Studies on the zinc(II)-azoimine system. Single-crystal X-ray structure of Zn(MeaaiMe)Cl2·H2O and Zn(HaaiMe)2(NCS)2 (MeaaiMe = 1-methyl-2-(p-tolylazo)imidazole, HaaiMe = 1-methyl-2-(phenylazo) imidazole)

Article abstract of DOI:10.1016/S0277-5387(03)00111-6

Dichloro zinc(II) complexes of 1-alkyl-2-(arylazo)imidazole, Zn(RaaiR′)Cl₂ · H₂O (RaaiR′ = p-R-C₆H₄-N = N-C₃H₂N-N(1)-R′: R = H, Me, Cl and R′ = Me, CH₂CH₃, CH₂Ph) are characterized by elemental analysis, IR, UV-Vis and ¹H NMR spectral data. The reaction of ZnCl₂ and RaaiR′ (excess) in the presence of NH₄CNS has synthesized Zn(RaaiR′)₂(NCS)₂. The single-crystal X-ray structure of dichloro-{1-methyl-2-(p-tolylazo)imidazole}zinc(II)-monohydrate (Zn(MeaaiMe)Cl₂ · H₂O) suggests that the complex is distorted trigonal bipyramidal (TBP) around Zn(II) with a trigonal plane constituted from Cl(1), Cl(2), N(imidazole) (N(1)). Molecular packing shows a 1D chain via a hydrogen bonded eight membered ring. The X-ray structure of di-thiocyanatobis-{1-methyl-2-(phenylazo)-imidazole}zinc(II) also shows a distorted TBP around Zn(II). The coordination sphere is ZnN₃ type: where one of the ligands acts as a bidentate chelating agent and the second one coordinates via only the imidazole-N along with two N-centers from the NCS group.

Full text of DOI:10.1016/S0277-5387(03)00111-6

Polyhedron 22 (2003) 1213Á1219
/
www.elsevier.com/locate/poly
Studies on the zinc(II)-azoimine system. Single-crystal X-ray structure
of Zn(MeaaiMe)Cl2×H2O and Zn(HaaiMe)2(NCS)2
(MeaaiMeꢀ1-methyl-2-(p-tolylazo)imidazole,
HaaiMeꢀ1-methyl-2-(phenylazo) imidazole)
/
/
/
B.G. Chand a, U.S. Ray a, J. Cheng b, T.-H. Lu b, C. Sinha a,*
a Department of Chemistry, The University of Burdwan, Burdwan 713104, India
b Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan, ROC
Received 15 May 2002; accepted 21 January 2003
Abstract
Dichloro zinc(II) complexes of 1-alkyl-2-(arylazo)imidazole, Zn(RaaiR?)Cl2×
R?: RꢀH, Me, Cl and R?ꢀMe, CH2CH3, CH2Ph) are characterized by elemental analysis, IR, UVÁ
data. The reaction of ZnCl2 and RaaiR? (excess) in the presence of NH4CNS has synthesized Zn(RaaiR?)2(NCS)2. The single-crystal
X-ray structure of dichloro-{1-methyl-2-(p-tolylazo)imidazole}zinc(II)-monohydrate (Zn(MeaaiMe)Cl2×H2O) suggests that the
/
H2O (RaaiR?ꢀ
/
p-Rꢀ
/
C6H4ꢀ
/
Nꢀ
/
Nꢀ
/
C3H2Nꢀ
/
N(1)ꢀ
/
1
/
/
/
Vis and H NMR spectral
/
complex is distorted trigonal bipyramidal (TBP) around Zn(II) with a trigonal plane constituted from Cl(1), Cl(2), N(imidazole)
(N(1)). Molecular packing shows a 1D chain via a hydrogen bonded eight membered ring. The X-ray structure of di-thiocyanato-
bis-{1-methyl-2-(phenylazo)-imidazole}zinc(II) also shows a distorted TBP around Zn(II). The coordination sphere is ZnN3 type:
where one of the ligands acts as a bidentate chelating agent and the second one coordinates via only the imidazole-N along with two
N-centers from the NCS group.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Arylazoimidazole; Zinc(II) complexes; Crystal structures; Hydrogen bonding; 1D chain
1. Introduction
redox inactive in most cases it provides the structural
integrity to the polymetallo-enzymes so that the redox
active metal ion may exhibit maximum efficiency. The
active site is connected to a network of hydrogen bonds
formed by adjacent residues and water molecules. It has
Zinc complexes with heterocyclic N-donor ligands
offer a potential agent to mimic different zinc containing
metalloproteins. For example, the active site coordina-
tion geometry of liver alcohol dehydrogenase is a
mononuclear zinc center ligated in a tetrahedral array
by two histidine nitrogen, two cystine thiolates and a
water molecule [1], super oxide dismutase binds Cu(II)
and Zn(II) via an imidazolate bridge [2,3], etc. The
specific role played by the imidazole active center of a
large number of metalloproteins has encouraged the
design of many imidazole containing ligands from view
points of coordination chemistry [4,5]. Although zinc is
been demonstrated that L3Znꢀ
/
OH2 (Lꢀcoordinated
/
parts of enzyme) is the active species in the dehydration
of HCO3ꢁ in carbonic anhydrase [6,7]. Design of model
zinc(II) complexes having labile H2O coordination is a
real challenge [8].
