Synthesis, spectral and thermal studies of the newly hydrogen bonded charge transfer complex of o-phenylenediamine with π acceptor picric acid

Article abstract of DOI:10.1016/j.saa.2010.06.011

Newly proton or charge transfer complex [(OPDH)⁺(PA) ^-] was synthesized by the reaction of the donor, o-phenylenediamine (OPD) with acceptor, 2,4,6-trinitrophenol (PAH). The chemical reaction has occurred via strong hydrogen bonding followed by migration of proton from acceptor to donor. UV-vis, ¹H NMR and FTIR spectra, in addition to the thermal and elemental analysis were used to confirm the proposed occurrence of the chemical reaction and to investigate the newly synthesized solid CT complex. The stoichiometry of the CT complex was found to be 1:1. The formation constant and molar extinction coefficient of the CT complex were evaluated by the Benesi-Hildebrand equation.

Full text of DOI:10.1016/j.saa.2010.06.011

Spectrochimica Acta Part A 77 (2010) 437–441
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectral and thermal studies of the newly hydrogen bonded charge
transfer complex of o-phenylenediamine with ꢀ acceptor picric acid
Ishaat M. Khan, Afaq Ahmad∗
Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh, Uttar Pradesh 202002, India
a r t i c l e i n f o
a b s t r a c t
Newly proton or charge transfer complex [(OPDH)+(PA)−] was synthesized by the reaction of the donor, o-
phenylenediamine (OPD) with acceptor, 2,4,6-trinitrophenol (PAH). The chemical reaction has occurred
via strong hydrogen bonding followed by migration of proton from acceptor to donor. UV–vis, 1H NMR
and FTIR spectra, in addition to the thermal and elemental analysis were used to confirm the proposed
occurrence of the chemical reaction and to investigate the newly synthesized solid CT complex. The
stoichiometry of the CT complex was found to be 1:1. The formation constant and molar extinction
coefficient of the CT complex were evaluated by the Benesi–Hildebrand equation.
Article history:
Received 31 March 2010
Received in revised form 19 May 2010
Accepted 4 June 2010
Keywords:
o-Phenylenediamine
2,4,6-Trinitrophenol
UV–vis
© 2010 Elsevier B.V. All rights reserved.
FTIR spectra
TGA–DTA
1H NMR
1. Introduction
ranilic acid were also studied due to their interesting properties
[23–25].
Proton or electron transfer complexes play an important role in
the field of magnetic, electrical conductivity and optical properties
[1–4]. Generally, the CT complexes are being regarded materials for
use as organic semiconductors and superconductors whose elec-
tronic properties depend strongly on the packing of the donor
radical cation [5–7]. They can also be used as corrosion inhibitors
[8] and micro-emulsion [9]. In addition, the CT complexes have
been found with wide application in biological field [10–14]. The
CT-interactions between aromatic electron acceptors and elec-
tron donors containing nitrogen, oxygen, or sulfur atoms have
attracted increased interest over the last years because of this type
of interactions play the important role in the field of drug-receptor
binding mechanism [15,16]. The proton-transfer from organic acid
to amines may take place readily with very low activation energy
in contrast with the attack of the amine to the carbonyl carbon
leading to amide formation [17]. In recent years, CT complexes
of 2,9-dimethyl-1,10-phenanthroline, phenanthrene, nitro aniline
7,7-bis(piperazino)-8,8-dicyanoquinodimethane and morpholine
as electron donors with 2,4,6-trinitrophenol as electron acceptor
were reported [18–22]. The complexation of o-phenylenediamine
and its derivatives with tetracyanoethylen, p-chloranil and chlo-
In continuation to our scope of interest, charge or proton trans-
fer interaction [26–29], this article presents the results obtained
from electronic and vibrational absorptions on the chemical prod-
uct formed in the reaction of o-phenylenediamine (OPD) as an
electron donor and the ꢀ acceptor 2,4,6-trinitrophenol. The aim of
the work is to determine the structure, reaction stoichiometry and
formation constant as well as investigate the thermal stability of the
CT complex, and interpreting the nature of these interactions using
1H NMR, FTIR spectra, elemental analysis, electronic absorption as
well as thermal analysis (TGA and DTA).
2. Experimental
2.1. Materials
Analytical grade of chemicals was used throughout. o-
Phenylenediamine was obtained from Merck, while picric acid
(PAH) was obtained from CDH. Methanol (Merck) and DMSO
(Merck) were all analytical grade (AR) and used without further
purification.
2.2. Preparation of standard solutions
A 1 × 10−2 M standard solution of o-phenylenediamine (donor)
and 1 × 10−2 M standard solution picric acid (acceptor) were pre-
pared in methanol.
∗
Corresponding author. Tel.: +91 5712703515.
E-mail addresses: drishaatamu@gmail.com (I.M. Khan),
drafaqahmad7@gmail.com (A. Ahmad).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.06.011

