Cobalt(II) complexes of 4-(1-H-1,3-benzimidazole-2-yl)-1,5-dimethyl-2- phenyl-1-2-dihydro-3-H-pyrazol-3-one incorporating various anions: Synthesis, characterization and crystal structures

Article abstract of DOI:10.1016/j.poly.2010.05.010

The structure of 4-(1-H-1,3-benzimidazole-2-yl)-1,5-dimethyl-2-phenyl-1-2- dihydro-3-H-pyrazol-3-one (Opdac) derived from the condensation of O-phenylenediamine and 4-antipyrine carboxaldehyde has been confirmed by X-ray crystallography. The free ligand, Opdac, crystallizes into a triclinic lattice with the space group P1?. Reaction of Opdac with salts of Co(II) affords five novel mononuclear complexes [Co(Opdac)Cl₂] (1), [Co(Opdac)Br₂] (2), [Co(Opdac)[₂] (3), [Co(Opdac) ₂NO₃]NO₃ (4) and [Co(Opdac)₂ClO ₄]-ClO₄ (5). All the reaction products were characterized by a variety of physicochemical techniques, viz. elemental analysis, molar conductivity, CHNS analysis, magnetic susceptibility measurements and spectroscopic studies. The solid-state structures of complexes 1, 2 and 4 were established by Single Crystal X-ray crystallography. The complex 1 crystallize in the monoclinic space group C2/c, where as complex 2 crystallizes in the orthorhombic space group Pbca. In complexes 1 and 2, the ratio of metal to ligand is 1:1 and both the anions are coordinated to the metal ion, in which central metal ion adopts a distorted tetrahedral geometry. The mononuclear units in 1 and 2 are engaged in weak intermolecular C-H?X and N-H?X (X = Cl or Br) hydrogen bonds to give a 2D structure. The complex 4 crystallized in monoclinic space group C2/c forming six membered chelate ring. The structural analysis show that Co(II) ion in 4 adopts a distorted octahedral geometry with a CoN2O₂ chromophore, ligated through two imine nitrogen and two pyrazolone oxygen atoms from two Opdac units; the hexacoordination is completed by O atom of the nitrate ion in a bidentate fashion. The hydrogen gas sorption isotherms at 77 K confirmed the permanent porosities of the complex 1.

Full text of DOI:10.1016/j.poly.2010.05.010

Polyhedron 29 (2010) 2335–2344
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Cobalt(II) complexes of 4-(1-H-1,3-benzimidazole-2-yl)-1,5-dimethyl-2-phenyl-1-2-
dihydro-3-H-pyrazol-3-one incorporating various anions: Synthesis,
characterization and crystal structures
*
P.M. Vimal Kumar, P.K. Radhakrishnan
School of Chemical Sciences, Mahatma Gandhi University, Kottayam 686560, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The structure of 4-(1-H-1,3-benzimidazole-2-yl)-1,5-dimethyl-2-phenyl-1-2-dihydro-3-H-pyrazol-3-one
(Opdac) derived from the condensation of O-phenylenediamine and 4-antipyrine carboxaldehyde has
been confirmed by X-ray crystallography. The free ligand, Opdac, crystallizes into a triclinic lattice with
Received 22 March 2010
Accepted 5 May 2010
Available online 26 May 2010
ꢀ
the space group P1. Reaction of Opdac with salts of Co(II) affords five novel mononuclear complexes
[Co(Opdac)Cl2] (1), [Co(Opdac)Br2] (2), [Co(Opdac)I2] (3), [Co(Opdac)2NO3]NO3 (4) and [Co(Opdac)2ClO4]-
ClO4 (5). All the reaction products were characterized by a variety of physicochemical techniques, viz. ele-
mental analysis, molar conductivity, CHNS analysis, magnetic susceptibility measurements and
spectroscopic studies. The solid-state structures of complexes 1, 2 and 4 were established by Single Crys-
tal X-ray crystallography. The complex 1 crystallize in the monoclinic space group C2/c, where as com-
plex 2 crystallizes in the orthorhombic space group Pbca. In complexes 1 and 2, the ratio of metal to
ligand is 1:1 and both the anions are coordinated to the metal ion, in which central metal ion adopts a
distorted tetrahedral geometry. The mononuclear units in 1 and 2 are engaged in weak intermolecular
C–HꢀꢀꢀX and N–HꢀꢀꢀX (X = Cl or Br) hydrogen bonds to give a 2D structure. The complex 4 crystallized
in monoclinic space group C2/c forming six membered chelate ring. The structural analysis show that
Co(II) ion in 4 adopts a distorted octahedral geometry with a CoN2O2 chromophore, ligated through
two imine nitrogen and two pyrazolone oxygen atoms from two Opdac units; the hexacoordination is
completed by O atom of the nitrate ion in a bidentate fashion. The hydrogen gas sorption isotherms at
77 K confirmed the permanent porosities of the complex 1.
Keywords:
Co(II) complexes
Spectroscopy
Magnetism
Crystal structures
Non-covalent interactions
2D supramolecular network
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
sidered. Among them, the choice of metal ions (cation) and anion
is the key factor governing the topology of coordination complex.
The design and synthesis of novel multi-functional materials are
a major focus of research over recent years. Transition metal com-
plexes with oxygen and nitrogen donor Schiff bases are of particular
interest for their ability to possess unusual configurations, struc-
tural lability and their sensitivity to molecular environments [1].
Cobalt(II) complexes of Schiff bases have been extensively studied
since they play an important role in both synthetic and structural
research [2]. Recently, there has been much interest in the con-
struction of coordination complexes through crystal engineering
due to their fascinating structural topologies as well as their poten-
tial applications as functional materials in the fields of molecular
magnetism, catalysis, gas sorption and optoelectronic devices [3].
