Spectral, thermal, XRD and SEM studies of charge-transfer complexation of hexamethylenediamine and three types of acceptors: π-, σ- And vacant orbital acceptors that include quinol, picric acid, bromine, iodine, SnCl 4 and ZnCl2 acceptors

Article abstract of DOI:10.1016/j.molstruc.2013.08.006

In this work, structural, thermal, morphological and pharmacological characterization was performed on the interactions between a hexamethylenediamine (HMDA) donor and three types of acceptors to understand the complexation behavior of diamines. The three types of acceptors include π-acceptors (i.e., quinol (QL) and picric acid (PA)), σ-acceptors (i.e., bromine and iodine) and vacant orbital acceptors (i.e., tin(IV) tetrachloride (SnCl₄) and zinc chloride (ZnCl₂)). The characterization of the obtained CT complexes was performed using elemental analysis, infrared (IR), Raman, ¹H NMR and electronic absorption spectroscopy, powder X-ray diffraction (XRD) and thermogravimetric (TG) analysis. Their morphologies were studied using scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX). The biological activities of the obtained CT complexes were tested for their antibacterial activities. The complex containing the QL acceptor exhibited a remarkable electronic spectrum with a strong, broad absorption band, which had an observed ^λmax that was at a much longer wavelength than those of the free reactants. In addition, this complex exhibited strong antimicrobial activities against various bacterial and fungal strains compared to standard drugs. The complexes containing the PA, iodine, Sn(IV) and Zn(II) acceptors exhibited good thermal stability up to 240, 330, 275 and 295 C, respectively. The complexes containing bromine, Sn(IV) and Zn(II) acceptors exhibited good crystallinity. In addition to its good crystallinity properties, the complex containing the bromine acceptor exhibits a remarkable morphology feature.

Full text of DOI:10.1016/j.molstruc.2013.08.006

Accepted Manuscript
Spectral, thermal, XRD and SEM studies of charge-transfer complexation of
hexamethylenediamine and three types of acceptors: π-, σ- and vacant orbital
acceptors that include quinol, picric acid, bromine, iodine, SnCl and ZnCl ac‐
4
2
ceptors
Abdel Majid A. Adam, Moamen S. Refat, Hosam A. Saad
PII:
S0022-2860(13)00682-0
DOI:
http://dx.doi.org/10.1016/j.molstruc.2013.08.006
MOLSTR 19941
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
9 March 2013
25 July 2013
5 August 2013
Please cite this article as: A.M.A. Adam, M.S. Refat, H.A. Saad, Spectral, thermal, XRD and SEM studies of charge-
transfer complexation of hexamethylenediamine and three types of acceptors: π-, σ- and vacant orbital acceptors
that include quinol, picric acid, bromine, iodine, SnCl and ZnCl acceptors, Journal of Molecular Structure (2013),
4
2
doi: http://dx.doi.org/10.1016/j.molstruc.2013.08.006
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Spectral, thermal, XRD and SEM studies of charge-transfer complexation of
hexamethylenediamine and three types of acceptors: -, - and vacant orbital
acceptors that include quinol, picric acid, bromine, iodine, SnCl4 and ZnCl2
acceptors
Abdel Majid A. Adama,*, Moamen S. Refata,b and Hosam A. Saada,c
a
Department of Chemistry, Faculty of Science, Taif University, Al-Haweiah, P.O.
Box 888, Zip Code 21974, Taif, Saudi Arabia
b Department of Chemistry, Faculty of Science, Port Said University, Port Said, Egypt
c Department of Chemistry, Faculty of Science, Zagazig University, Zagazig, Egypt
Abstract
In this work, structural, thermal, morphological and pharmacological
characterization was performed on the interactions between a hexamethylenediamine
(HMDA) donor and three types of acceptors to understand the complexation behavior
of diamines. The three types of acceptors include -acceptors (i.e., quinol (QL) and
picric acid (PA)), -acceptors (i.e., bromine and iodine) and vacant orbital acceptors
(i.e., tin(IV) tetrachloride (SnCl4) and zinc chloride (ZnCl2)). The characterization of
the obtained CT complexes was performed using elemental analysis, infrared (IR),
1
Raman, H NMR and electronic absorption spectroscopy, powder X-ray diffraction
(XRD) and thermogravimetric (TG) analysis. Their morphologies were studied using
scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX).
The biological activities of the obtained CT complexes were tested for their
antibacterial activities. The complex containing the QL acceptor exhibited a
remarkable electronic spectrum with a strong, broad absorption band, which had an
observed max that was at a much longer wavelength than those of the free reactants.
In addition, this complex exhibited strong antimicrobial activities against various
bacterial and fungal strains compared to standard drugs. The complexes containing
the PA, iodine, Sn(IV) and Zn(II) acceptors exhibited good thermal stability up to
240, 330, 275 and 295 C, respectively. The complexes containing bromine, Sn(IV)
and Zn(II) acceptors exhibited good crystallinity. In addition to its good crystallinity
properties, the complex containing the bromine acceptor exhibits a remarkable
morphology feature.
Key words: Hexamethylenediamine, Charge-transfer interaction, XRD, Morphology,
Thermal analysis.
_________
* Tel: +966502099808.
E-mail address: majidadam@yahoo.com
2

1. Introduction
The chemical and physical properties of electron donor-acceptor (EDA) or
charge-transfer (CT) complexes formed by the reaction of - and -electron acceptors
with different donors containing nitrogen, sulfur and oxygen atoms, as well as amines,
polysulfur and crown ether bases in solution and in the solid state have attracted
considerable attention and growing importance in recent years [1-5]. This attention is
due to the important role of charge-transfer (CT) complexes in biological systems as
well as the significant physical properties of the CT complexes including electrical
conductivity and their applications in electronic and solar cells, organic
semiconductor lasers, high efficiency non-linear optical materials and organic light
emitting diodes (OLEDs). Such complexes also find important applications in the
quantitative determination of drugs in pharmaceutical preparations [6-9]. In addition,
literature shows that the CT complexes exhibit potential antimicrobial properties
against Gram-positive and Gram-negative bacteria as well as fungi [10].
Nitrogen bases are generally of special interest as electron donors because
they can function as n- and -donors. To shed light on the role of nitrogen bases as
significant donors, we have previously investigated the CT complexation properties of
tetramethylethylenediamine with various acceptors [11]. Based on the interesting
results obtained for the complexation properties of this diamine, we are interested in
further investigating diamine compounds. Therefore, as a continuation of our earlier
work, we report the synthesis and characterization of six new CT complexes of
hexamethylenediamine (HMDA) with quinol (QL), picric acid (PA), bromine, iodine,
tin(IV) tetrachloride (SnCl4) and zinc chloride (ZnCl2). These complexes were
structurally, thermally, morphologically and pharmacologically characterized via
1
elemental analysis, infrared (IR), Raman, H NMR and electronic absorption
spectroscopy, powder X-ray diffraction, thermal analysis and scanning electron
microscopy (SEM) as well as with biological applications to study antimicrobial
activities.
c
b
a
H2N
NH2
a
c
b
Formula I. Chemical Structure of Hexamethylenediamine.
3