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I.M. Khan, A. Ahmad / Spectrochimica Acta Part A 77 (2010) 437–441
2.3. Synthesis of the solid CT complex
The solid CT complex [(OPDH)+(PA)−] was prepared by mix-
ing of 15 ml saturated solution of o-phenylenediamine (0.108 gm,
1 mmol) in CHCl3 with 15 ml of a saturated solution of picric
acid (0.229 gm, 1 mmol) in CHCl3. A yellow color solution was
obtained upon mixing that changed to yellow precipitate. The pre-
cipitate was filtered off and washed several times with CHCl3 and
dried under vacuum over CaCl2. It was ensured that the product
was not soluble in chloroform. The complex thus obtained was
characterized by elemental analysis (theoretical values are shown
in brackets): [(OPDH)+(PA)−], (C12H11N5O7) CT complex (M/W:
337.25 g): C, 42.80% (42.73%); H, 3.19% (3.25%); N, 19.94% (20.69%).
2.4. Spectrophotometric analyses
When 3 ml solution each of the acceptor and the donor were
mixed, a charge transfer complex was formed. The wavelength of
maximum absorption of the resulting solution was determined. The
CT complex of the 1:1 reaction mixture was kept over night at room
temperature to form stable complexes, were analyzed. The maxi-
mum wavelength of the charge transfer complex was determined
by spectrophotometer to be 224 nm.
Fig. 1. 1H NMR spectrum of charge transfer complex of o-phenylenediamine (donor)
and picric acid (acceptor).
The same was observed at ı = 6.651 ppm in free OPD but a multiple
peak of four protons of 2 × NH2 has been split into two mentioned
peak due to charge transfer.
2.5. Analyses
The FTIR spectra of the reactants and the resulting CT com-
plex were recorded using KBr disc on the spectroscopic 2020 FTIR
spectrometer. 1H NMR spectrum of the CT complex, donor and
acceptor was recorded in DMSO using Bruker DRX-300 NMR spec-
trometer and the thermal analysis (TGA and DTA) was carried out
under nitrogen atmosphere with a heating rate of 20 ◦C/min for TGA
and DTA using Shimadzu model DTG-60H thermal analyzers. The
electronic absorption spectra of the donor (o-phenylenediamine),
acceptor (picric acid) and the resulting CT complex in methanol
were recorded in the region of 700–200 nm using an Intra 10
UV–visible spectrophotometer with a 1 cm quartz cell paths length.
3.2. FTIR spectra
The FTIR spectra of OPD, PAH and their CT complex were
recorded and shown in Fig. 2 while the characteristic vibrational
frequencies of the functional groups in CT complex are listed in
Table 1 which reveal two broad absorption band and the absence
of phenolic group of PAH attributed to the CT interaction. The
new bands were observed in the spectrum of CT complex at 2591
and 2916 cm−1 is assigned to N+–H–O− species. They confirm the
formation of the new CT complex. The formation of CT complex
between donor and acceptor is strongly evidenced by the presence
of the main characteristic infra red bands of the OPD and PAH in
the spectrum of the product. However, the bands of the donor are
shifted lower wavenumbers, while that of the acceptor are shifted
to the higher frequency. The phenolic group stretching vibration
is disappeared in the spectrum of the CT complex which was nor-
mally observed at 3433 cm−1 for free picric acid. These shift and the
absence of OH peak have been attributed to transfer of phenolic pro-
3. Result and discussion
3.1. 1H NMR spectra
The 1H NMR spectra of the CT complex, acceptor and donor are
recorded in DMSO. The spectrum of CT complex is shown Fig. 1.
The protons in the picric acid can be assigned by following the
previously known literature [30] the phenolic proton is assigned
at ı = 11.94 ppm. The observed peaks in the spectrum of the indi-
vidual reactants are also observed in the spectrum of CT complex
except phenolic proton, suggesting its formation. The proton sig-
nals of the OPD are usually down field shifted to higher values
due to the phenolic proton transfer from acceptor to donor. The
triplet at ı = 6.901 ppm is assigned two aromatic protons of OPD
moiety whereas in free OPD this was observed at ı = 6.651 ppm
which was multiple peak for four aromatic protons of OPD. The
singlet at ı = 7.022 ppm for two aromatic protons of the same kind
in picric acid moiety of the CT complex but this was observed at
ı = 8.57 ppm in free picric acid. This upfield shift has been attributed
to increased electron density on picric acid part of the CT complex
due to the existence of the charge transfer interaction between the
donor and the acceptor molecules. The singlet peak at ı = 7.202 ppm
is assigned to the five protons of amine group of the donor in the
spectrum of CT complex because of one proton of OH group was
submerged in the bunch of amine protons of OPD moiety in that
region but four protons of 2 × NH2 group of free OPD are observed
at ı = 3.3 ppm. The intense singlet peak at ı = 8.623 ppm in the spec-
trum of CT complex is assigned to two aromatic protons of donor.
Fig. 2. FTIR spectra of (A) o-phenylenediamine, (B) picric acid, and (C) charge trans-
fer complex of OPD and PAH.

