Zn(II), Cd(II) and Hg(II) complexes with 1-(p-methoxybenzyl)-2-(p- methoxyphenyl)benzimidazole: Syntheses, structures and luminescence

Article abstract of DOI:10.1016/j.poly.2011.02.018

Five coordination compounds Zn(mbmpbi)₂Cl₂ (1), Zn(mbmpbi)₂Br₂ (2), Cd(mbmpbi)₂Cl₂ (3), Hg(mbmpbi)₂Cl₂ (4) and Hg(mbmpbi)₂Br ₂ (5) were synthesized by the reaction of 1-(p-methoxybenzyl)-2-(p- methoxyphenyl)benzimidazole (mbmpbi) with the corresponding metal halides. The complexes have been characterized by elemental analysis, conductance measurements, FT-IR, ¹H NMR and photoluminescence spectral studies. The ligand mbmpbi exhibits the N-benzimidazole coordination. The structures of 3-5 have been determined by single crystal X-ray diffraction. These three complexes are isostructural, crystallizing in the monoclinic system, P2/n space group with a distorted tetrahedral geometry around the metal ion. Zn(II) and Cd(II) complexes show strong blue emission in solid state at room temperature.

Full text of DOI:10.1016/j.poly.2011.02.018

Polyhedron 30 (2011) 1299–1304
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Polyhedron
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Zn(II), Cd(II) and Hg(II) complexes with 1-(p-methoxybenzyl)-2-(p-methoxy-
phenyl)benzimidazole: Syntheses, structures and luminescence
M.N. Manjunatha a, Amol G. Dikundwar b, K.R. Nagasundara a,
⇑
a Department of Studies in Chemistry, Central College Campus, Bangalore University, Bangalore 560 001, Karnataka, India
b Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Five coordination compounds Zn(mbmpbi)2Cl2 (1), Zn(mbmpbi)2Br2 (2), Cd(mbmpbi)2Cl2 (3),
Hg(mbmpbi)2Cl2 (4) and Hg(mbmpbi)2Br2 (5) were synthesized by the reaction of 1-(p-methoxyben-
zyl)-2-(p-methoxyphenyl)benzimidazole (mbmpbi) with the corresponding metal halides. The com-
plexes have been characterized by elemental analysis, conductance measurements, FT-IR, 1H NMR and
photoluminescence spectral studies. The ligand mbmpbi exhibits the N-benzimidazole coordination.
The structures of 3–5 have been determined by single crystal X-ray diffraction. These three complexes
are isostructural, crystallizing in the monoclinic system, P2/n space group with a distorted tetrahedral
geometry around the metal ion. Zn(II) and Cd(II) complexes show strong blue emission in solid state
at room temperature.
Received 21 October 2010
Accepted 9 February 2011
Available online 16 February 2011
Keywords:
Zn(II)
Cd(II) and Hg(II) complexes
1-(p-Methoxybenzyl)-2-(p-
methoxyphenyl)benzimidazole (mbmpbi)
X-ray structure
Ó 2011 Elsevier Ltd. All rights reserved.
Luminescence
1. Introduction
benzyl)-2-(p-methoxyphenyl)benzimidazole (mbmpbi) have not
been investigated earlier. The ligand mbmpbi is interesting from
During the last decade benzimidazole derivatives including
those containing substituents such as thiazolyl, ester, carboxyl, al-
kyl and amine groups have proved to be important antibacterial
and antifungicidal agents [1–14]. They have attracted a great deal
of attention in recent years due not only to their biological activi-
ties [10,11], but also their strong coordinating abilities as multid-
entate ligands. By varying the substituents on the benzimidazole
ring, the structure, photophysical and photochemical properties
of the complexes could be tuned [15,16]. Among coordination
compounds, d10 metal complexes are of interest due to their high
thermal stability and good photoluminescent and electrolumines-
cent properties [17–21]. For example, Zn(II) complexes with nitro-
gen and oxygen donor atoms exhibit blue [22–28] and white
emissions [29–31]. Blue luminescent complexes are very impor-
tant materials because they though rare are useful for full-color,
flat-panel display technology. They can be used as fluorescent
sensors for the detection of aromatic molecules, since the lumines-
cence intensity is sensitive to the environment and have absorp-
tion band in UV or near-UV region, where most aromatic
molecules show absorptions [32,33]. Our recent efforts have been
focused largely on synthesizing novel transition metal complexes
showing photoluminescence. The complexes of the 1-(p-methoxy-
the structural point of view because of its bulky nature. Here, we
report the synthesis, structure and photoluminescence studies of
Zn(II), Cd(II) and Hg(II) complexes of mbmpbi.
