Synthesis, spectral and electrochemical studies of 2-(arylazo)heterocycle complexes of zinc(II). Single-crystal X-ray structure of [Zn(papm)Cl2·CH3OH] (papm = 2-(phenylazo)pyrimidine)

Article abstract of DOI:10.1016/S0277-5387(01)00824-5

2-(Arylazo)pyridines (aap) (R-C₆H₄-N=N-C₅H₄N, 1) and 2-(arylazo)pyrimidines (aapm) (R-C₆H₄-N=N-C₄H₃N₂, 2); R = H (a), o-Me (b), m-Me (c), p-Me (d), p-Cl (e) are used to synthesise title compounds. The complexes are characterised by elemental analyses, IR, UV - Vis and ¹H NMR spectral data. Single-crystal X-ray structure of dichloro-{2-(phenylazo)pyrimidine}-zinc(II) methanol suggests that the complex is a distorted trigonal bipyramidal symmetric around Zn(II), and Cl(1), Cl(2), N(4) (N (azo)) make the trigonal plane. Zn(II) moves downwards by 0.12 ? from the centre of gravity of the plane. Two axial positions are occupied by N(1) (N(pyrimidine)) and methanol-O. Molecular packing shows one-dimensional infinite chain via hydrogen bonding. The EHMO calculation has been carried out to explain the electronic behaviour of the complexes and the results are compared with that of copper(I) complex.

Full text of DOI:10.1016/S0277-5387(01)00824-5

Polyhedron 20 (2001) 2253–2259
www.elsevier.com/locate/poly
Synthesis, spectral and electrochemical studies of
2-(arylazo)heterocycle complexes of zinc(II).
Single-crystal X-ray structure of [Zn(papm)Cl2·CH3OH]
(papm=2-(phenylazo)pyrimidine)
Jiban Kumar Nag a, Prasanta Kumar Santra a, Chittaranjan Sinha a,*, Fen-Ling Liao b,
Tian-Huey Lu c
a Department of Chemistry, The Uni6ersity of Burdwan, Burdwan 713104, India
b Department of Chemistry, National Tsing-Hua Uni6ersity, Hsinchu 300, Taiwan, ROC
c Department of Physics, National Tsing-Hua Uni6ersity, Hsinchu 300, Taiwan, ROC
Received 15 November 2000; accepted 27 April 2001
Abstract
2-(Arylazo)pyridines (aap) (RꢀC6H4ꢀNꢁNꢀC5H4N, 1) and 2-(arylazo)pyrimidines (aapm) (R-C6H4ꢀNꢁNꢀC4H3N2, 2); R=H
(a), o-Me (b), m-Me (c), p-Me (d), p-Cl (e) are used to synthesise title compounds. The complexes are characterised by elemental
analyses, IR, UV–Vis and 1H NMR spectral data. Single-crystal X-ray structure of dichloro-{2-(phenylazo)pyrimidine}-
zinc(II)·methanol suggests that the complex is a distorted trigonal bipyramidal symmetric around Zn(II), and Cl(1), Cl(2), N(4)
,
(N (azo)) make the trigonal plane. Zn(II) moves downwards by 0.12 A from the centre of gravity of the plane. Two axial positions
are occupied by N(1) (N(pyrimidine)) and methanol-O. Molecular packing shows one-dimensional infinite chain via hydrogen
bonding. The EHMO calculation has been carried out to explain the electronic behaviour of the complexes and the results are
compared with that of copper(I) complex. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Azopyridine; Azopyrimidine; Zinc(II) complexes; Crystal structures
1. Introduction
ture from our group [3–13] and others [14–21]. Major
developments have been carried out in the field of
transition metal systems [3–21] and non-transition
metal chemistry is scarce [22–24].
