squares using SHELXS-97 [13]. Organic hydrogens were generated
in ideal positions. Anisotropic thermal parameters were applied to all non-hydrogen
atoms. Crystallographic parameters and basic information pertaining to data collection
and structure refinement for 2 and 3 are summarized in table 1. Selected bond distances
and angles are given in table 2.
3. Results and discussion
3.1. Synthesis
Traditionally, 2-(2-hydroxyphenyl) benzimidazole (hpbm) is prepared from salicylic
acid and o-phenylenediamine in concentrated polyphospholic acid at 190–210ꢂC in low
yield [14]. Takashi et al. [15] have obtained it via photolysis of [N,N0-o-phenylenebis-
(salicylideneaminato)] diaquamanganese(III) with a 40% yield, however under harsh
conditions. Later Yuka et al. [16] synthesized it with stirring at 120ꢂC in the pres-
ence of activated carbon in xylene. Kiumars et al. [17] employed ceric ammonium
nitrate as catalyst in the presence of H2O2 for the preparation of hpbm and a series
of 2-substituted benzimidazole at 50ꢂC with short reaction times and excellent
yields (above 90%), by far the most efficient method for the synthesis
Table 1. Crystal data and structure reEnement details for 2 and 3.
2
3
Empirical formula
Formula weight
Temperature (K)
Color
Crystal system
Space group
Unit cell dimensions (A,
C62H56N10Mn2O11
1227.05
298(2)
Black
Orthorhombic
Pbcn
C26H18N4O2Co
477.37
273(2)
Dark purple
Monoclinic
P2(1)/n
ꢂ
˚
)
a
b
c
16.520(2)
14.162(2)
24.043(4)
90
5625.2(14), 4
1.449
2544
0.22 ꢃ 0.16 ꢃ 0.11
1.69–25.35
27,869
5142
1.0000
10.1905(19)
10.0322(9)
21.3639(18)
94.3700(10)
2177.7(3), 4
1.456
ꢂ
Volume (A ), Z
3
˚
Calculated density (g cmꢁ3
F(000)
)
980
Crystal size (mm3)
0.22 ꢃ 0.13 ꢃ 0.11
1.91–28.26
13,387
5270
0.976
ꢃ range for data collection (ꢂ)
Reflections collected
Independent reflections
Data completeness
Data/restraints/parameters
R1 [I 4 2ꢄ(I )]
5142/5/413
0.0426
0.0984
5270/0/304
0.0322
0.0891
wR2 [I 4 2ꢄ(I )]
Goodness-of-fit on F2
0.852
1.108

Bidentate Benzimidazole
93
ꢂ
˚
Table 2. Selected bond lengths (A) and angles ( ) of 2 and 3.
2
Mn(1)–O(1)
Mn(1)–N(3)
O(1)–Mn(1)–O(1)#1
O(1)–Mn(1)–O(3)
O(1)#1–Mn(1)–O(3)
O(1)–Mn(1)–O(3)#1
1.811(2)
1.990(2)
80.11(11)
173.11(8)
94.21(8)
94.21(8)
Mn(1)–O(3)
Mn(1)–Mn(2)
O(1)#1–Mn(2)–O(1)
O(1)#1–Mn(2)–O(2)#1
O(1)–Mn(2)–O(2)#1
Mn(1)–O(1)–Mn(2)
1.943(2)
2.7772(9)
79.92(11)
172.39(8)
93.66(8)
99.98(47)
3
Co(1)–O(1)
Co(1)–N(3)
O(1)–Co(1)–O(2)
O(2)–Co(1)–N(3)
1.9154(12)
1.9637(13)
114.09(6)
94.12(5)
Co(1)–O(2)
Co(1)–N(1)
O(1)–Co(1)–N(3)
O(1)–Co(1)–N(1)
1.9190(12)
1.9677(13)
112.40(5)
94.52(5)
Symmetry code #1: ꢁx, y, ꢁz þ 1/2.
H
N
H2N
OH
HN
H
1/2 O2
–H2O
H2N
H2N
CHO
OH
N
N
H
N
+
– H2O
OH
OH
Scheme 1. A possible mechanism forming 2-(2-hydroxyphenyl) benzimidazole (1).
of benzimidazole derivatives. The main advantage of our method is that it is
environmentally benign without any catalyst or other reagent. Schiff bases are prepared
by the condensation of aromatic carboxaldehydes with polyamines. However, closed-
ring production may occur under conditions of proper molar ratio, reaction
temperature, and solvent. The possible reaction mechanism (scheme 1) may be an
intramolecular nucleophilic attack resulting in unsaturated heterocyclic rings and then
the corresponding closed-ring product with the participation of oxygen in the reaction.
Therefore, it is an environmentally friendly, simple, and relatively high-yielding method
to synthesize hpbm [5, 7].
We found that single crystals of transition metal complexes are difficult to obtain
due to their insolubility in organic solvent. Fortunately, X-ray-quality crystals of two
neutral molecules, 2 and 3 were obtained using solvent diffusion and hydrothermal
methods, respectively.
3.2. Crystal structures of 2
Compound 2 consists of two six-coordinate manganese(IV) bridged by two O2ꢁ; the
Mn2O2 bridging unit is planar (figure 1). Each manganese is additionally ligated to two
hpbm through one nitrogen on the imidazole ring and one oxygen of the deprotonated
phenol. Although d3 is not subject to Jahn–Teller distortion, the Mn(IV) ions are
in a distorted octahedral environment, probably because of steric crowding around
˚
manganese centers. The Mn–Mn distance is 2.777 A in the range of Mn–Mn
˚
bonds (2.625–2.780 A) observed for other structurally characterized complexes with
a Mn2O2 unit [18–20]. The planar hpbm is strongly twisted when coordinated with
dihedral angles of 16.60 and 19.96ꢂ, respectively, between the benzimidazole ring

