Spectral, thermal and electrical properties of some new azo complexes

Article abstract of DOI:10.1016/S1386-1425(00)00249-3

Cobalt(II), nickel(II) and copper(II) acetates react with thymine compound (H₂L) to form complexes having the general formula [MH₂L(OAC)₂(H₂O)₂]nH₂O. However, the interaction of iron(III) chloride with thymine in acetic acid-water medium yields a new complex of the type [FeH₂L(OAC)₂H₂O]OAC.H₂O. All the thymine complexes have been characterized by elemental analyses, spectral and magnetic studies where thymine acts as a neutral ligand and the acetate ion behaves as a strong nucleophile during complexation. 6-aryl and thiazolylazo thymine compounds and their metal complexes were synthesized and characterized where the ligands act as a bidentate dibasic. The azo group is not involved in the structure. Thermal decomposition studies of the azo complexes were explained to give more information on the structure of the investigated materials. The effect of some transition metal cations such as Co^II, Ni^II and Cu^II on the electrical behaviour of 6-(2-thiazolylazo)thymine compound is studied. The data obtained obeyed the relation σ = σ° exp (-E/2kT) over the temperature range 30-150 °C. The observed conductivities of the different complexes follow the order Co-5 to 10^-9cm²/V s suggesting that the conduction of the studied complexes takes place by hopping mechanism.

Full text of DOI:10.1016/S1386-1425(00)00249-3

Spectrochimica Acta Part A 56 (2000) 1917–1923
www.elsevier.nl/locate/saa
Spectral, thermal and electrical properties of some new azo
complexes
Zenat M. Zaki
Chemistry Department, Faculty of Science, Cairo Uni6ersity (Beni-Suef Branch) Beni-Suef, Egypt
Received 10 November 1999; received in revised form 24 February 2000; accepted 24 February 2000
Abstract
Cobalt(II), nickel(II) and copper(II) acetates react with thymine compound (H2L) to form complexes having the
general formula [MH2L(OAC)2(H2O)2]nH2O. However, the interaction of iron(III) chloride with thymine in acetic
acid–water medium yields a new complex of the type [FeH2L(OAC)2H2O]OAC.H2O. All the thymine complexes have
been characterized by elemental analyses, spectral and magnetic studies where thymine acts as a neutral ligand and
the acetate ion behaves as a strong nucleophile during complexation. 6-aryl and thiazolylazo thymine compounds and
their metal complexes were synthesized and characterized where the ligands act as a bidentate dibasic. The azo group
is not involved in the structure. Thermal decomposition studies of the azo complexes were explained to give more
information on the structure of the investigated materials. The effect of some transition metal cations such as CoII,
NiII and CuII on the electrical behaviour of 6-(2-thiazolylazo)thymine compound is studied. The data obtained obeyed
the relation |=|° exp (−E/2kT) over the temperature range 30–150°C. The observed conductivities of the different
complexes follow the order CoBNiBCu. It is clear that this trend is depending on the decreasing of the ionic radii
and the increasing stability of metal complexes. The calculated mobility of charge carriers is ranged from 10−5 to
10−9cm2/V s suggesting that the conduction of the studied complexes takes place by hopping mechanism. © 2000
Elsevier Science B.V. All rights reserved.
Keywords: Thymine azo complexes; Spectral, thermal and electrical properties; Pyrimidine derivatives
elucidation of metal ion effects on the pyrimidine
nucleus could possibly lead to a better under-
1. Introduction
standing of complex biological processes occur-
Pyrimidine derivatives constitute a very impor-
ring in living systems. This work was directed
tant class of compounds because they are compo-
towards an understanding as to the nature of
nents of the biologically important nucleic acid
metal binding in thymine and its azo derivatives.
and have been shown to exert a pronounced
The DC electrical behaviour of 6-(2-thiazoly-
physiological effect [1–6]. Since most living sys-
lazo)thymine compound and its cobalt(II), nick-
tems contain metal ions, which are essential for
el(II) and copper(II) complexes at different
proper functioning, the question arises as to the
temperatures is investigated. The mechanism of
effect of such metal ions on nucleic acids. Any
conduction is explained.
1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(00)00249-3

