Preparation and spectroscopic studies on charge-transfer complexes of 2-hydroxypyridine with electron acceptors

Article abstract of DOI:10.1016/j.molstruc.2013.03.047

The CT-interactions of electron acceptors such as iodine (I₂), chloranilic acid (H₂CA) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) with 2-hydroxypyridine (HPyO) have been investigated in the defined solvent. The data indicate t

Full text of DOI:10.1016/j.molstruc.2013.03.047

Journal of Molecular Structure 1043 (2013) 91–102
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Preparation and spectroscopic studies on charge-transfer complexes
of 2-hydroxypyridine with electron acceptors
⇑
Akmal S. Gaballa
Faculty of Specific Education, Zagazig University, Zagazig, Egypt
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
Spectroscopic investigations on the interaction of 2-hydroxypyridine as an electron donor with
- and -acceptors used in this study are iodine, chloranilic acid and 2,3-dichloro-5,6-dicyano-p-benzoquinone.
The prepared CT-complexes have the general formula [(HPyO)(acceptor)].
Formation constant, charge transfer energy and thermal analyses data of CT-complexes are presented.
r- and p-acceptors.
r
p
a r t i c l e i n f o
a b s t r a c t
Article history:
The CT-interactions of electron acceptors such as iodine (I2), chloranilic acid (H2CA) and 2,3-dichloro-5,6-
dicyano-p-benzoquinone (DDQ) with 2-hydroxypyridine (HPyO) have been investigated in the defined
solvent. The data indicate the formation of CT-complexes with the general formula [(HPyO)(acceptor)].
The 1:1 stoichiometry of the (HPyO)–acceptors were based on elemental analysis, IR spectra and thermo-
gravimetric analysis of the solid CT-complexes along with the photometric titration measurements for
the reactions. 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 are discussed.
Ó 2013 Elsevier B.V. All rights reserved.
Received 11 February 2013
Received in revised form 24 March 2013
Accepted 25 March 2013
Available online 12 April 2013
Keywords:
Charge transfer
2-Hydroxypyridine
Electron acceptors
IR
UV-visible
Thermograms
1. Introduction
the study of its CT interactions can help elucidate many chemical
and biological phenomena they take part in. It is well known to
Many charge-transfer complexes were formed between
p
- and
form hydrogen bonded structures somewhat related to the base-
pairing mechanism found in RNA and DNA. It is also a classic case
of a molecule that exists as tautomers. The determination of which
of the two tautomeric forms is present in solution has been the
subject of many publications. The energy difference appears to
be very small and is dependent on the polarity of the solvent.
Non-polar solvents favor the formation of 2-hydroxypyridine
whereas polar solvents such as alcohols and water favor the forma-
tion of 2-pyridone [16–19]. By considering the equilibrium be-
tween 2-pyridone and 2-hydroxypyridine, it is clear that this
tautomerism is ruled by a proton transfer between nitrogen and
oxygen, Fig. 1 [20–24].
We have been studying the synthesis and spectroscopic charac-
terization of a variety of molecular donors and acceptors in order
to fully understand the nature of their CT interactions [8,9,25–
28]. In connection with such studies herein, we have prepared
and spectroscopically investigated the interaction of 2-hydroxy-
pyridine (HPyO) as an electron donor with iodine (I2), chloranilic
n-donor [1–10]. Charge-transfer complexes exhibit certain proper-
ties, which could be important in biological systems and other var-
ious applications in electronics, solar cells, optical devices and
others [11–13]. The mechanism of photosynthesis and oxidative
phosphorylation are the object of much intense study [1–7]. CT-
reaction is utilized for the assay of different pharmaceuticals
[14,15].
2-Hydroxypyridine is an aromatic heterocyclic compound rep-
resenting a very important class of compounds which possess a
system of
p- and n-electrons. This system in principle is able to
form two types of charge-transfer complexes. Furthermore, CT
complexes of 2-hydroxypyridine are particularly interesting be-
cause they provide an opportunity to examine the degree of inter-
action between p- and n-donor sites with different acceptors and
⇑
Tel.: +20 100 3627093.
E-mail addresses: akmalsg@yahoo.com, akmalsg@zu.edu.eg
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.03.047