94
M.-Y. Duan et al.
Figure 1. The crystal structure of 2. H atoms of C–H are omitted for clarity.
and phenol ring. This twisting is largely due to the octahedral environment and
formation of six-member rings.
The overall structure is highly solvated with two pyridines as well as five water
molecules, four around the fifth in the center, forming a distorted tetrahedron (figure 2).
Four hydrogen bonds in the structure are: (1) the nitrogen of two pyridines accept
protons of two water molecules to form O–H ꢀ ꢀ ꢀ N hydrogen bonds (N–O distance is
˚
2.831 A); (2) O–H ꢀ ꢀ ꢀ O hydrogen bonds between four H2O molecules and the fifth one
˚
in the center (O–O distances are 2.969 and 2.905 A); (3) four H2O molecules at the
vertices donate protons to oxygen in ligands of neighboring molecules (O–O distances
˚
are 2.837 and 2.799 A, respectively); (4) four H2O molecules at the vertices accept
protons of uncoordinated –NH– in ligands of neighboring molecules (N–O distances
˚
are 2.846 and 2.814 A, respectively). The latter three hydrogen bonds form an inEnite
arrangement with water playing a vital role in constructing a 2-D structure (figure 3).
The Mn together with the fifth water in the center are on the two-fold axes of space
group Pbcn. Packing along the two-fold axes is well organized giving room for the
H-bonding between the ligand oxygens and water. The numbers and orientations
of interaction sites, counteranions, and solvents, and the hydrogen bonds are the main
factors holding the subunits together [21].
3.3. Crystal structures of 3
Compound 3 (figure 3) involves a Co(II) coordinated to two deprotonated hpbm’s,
through oxygen and nitrogen from phenol and imidazole in a slightly twisted
tetrahedron with dihedral angles between the plane defined by N(1)–Co(1)–O(1) and

Bidentate Benzimidazole
95
Figure 2. 2-D arrangement of 2 via H-bonds. Hydrogens of C–H are omitted for clarity.
Figure 3. ORTEP diagram of 3.

96
M.-Y. Duan et al.
Figure 4. 2-D arrangement of 3 as a result of intermolecular N–H ꢀ ꢀ ꢀ O bonds and ꢅ–ꢅ stacking. Hydrogens
of C–H are omitted for clarity.
N(3)–Co(1)–O(2) of 89.73ꢂ. The coordinated ligands are almost planar with dihedral
angles of 6.15 and 5.92ꢂ, respectively, between benzimidazole and phenol. Proton from
uncoordinated –NH– in benzimidazole and oxygen of the deprotonated phenol have
a strong intermolecular hydrogen bond N–H ꢀ ꢀ ꢀ O with neighboring molecules to form
a 2-D supermolecular structure (figure 4). The N–O bond distances are 2.818 and
˚
˚
2.830 A, respectively. ꢅ–ꢅ stacking is also found between different layers at 3.372 A.
In Co(C13H9N2O)2 ꢀ 2DMF [22], Co(II) was also ligated to two hpbm, but possessing
two more DMF molecules compared with 3; two DMF molecules were on each side
of hpbm forming hydrogen bonds between the oxygen of DMF and uncoordinated
nitrogen of benzimidazole twisting hpbm, resulting in a more distorted tetrahedral
geometry with smaller dihedral angles (75.4ꢂ) than 3. The Co–N length was slightly
longer than in 3, while the Co–O length was the same as 3. Coordination between Co(II)
and phenol was not affected by the hydrogen bond.
A Zn complex of hpbm [5] resembles the Co(II) complex, isostructural in monoclinic
and P2(1)/n space group with Z ¼ 4; though the unit cell volume for the Zn(II) complex
3
3
˚
˚
(2169.2(3) A ) is slightly smaller than that of Co(II) (2177.7(3) A ), the unit cell
similarity index ꢅ was calculated to be 0.00075 [23], quite near to zero, indicating
considerable similarity in their 3-D array. For a Cu(II) [10] complex the dihedral angle
between two hpbm is 47.3(3)ꢂ, much smaller than that of Co(II) and Zn(II); perhaps
a compromise between the trend of Cu(II) to adopt square planar geometry, the special
hindrance of hpbm, and hydrogen bond forces between the solvent and ligands.
The trinuclear Ni(II) complex [9] is much more complicated in structure, probably due
to the relatively small ion radius creating hindrance, and the bridging oxygens in ꢆ3 and
ꢆ2 form making the flexible structure possible. For 2, the instability of Mn(III) in the
solution exposed to air leads to a di-ꢆ-oxo dinuclear structure.
The nature of ligands, and the radii and nature of metals, play significant roles
in determining the structure and properties of metal coordination complexes.
Though no general approach predicts and controls the structure of metal complexes,