2. Experimental methods
2.1 Chemicals and standard stock solutions
All of the chemicals used were of high reagent grade. Hexamethylenediamine
(HMDA; Hexane-1,6-diamine, C6H16N2) and the quinol (QL), picric acid (PA),
bromine, iodine, SnCl4 or ZnCl2 acceptors were purchased from the Aldrich and
Merck Chemical Companies and were used without further purification. Stock
solutions of the HMDA and each acceptor at a concentration of 5.010-3 M were
freshly prepared prior to each series of measurements by dissolving precisely weighed
quantities in the appropriate volume of the methanol or chloroform solvent. The stock
solutions were protected from light.
2.2 Instrumentation
The elemental analyses of the carbon, hydrogen and nitrogen content were
performed using the microanalysis unit at Cairo University, Egypt, on a Perkin-Elmer
CHN 2400 (USA). The electronic absorption spectra of the HMDA donor, the
acceptors (QL, PA, bromine and iodine) and the prepared CT complexes were
recorded in methanol or chloroform over a wavelength range of 200-800 nm using a
Perkin-Elmer Lambda 25 UV/Vis double-beam spectrophotometer at Taif University,
Saudi Arabia. The instrument was fitted with a quartz cell that had a path length of 1.0
cm. The infrared (IR) spectra using KBr discs within the range of 4000-400 cm-1 for
the solid CT complexes were recorded on a Shimadzu FT-IR spectrophotometer with
30 scans at 2 cm-1 resolution. The Raman laser spectra of the solid CT complexes
were measured on a Bruker FT-Raman spectrophotometer equipped with a 50 mW
1
laser at Taif University, Saudi Arabia. The H NMR spectra were recorded by the
Analytical Center at the King Abdul Aziz University, Saudi Arabia, on a Bruker
DRX-250 spectrometer operating at 250.13 MHz with a dual 5 mm probe head. The
measurements were performed at ambient temperature using CDCl3 as the solvent and
TMS (tetramethylsilane) as the internal reference. The 1H NMR data are expressed in
parts per million (ppm) and are internally referenced to the residual proton impurity in
the CDCl3 solvent. The thermogravimetric analysis (TGA) was performed under a
static air atmosphere to a temperature of 800 C at a heating rate of 10 C/min using a
Shimadzu TGA–50H thermal analyzer in the Central Lab at the Ain Shams
University, Egypt. The X-ray diffraction patterns for the obtained CT complexes were
collected on a PANalytical X’Pert PRO X-ray powder diffractometer in the Central
Lab at the Ain Shams University, Egypt. The instrument was equipped with a Ge(III)
secondary monochromator, and a copper K1 X-ray source with a wavelength of
0.154056 nm was employed. The scanning electron microscopy (SEM) images and
energy-dispersive X-ray detection (EDX) were recorded with Jeol JSM-6390
equipment at Taif University, Saudi Arabia. The instrument was operated at an
accelerating voltage of 20 kV.
2.3 Procedures
2.3.1 Synthesis of HMDA CT complexes procedure
The solid HMDA CT complexes with the QL, PA, bromine, iodine, SnCl2 or
ZnCl2 acceptor were prepared by mixing 1 mmol of the HMDA donor in pure–grade
methanol (10 ml) with 1 mmol of each acceptor in methanol (10 ml) (the bromine and
iodine were dissolved in chloroform). The solutions were stirred for approximately 20
min and allowed to evaporate slowly at room temperature resulting in the
precipitation of the solid CT complexes. The resultant complexes were filtered and
4

washed well with methanol or chloroform. Then, the synthesized complexes were
collected and dried under vacuum over anhydrous calcium chloride for 24 h.
2.3.2 Spectrophotometric titration procedure
The spectrophotometric titrations of the detectable CT bands at 341, 342, 295
and 370 nm were performed for the reactions of HMDA with QL, PA, bromine and
iodine acceptors as follows: 0.25, 0.50, 0.75, 1.00, 1.50, 2.0, 2.50, 3.00, 3.50 or 4.00
ml of a standard solution (5.010-4 M) of the appropriate acceptor in methanol or
chloroform was added to 1.00 ml of 5.010-4 M HMDA, which was also dissolved in
the same solvent. The final volume of the mixture was 5 ml. The concentration of
HMDA (Cd) was maintained at 5.010-4 M, whereas the concentration of the
acceptors (Ca) varied from 0.2510-4 M to 4.0010-4 M to produce solutions with a
donor: acceptor molar ratio that varied from 4:1 to 1:4. The stoichiometry of the
HMDA CT complexes was obtained from the determination of the conventional
spectrophotometric molar ratio according to previously published protocols [12] using
a plot of the absorbance of each CT complex as a function of the Cd:Ca ratio. The
Benesi–Hildebrand plots [13,14] were constructed to calculate the formation constant
(KCT) and the absorptivity (CT) for each CT complex.
2.3.3 Pharmacology analyses procedures
Antibacterial activity
The antibacterial activity of the prepared HMDA CT complexes and the pure
solvent was tested in vitro against two Gram-positive bacteria (i.e., Staphylococcus
aureus (MSSA 22) and Bacillus subtilis (ATCC 6051)) and two Gram-negative
bacteria (i.e., Escherichia coli (K 12) and Pseudomonas aeruginosa (MTCC 2488))
using the modified Bauer–Kirby disc diffusion method [15]. The microanalysis
facility at Cairo University, Egypt performed the investigations. Briefly, 100 l of the
test bacteria were grown in 10 ml fresh medium until they reached a count of
approximately 108 cells/ml for bacteria or 105 cells/ml for fungi [16]. Then, a 100 l
microbial suspension was spread onto agar plates. The nutrient agar medium for the
antibacterial tests consisted of 0.5% peptone, 0.1% beef extract, 0.2% yeast extract,
0.5% NaCl and 1.5% agar-agar [17]. Isolated colonies of each organism that may play
a pathogenic role were selected from the primary agar plates and tested for
susceptibility. After the plates were incubated for 48 h at 37 C, the inhibition (sterile)
zone diameters (including the disc) were measured using slipping calipers from the
National Committee for Clinical Laboratory Standards (NCCLS, 1993) [18] and are
expressed in mm. The screening was performed using 100 g/ml CT complex. An
antibiotic disc containing tetracycline (30 g/disc, Hi-Media) was employed as the
positive control. A filter disk impregnated with 10 L of solvent (distilled water,
chloroform, DMSO) was employed as the negative control.
Antifungal activity
The present HMDA CT complexes were also screened for their antifungal
property against Aspergillus flavus (laboratory isolate) and Candida albicans (IQA-
109) in DMSO using the modified Bauer–Kirby disc diffusion method [15]. The
complex was dissolved in DMSO. The medium for the antifungal tests consisted of
3% sucrose, 0.3% NaNO3, 0.1% K2HPO4, 0.05% KCl, 0.001% FeSO4 and 2% agar-
agar [17]. The disc diffusion method for the filamentous fungi was tested using the
approved standard method (M38-A) [19] developed by the NCCLS for evaluating the
5