2. Experimental
2.1. Materials and physical measurements
All the chemicals were reagent grade; they were used as such
without further purification. The solvents, methanol, ethanol,
N,N-dimethylformamide, acetonitrile, etc. were purified according
to the standard methods. The conductivity data of the complexes
in nitrobenzene (10ꢀ3 M) were obtained with a digital conductivity
meter (Elico model-180) at room temperature. Elemental analyses
were carried out using elementar Vario EI III and Carlo Erba-1108
instruments. The FT-IR spectra of mbmpbi and its complexes in
KBr pellet were recorded on a Nicolet impact 400D spectrometer
in the range 4000–400 cmꢀ1. The 1H NMR spectra of the complexes
in CDCl3 (using TMS as internal reference) were recorded on a Bru-
ker 400 MHz spectrometer. Mass spectrum of the mbmpbi was re-
corded using Z-spray electrospray ionization (ESI) source on
Esquire 300 plus, Bruker Daltonics. Melting points were obtained
using an electrothermal melting point apparatus. The lumines-
cence properties were measured using a LS-55 Perkin–Elmer fluo-
rescence spectrophotometer at room temperature (298 K).
⇑
Corresponding author. Tel.: +91 9980685848.
E-mail address: kudernag@yahoo.com (K.R. Nagasundara).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.02.018

1300
M.N. Manjunatha et al. / Polyhedron 30 (2011) 1299–1304
2.2. Syntheses of compounds
tained from the mother liquor on slow evaporation of the solvent
at ambient temperature over a week. X-ray diffraction data were
collected on an Oxford Xcalibur diffractometer [34] equipped with
2.2.1. Synthesis of the ligand 1-(p-methoxybenzyl)-2-(p-
methoxyphenyl)benzimidazole (mbmpbi)
an Eos CCD detector (Mo Ka radiation, graphite monochromator,
A mixture of o-phenylenediamine (0.1 mol) and p-methoxy-
benzaldehyde (0.2 mol) in benzene (100 mL) was refluxed for 2 h
on a steam bath. On keeping it overnight a crystalline solid sepa-
rates out. It was collected by filtration and washed with aqueous
n-hexane and dried. It was recrystallised from ethanol and dried
in vacuo over P2O5 to get pale yellow crystals (yield 90%).
k = 0.71073 Å). The structures were solved by direct methods using
SHELXS-97 and refined with full matrix least-squares technique by
using SHELXL-97 [35]. The non-hydrogen atoms were refined aniso-
tropically and all the hydrogen atoms were fixed geometrically and
refined isotropically. Geometry calculations were performed using
PLATON [36], ORTEP representations were prepared using ORTEP-3
[37] and packing diagrams were made using CAMERON [38] incorpo-
rated in the WINGX [39] software package.
2.2.2. Synthesis of metal complexes
The divalent salts of zinc, cadmium and mercury (1 mmol) were
separately dissolved in ethanol (25 mL) and to each of this was
added a solution of mbmpbi (2 mmol) in hot ethanol (15 mL). It
was refluxed for about 3 h for zinc halides and 6 h for other metal
halides. The resulting solid was filtered, washed thoroughly with
cold ethanol and dried in vacuo over P2O5 (yield ꢁ85–90%).
3. Results and discussion
3.1. Syntheses
The analytical data for mbmpbi and its metal complexes are gi-
ven in Table 1. The data show that the complexes have the general
formula [M(mbmpbi)2X2] (where M = Zn, Cd, Hg,; X = Clꢀ, Brꢀ).
They are quite stable at room temperature. The complexes are
insoluble in common organic solvents except in N,N-dimethyl-
formamide, nitrobenzene and dimethyl sulfoxide. The mass spec-
trum of mbmpbi showed the molecular ion peak at m/z 344
corresponding to the M+1 species. The molar conductance values
suggest the complexes to be non-electrolytes in nitrobenzene
(10ꢀ3 M) showing that the halogen atoms are coordinated to the
metal [40]. The compounds were characterized on the basis of
the following studies.
