Synthesis and characterization of 3D metal complexes based on 1-(4-nitrophenyl)imidazole

Article abstract of DOI:10.1246/bcsj.20100154

Synthesis and characterization of complexes with formulations [M(nopi) ₂Cl₂] (M= Co, 1; Ni, 3; Cu, 6; Zn, 9), [M(nopi) ₉](X)₂ (X^- = NO₃^-, M = Co, 2; Ni, 4; Zn, 10; X^-

Full text of DOI:10.1246/bcsj.20100154

© 2011 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 84, No. 2, 205–210 (2011)
205
Synthesis and Characterization of 3d Metal Complexes Based
on 1-(4-Nitrophenyl)imidazole
Ashish Kumar Singh, Mahendra Yadav, and Daya Shankar Pandey*
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi-221 005, U.P., India
Received May 31, 2010; E-mail: dspbhu@bhu.ac.in
Synthesis and characterization of complexes with formulations [M(nopi)2Cl2] (M = Co, 1; Ni, 3; Cu, 6; Zn, 9),
¹
¹
¹
¹
[M(nopi)6](X)2 (X = NO3 , M = Co, 2; Ni, 4; Zn, 10; X = ClO4 , M = Ni, 5; Cu, 8; Zn, 11) and [Cu(nopi)4(NO3)2]
(7) imparting 1-(4-nitrophenyl)imidazole (nopi) have been described. The complexes have been characterized by
elemental analyses and spectral (IR, 1H NMR, electronic absorption and emission) studies. Molecular structures of 7 and 9
have been determined crystallographically. Weak interaction studies on 7 and 9 revealed the presence of various
interesting motifs resulting from C-H£N, C-H£Cl, and ³-³ stacking interactions. Temperature-dependent (313-373 K)
pressed pellet conductivity and activation energy calculations from the plot of ln · vs. 1/T suggested semiconducting
behavior of 2, 6, and 7.
Rational design and synthesis of supramolecular coordina-
tion networks have seen a large surge during the past couple of
decades by virtue of possible applications in various areas.1 A
large number of networks have been successfully designed and
synthesized through judicious choice of the ligands and metal
ions.2 Rational design based on covalent or supramolecular
interactions is a key step in self-assembly supramolecular
chemistry.3 It is well established that noncovalent interactions
including electrostatic and hydrogen bonding play a vital role
in constructing supramolecular systems.4 The self-assembly of
metal-organic frameworks is greatly influenced by factors such
as the solvent system, pH of the solution, geometric require-
ments of the metal ions, and counter ions.5 Further, several
forces such as coordination bonding, hydrogen bonding, ³-³
stacking, and electrostatic interactions have been recognized
and used in the construction of extended inorganic networks.
Among these, coordination bonding and hydrogen-bonding
interactions are most important. One synthetic strategy gen-
erally employed in this area is controlled assembly of donor
and acceptor building blocks to generate supramolecular
buildings.6,7 High directionality of hydrogen bonds present in
supramolecular architectures makes them useful in the design-
ing of functional materials with controlled physicochemical,
catalytic, and optical properties.3a-3c,8
Further, imidazole and the ligands based on it have proven to
be very useful. Good coordination ability, diverse coordination
modes, and bridging ability of the imidazole and its derivatives
have prompted their use in the construction of extended solids.7
In this context, 1-(4-cyanophenyl)imidazole (cpi) has attracted
considerable current attention.7a,7b Substitution of the cyano-
phenyl group by nitrophenyl may be useful in fine tuning the
luminescent, catalytic and other properties of the resulting
systems based on 1-(4-nitrophenyl)imidazole (nopi). In con-
tinuation of our previous studies on metal complexes based on
substituted phenylimidazole ligands, we have examined reac-
tivity of nopi with various metal salts and have isolated self-
assembled systems resulting from weak bonding interactions.7
Through this work we present the synthesis and character-
ization of complexes with the formulations [M(nopi)2-
Cl2] (M = Co, 1; Ni, 3; Cu, 6; Zn, 9) and [M(nopi)6](X)2
(X = NO3 , M = Co, 2; Ni, 4; Zn, 10; X = ClO4 , M = Ni,
5; Cu, 8; Zn, 11) and [Cu(nopi)4(NO3)2] (7) and single-crystal
X-ray structures of 7 and 9. Also, we decribe herein results on
weak bonding interactions in 7 and 9, luminescence behavior
of the complexes and pressed pellet conductivity measurements
on 2, 6, and 7.
