Preparation, spectroscopic and antibacterial studies on charge-transfer complexes of 2-hydroxypyridine with picric acid and 7,7′,8,8′-tetracyano-p-quinodimethane

Article abstract of DOI:10.1016/j.saa.2015.03.005

The reactions of electron acceptors such as picric acid (HPA) and 7,7′,8,8′-tetracyano-p-quinodimethane (TCNQ) with 2-hydroxypyridine (HPyO) have been investigated in EtOH at room temperature. Based on elemental analysis and IR spectra of the solid CT-complexes along with the photometric titration curves for the reactions, the data obtained indicate the formation of 1:1 charge transfer complexes [(H₂PyO)(PA)] and [(PyO)(HTCNQ)], respectively. The infrared and ¹H NMR spectroscopic data indicate a charge transfer interaction associated with a proton migration from the acceptor to the donor followed by intramolecular hydrogen bonding in [(H₂PyO)(PA)] complex. Another charge transfer interaction was observed in [(PyO)(HTCNQ)] complex. The formation constants (K_CT) for the CT-complexes are shown to be strongly dependent on the type and structure of the electron acceptors. Factors affecting the CT-processes and the kinetics of thermal decomposition of the complexes have been studied. The CT complexes were screened for their antibacterial activities against selected bacterial strains.

Full text of DOI:10.1016/j.saa.2015.03.005

Accepted Manuscript
Preparation, spectroscopic and antibacterial studies on charge-transfer com-
plexes of 2-hydroxypyridine with picric acid and 7,7‘,8,8‘-tetracyano-p-quino-
dimethane
Akmal S. Gaballa, Alaa S. Amin
PII:
S1386-1425(15)00301-7
http://dx.doi.org/10.1016/j.saa.2015.03.005
SAA 13419
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
To appear in:
Received Date:
Revised Date:
Accepted Date:
1 October 2014
3 December 2014
1 March 2015
Please cite this article as: A.S. Gaballa, A.S. Amin, Preparation, spectroscopic and antibacterial studies on charge-
transfer complexes of 2-hydroxypyridine with picric acid and 7,7‘,8,8‘-tetracyano-p-quinodimethane,
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.
2015.03.005
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Preparation, spectroscopic and antibacterial studies on charge-transfer
complexes of 2-hydroxypyridine with picric acid and 7,7`,8,8`-tetracyano-p-
quinodimethane
a,
b
*
Akmal S. Gaballa and Alaa S. Amin
aFaculty of Specific Education, Zagazig University, Zagazig, Egypt
bChemistry Department, Faculty of Science, Benha University, Benha, Egypt
Received….
Abstract
The reactions of electron acceptors such as picric acid (HPA) and 7,7`,8,8`-tetracyano-p-
quinodimethane (TCNQ) with 2-hydroxypyridine (HPyO) have been investigated in EtOH at
room temperature. Based on elemental analysis and IR spectra of the solid CT-complexes along
with the photometric titration curves for the reactions, the data obtained indicate the formation of
1:1 charge transfer complexes [(H2PyO)(PA)] and [(PyO)(HTCNQ)], respectively. The infrared
and 1H NMR spectroscopic data indicate a charge transfer interaction associated with a proton
migration from the acceptor to the donor followed by intramolecular hydrogen bonding in
[(H2PyO)(PA)] complex. Another charge transfer interaction was observed in [(PyO)(HTCNQ)]
complex. The formation constants (KCT) for the CT-complexes are shown to be strongly
dependent on the type and structure of the electron acceptors. Factors affecting the CT-processes
and the kinetics of thermal decomposition of the complexes have been studied. The CT
complexes were screened for their antibacterial activities against selected bacterial strains.
* For correspondence
E-mail address: akmalsg@yahoo.com; akmalsg@zu.edu.eg Tel.: 002 0100 3627093
1

Key words: Charge transfer; 2-Hydroxypyridine; Electron acceptors; UV-visible; IR; 1H NMR;
TGA spectrometry; Kinetics of thermal decomposition; Antibacterial activity.
1. Introduction
Organic charge-transfer complexes [1-3] are of great importance in many chemical reactions
including addition, substitution, condensation [4, 5], biochemical and bio-electrochemical
energy-transfer processes [6] and biological systems [7]. The charge-transfer complexes that are
formed between n- and π-donors [1-3, 8-10] exhibit certain properties which could be important
in biological systems and other various applications in electronics, solar cells, and optical
devices [11-12].
Proton transfer reactions are important in many chemical and biological systems [13-16]. It
constitutes one of the simplest models for DNA base units and for molecules involved in
biological life cycles, where proton transfer causes genetic damage [17]. For some hydrogen
bond, particular type of proton transfer occurs in systems where catalysis molecules can conduct
the process by serving as a bridge between the donor and acceptor sites [18].
The donor molecule used in this study was 2-hydroxypyridine (HPyO), an aromatic
heterocyclic compound representing a very important class of compounds which possess a
system of π- and n-electrons. Regarding to the equilibrium between 2-pyridone and 2-
hydroxypyridine, it is clear that this tautomerism is ruled by a proton transfer between nitrogen
and oxygen, I. This system in principle is able to form two types of charge-transfer complexes
[19–23]. The π-electron acceptors 7,7',8,8'-tetracyanoquinodimethane (TCNQ) and picric acid
(HPA) are known to form stable colored CT-complexes with many donor bases [1, 24-27].
2