Bidentate Benzimidazole
97
investigation into relationships between the structures of complexes and properties of
metals, rigidity, and spacial hindrance of ligand increases understanding.
4. Conclusion
In summary, an environmental friendly, simple, and relatively high-yield method
was used to synthesize 2-(2-hydroxyphenyl)benzimidazole (1) and two transition metal
complexes, [Mn2O2(hpbm)4 ꢀ 2Py ꢀ 5H2O] (2) and [Co(hpbm)2] (3). In 2, two Mn(IV)
ions in distorted octahedral environment are bridged by two O2ꢁ with a Mn–Mn
˚
distance of 2.777 A. The hpbm planes are strongly twisted when coordinated with
dihedral angles 16.60 and 19.96ꢂ between the benzimidazole and phenol. In 3, Co(II)
is coordinated to two hpbm anions in a tetrahedron. The coordinated ligands have
dihedral angles of 6.15 and 5.92ꢂ between benzimidazole and phenol. The two
structures were compared with the reported Zn(II), Cu(II), and Ni(II) complexes with
hpbm, and suggest that metal ions play significant roles in determining the structure
and properties of metal coordination complexes. Hydrogen bonds are observed in both
complexes playing a significant role in 2-D structures.
Supplementary material
CCDC 715036, 697335, and 697336 contain the supplementary crystallographic data for
1, 2 and 3. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk,
or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; Fax: (þ44) 1223-336-033; or Email: deposit@ccdc.cam.ac.uk.
Acknowledgments
This work was supported by the National Natural Science Fund of China (20971103),
International Cooperation Foundation of Shaanxi Province and Educational
Committee Foundation of Shaanxi Province.
References
[1] C.K. Xia, C.Z. Lu, Q.Z. Zhang. Cryst. Growth Des., 5, 1569 (2005).
[2] Q.D. Liu, W.L. Jia, S. Wang. Inorg. Chem., 44, 1332 (2005).
[3] B. Xiao, W. Li, H. Hou, Y. Fan. Coord. Chem., 62, 1630 (2009).
[4] C.-M. Jin, Z.-F. Chen, C.-Y. Chen, J.-H. Dong. Coord. Chem., 62, 390 (2009).
[5] Y.P. Tong, S.L. Zheng, X.-M. Chen. Eur. J. Inorg. Chem., 3734 (2005).
[6] Y.P. Tong, S.L. Zheng, X.M. Chen. Inorg. Chem., 44, 4270 (2005).
[7] X.Q. Buendia, A.E. Ruiz, A.P. Hueso, N.B. Behrens. Inorg. Chim. Acta, 361, 2759 (2008).
[8] Y. Tong, S. Zheng. J. Mol. Struct., 841, 34 (2007).
[9] Y. Xi, J. Li, M. Jiang, C. Wang, J.F. Yan, F.X. Zhang. Acta Chim. Sinica, 64, 1183 (2006).

98
M.-Y. Duan et al.
[10] Y. Xi, J. Li, F.X. Zhang. Acta Cryst., E61, 1953 (2005).
[11] J. Li, F.X. Zhang, Z.X. Tang, Q.Z. Shi. Chin. J. Inorg. Chem., 84, 16 (2000).
[12] R. Blessing. Acta Crystallogr. A, 51, 33 (1995).
[13] G.M. Sheldrick. SHELX-97 Program for the Solution of Crystal Structures, University of Goettingen,
Germany (1997).
[14] D.W. Hein, R.J. Alheim, J.J. Leavitt. J. Am. Chem. Soc., 79, 427 (1957).
[15] F. Takashi, S. Fuminori, S. Minoru, N. Yoshiharu. Chem. Commun., 1391 (1998).
[16] K. Yuka, N. Natsuki, K. Hirotoshi, H. Masahiko. Org. Lett., 5, 3713 (2003).
[17] B. Kiumars, K.M. Mehdi, N. Fardin. J. Org. Chem., 73, 6835 (2008).
[18] J.W. Gohdes, W.H. Armstrong. Inorg. Chem., 31, 368 (1992).
[19] N.A. Law, J.W. Kampf, V.L. Pecoraro. Inorg. Chim. Acta, 297, 252 (2000).
[20] G. Blay, I. Fernandez, J.R. Pedro, R.R. Garcia. J. Mol. Catal. A: Chem., 20, 250 (2006).
[21] X.P. Li, M. Pan, S.R. Zheng, Y.R. Liu, Q.T. He, B.S. Kang, C.Y. Su. Cryst. Growth Des., 7, 2481 (2007).
[22] F. He, Y. Xi, J. Li, F. Zhang. Acta Cryst., E64, 1311 (2008).
[23] L. Fabian, A. Kalman. Acta Cryst., B55, 1099 (1999).