susceptibility of filamentous fungi to antifungal agents. The disk diffusion method for
yeast, which was developed as the standard method (M44-P) by the NCCLS, was
employed [20]. The plates inoculated with filamentous fungi or yeast were incubated
for 48 h at 25 C or 30 C, respectively. The antifungal activity of the CT complexes
was compared with that of amphotericin B (30 g/disc, Hi-Media), which was used as
the standard antifungal agent (positive control). A filter disk impregnated with 10 L
of solvent (distilled water, chloroform, DMSO) was employed as the negative control.
The antifungal activity was determined by measuring the diameters of the sterile zone
(mm) in triplicate.
3. Results and discussion
3.1 Analytical results
The analytical data, elemental analysis (C, Hand N), molar ratio and color of the
HMDA CT complexes are shown in Table 1. The elemental analysis data for the
prepared CT complexes are in agreement with the molar ratio obtained from the
spectrophotometric titrations. The resulting elemental analysis values are in good
agreement with the calculated values. The stoichiometry of the HMDA CT complexes
with the PA, bromine and iodine acceptors was found to have a 1:2 ratio, whereas its
complexes with the QL, SnCl4 and ZnCl2 acceptors was found to have a 1:1 ratio.
Based on the obtained data, the prepared HMDA CT complexes were formulated as
-
+
.
[(HMDA)(QL)], [(HMDA)(PA)2], [HMDA] 2Br2, [HMDA-I] I3 , [(HMDA)(SnCl4)]
and [(HMDA)(ZnCl2)]. The formation of 1:1 or 1:2 complexes was strongly
supported by IR, 1H NMR and thermal analysis. The QL, PA, bromine and iodine CT
complexes have a pale brown, orange, orange yellow and reddish brown color,
respectively, but the SnCl4 and ZnCl2 complexes are white powders.
3.2Electronic absorption spectra
3.2.1 Characteristics of the CT bands
Figure 1 shows the electronic absorption spectra of the reactants, HMDA
(5.010-4 M) and acceptors (QL, PA, bromine and iodine) (5.010-4), along with those
of the prepared CT complexes. These spectra revealed the presence of the absorption
bands that correspond to the CT interactions. These bands are observed at 341, 342,
.
295 and (275 and 370) nm for the [(HMDA)(QL)],[(HMDA)(PA)2], [HMDA] 2Br2
-
and [HMDA-I]+I3 complexes, respectively. The absorption spectrum of HMDA has a

max at 293 nm, while that of the QL acceptor is 294 nm. Interestingly, when HMDA
and QL are mixed together, a new broad band appears at a much longer wavelength
(341 nm). This longer wavelength is indicative of charge-transfer. The spectrum of
-
the [HMDA-I]+I3 complex contains two new strong absorption bands at 275 and 370
nm. These new bands do not exist in the spectra of the reactants.
3.2.2 Stoichiometry of the HMDA CT complexes
The stoichiometry of the HMDA CT complexes formed between the HMDA
donor and the acceptors was determined by the molar ratio method. The peak
absorbance values that appeared in the spectra assigned to the prepared CT complexes
were measured and plotted as a function of the Cd:Ca ratio according to a previously
published protocol. Representative spectrophotometric titration plots based on these
characterized absorption bands are shown in Figure 2. Based on this figure, the
complex formation occurred with a ratio (HMDA: acceptor) of 1:1 for the QL
6

acceptor and 1:2 for the PA, bromine and iodine acceptors. These stoichiometry
values are consistent with the elemental analyses.
3.2.3 Determination of the formation constant and the molar extinction coefficient
The formation constant and the molar extinction coefficient of the CT
complexes were determined spectrophotometrically using the Benesi–Hildebrand
equations. The 1:1 Benesi–Hildebrand equation (Eq. (1)) was employed to calculate
the formation constant (KCT) and the extinction coefficients () of the HMDA
complex with the QL acceptor:
(CaCd)/A = 1/K + (Ca + Cd)/
(1)
where Ca and Cd are the initial concentrations of the acceptor (QL) and the donor
(HMDA), respectively, and A is the absorbance of the strongly detected CT band at
341 nm. By plotting the (Ca Cd)/A values as a function of the corresponding (Ca + Cd)
values for this complex, a straight line was obtained, which supports the formation of
a 1:1 complex. In the plot, the slope and intercept equal 1/ and1/K, respectively.
The Benesi–Hildebrand plot is shown in Figure 3, and the KCT and values associated
with this complex are given in Table 2.
The 1:2 modified Benesi–Hildebrand equation (Eq. (2)) was employed to
calculate the formation constant (KCT) and the molar extinction coefficients () of the
HMDA complex with the PA, bromine and iodine acceptors:
(Ca)2Cd/A = 1/K + 1/ Ca (4Cd + Ca)
(2)
where Ca and Cd are the initial concentrations of the acceptor (PA, bromine and
iodine) and the donor (HMDA), respectively, and A is the absorbance of the strongly
detected CT band. By plotting the (Ca)2Cd/A values for the 1:2 CT complex as a
function of the corresponding Ca (4Cd + Ca) values, a straight line is obtained with a
slope of 1/ and an intercept at 1/K. The modified Benesi–Hildebrand plots are
shown in Figure3, and the obtained KCT and values associated with the PA, bromine
and iodine complexes are given in Table 2.
In general, the 1:2 complexes exhibit high values for the formation constants
(KCT). These high KCT values reflect the high stabilities of the prepared HMDA CT
complexes due to the strong donation of the HMDA donor, which contains two
nitrogen atoms. These high values suggest that the prepared CT complexes can be
classified as strong [21]. The linearity of the Benesi–Hildebrand plots (r>0.99) further
supports this result. The formation constant (KCT) is strongly dependent on the nature
of the acceptor including the type of electron withdrawing substituents on it (e.g.,
nitro and halo groups).The results revealed that the [(HMDA)(PA)2] complex exhibits
a higher KCT value compared to the other complexes reflecting the relatively higher
electron acceptance ability of PA, which contains three nitro groups. The data also
revealed that the [(HMDA)(PA)2] complex exhibits a very high  value. The  values
of the HMDA CT complexes decrease in the following order: [(HMDA)(PA)2] 
-
+
.
[HMDA] 2Br2  [(HMDA)(QL)]  [HMDA-I] I3 .
3.2.4 Determination of the spectroscopic and physical data
The spectroscopic and physical data, such as the energy of the CT complex
(ECT), the oscillator strength (f), the transition dipole moment () and the standard free
energy (G°), were estimated for the formed CT complexes dissolved in methanol or
chloroform at 25 C. The calculations are summarized below.
7

The energy values of the CT complexes (ECT) of the n* and –*
interactions between the HMDA donor and the acceptors were calculated using the
equation derived by Briegleb [22]:
ECT = (hCT) = 1243.667/CT (nm)
(3)
where CT is the wavelength of the complexation band.
The oscillator strength (f) is a dimensionless quantity used to express the
transition probability of the CT band. From the CT absorption spectra, the oscillator
strength (f) can be obtained using the approximate formula [23]:
f = 4.31910-9CT d
(4)
where CT d is the area under the curve of the extinction coefficient of the
absorption band in question plotted as a function of the frequency. To a first
approximation,
f = 4.31910-9 CT