The increased interest in the study of charge-transfer interactions stems from the various
applications of CT-complexes [1, 28]. We have been studying the synthesis and spectroscopic
characterization of a variety of molecular donors and acceptors in order to fully understand the
nature of their CT interactions [29-35]. In continuation of our previous studies made on the
charge transfer complexes of the donor 2-hydroxypyridine [35], we wish to report on the results
of preparation and spectroscopic investigation of the interaction of 2-hydroxypyridine (HPyO) as
an electron donor with picric acid (HPA) and 7,7`,8,8`-tetracyano-p-quinodimethane (TCNQ) as
electron acceptors to characterize the reaction products. This investigation contributes to a
deeper understanding of the mode of interaction, reaction stoichiometries and structures in the
different kinds of the charge-transfer complexes under investigation. Furthermore, the results
obtained here give some insight of analogous reactions with other related derivatives of n-donor
bases.
4
3
5
6
2
O
N
H
O
H
CN
NC
N
OH
O2N
NO2
H
O
H
N
O
N
NO2
NC
CN
2-Pyridone 2-hydroxypyridine (HPyO)
HPA
TCNQ
I. Structure of the donor and acceptors
2. Experimental
2.1. Materials and spectral measurements
All the used chemicals were of high grade. 2-Hydroxypyridine (HPyO) was obtained from
3

Aldrich, Picric acid (HPA) was obtained from BDH while 7,7`,8,8`-tetracyano-p-
quinodimethane (TCNQ) was purchased from Merck Chemical Co.
The electronic absorption spectra were recorded in the region of 250 900 nm using UV-
Vis. spectrophotometer model JASCO V-530 with quartz cell of 1.0 cm path length. The infrared
spectra of the reactants and the obtained complexes were recorded using KBr discs on Perkin-
Elmer 1430 ratio recording Infrared spectrometer. 1H NMR spectra were recorded on Varian
spectrophotometer Gemini 200 operating at 200 MHz using dimethylsulphoxide-d6 as a solvent
and TMS as an internal reference.
Photometric titrations at (354, 400 nm) and (743, 845 nm) were performed for the
reactions of HPA and TCNQ, respectively, with the donor (HPyO) in Ethanol at 25 oC using a
Helios Gamma Unicam UV-Vis. Spectrophotometer and Jenway Visible range
) were kept fixed at
spectrophotometer model 6300 as follows. The concentrations of HPyO (Cd
1.00 ×10−4 mol/L in the reaction with the acceptors HPA and TCNQ. Whereas the concentrations
were changed over a wide range of: 0.25 ×10−4 −3.00 ×10−4 mol/L for all
of the acceptors Ca
:
) obtained in this case varies over
acceptors, Table 1. The acceptor-donor molar ratio (Ca Cd
the range 0.25:1.00 to 3.00:1.00. The peak absorbances appeared in the spectra that assigned to
:
the formed CT-complexes were measured and plotted as a function of the ratioCa Cd
according to the known method [36], Table 2.
Elemental analyses of the dried and pure samples were carried out in microanalysis unit of Cairo
University, Egypt using CHNS-932 (LECO) and Vario EL elemental analysers. Chlorine was
determined by burning the substance in oxygen with platinum contact and following titration
with mercuric nitrate towards diphenylcarbazide. The results of elemental analyses of the solid
4