When preparing coordination complexes, there are some factors,
such as the choice of metal ions (cation) or anions, the ligand/metal
ratios, temperature, solvent and pH of the solution should be con-
The coordination geometry and the size of the cation are the two
key effects upon the structure of the final product. The anion has
two main effects on the final topology. Firstly, the coordinative abil-
ity of the anion can clearly influence the coordination sphere of the
metal ion used. Secondly, the size of the anion can influence the
long-range order and degree of interpenetration [4]. A typical
geometry can be extended throughout the crystal if suitable ligands
are designed that carry sites suitable for participation in non-cova-
lent interactions with nearest neighbors. In particular, hydrogen
bonding has been exploited for the molecular recognition associ-
ated with the biological activity and for an engineering of the
molecular solids. In addition to that, the existence of C–HꢀꢀꢀX and
N–HꢀꢀꢀX (X = Cl or Br) hydrogen bonds triggered by terminal MꢀꢀꢀX
bonds have been well recognized in recent years. It is also notice-
able that terminal MꢀꢀꢀX bonds are distinctly directional acceptors
of hydrogen bonds [5]. Hence to design specific architectures, suit-
able substrates should bear functional groups that are apt to devel-
op predefined interactions [6]; for this purpose, often, 4-antipyrine
* Corresponding author. Tel.: +91 4812731036; fax: +91 4812731002.
E-mail address: pkrkn@yahoo.co.in (P.K. Radhakrishnan).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.05.010

2336
P.M. Vimal Kumar, P.K. Radhakrishnan / Polyhedron 29 (2010) 2335–2344
carboxaldehyde and O-phenylenediamine were exploited as the
building blocks.
wise with constant stirring. The resulting blue solution on being
heated under reflux for 3 h yielded some blue crystalline precipi-
tate which was filtered off, washed with ethyl acetate, dried over
fused CaCl2 and kept in a desiccator. Anal. Calc. for C18H16N4OCl2Co
(434.18): C, 49.79; H, 3.71; N, 12.90; Co, 13.57; Cl, 16.33; Found: C,
49.77; H, 3.67; N, 12.87; Co, 13.53; Cl, 16.29%. IR (cm-1): 1623
As part of our continued involvement in the coordination chem-
istry of pyrazolone ligands [7–11], we have investigated the metal-
binding properties of the bidentate ligand to control the stereo-
chemistry at metal(II) center. In the present work our group was at-
tracted by the structural features of 4-(1-H-1,3-benzimidazole-2-
yl)-1,5-dimethyl-2-phenyl -1-2-dihydro-3-H-pyrazol-3-one (Op-
dac) and its five novel Co(II) complexes together with the X-ray
crystal structures of the free ligand (Opdac), [Co(Opdac)Cl2] (1),
[Co(Opdac)Br2] (2) and [Co(Opdac)2NO3]NO3 (4). Unfortunately, in
spite of several attempts, we failed to get diffraction quality single
crystals of [Co(Opdac)I2] (3) and [Co(Opdac)2ClO4]ClO4 (5).
[
[
m
m
(C@O)], 1604 [
m
m
(C@N)], 3320 [
m(N–H)], 455 [m(Co–N)], 552
(Co–O)], 286
[
(Co–Cl)]; (kM/Ohmꢁ1 cm2 molꢁ1): 32.52 (in
DMF),72.42 (in MeOH), 18.63 (in C6H5NO2); (kmax/nm) 378 (n–
p
*), 257 ( *), 650, 638, 590 (d–d transitions), 328 (C–T transi-
p–p
tion); leff 4.30 BM.
2.3.2. Preparation of [Co(Opdac)Br2] (2)
To a hot solution of Opdac (0.304 g, 1 mmol) in ethyl acetate
(20 mL), CoBr2ꢀ6H2O (0.371 g, 1 mmol) dissolved in methanol
(10 mL), was added drop wise with constant stirring. The resulting
blue solution on being heated under reflux for 3 h yielded some
blue precipitate which was filtered off, washed with ethyl acetate,
dried over fused CaCl2 and kept in a desiccator. Anal. Calc. for
2. Experimental
2.1. Materials and Instrumentation
High purity 4-antipyrine carboxaldehyde (Aldrich, USA), O-
phenylenediamine, Cobalt(II) nitrate hexahydrate, Cobalt(II) chlo-
ride hexahydrate, (E. Merck, India) were purchased from the
respective concerns and used as such. Metal bromide, iodide and
perchlorate salts were prepared from Analar BDH cobalt carbonate
and the respective 50% acids by crystallizing the salts after concen-
trating the solutions on a steam bath. All other chemicals and sol-
vents were AR grade and used as such.
C
18H16N4OBr2Co (523.10): C, 41.33; H, 3.08; N, 10.71; Co, 11.27;
Br, 30.55; Found: C, 41.30; H, 3.03; N, 10.68; Co, 11.24; Br,
30.52%. IR (cmꢁ1): 1620 [
(C@O)], 1600 [ (C@N)], 3319 [ (N–H)],
454 (Co–N)], 552 (Co–O)], 232 (Co–Br)]; (kM/
Ohmꢁ1 cm2 molꢁ1): 41.16 (in DMF), 68.23 (in MeOH), 16.93 (in
C6H5NO2); (kmax/nm): 376 (n–
*), 256 ( *), 649, 624, 575 (d–d
transitions), 330 (C–T transition); eff 4.26 BM.
m
m
m
[
m
[m
[m
p
p–p
l
The carbon, hydrogen and nitrogen content in the synthesized
compound was determined on a Perkin–Elmer 240 CHN elemental
analyzer. All the complexes were analyzed for their metal, halide
and perchlorate contents by standard methods [12,13]. Molar con-
ductivities were measured using freshly prepared 10ꢁ3 M solutions
in dimethylformamide, nitrobenzene and methanol at room tem-
perature, using a Toshniwalconductivitybridge with a dip-type con-
ductance cell (cell constant 0.9835) which was calibrated with
0.01 M KCl solution. The infrared spectra of the ligands and the com-
plexes were recorded in the range 4000–400 cmꢁ1 on a Shimadzu IR
470 spectrophotometer and in the range 400–200 cmꢁ1 were mea-
sured on a Perkin–Elmer FTIR-8400S, spectrophotometer. Solid state
electronic spectra in the range 1100–200 nm were recorded using a
UV–Vis-2450 spectrophotometer and in the range 1100–1500 nm
on a Cary 2390 UV–Vis–NIR spectrophotometer. Magnetic suscepti-
bility measurements were performed on pulverized samples at
room temperature on a Sherwood Magway MSB Mk1 balance. The
diamagneticcorrectionswere calculated by using Pascal’s constants.