In an effort towards the design of imidazole contain-
ing ligands we have synthesized arylazoimidazoles [9Á
11]. The molecule is p-acidic and the active function is
). They have been
/
the azoimine group (ꢀ
/
Nꢁ
/
Nꢀ
/
Cꢁ
/
Nꢀ
/
used successfully to stabilize lower metal oxidation
states viz. Cu(I), Ru(II), Os(II). We have intensively
developed the coordination chemistry [9Á11] and analy-
tical application of arylazoimidazoles [12,13]. Major
developments have been carried out in the field of
/
* Corresponding author. Tel.:
530452.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
ꢂ
/
91-342-557683; fax:
ꢂ/91-342-
0277-5387/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0277-5387(03)00111-6

1214
B.G. Chand et al. / Polyhedron 22 (2003) 1213Á1219
/
transitions metals [9Á
chemistry of this function is scarce [14Á
/
11]. The non-transition metal
16]. The
Table 1
Crystallographic data
/
coordination chemistry of zinc with 1-alkyl-2-(arylazo)-
imidazole has remained unexplored. In this article, we
report the synthesis, spectral properties, chemical re-
activity of zinc(II) complexes of 1-alkyl-2-(arylazo)imi-
dazole. The single-crystal X-ray structure of one of the
complexes has shown a hydrogen bonded 1D chain via
coordinated water. The synthesis and X-ray structure of
the thiocyanato derivative are also reported here.
Zn(MeaaiMe)Cl2×
/
H2O
Zn(HaaiMe)2(NCS)2
(7a)
(4b)
Empirical
formula
C11H14Cl2N4OZn
C22H20N10S2Zn
Formula weight 354.53
Temperature (K) 293 (2)
553.97
294
Crystal system
Space group
monoclinic
P21/c
monoclinic
P21/c
Crystal size (mm) 0.30ꢃ/0.30ꢃ/0.30
0.30ꢃ
/
0.30ꢃ0.40
/
Unit cell
dimensions
2. Experimental
˚
a (A)
7.878(2)
8.9562(8)
15.3366(13)
19.2868(17)
90
˚
b (A)
˚
c (A)
11.0625(14)
17.011(7)
90
2.1. Materials
a (8)
b (8)
98.69(3)
90
1465.4(8)
92.647(2)
90
2646.4(4)
1-Alkyl-2-(arylazo)imidazoles were prepared by the
reported procedure [9]. [n-Bu4N][ClO4] and MeCN for
electrochemical work were purified by an earlier proce-
dure [10]. All other chemicals and solvents were of
reagent grade and used as received.
g (8)
3
V(A )
˚
Z
4
4
2u-range (8)
˚
l (A)
4Á/56
4Á56
/
0.71073
2.037
1.607
181
0.71073
1.116
1.390
316
m (Mo Ka)
Dcalc (Mg mꢁ3
Refined para-
meters
Ra1 [I ꢀ
wRb2
Goodness of fit
2.2. Physical measurements
)
Details of physical measurements are described in
Ref. [14].
/2s (I)]
0.0250
0.0723
1.090
0.0462
0.1276
0.930
2.3. Synthesis of [Zn(MeaaiMe)Cl2×
/
H2O] (4b)
1-Methyl-2-(p-tolylazo)imidazole (0.1 g, 0.5 mmol) in
MeOH (10 cm3) was added dropwise to a stirred
methanolic solution (10 cm3) of ZnCl2 H2O (0.077 g,
0.5 mmol) at room temperature over a period of 3 h. The
It was collected and dried over CaCl2.The yield was
0.1892 g, 65%.
All other zinc complexes were prepared similarly and
the yield varied from 40 to 60%. The microanalytical
data of the complexes are given in Table 2.
volume of the orangeÁred solution was reduced to half
/
of its original volume by slow evaporation in air. The
solution was then kept in a refrigerator maintaining the
temperature at 5 8C. The bright orangeÁ/red crystalline
2.5. X-ray diffraction study
product was then precipitated out. The product was
filtered, washed with water and cold MeOH. The
Crystal parameters and refinement results are sum-
product was then recrystallised from MeOHÁ
(1:1, v/v) and dried over CaCl2. The yield was 0.124 g,
70%.
All other zinc complexes were prepared similarly and
the yield varied from 40 to 60%. The microanalytical
data of the complexes are given in Table 2. Zinc was
estimated by complexometric titration [17].