2.3. X-ray crystallographic analyses
Crystal structures of the three complexes were determined by
single crystal X-ray diffraction technique. Single crystals were ob-
Table 1
Physical and analytical data of the compounds.
Compound
Empirical formula
(mol. wt.)
Color
Anal. found (Calc.) (%)
C
H
N
mbmpbi
C22H20N2O2 (344)
Yellow
75.92
5.69
7.94
(76.74)
Colorless 65.06
(64.05)
Colorless 57.82
(57.78)
Colorless 60.51
(59.66)
Colorless 55.53
(55.03)
Colorless 50.62
(50.36)
(5.81) (8.14)
5.00 7.21
(4.85) (6.79)
4.38 6.21
(4.36) (6.19)
4.59 6.43
(4.40) (6.21)
4.16 5.51
(4.17) (5.83)
3.74 5.69
(3.81) (5.34)
3.2. Spectral studies
Zn(mbmpbi)2Cl2 C44H40N4O4Cl2Zn
(1) (824.3)
Zn(mbmpbi)2Br2 C44H40N4O4Br2Zn
(2) (913.2)
Cd(mbmpbi)2Cl2 C44H40N4O4Cl2Cd
(3) (871.3)
Hg(mbmpbi)2Cl2 C44H40N4O4Cl2Hg
(4) (959.5)
Hg(mbmpbi)2Br2 C44H40N4O4Br2Hg
(5) (1048.4)
The IR spectral data of mbmpbi and its complexes are presented
in Table 2. The IR spectra of the complexes are almost similar to
that of the spectrum of the uncoordinated mbmpbi except for min-
or shifts in the position of some of the peaks. A strong band around
1616 cmꢀ1 in the free ligand assigned to C@N stretching shifted to-
wards lower frequencies in the spectra of complexes, suggesting
that the tertiary nitrogen of the benzimidazole coordinates to the
metal atom [41–45]. The bands at 1284, 1016, 602 and 457 cmꢀ1
are assigned to benzimidazole ring vibrations [46]. The CAN
stretching band is found at 1320 cmꢀ1 [47,48].
Table 2
Infrared spectral data (cmꢀ1) of mbmpbi and its complexes.
NMR spectroscopy has proven to be useful for establishing the
structure of many benzimidazole ligands as well as their com-
plexes. The assignments for the 1H NMR spectra of the mbmpbi
and its complexes are as shown in Table 3. The peaks were as-
signed on the basis of earlier studies made on coordinated benz-
imidazole and its complexes [46]. The 1H NMR spectrum of the
free ligand exhibits a singlet at d 3.81 ppm corresponding to meth-
oxy protons. The peak at d 5.42 ppm arises from the methylene
protons of the benzyl ring while the phenyl protons of the benzyl
Compound
m(C@N)
m(OCH3)
m(CH2)
m(C@C)
m(CAN)
mbmpbi
1616
1616
1616
1616
1610
1610
2800
2836
2838
2841
2831
2836
3000
3000
3000
2940
2966
2955
1460
1471
1471
1460
1460
1460
1331
1305
1305
1300
1350
1351
1
2
3
4
5
Table 3
1H NMR spectral data (d, ppm) of the mbmpbi and its complexes.
Compound OCH3
ACH2A
H-200, 600
H-300, 500
6.85(d)
H-20 , 60
H-30 , 50
H-4
H-5, 6
H-7
mbmpbi
3.81(s)
5.4(s)
6.70(d)
7.63(d)
7.03(d)
7.84(d)
7.30(m)
7.21(d)
1
2
3.8(s) (ꢀ0.01) 5.1(s) (ꢀ0.3)
6.77(d) (ꢀ0.2) 6.56(d) (0.28)
7.48(d) (0.16)
7.47(d)
6.9(d) (0.16)
6.92(d)
8.7(d) (0.86)
8.69(d)
(0.85)
7.3(m) (0.01)
7.29(m)
7.1(d) (ꢀ0.12)
3.79(s)
5.2(s) (ꢀ0.2)
6.8(d) (0.10)
6.68(d)
7.2(d) (ꢀ0.01)
(ꢀ0.02)
(ꢀ0.17)
(ꢀ0.16)
(ꢀ0.11)
(ꢀ0.01)
3
4
5
3.79(s)
5.1(s) (ꢀ0.3)
6.77(d) (ꢀ0.2) 6.56(d) (0.28)
7.48(d) (0.16)
6.9(d) (0.16)
8.7(d) (0.86)
7.3(m) (0.01)
7.1(d) (ꢀ0.12)
(ꢀ0.02)
3.83(s) (0.01)
5.32(s)
6.94(d)
6.85(d)
7.56(d)
(ꢀ0.07)
6.98(d)
(ꢀ0.05)
8.01(d)
(0.17)
7.38(m) (0.10)
7.26(d) (0.05)
(ꢀ0.05)
(ꢀ0.02)
3.8(s) (ꢀ0.02) 5.3(s) (ꢀ0.09) 6.93(d)
(ꢀ0.03)
6.8(d) (ꢀ0.05) 7.46(d)
(ꢀ0.17)
6.9(d) (ꢀ0.13) 8.0(d)
(ꢀ0.16)
7.24(m)
(ꢀ0.04)
7.24(d)
(ꢀ0.03)
The values in the parenthesis are c.i.s.

