Supramolecular spectral studies on metal-ligand bonding of novel quinoline azodyes

Article abstract of DOI:10.1016/j.saa.2013.07.053

(Graph Presented) A series of novel bidentate azodye quinoline ligands were synthesized with various p-aromatic amines like p-(OCH₃, CH ₃, H, Cl and NO₂). All ligands and their complexes have been characterized on the basis of elemental analysis, IR, ¹H and ¹³C NMR data and spectroscopic studies. IR and ¹H NMR studies reveal that the ligands (HL_n) exists in the tautomeric azo/hydrazo form in both states with intramolecular hydrogen bonding. The ligands obtained contain N=N and phenolic functional groups in different positions with respect to the quinoline group. IR spectra show that the azo compounds (HL_n) act as monobasic bidentate ligand by coordinating via the azodye (-N=N-) and oxygen atom of the phenolic group. The ESR (g _∥ and g_⊥) and bonding α² parameters of the copper ion were greatly affected by substituting several groups position of ring of quinoline and p-aromatic ring. The ESR spectra of copper complexes in powder form show a broad signal with values in order g _∥ > g_⊥ > g_e > 2.0023. The value of covalency factor β and orbital reduction factor K accounts for the covalent nature of the complexes. All complexes possessed an octahedral and square planar geometry. The thermal properties of the complexes were investigated using TGA and DSC. It is found that the change of substituent affects the thermal properties of complexes.

Full text of DOI:10.1016/j.saa.2013.07.053

Accepted Manuscript
Supramolecular spectral studies on metal-ligand bonding of novel quinoline
azodyes
M.A. Diab, A.Z. El-Sonbati, A.A. El-Bindary, A.M. Barakat
PII:
S1386-1425(13)00801-9
http://dx.doi.org/10.1016/j.saa.2013.07.053
SAA 10797
DOI:
Reference:
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Spectrochimica Acta Part A: Molecular and Biomo‐
lecular Spectroscopy
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Revised Date:
Accepted Date:
25 May 2013
15 July 2013
22 July 2013
Please cite this article as: M.A. Diab, A.Z. El-Sonbati, A.A. El-Bindary, A.M. Barakat, Supramolecular spectral
studies on metal-ligand bonding of novel quinoline azodyes, Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy (2013), doi: http://dx.doi.org/10.1016/j.saa.2013.07.053
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Supramolecular spectral studies on metal-ligand bonding of novel
quinoline azodyes
M.A. Diab, A.Z. El-Sonbati+, A.A. El-Bindary, A.M. Barakat*
Chemistry Departments, Faculty of Science, Damietta University, Damietta, Egypt
Abstract
A series of novel bidentate azodye quinoline ligands were synthesized with
various p-aromatic amines like p-(OCH3, CH3, H, Cl and NO2). All ligands and their
1
complexes have been characterized on the basis of elemental analysis, IR, H and 13C
1
NMR data and spectroscopic studies. IR and H-NMR studies reveal that the ligands
(HLn) exists in the tautomeric azo/hydrazo form in both states with intramolecular
hydrogen bonding. The ligands obtained contain N═N and phenolic functional groups in
different positions with respect to the quinoline group. IR spectra show that the azo
compounds (HLn) act as monobasic bidentate ligand by coordinating via the azodye
(-N═N-) and oxygen atom of the phenolic group. The ESR (g|| and g) and bonding α2
parameters of the copper ion were greatly affected by substituting several groups position
of ring of quinoline and p-aromatic ring. The ESR spectra of copper complexes in
powder form show a broad signal with values in order g|| > g> ge > 2.0023. The value of
covalency factor β and orbital reduction factor K accounts for the covalent nature of the
complexes. All complexes possessed an octahedral and square planar geometry. The
thermal properties of the complexes were investigated using TGA and DSC. It is found
that the change of substituent affects the thermal properties of complexes.
Keywords: (4-alkylphenylazo)-5-sulfo-8-hydroxyquinolines (HLn), Hydrogen-bonding
stability, Supramolecular complexes, Spectroscopic studies, ESR, TGA and DSC.
___________
*Abstracted from her M.Sc.
+ Corresponding author Tel.: +20 1060081581; fax: +20 572403867.
E-mail address: elsonbatisch@yahoo.com (A.Z. El-Sonbati).
1

1. Introduction
Copper(II) complexes show a variety of stereochemistries and coordination
numbers. Their flexibility is due to the d9 configuration, where the nature of the dx2-y2
orbital containing the unpaired electron imposes a strong Jahn-Teller effect [1].
The copper(II) ion exhibit great affinity for nitrogen donor ligands and quinoline
complexes continue to be an intense field of investigation in coordination chemistry as
bioinorganic models of metaloproteins and as catalysts of important industrial processes
[2-5]. More recently the interest in the chemistry of azoquinoline ligand has been reborn
due to their applications in supramolecular chemistry and new materials [4-10].
The coordination chemistry of quinoline and its azo derivatives have much
attention by virtue of their applicability as potential ligands for large number of metal
ions [7-10]. The metal chelates thus produced have wide applications in the dye industry,
as analytical reagents for the microdetermination of metals and in biological uses [11].
Different methods were reported for the synthesis of azoquinoline [5-10]. The electronic
and infrared (IR) spectra of a large number of azoquinoline were investigated for
structure elucidation [8-10]. It was confirmed that these compounds exist mainly as a
chelated hydrazo-keto structure but under the influence of high electron accepting
substituents and in highly polar solvents, the azoquinoline compound was liable to exist
in hydrazone-azo tautomeric equilibrium [5-7]. Nuclear magnetic resonance was used as
a tool to determine the tautomeric forms of (4-methoxyphenylazo)-5-sulfo-8-
hydroxyquinoline (HL1), (4-methylphenylazo)-5-sulfo-8-hydroxyquinoline (HL2), (4-
phenylazo)-5-sulfo-8-hydroxyquinoline (HL3), (4-chlorophenylazo)-5-sulfo-8-hydroxy
quinoline (HL4), (4-nitrophenylazo)-5-sulfo-8-hydroxyquinoline (HL5) and several
related azo heterocyclic in different solvents [10].
The design of new coordination supramolecules and polymers based on transition
metals compounds and multidentate organic ligands has attracted much interest in recent
years [12,13]. Depending on the nature of the metal and the coordination behaviour of the
ligand one can develop synthetic strategies to influence the one-, two- or three-
dimensional arrangement in the crystal in a more direct way [14]. Furthermore, it is now
realized that weak hydrogen bond(s) that involve O-H…O and/or N-H...O hydrogen bond
stacking interactions also play a significant and predictable structure determining role.
(4-alkylphenylazo)-5-sulfo-8-hydroxyquinoline (HLn) and their related
compounds (Fig. 1 A`) have been extensively used as ligands in transition metal
2