There has been a continuous interest in the chemistry
of metal complexes of N-donor heterocycles [1,2]. We
have been engaged for the last several years in design-
ing arylazoheterocycles [3–13]. The molecule has
azoimine, ꢀNꢁNꢀCꢁNꢀ, functional group. Apart from
its electrochemical properties, the properties of this
functional group include stabilisation of low-valent
metal redox state [4–9], formation of oxo-metal com-
plexes [14], activation of the CꢀH function of the
pendent aryl group [15–21] in metal coordinated state
etc. These properties are well documented in the litera-
The chemistry of zinc with this functional group has
remained unexplored. Zinc has drawn special attention
because of its importance in bioinorganic chemistry
[25,26]. It plays vital role in many enzymatic processes
in biology. It is a constituent and activator of over 150
different enzymes of metabolic pathways; it is a con-
stituent of the hormone insulin [25,26]. The richness of
the chemistry of arylazoheterocycles [3–24] and the
importance of zinc [25,26] have prompted us to under-
take this work. Herein, we wish to report the synthesis,
spectral characterisation of zinc(II) complexes of aryla-
zopyridine and arylazopyrimidine. The structural confi-
rmation has been carried out by X-ray diffraction
study.
* Corresponding author. Tel.: +91-342-560-810; fax: +91-342-
564-452.
E-mail address: c
r sinha@yahoo.com (C. Sinha).
– –
0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(01)00824-5

2254
J.K. Nag et al. / Polyhedron 20 (2001) 2253–2259
Table 2
2. Experimental
,
Selected bond distances (A) and angles (°) for Zn(papm)Cl2·CH3OH
2.1. Material
Bond distances
ZnꢀN(4)
ZnꢀO
ZnꢀCl(1)
ZnꢀCl(2)
N(1)ꢀN(2)
2.100(3)
2.146(3)
2.2404(10)
2.2182(12)
1.224(4)
ZnꢀN(1)
2.435(3)
1.346(5)
1.322(5)
1.426(5)
1.453(5)
2-Aminopyridine and 2-aminopyrimidine were pur-
chased from Aldrich. ZnCl2·xH2O was obtained from
Loba Chemicals, Bombay, India. [Bu4N][ClO4] and
MeCN for electrochemical work were purified by a
reported procedure [7–9]. All other chemicals and sol-
vents were reagent grade.
C(7)ꢀN(4)
C(7)ꢀN(3)
C(7)ꢀN(2)
C(6)ꢀN(1)
Bond angles
N(4)ꢀZnꢀO
N(4)ꢀZnꢀCl(1)
N(4)ꢀZnꢀCl(2)
Cl(1)ꢀZnꢀCl(2)
Cl(1)ꢀZnꢀO
88.10(12)
118.76(9)
117.42(9)
122.23(5)
93.98(10)
Cl(2)ꢀZnꢀO
N(4)ꢀZnꢀN(1)
OꢀZnꢀN(1)
Cl(2)ꢀZnꢀN(1)
Cl(1)ꢀZnꢀN(1)
100.21(10)
69.97(11)
156.74(12)
96.87(8)
Table 1
Summarised crystallographic data for Zn(papm)Cl2·CH3OH
90.11(8)
Empirical formula
Formula weight
Temperature (K)
Crystal system
C11H12Cl2N4OZn
352.52
296(2)
monoclinic
P21/c
2.2. Physical measurements
Space group
Microanalytical (C, H, N) data were obtained from a
Perkin–Elmer 2400 CHNS/O elemental analyser. Spec-
troscopic data were obtained using the following instru-
ments: UV–Vis, JASCO UV/Vis/NIR model V-570; IR
(KBr disk, 4000–200 cm−1), JASCO FT-IR model 420
spectrophotometer, and 1H NMR, Bruker 300 MHz
FT-NMR spectrometer. An X-ray crystal structure de-
termination was carried out on a Siemens SMART CCD
diffractometer. Thermogravimetric data were collected
from DT/TG 50 thermal analyser under air. Electro-
chemical experiments were carried out using an EG&G
PARC model 270 electrochemical instrument. The re-
ported potentials are uncorrected for junction potential.
Unit cell dimensions
,
a (A)
7.1680(5)
14.4710(10)
13.9416(10)
99.1320(10)
1427.81(17)
4
0.7103
2.090
4
1.640
220
0.0383
0.0962
0.686
,
b (A)
,
c (A)
i (°)
3
,
V (A )
Z
,
u (A)
v (Mo Ka) (mm−1
)
Z
Dcalc (Mg m−3
)
Refined parameters
a
R1 [I\2|(I)]
b
wR2
Goodness-of-fit on F2 c
a R=ꢀꢁFo−Fcꢁ/ꢀFo.