1918
Z.M. Zaki / Spectrochimica Acta Part A 56 (2000) 1917–1923
tate was separated by filtration. The complex was
washed with hot H2O and dried at 60°C.
2.2. Preparation of thymine azo metal complexes
0.01 mol of an alcoholic solution of FeIII, CoII,
NiII and CuII chlorides was refluxed with 0.02
moles of the azo ligand dissolved in EtOH for 2 h.
The volume of the resultant solution was reduced
to ca. one half by evaporation and the new com-
plexes were precipitated by the addition of ammo-
nia. The precipitate was then filtered, washed with
EtOH, finally Et2O and dried at 60°C. The analyt-
2. Experimental
Thymine was supplied from BDH, UK. The
other chemicals used were of analytical grade.
6-aryl and thiazolylazo thymine compounds were
prepared by dissolving each of aniline and 2-
amino thiazole (0.1 mole) in HCl (0.2 mole /25
cm3 distilled water). The hydrochloride was diazo-
tised below 5°C with a solution of NaNO2 (0.1
mole/30 cm3 H2O). The diazonium chloride was
coupled with an alkaline solution of thymine (0.1
mole). The crude dye was collected by filtration
and was dried at 80°C.
1
ical data are listed in (Table 1). H n.m.r. spectra
were obtained with Varian EM 90 MHZ n.m.r.
spectrometer. IR spectra were recorded on a
Perkin-Elmer FT-IR Model 8001 spectrometer.
The solid reflectance spectra were measured on a
Schimadzu 3101 PC spectrophotometer. The mo-
lar magnetic susceptibilities were measured on
powered samples using the Faraday method. Dia-
magnetic corrections were made using Pascal’s
constants with Hg[Co(SCN)4] as calibrant. The
thermogravimetric analyses (TGA) were carried
out in dynamic nitrogen atmosphere (20 ml
min−1) with a heating rate of 10°C min−1 using a
Schimadzu TGA-50H thermal analyzer. Conduc-
tivity measurements were made on discs having
1mm thickness of 6-(2-thiazolylazo) thymine and
its complexes sandwiched between two copper
2.1. Preparation of thymine metal complexes
Metal complexes of thymine were prepared by
refluxing 0.01 mole of aqueous solution of cobalt,
nickel and copper acetates and ferric chloride with
0.02 moles of thymine dissolved in a mixture of
water-acetic acid (5:1 v/v) for 1 h. The reaction
mixture was concentrated where a brown precipi-
Table 1
Analytical data of thymine and its azo metal complexes
Complex
% Found (% calculated)
M
C
H
Thymine (H2L)[FeH2L(OAc)2H2O]OAc.H2O
[CoH2L(OAc)2H2O]2H2O
[NiH2L(OAc)2H2O]H2O
[CuH2L(OAc)2H2O]4H2O
6-(arylazo)thymine (H2L%)
[Co2L%(OH)2(H2O)4]
14.3 (14.2)
16.8 (16.8)
17.4 (17.6)
16.1 (16.1)
–
26.3 (26.4)
19.1 (19.5)
–
23.6 (23.8)
18.2 (18.1)
18.0 (18.1)
18.8 (19.1)
33.7 (33.4)
30.1 (30.2)
31.6 (31.8)
26.9 (27.1)
57.2 (57.4)
29.0 (29.1)
40.0 (40.2)
40.3 (40.5)
20.7 (20.4)
28.6 (28.9)
28.7 (28.9)
28.3 (28.6)
4.6 (4.8)
4.8 (5.0)
4.8 (4.7)
5.2 (5.5)
4.2 (4.3)
3.8 (4.0)
3.6 (3.7)
2.8 (3.0)
3.3 (3.6)
2.8 (2.7)
2.4 (2.7)
2.5 (2.7)
[CuL%(H2O)2]
6-(thiazolylazo)thymine (H2L%%)
[Fe2L%%(OH)4(H2O)2]H2O
[CoL%%(H2O)2]
[NiL%%(H2O)2]
[CuL%%(H2O)2]