coordination chemistry [15-19]. Ease of synthesis, favorable steric arrangement and
variability of donor sites that these ligands possess with suitable constituents, make this
family an excellent candidate for constructing new families of complexes which are of
great intriguing interest for the coordination chemistry. Although, no structural chemistry
or coordinating and biological studies have been reported on ligands containing both azo
and quinoline function groups, data from our laboratory [15-19] have demonstrated that
the bis-bidentate azodyes ligands play a key role in making new complexes with
transition metal ions. However, little is known concerning the constituents of these
complexes, as well as the chemistry involved in their preparation, or the structural and
coordination in such complexes. It has been shown from the IR spectral data [14-22] that
the hydrogen bonding plays an important role in biological systems. Due to the higher
number of hydrogen bonds, the Watson-Crick pair of guanine and cytosine is more stable
than the thymine-adenine complex [23]. Moreover, Jorgensen and El-Sonbati et al. [15-
19,24] found out that the stability of multiple hydrogen bonded "dimmers" depends not
only on the number of hydrogen bonds but also on the hydrogen bonding pattern. The
importance of clarifying the structure and stability of hydrogen-bonded complexes has
opened up an area of surface science that has attracted a considerable attention in the
environmental chemistry.
In this paper, we investigate the supramolecular chemistry of azo 8-hydroxy
quinoline derivatives regarding the metal coordination as well as the behavior of
hydrogen bonding of these molecules. These are achieved by reporting the studies of (i)
the synthesis of novel (4-alkylphenylazo)-5-sulfo-8-hydroxyquinoline (HLn) ligands, (ii)
the synthesis of Cu(II)/Ni(II) complexes derived from this ligands, (iii) determining the
vibrational mode of bonding, stability and structures of the hydrogen-bonding
complexes, (iv) investigating the stereochemistry of the complexes based on the
electronic spectra and other measurements, and (v) as well as study the thermal
gravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC) of
Cu(II) complexes.
In addition to the above mentioned aims, we will discuss the previous studies of
hydrogen bonding [16-18] and compare them with the results of the present paper in
order to provide a better explanation and justification to the chemical behavior of such
3

complexes and biological allowing the reversible formation of aggregates which are non-
covalently linked.
2. Experimental
2.1. Synthesis of ligands
The synthesis of ligand is summarized in Scheme 1. However detailed procedure
is given in the following reactions:
SO3H
SO3H
N
HCl
0-5οC
.
H2O
NaNO2
-
/
OH
N+
Cl
R
N
HCl
R
NH2
+
N
PH=10-12
0-5οC
N
N
OH
R
:
SO3H
N
SO3H
X
N
O
O
=
M
N
HO
N
N
N
N
N
N
O
R
R
O
H
X
H
Azo form
Metal complexes
(b)
Hydrazo form
(a)
M = Cu(II); X = nil
M = Ni(II); X = H2O
n=1, R = OCH3 (HL1); n=2, CH3 (HL2); n=3, H (HL3); n=4, Cl (HL4); and n=5, NO2 (HL5)
Scheme 1. The formation mechanism of azodye ligands (HLn).
The resulting formed ligands are:
(4-methoxyphenylazo)-5-sulfo-8-hydroxyquinoline (HL1)
(4-methylphenylazo)-5-sulfo-8-hydroxyquinoline (HL2)
(4-phenylazo)-5-sulfo-8-hydroxyquinoline (HL3)
(4-chlorophenylazo)-5-sulfo-8-hydroxyquinoline (HL4)
(4-nitrophenylazo)-5-sulfo-8-hydroxyquinoline (HL5)
4