2.3. Synthesis of [Zn(pap)Cl2·CH3OH] (3a)
b wR=[ꢀw(Fo2−Fc2)/ꢀwFo4]1/2
14.3394P], P=[(Fo2+2Fc2]/3.
,
w=1/[|2(Fo)2+(0.0473P)2+
c Goodness-of-fit is defined as [w(Fo−Fc)/(no−nv)]1/2, where no and
nv denote the numbers of data and variables, respectively.
2-(Phenylazo)pyridine (1a) (0.2 g, 1.09 mmol) in dry
MeOH (10 cm3) was added dropwise to a stirred
methanolic solution (10 cm3) of ZnCl2·H2O (0.17 g, 1.1
mmol) at room temperature. The orange–red solution
was stirred for 2 h and the volume reduced to half of its
original volume. The bright orange–brown crystalline
product was then slowly separated. The solution was
kept in freezing temperature and the precipitated crys-
tals were filtered, washed with water and cold MeOH.
The product was then recrystallised from MeOH and
dried in vacuo. The yield was 0.25 g (70%).
All other zinc complexes were prepared similarly and
the yield varied from 60 to 70%. The microanalytical
data of the complexes are given below. Zn(pap)-
Cl2·CH3OH (3a): Anal. Found: C, 40.9; H, 3.8; N, 12.1.
Calc. for C12H13Cl2N3OZn: C, 41.0; H, 3.7; N, 12.0%.
Zn(o-tap)Cl2·CH3OH (3b): Anal. Found: C, 42.8; H,
4.2; N, 11.6. Calc. for C13H15Cl2N3OZn: C, 42.7; H,
4.1; N, 11.5%. Zn(m-tap)Cl2·CH3OH (3c): Anal.
Found: C, 42.8; H, 4.2; N, 11.6. Calc. for C13H15-
Cl2N3OZn: C, 42.7; H, 4.11; N, 11.5%. Zn(p-tap)-
Cl2·CH3OH (3d): Anal. Found: C, 42.6; H, 4.0; N, 11.6.
Calc. for C13H15Cl2N3OZn: C, 42.7; H, 4.11; N,
Fig. 1. Single-crystal X-ray structure of Zn(papm)Cl2·CH3OH.

J.K. Nag et al. / Polyhedron 20 (2001) 2253–2259
2255
11.5%. Zn(p-Clpap)Cl2·CH3OH (3e): Anal. Found: C,
33.4; H, 3.20; N, 10.8. Calc. for C12H12Cl3N3OZn: C,
33.3; H, 3.1; N, 10.9%.
C12H14Cl2N4OZn: C, 39.3; H, 3.8; N, 15.3%. Zn(m-
tapm)Cl2·CH3OH (4c): Anal. Found: C, 39.4; H, 3.9; N,
15.4. Calc. for C12H14Cl2N4OZn: C, 39.3; H, 3.8; N,
15.3%. Zn(p-tapm)Cl2·CH3OH (4d): Anal. Found: C,
39.2; H, 3.9; N, 15.6. Calc. for C12H14Cl2N4OZn; C,
39.3; H, 3.8; N, 15.28%. Zn(p-Clpapm)Cl2·CH3OH
(4e): Anal. Found: C, 34.2; H, 2.75; N, 14.6. Calc. for
C11H11Cl3N4OZn: C, 34.1; H, 2.8; N, 14.5%.