complexes were in accordance with the stoichiometric ratios obtained from photometric
titrations.
Thermal analyses (TG, DTG) were carried out using a Shimadzu TGA-50 H computerized
thermal analysis system. The system includes program which process data from the thermal
analyzer with the ChromotPac C-R3A. The rate of heating of the samples was kept at 10 oC/min.
Sample masses 2.066, 2.896 and 1.967 mg for HPyO and [(H2PyO(PA)] (1)
[(PyO)(HTCNQ)] (2) complexes, respectively were analyzed under N2 flow at 20 ml/min.
<<<< Insert tables 1, 2 >>>>>
2.2. Preparation of the solid complexes
[(H2PyO(PA)] (1)
To a solution of HPyO (95.1 mg, 1.0 mmol) in EtOH (10 mL), a solution of HPA (458.2
mg, 2.0 mmol) in EtOH (50 mL) was added at room temperature with stirring. The formed
scarlet yellow precipitate was filtered off and washed with EtOH (3 × 1/2 mL). The precipitate
was also washed with Et2O (2 × 1/2 mL) and dried in vacuo overnight over CaCl2. Yield: 295.0
mg (90.15 %).
Anal. found (Calcd. for C11H8N4O8, 324.20): C, 40.98 (40.75); H, 2.61 (2.49); N, 17.09 (17.28).
[(PyO)(HTCNQ)] (2)
To a solution of HPyO (95.1 mg, 1.0 mmol) in EtOH (10 mL), a saturated solution of
TCNQ (205 mg, 1.0 mmol) in EtOH (90 mL) was added. The resultant green solution was stirred
for about 6 h. The volume of the reaction mixture was reduced to half then, left overnight to
obtain the dark green precipitate of complex 2, which was filtered off, washed with EtOH (3 ×1/2
mL) and dried in vacuo over CaCl2. Yield: 150.0 mg (50.12 %).
Anal. found (Calcd. for C17H9N5O, 299.29): C, 68.45 (68.22); H, 3.20 (3.03); N, 23.37 (23.40).
5

Preparation of (H2PyO)Cl
To a solution of HPyO (19.0 mg, 0.20 mmol) in MeOH (10 mL), HCl was added (0.40
mmol, 0.10 M) drop by drop with continuous stirring. By evaporation of solvent, the solid 2-
hydroxybipyridine-1-ium chloride was obtained.
Anal. found (Calcd. for C5H6ClNO, 131.56): C, 45.57 (45.65); H, 4.71 (4.60); N, 10.57 (10.65);
Cl, 27.32 (26.95).
2.3. Antibacterial activity
The in vitro antibacterial screening effects of the compounds were tested against four bacterial
strains namely Escherichia coli and Pseudomonas aeroginosa (Gram negative bacteria) and
Bacillus subtillis and Staphylococcus aureus (Gram positive bacteria) by agar well diffusion
method using nutrient agar medium for antibacterial activity [37-40]. All bacteria were
inoculated into Nutrient Broth and incubated for 24 h (1.0 mL of inocula were added to 50 mL of
agar media (50 ºC) and mixed). The agar was poured into 120 mm petri dishes and allowed to
cool to room temperature. In the agar well diffusion method, the dilution plate method was used
to enumerate microorganisms for 24 h [41, 42]. By using a sterilized cork borer (7 mm
diameter), wells were dug in the culture plates. 0.1 mL of the compounds dissolved in DMSO
(250 μmol/mL) were added to these wells. The petri dishes were left at 5 ºC for 2 h and then the
plates were incubated at 35 ºC for bacteria (22–24 h). At the end of the period, inhibition zones
formed on the medium were evaluated in milimeters (mm). DMSO was used as a control under
similar conditions for comparison (0.1 mL). The results were compared with a similar run of
standard Ampicilin as antibacterial. The zones of inhibition based upon zone size around the
wells were measured and calculated as a mean of three replicates.
6

3. Results and discussion
Two stable charge-transfer complexes, [(H2PyO)(PA)] (1) and [(PyO)(HTCNQ)] (2) with
the molar ratio of 1:1 (donor−accpetor) are obtained in good yields during the reaction of HPyO
in EtOH with HPA and TCNQ, respectively. This molar ratio was determined from the
photometric titration curves and in good agreement with the analytical data obtained from
elemental analyses.
3.1. Electronic spectra
Electronic absorption spectra of 2-hydroxypyridine donor (HPyO) and its obtained CT-
complexes (Fig. 1(A, B)) revealed strong absorption bands attributed to the CT-interaction.
These bands are not present in the spectra of the free reactants and observed at (354, 410 nm)
and (743, 845 nm) for the complexes [(H2PyO)(PA)] (1) and [(PyO)(HTCNQ)] (2), respectively.
The interaction of the donor with the π-acceptor in polar solvents such as ethanol, complete
electron transfer takes place with the formation of intensively coloured radical ions. The
interaction of HyO with HPA and TCNQ at room temperature gave coloured chromogens
showing different absorption maxima at (354, 410 nm) and (743, 845 nm) for HPA and TCNQ,
respectively. The predominant chromogen with TCNQ in EtOH is the bluish-green coloured
radical ion, which exhibits strong absorption maxima at 845 and 743 nm [1, 2, 24-27].
Furthermore, photometric titration measurements based on the characteristic absorption bands of
the CT-complexes, Fig. 2(A, B) confirmed the complex formation in a ratio, HPyO: acceptor =
1:1. Examination and comparison of the absorption spectra of the HPyO−acceptors system reveal
that the spectra are characterized by maxima at the defined wavelengths which are nearly similar
to the same absorption bands of such acceptors with other donors [24-27, 34, 35].
7