(4-alkylphenylazo)-5-sulfo-8-hydroxyquinolines (HLn) were typically prepared by
adding a 25 mL of distilled water containing hydrochloric acid (12 M, 2.68 mL,
32.19 mmol) were added to aniline (10.73 mmol) or p-derivatives. To the resulting
mixture, stirred and cooled to 0 ºC, a solution of sodium nitrite (10.73 mmol, in 20 mL of
water) was added drop wise. The so-formed diazonium chloride was consecutively
coupled with an alkaline solution (sulfonate) (10.73 mmol) in 20 mL of ethanol
containing 602 mg (10.73 mmol) of potassium hydroxide. The red precipitate, which
formed immediately was filtered and washed several times with water. The crude product
obtained was purified by crystallization from hot ethanol (yield ~ 60-80%). The
analytical data confirmed by expected composition, (Table 1). The ligands were also
characterized by 1H & 13C NMR and IR spectroscopy.
2.2. Synthesis of metal complexes
The appropriate ligand (0.01 mol) dissolved in ethanol (20 cm3) was added
dropwise into an a ethanolic solution (20 cm3) of metal salt (0.01 mol) with stirring. After
the complete addition, 0.50 g of sodium acetate was added to the solution and the
reaction mixture was refluxed for 2 h. The solution, thus obtained was concentrated to
half of its original volume by evaporation using a hot plate and allowed to cool to room
temperature. During this, a microcrystalline solid was separated, which was isolated by
filtration, washed with hot water followed by ethanol, ether and dried in air.
2.3. Analysis and physical measurements
All chemicals used were of highest available purity, were BDH, Analar, Sigma, or
Merck products. Copper chloride dihydrates, nickel chloride hexahydrates, 5-sulfo-8-
hydroxyquinoline,
4-alkylaniline,
absolute
ethyl
alcohol,
diethylether,
dimethylformamide and dimethylsulfoxide. Concentrated nitric acid was reagent grade
and used as supplied. Microanalyses of carbon, hydrogen, nitrogen and sulfur contents
were performed in the Microanalytical Center, Cairo University, Egypt. Metal contents
were determined complexometry after complete decomposition of their complexes with
concentrated nitric acid in a Kjeldal flask, while the chlorine content was determined
gravimetrically as AgCl [25]. Molar conductance measurements were measured in
solutions of the metal complexes in DMF (10-3 M) using using Sargent Welch scientific
Co., Skokie, IL, USA. X-ray diffraction patterns of [Cu(L5)2], is recorded on X-ray
5

diffractometer (Philips x` pert) with Ni-filtered CuK α-radiation (λ = 1.5418 Ao) in the
range of diffraction angle (2θo = 4-70o). The applied voltage and the tube current are 30
1
KV and 30 mV, respectively. The H-NMR spectrum was obtained with a JEOL FX90
Fourier transform spectrometer with DMSO-d6 as the solvent and TMS as an internal
reference. Infrared spectra were recorded as KBr pellets using a Pye Unicam SP 2000
spectrophotometer. Ultraviolet-Visible (UV-Vis) spectra of the compounds were
recorded in nuzol solution using a Unicom SP 8800 spectrophotometer. The magnetic
moment of the prepared solid complexes was determined at room temperature using the
Gouy’s method. Mercury(II) (tetrathiocyanato)cobalt(II), [Hg{Co(SCN)4}], was used for
the calibration of the Gouy tubes. Diamagnetic corrections were calculated from the
values given by Selwood [26] and Pascal’s constants. Magnetic moments were calculated
using the equation, eff.=2.84 [T coor.]1/2. ESR measurements of powdered samples were
M
recorded at room temperature (Tanta University, Egypt) using an X-band spectrometer
utilizing a 100 kHz magnetic field modulation with diphenyl picrylhydrazyle (DPPH) as
a reference material. Thermal properties are investigated using Shimadzu Thermal
Analyzer with a scan rate 10.0 oC/minute in air atmosphere in the temperature range 46-
800 ºC.
3. Results and discussion
The physical properties of the ligands and the complexes are grouped in Tables 1
and 2 .The elemental analysis data (Table 2) was found to agree well with the proposed
formula of the complexes and confirmed the [Cu(Ln)2] composition with 1:2 [M:L] ratio.
Generally all the complexes were colored, quite stable towards air, insoluble in water and
common organic solvents but soluble in coordinated solvents. The Molar conductance
values (Λm) of the complexs in DMSO (1×10-3 M) solution at 25 ºC were found in the
range 8.58-14.90 Ω-1 mol-1 cm2. These low values indicated that all of the complexes
have non-electrolytie nature [27]. Hence, the molecular conductance measurements of the
chelates confirm the proposed general formulae of those chelates as suggested depending
upon the results of elemental analyses and IR spectra.
As shown in Tables 1 & 2, the values of yield % is related to the nature of the
p-substituent as they increase according to the following order p-(NO2 > Cl >H > CH3 >
OCH3). This can be attributed to the fact that the effective charge experienced by the
d-electrons increased due to the electron withdrawing p-substituent (HL4 and HL5) while
6

it decreased by the electrons donating character of (HL1 and HL2). This is in accordance
with that expected from Hammett’s constants (σR) as in Fig. 2 correlate the yield %
values with (σR), it is clear that all these values increase with increasing σR. The above
results show clearly the effect of substitution in the para position of the benzene ring. It
is important to note that the existence of a methyl and/or methoxy group enhances the
electron density on the coordination sites and simultaneously decreases the values of
yield %.
The X-ray diffraction spectrum of complex [Cu(L5)2] (9) is shown in Fig. 3.
Spectrum of compound (9) exhibit many peaks with different intensities indicating
polycrystalline nature.
3.1. Structure and stability
The presence of a salfonate group in the quinoline ring confers special
characteristics to the ligand, introducing changes in spectroscopic and structural
properties of the metallic complexes.
It has been known [20-22,28,29] that 8-hydroxyquinoline exists, in solution, in a
monomer dimmer equilibrium. The results of this work suggest that in the monomeric
form a strong intramolecular hydrogen bond is present. This is in agreement with a
previous results [20-22,30]. The two such monomers lead to the dimmer by forming an
additional hydrogen bonding yielding the bifurcated hydrogen bonds and H-N-H nitrogen
bridges (Fig. 1 A).
In addition to the two bifurcated intra/intermolecular OH---N hydrogen bonds
(Scheme 1 and Fig. 4), two more intermolecular hydrogen bonding interactions are
observed between nitrogen atom of azo/azomethine group and phenolic hydroxyl
hydrogen oxygen atom. This additional H-bonding does not influence the intramolecular
distance which shows a band at a lower frequency than the intermolecular interaction.
Reason for this behavior might be due to the additional H-bond which influences the
hydrogen bonding ability of the sulfonyl group by electronic and/or steric factors. The
overall structure of the dimmer is close to planar with a slight shift of the two quinoline
units from the plane.
The two hydroxyquinoline units of the dimmer (Fig. 1 B` & 1 C2) are in one
plane. The intermolecular as well as intramolecular hydrogen bonding occur between the
hydroxyl group and the quinoline nitrogen atom. The intermolecular hydrogen bond
7