2.4. Synthesis of [Zn(aapm)Cl2·CH3OH] (4)
Zn(aapm)Cl2·CH3OH complexes were synthesised by
the same procedure described above with the use of
2-(arylazo)pyrimidine (2). The yield varied from 65 to
75%. The microanalytical data of the complexes are
given below. Zn(papm)Cl2·CH3OH (4a): Anal. Found:
C, 37.5; H, 3.5; N, 15.95. Calc. for C11H12N4OZn: C,
37.5; H, 3.4; N, 15.9%. Zn(o-tapm)Cl2·CH3OH (4b):
Anal. Found: C, 39.2; H, 3.8; N, 15.2. Calc. for
2.5. X-ray crystallography
Crystal suitable for X-ray diffraction study of
dichloro-{2(phenylazo)pyrimidine}zinc(II)·methanol
(Zn(papm)Cl2·CH3OH) was grown by slow evaporation
Table 3
UV–Vis spectral a, IR b and voltammetric data c
Compound
UV–Vis spectral data
IR data (w, cm−1
)
Reduction potential
umax (nm) (10−3×m (dm3 mol−1 cm−1))
NꢁN
CꢁN ZnꢀCl
E1/2 (V) (DEp (mV))
Zn(pap)Cl2·MeOH (3a)
445 (1.54), 312 (17.56), 226 (11.20)
446 (1.48), 315 (18.40), 230 (10.58)
444 (1.53), 310 (17.84), 225 (11.40)
439 (0.80), 322 (19.55), 232 (12.64)
440 (1.70), 320 (20.67), 225 (15.66)
442 (0.91), 315 (12.10), 229 (6.45)
445 (1.05), 320 (13.09), 230 (7.82)
444 (1.32), 310 (11.12), 232 (7.10)
445 (1.19), 323 (12.18), 233 (7.23)
436 (0.99), 310 (10.29), 232 (7.88)
1390
1390
1385
1390
1390
1395
1400
1395
1400
1395
1600 330, 290
1595 330, 290
1600 328, 295
1600 330, 290
1590 330, 280
1590 328, 280
1595 330, 295
1600 335, 300
1595 330, 285
1600 335, 290
−1.14 (135), −1.40 (180)
−1.26 (160), −1.54 (170)
−1.23 (170), −1.51 (190)
−1.19 (140), −1.47 (180)
−1.09 (150), −1.30 (180)
−0.84 (120), −1.24 (180)
−0.88 (130), −1.48 (170)
−0.87 (120), −1.45 (170)
−0.86 (120), −1.44 (180)
−0.78 (110), −1.20 (170)
Zn(o-tap)Cl2·MeOH (3b)
Zn(m-tap)Cl2·MeOH (3c)
Zn(p-tap)Cl2·MeOH (3d)
Zn(p-Clpap)Cl2·MeOH (3e)
Zn(papm)Cl2·MeOH (4a)
Zn(o-tapm)Cl2·MeOH (4b)
Zn(m-tapm)Cl2·MeOH (4c)
Zn(p-tapm)Cl2·MeOH (4d)
Zn(p-Clpapm)Cl2·MeOH (4e)
a Solvent: MeOH.
b In KBr disk, w (MeOH): 3120–3140 cm−1
.
c Solvent: MeCN, glassy carbon millielectrode, SCE reference, NBu4ClO4 supporting electrolyte, scan rate 50 mV s−1, potentials are in V vs.
SCE, DEp=(Epc−Epa), Epc, and Epa are cathodic and anodic peak potentials, respectively.
Table 4
1H NMR spectral data of the complexes in DMSO-d6
Compound l (ppm) (J (Hz))
3-H a
4-H
5-H b
6-H a
8-H
9-H
10-H
11-H
12-H a
-Me
3a
3b
3c
3d
3e
4a
4b
4c
4d
4e
7.86 (8.0)
7.84 (8.0)
7.85 (8.0)
7.84 (8.0)
7.90 (8.0)
8.08 b (7.5) 8.08 (7.5)
8.08 b (7.5) 8.08 (7.5)
8.07 b (7.5) 8.07 (7.5)
8.07 b (7.5) 8.07 (7.5)
8.08 b (7.5) 8.08 (7.5)
8.30 a (7.0) 8.20 (7.0)
8.28 a (7.0) 8.20 (7.0)
8.28 a (7.0) 8.21 (7.0)
8.26 a (7.5) 8.18 (7.5)
8.32 a (7.5) 8.23 (7.5)
8.83 (7.0)
8.81 (7.0)
8.82 (7.0)
8.83 (7.0)
8.85 (8.0)
9.02 (7.0)
9.00 (7.0)
9.00 (7.0)
9.00 (7.5)
9.04 (7.5)
7.97 a (8.0) 7.55 d
7.55 d
7.50 d
7.97 (8.0)
7.11 a (8.0) 7.48 b (8.0) 7.48 b (8.0) 7.88 (8.0)
7.28 a (8.0) 7.40 b (8.0) 7.90 (8.0)
2.65 e
2.55 f
2.44 g
7.65 c
7.89 a (7.0) 7.44 a (8.4)
8.03 a (8.0) 7.58 a (8.0)
7.88 a (8.0) 7.54 d
7.44 a (8.4) 7.89 (7.0)
7.58 a (8.0) 8.03 (8.0)
7.60 d
7.54 d
7.88 (8.0)
7.15 a (8.0) 7.50 b (8.0) 7.50 b (8.0) 7.90 (8.0)
7.30 a (8.0) 7.43 b (8.0) 7.90 (8.0)
2.70 e
2.58 f
7.66 c
7.76 a (8.0) 7.38 a (8.0)
7.95 a (8.0) 7.60 a (8.0)
7.38 a (8.0) 7.76 (8.0)
7.60 a (8.0) 7.95 (8.0)
2.46
g
a Doublet.