¹
¹
¹
¹
Experimental
Reagents. All the synthetic manipulations were performed
under nitrogen atmosphere in de-aerated solvents. The solvents
were purified rigorously by standard procedures prior to their
use.9 Metal salts CoCl2, ZnCl2, CuCl2, NiCl2¢6H2O, Co-
(NO3)2¢6H2O, Ni(NO3)2¢6H2O, Cu(NO3)2¢3H2O, Ni(ClO4)2¢
6H2O, Cu(ClO4)2¢6H2O, Zn(ClO4)2¢6H2O, and other reagents
were procured from Sigma-Aldrich and were used as received
without further purifications. 1-(4-Nitrophenyl)imidazole
(nopi) has been prepared by literature procedure using imi-
dazole and 4-fluoronitrobenzene (all Aldrich).10
Instrumentation and General Methods.
Elemental
analyses for C, H, N in the complexes were performed on an
Exter CE-440 CHN analyzer. IR in KBr discs and electronic
spectra in DMSO solution were recorded on a Shimadzu-
8201PC and Shimadzu UV-1700 spectrophotometers, respec-
tively. 1H NMR spectra of 9-11 were acquired in DMSO-d6 on
a JEOL 300 MHz NMR instrument using TMS as an internal
reference. Pressed pellet conductivity of the complexes was
measured on a Keithley 236 Source Measure Unit by employ-
ing a conventional two probe technique. Emission spectra were
recorded on a Varian Carry Eclipse Fluorescence spectropho-
tometer at room temperature.
Synthesis. The following general procedure was employed
for the synthesis of all the complexes: In a typical reaction,
Published on the web February 4, 2011; doi:10.1246/bcsj.20100154

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Bull. Chem. Soc. Jpn. Vol. 84, No. 2 (2011)
Complexes Based on 1-(4-Nitrophenyl)imidazole
respective metal salts (1.0 mM) were treated with 1-(4-nitro-
phenyl)imidazole (nopi) (2.0 mM) in ethanol-water mixture
(1:4 v/v). After stirring well, the reaction mixture was allowed
to heat in a sealed tube at 120 °C for 72 h and the resulting
solution was allowed to cool slowly to ambient temperature.
Crystalline products were separated from the reaction mixture
in a couple of days. It was separated by filtration, washed
several times with water, ethanol, and diethyl ether and dried in
air. Selected characterization data of the complexes are given
below.
1382 (br), 1343, 1305 (vs), 1265 (vs), 1186 (w), 1115, 1063,
963 (w), 853, 783, 684, 614 (s), 520.
[Zn(nopi)2Cl2] (9):
Yield of ca. 59% (0.303 g). MW:
514.63. Anal. Calcd for 9 (C18H14N6O4Cl2Zn): C, 42.01; H,
2.74; N, 16.33%. Found: C, 41.86; H, 2.66; N, 16.04%. IR
(KBr cm¹1): 3421, 3221, 1600 (vs), 1522 (vs), 1385 (s), 1338,
1310 (vs), 1260 (s), 1189 (w), 1059 (s), 962 (w), 825 (vs), 761
(s), 476. -max/nm (-ex/nm) = 381 (298). 1H NMR (DMSO): ¤
7.17 (s, 1H), 7.93 (d, 3H, J = 7.2 Hz), 8.01 (d, 2H, J = 6.6 Hz),
8.46 (s, 1H).
[Co(nopi)2Cl2] (1):
Yield of ca. 65% (0.330 g). MW:
[Zn(nopi)6](NO3)2 (10): Yield of ca. 60% (0.265 g). MW:
1324.43 Anal. Calcd for 10 (C54H42N20O18Zn): C, 48.97; H,
3.20; N, 21.15%. Found: C, 48.43; H, 3.05; N, 21.08%. IR
(KBr pellets, cm¹1): 3470, 3123 (s), 2929, 2851, 1600 (vs),
1521 (s), 1381 (br), 1340, 1307, 1263, 1187 (s), 1111 (s), 1064
(s), 963 (w), 853, 748 (s), 685, 650, 507. -max/nm (-ex/nm) =
378 (288).1H NMR (DMSO): ¤ 7.19 (s, 1H), 7.99 (d, 3H,
J = 9.6 Hz), 8.37 (d, 2H, J = 8.7 Hz), 8.50 (s, 1H).