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A.S. Gaballa / Journal of Molecular Structure 1043 (2013) 91–102
with mercuric nitrate towards diphenylcarbazide. The results of
elemental analyses of the solid 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 in-
cludes program which process data from the thermal analyzer with
the ChromotPac C-R3A. The rate of heating of the samples was kept
at 10 °C/min. Sample masses 2.066, 2.811 and 1.456 mg for HPyO
and complexes 2 and 3, respectively were analyzed under N2 flow
at 20 ml/min.
O
N
H
O
H
N
H
O
H
N
O
N
2.2. Preparation of the solid complexes
2-Pyridone
2-hydroxypyridine (HPyO)
2.2.1. [(HPyO)I]ꢃI (1),
To a solution of HPyO (47.6 mg, 0.50 mmol) in EtOH (10 mL), a
solution of the acceptor (280.0 mg, 1.10 mmol I2) in EtOH (50 mL)
was added and stirred for 3 h then left overnight. The dark brown
precipitate formed was filtered off, washed with the least amount
of EtOH (2 ꢁ 1/2 mL). The precipitate was also washed with Et2O
(3 ꢁ 1 mL) and dried in vacuo overnight over CaCl2. Yield:
140.0 mg (80.16%).
(dimers and hydrogen bonds formations)
O
O
Cl
HO
Cl
Cl
CN
CN
Anal. found (Calcd. for C5H5I2NO, 348.91): C, 16.98 (17.21); H,
1.51 (1.44); N, 4.11 (4.01).
OH
Cl
O
O
2.2.2. [(H2PyO)(HCA)] (2)
To a solution of HPyO (95.2 mg 1.0 mmol) in EtOH (20 mL), a
solution of H2CA (211.0 mg, 1.01 mmol) in EtOH (30 mL) was
added at room temperature. The formed violet precipitate was fil-
tered off, washed with the same solvent (3 ꢁ 1/2 mL) and dried in
vacuo overnight over CaCl2. Yield: 240.0 mg (78.84%).
Anal. found (Calcd. for C11H7Cl2NO5, 304.08): C, 43.67 (43.45);
H, 2.44 (2.32); N, 4.69 (4.61); Cl, 22.99 (23.32).
H2CA
DDQ
Fig. 1. Structure of the studied donor and acceptors.
acid (H2CA) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ)
as electron acceptors to characterize the reaction products.
2.2.3. [(HPyO)(DDQ)] (3)
2. Experimental
To a solution of HPyO (95.1 mg, 1.0 mmol) in EtOH (10 mL), a
solution of DDQ (228.0 mg, 1.0 mmol) in EtOH (40 mL) was added
at room temperature. The formed dark reddish-brown precipitate
was filtered off, washed with EtOH (3 ꢁ 1/2 mL) and air dried.
Yield: 230.0 mg (71.41%).
2.1. Materials and spectral measurements
All chemicals used were of high grade. 2-Hydroxypyridine
(HPyO) was obtained from Aldrich, 2,3-dichloro-5,6-dicyano-p-
benzoquinone (DDQ) was obtained from BDH while chloranilic
acid (H2CA) and Iodine were purchased from Merck Chemical Co.
and were used as received.
Anal. found (Calcd. for C13H5Cl2N3O3, 322.10): C, 48.52 (48.47);
H, 1.61 (1.56); N, 13.23 (13.05); Cl, 22.52 (22.01).
3. Results and discussion
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
disks on Perkin–Elmer 1430 ratio recording infrared spectrometer.
Photometric titrations at (294, 360), 520 and 396 nm were per-
formed for the reactions of I2, H2CA and DDQ, respectively, with the
donor (HPyO) in the defined solvent at 25 °C using a Helios Gamma
Unicam UV–Vis. Spectrophotometer and Jenway Visible range
spectrophotometer model 6300 as follows. The concentrations of
HPyO (Cd) were kept fixed at 1.00 ꢁ 10ꢂ4 mol/L in the reaction with
the acceptors I2, H2CA and DDQ, respectively, whereas the concen-
trations of the acceptors Ca were changed over a wide range of:
0.25 ꢁ 10ꢂ4–3.00 ꢁ 10ꢂ4 mol/L for all acceptors. The acceptor–do-
nor molar ratio (Ca:Cd) obtained in this case varies over the range
0.25:1.00–3.00:1.00. The peak absorbances appeared in the spectra
that assigned to the formed CT-complexes were measured and
plotted as a function of the ratio Ca:Cd according to the known
method [29].
Three stable charge-transfer complexes, [(HPyO)I]ꢃI (1), [(H2-
PyO)(HCA)] (2) and [(HPyO)(DDQ)] (3) with the molar ratio of
1:1 (donor–acceptor) are obtained in good yields during the reac-
tion of HPyO in EtOH with I2, H2CA and DDQ, respectively.
3.1. Electronic spectra
Figs. 2A–2D show the electronic absorption spectra of the donor
HPyO and of the corresponding formed CT-complexes. The spectra
revealed new absorption bands attributed to the CT-interactions.
These bands are not present in the spectra of the free reactants
and are observed at (294, 360), 520 and 396 nm for the complexes
[(HPyO)I]ꢃI, [(H2PyO)(HCA)] and [(HPyO)(DDQ)], respectively.
These absorptions are associated with the strong change in color
observed upon mixing of reactants and reflect the electronic tran-
sitions in the formed CT-complexes. Furthermore, photometric
titration measurements based on these characteristic CT-absorp-
tion bands of the CT-complexes, Figs. 3A, 3B, 3C confirmed the
complex formation in a ratio, HPyO: acceptor of 1:1 in all cases.
This is in good agreement with the obtained elemental analysis
of the solid CT-complexes. Examination and comparison of the
Elemental analyses were carried out in microanalysis unit of
Cairo University, Egypt using CHNS-932 (LECO) and Vario EL ele-
mental analyzers. Chlorine was determined by burning the sub-
stance in oxygen with platinum contact and following titration