I.M. Khan, A. Ahmad / Spectrochimica Acta Part A 77 (2010) 437–441
439
Table 1
Infrared frequenciesa (cm−1) and tentative assignmentsb for o-phenylenediamine (OPD), picric acid (PAH) and their CT complex [(OPDH)+(PA)−].
OPD
PAH
[(OPDH)+(PA)−
3219 s
]
Assignmentsb
3368 ms
3287 w
3572 s br
3450 m br
ꢁ(OH); PAH ꢁ(NH); free OPD and CT complex
3190 m br
3033 w
3104 s
3083 w
ꢁ(C–H); C C–H and combination aromatic
2854 m br
1969 m br
1680 s
1877 w
1628 vs
2916 br
2591 m br
Overtone of ıCH; (out-of-plane) and combination
aromatic + ꢁ(C–H); OPD, PAH and CTC
1592 v
1496 vs
1609 s
1528 s
1447 s
1624 vs
1559 sh
1533 vs
1422 w
1341 vs vs
1270 s
ı(NH2), ı(NH), ı(CH), (C C) aromatic, ꢁas (NO2); OPD,
PAH and CT complex
1267 vs
1149 m
1341 vs
1265 vs
ꢁs (NO2); PAH + ꢁ(C–O); PAH, C–NH2; OPD and CT
complex, and in-plane bending of aromatic CH
1116 w
1058 w
1030 vw
926 w
1164 vs
1080 s
1164 m
1088 m br
ꢁs (C–O); C–NH2 + in-plane bending of aromatic CH
915 m
824 s
915 w
834 m
769 w
713 s
OH and CH out-of-plane def. (aromatic ring + ꢁ(C–N);
C–NH2 + ꢁ(C–N); C–NO2)
747 v br
442 ms
703 vs
620 ms
505 m br
581 w
526 w
450 w
Aromatic ring character of ortho substitution + NH2
def. + ıb (CNO); NO2
a
br, broad; m, medium; s, strong; sh, shoulder; w, weak; ꢁ, stretching, ı, bending.
Stretching and bending.
b
ton from acceptor to the donor for making hydrogen bonding via
N+–H–O− (between the nitrogen atom of one of the two NH2 group
of the donor and oxygen of acceptor). This is due to the ring cur-
rent influence of the magnetic anisotropy of nitrogen atoms and the
field effect of the electronic dipoles located on the nitrogen atoms.
The absorptions at 1624, 1569 and 1496 cm−1 confirm the presence
of C C stretching of the aromatic ring. This band overlaps with the
–NH2 deformation mode. The presence of the asymmetric and sym-
metric stretching vibrations of NO2 group at 1533 cm−1 (strong)
and 1341 cm−1 (very strong) and 1343 cm−1 (strong) respectively
has been confirmed in the CT complex but in free picric acid, these
were observed at 1536 cm−1 (very strong) and 1343 cm−1 (strong)
respectively owes to the large electron density on the picric acid
part as a result of the CT interaction due to migration of proton from
the acceptor to donor. In addition, the absorption at 1164 cm−1
was observed due to the C–O vibration stretching in the CT com-
plex whereas in free donor, this was observed at 1149 cm−1. The
C–NO2 stretching is observed at 915 cm−1 (weak) but in free picric
acid this was observed at 920 cm−1 (very strong). The broad strong
band in the FTIR spectrum of free OPD was observed in the range of
800–1000 cm−1 but this band was disappeared in the spectrum of
CT complex due to transfer of proton from acceptor to donor. The
broad strong band at 747 cm−1 is due to the ring bending vibration
in free donor but this was observed at 713 cm−1 (medium).
position. The second step occurs within the range 199–400 ◦C
which corresponds to 66.49% weight loss of the acceptor (PAH).
The third and the fourth steps are the decomposition of the donor,
OPD which are decomposition within the range 400–600 and
600–800 ◦C with weight loss 21.08 and 7.18% respectively. It was
also observed that CT complex has been completed with endother-
mic process, as indicated by absorption of 22.015, 23.770 and
251.960 J/g in the 1st, 2nd and 3rd steps of its degradation respec-
tively, and left is exothermic process which correspond 11,717 J/g.
This has been attributed that the CT complex is more stable its
constituents.
3.4. Observation of CT bands
The spectrophotometric data were used to calculate both for-
mation constant (KCT), and extinction coefficient (εCT) of the CT
complex in the defined solvent based on Benesi–Hildebrand equa-
3.3. Study of thermogram of the CT complex of
o-phenylenediamine and picric acid
Thermogravimetric (TG) and differential thermogravimetric
(DTG) analysis were carried out for CT complex of OPD and PAH
under a N2 flow in order to confirm its formula, structure and ther-
mal stability. The thermogram for CT complex is depicted in Fig. 3.
The degradation data clearly support the proposed molecular struc-
ture of the CT complex. However, it is difficult to propose a definite
decomposition reaction.
It was observed that the thermal decomposition of the CT com-
plex of OPD and PAH exhibits different degradation steps. The
thermogram for the CT complex (Fig. 3) shows four stages of decom-
Fig. 3. TGA–DTA curves of charge transfer complex of o-phenylenediamine (donor)
and picric acid (acceptor).