M.N. Manjunatha et al. / Polyhedron 30 (2011) 1299–1304
1301
Table 4
Table 5
Crystallographic data and structure refinements summary for the metal complexes
Coordination geometry around Cd(II) and Hg(II) atoms in complexes 3–5A.
3–5.
Bond distances (Å)
Bond angles (°)
3
4
5
3
CCDC number
Empirical
formula
Formula weight
Crystal
776526
776528
776527
Cd(1)–Cl(1)
Cd(1)–N(1)
Cd(1)–Cl(1)a
Cd(1)–N(1)a
2.441(17)
2.252(2)
2.441(17)
2.252(2)
Cl(1)–Cd(1)–N(1)
Cl(1)–Cd(1)–Cl(1)a
Cl(1)–Cd(1)–N(1)a
N(1)–Cd(1)–Cl(1)a
N(1)–Cd(1)–N(1)a
Cl(1)a–Cd(1)–N(1)a
109.9(6)
C44H40N4O4Cl2Cd C44H40N4O4Cl2Hg C44H40N4O4Br2Hg
111.2(4)
107.8(6)
107.8(6)
110.2(8)
109.9(6)
871.3
block
959.5
block
1048.4
block
morphology
T (K)
293(1)
293(1)
293(1)
4
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
0.71073
monoclinic
P2/n
0.71073
monoclinic
P2/n
0.71073
monoclinic
P2/n
Hg(1)–Cl(1)
Hg(1)–N(1)
Hg(1)–Cl(1)a
Hg(1)–N(1)a
2.461(13)
2.288(2)
2.461(13)
2.288(2)
Cl(1)–Hg(1)–N(1)
Cl(1)–Hg(1)–Cl(1)a
Cl(1)–Hg(1)–N(1)a
N(1)–Hg(1)–Cl(1)a
N(1)–Hg(1)–N(1)a
Cl(1)a–Hg(1)–N(1)a
107.3(6)
114.0(4)
109.9(6)
109.9(6)
108.3(7)
107.3(6)
12.912(50)
9.952(50)
15.533(50)
91.867(5)
1994.9(14)
2
1.45
0.729
891.9
2.4, 26.00
12.915(6)
9.982(5)
15.534(8)
91.803(1)
2001.7(2)
2
1.59
4.026
955.8
2.0, 26.4
12.803(4)
10.228(3)
15.551(6)
90.818(3)
2036.2(1)
2
1.71
5.789
1027.8
3.2, 26.0
b (Å)
c (Å)
5
b (°)
V (Å3)
Hg(1)–Br(1)
Hg(1)–N(1)
Hg(1)–Br(1)a
Hg(1)–N(1)a
2.569(6)
2.304(2)
2.569(6)
2.304(2)
Br(1)–Hg(1)–N(1)
Br(1)–Hg(1)–Br(1)a
Br(1)–Hg(1)–N(1)a
N(1)–Hg(1)–Br(1)a
N(1)–Hg(1)–N(1)a
Br(1)a–Hg(1)–N(1)a
106.1(6)
113.7(2)
111.4(6)
111.4(6)
108.2(8)
106.1(6)
Z
Dcalc (g/ml)
l
(mmꢀ1
)
F (0 0 0)
h (minimum,
maximum)
Independent
reflections
No. of
A
a = 1/2 ꢀ x, y, 3/2 ꢀ z.