b Triplet.
c Singlet.
d Multiplet
e d(8-Me).
f d(9-Me).
g d(10-Me).

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J.K. Nag et al. / Polyhedron 20 (2001) 2253–2259
Fig. 2. Frontier orbitals, HOMO and LUMO of the complexes: (a) Zn(papm)Cl2·CH3OH and (b) [Cu(papm)2]+
.
scan technique. Data were corrected for Lp effects and
for linear-decay on c-scans were applied [27]. The
structure was solved by heavy-atom methods using
SHELXS-97 [28] and successive difference Fourier syn-
thesis. All non-hydrogen atoms were refined anisotropi-
cally. The hydrogen atoms were fixed geometrically and
refined using riding model. In the final difference
Fourier map the residual maxima and minima were
of methanol solution of the complex at 283 K. The
crystal size was 0.65×0.47×0.88 mm3. X-ray diffrac-
tion data were collected at 296(2) K with the Siemens
SMART CCD using graphite-monochromatised Mo Ka
,
radiation (u=0.71073 A). Unit cell parameters were
determined from least-squares method. Data collections
were performed in the 2q range 4–55°. A summary of
the crystallographic data and structure refinement
parameters is given in Table 1. Of 8459 collected reflec-
tions 3201 unique reflections were recorded using ꢀ-
0.613 and −0.338 e A−3. All calculations were carried
,
out using the SHELXL-97 [29].

J.K. Nag et al. / Polyhedron 20 (2001) 2253–2259
2257
2
,
,
3. Results and discussion
1.40 A, N (sp ): 1.55 A). The elongation may be due to
axial coordination of N(1) with reference to the trigonal
plane of Zn(II) and from the small chelate bite angle.
2-(Phenylazo)pyrimidine is planar with no atom deviat-
ing \0.08°. Two rings are joined by azo group and the
dihedral angle is 0.85°.The pendant phenyl ring is al-
most planar to the chelate ring plane (dihedral angle is
3.1. Synthesis
Ligands used are 2-(arylazo)pyridines (aap, 1) and
2-(arylazo)pyrimidines (aapm, 2). Methanolic solution
of ZnCl2 and aap/aapm in 1:1 mole proportion have
isolated orange–yellow to orange–brown crystalline
products of composition Zn(aap)Cl2·CH3OH (3)/
Zn(aapm)Cl2·CH3OH (4). Addition of excess amount
of ligand (\3 mole) under refluxing conditions also
isolates the complex of this composition. Microanalyti-
cal and thermal data support the composition of com-
plex and in one case it is established by single-crystal
X-ray diffraction study.
