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Abstract
Zinc II, cadmium II and mercury II
complexes derived from barbituric acid (BA), 5-nitrobarbituric acid (NBA),
phenobarbital (PB) and 2-thiouracil (TU) were synthesized. The analytical
results assigned the formation of complexes with the stoichiometries 1:1 and
1:2. The infrared spectral measurements assigned, and bands. The tetrahedral
geometries are given for these complexes. The capacitance (CP) and the
dielectric constant of the complexes are decreased with increasing the applied
frequency and increased with increasing temperature. The behavior of the
dielectric loss (e”)
indicated a polar polarization mechanism. The loss tangent (tan d) is decreased with increasing frequency and increased with
increasing temperature while the impedance (Z) is mostly decreased with
increasing both of frequency and temperature. Cole-Cole diagrams for the
complexes at different temperatures reveal non-Debye type of the complexes. The
relaxation time (t) for each relaxator becomes smaller as the temperature
increases. In most complexes, the conductivity – temperature relationship is
characterized by a phase transition temperature. Two pathways for the
conduction of electricity may be expected at lower and upper temperature
regions: n ® p* and p
® p* transitions, respectively. The relative permittivity,
dielectric loss and conductivity values for the complexes revealed
semiconducting features based mainly on the hopping mechanism. The lower values
of the activation energy (DE)
may be understood assuming that the metal ion forms a bridge with the ligands,
thus facilitating the transfer of current carriers with some degree of
delocalization in the excited state.
Keywords: Ligands and Complexes; IR Spectra; Dielectric Properties;
Electrical Conductivity; Cole-Cole Diagrams and Activation Energy
Introduction
In today’s age of molecular biology
purines and pyrimidines are probably best known as the basic constituents of
the nucleic acids which are biomolecules that store genetic information in
cells or that transfer this information from old cells to new cells. A number
of pyrimidines were tested for their ability to inhibit nuclear and
mitochondrial (uracil- deoxyribonucleic acid (DNA) glycosylase) activities
also, 2-thiouracil, a ribonucleic acid (RNA) synthesis inhibitor, reduces the
fertility of photoperiod sensitive genic male- sterile rice. Some nucleobase
analogous were screened as inhibitors of dihydrouracil dehydrogenase (DHU
dehydrogenase) from mouse liver. 5-Nitrobarbituric acid was identified as a
potent inhibitor [1]. Since most living systems contain metal ions which are
essential for proper functioning, question arises as to study the effect of
such metal ions on nucleic acids. Any elucidation of metal ions effects on the
pyrimidine nucleus could possibly lead to a better understanding of complex
biological processes occurring in living system. Transition metals possess
great biological activity when associated with certain metal-protein complexes
which participate in the transport of oxygen and electronic transfer reactions
[2]. Platinum group metal complexes of nucleic acid bases and their derivatives
attracted considerable attention because of their antitumor and antibacterial
activity [3]. The biological activity of cisplatin is due to its ability to
bind the guanine-cytosine of the DNA strand and stop the replication process
[4]. Cisplatin has been used in treating several human tumors of the
genito-urinary type [5].
DNA strands can curl up to produce
some amazing structures, and they can bind to metal ions. DNA has served as an
ingenious storage device for genetic data for more than three billion years.
But only few years ago, it has also emerged as a powerful material for building
complex structures at the nanometer scale. Masoud and coworkers published a
series of papers about pyrimidine complexes, the most recent references are
cited [6-12]. So, in a sequel of continuation, the present paper is focused to
study the complexing properties and electrical applications of some
biologically active nucleic acid constituents (barbituric acid,
5-nitrobarbituric acid, phenobarbital, and 2-thiouracil).
Experimental
A-
Synthesis of Complexes
The required metal salts were
dissolved and mixed with the required weight of the ligand solutions. The
selected ligands are shown in the following (Scheme 1).
Barbituric acid (BA),
5-nitrobarbituric acid (NBA), phenobarbital (PB), and 2-thiouracil (TU).
I. Metal ion content
The complexes were digested and
decomposed with aqua regia. The contents of Zn2+, Cd2+
and Hg2+ were determined by the usual complexometric titration
procedures [13].
II. Carbon, hydrogen, nitrogen and
sulfur contents were analyzed as usual.
C-
Instruments and Working Procedures
Infrared Spectrophotometer
The spectra of ligands and their
complexes were recorded using Perkin-Elmer Spectrophotometer model 1430
covering the frequency range 4000-200cm-1, by the KBr disc method.