The obtained spectrophotometric data, Table 2 were used to calculate the values of both
equilibrium constant, KC, and extinction coefficient, εof the CT-complexes in EtOH for
complexes 1 and 2 based on the known equation (1) for the 1:1 stoichiometry [43].
Cdꢀ ⋅ Caꢀ
Cdꢀ + Caꢀ
1
=
+
(1)
ε
A
εKc
where Caꢀ andCdꢀ are the initial concentration of the acceptor and the donor, respectively, and A
is the absorbance of the CT band. Plotting Cdꢀ .Caꢀ /A vs (Cdꢀ +Caꢀ ) for the complexes 1and 2,
straight lines were obtained supporting our conclusion of the formation of the 1:1 complexes,
Fig. 3(A,B). In these plots, the slope and intercept for each case equal 1/εand 1/εKC, respectively.
The values of both KC and εassociated with the complexes are given in Table 3. These data
reveals that [(PyO)(HTCNQ)] (2) complex shows a higher value of the formation constant (KC)
in comparison with [(H2PyO)(PA)] (1) complex. This confirms the expected high stability of
complex 2 as a result of the expected higher accepting properties of TCNQ compared with picric
acid. The values of the equilibrium constants are dependent on the nature of the acceptor
including the type of electron withdrawing substituents such as cyano and nitro groups.
<<<< Insert Figs 1, 2,3 >>>>>
3.2. Determination of the spectroscopic and physical data of CT-complexes
The oscillator strength (f) of the CT absorption spectra in ethanol at room temperature
was obtained from the approximate formula [44].
(2)
f = 4.319×10−9 (εmax ⋅ Δυ1/ 2
)
where ν is the band-width for half-intensity in cm−1 and εmax is the maximum extinction
1/2
coefficient of the CT-band. The oscillator strength values are given in Table 3. The data resulted
8

reveals several items: The HPyO−HPA and HPyO−TCNQ systems show different values of both
formation constant (Kc) and molar absorptivity (ε). The different values of the oscillator strength,
f , increase with increasing the accepting property of the acceptor. This result could also be
explained on the basis of competitive solvent interactions with the acceptors [29-35].
The transition dipole moment (μ) which determines the transitions from a donor πbonding
orbital to an acceptor π* anti-bonding orbital is calculated [45] according to equation 3 and is
given in Table 3.
εmax ⋅Δυ1/ 2
1/ 2
(3)
μ(Debye) = 0.958(
)
υmax
The ionization potential (Ip) of the HPyO donor in the charge transfer complexes 1 and 2
are calculated using empirical equation 4 derived by Aloisi and Piganatro [46];
ID(eV ) = 5.76 +1.53×10−4 ⋅υCT
(4)
where νCT is the wavenumber in cm−1 corresponding to the CT band formed from the interaction
between donor and acceptor. The electron donating power of a donor molecule is measured by its
ionization potential which is the energy required to remove an electron from the highest occupied
molecular orbital.
The energy of the charge-transfer complexes, ECT of the n→π* and π–π* interactions between
HPyO and acceptors were calculated using equation 5 [45];
(5)
ECT = (hυCT ) =1243.667/λCT (nm)
where, λCT is the wavelength of the complexation band of the studied complexes.
The standard free energy changes of complexation (ΔGo) were calculated from the
formation constants by the following equation 6 [47];
ΔGꢀ = −2.303RT log KCT
(6)
9

where R is the gas constant (8.314 J K−1 mol−1), T is the temperature in Kelvin degrees and KC
is the formation constant of the complexes at room temperature (L mol-1). ΔGo of interactions are
negative in all CT complexes which indicate endothermic processes and reactions are
spontaneously proceeding to the direction of CT complex formations, Table 3.
<<<< Insert table 3 >>>>>
3.3. IR Analysis
Assignments of the well characterized bands in the infrared spectra of reactants and the
obtained products are given in Table 4. The formation of CT-complexes during the reaction of
HPyO with picric acid and TCNQ is strongly supported by observing of the main infrared bands
of the donor (HPyO) and acceptors (HPA and TCNQ) in the product spectra. However, the bands
of the donor and acceptors in the complexes spectra reveal small shifts in intensities compared
with those of the free donor and acceptors. This should be attributed to the expected symmetry
and electronic structure changes upon the formation of CT-complexes.
In general for acid-base interaction, a proton transfer from the acceptor (picric acid) to
the donor (base) is expected to occur. Such assumption is strongly supported by the appearance
of a new band of medium intensity in the spectrum of complex 1. This band is observed at 3100
and 3047 cm−1 for the HPyO−HPA CT-complex, and should be due to the ν(N−H) stretching
vibration of hydrogen against positively charged nitrogen [48-52]. The complex spectrum
revealed also a broad absorption lying in the region of 2365cm−1 and could be attributed to the
expected hydrogen bonding in the complex [48]. Depending on these observations, we may
suggest that, the acid−base interaction is associated with a proton migration followed by
hydrogen bonding formation. The HPyO−HPA interaction involves nitrogen protonation.
10