[Zn(nopi)6](ClO4)2 (11): Yield of ca. 58% (0.270 g). MW:
1399.32. Anal. Calcd for 11 (C54H42N18O20Cl2Zn): C, 46.35;
H, 3.03; N, 18.02%. Found: C, 45.99; H, 3.09; N, 18.24%. IR
(KBr pellets, cm¹1): 3317 (vbr), 3212 (s), 3121 (br), 2230.9
(vs), 1609 (vs), 1521 (s), 1425 (s), 1373 (br), 1308 (vs), 1268
(vs), 1188 (w), 1118 (s), 1065 (vs), 962 (w), 838 (s), 753 (s),
655 (w), 552 (w). -max/nm (-ex/nm) = 369 (278).1H NMR
(DMSO): ¤ 7.28 (s, 1H), 8.03 (d, 3H, J = 8.7 Hz), 8.38 (d, 2H,
J = 8.7 Hz), 8.66 (s, 1H).
Crystallographic Data. Crystals suitable for single-crystal
X-ray diffraction analyses for 7 and 9 were obtained directly
from the reaction mixture after cooling. Preliminary data on the
space group and unit cell dimensions as well as intensity data
were collected for complex 7 on OXFORD DIFFRACTION X
CAUBER-S and 9 on a BRUKER SMART APEX diffractom-
eter using graphite monochromatized Mo K¡ radiation. The
crystal orientation, cell refinement, and intensity measurements
were made using the program CAD-4 PC. The structures were
solved by direct methods using SHELXS-97,11 and full-matrix
least-squares refinement against F2 using SHELXL-97.12 Non-
hydrogen atoms were refined with anisotropic thermal param-
eters. The hydrogen atoms were geometrically fixed and
allowed to refine using a riding model. PLATON was used
for analyzing the interaction and stacking distances.13 CCDC-
776179 for 9 and -776180 for 7 contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
508.19, Anal. Calcd for 1 (C18H14N6O4Cl2Co): C, 42.54; H,
2.78; N, 16.54%. Found: C, 42.39; H, 2.46; N, 16.33%. IR
(KBr pellets, cm¹1): 3086 (s), 1597 (vs), 1513 (vs), 1433 (s),
1340 (vs), 1305 (vs), 1270 (s), 1200 (w), 1112 (s), 1064 (s),
1008 (w), 958 (w), 844 (vs), 747 (s), 622 (w), 506 (w). -max
nm (-ex/nm) = 395 (284).
[Co(nopi)6](NO3)2 (2): Yield of ca. 67% (0.294 g). MW:
1317.98. Anal. Calcd for 2 (C54H42N20O18Co): C, 49.21; H,
3.21; N, 21.25%. Found: C, 49.13; H, 3.19; N, 21.18%. IR
(KBr pellets, cm¹1): 3484 (vbr), 3123 (s), 2880 (w), 1602 (vs),
1526 (vs), 1458 (s), 1398 (s), 1271 (w), 1218 (w), 1176 (s),
1126 (s), 1064 (s), 958 (w), 828 (vs), 751 (s), 646 (w), 514 (s).
/
-
max/nm (-ex/nm) = 392 (275).
[Ni(nopi)2Cl2] (3): Yield of ca. 67% (0.340 g). MW:
507.94. Anal. Calcd for 3 (C18H14N6O4Cl2Ni): C, 42.56; H,
2.78; N, 16.55%. Found: C, 42.31; H, 2.86; N, 16.63%. IR
(KBr pellets, cm¹1): 3450, 3084, 1596, 1514, 1433, 1339,
1305, 1270, 1200, 1111, 1065, 1009, 957, 846, 745, 688, 621,
506. -max, nm (-ex/nm) = 393 (269).
[Ni(nopi)6](NO3)2 (4): Yield of ca. 64% (0.281 g). MW:
1317.74. Anal. Calcd for 4 (C54H42N20O18Ni): C, 49.22; H,
3.21; N, 21.26%. Found: C, 49.17; H, 3.15; N, 21.18%. IR
(KBr pellets, cm¹1): 3450, 3123, 1599 (vs), 1520 (s), 1382 (br),
1343, 1305, 1112 (s), 1060 (s), 962 (w), 855, 750 (s), 684 (w),
657 (w), 510. -max/nm (-ex/nm) = 387 (289).