,
3.933°). The NꢁN bond distance is 1.22 A and is less
than that of the results reported [12,13]. Free NꢁN for
arylazopyrimidine is not known. However, the NꢁN
bond length in free 1-methyl-2-(phenylazo)imidazole is
,
1.25 A [31]. In the coordination complexes of 2-(ary-
lazo)pyrimidines of copper(I) [12], ruthenium(II) [13]
and palladium(II) [32], the NꢁN distances are 1.26, 1.29
,
and 1.26 A, respectively. Thus the NꢁN bond in the
,
zinc complex is shortened by 0.03–0.09 A from that of
the results reported. This is quite unusual because of
the p-acidity of arylazopyrimidine [12,13]. In the TBP
geometry p* (azo) may not involve symmetrically with
the d (Zn) orbitals in performing the p-back bonding
[33]. The coordination to Zn2+ (d10) enhances the
s-electron density and may cause shortening of azo
bond length. The molecular packing shows one-dimen-
sional infinite chain via hydrogen bonding through
,
,
OꢀH(9)···N(3) (OꢀH, 0.9229 A; H···N(3), 1.9741 A and
,
O···N(3), 2.8865 A) and the angle extended by the
hydrogen bridging three-atom system is 169.53° where
N(3) is coming from the neighbouring molecule.
3.3. Thermal analysis
Thermal studies were carried out in air under non-
isothermal conditions. Differential thermal analyses
(DTA) of the complexes give two peaks, one exo- and
another endothermic peak; the first peak at 350–353 K
is attributed to the loss of coordinated methanol and
second DTA peak appears at 508–510 K, may be
attributed to complete elimination of the ligand and the
residue after 650 K is ZnO.
3.2. Molecular structure
The crystal structure of Zn(papm)Cl2·CH3OH (4a) is
shown in Fig. 1 and bond parameters are listed in Table
2. The structure shows a distorted trigonal bipyramidal
(TBP) geometry with penta-coordination around
Zn(II). The coordination includes the chelated azoimine
(ꢀNꢁNꢀCꢁNꢀ) group; two ZnꢀCl bonds and one
ZnꢀO(H)CH3 bond. Best fit trigonal plane is consti-
tuted by Zn, Cl(1), Cl(2), N(4), and Zn is deviated
3.4. Spectral studies
The complexes exhibit a broad, medium intense band
centred at 3140 cm−1 and is assigned to w (MeOH).
The NꢁN and CꢁN vibrations are observed at 1390–
1400 and 1590–1600 cm−1, respectively (Table 3). The
w (ZnꢀCl) is observed at 328–335 and 280–300 cm−1
.
,
downwards from the trigonal plane of Cl2N by 0.12 A.
The complex does not exhibit the broad stretch corre-
sponding to w (MeOH) after exposure to 140°C in hot
air oven.
The chelate ring N(1)ꢀZnꢀN(4) is unusually short
(69.97(11)°) and the shortest amongst the chelated
azoimine complexes so far reported [12,13]. The
ZnꢀN(1) (ZnꢀN (azo)) bond length is unusually long
UV–Vis spectral studies of the complexes exhibit
transitions B400 nm corresponding to intramolecular
(n“p*, p“p*) charge-transfer transitions. A weak
additional band is observed at 440–465 nm in the
complexes may be assigned to charge transfer between
zinc(II) and ligand [34]. The spectral data are shown in
Table 3.
,
(2.4351(3) A) compared to ZnꢀN(4) (ZnꢀN (pyrim-
,
idine)) bond length, 2.100(3) A. The bond length ZnꢀN
2
,
(sp ) lies in the range of 1.95–2.15 A [30]. The ZnꢀN(1)
is considered to be a long bond although it is well
,
below (0.5 A) the sum of the van der Waals radii (Zn:

2258
J.K. Nag et al. / Polyhedron 20 (2001) 2253–2259
1
The H NMR spectra of the complexes were com-
pared to the free ligand values [12,13,24] to determine
the binding mode of the complex (Table 4). The pro-
ton-numbering pattern is shown for aap (1) and aapm
(2) and are assigned on the basis of spin–spin interac-
tion and changes therein on substitution [22–24,34].
There are two distinct parts at the aromatic region of
the spectra. Upfield signals perturb significantly with
the substituent in the aryl ring while downfield signals
remain almost unchanged. The former correspond to
aryl ring protons (8-H–12-H) and the latter to hetero-
cycle protons (3-H–6-H for aap/4-H–6-H for aapm).