/
H2O
marized in Table 1. For Zn(MeaaiMe)Cl2×
/
H2O (4b)
diffraction data were collected from an Enraf-Nonius
CAD-4 diffractometer using graphite monochromatised
˚
0.71073 A). Data collections were
Mo Ka radiation (lꢀ
/
performed in the 2u range 4Á568. For Zn(Haai-
/
Me)2(NCS)2 (7a) the data were collected by a SMART
CCD diffractometer at 293(2) K using graphite mono-
chromatized Mo Ka radiation. Data collections were
performed in the 2u range 3 to 568. The data were
corrected for linear decay as three reference reflections
were monitored throughout the data collection. Empiri-
cal absorption corrections were carried out based on v-
scans. The structure was solved by the heavy atom
method and refined by full-matrix least-squares refine-
ment based on Fo2 and including all reflections. All non-
hydrogen atoms were refined anisotropically. Hydrogen
atoms bonded to aromatic carbons were included at
2.4. Synthesis of Zn(HaaiMe)2(NCS)2 (7a)
A methanol solution (15 cm3) of MeaaiMe (0.186 g, 1
mmol) was added to a stirred solution of ZnCl2×
/
H2O
(0.077 g, 0.5 mmol) and NH4CNS (0.076 g, 1 mmol) at
room temperature The stirring was continued for 1 h
and then the orangeÁyellow solution was filtered. The
/
solution was evaporated slowly in air and an orange
crystalline product deposited on the wall of the beaker.

B.G. Chand et al. / Polyhedron 22 (2003) 1213Á
/1219
1215
Table 2
Microanalytical a, UVÁVis b and voltammetric data c
/
Compounds
Microanalytical data
Found (calc.)%
UVÁ
/
Vis spectral data
Ligand reduction
lmax (nm) (10ꢁ3 o (dm3 molꢁ1 cmꢁ1))
V
C
H
N
Zn(HaaiMe)Cl2×
Zn(MeaaiMe)Cl2×
Zn(ClaaiMe)Cl2×H2O (4c)
Zn(HaaiEt)Cl2×H2O (5a)
Zn(MeaaiEt)Cl2×H2O (5b)
Zn(ClaaiEt)Cl2×H2O (5c)
Zn(HaaiCH2Ph)Cl2×
Zn(MeaaiCH2Ph)Cl2×
Zn(ClaaiCH2Ph)Cl2×H2O (6c) 41.95(42.58) 3.22(3.32) 12.44(12.42) 444(5.38), 380(18.14), 280(4.18) 245(9.88)
/
H2O (4a)
35.15(35.25) 3.4(3.52) 16.48(16.45) 435(7.37), 378(17.62), 278(3.93), 232(9.06)
37.14(37.24) 3.85(3.95) 15.93(15.80) 444(7.18), 383(17.62), 275(5.95), 244(7.08)
31.91(32.01) 2.83(2.93) 14.97(14.93) 440(6.92), 380(18.20), 278(6.18), 240(7.08)
37.10(37.24) 3.89(3.95) 15.85(15.80) 420(7.28), 370(18.14), 278(3.62), 230(7.98)
38.99(39.09) 4.32(4.34) 15.21(15.20) 418(6.60), 382(17.64)s, 370(18.41), 278(4.03), 235(8.52)
33.88(33.94) 3.24(3.34) 14.42(14.40) 438(6.78), 375(19.31), 282(5.19), 245(8.94)
ꢁ
/
0.48, ꢁ
0.50, ꢁ
0.43, ꢁ
0.45, ꢁ
0.49, ꢁ
0.40, ꢁ
0.47, ꢁ
0.50, ꢁ
0.42, ꢁ
0.48, ꢁ
0.50, ꢁ
0.43, ꢁ
0.46, ꢁ
0.50, ꢁ
0.43, ꢁ
0.42, ꢁ
0.50, ꢁ
0.45, ꢁ
/
1.22
1.38
1.15
1.22
1.27
1.12
1.24
1.38
1.20
1.22
1.38
1.15
1.23
1.28
1.14
1.21
1.39
1.21
/
H2O (4b)
ꢁ
/
/
/
ꢁ
/
/
/
ꢁ
/
/
/
ꢁ
/
/
/
ꢁ
/
/
/
H2O (6a) 46.01(46.11) 3.81(3.84) 13.49(13.44) 441(7.41), 377(25.61), 278(4.05), 240(10.46)
H2O (6b) 47.19(47.39) 4.10(4.18) 13.09(13.01) 484(6.67), 384(19.07), 279(3.27), 241(8.15)
ꢁ
/
/
/
ꢁ
/
/
/
ꢁ
/
/
Zn(HaaiMe)2(NCS)2 (7a)
Zn(MeaaiMe)2(NCS)2 (7b)
Zn(ClaaiMe)2(NCS)2 (7c)