2.3.3. Preparation of [Co(Opdac)I2] (3)
The ligand Opdac (0.304 g, 1 mmol) was dissolved in (20 mL)
methanol. A clear solution of CoI2ꢀ6H2O (0.182 g, 1 mmol) in meth-
anol (10 mL) was added drop wise to the boiling ligand solution
and the mixture was refluxed for 3 h. A blue coloured solid was ob-
tained, which was filtered and thoroughly washed by ethyl acetate
solution, dried over fused CaCl2 and kept in a desiccator. Anal. Calc.
for C18H16N4OI2Co (617.09): C, 35.03; H, 2.61; N, 9.08; Co, 9.55; I,
41.13; Found: C, 34.99; H, 2.57; N, 9.05; Co, 9.52; I, 41.11%. IR
(cmꢁ1): 1620
(Co–N)], 550 [
[
m
m
(C@O)], 1602
(Co–O)]; 198 [m
[
m
(C@N)], 3318
[m(N–H)], 452
[m
(Co–I)]; (kM/Ohmꢁ1 cm2 molꢁ1):
46.24 (in DMF), 64.36 (in MeOH), 15.41 (in C6H5NO2); (kmax/nm):
*
*
), 642, 620, 584 (d–d transitions), 324 (C–T
382 (n–
p
), 258 (p–p
transition); leff 4.20 BM.
2.3.4. Preparation of [Co(Opdac)2NO3]NO3 (4)
A solution of Co(NO3)2ꢀ6H2O (0.291 g, 1 mmol) in methanol
(10 mL) was added to a hot solution of Opdac (0.608 g, 2 mmol)
in acetonitrile (20 mL). The reaction mixture was refluxed for
about 3 h. The precipitated pink coloured complex was filtered
off, washed with ethyl acetate and dried under fused CaCl2 and
kept in desiccator. Anal. Calc. for C36H32N10O8Co (791.65): C,
54.62; H, 4.07; N,17.69; Co, 7.44; Found: C, 54.58; H, 4.02; N,
2.2. Preparation and characterization of 4-(1-H-1,3-benzimidazole-2-
yl)-1,5-dimethyl-2-phenyl-1-2-dihydro-3-H-pyrazol-3-one (Opdac)
The Schiff base ligand (Opdac) was synthesized by refluxing eth-
anolic solutions of 4-antipyrine carboxaldehyde (0.432 g, 2 mmol)
and O-phenylenediamine (0.108 g, 1 mmol). Reflux was continued
for ca. 3 h at water bath temperature during which a solid microcrys-
talline compound was separated. It was filtered off, washed several
times with hot ethanol and dried in vacuum over fused CaCl2. Single
crystalsofOpdac suitable forX-raydiffractionwere obtained by slow
evaporation of ethyl acetate solution of the ligand. Anal. Calc. for
17.67; Co, 7.41%. IR (cmꢁ1): 1620 [
m
(C@O)], 1604 [
(NO3coordinated)], 1384, 821
(Co–N)], 554 (Co–O)]; (kM/
m(C@N)], 3320
[
m
(N–H)], 1438, 1234, 1054
(NO3uncoordinated)], 454
[m
[m
[
m
[m
Ohmꢁ1 cm2 molꢁ1): 81.46 (in DMF), 104.21 (in MeOH), 20.09 (in
*
*
), 1120, 548, 470 (d–
C6H5NO2); (kmax/nm): 384 (n–
p
), 254 (
p–p
d transitions), 321 (C–T transition); leff 3.91 BM.
C
18H16N4O (304.35): C, 71.04; H, 5.30; N, 18.41. Found: C, 71.00; H,
5.25; N, 18.40%; IR (cmꢁ1): 1631 [
(C@O)]; 1612 [ (C@N)]; 3321
(N–H)]; UV–Vis: (kmax/nm): 387 (n–
2.3.5. [Co(Opdac)2ClO4]ClO4 (5)
v
v
p–
Complex 5 was prepared by the similar procedure as described
for 4, with Co(NO3)2ꢀ6H2O replaced by Co(ClO4)2.6H2O (0.291 g,
1 mmol). Anal. Calc for C36H32N8O10Cl2Co (866.53): C, 49.90; H,
3.72; N, 12.93; Co, 6.80; ClO4, 22.95; Found: C, 49.86; H, 3.68;
*
*
p ).
[v
p
) and 252 (
2.3. Synthesis of metal complexes
2.3.1. Preparation of [Co(Opdac)Cl2] (1)
N,12.90; Co, 6.76; ClO4, 22.91%. IR (cmꢁ1): 1621[
m
(C@O)], 1604
(N–H)], 1145, 1112, 1022, 942, 632 [ (ClO4coordi-
nated)], 1095, 624 [ (ClO4 uncoordinated)], 455 [ (Co–N)], 554
[m
(Co–O)]; (kM/Ohmꢁ1 cm2 molꢁ1): 78.46 (in DMF), 97.68 (in
[m
(C@N)], 3320 [
m
m
To a hot solution of Opdac (0.304 g, 1 mmol) in ethanol (20 mL),
m
m
CoCl2ꢀ6H2O (0.237 g, 1 mmol) in ethanol (10 mL) was added drop

P.M. Vimal Kumar, P.K. Radhakrishnan / Polyhedron 29 (2010) 2335–2344
2337
uum (5 ꢂ 10ꢁ3 mmHg) and the temperature and vacuum was
maintained for 8 h prior to the adsorption measurements. Initially
100 mg of the sample was taken before activation. The amount of
activated sample was determined from the weight of the samples
before as well after activation. Prior to hydrogen sorption analysis
the manifold and the lining of the system was evacuated and
flushed with hydrogen at least three times by utilizing the ‘‘flush-
ing lines” function of the program to ensure the purity of H2.The er-
rors in the measurements of hydrogen uptake values were within
the range of 0.5%.
*
*
),
MeOH), 28.12 (in C6H5NO2); (kmax/nm): 381 (n–
p
), 255 (
l
p
–
p
1116, 545, 452 (d–d transitions), 325 (C–T transition);
eff 3.65 BM.
Caution! Though we have not met with any incident while
working with the perchlorate compound described here, care
should be taken in handling them as the perchlorates are poten-
tially explosive. They should not be prepared and stored in large
amounts.