Accordingly, we may formulate complex 1 as [(HPyOH+)(PA−)]. The dipole caused by the
semipolar N→O bond makes the band position, especially the antisymmetric stretching vibration
ν (NO2) sensitive for polar influences and the electronic states of the nucleus. Therefore, one
as
could attribute the shift to a lower wavenumber of ν (NO2) vibration (1535 cm−1) in the
as
[(H2PyO)(PA)] (1) complex spectrum compared with its value in the free HPA (1607 cm−1) due
to the increased electronic density on the picrate unit as a result of charge transfer interaction and
deprotonation of picric acid upon complexation [49].
The IR spectrum of the free TCNQ shows CN stretching frequency at 2222 cm−1. The
significant shift of this vibration toward a different frequency (2252 cm−1) on complexation is
*
indicative of charge transfer from HPyO to π of TCNQ and more than one type of CN group
might be found. Because TCNQ lacks acidic centers, the molecular complexes can be concluded
to form through π→π* and/or n→π* charge migration from the HOMO of the donor HPyO to the
LUMO of the acceptor TCNQ. The π→π* CT complex is formed via the benzene ring (electron-rich
group) of the HPyO and the electron acceptor (TCNQ) reagents [53, 54]. The cyano group (–
C≡N) is an electron-withdrawing group that exists in TCNQ in a conjugated bonding system. The
CN groups in TCNQ withdraw electrons from the aromatic ring, and such a process will make
the aromatic ring an electron-accepting region. The π* electron density in CN appears to increase
and more easily accept a proton from the donor because of the electron-withdrawing process and
the conjugated electron system. So, the interactions mode between HPyO and the TCNQ
acceptor occurs through the migration of a H+ ion to one of the cyano groups in the acceptor to
form a positive ion (–C≡N– H+) that associates with the anion to form ion pairs [55, 56].
11

Hydrogen bond formation was confirmed with observation of medium broad bands
around 3400 cm−1 in the vibrational region of ν(O−H) and medium to week broad bands around
2366 cm−1 in the vibrational region of ν(N−H).
<<<< Insert table 4 >>>>>
3.4. 1H NMR spectra
1
The H NMR spectra of the reactants and the formed CT-complexes in dmso-d6 were
obtained and the chemical shifts in ppm for all peaks observed in the spectra are given in Table
5. The aromatic protons in the free 2-hydroxypyridine, picric acid and TCNQ were assigned. The
1H NMR spectra of the complexes reveal several observations. All the observed peaks in the
spectra of the individual components are also present in the complexes spectra suggesting their
formation. The proton signals of the donor HPyO are downfield shifted to higher ppm values
indicating a charge migration from the donor towards the acceptors.
In the 1H NMR spectrum of complex 1, the signal due to the phenolic proton in the free
acceptor (HPA) was disappeared in the complex spectrum indicating deprotonation. Instead a
new peak is observed in the complex spectrum around 5.00 ppm and assigned to N+−H proton
indicating protonation of the donor HPyO. This interaction mode was strongly supported by
1
obtaining a similar H NMR spectrum for the protonated 2-hydroxypyridinium unit in the
compound (H2PyO)Cl [30, 57, 58].
<<<< Insert table 5 >>>>>
The 1H NMR spectrum for complex 2 that formed between HPyO and TCNQ is
summarized in Table 5. The spectrum indicated a charge transfer interaction between HPyO and
12

TCNQ, the proton of N1−H/ O2−H in HPyO is transferred to nitrogen atom of (C≡N) in TCNQ to
form an ion-pair compound, namely (2-cyano-2-(4- (dicyanomethylene) cyclohexa-2,5-
dienylidene) ethylidyne) 2-oxopyridinium.
This mode of interaction was confirmed by the absence of the singlet signal of N1−H/O2−H
proton in the free HPyO and appearance of new singlet signal at 6.78, due to N+−H proton in the
complex spectrum. This new signal was not observed in the spectrum of the free TCNQ, which
indicated that the proton of N1−H/O2−H in the free HPyO and C≡N group primarily involved in
the formation of CT complex between HPyO and TCNQ. Transfer of hydrogen proton from
HPyO to one of the four C≡N groups in TCNQ resulted in formation of positive ion (C≡N+−H)
which associated with the anion (PyO−).
Finally, we could suggest that the interaction of 2-hydroxypyridine as a donor with
picric acid and 7,7`,8,8`-tetracyano-p-quinodimethane (TCNQ) proceeds in a molar ratio of
1: 1 according to the following equations:
OH
O2N
O
H
O2N
NO2
O
N
H
O
N
H
NO2
O2N
NO2
CN
CN
NC
NC
O
O
N
H
N
CN
HNC
CN
NC
13