Zn(HaaiEt)2(NCS)2 (8a)
Zn(MeaaiEt)2(NCS)2 (8b)
Zn(ClaaiEt)2(NCS)2 (8c)
Zn(HaaiCH2Ph)2(NCS)2 (9a)
47.61(47.69) 3.57(3.68) 25.23(25.29) 420(3.63), 372(22.34), 338(15.38), 226(8.73)
49.48(49.52) 4.09(4.12) 24.00(24.07) 419(2.99), 376(11.03), 338(6.27), 234(4.17)
42.35(42.40) 2.83(2.89) 22.43(22.48) 440(7.18), 378(17.62), 360(23.45), 232(8.56)
49.49(49.52) 4.08(4.12) 24.00(24.07) 419(1.87), 372(12.43), 338(13.16), 230(5.58)
51.09(51.18) 4.52(4.59) 22.88(22.96) 420(7.75), 376(28.20), 338(16.16), 236(10.20)
44.20(44.27) 3.32(3.38) 21.48(21.52) 436(6.99), 376(17.54), 362(23.68), 228(6.87)
57.79(57.83) 3.91(3.96) 19.78(19.84) 420(8.15), 376(18.71), 338(13.71), 234(8.07)
ꢁ
/
/
ꢁ
/
/
ꢁ
/
/
ꢁ
/
/
ꢁ
/
/
ꢁ
/
/
ꢁ
/
/
Zn(MeaaiCH2Ph)2(NCS)2 (9b) 58.81(58.89) 4.31(4.36) 19.00(19.08) 418(5.26), 380(17.89), 338(13.71), 234(8.07)
Zn(ClaaiCH2Ph)2(NCS)2 (9c)
ꢁ
/
/
52.62(52.67) 3.30(3.35) 18.01(18.07) 438(6.99), 376(17.54), 362(23.68), 228(6.87)
ꢁ/
/
a
Calculated values are in parenthesis.
Solvent: CHCl3.
b
c
Solvent: CH3CN, electrodes: Pt-disk (working), Pt-wire (auxiliary), SCE (reference), supporting electrolyte [n-Bu4N][ClO4], scan rate, 50
10ꢁ3 M, EPC (cathodic peak potential) in V.
mV sꢁ1, solute concentration ꢁ
/
idealized, calculated positions while water hydrogen
atoms were located in the difference Fourier maps and
subsequently refined according to the riding model.
Data reduction, structure solution and refinement were
performed with SHELX-97,
work. Zn(RaaiR?)2(NCS)2 on heating explodes at ꢀ675
/
K and was not studied further. Microanalytical data and
zinc analysis (complexometrically) [17] support the
composition of the complex and for one case of each
type of complex the structures are established by a
single-crystal X-ray diffraction study.
3. Results and discussion
3.2. Molecular structure and 1D polymer of
H2O (4b)
Zn(MeaaiMe)Cl2×
/
3.1. Synthesis and formulation
The crystal structure of Zn(MeaaiMe)Cl2×
/
H2O (4b) is
shown in Fig. 1 and bond parameters are listed in Table
1-Alkyl-2-(arylazo)imidazoles (RaaiR?, 1Á3) are a
/
N,N?-bidentate donor system. The donor centers are
abbreviated as N (3) (imidazole) as N and N(azo) as N?.
From a methanolic solution of ZnCl2 and RaaiR? in 1:1
mole proportion we have isolated an orangeÁ
crystalline product of composition Zn(RaaiR?)Cl2×
(4Á6). Even in the presence of an excess amount of
ligand (ꢀ3 mol) under refluxing conditions the complex
/yellow
/H2O
/
/
of this composition is inserted. The reaction of ZnCl2
with RaaiR? in the presence of NH4CNS however has
synthesized Zn(RaaiR?)2(NCS)2 (7Á
instead of ZnCl2 has also isolated Zn(RaaiR?)2 (NCS)2
(7Á9).
Thermogravimetric studies of Zn(RaaiR?)Cl2×
show water losses at 403Á410 K followed by the ligand
elimination at 515Á525 K. This observation accounts
for the coordinated water in a hydrogen bonded net-
/
9). Use of Zn(NO3)2
/
/H2O
/
/
Fig. 1. X-ray structure of Zn(MeaaiMe)Cl2×OH2 (4b).