[Ni(nopi)6](ClO4)2 (5): Yield of ca. 61% (0.283 g). MW:
1392.63. Anal. Calcd for 5 (C54H42N18O20Cl2Ni): C, 46.57; H,
3.04; N, 18.10%. Found: C, 46.06; H, 2.99; N, 18.30%. IR
(KBr pellets, cm¹1): 3468, 3123, 2930, 1600 (vs), 1522 (s),
1381 (br), 1343, 1307, 1263 (vs), 1185 (w), 1112 (vs), 1064,
963, 854, 784, 685 (w), 651 (w), 509. -max/nm (-ex/nm) =
383 (285).
[Cu(nopi)2Cl2] (6):
Yield of ca. 65% (0.333 g). MW:
512.80. Anal. Calcd for 6 (C18H14N6O4Cl2Cu): C, 42.16; H,
2.75; N, 16.39%. Found: C, 42.05; H, 2.66; N, 16.43%. IR
(KBr pellets, cm¹1): 3454, 3125, 2918, 1602 (vs), 1517, 1489,
1376 (s), 1304 (vs), 1213, 1168, 1120 (w), 1061 (s), 958 (w),
829 (vs), 759 (s), 478 (w). -max/nm (-ex/nm) = 393 (292).
[Cu(nopi)4(NO3)2] (7): Yield of ca. 68% (0.321 g). MW:
944.25. Anal. Calcd for 7 (C36H28N14O14Cu): C, 45.79; H,
2.99; N, 20.77%. Found: C, 45.23; H, 3.09; N, 20.28%. IR
(KBr pellets, cm¹1): 3448, 3121, 1599 (vs), 1518 (s), 1383 (br),
1343, 1304, 1113 (s), 1060 (s), 962 (w), 853, 750 (s), 688 (w),
657 (w), 510.
Crystallographic Data for 9: Formula: C18H14Cl2N6O4Zn,
MW: 514.62, specimen 0.16mm © 0.14mm © 0.12 mm, mono-
clinic, space group P21/n, a = 7.6352(15) ¡, b = 17.233(3) ¡,
c = 16.144(3) ¡, ¢ = 93.27(3)°, V = 2120.7(7) ¡3, Z = 4,
¹3
¹1
D
calcd = 1.612 g cm
,
F(000) = 1040, ® = 1.448 mm ,
T/K = 153(2), - = 0.71073 ¡, No. of reflection/parame-
ters = 4827/280, R(all) = 0.0769, R[I > 2·(I)] = 0.0389,
wR2(all) = 0.0857, wR2 [I > 2·(I)] = 0.0714, GOF = 1.015.
Crystallographic Data for 7: Formula: C36H28CuN14O14,
MW: 944.25, specimen 0.23 mm © 0.17 mm © 0.15 mm, tri-
[Cu(nopi)6](ClO4)2 (8): Yield of ca. 61% (0.284 g). MW:
1397.49. Anal. Calcd for 8 (C54H42N18O20Cl2Cu): C, 46.41; H,
3.03; N, 18.04%. Found: C, 46.06; H, 3.12; N, 17.94%. IR
(KBr pellets, cm¹1): 3944, 3118, 2945, 1598 (vs), 1517 (s),
ꢀ
clinic space group P1, a = 8.7913(14) ¡, b = 14.2033(17) ¡,
c = 15.803(2) ¡, ¡ = 92.753(10)°, ¢ = 93.600(12)°, £ =
¹3
94.014(12)°, V = 1961.8(5) ¡3, Z = 2, Dcalcd = 1.599 g cm
,

A. K. Singh et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 2 (2011)
207
F
Cl
N
Cl
H
M
N
N
+
N
N
N
NO2
NO2
N
NO2
O2N
(X)2
O2N
M = Co (1), Ni (3), Cu (6), Zn (9)
NO2
N
N
N
N
N
N
O2N
O
O
N
N
N
NO2
O2N
N
M
N
N
NO2
N
N
O
N
N
N
Cu
N
N
N
N
O
N
O2N
N
NO2
NO2
O2N
O
O
X = NO3 M = Co (2), Ni (4), Zn (10),
X = ClO4 M = Ni (5), Cu (8), Zn (11)
7
F(000) = 966, ® = 0.646 mm¹1, T/K = 150(2), - = 0.71073
¡, No. of reflection/parameters = 6863/586, R(all) = 0.0863,
R[I > 2·(I)] = 0.0521, wR2 = 0.1196, wR2[I > 2·(I)] =
0.1088, GOF = 0.950.