A.S. Gaballa / Journal of Molecular Structure 1043 (2013) 91–102
93
Fig. 2A. Electronic absorption spectrum of HPyO in EtOH, ([HPyO] = 1.0 ꢁ 10ꢂ4 M).
Fig. 2B. Electronic absorption spectrum of HPyO–I2 reaction in EtOH ([HPyO] = 1.0 ꢁ 10ꢂ4 M and [I2] = 1.0 ꢁ 10ꢂ4 M).
absorption spectra of the HPyO–acceptor systems reveal that the
spectra are characterized by a maxima at the wavelengths (294,
360), 520 and 396 nm which are nearly similar to the same absorp-
tion bands of such acceptors with other donors [30–32].

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A.S. Gaballa / Journal of Molecular Structure 1043 (2013) 91–102
Fig. 2C. Electronic absorption spectrum of HPyO–H2CA reaction in EtOH ([HPyO] = 1.0 ꢁ 10ꢂ4 M and [H2CA] = 1.0 ꢁ 10ꢂ4 M).
Fig. 2D. Electronic absorption spectrum of HPyO–DDQ reaction in EtOH ([HPyO] = 1.0 ꢁ 10ꢂ4 M and [DDQ] = 1.0 ꢁ 10ꢂ4 M).

A.S. Gaballa / Journal of Molecular Structure 1043 (2013) 91–102
95
Fig. 3A. Photometric titration curves of HPyO–I2 reaction in EtOH at 294 and 360 nm.
As previously indicated the absorption spectrum of the iodine
complex, Fig. 2B, shows new strong absorption bands at 294 and
360 nm. Neither the free reactants HPyO nor iodine show these
absorptions. These two absorptions of iodine complex are well
known [4–7,28] to be characteristic of the iodide ion, Iꢂ. Based
on the reaction stoichiometry of 1:1 (HPyO:I2) as well as on the
elemental analysis data, the formed CT-complex of HPyO-iodide
is identified as [(HPyO)I]+ꢃIꢂ. The formation of [(HPyO)I]+ꢃIꢂ could
be understood as follows:
obtained with the same molar ratio [30b] which means that the
donor has no effect on the reaction stoichiometries.
3.2. Formation constant and charge transfer energy of CT-complexes
The obtained spectrophotometric data, Table 1 were used to
calculate the values of both equilibrium constants, KC, and extinc-
tion coefficient,
e of the CT-complexes in EtOH for [(HPyO)I] I (1),
[(H2PyO)(HCA)] (2) and [(HPyO)(DDQ)] (3) for the 1:1 stoichiome-
try using following equation [33,34]:
(i) Formation of the outer complex, (HPyO) + I2 ? [(HPyO)] I2
(ii) This is followed by the formation of inner complex, [(HPyO)]
I2 ? [(HPyO)I]+ Iꢂ.
Cꢄd ꢃ Cꢄa
1
Cꢄd þ Cꢄa
¼
þ
ð1Þ
A
eKc
e
Photometric titration for HPyO–DDQ system based on the
absorption at 396 nm shows a reaction stoichiometry of 1:1
(HPyO:DDQ), indicate that this absorption belongs to the charge
transfer and the complex could be formulated as [(HPyO)(DDQ)].
Similar complexes with 2-aminopridine and 3-aminopyridine were
where Cꢄa and Cꢄd are the initial concentration of the acceptor and the
donor, respectively, and A is the absorbance of the CT band. Plotting
Cꢄd ꢃ Cꢄa/A vs (Cdꢄ + Cꢄa) for the complexes 1, 2 and 3, straight lines were
obtained supporting our conclusion of the formation of the 1:1
complexes, Figs. 4A–4C. In these plots, the slope and intercept for
Fig. 3B. Photometric titration curves of HPyO–H2CA reaction in EtOH at 520 nm.

96
A.S. Gaballa / Journal of Molecular Structure 1043 (2013) 91–102
Fig. 3C. Photometric titration curves of HPyO–DDQ reaction in EtOH at 396 nm.
each case equal 1/e and 1/e KC, respectively. The values of both KC