/

1216
B.G. Chand et al. / Polyhedron 22 (2003) 1213Á1219
/
Table 3
˚
Selected bond lengths (A) and angles (8) of Zn(MeaaiMe)Cl2×
/
H2O (4b) with esd
Bond lengths
Angles
Znꢀ
Znꢀ
Znꢀ
Znꢀ
/
N(1)
O
2.017(2)
2.159(2)
2.223(1)
2.256(1)
1.325(3)
1.372 (3)
1.269(3)
2.546(3)
N(1)ꢀ
N(1)ꢀ
N(1)ꢀ
Cl(1)ꢀ
Cl(1)ꢀ
ClꢀZnꢀ
N(4)ꢀZnꢀ
N(4)ꢀZnꢀ
N(4)ꢀZnꢀ
N(4)ꢀZnꢀ
/
Znꢀ
Znꢀ
Znꢀ
Znꢀ
Znꢀ
/
O
92.15(9)
108.31(7)
134.07(7)
116.00(4)
100.84(7)
91.01(7)
70.6(4)
/
/
/
Cl(1)
Cl(2)
/
Cl(2)
Cl(1)
/
/
/
/
/Cl(2)
N(1)ꢀ
N(1)ꢀ
N(3)ꢀ
Znꢀ
/
C(4)
C(1)
N(4)
/
/O
/
/
/O
/
/
/
N(1)
Cl(1)
Cl(2)
O
/
N(4)
/
/
90.5(6)
/
/
96.9(7)
/
/
161.8(7)
˚
Selected bond lengths (A) and angles (8) of [Zn(HaaiMe)2(NCS)2 (7a) with esd
Bond lengths Angles
Znꢀ
Znꢀ
Znꢀ
Znꢀ
Znꢀ
Znꢀ
N(3)ꢀ
N(7)ꢀ
/
N(1)
N(2)
N(3)
N(5)
N(7)
N(9)
1.943(3)
1.987(3)
2.726(1)
1.995(3)
2.976(2)
2.079(3)
1.262(4)
1.258(4)
N(1)ꢀ
N(1)ꢀ
N(1)ꢀ
N(1)ꢀ
N(2)ꢀ
N(2)ꢀ
N(2)ꢀ
N(3)ꢀ
N(3)ꢀ
N(5)ꢀ
C(13)ꢀ
C(19)ꢀ
/
Znꢀ
Znꢀ
Znꢀ
Znꢀ
Znꢀ
Znꢀ
Znꢀ
Znꢀ
Znꢀ
Znꢀ
N(7)ꢀ
N(9)ꢀ
/
N(2)
N(3)
N(5)
N(9)
N(3)
N(5)
N(9)
N(5)
N(9)
N(9)
110.05(12)
83.2(8)
/
/
/
/
/
/
126.98(12)
102.74(11)
86.7(2)
/
/
/
/
/
/
/
/
/
110.72(11)
95.91(11)
66.86(5)
/
N(4)
N(8)
/
/
/
/
/
/
/
172.1(1)
/
/
105.23(10)
114.4(3)
/
/N(8)
/
/Zn
127.9(2)
ligand values [10,11,21Á
good p back donor to affect the Nꢀ
acute chelate angle may develop a strain that is relieved
partially by structural distortion and bond length
elongation.
/
23]. Zn(II) is not a sufficiently
3. The structure shows a distorted trigonal bipyramidal
(TBP) geometry around Zn(II). The coordination in-
/N bond length. The
ꢂ
ꢄ
cludes the chelated azoimine Znꢀ(NꢁCꢀNꢁN
two ZnꢀCl bonds and one ZnꢀOH2 bond. The atomic
arrangements Zn, Cl (1), Cl (2), N(1) constitute a
/
ꢀ
/
) group,
/
/
The molecular packing shows a 1D polymer running
along the a-axis (Fig. 2). The network is constituted via
hydrogen bonding of two rows of Zn(N,N?)Cl2. OH2 in
an interlocking fashion [24]. The coordinated H2O of
one molecule in a row interacts via hydrogen bonding
˚
trigonal plane and Zn is deviated by 0.16 A from the
˚
mean plane. The chelate angle is 70.6(4) A and is the
shortest so far reported [10,11] in the series of chelated
arylazoimidazole complexes. This may be the reason for
the distortion from the symmetric TBP geometry. The
N(4), [N(azo)] and coordinated OH2 appear as the
with two adjacent molecules [(Zn?)Cl?
Cl?(Zn?)
Á Á Á] of the second row nd vice-versa. This
constitutes an eight-member chair-type bi-hydrogen
bonded ring and propagates via edge-sharing of Znꢀ
bonds. The angle extended by the hydrogen bonding
three atom system (Oꢀ Á Á ÁCl) are ꢁ1668 where Cl is
coming from the neighbouring atom. Sequence of H-
bond parameters: Dꢀ Á Á ÁA, distances of DꢀH, Hꢀ
A, (A) and angles Dꢀ Á Á ÁA, (8) are as given:
H(1A), 0.7426; H(1A) Cl(2), 2.4241; OꢀCl(2),
Á Á Á
3.1486; OꢀH(2B)), 0.8023; H(2B)Á Á ÁCl(1), 2.3684; Oꢀ
Cl(1), 3.1578; OꢀH(1A)ꢀCl(2), 165.54; OꢀH(2B)Á Á Á
Cl(1), 167.85.