distance is shorter than the intramolecular one. This observation was also reported for
other 8-hydroxyquinoline dimmers and might be due to an unflavored small O-H-N angle
for the intramolecular interaction [29-31].
Hydrogen bonding represents one of the most versatile interactions that could be
used for molecular recognition. In view of the large differences in the substituent effects
(e.g., the Hammett-type substituent constants for p-positions and salfonyl group); it might
be possible to tune the strength of the hydrogen bond effectively by linking the hydrogen-
bonding site to a reaction center through a conjugated spacer, and by altering the charge
state of the reaction center in the solution. At the hydrogen-bonding end, azo/azomethine
is used as a proton acceptor to form a hydrogen bond with OH group of ligand.
An electron-withdrawing bridge would be expected to increase the acidity of the
proton donor and hence increase its binding ability. As the electron-withdrawing
character of an azo group is relevant to the interesting signal-amplifying behavior [32].
The results indicate that in the HLn, the effects of the bridges are electron-withdrawing
and electron-donating, respectively. Accordingly, the efficiency of sp2-hybridized bridges
is N═N > C═N.
Coggeshall [33] and El-Sonbati et al. [16-22] found three kinds of bonded –OH
structures on the basis of the frequencies: i) only the oxygen is in the bridge while the
hydrogen is free, ii) a polymer chain is formed in which both hydrogen and oxygen atoms
participate in the hydrogen bond, iii) dimmer associates are formed. Intramolecular hydrogen
bond between the nitrogen atom of C═N(CNPy)/–N═N- (five/six-membered) system and
hydrogen atom of the phenolic hydroxyl hydrogen atom and hydrogen (C8-OH) are
illustrated in (Fig. 1 B, C and C`). Intermolecular hydrogen bonding can form cyclic dimmer
through the O-H---OH type between C8-OH/N═N of one molecule and C8-OH/N═N group
of another one (Fig. 4 H, G & F) and/or ----N type between C8-OH of one molecule and
CNpy/–N═N- of another (Fig. 4 D).
In general, hydrogen bonding involving OH groups are proton donors and their O
atoms are proton acceptors. Both intra and intermolecular OH-N may form a number of
structures in a simultaneous equilibrium.
8

3.2.
1H & 13C NMR spectra
1
The H NMR spectra of all the ligands were recorded in DMSO-d6 at room
temperature. The signal due to methyl and methoxy proton appeared as singlet at 1.55
and 3.84 ppm, respectively. In the aromatic region, a few doublets and in few cases some
overlapping doublets/multiplets are observed in the range δ 6.78-8.40 ppm (J = 12 Hz).
These doublets/multiplets are due to aryl protons of three benzene rings. Another singlet
corresponding to one proton for all compounds is observed in the range δ ~ 9.2-10.40
ppm. This signal disappeared when a D2O exchange experiment was carried out. It can be
assigned either to OH or NH, in either case; it is strongly deshielded because of formation
of strong hydrogen bonding with the other atom (N/O) (Figs. 1 & 4). It may be noted that
the integration of this signal perfectly matches with one proton and there is no other
fragment(s) of this signal, which suggests that only one tautomeric form of the ligand
exist in solution under the experimental conditions. Comparing with the solid state study,
we prefer to assign this signal to OH, however, assignment of this peak to NH cannot be
ruled out provided solid state structural evidence is not considered [34]. As reported in a
previous study [36], this hydrogen bonding leads to a large deshielding of these protons.
The shifts are in the sequence: p-NO2 > p-Cl > H > p-OCH3 > p-CH3. The appearance of
signals due to HC═N [~ 8.94 ppm (1H)] protons of the same positions in the ligand.
1
The assignments made from H & 13C NMR spectrum of the ligand (HL5) are
shown in Table 3.
3.3.
Infrared spectra
The significant IR bands with the tentative assignments of the HLn and copper(II)
Characterization of the complexes
complexes are presented in Table 4. The characteristic IR bands of the complexes differ
from their uncomplexed HLn and propvide significant indications regarding the
coordination and bonding sites of the ligand. The IR spectroscopy is known to be a
powerful tool for structural determinations of the ligand and metal chelates.
By tracing the IR spectra of the azo compounds, no NH2 stretching vibrations are
apparent. This supports the formation of azodye ligands. The mode of bonding of the HLn
to the metal ions was elucidated by investigating the IR spectra of the complexes on the
basis of a comparative analysis of the results with respect to literature data of related
9