creases with increasing the accepting property of the acceptor. This
result could also be explained on the basis of competitive solvent
interactions with the acceptors [36].
and
e associated with the complexes are given in Table 2.
The high values of (KC) reflects the high stability of the com-
plexes as a result of the expected high donation of the HPyO which
contains one hetero nitrogen atom and one hydroxyl group. The
data also reveals that [(HPyO)(DDQ)] complex shows a higher va-
lue of the formation constant (KC) in comparison with [(HPyO)I] I
and [(H2PyO)(HCA)] complexes. The values of the equilibrium con-
stants are dependent on the nature of the acceptor including the
type of electron withdrawing substituents such as cyano and halo
groups.
The transition dipole moment (
tions from a donor bonding orbital to an acceptor antibonding
orbital is calculated using Eq. (3) [37–39]. A values were found
in the following order for CT complexes, 1 > 3 > 2.
l) which determines the transi-
l
ꢀ
ꢁ
1=2
emax
ꢃ
D
t1=2
lðDebyeÞ ¼ 0:958
ð3Þ
tmax
The ionization potential (Ip) of the HPyO donor in the charge
transfer complexes 1, 2 and 3 are calculated using Eq. (4) derived
by Aloisi and Pignataro [40–42];
The oscillator strength (f) was obtained from the following
equation [35]:
IDðeVÞ ¼ 5:76 þ 1:53 ꢁ 10ꢂ4
where m
CT is the wavenumber in cmꢂ1 corresponding to the CT band
ꢃ
tCT
ð4Þ
f ¼ 4:319 ꢁ 10ꢂ9
ð
emax
ꢃ
D
t1=2
Þ
ð2Þ
where
t
1/2 is the band-width for half-intensity in cmꢂ1 and emax
is the maximum extinction coefficient of the CT-band. The oscilla-
tor strength values are given in Table 2. The data resulted reveals
several items: The HPyO–I2, HPyO–H2CA and HPyO–DDQ systems
show different values of both formation constant (KC) and molar
formed from the interaction between donor and acceptor. The elec-
tron donating power of a donor molecule is measured by its ioniza-
tion potential which is the energy required to remove an electron
from the highest occupied molecular orbital. Ip values were found
in the following order for CT complexes, 1 > 3 > 2.
absorptivity (e). The different values of the oscillator strength, f, in-
Table 1
The values of Cꢄd ꢃ Caꢄ /A and Cdꢄ + Cꢄa for [(HPyO)(acceptors)] complexes.
Caꢄ
Cꢄd + Cꢄa
Cꢄd ꢃ Cꢄa; (ꢁ
Cꢄd ꢃ Cꢄa/A (ꢁ 10ꢂ8
)
Va, ml
Ratio (a/
d)
A
(5.00 ꢁ 10ꢂ4 M)a
(ꢁ 10ꢂ4
)
(ꢁ 10ꢂ4
)
10ꢂ8
)
1,
1,
2,
3,
1,
1,
2,
3,
294 nm
360 nm
520 nm
390 nm
294 nm
360 nm
529 nm
390 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
0.451
0.902
1.365
1.811
1.935
2.064
2.193
2.325
2.601
2.835
0.122
0.245
0.356
0.475
0.490
0.511
0.532
0.556
0.622
0.655
0.019
0.035
0.052
0.071
0.078
0.088
0.096
0.101
0.124
0.140
0.105
0.233
0.314
0.399
0.434
0.451
0.472
0.493
0.482
0.585
1.25
1.50
1.75
2.00
2.25
2.50
2.75
3.00
3.50
4.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.50
3.00
0.554
0.554
0.549
0.552
0.646
0.727
0.798
0.860
0.961
1.058
2.049
2.041
2.107
2.105
2.551
2.935
3.289
3.597
4.019
4.580
13.16
14.29
14.42
14.08
16.03
17.05
18.23
19.80
20.16
21.43
2.38
2.15
2.39
2.51
2.88
3.66
3.71
4.06
5.19
5.13
Vd, is 1 ml (5.00 ꢁ 10ꢂ4 M); Cꢄd is 1.00 ꢁ 10ꢂ4 M in all systems.
a