10
Results and Discussion
9
All the complexes have been prepared by a general
procedure wherein respective metal salts (1.0 mmol) and nopi
(2.0 mmol) were heated in water-ethanol mixture (10 mL, 4:1)
in a sealed tube at 120 °C. Upon cooling to room temperature
diffraction quality crystals of 7 and 9 were directly obtained
from the reaction mixture. A simple scheme showing the
synthesis of complexes is depicted in Scheme 1. Interestingly,
reaction of the metal salts with nopi afforded three types of
complexes (i) tetrahedral complexes of the formulations
[M(nopi)2Cl2], (ii) octahedral complexes [M(nopi)6](X)2, and
(iii) square-planar complexes of the type [M(nopi)4(NO3)2],
which was obtained only in the case of copper. These
observations are consistent with our earlier reports.7a,7b The
complexes under investigation are air-stable solids, insoluble
in common organic solvents like dichloromethane, methanol,
ethanol, hexane, diethyl ether, acetone, chloroform, benzene,
etc. and are soluble in solvents with high dielectric constant
e.g., DMF, DMSO, acetonitrile, etc.
nopi
Figure 1. Overlay NMR spectra of nopi (in CHCl3), 9, and
10 (in DMSO).
nitrogen. In the 1H NMR spectra of zinc based complexes 9-11,
the phenyl ring protons displayed a significant shift and
resonated between µ7.2-8.6 ppm (Figure 1). Integral intensity
and shift in the position of various signals suggested the
coordination of nopi with respective metal centers.
Single-Crystal X-ray Diffraction Studies. The structure
of tetrahedral complexes of the type [M(nopi)2Cl2] 1, 3, 6, and
9 have been verified by single-crystal X-ray diffraction
analyses of the representative complex 9. Selected crystallo-
graphic data are summarized in the crystallographic section and
geometric parameters are shown below in Figure 2. Complex 9
crystallizes in a monoclinic system having P21/n space group.
In this complex coordination geometry about the Zn center
is distorted tetrahedral with N2Cl2 donor groups which is
analogous to that observed in [Zn(cpi)2Cl2] (cpi: 1-(4-cyano-
phenyl)imidazole).7b Two of the coordination sites about the
Characterization of the complexes has been achieved by
elemental analyses and spectral studies. Selected character-
ization data are summarized in the experimental section.
Infrared spectra of the respective complexes exhibited
characteristic bands due to -NO2 of the nitrophenyl moiety
¹1
along with imidazole ring vibrations at 1339 and µ1598 cm
,
respectively. The bands corresponding to imidazole ring
¹1
vibrations appeared at µ1600 cm while that in the free nopi
¹1 7a,7b
appears at 1610 cm
.
A shift in the position of ¯(C=N)
suggested linkage of nopi to the metal center through imidazole

208
Bull. Chem. Soc. Jpn. Vol. 84, No. 2 (2011)
Complexes Based on 1-(4-Nitrophenyl)imidazole
Figure 2. Ellipsoid view of 9 at 30% probability (H atoms
excluded for clarity): Selected bond length/¡ and angles/°:
Zn(1)-N(1) = 2.031(2), Zn(1)-N(4) = 2.020(2), Zn(1)-
Cl(1) = 2.2166(10), Zn(1)-Cl(2) = 2.2117(10), N(4)-
Zn(1)-N(1) = 102.16(10), N(4)-Zn(1)-Cl(2) = 106.84(7),
Figure 3. ORTEP view of 7 at 30% probability of thermal
ellipsoids (H atoms excluded for clarity). Selected bond
lengths and angles: Cu(1)-N(9) = 1.994(3) ¡, Cu(1)-
N(3) = 2.007(3) ¡, Cu(1)-N(12) = 2.015(3) ¡, Cu(1)-
N(1)-Zn(1)-Cl(2) = 109.64(7),
N(4)-Zn(1)-Cl(1) =
109.86(8), N(1)-Zn(1)-Cl(1) = 105.38(7), Cl(2)-Zn(1)-
Cl(1) = 121.33(4).