Dielectric and Electrical
Conductivity Measurements
i Four test parameters including
impedance |Z|, phase angle θ, parallel equivalent static capacitance CP and
loss tangent tan δ were measured for the complexes Zn(BA)2, Cd(BA)2.2H2O,
Zn(NBA).2H2O, Cd(PB)2.H2O and Zn(TU).H2O
in the solid state at constant voltage 0.80 volt. The measurements were taken
at different temperatures (26-190°C) and variable frequencies (4 kHz-100 kHz)
using HIOKI “3532-50 LCR HITESTER” instrument.
ii The complexes were prepared in
the form of tablets at a pressure of 4 tons/cm2.The tablets were hold between
two copper electrodes and then inserted with the holder vertically into
cylindrical electric furnace. The potential drop across the heater was varied
gradually through variable transformer to produce slow rate of increasing the temperature
to get accurate temperature measurements using a pre-calibrated Cuconstantan
thermocouple attached to the sample.
iii The dielectric constant ε, the
dielectric loss e²,
real part of impedance Z′, imaginary part Z″, the conductivities σa.c.
the relaxation times τo , τ and the activation energies ΔE of the
complexes were calculated [14] and correlated with the structures.
Results and Discussion
Mode
of Bonding and Stereochemistry of the Prepared Complexes
The IR spectra of the free ligands
and their metal complexes were studied, usually, a charge transfer takes place
from the ligand to the metal ion resulting in a decrease in the force constant
of the bond reflecting a red shift of the band position. In some cases, a blue
shift occurs for a reverse process, i.e. electrons are donated from the metal
ion to the coordinated groups leading to increase the bond order of the groups
bonded to the metal ion [15]. Most of the prepared complexes contain water.
Generally, lattice water absorbs at 3550-3200cm-1 (asymmetric and
symmetric OH stretchings) [16], and at 1630-1600cm-1 (HOH bending).
Also, the rocking and metaloxygen stretching modes will become infrared active
if the metaloxygen bond is sufficiently covalent. The presence of these bands
in aqua complexes was reported at 880-850cm-1 and assigned to the
rocking mode of coordinated water [17]. Infrared data illustrated the following
main points
b) Shifts of the band of the free
ligand occurred upon complexation, due to the existence of coordinated water
molecules or M-O and hydrogen bond formations [19], However, 
is assigned.
c) The shifts or disappearance of
both the 
bands, (Table 1) suggest that
these groups are strongly involved in the structural chemistry of the
complexes. This is supported either by the probable existence of M-N bands or
the free ligand may be subjected to keto
enol tautomerism [20,21].
d) New IR bands of the complexes appeared
at (528- 523cm-1) and (316-315cm-1) assigned as 
, respectively. The 
bands of BA are shifted on
complexation, indicating M-O interaction.
e) Barbituric acid is of bidentate
or tridentate bonding according to the complex stoichiometries, (Scheme 1) The
bidentate chelation is suggested to be through N(1) and C(2) O while the
tridentate interaction is via C(2)O, N(3) and C(4)O.
In
Case Of NBA, (Table 2) The Infrared Data, Illustrated the Following Main
Points:
a) The broad band at 3433cm-1
in the free ligand is shifted to 3561 and 3559cm-1 for ZnII and CdII
complexes, respectively [22]. Two 
bands for the free ligand at 3173
and 3028cm-1 are identified. On complexation, the former band for
the ligand becomes doublet at (3170, 3146cm-1) and (3170, 3141 cm-1)
for ZnII and CdII complexes, respectively. However, the latter band is shifted to 3024 and 3025cm-1
for ZnII and CdII complexes, respectively.
b) The medium CH υ band at 2836cm-1
for the free ligand excluded the possibility of the formation of an
organometallic compound. However, the band in the free ligand (NBA) is not
remarkably affected on complexation suggesting that the C=O in position (6) is
still exist in the complexes.
c) The observed medium C N υ = band
in the free ligand at 1651cm-1 may be due to tautomerism. It is
shifted (-3cm-1) for both ZnII and CdII complexes in strong feature.
Such data suggest that the nitrogen atom of the pyrimidine ring formed by
tautomerism is bonded to the metal ion.
d) The nitro group is not involved
in coordination.