3.5. Thermal analysis
To make sure about the proposed structures for the complexes under investigation,
thermogravimetric analyses TGA and DTG (Fig. 4) are measured under nitrogen flow. The
thermal data obtained for complexes 1 and 2 together with the donor HPyO are summarized and
given in Table 6.
The decomposition reaction of complex 1 occurs in one unexpecting stage with a weight
loss value of 99.99 % at a temperature of approximately 237 oC. This stage of decomposition
might be associated with the total loss of complex molecule in good agreement with the
calculated value of 100.00 %.
The obtained data for complex 2 indicate that, the decomposition reactions occur in two
stages. The donor molecule, PyO− is lost at a temperature maximum of 137 oC. The calculated
weight loss of PyO− unit in this complex corresponds to 31.44 % in agreement with the obtained
value of 30.50 %. The second stage of decomposition proceeds at a maximum temperature of
approximately 265 oC and associated with a weight loss value of 68.50 %. This could be
attributed to the loss of the protonated acceptor unit, TCNQH+ in agreement with the calculated
values of 68.56 %.
<<<< Insert Fig. 4, table 6 >>>>>
3.6. Thermodynamics parameters of the thermal decomposition of the complexes
It is well known that the temperature has a great influence on any chemical process. So
that, it may enhance or retard such dependence on the nature of the reactants and/or the products.
The thermodynamic parameters, namely; the equilibrium constants (Kc), the enthalpy (ΔH), the
Gibbs free energy (ΔG) and the entropy (ΔS) were calculated using the following equations [59-
63]
14

ΔG = −RT lnKc
(7)
(8)
(9)
log Kc = ΔS/2.203R−ΔH/2.303RT
ΔG = ΔH −TΔS
where R is an ideal gas constant (8.314 J K−1 mol−1) and T is the absolute temperature (in K).
Plotting log Kc against 1/T gives straight lines, (Fig. 5) with slope and intercept equal
−ΔH/2.303R and ΔS/2.303R were calculated. The thermodynamic parameters are reported in
Table 7.
The calculated data indicates the high values of entropy (ΔS) and enthalpy (ΔH) and low
values of the Gibbs free energy change (ΔG). This may be attributed to the ease of decomposition
of complexes 1 and 2 in comparison with the free donor. The positive values of ΔH confirm the
endothermic nature of compounds and the decomposition of chemical bonds. The positive values
of ΔS indicate a less orderly structured.
<<< Insert Table 7 >>>
<<< Insert Fig. 5 >>>
3.7. Antibacterial activity
The in vitro antibacterial activity of the donor and its CT complexes were tested against
four bacterial strains, Bacillus subtillis and Staphylococcus aureus as gram positive bacteria and
Escherichia coli and Pseudomonas aeroginosa as gram negative bacteria by agar well diffusion
method, Table 8.
The activity was determined by measuring the inhibition zone diameter values (mm) of the
complexes against the early mentioned microorganisms. Ampicilin was used as a positive control.
The screening data are reported in Table 8. The results indicated that the HPyO complexes showed
varying degrees of inhabitation against the tested bacterial species. Regarding the inhabitation zone
15

diameter, complex 1 had the highest antibacterial activity against the growth of the tested bacterial
species compared to complex 2. It gained approximately 69% of activity of tested antibiotic
(Ampicilin). Complex 1 exhibited moderate inhibitory results against the tested Gram-positive and
low inhibitory results against Gram-negative bacterial species. The marked activity of complex 1
might be due to the outer membrane of the tested bacterial species is more permeable for complex 1
than complex 2 [64-67].
<<<< Insert table 8 >>>>>
4. Conclusion
The interactions between the electron donor HPyO with the acceptors picric acid (HPA)
and 7,7',8,8'-tetracyanoquinodimethane (TCNQ) were studied spectrophotometrically in ethanol.
New charge-transfer complexes were isolated and characterized using microanalyses, electronic,
IR, and 1H NMR, spectra as well as thermal and kinetic thermodynamic studies. The
stoichiometry of the products was found to be 1:1 in both cases. Accordingly, the formed CT-
complexes have the formulas [(H2PyO)(PA)] (1) and [(PyO)(HTCNQ)] (2). The antibacterial
activity test results showed that complex 1 have antibacterial activity against some bacterial
strains.
Acknowledgement
The author wishes to acknowledge Dr W.F. Hessen, Microbiology department for antimicrobial
activity measurements of this research and Dr A.A. Hamouda, Hydrogeo-chemistry Department,
Desert Research Center, Egypt for Thermodynamics parameters calculations.
References
[1] F. Gutmann, C. Johnson, H. Keyzer, J. Molnár “Charge-Transfer Complexes in
16

