Stereochemistry of new nitrogen containing heterocyclic compounds.: X. Supramolecular structures and stereochemical versatility of polymeric complexes

Article abstract of DOI:10.1016/S1386-1425(03)00326-3

New polymeric complexes of Cu(II), Co(II), Ni(II) and UO₂(II) with 5-(2¹-carboxyphenylazo)-8-hydroxyquinoline (LH₂) have been prepared and characterized on the basis of analytical, magnetic, ¹H NMR, EPR and electronic spectral studies. Tentative structures for the polymeric complexes are proposed. The important infrared (IR) bands and the main ¹H NMR signals are assigned and discussed relative to the molecular structure. EPR spectrum of copper(II) complex has been studied with a view to assigning their stereochemistries. Various EPR parameters have been calculated. Each metal has six-coordinates in an octahedral environment. The ligand acts as a dibasic (bis-bidentate chelating agent coordinating through CN, N=6;N, COOH and OH groups by replacement of a proton from the two latter groups. Considerable interest has also been focused on the synthesis of the azo compounds and its polymeric metal complex due to its wide potential applications. The thermal decomposition behavior of the complexes is also discussed.

Full text of DOI:10.1016/S1386-1425(03)00326-3

Spectrochimica Acta Part A 60 (2004) 965–972
Stereochemistry of new nitrogen containing heterocyclic compounds.
X. Supramolecular structures and stereochemical versatility of polymeric
complexesଝ
A.Z. El-Sonbatia,∗, A.A.M. Belalb, S.I. El-Wakeelc, M.A. Hussienb
a
Department of Chemistry, Faculty of Science, Mansoura University, Demiatta, Egypt
b
Department of Chemistry, Faculty of Education, Suez Canal University, Port Said, Egypt
Department of Chemistry, Faculty of Science, Suez Canal University, Ismailia, Egypt
c
Received 5 April 2003; accepted 20 June 2003
Abstract
New polymeric complexes of Cu(II), Co(II), Ni(II) and UO2(II) with 5-(21-carboxyphenylazo)-8-hydroxyquinoline (LH2) have been pre-
1
pared and characterized on the basis of analytical, magnetic, H NMR, EPR and electronic spectral studies. Tentative structures for the
polymeric complexes are proposed. The important infrared (IR) bands and the main 1H NMR signals are assigned and discussed relative to
the molecular structure. EPR spectrum of copper(II) complex has been studied with a view to assigning their stereochemistries. Various EPR
parameters have been calculated. Each metal has six-coordinates in an octahedral environment. The ligand acts as a dibasic (bis-bidentate
=
chelating agent coordinating through CN, N N, COOH and OH groups by replacement of a proton from the two latter groups. Considerable
interest has also been focused on the synthesis of the azo compounds and its polymeric metal complex due to its wide potential applications.
The thermal decomposition behavior of the complexes is also discussed.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Supramolecular structures; Thermal; EPR; Electronic spectra
1. Introduction
studies have been reported for ligands containing both azo
and quinoline functional groups.
In continuation of our theoretical and practical interest in
the synthesis and study of new families of complexes with
quinoline derivatives, much interest has been given to the
synthesis of novel chelating bis-bidentate azodye models.
Transition metal complexes of azoquinoline functional
groups have been interesting owing to the design of new in-
organic materials as well as their various geometry aspects
[1–7].
The azo compounds have also gained much attention be-
cause of their use as models of biological systems [1]. In
our laboratory, the coordinating behavior and chemical equi-
libria of some azo compounds have been investigated in
depth. However, no structural chemistry and coordinating
El-Sonbati and co-workers [2–7] found that the bis-biden-
tate azodye ligands play a key role in our understanding
of the coordination chemistry of transition metal ions. The
major interests of this paper are: (i) the synthesis of new
substituted phenylazo-8-hydroxyquinoline (LH2) ligand, (ii)
the synthesis of Cu(II), Co(II) and Ni(II) complexes de-
rived from this ligand, (iii) assigning the stereochemistry of
the complexes based on the electronic spectra and magnetic
susceptibility measurements, and (iv) knowing the mode of
bonding in these complexes from the infrared (IR) spectral
data.
2. Experimental
2.1. Synthesis of the ligand
ଝ
This article has been abstracted from the M.Sc. thesis of M.A.
Hu∗ssien.
The standard chemicals 8-hydroxyquinoline and 2-car-
boxyaniline was used without any further purification.
Corresponding author. Fax: +20-5740-3868.
E-mail address: elsonbatisch@yahoo.com (A.Z. El-Sonbati).
1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S1386-1425(03)00326-3