In case of thiouracil (TU), the NH
group either participates in bond formation with the metal ion or tautomerised
with the adjacent C=S and C=O groups to form the enol-thiol tautomers. The
latter view is verified by the presence of C N υ = , C O υ − and bands at 1626,
1390 and 1001cm-1, respectively [23]. 2-Thiouracil acts as dianionic
and tridentate chelator through C(2)S, N(3) and C(4)O. From the previous
findings, together with the elemental analyses, the following structures for
NBA complexes are given: (Scheme 2)
B- Dielectric and Electrical
Conductivity Measurements
Dielectric
Measurements
For a parallel-plate condenser in
which a dielectric tablet fills the space between the plates, the capacitance
is given by [26]:
where o ε is the permittivity of a
vacuum and its value is approximately 8.854 × 10-12 F m-1,
ε is the dielectric constant of a
dielectric, A and d are the area and thickness of the tablet, respectively.
The real and imaginary parts of the
complex impedance are given by:
where Z′ and Z″ are the real and
imaginary parts of the impedance, respectively.
Dispersion arising during the
transition from full orientational polarization at zero or low frequencies to
negligible orientational polarization at high radio frequencies is referred to
as dielectric relaxation. The rate of decay and build-up of the orientational
polarization, as given by the relaxation time τ, will depend upon the thermal
energy of the dipoles as well as upon the internal or molecular friction forces
encountered by the rotating dipoles. The dielectric parameters are given in
terms of temperature and frequency changes, e.g. Zn(BA)2 (Figure 1).
The more spotlight points could be given as follows:
I. The capacitance (CP) and the
dielectric constant decreased with increasing the applied frequency in some
different ranges which probably due to that the polarization does not occur
instantaneously with the application of the electric field.
II. The variation of the
permittivity values with increasing temperature at certain constant frequency
revealed small dielectric constant at lower temperatures, where the molecules
are rigid, i.e. less oriented forces. By increasing the temperature, the number
of molecules capable of rotating about their long axes increased with higher
permittivity values. The behavior of the dielectric loss e² values, (Figure 1), indicated a polar polarization
mechanism [28], where its values are affected by both temperature and
frequency.
III. The relative permittivity and
dielectric loss values for the complexes, (Figure 1), revealed semiconducting
features based mainly on the hopping mechanism [29].
IV. The loss tangent (tan δ) is
decreased with increasing frequency and increased with increasing temperature
in most cases, (Figure 1).
V. The impedance (Z) is mostly
decreased and illustrated for Zn(BA)2 and Cd(BA)2.2H2O
as two different examples at different temperatures, (Figure 2).
The evaluation of experimental
dielectric data is much facilitated by certain graphical methods of display,
which permit the derivation of parameters by geometrical construction. The
earliest and most used of these methods consists of plotting e²(ω) for certain frequency against ε′(ω) at the same
frequency, in cartesian coordinates or in the complex plane. For a dielectric
with a single relaxation time the Cole-Cole plot is a semi-circle which
provides an elegant method of finding out whether a system has a single
relaxation time or more [30]. The semi-circle diagram has been used to
determine the distribution parameter α [31], which measures the width of
distribution of relaxation time and evaluated by measuring the angle between
the real part of dielectric constant and radius of the circle. Also, the
macroscopic relaxation time to and the molecular relaxation time τ can be
determined [30,32]. If the centers of semi-circles lie ε′(ω) axis, α is zero
(Debye type). Otherwise the centre is below ε′(ω) axis and α ≠ 0 (non-Debye
type). Two intersections between the real axis ε′(ω) and the circular arc, give
the relative permittivity at zero frequency (static dielectric constant es) and that at infinite frequency approaching the
frequencies of light oscillators (optical dielectric constant ε∞) [32]. A point
on the semi-circle defines two vectors u and v. v is the distance on the
Cole-Cole diagram between the static dielectric constant es and the
experimental point, u is the distance between that point and the optical
dielectric constant ε∞. Cole and Cole generalized the representation of a Debye
dielectric by a circular arc plot in the complex plane so that it is applied to
a certain type of distributions of relaxation times, so
The extent of the distribution of
relaxation times increases with increasing parameter α. On the other hand, the
value of to decreases with increasing temperature. The molecular relaxation
time τ could be determined based on the following equation [30]:
The temperature dependence of τ can
be expressed for thermally activated processes as [32]:
where to is a constant
characteristic relaxation time and represents the time of a single oscillation
of a dipole in a potential well, Eo is the energy of activation for the
relaxation of the dipole, k is the Boltzmann constant and τ represents the
average or most probable value of the spread of the relaxation times. A
representative Cole-Cole diagrams for Zn(BA)2 complex at 30 and
50°C, (Figure 3), reveal non-Debye type of the complex. The dielectric data
obtained from the analysis of Cole-Cole diagrams for different complexes are
collected in Table 3. The change of central metal ion from Zn to Cd in the
complexes results mainly in a decrease of the relaxation time values. to for
Cd(PB)2.H2O complex is much higher than that for Cd(BA)2.2H2O
complex at the same temperature in most cases, (Table 3). One must focus the
attention that the molecular orientation of Cd(PB)2.H2O
gave its high restriction. So, this complex is probably associated in its
molecular structure.