A.S. Gaballa / Journal of Molecular Structure 1043 (2013) 91–102
97
Fig. 4A. Relation between Cꢄd ꢃ Caꢄ /A and Cdꢄ + Caꢄ for HPyO–I2 system in EtOH at 294 and 360 nm.
Fig. 4B. Relation between Cdꢄ ꢃ Caꢄ /A and Cdꢄ + Caꢄ for HPyO–H2CA system in EtOH at 520 nm.
Fig. 4C. Relation between Cꢄd ꢃ Caꢄ /A and (Cdꢄ + Cꢄa) for HPyO–DDQ system in EtOH at 396 nm.

98
A.S. Gaballa / Journal of Molecular Structure 1043 (2013) 91–102
Table 2
Spectrophotometric results for HPyO CT-complexes in EtOH.
Complex
kmax (nm)
KC (l molꢂ1
)
emax (l molꢂ1 cmꢂ1
)
ECT (eV)
f
l
Ip (eV)
D )
Gꢄ (25 °C) (k J mol-1
[(HPyO)I]I
294
360
520
396
2.35 ꢁ 104
0.95 ꢁ 104
0.36 ꢁ 104
3.83 ꢁ 104
0.98 ꢁ 104
4.83 ꢁ 104
0.31 ꢁ 104
0.81 ꢁ 104
4.23
3.46
2.39
3.14
19.77
46.73
1.08
34.88
59.22
11.01
19.63
10.96
9.10
8.67
9.89
ꢂ2.49 ꢁ 104
ꢂ2.27 ꢁ 104
ꢂ1.99 ꢁ 104
ꢂ2.62 ꢁ 104
[(H2PyO)(HCA)]
[(HPyO)(DDQ)]
5.04
Fig. 5. Infrared spectra for HPyO and complexes (1, 2, 3).

A.S. Gaballa / Journal of Molecular Structure 1043 (2013) 91–102
99
Fig. 5. (continued)
The energy of the charge-transfer complexes, ECT of the HPyO
The standard free energy changes of complexation (
D
Gꢄ) were
complexes is calculated using following equation [37–39]:
calculated from the formation constants by following equation
[43]:
D
Gꢄ ¼ ꢂ2:303RT log KCT
ð6Þ
ECT ¼ ðhtCTÞ ¼ 1243:667=kCTðnmÞ
ð5Þ
where R is the gas constant (8.314 J molꢂ1 K), T is the temperature
in Kelvin degrees and KC is the formation constant of the complexes
at room temperature.
plexes which indicate exothermic processes, Table 2.
where kCT is the wavelength of the complexation band of the stud-
ied complexes.
D
Gꢄ of interactions are negative in all CT com-