3930
251
4105
251
4002
251
parameters
R_obs, wR2_obs
0.043, 0.102
ꢀ0.545, 0.510
0.026, 0.060
ꢀ0.361, 0.772
0.029, 0.061
ꢀ0.947, 0.889
3.3. Crystal structures
D
qmin
,
D
qmax
(e Åꢀ3
)
Crystal structure of the ligand mbmpbi has been reported by
Yang et al. [49]. The crystal structures of the complexes 3–5 have
been determined by single crystal X-ray diffraction. The complexes
Zn(mbmpbi)2Cl2 (1) and Zn(mbmpbi)2Br2 (2) did not yield crystals
suitable for single crystal X-ray diffraction. Crystallographic data
for the complexes Cd(mbmpbi)2Cl2 (3), Hg(mbmpbi)2Cl2 (4) and
Hg(mbmpbi)2Br2 (5) are summarized in Table 4. Selected bond
distances and bond angles are given in Table 5. Chemically
significant intramolecular and intermolecular interactions in the
Goodness-of-fit
(GOF)
0.960
1.038
0.960
ring are observed in the range d 7.20–8.21 ppm. The signals due to
the phenyl ring protons are found between d 6.63 and 7.60 ppm. A
comparison of the spectra of complexes with that of the free ligand
spectrum indicated both positive and negative coordination in-
duced shift (c.i.s = dcomplex ꢀ dligand) showing that there is consider-
able deshielding effect upon coordination. Therefore, it is proposed
that mbmpbi acts as a monodentate ligand bonding through the
imidazole nitrogen.
complexes are given in Table 6. Torsion angle
u, (N1–C1–C8–C9)
in the crystal structures of mbmpbi and its complexes 3–5 are
given in Table 7.
Table 6
Chemically significant intramolecular and intermolecular interactions in the complexes 3–5.
D–Hꢂ ꢂ ꢂAa
R(D–H) (Å)
r(D–A) (Å)
r(Hꢂ ꢂ ꢂA) (Å)
\D–H (°)
Symmetry
3
C4–H4ꢂ ꢂ ꢂCl1
0.93
0.93
0.93
0.93
0.97
0.96
3.605(3)
3.766(4)
3.640(4)
3.444(4)
3.489(4)
3.710(4)
2.779(1)
2.923(2)
2.848(2)
2.577(2)
2.62
148.5(2)
151.4(2)
143.7(2)
155.2(2)
150
x, y, z
C12–H12ꢂ ꢂ ꢂCl1
C9–H9ꢂ ꢂ ꢂCl1
ꢀx + 1/2, y, ꢀz + 1/2
ꢀx + 1/2, y, ꢀz + 1/2
ꢀx ꢀ 1/2, y, ꢀz + 1/2
ꢀx, 1 ꢀ y, 1 ꢀ z
ꢀx, ꢀy, 1 ꢀ z
C20–H20ꢂ ꢂ ꢂO2
C(15)–H(15B)ꢂ ꢂ ꢂCg(2)
C(22)–H(22C)ꢂ ꢂ ꢂCg(3)
2.84
152
4
C9–H9ꢂ ꢂ ꢂCl1
0.93
0.93
0.93
0.93
0.97
0.96
3.650(3)
3.621(3)
3.782(3)
3.452(3)
3.492(3)
3.711(4)
2.858(1)
2.790(1)
2.939(1)
2.587(2)
2.62
143.8(1)
149.2(2)
151.5(1)
155.0(1)
150
x, y, z
C4–H4ꢂ ꢂ ꢂCl1
ꢀx + 1/2, y, ꢀz + ½
C12–H12ꢂ ꢂ ꢂCl1
C20–H20ꢂ ꢂ ꢂO2
C(15)–H(15A)ꢂ ꢂ ꢂCg(2)
C(22)–H(22A)ꢂ ꢂ ꢂCg(3)
x, y + 1, z
ꢀx + 1/2, y, ꢀz + 1/2
1 ꢀ x, ꢀy, ꢀz
1 ꢀ x, 1 ꢀ y, ꢀz
2.84
151
5
C9–H9ꢂ ꢂ ꢂBr1
0.93
0.93
0.93
0.93
0.97
0.96
0.96
3.746(3)
3.745(3)
3.475(3)
3.296(4)
3.533(3)
3.710(3)
3.810(4)
2.934(1)
2.912(1)
2.631(2)
2.560(2)
2.67
146.6(2)
149.8(2)
151.3(2)
136.3(2)
148
x, y, z
C4–H4ꢂ ꢂ ꢂBr1
ꢀx + 1/2, y, ꢀz + 1/2
ꢀx ꢀ 1/2, y, ꢀz + 1/2
x, y + 1, z
ꢀx, 2 ꢀ y, ꢀz
ꢀx, 1 ꢀ y, ꢀz
1 + x, y, z
C20–H20ꢂ ꢂ ꢂO2
C5–H5ꢂ ꢂ ꢂO2
C(15)–H(15B)ꢂ ꢂ ꢂCg(2)
C(22)–H(22A)ꢂ ꢂ ꢂCg(3)
C(14)–H(14B)ꢂ ꢂ ꢂCg(4)
2.85
2.98
149
145
a
Cg(2), Cg(3) and Cg(4) are the centroids of the rings C(2)–C(3)–C(4)–C(5)–C(6)–C(7), C(8)–C(9)–C(10)–C(11)–C(12)–C(13) and C(16)–C(17)–C(18)–C(19)–C(20)–C(21),
respectively.