Z.M. Zaki / Spectrochimica Acta Part A 56 (2000) 1917–1923
1919
electrodes. The conductivity cell used was the
same as that reported before [7]. The electrical
conductivity was measured in the temperature
range 30–150°C. The activation energies were
calculated using the equation |=|° exp (−E/
2kT).
nasy(CꢁO) and nsym(CꢁO) of the acetate group [8].
The presence of acetate ions in the structural
configuration of the complexes suggests that
thymine acts as a neutral molecule during com-
plexation with the metal ions. The spectrum of
6-(2-thiazolylazo) thymine ligand shows bands at
1540, 1320 and 830 cm−1 assigned for nCꢀN
cyclic, nCꢁN and nCꢁSꢁC of the thiazole moiety.
The position of these bands remain unaltered in
the metal complexes, ruling out the possibility of
coordination of both nitrogen and sulphur atoms
of the thiazole ring. The spectra of the azo com-
plexes do not show any shift in the frequencies of
either nC2ꢀO or nNꢀN indicating that neither
C2ꢀO nor NꢀN is taking part in coordination with
the metal ions. However, all the spectra of the azo
complexes show the absence of dN3H and nC4ꢀO
and the appearance of nCꢀN and nCꢁO indicating
that N3H is tautomerized with C4ꢀO to the iminol
structure before complexation. Again, the dN1H
observed at 1482 cm−1 in the free ligand is com-
pletely disappeared during complexation indicat-
ing that N1H group is coordinated to the metal
ion through deprotonation. The nOH bending
frequencies in the range 950–930 cm−1 for
cobalt–arylazo thymine complex and both iron
and copper complexes of thiazolylazo thymine
compound show a bridge structure formation
through OH groups [9]. Since all the complexes
are insoluble in common organic solvents, poly-
meric structures for the complexes are proposed.
The polymerization involves bridging through OH
for cobalt–arylazo thymine and both iron and
copper–thiazolylazo thymine complexes and
through O and N atoms of the thymine moiety
for the rest of the complexes.
3. Results and discussion
1H n.m.r. spectrum of 6-(arylazo) thymine com-
pound in d6 DMSO shows signals in the range
7–7.7 ppm due to phenyl protons calculated as 5.
Two broad bands appeared at 10.2 and 11 ppm
assigned for NH groups. Concordant results were
obtained from IR studies where two strong bands
are appeared at 1745 and 1685 cm−1 due to
nC2ꢀO and nC4ꢀO. A medium broad band at 3240
cm−1 may be due to nNH. Extra sharp signal
appeared at 1.6 ppm due to protons of the methyl
group. For 6-(2-thiazolylazo) thymine ligand, the
spectrum shows two doublets in the vicinity of d
6.5–7.4 ppm assigned for H4 and H5 protons of
the thiazole ring. Broad peaks in the 10.1–11.9
ppm ranges are assigned for NH groups.
3.1. IR spectra and mode of bonding
Thymine compound has various potential
donor sites. A comparison between the IR spectra
of thymine and those of transition metal com-
plexes provide evidence regarding the bonding
sites in the thymine complexes. The IR spectra of
all thymine complexes (Table 2) show bands at
3500–3400, 750–730 and 600 cm−1 characteristic
of stretching, rocking and waging modes of coor-
dinated H2O molecules. The absence of both nNH
and nCꢀO bands of the thymine compound and
the appearance of new bands in the region 2633–
2513, 1610 and 1315–1310 cm−1 in the spectra of
thymine complexes assigned for nOHiminol, nCꢀN
and nCꢁO is strong evidence that tautomerization
of the NH groups with the adjacent carbonyls
takes place before complexation. The pseudo aro-
maticity of the iminol structure may stabilize the
enol form in comparison with the keto form. All
the metal complexes display two intense bands at
1538–1536 and 1395–1380 cm−1 assigned for
3.2. Electronic spectra and magnetic properties
The FeIII thymine complex being a d5 system.
Its reflectance spectrum does not show any peak
in the visible region but the colour may be due to
charge transfer transition [10]. For the cobalt
thymine complex, the spectrum displays two
bands at 19500 and 7800cm−1 assigned respec-
4
tively to the transitions T1g“4T1g(P)n3 and
4
4T1g“4T2g(F)n1. The transition T1g“4A2gn2 is a
two electron process and is normally weak. The