/
Á Á Á
/
H(O)H
/
Á Á Á
/
/
apices to the trigonal plane. The Znꢀ
/
N(4) [Znꢀ
N(azo)] is the longest (2.546(3) A) one in the family of
MꢀN(azo) bond lengths so far know in the literature
/
˚
/O
/
[10,11]. The elongation may be due to axial coordination
of N(4) with reference to the trigonal plane described by
ZnNCl2 and from the small chelate bite angle. 1-Methyl-
/
H/
/
/
/
H
/
/
/
/A
2-(p-tolylazo)imidazole is planar and N(2) (N-Methyl)
˚
0.106 A.
˚
and D
/
Á Á Á
/
/
H
/
/
suffers maximum deviation to downward by ꢁ
/
Oꢀ
/
/
/
/
Imidazole and p-tolyl rings are joined by an azo group
and the dihedral angle is 7.818. The NꢀN bond length
is 1.269(3) A and is slightly elongated compared to that
/
/
/
/
/
/
/
/
/
/
˚
˚
of the free ligand value (1.250(1) A) [18Á
/
20]. The
ꢄ
ꢂ
stability of the chelated azoimine Mꢀ(NꢁCꢀNꢁN
/
ꢀ
/
) is
due to the metal-to-ligand p-back bonding and the azo
group is directly involved in this process [10,11]. This
3.3. Molecular structure of Zn(HaaiMe)2(NCS)2 (7a)
reduces the Mꢀ
/
N(azo) bond length and subsequently
The molecular structure of Zn(HaaiMe)2(NCS)2 (7a)
is shown in Fig. 3 and bond parameters are given in
increases the Nꢁ
/N bond length from that of the free

B.G. Chand et al. / Polyhedron 22 (2003) 1213Á
/1219
1217
Fig. 2. Hydrogen bonded 1D chain of 4b.
˚
sum of the van der Waals radii (2.96 A) of the members
˚
N(7) length (2.97(6) A) is
[25] while the estimated Znꢀ
/
˚
greater than 2.96 A. Thus ligand (ii) behaves as a
monodentate N-donor system. Because of the higher
affinity of imidazole-N to Zn(II) out of N(imidazole)
and N(azo) in the ligand, the Zn(II) prefers to bind
N(imidazole). This is also supported from the structures
of zinc-metalloenzymes/proteins [2,3]. The trans angle
N(3)ꢀ
/
ZnꢀN(9) is 172.1(1)8 and is in accordance with the
/
structural distortion. This is again supported by the Nꢁ
/
˚
N(4), 1.262(4) A is longer
N(8), 1.258(9) A. The later bond length
N bond distance data: N(3)ꢀ
than N(7)ꢀ
(N(7)ꢀN(8)) is closer to the free ligand values [18Á
/
˚
/
/
/20].
The azophenyl group is freely suspended from the
imidazole back bone and makes a dihedral angled
10.908 in the chelated ligand (i). The monodentate
ligand (ii) is more planar than ligand (i) and the pendant
phenyl group makes a dihedral angle of the 3.88 with the
imidazole ring.
Fig. 3. X-ray structure of Zn(HaaiMe)2(NCS)2 (7a).
Table 3. The structure shows distorted TBP symmetry
around Zn(II). This is a rare example where one
HaaiMe binds as a N,N?-chelator and the second
molecule acts as a monodentate N-donor ligand. The
coordination sphere ZnN5 is coming from a N,N?-
chelator (one of the HaaiMe ligands forms a chelate ring
and is abbreviated as ligand (i)) and N(imidazole) (from
the second HaaiMe molecule which is abbreviated as
ligand (ii)) and two N-centers from the NCS groups.
3.4. Spectral studies
The infrared spectra of 4Á
intense band centered at 3400Á
assigned to n(H2O). The NꢁN and Cꢁ
are observed at 1380Á
1390 cmꢁ1, respectively. The
/
6 show a broad medium
3500 cmꢁ1 and is
N vibrations
/
/
/
¯
The trigonal plane, ZnN3 is constituted of two NCS
/
groups and N(imidazole) of ligand (i). Two axial
positions are occupied by N(3) (N(azo) of ligand (i))
and N(9))N(imidazole) of ligand (ii)). The structural
distortion of the complex is reflected from the inequi-
(Znꢀ
/
Cl) vibrations appear at 315Á
/
355 and 280Á300
/
cmꢁ1. On exposure to hotair at 425 K in an oven the
complexes turned brown and do not exhibit the broad
stretch corresponding to n(H2O). In complexes 7Á
high intense stretch at 2090 cmꢁ1is assigned to n (NCS).
UVÁVis spectra of the complexes in chloroform
solution exhibit intense transitions around 280 and 380
nm along with a weak transition at 415Á450 nm. On
/9 a
valence of all five Znꢀ
angles; even the two ZnꢀNCS bond lengths are different
(ZnꢀN(1)CS, 1.943(3); Znꢀ
distortion may be due to the small chelate bite angle,
N(5)ꢀZnꢀN(3), 66.8(6)8 which allows closer approach
of ZnꢀN(5) to ZnꢀN(2). Thus the N(5)ꢀZnꢀN(2) angle
(110.7(2)8) is less than that of N(5)ꢀZnꢀN(1), 126.9(8)8.