1302
M.N. Manjunatha et al. / Polyhedron 30 (2011) 1299–1304
Table 7
Torsion angle
complexes 3–5.
adopt a nonplanar geometry about benzimidazole and the para-
methoxyphenyl rings, the torsion angle ( ), N1–C1–C8–C9 being
36.7°. The packing of the molecules in the crystal lattice is mainly
u, (N1–C1–C8–C9) in the crystal structures of mbmpbi and the
u
Compound
mbmpbi
36.7(5)
3
4
5
governed by C–Hꢂ ꢂ ꢂO hydrogen bonds and C–Hꢂ ꢂ ꢂ
p intermolecular
interactions.
u
(°)
46.8(4)
47.4(3)
47.2(4)
3.5. Metal complexes
3.4. Ligand mbmpbi
All the three complexes exhibit dissymmetric molecular struc-
tures with a twofold symmetry axis and crystallize in the same
space group, P2/n with two molecules in a unit cell. The molecular
structure remains almost similar in all the complexes irrespective
of the metal ion and/or the terminal halide ligands. Notably the
molecular conformation of the ligand changes significantly upon
coordination with the metal ion and remains almost similar in all
the three complexes 3–5. Departure from coplanarity of the benz-
imidazole and the paramethoxyphenyl rings, is given by the tor-
The compound mbmpbi crystallizes in orthorhombic system,
space group, Pna21 with Z = 4. Fig. 1 shows the molecular structure
of mbmpbi with the numbering scheme. Molecules of mbmpbi
sion angle (
u), N1–C1–C8–C9 (Table 7). It can be seen that the
value of is almost similar in all the complexes increasing from
u
36.7° in mbmpbi to ꢁ47° in the complexes, regardless of the metal
ion or the terminal halide ligands (Cl/Br). Further the intermolecu-
lar C–Hꢂ ꢂ ꢂO interaction, as in the mbmpbi, is retained in all the
three complexes in addition to the intramolecular and intermolec-
ular interactions involving the halogen atoms (Cl for complexes 3
and 4 and Br in the case of 5).
In the crystal structures 3–5, the metal ions (Cd(II) and Hg(II))
possess a distorted tetrahedral geometry and are located on a two-
fold rotation axis. Fig. 2(a–c) shows the molecular structures of 3, 4
and 5, respectively. The tetrahedral geometry around the metal ion
is completed by two halogen atoms (Cl or Br) and two coordinating
nitrogen atoms from mbmpbi. The HgACl bond distances in 4 are
within a range of divalent HgACl bond lengths reported in other
studies [50,51]. The distortion from tetrahedral coordination
around the metal ion is reflected in the deviations from the normal
tetrahedral angle which ranges from 106.1° to 113.7° (Table 2) in
the structures 3–5.