1920
Z.M. Zaki / Spectrochimica Acta Part A 56 (2000) 1917–1923

Z.M. Zaki / Spectrochimica Acta Part A 56 (2000) 1917–1923
1921
Racah parameter is calculated using the equa-
tion [11]:
10 000 cm−1. However, for pseudo-tetrahedral
CuII complexes three peaks have been reported
[16]. Therefore, energy peaks observed in the
present CuII complex may be assigned to a
pseudo-tetrahedral configuration. The magnetic
moment can not be used to distinguish between
distorted tetrahedral and octahedral complexes
of copper but the value of 2.09 B.M. places in
the range of known pseudo-tetrahedral com-
plexes [17]. For the complexes derived from 6-
(2-thiazolylazo)thymine compound, the spectrum
of the FeIII complex displays two intense bands
at 12180 and 7550 cm−1 suggesting penta-coor-
dination of the iron (III) as no such bands can
be observed for six coordinate iron(III) com-
plexes due to spin forbidden transitions [18].
The solid reflectance spectrum of the cobalt
complex shows two intense bands n43A2“4T1(P)
(13200 cm−1) and n24A2“4T1(F) (5618 cm−1).
The parameters Z (10Dq) and B% could be cal-
culated by solving the following equations [19]
using the values of n2 and n3.
B=1/30[−(2n1−n3)9(−n21+n32+n1n3)1/2
]
The B parameter is a measure of the elec-
tron–electron repulsion terms which is lower in
the complex (853 cm−1) than in the free ion
(985 cm−1) with b value of 0.866. From the
position of the bands and the calculated Racah
parameter, the complex is octahedral with cova-
lent bonds between the organic ligand and the
metal ion. The d–d spectrum of the NiII
thymine complex shows bands at 21800, 17857
and 8750 cm−1 suggesting the existence of
3
3
3A2g“3T1g(P)n3, A2g“3T1g(F)n2 and A2g“
3T2gn1 transitions with an octahedral spatial
configuration. The Racah parameter is calcu-
lated according to B=n2+n3−3n1/15, n1=
10Dq. The 10Dq value of 8750 cm−1 confirms
the octahedral configuration of the complex.
The smaller B value (893 cm−1) compared to
the free metal ion (1040 cm−1) is due to strong
interaction between the reaction site of the or-
ganic compound with nickel. The b value was
less than unity (0.859) suggesting a largely cova-
lent bond between the organic ligand with NiII
ion. The reflectance spectrum of CuII thymine
complex shows a broad band centered at ꢀ
17544 cm−1 which may be assigned to the tran-
n1=10Dq=Z,
n2=1.5Z+7.5B%−Q,
n3=1.5Z+7.5B%+Q
and
Q=12[(0.6Z−
1
15B%)2+0.64D2] 2. The ligand field parameters
Dq (321.8 cm−1) and B% (611 cm−1) are indica-
tive of tetrahedral geometry for the CoII com-
plex. For the copper complex, the reflectance
spectrum shows a broad band, which has three
components with maxima at 13750, 10300 and
8650 cm−1. This spectrum resembles those of
the pseudotetrahedral CuII complex [20].
2
sition Eg“2T2g in an approximately octahedral
(
environment [12]. The observed magnetic mo-
ments of the CoII, NiII and CuII thymine com-
plexes are 3.7, 2.6 and 1.54 B.M. respectively.
These values indicate some sort of molecular as-
sociation that could be achieved through
bridged ligand or coordinated anion [13]. How-
ever, the spectrum of [CoL%(OH)2(H2O)4] shows
three absorption bands at 19850, 13400 and
3.3. Thermal and electrical properties of the azo
thymine complexes
6350 cm−1 assigned to 4A%(F)“4E%(P),
The thermogram of the iron (III) complex
derived from 6-(2-thiazolylazo) thymine com-
pound shows that the weight loss begins at 40°C
assigned to desolvation (loss of water of crystal-
ization). Sharp peaks at 285 and 370°C in the
DTG curve demonstrate the decomposition of
the complex in two steps. The overall weight
loss 67.9% theoretically 66.0% may be in accor-
dance with the proposed structure of the com-
plex shown in form (1).
2
4A2%(F)“4A%(P) and 4A%(F)“4E%(F) transitions
2
2
indicating a trigonal bipyramidal structure is
proposed for this complex [14]. The veff value
4.6 B.M. suggests high spin five coordinate ge-
ometry. The electronic spectrum of [Cu
L%(H2O)2] complex exhibits three peaks at 18650,
13600 and 8900 cm−1. The crystal field theory
of tetrahedral CuII complexes [15] predicts only
one transition 2T2“2E which occurs below