N(6) = 2.019(3) ¡,
N(9)-Cu(1)-N(12) = 86.46(11)°,
N(9)-Cu(1)-N(3) = 177.45(12)°,
N(3)-Cu(1)-N(12) =
zinc center are occupied by imidazolyl nitrogen of nopi, while
other two by chloro groups. The angles about the metal center
are comparable to those in tetrahedral systems with small
deviations. One of the major differences between [Zn(cpi)2Cl2]
and [Zn(nopi)2Cl2] lies in the absence of a 2-fold symmetry
axis in [Zn(nopi)2Cl2]. The Zn-N and Zn-Cl bond distances
[Zn(1)-N(1) = 2.031(2) ¡, Zn(1)-N(4) = 2.020(2) ¡, Zn(1)-
Cl(1) = 2.2166(10) ¡, and Zn(1)-Cl(2) = 2.2117(10) ¡] are
consistent with the values reported in the literature.7b The
nitrophenyl ring is twisted with respect to the imidazole ring.
Dihedral angles between the phenyl and imidazole ring planes
are ¹32.7(4) and ¹28.6(4)°, respectively. Geometry about the
nitrogen of -NO2 groups is trigonal planer with sp2 hybrid-
ization. The N-O bond distances differ slightly among
themselves (N(3)-O(2) = 1.220(4) ¡, N(3)-O(1) = 1.209(4) ¡
and N(6)-O(3) = 1.211(4) ¡, N(6)-O(4) = 1.219(4) ¡) and lie
within the range for N-O single and N=O double bonds,
indicating resonance in the -NO2 group. The nitro group
deviated a little from the plane of the phenyl ring suggesting
extended conjugation with the phenyl ring.
95.68(11)°, N(9)-Cu(1)-N(6) = 92.43(11)°, N(3)-Cu(1)-
N(6) = 85.38(11)°, N(12)-Cu(1)-N(6) = 177.17(11)°.
Figure 4. Zig-zag motif in 9 via O£H weak interaction and
³£³ stacking interaction.
between the plane of phenyl and imidazole rings are between
26.69 to 34.70°, while -NO2 groups deviated from the phenyl
ring plane from 3.05 to 13.65°.
Weak Interaction Studies. Crystal structures of 9 and 7
revealed the presence of extensive intra- and intermolecular C-
H£X (X = O, N, and Cl) and ³£³ interactions which play a
significant role in the building of supramolecular moieties.4e,14
Interesting zig-zag motifs resulting from C-H£O and ³£³ are
shown in Figure 4. Weak bonding interactions between oxygen
of -NO2 and C6 hydrogen in 9 leads to a unique zig-zag chain
along crystallographic a axis (Figure 4). Centre to center
separation of the phenyl rings from two different molecules are
3.648 ¡ (Figure S1), suggesting strong ³£³ stacking inter-
actions between these molecules. The stacking is stabilized by
zig-zag arrangement of these molecules.
Similarly in 7, intermolecular ³-³ stacking interactions
resulted in the creation of huge coordination networks from
discrete molecules (Figures 5 and S2).4e,14 Intermolecular face-
to-face ³-³ interactions between discrete [Cu(nopi)4]2+ moie-
ties involving nitrophenyl rings leads to a ladder motif shown
in Figures 5 and S3. In addition, C-H£N interaction also leads
to formation of an interesting structural motif (Figure S4).
The second category of the complexes are represented by
¹
¹
formulations [M(nopi)6](X)2 (X = NO3 and ClO4 ). Due to
the poor quality of the crystals, attempts to verify the structure
of this type of complexes by single-crystal X-ray diffraction
analyses were unsuccessful in our hands, however, spectral and
analytical data corroborated well to proposed formulations.