100
A.S. Gaballa / Journal of Molecular Structure 1043 (2013) 91–102
Table 3
Characteristic infrared frequenciesa (cm) and tentative assignments for complexes 1, 2 and 3.
H2CA
DDQ
HPyO
1
2
3
Assignments
3235 mbr
3442 w
3339 w
3220 wbr
3436, br, m
3119, m
3072, m
2921, m
2815, m
3432, br
3243, m, br
3069, m
2926, w
2810, w
3430, br
3127, m
m
m
m
m
m
m
(OAH); H bonded
(NAH)
(CAH)
s(CAH)
as(CAH);
(NAH)
3077, vs, br
2986, m
2961, br
2680, m
2364, w
2366, w
2363, w
2360, m
2217, m
1715, sh
1656, vs
Hydrogen bonding
2234 w
1674 vs
m
m
m
m
m
(CBN)
(C@O),
(C@O), quinone
(C@N)
(C@C)
1682, s
1638, s
1613, s
1866, vs
1610, s
1665 m
1632 s
1641, sh
1531, m
1459, m
1572, vs
1540 w
1452 s
1554 s
ring breathing bands
1533, s
1463, w
1365, m
1588, s
1476, m
1416, m
1340, vs
1279, s
1406 s
1274, s
CAH deformation
1369 m
1290 vs
1225
1425, m
1364, sh
1250, w
1144, w
1101, w
1030, vw
986, m
m
m
m
(CAC)
(CAN)
(CAO)
1268 ms
1216 w
1174 vs
1073 vw
1249, m
1040, m
1190 vs
1079 w
1172 w
1159, w
1078, w
917, m
982 vs
853 m
993, s
883, w
845, w
999 w
891 m
896 ms
919, m
CAH bend
(CACl)
840, m
m
802 s
722 ms
767, m
696, w
560, w
541, w
511, w
769 w
708 m
622 m
550 w
513 w
773 w
CH2 Rock
Skeletal vibration,
752 m
690 m
866, m
769, s
720, m
609, m
575, vw
510, m
620 w
577 m
m
m
(CACl)
(CAOAC)
623 vw
571 ms
510 m
433 w
432, w
434w
479 w
CNC def.
457 w
CH out of plan bend, skeletal vibration
a
s, Strong; w, weak; m, medium; sh, shoulder; v, very; br, broad.
3.3. IR Analysis
ingly, we may formulate the complex as [(H2PyO+)(HCAꢂ)]. The
increased electron density on the chloranilate unit as a result of
charge transfer interaction and deprotonation of chloranilic acid,
resulted in a pronounced shift to lower wavenumber in the
The infrared absorption spectra of the donor and the formed
complexes are shown in Fig. 5. Assignments of the well character-
ized bands in the infrared spectra of reactants and the obtained
products are given in Table 3. The formation of CT-complexes dur-
ing the reaction of HPyO with I2, H2CA and DDQ is strongly sup-
ported by observing of main infrared bands of the donor (HPyO)
and acceptors (H2CA, DDQ) in the product spectra. However, the
bands of the donor and acceptors in the complexes spectra reveal
small shifts in wavenumber values and 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.
m
(CACl) in [(H2PyO)(HCA)] complex spectrum (840 cmꢂ1) com-
pared with the free chloranilic acid (853 cmꢂ1) [46].
The significant shift of the CN stretching frequency from
2234 cmꢂ1 towards lower frequency (2217 cmꢂ1) on complexation
of DDQ with HPyO is indicative of charge transfer from HPyO to pꢅ
of CN groups of DDQ which leads to a weakening of this bond. Sim-
ilarly for DDQ, the CO stretching frequencies that appeared at
1674 cmꢂ1 of the free acceptor was shifted to 1715 cmꢂ1 and
emerged with 1656 cmꢂ1 upon complex formation.
Hydrogen bond formation in all CT complexes was confirmed
3.4. Thermal analysis
with observation of medium to broad bands around 3430 cmꢂ1
in the vibrational region of
m
(OAH) and medium to week bands
(NAH) [44].
The proposed structures for the complexes under investigation
were confirmed by measuring TGA, and DTG thermograms (Fig. 6)
under nitrogen flow. The thermal data obtained for complexes 2
and 3 together with the donor HPyO are summarized and given
in Table 4.
The decomposition reactions of complex 2 occur in two expect-
ing stages. The first stage of decomposition proceeds with a weight
loss value of 29.50% at a lower temperature of approximately
150 °C in comparison with the thermogram of the free donor. This
stage of decomposition might be associated with loss of the donor
molecule in agreement with the calculated value of 31.27%. The
second stage of decomposition proceeds at a maximum tempera-
around 2360 cmꢂ1 in the vibrational region of
m
In general for acid–base interaction, a proton transfer from the
acceptor (acid) to the donor (base) is expected to occur in complex
2. Such assumption is strongly supported by the appearance of a
new band of medium intensity in the spectrum of complex 2. This
band is observed at 2680 cmꢂ1 for HPyO–H2CA CT-complex and
may be due to the m(NAH) stretching vibration of hydrogen against
positively charged nitrogen [45]. We may suggest that, the acid–
base interaction is associated with a proton migration followed
by hydrogen bonding formation. The HPyO–H2CA interaction in-
volves a protonation for the basic nitrogen of the HPyO. Accord-