systems. The positions of the most relevant and characteristic bands are due to:, i) azo
nitrogen, ii) OH group and iii) CNPy group.
Much has been discussed about the –OH stretching frequency and intensity of
phenol derivatives depending on many factors (e.g., constituents, medium, etc.). Hoyer
[37] reviewed the relationship between the hydrogen bond band and the –OH stretching
frequency. In addition to that, there have been many other research workers who
examined the problem in finer and more quantitative data. This has attracted our attention
to acquire the IR technique to explain and justify the bonding in this study.
The IR spectrum of the ligands showed bands at 3390-3410 and 1285-1295 cm-1
that may be assigned to υ(OH) and υ(C-O) phenolic, respectively. This latter band was
shifted to higher frequencies (1345-1358 cm-1) in the complexes, confirms that the
C8-OH group takes part in the complex formation through O- owing to strong hydrogen
bonding both intramolecular [O-H..N (Fig. 1 B), O-H…N (Fig. 1 C)] and intermolecular
hydrogen bonding of the O-H..N type between N=N of one molecule and OH group of
another one (Fig. 4 E), the frequency of the hydrogen bonded OH is probably lowered to
considerable extent and overlaps with the OH vibrations, thus appearing as a broad band in
the region 3390-3410 cm-1 [10,28]. All the complexes exhibit (C=C) in 1498 cm-1. The
presence of a p-substituted benzene ring in the ligands as well as in the complexes is
indicated by strong and sharp bands around 720-740 cm-1. Bands at 2950-3040 cm-1 for the
ligands and at ~ 2960-3040 cm-1 for the complexes are assigned to (C-H) vibrations of the
aromatic system. This is also in agreement with the data reported by El-Sonbati and co-
workers [16-22,36].
The assignment of the 1570 cm-1 (N=N) and 1585 cm-1 (CN) (non-bonded groups)
stretching modes is based on our previous work [16-22], El-Sonbati et al. [35] and Henry
et al. [38]. Comparing the position of these bands for the different ligands, it is obvious
that ring substitution results in a shift to lower frequencies. This behavior can probably be
explained by the lowering in the electronic density caused by electronegative groups.
1570 cm-1 (N=N) occurs at higher frequency in the ligands than in the respective
complexes by 20-35 cm-1, revealing its involvement in complexation [20-22]. In this
case, the decrease of the electronic density of the ring may be correlated with the donor
character of the oxygen and/or N atoms in the M-O and M-N bond. These is also another
correlation, observed when comparing the position of the (N=N) band in the free ligand
10

and in the corresponding complexes. In these cases, the higher shift corresponds to higher
electronegativity. Evidently, the presence of electronegative groups in the ring affects the
donor capacity of the O and N atoms. The coordination of the azo-nitrogen and phenolic
oxygen were further supported by the appearance of two non-ligand bands. According to
Stefov et al. and El-Sonbati el al. [35,39], coordinated water should exhibit frequencies at
~ 830, ~ 570 and ~ 495 cm-1. The absence of spectral bands in these regions in the
spectral of complexes 1, 3, 5, 7 and 9 indicates that the water molecules in these
complexes are not coordinated. These were not present in the spectra of the ligands. From
the above arguments together with the elemental analysis, it was concluded that ligands
behave as a monobasic bidentate azodye ligands with NO donor sites.
These comparisons show that the electronegativity of the ring substituents produces
not only a decrease of the electronic density over the ring, generating a dimination in the
(N=N) stretching frequency, but also causes a lowering of the donor character of the
N atom.
3.4.
Electronic spectra
In general, most of the azo compounds give spectral localized bands in the
wavelength range 46620-34480 and 31250-270370 cm-1. The first region is due to the
absorption of the aromatic ring compared to 1Bb and 1Lb of mono substituted benzene and
the second region is due to the conjugation between the azo group and the aromatic
nuclei with intermolecular charge transfer resulting from -electron migration to the diazo
group from electron donating substituents. The p-substituents increase the conjugation
with a shift to a longer wavelength. Most of the simple p-substituted compounds are in
the azoid form in cyclohexane and alcohols. The substituted effect is related to the
Hammett's constants values [13,21]. The position of the - * transition of the azo groups
remains as one of the more interesting unanswerable questions of molecular
spectroscopy. For azo benzenes, as the possibilities of the mesomerism became greater,
the stabilization of the excited state is increased relative to that of the ground state and a
bathochromic shift of the absorption bands follow [13]. Based on MO
theory [40] the
energy terms of the molecular orbital became more closely spaced as the size of the
conjugated system increases. Therefore, with every additional conjugated double bond
the energy difference between the highest occupied and the lowest vacant
11
-

electron level became smaller and the wavelength of the first absorption band
corresponds to this transition is increased. The azo group can act as a proton acceptor in
hydrogen bonds [13,21]. The role of hydrogen bonding in azo aggregation has been
accepted for some time.
HLn exhibited bands at 32500-32150 cm-1 (CN) ( - *), 33450-33340 cm-1 (H-
bonding and association), 40038-39460 cm-1 (phenyl) (Ph-Ph*, - *) [13,41] and 29340-
29230 cm-1 transition of phenyl rings overlapped by composite broad ( - *) of azo
structure. The band due to the n → * transition obtained in the visible region is
associated mainly with the color of the respective compound [13]. The band due to
*
transition moves to lower energy. The last band in the visible range is considered as being
due to an intramolecular charge transfer involving the rote molecule in analogy to the
behavior of azomethine compound [36].These shifts or the disappearance of the bands are
indicative of coordination of the ligands to M(II). The position of their bands is varied
from one dye to the other which may be due attributed to the p-phenylazo substituent
variable donating power.
3
The nickel(II) complexes displayed three bands 8860-8890 cm-1 [(3A2g → T2g
(F)]υ1, 14840-14960 cm-1 [3A2g→3T1g (F)]υ2 and 25000-25105 cm-1 [3A2g→3T1g (P)]υ3,
transitions, suggestive of octahedral geometry [42]. The value of transition ratio υ2/υ1 and
β lies in ~ 1.677 and ~ 0.85 cm-1 providing further evidence for octahedral geometry for
nickel(II) complexes. The β values for the complexes are lower than the free ion value,
thereby indicating orbital overlap and delocalization of d-orbitals. The β-values obtained
are less than unity suggestive considerable amount of covalent character of the metal-
ligand bonds. Nickel(II) complexes are paramagnetic and the room temperature magnetic
moment value 2.94-3.15 B.M. corresponding to three unpaired electron and in the range
of octahedral nickel(II) complexes [43].
For square planar complexes, the lower energy d-d bands is usually observed in the
region 18280-16000 cm-1 nm .The complexes reported in the present work show the
lower energy d-d band in the region 15575-13640 cm-1, and this red shift may be due to
the tetrahedral distortion from square planar environment of the Cu(II) ion [44]. This
behavior probably due to the steric effects of the bulky group in the pyridine of the
ligands. Copper(II) complexes are paramagnetic and the room temperature magnetic
moment values ranging from 1.76 to 1.86 B.M. are close to the spin only value of 1.70
12