The variation of ln τ as a function
of reciprocal absolute temperature for different complexes, (Figure 4), showed
the above relation for Zn(NBA).2H2O and that for Cd(PB)2.H2O
assigned that as the temperature increases, the relaxation time for each
relaxator becomes smaller in some ranges. The activation energies for the
relaxation processes of different complexes are given in Table 4.
Electrical
Conductivity Measurements
The frequency dependence of a. c.
conductivity for the complexes at different temperatures is illustrated in
Figure 5. The behavior shows that the a. c. conductivity increases with increasing
the frequency. In the present complexes, the conductivities have a magnitude
close to that of semiconductors, where the electrons in the orbitals are not of
sufficient mobility to be promoted. The study of the conduction mechanism of
organic materials leads to an increasing use of these materials in commercial
devices such as solar energy panels, scintillation counter and also in some
technological applications such as photocopy process. The electrical
conductivity of substances at a given frequency varies exponentially with the
absolute temperature according to the Arrhenius relation [33]:
where σ is the electrical
conductivity at an absolute temperature T, so is the pre-exponential factor, ΔE
is the activation energy and k is the Boltzmann constant. Therefore, the
temperature dependence of the electrical conductivity is characterized by the
two constants: the activation energy (ΔE) and the pre-exponential factor (so).
The variations of ln σ as a function of reciprocal absolute temperature for
Zn(NBA)2.2H2O and Cd(PB)2.H2O
complexes at different frequencies are illustrated in Figure 6. The activation
energy data and ln so values for the complexes are given in Table 4, from which
the ΔE values are in harmony with those calculated from relaxation processes.
For the complexes, the curves are characterized by breaks at a transition
temperature. So, the behavior is nearly the same till the phase transition
temperatures (343-403K) followed by large increase in conductivity by further
increase of temperature. This can be ascribed to a molecular rearrangement or
different crystallographic or phase transitions [34,35]. The magnitude of the
conductivities of the complexes, along with the values of the energy gaps
indicated slight semiconducting properties. The most realistic description of
the complexes involves an interaction of the metal orbitals with the ligands to
give new molecular orbitals (MO), which are delocalized over the whole
molecular complex. In view of the high degree of covalency in the M-O and M-N
bonds, it is no longer permissible to distinguish the central metal from the
ligands, the complexes must be regarded as individual entities.
The conductivity for amorphous
semiconductor could be interpreted with an intrinsic two-carrier model which
originates with thermally assisted hopping conduction [29]. The relationship
between molecular structure and electrical properties was deduced. On the basis
of electronic transition within molecules, two pathways for the conduction of
electricity may by expected. The first conducting process occurring in the
lower temperature region is attributed to n → π* transitions which require less
energy to be performed. While in the upper temperature region, conduction could
be attributed to π → π* transitions which need more energy to participate in
electronic conduction. The observed increment of conduction in the upper
temperature region may be attributed to interactions between n → π* and π → π*
transitions. The lower temperature range is the region of extrinsic
semiconductor where the conduction is due to the excitation of carriers from
donor localized level to the conduction band. In the upper temperature range,
the intrinsic region is reached where carriers are thermally activated from the
valence band to the conduction band. This behavior can be explained as follows:
the upper temperature range may be attributed to the interaction between the
electrons of d-orbitals and the p-orbitals of the ligand. This interaction will
lead to small delocalization of the p-electronic charge on the ligand which
tends to increase the activation energy. The presence of d-electrons in a
narrow energy band leads to magnetic ordering and degeneracy of d-bands with
respect to the orbital quantum number, which is only partially lifted in a
crystal field [36].
In all complexes, during temperature
increase, an additional increase in electrical conductivity occurs. This is a
useful criterion for ascertaining the nature of the metal-ligand bonding [37],
so
a) The electrical conductivities
increased by increasing the molecular weight of the complexes
b) The activation energy decreased
with increasing the atomic number of the metal, which indicates that the
presence of holes in the system has little effect on the mobility of charges
[38].
The lower values of ΔE may be
understood assuming that the metal ion forms a bridge with the ligands, thus
facilitating the transfer of current carriers with some degree of
delocalization in the excited state during measurements. Meanwhile, this leads
to an increase of the electrical conductivity with a decrease in energy of
activation [39].
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