1922
Z.M. Zaki / Spectrochimica Acta Part A 56 (2000) 1917–1923
Table 3
Thermal and electrical data of 6-(2-thiazolylazo)thymine and its metal complexes at 303 K
Compound
Decomposition temperature (°C)
| (Ohm−1 cm−1
)
E (ev)
n (cm−3
)
v (cm2/V s)
Ligand (H2L%)
[CoL%%(H2O)2]
[Ni L%%(H2O)2]
[CuL%%(H2O)2]
–
230
245
265
1.45×10−8
1.66×10−8
2.09×10−8
2.5×10−8
0.33
0.38
0.43
0.56
8.56×1019
12.8×1018
1.7×1018
1.29×1016
1.0×10−7
8.0×10−9
7.7×10−8
1.2×10−5
63.3%) agrees with the proposed structure shown
in form (3).
The TGA curves of the CoII, NiII and CuII
complexes of 6-(2-thiazolylazo)thymine ligand
show weight losses of 10.9, 10.88 and 10.7% in the
range 120–265°C corresponding to the loss of two
coordinated water molecules. From the procedural
decomposition temperature (Table 3), it can be
concluded that the thermal stability order of the
complexes is CoBNiBCu. The analysis of the
thermograms indicates that the complexes decom-
posed in two stages after loss of water molecules.
In all the chelates, the rate of decomposition in the
first stage is slow as compared to the second stage
and decomposition completes at about 600°C.
According to the stereochemistry of these com-
plexes obtained from electronic spectra and mag-
netic susceptibility measurements and from the
mode of bonding based on the IR spectra, the
structure is as shown in form (2).
The [CuL%(H2O)2] complex yields a horizontal
line extending up to 120°C showing stability of the
complex up to this temperature. The decomposi-
tion starts at 120°C and extends up to 230°C. The
weight loss at this stage (10.6%) corresponds to the
escape of two coordinated water molecules. A
further decomposition starts beyond 230°C and
The TG curve of cobalt–arylazo thymine com-
plex shows that the weight loss begins at 120° up
to 190°C corresponding to the loss of four water
molecules. The ligand moiety starts dissociation at
190° up to 700°C leading to the formation of
Co3O4. The overall weight loss (62.3%, theoretically
Fig. 1. The electrical conductivity of 6-(2-thiazolyazo)thymine
and its complexes.

Z.M. Zaki / Spectrochimica Acta Part A 56 (2000) 1917–1923
1923
continues up to 650°C corresponding to the loss of
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