B.M. indicate the presence of one unpaired electron and demonstrate that these
complexes are monomeric in nature and the presence of metal-metal interaction.
3.5.
ESR spectra
ESR spectra of the polycrystalline powders were recorded at room temperature. The
values of g factors were assessed following the method described by Searl et al. [45] and
are presented in Table 5. The g tensor values of Cu(II) complex can be used to derive the
ground state. In tetragonal and square planar complexes the unpaired electron lies in the
2
2
2
dx -y orbital giving B1g as the ground state with the gll > g > 2.0023 [44]. From the
observed values for complexes (1, 5 & 9), it is clear that gll > g. These data are in
agreement with those obtained from the electronic spectra and confirm the square planar
2
geometry. The term of the fundamental state is thus defined by the obtained dx -y2. The g-
values are related by the expression [44], G = gll-2/g-2. If G > 4.0, then local tetragonal
axes are aligned parallel or only slightly misaligned, the exchange interaction are
2
neglected, if G < 4.0, suggesting dx -y2 ground state with a small exchange coupling [44],
i.e. it is considered the existence of some exchange interaction between the Cu(II)
centers. Thus, in case of complexes (5) and (9), the geometric parameter G = 3.73 and
3.29, respectively, confirmed the existence of some exchange interactions between the
Cu(II) centers. The observed value for the exchange interaction parameter for the
complex (1) was greater than 4.0 (G = 4.24) which confirmed the subsistence of no
exchange interaction between the Cu(II) centers. These results were further supported by
their magnetic moments. The empirical ratio gll/All was frequently used to evaluate
distortions in tetra coordinated copper(II) complexes [44]. The close to 100, indicated a
roughly square planar structure around the copper(II) ion [44] and the values from 170 to
250 indicated a distorted tetrahedral geometry. Hence, the value of the gll/All ratio for
Also, the gll/All value considered as a diagnostic of stereochemistry [44], the range
reported for square planar complexes, are 105 to 135 cm-1 and for tetragonal distorted
complexes 150 to 250 cm-1. The gll/All values lie just within the range expected for the
complexes.
2
The g-values of the copper(II) complexes with a B1g ground state (gll > g) may
be expressed [21,44] by
gll = 2.0023 – (8K2 λo/ΔExy)
ll
13

g = 2.0023 – (2Kλo/ΔExz)
where Kll and K are the parallel and perpendicular components respectively of the orbital
reduction factor (K), λo is the spin-orbit coupling constant for the free copper, ΔExy and
ΔExz are the electron transition energies of 2B1g → 2B2g and 2B1g → 2Eg. From the above
relations, the orbital reduction factors (Kll, K, K), which are a measure of covalency
[21,44], can be calculated. For an ionic environment, K = 1 and for a covalent
environment K < 1, the lower value of K, the greater is the covalent.
K2 = (g – 2.0023) ΔExz/2λo
Kll = (gll – 2.0023) ΔExy/8λo
K2 = (K2 + 2K2 )/3
ll
The observed Kll (0.68 – 0.77) < K (0.69-0.95) relation indicated the presence of
significant in-plane π-bonding. Thus, the ESR study of the copper(II) complex has
provided supportive evidence to the conclusion obtained on the basis of electronic
spectrum and magnetic moment value.
K for the copper(II) complexes are indicating of their covalent nature [46,47].
Kivelson and Neiman [48] noted environment, gll < 2.3. Theoretical work by Smith [49]
seems to confirm this view. The gll-values reported here show considerable covalent
bonding character [21,44]. Also, the in-plane σ-covalency parameter α2(Cu) was
calculated by
α2 = All/P + (gll – 2.0023) + 3/7 (g – 2.0023) + 0.04
The in-plane and out-of-plane π-bonding coefficients (β2 & β2), respectively are
1
dependent upon the values of ΔExy and ΔExz in the following equations [21]
α2β2 = (g – 2.0023) ΔExz/2λo
α2β2 = (gll – 2.0023) ΔExy/8λo
1
In this work, the complexes show β2 values 1.03, 0.97 and 0.93 indicating a
1
moderate degree of covalence in the in-plane π-bonding, while β2 are 1.05, 1.33 and 1.22
indicating ionic and covalent character of the out-of-plane π-bonding [21].
The ESR study of the Cu(II) complexes has provided supportive evidence to the
conclusion obtained on the basis of electronic spectrum and magnetic moment value. The
plot of (β2)2 and (β1)2 vs. of (σR) gives straight line with increase the value of (σR)
14