(5)
½
where CT is the maximum extinction coefficient of the CT band and  is the half-
½
bandwidth (i.e., the bandwidth at half of the maximum extinction coefficient value) in
cm-1.
The transition dipole moments () of the HMDA CT complexes are calculated
from Eq. (6) [24]:
 (Debye) = 0.0958 [CT /max]
½
(6)
½
The transition dipole moment () can be employed to determine if a particular
transition is allowed. The transition from a bonding  orbital to an antibonding *
orbital is allowed because the integral that defines the transition dipole moment is
nonzero.
The standard free energy changes of complexation (G°) for each complex
were calculated from the formation constants using Eq. [25]:
G = -2.303RT log KCT
(7)
where G° is the standard free energy change of the CT complexes (kJ mol-1), R is the
gas constant (8.314 Jmol-1K-1), T is the absolute temperature in Kelvin, and KCT is the
formation constant of the complex (L mol-1) at room temperature.
The calculated values of the spectroscopic and physical data for the HMDA
CT complexes deduced from Equations 3-7 are presented in Table 2. First, the
[(HMDA)(PA)2] complex exhibits a considerably higher value for both the oscillator
strength (f) and the transition dipole moment (), which indicates a strong interaction
between the donor-acceptor pairs (HMDA and PA) with relatively high probabilities
for CT-transitions [26]. Secondly, the obtained values of the standard free energy
-
+
.
change (G) for [HMDA-I] I3 , [(HMDA)(PA)2], [(HMDA)] 2Br2, [(HMDA)(QL)]
are -48.6, -44.9, -44.8 and -34.9 KJmol-1, respectively. These negative values indicate
that the interaction between the HMDA and the acceptors is spontaneous [27].
3.3 Interpretation of the IR spectra
The infrared absorption spectra of the HMDA solid CT complexes were
recorded in the frequency range 4000-400 cm-1 using a KBr disc, and their
characteristic bands are provided in Table 3. The full IR spectra of the HMDA CT
complexes are shown in Figure 4. The IR spectrum of HMDA contains the following
characteristic absorption bands: (i) 3339 cm-1, which was assigned to the stretching
vibrational band of (N-H); (ii) 2934 and 2856 cm-1, which were assigned to the
asymmetric and symmetric stretching vibrational bands of –CH2, respectively; (iii)
1660 cm-1, which corresponds to the bending deformation bands of N-H; (iv) the
bending deformation bands of –CH2 are assigned to the bands at 1476, 1455, 1395
8

and 1360 cm-1; (v) the asymmetric and symmetric stretching bands (i.e., as(C-
N)+s(C-N)) of C-N appear at 1284 and 1237 cm-1 and 1061 and 1014 cm-1,
respectively; (vi) both rocking (NH) and in-plane bending (CH) have detectable
bands at 970 and 878 cm-1 and 823 and 733 cm-1, respectively; and (vii) the band at
611 cm-1 is assigned to the out-of-plane bending of (C-N). The IR spectra of the
prepared CT complexes showed significant changes compared to that of the HMDA
donor, which indicated complexation between the HMDA donor and the acceptors.
However, the bands associated with the prepared CT complexes exhibit variation in
their intensities and, in some cases, show small shifts in the frequency values
compared to those of the free acceptor and HMDA donor. This variation could be due
to symmetry and electronic structure changes in both the acceptor and HMDA in the
prepared CT complexes compared to those of the free molecules. In the IR spectra,
the [(HMDA)(QL)] and [(HMDA)(PA)2] complexes are characterized by the
appearance of absorption bands at 3267 cm-1 for the QL complex and at 3240 cm-1 for
the PA complex. These absorption bands do not appear in the spectra of the free
HMDA donor or those of the QL and PA acceptors. These broadened peaks can be
attributed to the stretching vibration of the intermolecular hydrogen bond formed
through the proton transfer phenomena from the acidic center of each acceptor (QL
and PA) to the lone pair of electrons on the nitrogen as dictated by acid–base theory
[28-30]. As expected, the characteristic IR bands of the HMDA donor in the
-
[HMDA] 2Br2 and [HMDA-I]+I3 , [(HMDA)(SnCl4)] and [(HMDA)(ZnCl2)]
.
complexes exhibit small changes in the band intensities and frequency values. In the
IR spectra of the [(HMDA)(SnCl4)] and [(HMDA)(ZnCl2)] complexes, these two
complexes exhibited non-ligand bands at 460-570 cm-1, which were assigned to the
(M-N) vibrations [31].
3.4 Interpretation of the laser Raman spectra
Figure 5 shows the representative laser Raman spectra of the HMDA CT
complexes. The [(HMDA)(PA)2] complex exhibits vibrations at 1561 and 1336 cm-1,
which clearly correspond to the symmetric vibrational modes of (C=C) and the
bending deformation of (CH), respectively. The (C=C) vibrational band of the QL
acceptor appears at 1594 cm-1 in the Raman spectrum of the [(HMDA)(QL)] complex.
Based on the Raman spectra of the [(HMDA)(SnCl4)] and [(HMDA)(ZnCl2)]
complexes, the non-ligand bands occurred at 570 and 292 cm-1 and 315 and 168 cm-1,
which correspond to the (M-N) and (M-Cl) vibrations, respectively [31]. Raman
spectroscopy can only provide valuable information on the nature and structural
features of polyiodide anions. The laser Raman spectrum of the HMDA-iodine
-
complex contained the characteristic bands for the triiodine (I3 ) unit, which was
-
observed at 164, 123 and 98 cm-1 and assigned to as(I-I), s(I-I) and (I3 ),
respectively [32]. Maki and Forneris [33] assigned the lines of the Raman spectra of
-
I3 that occurred at 111 and 161 cm-1 to the symmetric and anti-symmetric stretching
vibrations, respectively. The observed Raman spectrum confirmed the formation of
-
the [HMDA-I]+I3 complex, which was also supported by the electronic absorption
spectrum of the iodine complex. The appearance of the two absorption bands at
approximately 370 and 275 nm is well known to be characteristic of the formation of
-
-
the triiodine ion (I3 ) [34-36]. However, the I3 ion may be linear (Dh) or nonlinear
-
(C2). Group theoretical analysis indicates that the I3 has C2 symmetry and displays
-
three vibrations including as(I-I), A1; s(I-I), B2; and , (I3 ); A1. All of these
9