Biochemical Systems” Marcel Dekker Inc., 1997.
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21

Table 1.
Molar concentrations for HPyO with the acceptors in the reaction mixtures
Base conc., ×10−4 M
Acceptor
Concentration range of
the acceptors, M
Acceptor-base molar ratio
0.25 ×10−4 – 3 ×10−4
1.00
1.00
HPA
TCNQ
0.25: 1.00 – 3.00:1.00
0.25: 1.00 – 3.00:1.00
0.25 ×10−4 – 3 ×10−4
Table 2.
The Values of Cdꢀ .Caꢀ /A and Cdꢀ +Caꢀ for complexes 1 and 2
Cdꢀ . Caꢀ /A,
Va, ml
Ratio
(a/d)
A
Caꢀ
,
Cdꢀ + Caꢀ
Cdꢀ . Caꢀ
,
(5.00 ×10-
4 M)*
×10-8
×10-4
×10-8
,
×10-4
1,
354 nm
1,
2,
2,
1,
354 nm
1,
2,
2,
400 nm 845 nm 743 nm
400 nm 845 nm 743 nm
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.50
3.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.50
3.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.50
3.00
0.601
0.788
0.845
0.861
0.873
0.862
0.861
0.860
0.855
0.850
0.550 0.198 0.065 1.25
0.954 0.381 0.125 1.50
1.432 0.525 0.190 1.75
1.525 0.682 0.250 2.00
1.535 0.782 0.284 2.25
1.539 0.903 0.317 2.50
1.538 1.025 0.349 2.75
1.539 1.125 0.380 3.00
1.541 1.364 0.454 3.50
1.544 1.582 0.510 4.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.50
3.00
0.42
0.63
0.89
1.16
1.43
1.74
2.03
2.33
2.92
3.53
0.45
0.52
0.52
0.66
0.81
0.97
1.14
1.30
1.62
1.94
1.26
1.31
1.43
1.47
1.60
1.66
1.71
1.78
1.83
1.90
3.85
4.00
3.95
4.00
5.04
4.73
5.01
5.26
5.51
5.88
*Vd, is 1 ml (5.00 ×10-4 M); Cdꢀ is 1.00 ×10-4 M in all systems.
22

Table 3.
Spectrophotometric results for HPyO CT-complexes in EtOH
complex
KC
ECT
(eV)
f
µ
Ip
(ev)
ΔGo (25oC)
(K⋅J⋅mol−1)
λ
ε
max
max
(l⋅mol−1)
(l⋅mol−1⋅cm−1)
(nm)
-1.04 ×104
-1.43 ×104
0.87 ×104
1.76 ×104
-2.29 ×104
-2.37 ×104
[(H2PyO)(PA)]
354
400
3.51
3.11
17.28
15.20
36.22
35.95
10.04
15.32
0.29×104
1.26×104
-1.98×104
[(PyO)(HTCNQ)] 743
845
1.67
1.47
11.37
40.55
42.18
85.45
7.75
7.57
0.25×104
4.10×104
-1.94×104
23