Heterocycle protons in the complexes are shifted
downfield by 0.1–0.8 ppm compared to the free ligand
values [12,13,22–24] while aryl protons remain almost
unshifted. This supports the tight binding of zinc(II)
with heterocycle-N. The spectra were drawn in DMSO-
d6 because of the insolubility in CDCl3. The solvent
may substitute the coordinated MeOH and is not
observed.
calculation is carried out using crystallographic
parameters reported by us [12]. The HOMO is consti-
tuted by 22% metal p- and d-orbitals and 72% azopy-
rimidine orbitals. The LUMO is made up of 93% ligand
orbitals (Fig. 2(b)). Thus absorption spectra are charac-
terised by MLCT transitions. Cyclic voltammetric ex-
periment shows the oxidation couple positive to SCE
and is referred to CuII/CuI couple. Reduction is associ-
ated with the accommodation of electron at LUMO
and is referred to ligand reduction. Thus Zn(II) com-
plexes are redox innocent at the metal centre and they
participate in charge transitions at UV–Vis region and
redox reactions because of the available LUMOs at the
ligands.
4. Supplementary material
Crystallographic data for structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 148065 for Zn(papm)-
Cl2·CH3OH. Copies of this information may be ob-
tained free of charge from The Director, CCDC, 12
Union Road, Cambridge, CB2 1EZ, UK (fax: +44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
3.5. Electrochemical studies
The electrochemical behaviour of the complexes in
MeCN (1:1) was examined by cyclic voltammetry. The
voltammogram displayed the ligand reductions at the
negative side with respect to SCE. In the potential
range 0.0 to −1.8 V two quasi-reversible responses are
observed. On comparing the voltammogram of free
ligand, these two responses may correspond to azo
reductions (Eq. (1)) [12,13,34]. The anodic shifting of
potential data in the complexes is suggestive of electron
drifting by metal ion on coordination to the ligand.
Acknowledgements
Financial support from University Grants Commis-
sion, New Delhi, is gratefully acknowledged. We thank
Professor N. Roychowdhury, Indian Association for
the Cultivation of Science, Calcutta for thermal studies.
J.K.N. is thankful to Dr S. Chattopadhyay, Vidyasagar
University, Midnapore for his help. We thank the
referees for their meaningful suggestions.
+e
+e
[ꢀNꢁNꢀ] X [ꢀN···Nꢀ]− X [ꢀNꢀNꢀ]
(1)
3.6. EHMO calculation. Spectra and redox properties
In order to gain insight into the approximate compo-
sition of the frontier orbitals in zinc(II) complexes of
2-(arylazo)pyrimidines, extended Hu¨ckel calculations
were performed on a model of complex of dichloro-
(2-(phenylazo)pyrimidine)zinc(II)·methanol. Crystallo-
graphic data were used as the parameters for bond
distances and bond angles. The HOMO and LUMO of
the compound are depicted in Fig. 2(a). Both of them
are constituted by ligand orbitals: the composition of
HOMO is 95% azopyrimidine orbitals and LUMO is
100% ligand orbitals. Thus, the spectral transitions in
the UV–Vis region are intramolecular (n“p*, p“p*)
charge-transfer transitions. Due to the filled cell elec-
tronic configuration the redox couple negative to SCE
in cyclic voltammetry is attributed to electron accom-
modation at LUMO. The observation may be com-
pared with isoelectronic Cu(I) complexes of
2-(phenylazo)pyrimidine. In [Cu(papm)2]+ the EHMO
References
[1] J. Reedijk, in: G. Wilkinson, R.D. Gillard, J.A. McCleverty
(Eds.), Comprehensive Coordination Chemistry, vol. 2, Perga-
mon Press, Oxford, 1987, p. 73.
[2] E.C. Constable, P.S. Steel, Coord. Chem. Rev. 106 (1990) 227.
[3] R. Roy, P. Chattopadhyay, C. Sinha, S. Chattopadhyay, Polyhe-
dron 15 (1996) 3361.
[4] T.K. Misra, D. Das, C. Sinha, Polyhedron 16 (1997) 4163.
[5] D. Das, T.K. Misra, C. Sinha, Transition Met. Chem. 23 (1998)
73.
[6] T.K. Misra, P.K. Santra, C. Sinha, Proc. Indian Acad. Sci.
(Chem. Sci.) 110 (1998) 445.