vibrations are IR active, which is in good agreement with the observation of three
-
Raman bands for [HMDA-I]+I3 . According to the above discussion, a general
-
mechanism proposed for the formation of the [HMDA-I]+I3 complex is as follows:
.
HMDA + I2  HMDA I2 (outer complex)
-
+
.
HMDA I2  [HMDA-I] I (inner complex)
[HMDA-I]+I- + I2  [HMDA-I]+I3 (triiodine complex)
-
The Raman spectrum for the free bromine exhibited a very intense band at 310 cm-1
.
[11]. In the Raman spectrum of the [HMDA] 2Br2 complex, the halogen vibration
band ((Br-Br)) is shifted to a lower frequency of 271 cm-1 upon complexation.
3.5 Interpretation of the 1H NMR spectra
The nuclear magnetic resonance spectra provide evidence of the complexation
1
pathway. The 400 MHz H NMR spectra of the [(HMDA)(QL)], [(HMDA)(PA)],
[(HMDA)(SnCl4)] and [(HMDA)(ZnCl2)] CT complexes were measured in CDCl3 at
room temperature and are provided in Figure 6. The chemical shifts () of the
different types of protons in these CT complexes include the following:
(1) HMDA donor: δ = 1.22 (s, 4H, 2NH2), 1.36 (t, 4H, 2CH2, C(c)), 1.49 (m, 4H,
2CH2, C(b)), 2.67 (t, 4H, 2CH2, C(a)).
(2) [(HMDA)(QL)] complex: δ = 1.31 (t, 4H, 2CH2, C(c)), 1.52 (m, 4H, 2CH2, C(b)),
2.88 (t, 4H, 2CH2, C(a)), 3.17 (b, 4H, 2NH2, Hydrogen bonded with quinol OH),
5.23 (s, 2H, Ar-OH, quinol OH protons hydrogen bonded with NH2), 6.55 (s, 4H,
Ar-H of quinol).
(3) [(HMDA)(PA)2] complex: δ = 1.31 (t, 4H, 2CH2, C(c)), 1.51 (m, 4H, 2CH2, C(b)),
2.76 (t, 4H, 2CH2, C(a)), 3.42 (b, 4H, 2NH2, Hydrogen bonded with picric acid
OH), 7.61 (b, 2H, Ar-OH, picric acid OH protons hydrogen bonded with NH2),
8.65 (s, 4H, Ar-H of picric acid).
(4) [(HMDA)(SnCl4)] complex: δ = 1.31 (t, 4H, 2CH2, C(c)), 1.55 (m, 4H, 2CH2, C(b)),
2.74 (t, 4H, 2CH2, C(a)), 3.42 (b, 4H, 2NH2).
(5) [(HMDA)(ZnCl2)] complex: δ = 1.26 (t, 4H, 2CH2, C(c)), 1.48 (m, 4H, 2CH2,
C(b)), 2.63 (t, 4H, 2CH2, C(a)), 3.40 (b, 4H, 2NH2).
The HMDA free donor exhibited four characteristic bands at δ = 1.22 ppm for
the two aliphatic -NH2 groups along with three peaks at 1.36, 1.49 and 2.67 ppm
corresponding to three different CH2 groups (6H; 6CH2); CH2 C(c),CH2 C(b) and CH2
C(a), respectively. In the [(Qui)(QL)] complex, the high upfield shift of the phenolic
proton (–OH) band of quinol, which is observed at approximately 8.59 ppm in the
spectrum of the QL acceptor [30], to 5.23 ppm as well as a high downfield shift of the
NH2 band in HMDA (δ = 3.17 ppm) indicates the involvement of the –NH2 groups of
the HMDA donor and the (–OH) group of the QL acceptor in chelating the QL
acceptor to the HMDA donor via deprotonation. The downfield chemical shift of all
of the protons in the –CH2 groups is due to the formation of the intermolecular CT
complex. In the [(HMDA)(PA)2] complex, the high upfield shift of the band
associated with the OH proton of picric acid, which is observed at δ =11.94 for the
free picric acid [37], to 7.61 ppm along with a high downfield shift of the NH2 signal
of the HMDA (δ = 3.42 ppm) confirms the formation of intermolecular hydrogen
bonds between PA and HMDA [38]. In addition, the electronic environments of the
HMDA proton change during the formation of the CT complexes, and the proton
signal of the aromatic rings in the PA was shifted slightly downfield from 8.624 (pure
1
PA) to 8.650 ppm after complexation. Based on the H NMR of the OL and PA
complex with HMDA, the complexes are created by the formation of a hydrogen bond
10

between the N atom of the HMDA and the H atom of the –OH groups in each
acceptor. After complexation of [(HMDA)(ZnCl2)], the characteristic signal for the –
NH2 protons, which occurs at 1.22 ppm in free HMDA donor, was shifted downfield
to 3.4 ppm due to the deshielding effect of the Zn(II) ion. In addition, a small
downfield shift in the CH2 bands was observed. The more positive Sn(IV) cation
exhibited a larger deshielding effect in its complex with HMDA, resulting in a larger
downfield shift for all of the absorption bands compared to the Zn(II) complex.
3.6 Thermal results
To determine the formula and structure of the new HMDA CT complexes,
thermogravimetric analysis (TG) was performed for these complexes over a
temperature range of 25-800 C under a static air atmosphere. The TG curves were
redrawn with the mg mass loss as a function of temperature. Fig. 7 shows the
thermograms for the HMDA CT complexes. Table 4 provides the TG temperature
range together with the corresponding weight loss for each step in the decomposition
process of these complexes.
The TG curve of the [(HMDA)(QL)] complex indicates that this complex is
thermally stable in the 25-100C temperature range. The decomposition of the
complex began at 100 C and was complete at 650 C. The thermal decomposition
of this complex occurs in three steps within the temperature range of 25-800 C. The
first mass loss step occurs between 25-230 C, which corresponds to the loss of
C2H8N2 molecules and a weight loss of 26.29%. The second degradation step at 230-
514 C is attributed to the loss of C5H4O2 molecules, which represents a weight loss
of (obs.=42.81, cal.= 42.42%). The last mass loss step occurs between 514-800 C
and corresponds to the loss of C3H10 resulting in a few carbon atoms that remain as a
residue. The thermal analysis curve of the [(HMDA)(PA)2] complex indicates that
decomposition occurs in two decomposition steps within the 25-800 C temperature
range. The first decomposition step in the temperature range of 25-370C has a
weight loss of approximately 70.49% and is attributed to the formation of C7H12N8O14
molecules. The second decomposition step occurred within the 370-800 C
temperature range and was assigned to the removal of C11H10 molecules.
Theoretically, the loss of this moiety corresponds to the loss of 29.51%. The thermal
.
decomposition of the [HMDA] 2Br2 complex proceeds in three successive
degradation steps within the temperature range of 25-800 C. The decomposition
steps occur in the temperature ranges of 25-265, 265-468 and 468-800C with weight
losses of 43.86, 46.8 and 9.34%, which correspond to the loss of Br2 + 2CH4, Br2 +
CH4 + N2 and CH4 + 2CO2, respectively. The total percentage of weight loss observed
(100%) matched the calculated weight loss of 99.96%. The thermal degradation of the
-
[HMDA-I]+I3 complex occurs in two degradation stages within the 25-800 C
temperature range. The mass change begins at 330C and continues to 800 C. The
first stage of decomposition occurs within the temperature range of 25-460 C,
corresponding to the loss of two iodine molecules with a weight loss of 81.40%,
which is in good agreement with the calculated value (81.31%). The second
decomposition step occurred in the temperature range of 460-800 C and is assigned
to the removal of four methane molecules and one nitrogen molecule. The observed
weight loss of 14.75% is in good agreement with the theoretical weight loss value of
14.64%. The decomposition of this organic moiety led to residual carbon atoms as a
final product. The TG thermogram of the [(HMDA)(SnCl4)] complex indicated that
the thermal decomposition of this complex proceeds via one degradation step within
11