A.S. Gaballa / Journal of Molecular Structure 1043 (2013) 91–102
101
Fig. 6. Thermogravimeteric (TGA) and derivative (DTG) of HPyO and complexes 2 and 3.
ture of 222 °C. This could be attributed to the loss of the acceptor,
HCA. The weight loss associated with this stage of decomposition
(70.50%) is in good agreement with the calculated value of 68.73%.
The obtained data for complex 3, indicate that, the decomposi-
tion reactions occur in two stages. The donor molecule, HPyO is
lost at a temperature maximum of 193.5 °C. The calculated weight
loss of HPyO molecule in this complex corresponds to 29.52% in
good agreement with the obtained value of 29.00%. The main sec-
ond stage of decomposition proceeds at a maximum temperature
of approximately 290 °C and associated with a weight loss value
of 70.60%. This could be attributed to the loss of acceptor molecule,
DDQ in agreement with the calculated values of 70.48%.

102
A.S. Gaballa / Journal of Molecular Structure 1043 (2013) 91–102
Table 4
The maximum temperature values for the decomposition along with the species lost in each step of the decomposition reactions of HPyO and complexes 2 and 3.
Complex
Decomposition
Tmax/°C
Lost species
% Weight loss
Found
Calc.
HPyO
[(H2PyO)(HCA)] (2)
First step (total loss) residue
First step
Second step
Total loss
207
150
222
C5H5NO
C5H5NO
C6H2Cl2O4
99.86 00.14
29.50
70.50
100.00
00.00
100.00 00.00
31.27
68.73
100.00
00.00
C
–
11H7Cl2NO5
Residue
[(HPyO)(DDQ)] (3)
First step
Second step
Total loss
193.50
290.00
C5H5NO
C8Cl2N2O2
29.00
70.60
99.60
00.40
29.52
70.48
100.00
00.00
C
–
13H5Cl2N3O3
Residue
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A comprehensive study of the charge transfer interactions of the
donor 2-hydroxypyridine (HPyO) with the
r-acceptor, Iodine and
p-acceptors, H2CA and DDQ, was conducted in Ethanol. We were
able to show that the reaction stoichiometry donor:acceptor is
the same for all acceptors, 1:1 and the resulting CT-complexes
were shown to have the formulas: [(HPyO)(acceptor)]. Our ob-
tained results indicate that the nitrogen atom and hydroxyl group
in 2-hydroxypyridine are involved in the complexation with accep-
tors. In comparison with the previous studies of similar CT-com-
plexes, changing of derivative on the ligand (AOH instead of
ANH2), has no effect on complexation with
The type of solvent (ethanol) played important role in complex for-
mation with I2 as -acceptor and resulted in formation of new type
of CT-complex with iodine in comparison with using dichloro-
methane solution [47]. Spectrophotometric results (kmax, KC, emax
ECT, f, , Ip,
Gꢄ) for the new prepared HPyO CT-complexes are
p-acceptors [30b].
r
,
l
D
dependant on the nature of acceptor used.
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