decrease (β2)2 and increase the (β1)2 (Fig. 5). It seems that the electron withdrawing p-
substituent increase the positive charge on the metal ion leading to a increase in gll and All
× 10-4 cm-1 and subsequently a increase in (σR) (Fig. 6). In addition, Fig. 7 shows clearly
that the shift is in the direction to that of 2 and giso.
The above results show clearly the effect of electron density on the ESR parameter
on the stereochemistry of Cu(II) complexes. It is important to note that the existence of
high electron density enhances and activate the coordination sites and simultaneously
increases the value of ESR parameters (Table 5).
3.6.
Thermal studies
The thermal properties of the copper(II) complexes of ligands (HLn) were investigated
by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in the
temperature range 46-800 ºC are shown in Fig. 8. The thermal analyses data for the
copper(II) complexes are summarized in Table 6. It can be seen that the TG curves of the
complexes show mass loss down to 150 ºC, indicating the presence of adsorptive solvent
molecules in the coordination sphere. The TG curves of the complexes show that for the
copper(II) complexes three steps of the losses of mass. The DSC data for copper(II)
complexes show many exothermic peaks except [Cu(L5)2] complex show one exothermic
peak as shown in Fig. 8. These peaks can be explained in terms of TGA curves [50].
According to literature the azo bonds in the azo metal complexes breakdown when the
temperature is higher than 250 ºC resulting in the exothermic peaks [51].
For the [Cu(L1)2] complex the recorded DSC curve show three exothermic peaks at ~
338.8, 393.7 and 439.2 ºC. Simultaneously, the recorded DSC curve of the [Cu(L2)2]
complex show three exothermic peaks at 270.3, 364.1 and 520.5 ºC, while the recorded
DSC curve of the [Cu(L3)2] complex show only two exothermic peaks at 281.8 and 453.4
ºC. For the [Cu(L4)2] complex the recorded DSC curve show three exothermic peaks at
279.6, 378.3 and 506.5 ºC and for the [Cu(L5)2] complex the recorded DSC curve show
only one peak at 360.1 ºC (Fig. 8).
The effective activation energies of the thermal degradation of copper(II) complexes were
determined using TGA thermograms. The rate constant of the thermal degradation plotted
according to the Arrhenius relationship equation: [52]
15

lnK = lnA− Ea/RT,
where K is the rate constant of the thermal degradation in the initial stages of
decomposition, A is a constant, R is the gas constant (= 8.314 JK−1 mol−1), T is the absolute
temperature and Ea is the thermal activation energies of degradation. The values of the
thermal activation energies of degradation for copper(II) complexes are calculated from the
slope of the straight line obtained from the plot lnK versus 1/T as shown in Fig. 9. The
values Ea for [Cu(L1)2], [Cu(L2)2], [Cu(L3)2], [Cu(L4)2] and [Cu(L5)2] are found to be 62.27,
141.55, 139.23, 74.79, and 44.70 kJ/mol, respectively. The [Cu(L3)2] and [Cu(L2)2] have the
highest value of Ea. This indicates that, [Cu(L3)2] and [Cu(L2)2] are more thermally stable
than the other complexes [53].
4. Conclusion
The results arising from the present investigations confirm that the hydroxyquinoline
exist, in solution, in a monomer dimer equilibrium. The results suggest that in the
monomeric form a strong intramolecular hydrogen bond is present. Two monomers lead to
the dimer by formation of additional hydrogen bonding yielding the bifureated hydrogen
bonds and H-N-H nitrogen bridges (Fig. 1 A). The dimmer is able to dissociate, while the
intramolecular interaction can be broken if appropriate hydrogen bond acceptors are
attached which acts as competitors to the quinoline nitrogen atoms as was observed (Fig. 1
B`). In conclusion, the results of the present study indicate that the selected (4-
alkylphenylazo)-5-sulfo-8-hydroxyquinoline (HLn) ligands are suitable for building a
supramolecular structure. Moreover, since the azo and/or hydrazo compounds experience
photochemical isomerization and are, therefore, of interest for applicative purposes.
The satisfactory analytical data coupled with the studies presented above indicate that
the complexes under this investigation are of the general composition, [M(Ln)22X] [M = Cu;
X = nil or M = Ni; X = H2O], where HLn = (4-alkylphenylazo)-5sulfo-8-hydroxyquinoline.
In
view
of
the
well
characterized
of
bis[(4-alkylphenylazo)-5-sulfo-8-
hydroxyquinoline]Cu(II)/Ni(II) (involving a monoprotic bidentate (O, N-donor) azo ligand,
square planar/octahedral structures with O, N-donor at the equatorial positions in cis-
arrangement have been proposed for these complexes. It is also clear that these ligands have
high affinity for chelation with metal ions under study due to the increasing charge density
of the metal ion and hence to the increasing of their coordination affinities.
16

The present study demonstrates perfectly an application of IR spectra and molecular
modeling in revealing not only the multiple inter/intramolecular of π–π stacking among the
aromatic groups as well as the hydrophobic interactions among hydrogen bonding species
between hydrazide moieties involving in a series of novel hydrazide based azo quinoline
derivatives.
Further studies with the title ligand, using different metal ions, are in progress and
will be published in course.
17

Table 1
a
Elemental analysis (C, H, S and N) , color and yield (%) of the ligands.
Exp. (Calecd.)%
Yield
Compoundb
Color
M+
C
H
N
S
(%)
HL1
358
Black
Red
60.4
53.88
3.82
(3.62)
12.21
(11.70)
9.35
(C16H13N3O5S)
(53.48)
(8.91)
HL2
343
329
62.0
56.12
3.90
12.72
9.71
(C16H13N3O4S)
(55.98)
(3.79)
(12.25)
(9.33)
HL3
Pale red 64.7
54.87
3.45
13.22
10.10
(9.73)
(C15H12N3O4S)
(54.71)
(3.34)
(12.77)
HL4
363.5
374
Red
68.8
49.64
2.84
11.93
9.21
(C15H10N3O4SCl)
(49.52)
(2.75)
(11.55)
(8.80)
HL5
Dark red 78.0
48.30
2.72
15.30
8.83
(C15H10N4O6S)
(48.13)
(2.67)
(14.97)
(8.56)
aThe excellent agreement between calculated and experimental data supports the assignment
suggested in the present work, insoluble in water and partially in common organic solvents
but soluble in coordinated solvents.
bHL1− HL5 are the ligands as given in Scheme 1.
18