the 25-800 C temperature range. This decomposition begins at  275 C, and the
observed weight loss associated with this step is 68.52%, which can be attributed to
the loss of the C6H16N2Cl4 moiety and is in excellent agreement with the calculated
value of 68.50%. The continuous weight loss in this stage resulted in a residual tin
metal remaining as the final fragment. The obtained data indicate that the
[(HMDA)(ZnCl2)] complex was thermally decomposed in three successive
decomposition steps within the temperature range of 25-800 C. The decomposition
steps occurred at 25-367, 367-546 and 546-800C with a weight loss of 23.71, 36.4
and 14.1%, which corresponds to the loss of C2H8N2, C4H8Cl and ½Cl2, respectively.
The final thermal product obtained at 800 C is zinc metal. These steps are associated
with a total weight loss of 74.20%, which is in good agreement with the calculated
value (74.05%).
3.6 Kinetic results
Several equations have been proposed to analyze a TG curve and to obtain the
kinetic parameters. Two different methods were employed to evaluate the kinetic
thermodynamic parameters: the Coats–Redfern method [39] and the Horowitz–
Metzger method [40]. The kinetic thermodynamic parameters (i.e., the activation
energy (E*), the frequency factor (A), the enthalpy of activation (H*), the entropy of
activation (S*) and the free energy of activation (G*)) for the decomposition processes
of the HMDA CT complexes were evaluated graphically (Figure 8) by employing the
Coats-Redfern and Horowitz-Metzger methods, and the evaluated data are listed in
Table 5. The activation energy (E*) of the complexes is expected to increase with the
increasing thermal stability of complexes. Therefore, the E* value for the
[(HMDA)(PA)2] complex is higher compared to the other complexes, which indicates
the higher thermal stability of the [(HMDA)(PA)2] complex. By comparing the
E*values for the main decomposition stage of the HMDA CT complexes, we observed
the following trend for the different acceptors: PA  I2  Zn(II)  Sn(IV)  QL  Br2.
These differences may be due to the reactivity of the complexes and the electronic
configuration of the acceptor when complexed with the HMDA donor. The calculated
E* values using the Coats–Redfern and Horowitz–Metzger methods for the main
decomposition stage of the complexes are 1×106 kJ mol-1 for [(HMDA)(PA)2],
-
3.1×105 kJ mol-1 for [HMDA-I]+I3 , 1.3×105 kJ mol-1 for [(HMDA)(ZnCl2)], 8.4×104
kJ mol-1 for [(HMDA)(SnCl4)], 4.2×104 kJ mol-1 for [(HMDA)(QL)] and 3.9×104 kJ
-1
*
.
mol for [HMDA] 2Br2. The entropy of activation (S ) is found to be negative in
most cases, which indicates that the decomposition reactions proceed spontaneously.
The S* values of the HMDA CT complexes occur in a decreasing order as follows:
-
[(HMDA)(PA)2]  [HMDA-I]+I3  [(HMDA)(ZnCl2)]  [(HMDA)(SnCl4)] 
.
[HMDA] 2Br2  [(HMDA)(QL)]. The satisfactory values for the correlation
coefficients from the Arrhenius plots of the thermal decomposition steps were
observed to be r  1 for all cases, which indicates a good fit with the linear function
and reasonable agreement between the experimental data and the kinetic parameters.
3.8 Structural interpretation
The structures of the CT complexes of the HMDA donor with the QL, PA,
Br2, I2, SnCl4 or ZnCl2 acceptors are confirmed by elemental analysis,
1
spectrophotometric titration, IR and H NMR spectra, and thermal analysis. The
proposed structures of these CT complexes are shown in Formulas II, III, IV, V, VI
and VII.
12

H2N
HO
NH2
OH
Formula II. Proposed structural formula of the [(HMDA)(QL)] complex.
O N
NO
2
2
NO
2
H2
N
H
O
O
N
H
H2
NO
2
O N
NO
2
2
Formula III. Proposed structural formula of the [(HMDA)(PA)2] complex.
Br
Br
H2N
NH2
Br
Br
.
Formula IV. Proposed structural formula of the [HMDA] 2Br2 complex.
13

H2N
I
NH2
I
I
I
-
Formula V. Proposed structural formula of the [HMDA-I]+I3 complex.
Cl
Cl
Cl
NH2
NH2
Sn
Cl
Formula VI. Proposed structural formula of the [(HMDA)(SnCl4)] complex.
NH2
NH2
Zn
Cl
Cl
Formula VII. Proposed structural formula of the [(HMDA)(ZnCl2)] complex.
14

3.9 XRD characterization
To obtain further evidence regarding the structure of the HMDA CT
complexes and to demonstrate their crystallinity, powder X-ray diffraction (XRD)
studies were performed using a PANalytical model X’PERT-PRO X-ray powder
diffractometer system. The indexed diffractograms obtained in the range of 5 2
70 for these complexes are shown in Figure 9. The particle size of these two
complexes was estimated from their XRD patterns based on the highest intensity
value compared to the other peaks using the well-known Debye–Scherrer formula
given in (Eq. 13) [41].
D = K / cos 
(13)
where D is the apparent particle size of the grains, K is a constant (0.94 for Cu grid), 
is the X-ray wavelength used (1.5406 Å),  is half the scattering angle (the Bragg
diffraction angle), and  is the full-width at half-maximum (FWHM) of the X-ray
diffraction line (additional peak broadening) in radians. Table 6 presents the XRD
spectral data (i.e., 2, , d (the interplanar spacing between atoms) and D in nm) for
.
the [(HMDA)(QL)], [(HMDA)(PA)2], [HMDA] 2Br2, [(HMDA)(SnCl4)] and
[(HMDA)(ZnCl2)] complexes.
Based on the XRD investigations, the following results were obtained:
(1) The primary characteristic scattering peak of the [(HMDA)(QL)],
.
[(HMDA)(PA)2], [HMDA] 2Br2, [(HMDA)(SnCl4)] and [(HMDA)(ZnCl2)]
complex occurred at 21.471, 28.121, 32.525, 9.475 and 9.137 in the
diffraction pattern, respectively.
(2) The XRD patterns of the HMDA CT complexes differ from each other.
(3) For the complexes with the bromine, Sn(IV) and Zn(IV) acceptors, the appearance
of a sharp and strong Bragg peak indicates the formation of a well-defined
distorted crystalline structure.
(4) Using the Debye–Scherrer equation, the particle size values of the complexes
were found to be 4.9, 7.5, 10, 11.6 and 7.3 nm for the [(HMDA)(QL)],
.
[(HMDA)(PA)2], [HMDA] 2Br2, [(HMDA)(SnCl4)] and [(HMDA)(ZnCl2)]
complexes, respectively.
(5) Based on these values, the particle sizes are on the nanoscale.
3.10 Morphology characterization
Scanning electron microscopy (SEM) was employed to observe the
morphology and particle size of the HMDA CT complexes. The surface images
obtained with the SEM technique provide general information regarding the
microstructure, surface morphology, particle size, chemical composition, and porous
structures of the surfaces. In addition, the chemical compositions of the complexes
were determined using energy-dispersive X-ray diffraction (EDX). Figure 10 depicts
the SEM photographs of the synthesized HMDA CT complexes along with their EDX
spectra. Analyses of the SEM photographs and EDX profiles of the HMDA CT
complexes provided the following observations:
(1) The morphological phases of the synthesized complexes showed a uniform matrix
in the SEM images indicating the formation of a homogeneous material.
(2) The size of the particles differed substantially for each acceptors.
(3) The particles of the QL complex appear as agglomerates and display a different
granule size and shape with a particle size in the range of 10-50 m.
(4) The particles of the PA complex are flake-shaped, where most of the particles
exhibit angular shapes with estimated sizes in the range of 50-100 m.
15