2.4. X-ray crystallographic study
An yellow needle crystal of Opdac, a block blue crystals of
[Co(Opdac)Cl2] (1), [Co(Opdac)Br2] (2) and a needle pink crystal
of [Co(Opdac)2NO3]NO3 (4) were directly picked up from the
mother liquor, attached to a glass fiber, and transferred for the data
3. Results and discussion
3.1. Synthesis and properties
collection. X-ray single crystal data were collected using Mo K
a
(k = 0.7107 Å) radiation on BRUKER APEX II diffractometer
a
The complexes 1–5 were prepared by a straightforward stoichi-
ometric reaction between the ligand (Opdac) and Co(II)X2ꢀnH2O
(X = Cl, Br, I, NO3 and ClO4) in respective solvents as mentioned
above. The air-stable, micro crystalline compounds were soluble
in common organic solvents such as methanol, ethanol, dimethyl-
formamide and nitrobenzene, but insoluble in ethyl acetate, aceto-
nitrile and acetone. The elemental analyses data suggest that the
complexes can be formulated: Co(Opdac)X2 (X = Clꢁ, Brꢁ or Iꢁ)
and Co(Opdac)2X2 (X = NO3ꢁ or ClO4ꢁ). The molar conductance data
reveals that [Co(Opdac)Cl2] (1), [Co(Opdac)Br2] (2) and [Co(Opda-
c)I2] (3) behave essentially as non-electrolytes where as [Co(Op-
dac)2NO3]NO3 (4) and [Co(Opdac)2ClO4]ClO4 (5) complexes
behave as 1:1 electrolytic type [18].
equipped with CCD area detector. Unit cell refinement Bruker
[14] data reduction (SAINT) and structure solution as well as refine-
ment (SHELXL97) [15] were carried out using the software package of
SMART APEX. All structures were solved by direct method and refined
in a routine manner. In most of the cases, nonhydrogen atoms were
treated anisotropically. Whenever possible, the hydrogen atoms
were located on a difference Fourier map and refined. In other
cases, the hydrogen atoms were geometrically fixed. Molecular
graphics were generated by using the softwares MERCURY 2.3 [16]
and ORTEP 3 [17]. The details of the X-ray crystal data and the struc-
ture solution as well as the refinement are given in Table 1.
2.5. Hydrogen adsorption measurements
3.2. Infrared spectra
Hydrogen adsorption–desorption measurements were per-
formed using high purity hydrogen gas (99.9995%) which was
again passed through a molecular sieve trap to remove traces of
moisture. Hydrogen adsorption isotherms at 77 K were carried
out in a static volumetric system (Micromeritics Instrument Corpo-
ration, USA, model ASAP 2010) up to 1 bar pressure. The samples
were activated at a heating rate of 1 K minꢁ1, to 393 K under vac-
A comparative study of IR spectral data of the reported com-
plexes with those of the free ligand gives meaningful information
regarding bonding sites of the ligand molecule. The strong bands
observed at 1631 and 1612 cmꢁ1 in the spectrum of Opdac are
attributed to the stretching vibrations of the pyrazolone carbonyl
oxygen [19] and benzimidazole imine nitrogen [20], respectively.
Table 1
Crystallographic data and refinement parameters for Opdac, 1, 2 and 4.
Crystal parameters
Opdac
1
2
4
CCDC number
Empirical formula
Formula weight
Crystal size (mm)
Crystal system
Space group
a (Å)
761775
761773
C18H16Cl2CoN4O
434.18
0.24 ꢂ 0.18 ꢂ 0.12
monoclinic
C2/c
32.4991(11)
7.1761(2)
19.7537(7)
90.00
122.778(2)
90.00
3873.4(2)
8
1.489
1768
761774
C18H16Br2CoN4O
523.10
0.20 ꢂ 0.15 ꢂ 0.08
orthorhombic
Pbca
761776
C36H32CoN10O8
791.65
0.30 ꢂ 0.20 ꢂ 0.20
monoclinic
C2/c
21.4562(6)
17.0234(6)
14.8712(6)
90.00
131.871(3)
90.00
4044.8(2)
4
1.300
1636
C18H16N4O
304.35
0.40 ꢂ 0.35 ꢂ 0.15
triclinic
ꢀ
P1
7.1969(5)
7.4570(5)
14.4426(10)
103.409(3)
91.158(4)
95.516(4)
749.75(9)
2
10.1140(18)
19.167(4)
19.986(3)
90.00
90.00
90.00
3874.4(12)
8
1.794
2056
5.025
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
Dcalc (g cmꢁ3
F(0 0 0)
)
1.348
320
0.087
l
Mo K
a
(mmꢁ1
)
1.176
0.485
Temperature (K)
100(2)
100(2)
100(2)
298(2)
Rint
0.0357
0.0526
0.0879
0.0581
Range of h, k, l
ꢁ8/8, ꢁ9/9, ꢁ17/17
1.45/25.71
10430/2822/2045
2822/0/273
1.096
ꢁ39/35, ꢁ7/8, ꢁ24/24
2.07/25.68
11883/3531/2504
3531/0/239
1.056
ꢁ10/10, ꢁ20/20, ꢁ16/21
2.04 /22.22
13517/2447/1822
2447/1/241
1.020
ꢁ22/22, ꢁ18/18, ꢁ15/15
1.75/22.43
30171/2608/2343
2608/5/257
1.225
hmin
/
(°)
max
Reflections collected/unique/observed [I > 2
r(I)]
Data/restraints/parameters
Goodness of fit on F2
Final R indices [I > 2
r(I)]
R1 = 0.0471
wR2 = 0.1123
R1 = 0.0731
wR2 = 0.1355
R1 = 0.0486
wR2 = 0.1024
R1 = 0.0832
wR2 = 0.1133
R1 = 0.0435
wR2 = 0.0994
R1 = 0.0659
wR2 = 0.1112
R1 = 0.0773
wR2 = 0.2219
R1 = 0.0860
wR2 = 0.2321
R indices (all data)

2338
P.M. Vimal Kumar, P.K. Radhakrishnan / Polyhedron 29 (2010) 2335–2344
4
4
While in the spectra of the complexes 1–5 indicate that
and (C@N) stretching bands show a red shift with respect to those
of the free ligand by 10–12 and 8–12 cmꢁ1, respectively. The N–H
stretching frequency observed in the spectra of pure ligand [ (N–
m
(C@O)
545 nm T1g(F) ? 4A2g(F) and
m
3 = 470–452 nm T1g(F) ? 4T1g(P),
m
and these are characteristic of a six-coordinate high spin Co(II)
complex in a distorted octahedral geometry [29,30]. The room tem-
perature magnetic moments of the complexes 4 and 5 were found
at 3.91 and 3.65 BM, respectively indicative of three unpaired
electrons per Co(II) ion in a distorted octahedral environment
[30]. Furthermore, all the complexes 1–5 shows a strong band in
the 330–321 nm range can be assigned to the charge transfer
transition.