N(3) bond distance (2.72(6) A) is will below the
/N bond distances and bond
/
/
˚
N(2)CS, 1.987(3) A). This
/
/
/
/
/
comparing with the free ligand spectra [9] we may
conclude that the first two transitions refer to intramo-
/
/
/
/
/
/
lecular charge transfer transitions (n 0
/
p* and p0p*)
/
˚
The Znꢀ
/
and the third transition may be assigned to charge

1218
B.G. Chand et al. / Polyhedron 22 (2003) 1213Á1219
/
values [9]. This supports the zincophilicity of imidazole-
N, which is a normal feature in zinc-metalloenzymes
[26]. Zinc(II)-arylazopyrimidine complexes (Zn(aapm)-
Cl2×
/
CH3OH) also exhibit a similar proton signal pattern
[14]. The most significant feature is the difference in
frequency (Dd) of heterocyclic protons from that of free
ligand values; the
4 and 5-H for imidazole in
Zn(RaaiR?)Cl2 and 4,5, and 6-H for pyrimidine in
Zn(aapm)Cl2. The present series of complexes exhibit
higher Dd ꢀ
/
1.00 ppm than that of Zn(aapm)Cl2.-
Scheme 1. R?ꢀ
(a), CH3 (b), Cl (c).
/
Me (1/4/7), CH2CH3 (2/5/8), CH2Ph (3/6/9); RꢀH
/
CH3OH (Ddꢀ
/
0.1Á0.8 ppm) although pyrimidine is
/
more p-acidic than imidazole [26].This supports stronger
interaction between Zn(II)-imidazole than that of
Zn(II)-pyrimidine. Imidazole being a weaker p-acidic
heterocycle interacts strongly with borderline class of
metal ion, zinc(II), than that of pyrimidine. In the
transfer between zinc(II) and the ligand [14]. The
spectral data are collected in Table 2.
The 1H NMR spectra of the complexes were collected
in CD3CN and were compared with the free ligand
values [9] to determine the binding mode of the
complexes. The proton numbering scheme is shown in
Scheme 1. The assignment has been made on the basis of
complexes [Zn(RaaiR?)2(NCS)2] (7Á
tons,(4-H and 5-H) shift to a more downfield position
compared to Zn(RaaiR?)Cl2 (4Á6). It may be due to the
combined effect of a more electron withdrawing ꢀNCS
compared to ꢀCl and strong ZnꢀN(imidazole) bonding.
Aryl protons (7HÁ11H) remain almost unperturbed
/9) imidazole pro-
/
spinÁ
tion [9,21] (Table 4). The lower frequency resonances
(B7.5 ppm) suffer significant perturbation on substitu-
tion (R) and have been referred to aryl-H (7-HÁ11-H)
/spin interaction and changes therein on substitu-
/
/
/
/
/
/
compared to the free ligand values.
signals. The protons, 8,10-H have been particular
perturbed by the substituent, 9-R, because of the direct
3.5. Electrochemistry
influence of R at the CÁ
Imidazolic protons (4- and 5-H) appear at a higher
frequency position, ꢀ8 ppm and have been downfield
shifted by ꢀ1.00 ppm compared to the free ligand
/
H group in the ortho position.
Arylazoimidazoles are non-innocent molecules. They
exhibit quasireversible to irreversible azo reductions
/
ꢁ
2ꢁ
/
(ꢀ
/
Nꢁ
/
Nꢀ
/
)/(ꢀ
/
Nꢁ
/
Nꢀ/)
and (ꢀ
/
N*
/
/
Nꢀ
/
)ꢁ/(ꢀ
/
N*
/
Nꢀ
/
)
.
t
Table 4
1H NMR spectral data a of zinc(II) complexes (4Á
/9) at 300 K
Compound
d (ppm) (J (Hz))
b
e
4-H
5-H b
7, 11-H b
8,10-H b
9-R
1-CH3
1-CH2
4.63 g
(1-CH2)CH3
Ph-H
4a
4b
4c
5a
5b
5c
6a
6b
6c
7a
7b
7c
8a
8b
8c
9a
9b
9c
8.13 (7.5)
8.07 (7.5)
8.15 (7.2)
8.18 (7.5)
8.09 (7.2)
8.21 (7.2)
8.14 (7.5)
8.16 (7.5)
8.20 (7.5)
8.30 (7.0)
8.20 (7.5)
8.36 (7.5)
8.36 (7.0)
8.27 (7.5)
8.42 (7.5)
8.35 (7.8)
8.32 (7.5)
8.48 (7.8)
8.10 (7.8)
8.05 (7.8)
8.10 (7.5)
8.11 (7.5)
8.06 (7.5)
8.13 (7.8)
8.11 (7.5)
8.14 (7.8)
8.14 (7.5)
8.24 (7.5)
8.16 (8.0)
8.26 (8.0)
8.25 (8.0)
8.21 (7.8)
8.34 (8.1)
8.25 (8.1)
8.22 (7.8)
8.39 (7.8)
7.65 (8.1)
7.53 (8.1)
7.70 (7.8)
7.70 (7.8)
7.61 (8.1)
7.74 (8.1)
7.69 (7.8)
7.65 (7.8
7.69 (7.8)
7.60 (8.1)
7.52 (7.8)
7.66 (8.1)
7.65 (8.1)
7.56 (8.1)
7.69 (8.1)
7.61 (8.1)
7.58 (7.8)
7.63 (7.8)
7.50 c
7.33 (6.0)
7.57 (6.4)
7.53 c
7.36 (6.0)
7.60 (6.4)
7.49 c
7.37 (6.4)
7.52 (6.0)
7.50 c
7.35 (7.2)
7.55 (6.4)
7.50 c
7.32 (6.9)
7.55 (6.9)
7.45 c
7.50 d
2.45 f
4.06
4.04
4.10
7.53 d
2.45 f
1.59h (6.0)
1.53h (6.0)
1.58h (6.0)
4.52 g (8.0)
4.63 g (11.0)
5.53 e
7.49 d
2.47 f
7.30Á
7.30Á
7.35Á
/
7.40
7.45
7.45
5.62 e
/
5.65 e
/
7.50 d
2.42 f
4.11
4.10
4.15
7.48 d
2.40 c
4.70 g
4.58 g
4.68 g
5.60 e
5.65 e
5.70 e
1.58h (7.5)
1.55 (7.5)
1.55h (6.0)
7.45 d
2.42 f
7.30Á
7.30Á
7.30Á
/
7.40
7.40
7.45
7.33 (7.2)
7.47 (7.2)
/
/
a
Solvent: CD3CN.