(5) The complexation of HMDA with bromine leads to a very interesting
morphology. The image of the obtained complex shows coherent a rock-like-
shaped morphology with a particle of size 10 m. From the multi SEM
photographs with various degrees of enlargement (i.e., x500, x1000, x1500,
x2000) (Figure 10C) , the particles of this complex have a well-defined
morphology, and smaller granules are clearly observed on the surface of each of
the larger granules.
(6) A gelatin-like shape is observed for the iodine complex, which has clearly visible
holes and a particle size in the range of 50-100 m. The surface of this complex is
smoother than the surface of the other complexes.
(7) The Sn(IV) complex particles have a cave-like-shaped morphology with a particle
size of 10 m.
(8) The Zn(II) complex has a crushed ice granular-shaped morphology with a particle
size of 10 m. In addition, bright white particles were observed.
(9) The peaks refer to essential elements, such as carbon, nitrogen, oxygen and
subsidiary elements, that constitute the molecules of these complexes. These
elements were clearly identifiable, and the results confirmed the proposed
structures.
3.11 Pharmacology assessment
The pharmacological evaluation of HMDA and its CT complexes involved
antibacterial and antifungal activity studies. The screenings were performed with a
100 g/ml concentration of the test CT complexes and an antibiotic disk. The
diameter zones were measured to determine their effects on the growth of the tested
microorganisms. The antibacterial activity of HMDA and its CT complexes was
screened in vitro against two Gram-positive bacterial strains (i.e., Staphylococcus
aureus (S. aureus) and Bacillus subtilis) and two Gram-negative bacterial strains (i.e.,
Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa)). The activity
was determined by measuring the inhibition zone diameter (mm) of the complexes
against the microorganisms. Tetracycline was employed as the standard drug (positive
control) for comparison of the bacterial results. The results from the agar disk
diffusion tests are presented in Table 7 and statistically illustrated in Figure 11. The
synthesized HMDA CT complexes were also examined for their antifungal properties
against two fungal species including Aspergillus flavus and Candida albicans.
Amphotericin B was employed as the standard drug (positive control) for comparison
of the antifungal results. The screening data are reported in Table 7 and statistically
represented in Figure 12. By comparing the biological activity of HMDA and its CT
complexes, the following results were obtained:
(1) The [(HMDA)(QL)] complex showed excellent antifungal activity against the
growth of the tested fungal strains. The free HMDA exhibited no such activity,
which makes this an interesting complex. This complex is comparable to
Amphotericin B.
(2) The [(HMDA)(QL)] and [(HMDA)(SnCl4)] complexes exhibited good inhibitory
results against all of the Gram-positive and Gram-negative bacterial species.
(3) The other HMDA complexes exhibited moderate antibacterial activity against all
of the tested bacterial strains but had no inhibitory activity against either fungal
strain.
(4) The biological activity against the Gram-positive and Gram-negative bacteria
followed the order: [(HMDA)(QL)] > [(HMDA)(SnCl4)] > [(HMDA)(ZnCl2)] >
-
+
.
[HMDA] 2Br2 > [HMDA-I] I3 > [(HMDA)(PA)2].
16

(5) The antibacterial activity of the HMDA CT complexes is higher than that of the
free HMDA.
(6) The antibacterial activity of the HMDA and its complexes are lower than that of
the tetracycline standard.
(7) The Sn(IV) complex was found to be an efficient antibacterial agent, while the QL
complex was found to be an efficient antibacterial and antifungal agent.
References
[1] I.M. Khan, A. Ahmad, S. Kumar, J. Mol. Struct. 1035 (2013) 38.
[2] M. Shukla, N. Srivastava, S. Saha, J. Mol. Struct. 1021 (2012) 153.
[3] M.S. Refat, H.A. Saad, A.M.A. Adam, J. Mol. Struct. 995 (2011) 116.
[4] E.M. Nour, M.S. Refat, J. Mol. Struct. 994 (2011) 289.
[5] M.S. Refat, A. Elfalaky, E. Elesh, J. Mol. Struct. 990 (2011) 217.
[6] S.Y. AlQaradawi, A. Mostafa, H.S. Bazzi, J. Mol. Struct. 1011 (2012) 172.
[7] R. Foster, Organic Charge-Transfer Complexes, Academic Press Inc., New York,
1969.
[8] F. Gutmann, C. Johnson, H. Keyzer, J. Molnar, Charge-Transfer Complexes in
Biochemical Systems, Marcel Dekker Inc., 1997.
[9] I.M. Khan, A. Ahmad, M. Aatif, J. Photochem. Photobiol. B: Biol. 105 (2011) 6.
[10] A.M.A. Adam, Spectrochim. Acta A 104 (2013) 1.
[11] A.M.A Adam, M.S. Refat, T. Sharshar, Z.K. Heiba, Spectrochim. Acta A 95
(2012) 458.
[12] D.A. Skoog, Principle of Instrumental Analysis, third edn., Saunders College
Publishing, New York, USA, 1985, Ch. 7.
[13] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703.
[14] R. Abu-Eittah, F. Al-Sugeir, Can. J. Chem. 54 (1976) 3705.
[15] A.W. Bauer, W.M. Kirby, C. Sherris, M. Turck, Am. J. Clin. Pathol. 45 (1966)
493.
[16] M.A. Pfaller, L. Burmeister, M.A. Bartlett, M.G. Rinaldi, J. Clin. Microbiol. 26
(1988) 1437.
[17] D.J. Beecher, A.C. Wong, Appl. and Environ. Microbiol. 60 (1994) 1646.
[18] National Committee for Clinical Laboratory Standards. Methods for
Antimicrobial Susceptibility Testing of Anaerobic Bacteria: Approved Standard M11-
A3. NCCLS, Wayne, PA, USA (1993).
[19] National Committee for Clinical Laboratory Standards. Reference Method for
Broth Dilution Antifungal Susceptibility Testing of Conidium-Forming Filamentous
Fungi: Proposed Standard M38-A. NCCLS, Wayne, PA, USA (2002).
[20] National Committee for Clinical Laboratory Standards. Method for Antifungal
Disk Diffusion Susceptibility Testing of Yeast: Proposed Guideline M44-P. NCCLS,
Wayne, PA, USA (2003).
[21] M. Pandeeswaran, K.P. Elango, Spectrochim. Acta A 72 (2009) 789.
[22] G. Briegleb, Z. Angew. Chem. 76 (1964) 326.
[23] H. Tsubomura, R.P. Lang, J. Am. Chem. Soc. 83 (1961) 2085.
[24] R. Rathone, S.V. Lindeman, J.K. Kochi, J. Am. Chem. Soc. 119 (1997) 9393.
[25] A.N. Martin, J. Swarbrick, A. Cammarata, Physical Pharmacy, third edn., Lee
and Febiger, Philadelphia, PA, 1969, p. 344.
[26] M. Pandeeswaran, K.P. Elango, J. Indian Chem. Soc. 85 (2008) 399.
[27] K.M. Al-Ahmary, M.M. Habeeb, E.A. Al-Solmy, J. Mol. Liq. 162 (2011) 129.
[28] M.S. Refat, H.A. Saad, A.A. Adam, J. Mol. Strut. 995 (2011) 116.
17

[29] A.S. Gaballa, S.M. Teleb, E. Nour, J. Mol. Struct. 1024 (2012) 32.
[30] A.A. Adam, J. Mol. Struct. 1030 (2012) 26.
[31] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, Parts A and B, John Wiley and Sons, New York, 1997.
[32] M. Pandeeswaran, K.P. Elango, Spectrochim. Acta A 72 (2009) 789.
[33] A.G. Maki, R. Forneris, Spectrochim. Acta A 23 (1967) 867.
[34] W. Kiefer, H.J. Bernstein, Chem. Phys. Lett. 16 (1972) 5.
[35] L. Andrews, E.S. Prochaska, A. Loewenschuss, Inorg. Chem. 19 (1980) 463.
[36] K. Kaya, N. Mikami, Y. Udagawa, M. Ito, Chem. Phys. Lett. 16 (1972) 151.
[37] R.D. Kross, V.A. Fassel, J. Am. Chem. Soc. 79 (1957) 38.
[38] N. Singh, A. Ahmad, J. Mol. Struct. 977 (2010) 197.
[39] A.W. Coats, J.P. Redfern, Nature (London) 201 (1964) 68.
[40] H.H. Horowitz, G. Metzger, Anal. Chem. 35 (1963) 1464.
[41] S.J.S. Qazi, A.R. Rennie, J.K. Cockcroft, M. Vickers, J. Colloid Interface Sci.
338 (2009) 105.
18