m
H) = 3321 cmꢁ1] remains almost unchanged in all the complexes
containing the benzimidazole moiety, indicating non-participation
of the N–H nitrogen in coordination [20]. Thus the IR spectral data
of the solid state complexes show that the ligand, Opdac, acts as a
neutral bidentate ligand coordinating through the imine nitrogen
and the pyrazolone oxygen forming a six membered ring around
the central metal ion. The presence of additional non-ligand bands
in the spectra of all the complexes around 452–455 cmꢁ1 and 550–
554 cmꢁ1 are assignable to M–N and M–O stretching frequencies,
respectively [10].
Based on the above observations the following tentative struc-
ture in Figs. 1–3 may be assigned to the complexes.
CH3
The nitrate complex exhibits vibrational frequencies character-
istic of both uncoordinated and coordinated nitrate ions. A very
strong band at 1384 cmꢁ1 and a medium band at 821 cmꢁ1 are
attributed to the m3 and m2 vibrations, respectively of ionic nitrate
of D3h symmetry [21]. The presence of coordinated nitrate is indi-
cated by two bands at 1438 and 1246 cmꢁ1 due to the m4 and m1
vibrations, respectively of the nitrate of C2v symmetry [10]. Since
H3C
N
N
H
N
O
N
Co
(m4–m
1) = 192 cmꢁ1, the nitrate ion is coordinated in a bidentate
X
fashion [22].
X
In the perchlorate complex, the triply split band maxima at
1145, 1112 and 1022 cmꢁ1 are due to the m8
, m6 and m1 vibrations,
respectively, of the perchlorate ion of C2v symmetry, indicating the
coordination of the perchlorate ion in a bidentate fashion [23]. But
the presence of a very strong band at around 1095 cmꢁ1 is attrib-
Fig. 1. [Co(Opdac)Cl2] (1), [Co(Opdac)Br2] (2) and [Co(Opdac)I2] (where X = Clꢁ, Brꢁ
or Iꢁ).
uted to the m3 vibration of uncoordinated perchlorate ion of Td sym-
metry [24]. The bands observed around 942 and 632 cmꢁ1 due to
the m2 and m3 vibrations, respectively of the coordinated perchlo-
rate (C2v) ion and the band at 624 cmꢁ1 due to the m4 vibration of
the uncoordinated perchlorate (Td) ion also support the co-exis-
tence of both coordinated and uncoordinated perchlorate ion in
the complex [23].
CH3
H3C
N
N
H
N
O
N
In the far infrared spectra of the halide complexes 1, 2 and 3 the
O
bands at 286, 232 and 198 cmꢁ1 are assigned to the
m(Co–Cl),
m(Co–
N
O
Co
NO3
O
Br) and
m(Co–I) vibrations, respectively [10] which are absent in
O
the ligand spectrum.
N
3.3. Electronic spectra and magnetism
N
N
H
N
The electronic spectra of Opdac exhibit two main peaks at about
387 and 252 nm which are assignable to the n–
H3C
CH3
*
p
*
tran-
and
p–p
sitions, respectively. In all the complexes, the n–
p
* band is blue-
*
shifted to the 376–384 nm region while the
p–p band is red-
Fig. 2. [Co(Opdac)2NO3]NO3 (4).
shifted to the 254–258 nm region compared to Opdac. The reflec-
tance spectrum of blue coloured complexes 1–3 show three closely
spaced weak bands in the visible region and a very intense band in
the UV region. In the regular tetrahedral and near tetrahedral Co(II)
CH3
complexes only one d–d transition [4A2(F)–4T1(P), assigned as
m3] is
H3C
N
observed in the visible region. This transition has been reported for
the tetrahedral [Co(NCS)4]2ꢁ at 615 nm and for the pseudotetrahe-
dral [Co(morpholine)2(NCS)2] at 616 nm [25]. The three closely
spaced transitions in the spectrum of complexes 1–3 arise from
the distortion in the tetrahedral symmetry around the Co(II) cen-
ter. This splitting originates from the reduction of the orbital
degeneracy due to the difference in the ligand field strength and
coordinated halide ions as well as the restricted bite angle of the
O(1)–Co(1)–N(4) chelate ligand [26,27]. The room temperature leff
values 4.30 for 1, 4.26 for 2 and 4.20 BM for 3 are in good agree-
ment with those observed for most distorted tetrahedral cobalt(II)
complexes [28].
N
H
N
O
N
O
O
Cl
ClO4
Co
O
O
O
N
N
N
H
N
H3C
CH3
The diffuse reflectance spectrum in the near IR–Vis region of the
pink coloured complexes 4 and 5 shows the presence of three bands
4
in the regions
m
1 = 1120–1116 nm T1g(F) ? 4T2g (F),
m
2 = 548–
Fig. 3. [Co(Opdac)2ClO4]ClO4 (5).

P.M. Vimal Kumar, P.K. Radhakrishnan / Polyhedron 29 (2010) 2335–2344
2339
Fig. 4. (a) Molecular structure of Opdac with atom labeling scheme. Atomic displacement parameters are shown at 50% probability level. Hydrogen atoms are omitted for
clarity, (b) hydrogen bonded dimer, (c) 1D hydrogen bonded network view along b axis.
Table 2
Selected bond distances (Å) and angles (°) of Opdac.