Table 2
Elemental analysisa, yield (%) and magnetic moment data of complexes
Exp.(Calecd.)%
Yield
(%)
μeff.
Compoundb
C
H
N
M
(B.M.)
60.2
41.2
70.2
[Cu(L1)2]
(1)
49.19
7.89
2.95
(3.08) (10.78)
10.57
1.76
2.94
1.78
(49.26)
(8.15)
[NiL1)2(OH2)2]
(2)
47.42
7.45
3.50 10.61
(3.45) (10.36)
3.07 10.89
(3.21) (11.24)
3.68 11.10
(47.37)
(7.24)
[Cu(L2)2]
(3)
51.19
8.34
(51.47)
(8.50)
47.6
72.2
[Ni(L2)2(OH2)2]
(4)
49.45
7.76
2.96
1.82
2.98
1.85
3.13
1.86
3.15
(49.31) (3.60) (10.79) (7.54)
50.12 2.66 11.42 8.62
(50.03) (2.78) (11.67) (8.83)
48.20 3.27 11.46 8.36
(47.96) (3.20) (11.19) (7.82)
45.55 2.17 10.41 5.88
(45.65) (2.28) (10.65) (8.06)
44.06 2.73 10.56 7.50
(43.92) (2.58) (10.25) (7.15)
44.29 2.10 13.60 7.62
(44.47) (2.22) (13.84) (7.85)
42.75 2.94 13.58 7.33
(42.72) (2.85) (13.29) (6.97)
[Cu(L3)2]
(5)
[Ni(L3)2(OH2)2]
(6)
54.0
76.5
[Cu(L4)2]
(7)
56.9
79.0
67.4
[Ni(L4)2(OH2)2]
(8)
[Cu(L5)2]
(9)
[Ni(L5)2(OH2)2]
(10)
aThe excellent agreement between calculated and experimental data supports
the assignment suggested in the present work.
b L1 – L5 are the anions of the ligands HL1 – HL5 as given in Scheme 1.
19

Table 3
13C NMR and 1H NMR spectrum for HL5 (ppm vs. TMS)a.
13C NMR
δ, ppm (C atoms,
peak assignment)
157.7
1H NMR
δ, ppm (H atoms,
peak assignment)
-
b
HL5
14
11
08
02
05
07
04
13
15
06
12
16
03
09
10
157.7
147.2
147.0
146.5
136.9
135.9
124.9
124.9
123.6
121.1
121.1
116.9
128.2
129.4
-
9.87
8.44
-
-
8.77
8.26
8.26
8.26
7.77
7.77
7.38
-
-
a
The excellent agreement experimental data supports the assignment
suggested in the present work.
15
16
-O
11
14
OH
9
N+
N
N
6
7
5
8
O
12
1
N
13
10
O
2
S
3
4
O
HO
20

Table 4
The infrared (cm-1) data of the azodye ligands (HLn) and their complexes
Compounda υ(OH) υ(C-O)
υ(M-O) υ(M-N)
υ(OH2)
HL1
1
3390
1265
1345
1343
1287
1347
1349
1290
1350
1352
1293
1353
1354
1295
1355
1357
-
-
-
-
-
420
465
-
510
508
-
-
2
830,570,495
HL2
3
3395
-
-
425
470
-
518
517
-
-
4
-
833,572,495
HL3
5
3340
-
-
428
476
-
524
522
-
-
6
-
836,575,498
HL4
7
3405
-
-
433
479
-
527
526
-
-
8
-
842,580,505
HL5
9
3410
-
-
-
438
484
532
530
-
10
850,585,508
a Numbers as given in Table 2.
21

Table 5
ESR parameters for the copper(II) complexes
β2
β2
b
b
Complexa gll
gl
giso
All Aiso
G
gll/All ΔExy
ΔExz
Kll
Kl
K
α2
1
1.03 1.05
0.97 1.13
0.93 1.22
1
5
9
2.247 2.06 2.122 126 213 4.24 178
2.255 2.07 2.132 147 226 3.73 153
2.258 2.08 2.139 159 242 3.29 142
18320 19800 0.68 0.69 0.69 0.66
18522 20085 0.71 0.82 0.78 0.73
18585 20212 0.72 0.95 0.87 0.77
aNumbers as given in Table 2.
bExpressed in units of cm-1 multiplied by a factor of 10-4
22

Table 6
The thermal analyses data for the copper(II) complexes.
Complexa
First stage
Second stage
Third stage
Remaining
Wt loss%
after 650 oC
9.9
Temp.
oC
Wt
loss%
10.4
8.2
Temp.
oC
Wt
loss%
29.8
7.8
Temp.
Wt
loss%
45.2
50.1
55.9
49.5
49.9
oC
1
3
5
7
9
190
250
260
254
215
300
331
285
286
270
345
378
419
393
325
26.0
4.7
19. 3
11.5
30.9
13.2
6.3
23.1
6.5
9.5
a Numbers as given in Table 2.
Remaining Wt loss% corresponding to metal oxides.
23

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26

S O 3
H
O
S O 3
H
H O 3 S
O
C o u p lin g
H
H
H
N
P -alk y lam in e, 0 -5 °C
N
2
N
N
N
N
O H
R
O
S O 3
H
(A /
)
(A )
S O 3
H
S O 3
H
S O 3
H
N
N
N
N
N
N
N
O
N
R
O
N
R
H
R
O
H
H
(C /
)
(B )
(C )
R
N
N
N
R
N
O
S O 3 H
H O 3
S
O
I
H
II
N
N
O
II
H
I
N
N
H
N
N
H
H O 3
S
O
S O 3 H
N
R
N
R
(C 2
)
(B /)
Fig. 1. Representation of the dimeric structure and intramolecular hydrogen bond.
27

90
80
70
60
50
40
30
20
HL
n
Cu(II) Complexes
Ni(II) Complexes
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
R
Hammett's substituent coefficient (σ )
Fig. 2. The relation between Hammett's substituent coefficient (σR ) and yield %.
28

10
20
30
40
50
60
2θ0
Fig. 3. XRD patterns for [Cu(L5)2]
29