Reaction between the nopi and Cu(NO3)2¢3H2O afforded
complex [Cu(nopi)4(NO3)2] (7) possessing square-planner
arrangement of nopi ligands about the metal center, unlike its
cpi analog (Figure 3).7a In this complex four nopi ligands
coordinated with the metal center along equatorial positions,
while axial positions are occupied by weakly interacting nitrate
ions leading to an overall pseudo-octahedral geometry. The
Cu-N bond distances are consistent with coordination of the
ligands with the metal center.7a The Cu-O distances are greater
than the sum of covalent radii of Cu and O (Cu(1)-O(1) =
2.463 ¡, Cu(1)-O(4) = 2.577 ¡), suggesting absence of a
covalent bond between them. Analogous to cpi and other nopi
complexes, the ligand is not completely planar. Torsion angles
Emission Study.
The photoluminescence properties of
complexes 1-6 were followed in DMSO at room temperature.

A. K. Singh et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 2 (2011)
209
Measurements were carried out under analogous conditions.
Upon excitation at their MLCT or d-d transition bands these do
not exhibit any significant luminescence, while upon excitation
at their intra-ligand transitions these exhibit moderate lumi-
nescence. Resulting data are summarized in the experimental
section and comparative spectra of 1-6 are depicted in
Figure 6. Interestingly, all the complexes upon excitation at
260-296 nm exhibited luminescence with emission maxima
centered at 380-400 nm. It is analogous to that observed in the
ligand nopi (emission maximum at 438 nm upon excitation at
284 nm). The emissions observed in these complexes have been
tentatively assigned to the ³ ¼ ³* intra-ligand fluorescence
due to their close similarity with the emission bands in nopi.7c
Pressed Pellet Conductivity Measurement. The electrical
conductivity measurements on 1-8 pellets were made by
conventional two probe technique in the temperature range
313-373 K. Figure 7 shows that the ln · of the complexes 2, 6,
and 7 decreases continuously with 1/T, indicating that the
complexes show typical semiconducting behavior over the
entire temperature range. This plot is almost linear. The band
gaps evaluated from these plots were 1.551 eV for 2, 0.543 eV
for 6, 0.676 eV for 7 and these compounds exhibit very weakly
¹1
conducting ·rt in the range of rt 10¹9-10¹5 S cm
.
Conclusion
Through this work we have described synthesis and
characterization of novel monometallic complexes based on
nopi and different 3d metal ions. All the complexes have been
characterized by elemental analyses and spectral studies.
Structure of [Cu(nopi)4(NO3)2] (7) and [Zn(nopi)2Cl2] (9) have
been determined by single-crystal X-ray diffraction analyses.
Solid-state electrical conductance studies suggested that the
cobalt complex 2 and copper complexes 6 and 7 exhibit good
semiconducting behavior at room temperature. In addition,
these complexes show moderate emissions at room temper-
ature.
(a)
(b)
Complex 1
2
3
4
5
6
Figure 5. ³-³ stacking interaction in complex 7 (a) along
crystallographic a axis and (b) along crystallographic c
axis.
Wavelength/nm
Figure 6. Emission spectra of complexes 1-6.
T
−1 × 103/K−1
Figure 7. Temperature dependence of electrical conductivities of the complexes: rectangular bar 2, cubic bar 6, triangular bar 7.

210
Bull. Chem. Soc. Jpn. Vol. 84, No. 2 (2011)
Complexes Based on 1-(4-Nitrophenyl)imidazole
Matsuda, R. Kitaura, S. Kitagawa, Y. Kubota, R. V. Belosludov,
T. C. Kobayashi, H. Sakamoto, T. Chiba, M. Takata, Y. Kawazoe,
Y. Mita, Nature 2005, 436, 238.
Thanks are due to the Department of Science and Technol-
ogy, New Delhi, India for providing financial assistance SR/
S1/IC-15/2006. A.K.S. acknowledges CSIR for awarding JRF
09/013(0127)/2007, EMR-I. The authors are also grateful to
the Head Department of Chemistry, Faculty of Science,
Banaras Hindu University, Varanasi (U.P.), India for extending
facilities. Special thanks are due to Prof. Pradeep Mathur, In-
charge, National Single Crystal X-ray Diffraction Facility,
Indian Institute of Technology, Mumbai for providing single-
crystal X-ray data.
5
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Supporting Information
Structural motifs resulting from C-H£N and ³-³ stacking
interactions and matrices for weak bonding interactions in 7
and 9 are gathered in supporting materials. Supplementary data
associated with this article can be found free of cost. This
material is available free of charge on the web at http://
www.csj.jp/journals/bcsj/.
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