Table 4.
Characteristic infrared frequencies* (cm−1) and tentative assignments for complexes 1 and 2
HPA
TCNQ
HPyO
1
2
Assignments
3433 sh, br
3428 wbr
3436, br, m
3399 br, m
ν(O−H); H bonded
3101 s
3137 w
3050 s
2969 w
2928 w
2851 w
3119, m
3072, m
2921,m
2815, m
3100, sh
3047, m
2960, w
3100, m
3069 m, br
2985 m
2872 m
2467, w
2366, w
ν(N−H)
2976 sh
2873 w
ν(C−H)
s
ν (C−H)
as
2366, w
2365, m
Hydrogen bonding
ν(NO2)
1851 w
2222 vs
1994 w
ν(C≡N); TCNQ
2252 w
1850 w
1682, s
ν(C=O)
1641, sh
1531, m
1459, m
1671 w
1542 vs
1646 vs
1573 vs
ν(C=N)
ν(C=C)
ring breathing bands
1659, s
1640 vs
1607 vs
1559 sh
1429 s
1535 s
1467 w
νas(NO2), PA
C−H deformation
νs(NO2); PA
1347 vs
1316 m
1267 s
1183 w
1147 s
1083 s
1426 w
1342 m,br
1352 m
1285 vw
1205 vw
1132 w
1425, m
1364, sh
1250, w
1144, w
1101, w
1030, vw
1422 s
1237 s
C−H deformation
ν(C−C)
ν(C−N)
1115 m
ν(C−O)
1045 vw
1150 m
1092 m
942 w
915 s
826 w
997 vw
960 vw
986, m
919, m
997 m
861 s
979 s
920 m
C−H bend
δ(ONO); PA
783 s
861 vs
808 w
622 vw
866, m
769, s
720, m
609, m
575, vw
775 w
688 w
776 s
733 vs
698 vs
646 sh
724 m
610 w
552 m
519 m
Skeletal vibration,
ν(C−O−C)
510, m
550 m
530 m
448 vw
CNC def.
CH out of plan bend,
skeletal vibration
(ONO); PA
496 w
474 s
432, w
479 m
470 m
*: s, strong; w, weak; m, medium; sh, shoulder; v, very; br, broad.
24

Table 5.
1H NMR δvalues (ppm) of reactants and CT-complexes in dmso-d6
N+−H
Complex
H3
H4
H5
H6
H2,3/H5,6
(TCNQ)
H3/H5 OH
(HPA)
N1−H
O2−H
8.02, s
HPyO
6.48, dd
6.99, m
5.79, m
7.29,d
HPA
8.57, s
11.94*
TCNQ
9.06, s
8.99, s
(H2PyO)Cl
[(H2PyO)(PA)]
6.81, dd
6.84, dd
7.30, m
7.32, m
5.96, m
5.98, m
7.61, d 9.00, br
7.65, d 9.11, br
5.00, br
5.00, br 8.55, s
[(PyO)(HTCNQ)] 6.62, dd
* Observed in CDCl3.
7.25, m
5.86, m
7.45, d Not
observed
6.78, s
25

Table 6.
The maximum temperature values for the decomposition along with the species lost in each step
of the decomposition reactions of complexes 1 and 2
Complex
HPyO
Decomposition
Tmax/ oC
Lost species % Weight loss
Found
Calc.
First step
(Total loss)
Residue
207
C5H5NO
99.86
100.00
--
00.14
99.99
00.00
First step
(Total loss)
Residue
237
C11H8N4O8
100.00
[(H2PyO)(PA)] (1)
00.01
30.50
00.00
31.44
First step
137
265
C5H4NO
C12H5N4
C17H9N5O
[(PyO)(HTCNQ)] (2)
Second step
68.50
68.56
Total loss
Residue
99.00
01.00
100.00
00.00
26

Table 7.
Thermodynamics parameters of the decomposition of HPyO and its complexes (1, 2)
Linear
regression
coefficient (r)
Compound
HPyO
T (K)
∆G°
(kJ/mol)
∆H°
(kJ/mol)
∆S°
(kJ/mol)
423
473
493
-28.4204
-18.7405
-13.3609
270.3194
172.9944
462.4725
269.3696
99
99
425
473
510
-24.902
-21.0742
-14.2433
1
2
423
473
523
-29.8824
-25.9452
-19.1517
172.2668
231.8773
99
Table 8.
Antibacterial activity of HPyO and its complexes
Antibacterial activity (inhibition zone diameter in mm)
Bacteria
Bacillus
Esherichia. Pseudomonas
Compounds
Staphylococcus
subtillis,
aureus, G+
Coli, G−
aeroginosa, G−
G+
HPyO
9
6
8
7
[(H2PyO)(PA)] (1)
[(PyO)(HTCNQ)] (2)
DMSO
17.0
12.0
00.0
25.0
18.0
07.0
00.0
26.0
14.0
13.0
00.0
25.0
13.0
11.0
00.0
24.0
Ampicilin
27

Fig.1A. Electronic absorption spectra of HPyO−HPA reaction (a: [HPyO] = 1.0 ×10−4 M, b: [HPA]
= 1.0 ×10−4 M and c: HPyO−HPA product = 1.0 ×10−4 M) in EtOH.
Fig.1B. Electronic absorption spectra of HPyO −TCNQ reaction (a: [HPyO] = 1.0 ×10−4 M, b:
[TCNQ] = 1.0 ×10−4 M and c: HPyO−TCNQ product = 1.0 ×10−4 M) in EtOH.
28

Fig. 2A. Photometric titration curves of HPyO−HPA reaction in EtOH at 354 and 400 nm.
Fig. 2B. Photometric titration curves of HPyO−TCNQ reaction in EtOH at 743 and 845 nm.
29