Bond lengths
Bond angles
O(1)–C(16)
N(4)–C(18)
N(4)–C(7)
N(3)–C(12)
N(3)–C(18)
N(3)–H(3B)
N(2)–C(1)
N(1)–C(13)
N(1)–N(2)
N(2)–C(16)
C(16)–C(17)
C(17)–C(18)
1.246(19)
1.324(2)
1.389(2)
1.381(2)
1.362(2)
0.84(2)
1.433(2)
1.337(2)
1.3962(19)
1.387(2)
1.426(2)
1.446(2)
O(1)–C(16)–C(17)
O(1)–C(16)–N(2)
N(1)–C(13)–C(17)
N(2)–C(16)–C(17)
N(4)–C(18)–C(17)
N(4)–C(18)–N(3)
C(18)–N(4)–C(7)
C(16)–C(17)–C(18)
C(13)–N(1)–N(2)
C(13)–C(17)–C(16)
C(16)–N(2)–N(1)
N(3)–C(18)–C(17)
131.20(15)
123.47(15)
109.16(15)
105.33(14)
125.47(16)
112.81(16)
104.44(15)
124.52(16)
108.60(14)
108.03(15)
108.82(13)
121.67(15)
N(3)–C(12)–C(7)
N(4)–C(7)–C(12)
N(4)–C(7)–C(8)
C(18)–N(3)–C(12)
C(18)–N(3)–H(3B)
C(12)–N(3)–H(3B)
C(16)–N(2)–C(1)
105.05(15)
110.53(15)
129.77(17)
107.16(15)
125.1(14)
127.5(14)
126.52(14)
Table 3
Relevant hydrogen bonding parameters for Opdac, 1, 2 and 4.
D–H–A
D–H (Å)
H–A (Å)
D–A (Å)
D–H–A (°)
Symmetry operation
Opdac
N(3)–H(3B)–O(1)
N(3)–H(3B)–O(1)
C(14) –H(14A)–N(4)
C(14) –H(14C)–O(1)
0.84(2)
0.84(2)
0.99(3)
0.97(3)
2.41(2)
2.32(2)
2.48(3)
2.59(3)
2.9387(19)
3.026(2)
3.104(3)
3.555(3)
121.6(17)
142.9(18)
121(2)
x, y, z
1 ꢁ x, 1 ꢁ y, 1 ꢁ z
x, y, z
172(2)
ꢁ1 + x, y, z
1
N(3) –H(3B)–Cl(1)
C(15) –H(15C)–Cl(2)
0.74(4)
0.96
2.69(4)
2.68
3.338(3)
3.614(4)
148(4)
163.7
ꢁx, 1 ꢁ y, ꢁz
x, ꢁy, ꢁ1/2 + z
2
N(3) –H(3A)–Br(2)
C(9) –H(9)–Br(2)
C(14) –H(14A)–Br(1)
0.87(2)
0.93
0.96
2.82(6)
2.91
2.81
3.497(6)
3.736(8)
3.689(7)
135(7)
148.3
152.1
ꢁx, ꢁy, ꢁz
1/2ꢁx, ꢁ1/2 + y, z
ꢁ1 + x, y, z
4
N(3) –H(3B)–O(4B)
N(3) –H(3B)–O(4A)
N(3) –H(3B)–O(5A)
0.877(11)
0.877(11)
0.877(11)
1.89(4)
2.02(6)
2.18(4)
2.72(2)
2.748(14)
3.03(2)
159(9)
140(8)
162(9)
1 ꢁ x, y, 3/2 ꢁ z
1 ꢁ x, y, 3/2 ꢁ z
x, y, z

2340
P.M. Vimal Kumar, P.K. Radhakrishnan / Polyhedron 29 (2010) 2335–2344
3.4. The crystal structure of 4-(1-H-1,3-benzimidazole-2-yl)-1,5-
dimethyl-2-phenyl-1-2-dihydro-3-H-pyrazol-3-one (Opdac)
conformation [the torsion angle between benzimidazole ring and
pyrazolone ring is approximately 9.09°; the torsion angle between
phenyl ring and pyrazolone ring is approximately 65.59°]. The
C18–N4 bond distance [1.324(2) Å] is appreciably close to that of
a C@N bond 1.311(5) Å [31], confirming the imine bond formation.
The C16–O1 bond distance of 1.246(19) Å is also consistent with
the value of C@O double bond, as already observed in similar com-
pounds [32,33]. In the free ligand moiety Opdac, imine nitrogen
(N4) and pyrazolone oxygen (O1) are trans oriented to each other.
During chelation, they rearrange to cis fashion in all the complexes.
Single crystals suitable for X-ray analysis were grown from the
slow evaporation of a ethyl acetate solution of Opdac. Crystal data
and structure refinement are given in Table 1. The Crystal structure
of Opdac with the atom numbering scheme is given in Fig. 4a and
selected bond distances and angles are presented in Table 2. It was
ꢀ
crystallized in the triclinic centrosymmetric space group P1. From
the crystal structure, it is clear that the ligand adopts non-planar
Fig. 5. (a) Molecular structure of 1 with atom labeling scheme. Atomic displacement parameters are shown at 50% probability level. Hydrogen atoms are omitted for clarity,
(b) hydrogen bonded dimer, (c) propagation of hydrogen bonded dimer, (d) 2D hydrogen bonded microporous network.

P.M. Vimal Kumar, P.K. Radhakrishnan / Polyhedron 29 (2010) 2335–2344
2341
This cis conformation of the ligand is necessary for its participation
as a bidentate NO chelate ligand.
In the solid state, the ligand molecules are stabilized through a
network of weak hydrogen bond interactions. Complementary ben-
zimidzole N–H and pyrazolone C@O are involved in intra
[N(3)ꢀꢀꢀO(1) = 2.9387(19) Å; \N(3)–H(3B)ꢀꢀꢀO(1) = 121.6(17)°] and
inter [N(3)ꢀꢀꢀO(1) = 3.026(2) Å; \N(3)–H(3B)ꢀꢀꢀO(1) = 142.9(18)°]
molecular hydrogen bond interaction, which connects the molecule
to form a dimer Fig. 4b. Each hydrogen bonded dimer is further prop-
agated to a 1D hydrogen bonded network (Fig. 4c) via C–HꢀꢀꢀO inter-
Fig. 6. (a) Molecular structure of 2 with atom labeling scheme. Atomic displacement parameters are shown at 50% probability level. Hydrogen atoms are omitted for clarity,
(b) hydrogen bonded dimer, (c) Propagation of hydrogen bonded dimer to 2D corrugated sheet in (c) wireframe model and (d) spacefill.

2342
P.M. Vimal Kumar, P.K. Radhakrishnan / Polyhedron 29 (2010) 2335–2344
action [C(14)ꢀꢀꢀO(1) = 3.555(3) Å; \C(14)–H(14C)ꢀꢀꢀO(1) = 172(2)°].