b
c
d
e
f
Doublet.
Multiplet.
d(9-H).
Singlet.
d(9-Me).
Quartet.
g

B.G. Chand et al. / Polyhedron 22 (2003) 1213Á
/1219
1219
Zinc(II) is electroinactive in the potential range 1.5 to
1.5 V versus SCE. The complexes exhibit two/three
irreversible cyclic voltammetric responses in the poten-
tial range 0.0 to ꢁ1.5 V versus SCE (Table 2).
Voltammetric waves exhibit cathodic response (EPC
cathodic peak potential, V) and on scan reversal anodic
peaks are rarely obtained. This accounts instability of
the reduced species. On comparing with the voltammo-
gram of the free ligand, these responses may correspond
to azo reductions [9,21]. The shifting of potential data to
a more positive value in the complexes than the free
ligand may be due to electron drifting by the metal ion
on coordination to the ligand.
for thermal studies. Our sincere thanks are due to Dr
Babu Vargheese, Regional Sophisticated Instrumenta-
tion Center, IIT, Madras and Dr Golam Mostafa,
Department of Physics, National Tsing Hua University,
Hsinchu 300, Taiwan, ROC for his help in solving the
X-ray structures.
ꢁ
/
/
,
References
[1] H. Eklund, C.-I. Branden, in: T.G. Spiro (Ed.), Zinc Enzymes,
Wiley, New York, 1983, p. 124.
[2] W. Kaim, B. Schwederski, Bioinorganic Chemistry: Inorganic
Elements in the Chemistry of Life, Wiley, New York, 1994.
[3] R.G. Matthews, Acc. Chem. Res. 34 (2000) 618 (and references
therein).
3.6. EHMO calculation
Crystallographic data of Zn(MeaaiMe)Cl2×
/
H2O and
[4] S. Nagasato, Y. Sunatsuki, S. Ohsato, T. Kido, N. Matsumoto,
M. Kojima, Chem. Commun. 2 (2002) 14.
[Zn(HaaiMe)2(NCS)2 were used as a model for extended
Huckel calculation to find out the approximate compo-
sition of the frontier orbitals.It is observed that both the
HOMO and LUMO are constituted by ligand orbitals.
The HOMO is 89% and the LUMO is 86% ligand
orbitals. The HOMO contains 62% imidazole and 27%
arylazo functions. The LUMO is made up of 75% azo
[5] C. Plaee, J.-L. Zimmermann, E. Mulliez, G. Guillot, C. Bois, J.-C.
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[8] E. Kimura, Acc. Chem. Res. 34 (2001) 171.
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(2001) 313.
orbitals. Thus, the spectral transitions in the UVÁ
region are intramolecular (n 0p, p0p*) charge-transfer
transition. The oxidation is regarded as an electron
extraction from the HOMO and the reduction is referred
to as electron accommodation at the LUMO. The
complexes exhibit only reduction. We conclude that
reduction of the azo function is taking place.
/Vis
/
/
[11] P. Byabartta, S. Pal, T.K. Misra, C. Sinha, F.-L. Liao, K.
Panneerselvam, T.-H. Lu, J. Coord. Chem. 55 (2002) 479 (and
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4. Supplementary material
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(Chem. Sci.) 109 (1997) 159.
Crystallographic data for the structural analysis has
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 186346 for Zn(MeaaiMe)Cl2×
H2O and CCDC No. 184362 for [Zn(HaaiMe)2(NCS)2.
Copies of this information may be obtained free of
charge from the Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK. (fax: ꢂ44-1223-336033; e-
mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
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/
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Acknowledgements
Financial support from the University Grants Com-
mission New Delhi is gratefully acknowledged. We
thank professor N. Roychowdhury, IACS, Kolkatta
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