The chlorine atoms in the structures 3 and 4 form bifurcated
intramolecular C–Hꢂ ꢂ ꢂCl and intermolecular C–Hꢂ ꢂ ꢂCl hydrogen
bonds. A weak intermolecular C–Hꢂ ꢂ ꢂO hydrogen bond further
links the molecules in a three-dimensional network, leading to a
close packed arrangement as shown in Fig. 3. Intermolecular C–
Hꢂ ꢂ ꢂ
p interactions provides additional stability to the crystal struc-
tures (Table 6). Unlike complexes 3 and 4, complex 5 does not
show intermolecular C–Hꢂ ꢂ ꢂhalogen (Br) interaction. Instead the
bromine atom forms an intramolecular (C4–H4ꢂ ꢂ ꢂBr1) hydrogen
bonding. Weak intermolecular C–Hꢂ ꢂ ꢂO and C–Hꢂ ꢂ ꢂ
p interactions,
a common feature of the complexes 3 and 4, link the molecules
in a three-dimensional network, leading to a close packed arrange-
ment of the molecules (Table 6).
3.6. Luminescent properties
Photoluminescence properties of d10 metal coordination com-
pounds have been extensively studied due to their potential
applications as luminescent materials [52]. Coordination com-
pounds have been reported to have ability to adjust the emission
wavelength of organic substances through the incorporation of
metal centers and the anions [53,54]. Therefore, it is important
to investigate the luminescence properties of coordination com-
pounds in view of their potential applications as light-emitting
diodes (LEDs) [53,54]. The luminescent behaviors of the ligand
and its complexes 1–5 were studied in the solid state at room
temperature. The complexes 2, 4 and 5 do not show any lumines-
cence. Intense photoluminescence was observed only for the
complexes 1 and 3, and the emission spectra are shown in Figs.
3 and 4. The complexes of zinc chloride and cadmium chloride
Fig. 1. ORTEP views of the complexes 3 (a), 4 (b) and 5 (c) drawn at 30% probability
thermal ellipsoids showing the numbering scheme for atoms coordinated to the
metal ions.

M.N. Manjunatha et al. / Polyhedron 30 (2011) 1299–1304
1303
Fig. 2. A general packing diagram of complexes 3–5 viewed down the a-axis showing intermolecular interactions by the dotted lines.
are luminescent in the solid state at room temperature. The zinc
chloride complex displays emissions at 450 nm (exc. 254 nm and
350 nm). In the case of cadmium chloride complex, the emission
occurs at a higher energy 365 nm (exc. 254 nm). The intensity of
the emission is found to be higher than that of the free ligand.
The variation in emission intensities can also be rationalized in
terms of the coordination effect and the heavy atom effect. No
emissions originating from metal-centered excited states are
expected for the Zn(II) and Cd(II) complexes, since they are diffi-
cult to oxidize or reduce due to their d10 configuration. Thus the
1000
800
600
400
200
0
- Emiss: 450 nm, Exc: 254 nm
- Emiss: 450 nm, Exc: 350 nm
emission observed in the complexes is assigned to the
p ?
p⁄
intraligand fluorescence [55]. The observed red or blue shift of
the emission maxima between the ligand and its complexes is
attributed to the influence of the coordination of the metal to
the ligand [56]. The luminescent properties showed that the
photoluminescence arose from the intraligand emission from
the excited state, and that they are novel potential candidates
for applications in optoelectronic devices.
400
500
600
700
Wavelength nm
Fig. 3. Solid-state emission spectrum of complex 1 at room temperature.
4. Conclusion
Mononuclear M(mbmpbi)2X2 complexes (M = Zn, Cd, Hg; X = Cl,
Br) have been discussed from both synthetic and physicochemical
properties point of view. The crystal structures of 3, 4 and 5 have
brought out the isostructural features of these complexes with
the metal ion in distorted tetrahedral environment. The complexes
show intraligand photoluminescent properties. The zinc complex
shows emissions with different intensities depending on the exc.
wavelength.
1000
-Emiss: 365 nm, Exc: 254 nm
800
600
400
200
0
Acknowledgement
We thank the UGC-DRS for their financial assistance. Our sin-
cere thanks are due to the Raman Research Institute, Bangalore,
for the elemental analysis and to the NMR Research Centre, Indian
Institute of Science, Bangalore, for NMR spectra. Our thanks are
also due to Prof. D.N. Sathyanarayana and Prof. T.N. Guru Row, In-
dian Institute of Science, Bangalore for their valuable suggestions.
MNM thanks M.S. Ramaiah Institute of Technology, Bangalore, for
their support and encouragement and AGD thanks CSIR New Delhi,
for a fellowship.
200
300
400
500
Wavelength in nm
Fig. 4. Solid-state emission spectrum of complex 3 at room temperature.

1304
M.N. Manjunatha et al. / Polyhedron 30 (2011) 1299–1304
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