Br = 115.55(4)°; \N–Co–Br = 107.83(15)–109.33(15)°; \O–Co–
Br = 112.02(14)–114.04(14)°; \O–Co–N = 96.13(19)° in 2]. In com-
plex 1 the average Co–Cl bond length of 2.22 Å agrees approximately
with the same distance in other distorted tetrahedral cobalt com-
plexes: Co(Phca2en)Cl2, 2.22 Å Co(bdmpab)Cl2, 2.23 Å, Co(ethylene-
dimorpholine) Cl2, 2.23 Å, Co(p-toluidine)2Cl2, 2.26 Å [34–38].
While for complex 2 the average Co–Br bond length is found to be
2.373 Å which agrees with those reported for other distorted tetra-
hedral cobalt complexes [39,40]
The structure of [Co(Opdac)2NO3]NO3 (4) shows the presence of
a Co(II) mononuclear complex, which crystallized in monoclinic
C2/c space group. Asymmetric unit consists of one half of the com-
plex [with Co(II) sitting on a two fold symmetry axis, ligand and
half nitrate – both are coordinated to Co(II)] and rotationally disor-
dered counter anion nitrate. Hence the Co(II) ion presents a six
Relevant hydrogen bonding parameters are collected in Table 3.
3.5. Crystal structures of [Co(Opdac)Cl2] (1), [Co(Opdac)Br2] (2) and
[Co(Opdac)2NO3]NO3 (4)
The crystals of 1, 2 and 4 were recrystallised from methanol by
slow vapour diffusion of ethyl acetate from an adjacent container
which were suitable for X-ray crystallography. The crystallo-
graphic data are summarized in Table 1. The crystal structures of
the complex 1, 2 and 4 with the atom numbering scheme is given
in Figs. 5a, 6a and 7a. Selected bond lengths and bond angles are
given in Table 4.
Single crystal data reveal that the crystal of 1 belongs to the
monoclinic space group C2/c and its counterpart bromide derivative,
2 is crystallized in orthorhombic Pbca space group. The cobalt(II) ion
in both complexes is coordinated by the neutral bidentate ligand Op-
dac and two halide ions. The equatorial positions of the Co(II) metal
center are occupied by the benzimidazole imine nitrogen (N4) and
pyrazolone oxygen atom (O1) while the axial position is occupied
by two halide ions; mutually cis to each other. Thus the ligand Opdac
forms a six membered chelate ring with the Co(II) metal centre. In
the complexes, 1 and 2 Co(II) metal centre exhibits slightly distorted
tetrahedral coordination sphere with the angles [\Cl–Co–
Cl = 112.53(4)°; \N–Co–Cl = 107.22(8)–113.86(9)°; \O–Co–Cl =
109.03(8)–117.28(8)°; \O–Co–N = 95.37(11)° in 1 and \Br–Co–
coordination number with
a CoN2O4 chromophore, ligated
through the imine nitrogen N(4) and the pyrazolone oxygen O(1)
atoms of two bidentate Opdac units; the hexacoordination is com-
pleted by two O atoms from a bidentate nitrate ion. The Co(II) me-
tal centre exhibits slightly distorted octahedral geometry [\O–Co–
O = 92.37(18)–117.5(3)°;
\O–Co–N = 85.19(19)–92.99(19)°],
whilst the main distortion from a regular octahedral geometry is
attributable to the small bite of the bidentate nitrate ion [O(2)–
Co(1)–O(2), 58.3(3)° [30]. The average value of the Co–N bond
length is 2.12 (5) Å which is similar to that found in other macro-
cyclic nitrate complexes of octahedral Co(II) [41]. Within the com-
plexes 1, 2 and 4 the Co–N(imine) [1.997(3) for 1, 1.988(5) for 2
Fig. 7. (a) Molecular structure of 4 with atom labeling scheme. Atomic displacement parameters are shown at 50% probability level. Hydrogen atoms and disordered free
nitrate group are omitted for clarity, (b) 1D hydrogen bonded network in 4 view along b axis.

P.M. Vimal Kumar, P.K. Radhakrishnan / Polyhedron 29 (2010) 2335–2344
2343
and 2.11(5) for 4] and Co–O(pyrazolone) [1.967(2) for 1, 1.963(4)
for 2 and 2.003(4) for 4] bond length (Å) follow the trend 1 ffi 2 < 4.
We have investigated the non covalent interactions in the crys-
tal packing of 1, 2 and 4. The complex 1 found to be involved in
weak hydrogen bonding with the metal coordinated chloride and
bonded network (Fig. 5c and d). The complex 2 was found to be in-
volved in weak hydrogen bonding with the metal coordinated bro-
mide and N–H of benzimidazole ring via N(3)–H(3A)ꢀꢀꢀBr(2)
0
interactions [NꢀꢀꢀBr = 3.497(6) ÅA; \N–HꢀꢀꢀBr = 135(7)°] resulting in
the formation of a hydrogen bonded dimer (Fig. 6b) which on fur-
N–H of benzimidazole ring via N(3)–H(3B)ꢀꢀꢀCl(1) interactions
ther propagation via C(14)–H(14A)ꢀꢀꢀBr(1)[CꢀꢀꢀBr = 3.689(7)–
0
0
[NꢀꢀꢀCl = 3.338(3) ÅA; \N–HꢀꢀꢀCl = 148(4)°] resulting in the formation
3.736(8) ÅA; \C–HꢀꢀꢀBr = 148.3–152.1°] interaction lead to the for-
mation of 2D hydrogen bonded corrugated sheet (Fig. 6c and d).
The complex 4 was found to be involved in weak hydrogen bond-
of a hydrogen bonded dimer (Fig. 5b) which on further propagation
0
via C(15)–H(15C)ꢀꢀꢀCl(2) [CꢀꢀꢀCl = 3.614(4) ÅA; \C–HꢀꢀꢀCl = 163.7°]
interaction lead to the formation of 2D microporous hydrogen
ing with N–H of benzimidazole ring and nitrate counter anion via
0
N–HꢀꢀꢀO interactions [NꢀꢀꢀO = 2.72(2)–3.03(2) ÅA; \N–HꢀꢀꢀO =
140(8)–162(9)°] leading to the formation of 1D hydrogen bonded
network (Fig. 7b). Relevant hydrogen bonding parameters are col-
lected in Table 3.