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I.M. Khan, A. Ahmad / Spectrochimica Acta Part A 77 (2010) 437–441
Table 2
The electronic absorption spectral data, formation constant and molar extinction coefficient for charge transfer complex of OPD with PAH in methanol at room temperature.
Conc. of donor (×10−4 M)
Conc. of acceptor (M)
Absorbance at ꢂCT 224 nm
Formation constant
Molar extinction coefficient (εCT)
(KCT) (l mol−1
1.045 × 105
)
(l cm−1 mol−1
)
0.40
0.60
0.80
1.00
1.20
1.50
1 × 10−5
1 × 10−5
1 × 10−5
1 × 10−5
1 × 10−5
1 × 10−5
0.824
1.052
1.205
1.266
1.481
1.675
2.631 × 106
tion [31,32].
[A]0
A
1
1
1
εCT
=
·
+
KCTεCT [D]0
where [D]0 and [A]0 are the initial concentration of the donor and
acceptor, respectively and A is the absorbance of the CT band.
The concentration of the donor in the reaction mixture was kept
greater than acceptor ([D]0 ꢀ [A]0) [31,32], and changed over a
wide range of concentration from 0.4 × 10−4 to 1.5 × 10−4 M while
concentration of the ꢀ acceptor (PHA) in each of the reaction mix-
ture was kept fixed at 1 × 10−5 M. These produced solutions with
donor:acceptor molar ratios varying from 4:1 to 15:1 as reported
in Table 2. The electronic absorption spectra of 1 × 10−5 M OPD,
1 × 10−5 M PAH and charge transfer complex were recorded against
methanol as references are shown in Fig. 4. To obtain the electronic
absorption spectra of CT complex, solutions of donor and acceptor
were prepared in the same solvents. The straight line was obtained
when [A]0/A was plotted against 1/[D]0 which supports the for-
mation of the charge transfer complex as shown in Fig. 5. In this
plot, the slope and intercept equals 1/KCT εCT and 1/εCT respec-
tively. It was observed that the correlation coefficient (r) value for
the straight line was found to be 0.988. The formation constant
and molar extinction coefficient of the CT complex were obtained
by calculation of intercept on y-axis and slope, and were found
to be 1.045 × 105 and 2.631 × 105 l cm−1 mol−1 respectively. It is
observed that new absorption peak appear in the UV region. In
some cases multiple peaks were obtained, the longest wavelength
peak was considered as CT peak [33], which is found at 224 nm. The
change of the absorption intensities to higher values for complex
on the addition of the donor is reported in Table 2. These measure-
ments are based on the CT absorption bands in the spectra of the
systems and are mentioned and given in Figs. 4 and 5.
Fig. 5. Benesi–Hildebrand plot of charge transfer complex of OPD with PAH [A]0/A
vs 1/[D]0 in methanol at room temperature.
4. Conclusions
The formation of 1:1 hydrogen bonded proton or charge trans-
fer complex of OPD and PAH was studied in solid phase as well
as spectrophotometrically in methanol at room temperature. The
stoichiometry of the CT complex was found to be 1:1 and shown to
have the structural formula [(OPDH)+(PA)−]. It was observed that
FTIR, 1H NMR and electronic absorption spectra provide evidence
for the existence of new bands of the CT complex which indicate a
charge transfer interaction associated with proton migration from
the acceptor to the donor followed by intermolecular hydrogen
bonding. TGA–DTA studies showed thermal stability of the CT com-
plex as well as good agreement with elemental and other analysis
for determination of structure of the CT complex. The formation
constant and molar extinction coefficient of the CTC, were esti-
mated.
Acknowledgements
Thanks are due to the Chairman, Department of Chemistry, AMU
for providing research facilities at physical chemistry Lab and to
UGC, New Delhi for financial assistance to one of us (I.M.K.).
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