CT band (341 nm)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
CT band (342 nm)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
HMDA
PA
CT complex
HMDA
QL
CT complex
250
300
350
400
450
500
250
300
350
400
450
500
550
600
 (nm)
 (nm)
0.4
0.3
0.2
0.1
0.0
1.0
0.8
0.6
0.4
0.2
0.0
CT band (295 nm)
CT bands (275 and 370 nm)
HMDA
I2
HMDA
Br2
CT complex
CT complex
300
400
500
600
700
300
400
500
600
700
 (nm)
 (nm)
.
Figure 1. Electronic spectra of [(HMDA)(QL)], [(HMDA)(PA)2], [HMDA] 2Br2 and
-
[HMDA-I]+I3 charge-transfer complexes in methanol solvent.
1

4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
1.8
1.6
1.4
1.2
1.0
0.8
HMDA-PA complex
1:2 molar ratio
HMDA-QL complex
1:1 molar ratio
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
ml added of OL
ml added of PA
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
HMDA-I2 complex
1:2 molar ratio
HMDA-Br2 complex
1:2 molar ratio
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
ml added of I2
ml added of Br2
Figure 2. Spectrophotometric titration curves of [(HMDA)(QL)], [(HMDA)(PA)2],
-
+
.
[HMDA] 2Br2 and [HMDA-I] I3 charge-transfer complexes in methanol solvent.
2

R=0.99946
2.5
2.0
1.5
1.0
0.5
0.0
R=0.99493
4
3
2
1
0
HMDA-QL complex
HMDA-PA complex
0
5
10
15
20
25
30
35
100 150 200 250 300 350 400 450 500 550
Ca(4Cd+ Ca)x10-8
(Cd+Ca)x10-6
R=0.99919
R=0.99362
20
16
12
8
16
12
8
4
4
HMDA-Br2 complex
HMDA-I2 complex
0
0
0
5
10
15
20
25
30
35
0
5
10
15
20
25
30
35
Ca(4Cd+ Ca)x10-8
Ca(4Cd+ Ca)x10-8
Figure 3. The 1:1 and 1:2 Benesi-Hildebrand plot of [(HMDA)(QL)],
-
+
.
[(HMDA)(PA)2], [HMDA] 2Br2 and [HMDA-I] I3 charge-transfer complexes.
3

[(HMDA)(QL)] complex
[(HMDA)(PA)2] complex
3500
3000
2500
2000
1500
1000
500
3500
3500
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Wavenumbers (cm-1)
[HMDA-I]+I3- complex
[HMDA].2Br2 complex
3000
2500
2000
1500
1000
500
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Wavenumbers (cm-1)
[(HMDA)(ZnCl2)] complex
[(HMDA)(SnCl4)] complex
3000
2500
2000
1500
1000
500
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Wavenumbers (cm-1)
Figure 4. Infrared spectra of HMDA charge-transfer complexes.
4

0.08
0.06
0.04
0.02
0.00
[(HMDA)(PA)2] complex
[(HMDA)(QL)] complex
0.6
0.5
0.4
0.3
0.2
0.1
0.0
3500
3000
2500
2000
1500
1000
500
0
3500
3000
2500
2000
1500
1000
500
0
Wavenumbers (cm-1)
[HMDA-I]+I3- complex
Wavenumbers (cm-1)
0.020
0.015
0.010
0.005
0.000
[HMDA].2Br2 complex
0.4
0.3
0.2
0.1
0.0
600
500
400
300
200
100
0
500
400
300
200
100
0
Wavenumbers (cm-1)
[(HMDA)(SnCl4)] complex
Wavenumbers (cm-1)
0.025
0.020
0.015
0.010
0.005
0.000
0.10
0.08
0.06
0.04
0.02
0.00
[(HMDA)(ZnCl2)] complex
600
500
400
300
200
100
0
600
500
400
300
200
100
0
Wavenumbers (cm-1)
Wavenumbers (cm-1)
Figure 5. Laser Raman spectra of HMDA charge-transfer complexes.
5

Figure 6. 1H NMR spectrum of (A) [(HMDA)(QL)], (B) [(HMDA)(PA)2], (C)
[(HMDA)(SnCl4)] and [(HMDA)(ZnCl2)] CT complexes.
6

7
6
5
4
3
2
1
0
10
8
6
4
2
[(HMDA)(PA)2] complex
[(HMDA)(QL)] complex
100 200 300
0
0
100
200
300
400
500
600
700
800
0
400
500
600
700
800
Temp [ OC]
Temp [ OC]
14
12
10
8
16
14
12
10
8
6
6
4
4
2
2
[HMDA-I]+I3- complex
100 200 300
[HMDA].Br2 complex
100 200 300
0
0
0
400
500
600
700
0
400
500
600
700
800
Temp [ OC]
Temp [ OC]
16
14
12
10
8
7
6
5
4
3
2
6
[(HMDA)(ZnCl2)] complex
[(HMDA)(SnCl4)] complex
0
100
200
300
400
500
600
700
800
0
100
200
300
400
500
600
700
800
Temp [ OC]
Temp [ OC]
Figure 7. TG curves of HMDA charge-transfer complexes.
7

-12
-13
-14
-15
-16
CR method
[HMDA].2Br2
[HMDA-I]+I3-
[(HMDA)(QL)]
[(HMDA)(PA)2]
[(HMDA)SnCl4)]
[(HMDA)(ZnCl2)]
0.0016
0.0020
0.0024
0.0028
1000/T (K-1)
0.0
-0.5
-1.0
-1.5
HM method
[HMDA].2Br2
[HMDA-I]+I3-
[(HMDA)(QL)]
[(HMDA)(PA)2]
[(HMDA)SnCl4)]
[(HMDA)(ZnCl2)]
-75
-50
-25
0
25
50
75

Figure 8. Kinetic curves for HMDA charge-transfer complexes (CR; Coats-Redfern
equation, HM; Horowitz-Metzger (HM) equation).
8

25.388o
28.121o
21.471o
3000
2500
2000
1500
1000
500
3000
2500
2000
1500
1000
500
[(HMDA)(PA)2] complex
[(HMDA)(QL)] complex
0
0
0
10
20
30
40
50
60
70
0
10
20
30
40
50
60
70


9.475o
32.525o
1200
1000
800
600
400
200
0
4000
3000
2000
1000
0
[HMDA].Br2 complex
[(HMDA)(SnCl4)] complex
0
10
20
30
40
50
60
70
0
10
20
30
40
50
60
70


9.137o
2500
2000
1500
1000
500
[(HMDA)(ZnCl2] complex
0
0
10
20
30
40
50
60

Figure 9. XRD spectra of HMDA CT complexes.
9

2400
2100
1800
1500
1200
900
600
300
0
0.00
1.00
2.00
3.00
4.00
5.00
keV
6.00
7.00
8.00
9.00
10.00
Figure 10A. SEM images and EDX spectrum of [(HMDA)(QL)] complex.
10

3600
3200
2800
2400
2000
1600
1200
800
400
0
0.00
0.40
0.80
1.20
1.60
2.00
keV
2.40
2.80
3.20
3.60
Figure 10B. SEM images and EDX spectrum of [(HMDA)(PA)2] complex.
11

8000
7200
6400
5600
4800
4000
3200
2400
1600
800
0
0.00
1.00
2.00
3.00
4.00
5.00
keV
6.00
7.00
8.00
9.00
10.00
.
Figure 10C. SEM images and EDX spectrum of [HMDA] Br2 complex.
12