Table 4
Selected bond lengths (Å) and angles (0) for 1, 2 and 4.
Bond lengths
Bond angles
3.6. Gas adsorption–desorption studies
1
Co(1)–O(1) 1.967(2)
O(1)–Co(1)–N(4)
95.37(11)
O(1)–Co(1)–Cl(2)
117.28(8)
N(4)–Co(1)–Cl(2)
113.86(9)
N(4)–Co(1)–Cl(1)
107.22(8)
Cl(2)–Co(1)–Cl(1)
112.53(4)
C(16)–O(1)–Co(1)
119.5(2)
C(18)–N(4)–C(12)
106.3(3)
C(18)–N(4)–Co(1)
124.7(2)
C(12)–N(4)–Co(1)
128.0(2)
C(16)–C(17)–C(18)
123.2(3)
N(4)–C(18)–N(3)
110.8(3)
N(4)–C(18)–C(17)
122.3(3)
Since the X-ray crystallographic study reveals that [Co(Op-
dac)Cl2] (1) has a 2D porous structure, it is selected for the hydro-
gen sorption measurement. The hydrogen sorption isotherms of
the complex were measured at 77 K up to 1 bar pressure. The
hydrogen adsorption–desorption isotherm (Fig. 8) exhibits Type-I
sorption behavior according to Brauner classification, typical for
adsorption in microporous materials [42,43]. This type of isotherm
shows an initial steep increase, corresponding to progressive filling
of the micropores, and then a plateau is reached at higher pres-
sures, corresponding to monolayer coverage. Indeed, for micropo-
rous materials the small pore dimensions of the adsorbent limit
the adsorption only to one or a few molecular layers [44]. The
Langmuir model describes Type-I isotherms with a kinetic ap-
proach and assuming that the adsorption enthalpy is independent
from the coverage [45]. The hydrogen adsorption isotherm was
reversible with pressure which means that the adsorbed hydrogen
can be easily desorbed by reducing the pressure. Also the shape of
isotherm shows that hydrogen sorption capacity was not saturated
at 1 bar pressure; higher hydrogen sorption capacities can be ob-
tained by increasing the hydrogen adsorption pressure.
Co(1)–N(4) 1.997(3)
Co(1)–Cl(2)
2.2176(11)
Co(1)–Cl(1)
2.2360(11)
O(1)–C(16) 1.270(4)
N(4)–C(18) 1.339(4)
C(17)–C(18)
1.445(5)
C(16)–N(2)–N(1) 108.8(3) N(3)–C(18)–C(17)
126.9(3)
O(1)–C(16)–N(2) 120.6(3) O(1)–C(16)–C(17)
133.0(3)
N(2)–C(16)–C(17)
106.4(3)
2
Co(1)–O(1) 1.963(4)
O(1)–Co(1)–N(4)
96.13(19)
O(1)–Co(1)–Br(2)
114.04(14)
N(4)–Co(1)–Br(2)
107.83(15)
N(4)–Co(1)–Br(1)
109.33(15)
C(12)–N(4)–Co(1)
129.2(4)
C(16)–C(17)–C(18)
124.1(6)
N(4)–C(18)–N(3)
110.2(5)
N(4)–C(18)–C(17)
122.7(6)
Co(1)–N(4) 1.988(5)
Co(1)–Br(1)
2.3482(12)
Co(1)–Br(2)
2.3983(12)
O(1)–C(16) 1.275(7)
Br(1)–Co(1)–Br(2)
115.55(4)
N(3)–C(18)–C(17)
127.1(6)
3.6.1. Surface area measurement
The Langmuir surface area of the synthesized material was calcu-
lated by fitting the hydrogen adsorption isotherm data at 77 K into
Langmuir isotherm model. The cross-sectional area of one hydrogen
molecule is taken as 0.142 nm2 for the calculation of Langmuir sur-
N(4)–C(18) 1.342(7)
C(16)–O(1)–Co(1)
119.9(4)
C(16)–N(2)–N(1) 108.5(5) O(1)–C(16)–C(17)
132.4(6)
C(18)–N(4)–C(12)
106.2(5)
O(1)–C(16)–N(2)
120.7(6)
C(17)–C(18)
1.427(8)
N(2)–C(16)–C(17)
106.9(5)
C(18)–N(4)–Co(1)
124.6(4)
6
5
4
3
4
Co(1)–O(1) 2.003(4)
O(1)–Co(1)–N(4)
90.96(17)
O(1)–Co(1)–O(1) 117.5(3)
N(4)–Co(1)–N(4) 177.9(3)
O(1)–Co(1)–O(2)
149.91(19)
Co(1)–N(4) 2.120(5)
Co(1)–O(2) 2.192(5)
O(1)–C(16) 1.258(7)
O(2)–N(5) 1.259(6)
N(4)–Co(1)–O(2)
92.99(19)
O(3)–N(5) 1.226(12) O(2)–Co(1)–O(2) 58.3(3)
O(4A)–N(6) 1.284(8) C(16)–O(1)–Co(1)
123.3(4)
2
O(4B)–N(6)
1.383(10)
N(5)–O(2)–Co(1) 92.8(4)
Adsorption
Desorption
O(5A)–N(6) 1.444(9) N(3)–C(18)–C(17)
126.2(5)
1
0
O(5B)–N(6)
O(1)–C(16)–N(2) 121.6(5)
1.386(10)
O(4A)–O(5B) 0.77(3) O(1)–C(16)–C(17)
0.0
0.2
0.4
0.6
0.8
1.0
132.2(5)
O(5A)–O(5B) 1.36(4) N(2)–C(16)–C(17)
106.2(5)
Pressure (bar)
Fig. 8. H2 adsorption–desorption isotherms in [Co(Opdac)Cl2] (1) at 77 K.

2344
P.M. Vimal Kumar, P.K. Radhakrishnan / Polyhedron 29 (2010) 2335–2344
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Langmuir surface area obtained for the material was 41.2 m2/g.
4. Conclusion
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hydrogen bonded network. These intermolecular interactions are
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Acknowledgements
We are thankful to the Head, SAIF and IIT Madras for the single
crystal X-ray diffraction measurements.
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