[7] T.K. Misra, D. Das, C. Sinha, P. Ghosh, C.K. Pal, Inorg. Chem.
37 (1998) 1672.
[8] T.K. Misra, C. Sinha, Transition Met. Chem. 24 (1999) 172.
[9] T.K. Misra, C. Sinha, Transition Met. Chem. 24 (1999) 467.
[10] D. Das, C. Sinha, Transition Met. Chem. 23 (1998) 309.

J.K. Nag et al. / Polyhedron 20 (2001) 2253–2259
2259
[11] D. Das, T.K. Misra, C. Sinha, Transition Met. Chem. 23 (1998)
233.
[22] P.V. Rolling, D.D. Kirt, J.L. Dill, S. Hall, C. Houston, J.
Organomet. Chem. 116 (1976) 39.
[12] P.K. Santra, D. Das, T.K. Misra, R. Roy, C. Sinha, S.-M. Peng,
Polyhedron 18 (1999) 1909.
[23] T.S. Basubaul, D. Dey, Synth. React. Inorg. Met.-Org. Chem. 20
(1990) 541.
[13] P.K. Santra, T.K. Misra, D. Das, C. Sinha, A.M.Z. Slawin, J.D.
Woollins, Polyhedron 18 (1999) 2869.
[24] K. Bag, N.K. De, B.R. De, C. Sinha, Proc. Indian Acad. Sci.
(Chem. Sci.) 109 (1997) 159.
[14] S. Goswami, A.R. Chakravarty, A. Chakravorty, J. Chem. Soc.,
Chem. Commun. (1982) 1288.
[25] H. Sigel (Ed.), Metal Ions in Biological Systems, vol. 6, Marcel
Dekker, New York, 1976.
[15] P. Bandyopadhyay, D. Bandyopadhyay, A. Chakravorty, F.A.
Cotton, L.R. Falvello, S. Han, J. Am. Chem. Soc. 105 (1983)
6327.
[26] H. Hayashi, M. Goto, M. Shionoya, E. Kimura, Inorg. Chem.
Commun. 3 (2000) 107.
[27] G.M. Sheldrick, SHELXL-PLUS, Siemens, Analytical X-ray Instru-
[16] C.K. Pal, S. Chattopadhyay, C. Sinha, D. Bandyopadhyay, A.
Chakravorty, Polyhedron 13 (1994) 999.
[17] B.K. Santra, G.A. Thakur, P. Ghosh, K. Pramanik, G.K. Lahiri,
Inorg. Chem. 35 (1996) 3050.
[18] B.K. Santra, G.K. Lahiri, J. Chem. Soc., Dalton Trans. (1997)
1883.
[19] B.K. Santra, P. Munshi, G. Das, P. Bharadwaj, G.K. Lahiri,
Polyhedron 18 (1999) 617.
ments Inc., Madison, Wisconsin, USA, 1990.
[28] G.M. Sheldrick, Acta Crystallogr. Sect. A 46 (1990) 467.
[29] G.M. Sheldrick, Program for Refinement of Crystal Structures,
University of Go¨ttingen, Go¨ttingen, Germany, 1997.
[30] A.G. Orpen, L. Brmmer, F.H. Allen, O. Kennard, D.G. Waton,
R. Taylor, J. Chem. Soc., Dalton Trans. (1983) S1.
[31] D. Das, PhD thesis, Burdwan University, Burdwan, India, 1998.
[32] P.K. Santra, P. Byabartta, S. Chattopadhyay, L.R. Falvello, C.
Sinha, Inorg. Chem., submitted for publication.
[20] A. Saha, A.K. Ghosh, P. Majumdar, K.N. Mitra, S. Mondal,
K.K. Rajak, L.R. Falvello, S. Goswami, Organometallics 18
(1999) 3772.
[33] F.A. Cotton, Chemical Applications of Group Theory, 3rd ed.,
Wiley, New York, 1990.
[21] A.K. Ghosh, P. Majumdar, L.R. Falvello, G. Mostafa, S.
Goswami, Organometallics 18 (1999) 5086.
[34] R. Roy, P.K. Santra, D. Das, C. Sinha, Synth. React. Inorg.
Met.-Org. Chem. 30 (2000) 1975.
.