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2.2. Preparation of 5-(21-carboxyphenylazo)-8-
hydroxyquinolinato (LH2)
2.6. Measurements
Microanalysis of all the samples was carried out at Cairo
University Analytical Center, and the metal contents of the
polymeric complexes was calculated by standard technique
[2–7]. IR spectra were recorded on a Perkin-Elmer 1340
spectrophotometer. UV-Vis spectra were measured (Nuzol
mull) on a Pye Unicam 8800 spectrophotometer. Magnetic
measurements were carried out at room temperature us-
ing Gouy’s method, employing Hg[Co(SCN)4] for calibra-
tion purposes, and were corrected for dimagnetism by using
Pascal’s constant. Magnetic moments were calculated using
In a typical preparation, 25 ml of distilled water con-
taining hydrochloric acid (12 M, 2.68 ml, 32.19 mmol) was
added to 2-carboxyaniline (1.370 ml, 10.73 mmol). The re-
sulting mixture was stirred and cooled to 0 ◦C to which a
solution of sodium nitrite (740 mg, 10.73 mmol, in 20 ml
of water) was added dropwise. The so-formed diazonium
chloride was consecutively coupled with an alkaline solu-
tion of 8-hydroxyquinoline (1.557 g, 10.73 mmol) in 20 ml
of ethanol containing 602 mg (10.73 mmol) of potassium hy-
droxide. The orange precipitate which formed immediately
was filtered and washed several times with water. The crude
product obtained was purified by recrystallization from hot
ethanol (yield 60.9%). Anal. Calc. for C16H11N3O3: C, 65.5;
H, 3.8; N, 14.3. Found: C, 65.4; H, 3.8; N, 14.6%.
the equation: µeff = 2.84[TχM
]
corr. 1/2. 1H NMR spectra were
obtained on a JEOL Fx 900 Q Fourier transform spectrome-
ter with d6-DMSO as solvent and TMS as internal reference.
EPR measurements of powder samples were recorded at
room temperature (Tanta University, Egypt) using X-band
microwave frequency, as the first derivative, on a JEOL JES
FE 2XG spectrometer utilizing 100 kHz magnetic field mod-
ulation, with diphenyl picrylhydrazy (DPPH) as the refer-
ence material.
2.3. Synthesis of the complex
The Co(II), Cu(II) and Ni(II) salts (Aldrich) were used as
supplied.
Thermogravimetric analysis (TGA) and differential ther-
mal analysis (DTA) were performed as a Mettler TA2 in-
strument using micro-Pt cups. DTA peak areas were used to
calculate enthalpies. The ꢀH value was estimated from the
relation ꢀH = Aka/m, where A is the area, ka the calibration
constant of the apparatus, and m is the sample weight. The
instrument, and the precision expected was calibrated with
standards in the conventional way, and the expected preci-
sion was around 5%. The area under the peak was traced on
high-quality tracing paper, cut out, and weighed. The exper-
imental conditions for the DTA runs were as follows: heat-
2.4. Preparation of the metal complexes
All the metal complexes were prepared using a 1:1
(metal:ligand) molar ratio following a general method.
Hot solutions of anhydrous metal salts (10 mmol) in
EtOH (30 ml) and solutions of the required amount
of 5-(21-carboxyphenylazo)-8-hydroxyquinoline (LH2)
(2.5 mmol) in EtOH (30 ml) were stirred at room tempera-
ture and then maintained at reflux temperature on a water
bath for 3 h. The products were filtered, washed with EtOH,
Et2O and dried in vacuo to give the polymeric complexes
as powders.
ing rate: 2 ◦C min−1, Purge gas dry nitrogen: 100 ml min−1
and sample weight: ca. 10 mg.
,
The halogen content was determined by combustion of
the solid complex (30 mg) in an oxygen flask in the pres-
ence of a KOH–H2O2 mixture. The halide content was then
determined by titration with a standard Hg(NO3)2 solution
using diphenyl carbazone indicator [9].
However, the yield purity of the desired product depends,
to a great extent, on the particular reaction employed. For
example, reaction of LH2 with metal chloride does not oc-
cur even on prolonged refluxing and on the other hand, the
reaction is quit generally, though the yield is usually low.
Reaction metal acetate is found to the most satisfactory.
The yields are 75–90%, depending on the ligand. The pres-
ence of an –OH group at the eight positions in the coupling
2.5. Calculations
In the nickel complex, the following methods (a–d) were
used for the determination of the parameters of the interele-
cronic repulsion and of the ligand field, B and 10Dq [8].
(a) 10Dq = ν1, B = (2ν12 + ν22 − 3ν1ν2)/(15ν2 − 27ν1)
(b) 10Dq = ν1, B = 2ν22 + ν32 − 3ν1ν3 − 27ν1
(c) 10Dq = ν1, B = (ν2 + ν3 − 3ν1)/15
(d) 10Dq = ν1, B = [3ν1 {25(ν3 − ν2)2 − 16ν12}1/2]/75
The methods listed below were followed to determine
10Dq and B in the cobalt complex [8]:
(a) 10Dq = ν2 − ν1, B = (2ν12 − ν1ν2)/(12ν2 − 27ν1)
(b) 10Dq = 2ν1 − ν3 + 15B, B = [−(2ν1 − ν3) {−ν12 +
ν32 + ν1ν3}1/2]/30
(c) 10Dq = (2ν2−ν3)/3+5B, B = [7(ν3−2ν2) 3{81ν32−
16ν2(ν2 − ν3)}1/2]/510
(d) 10Dq = ν2 − ν1, B = (ν2 + ν3 − 3ν1)/15
Fig. 1. Hydrazone form.

A.Z. El-Sonbati et al. / Spectrochimica Acta Part A 60 (2004) 965–972
967
certainly be of lower energy in the H-bonding form due to
a shift of electron charge cloud from the N atom towards
the H atom [9]. Thus, the observed hypsochromic shifts are
consistent with intramolecular H-bonding. Such bonding is
not favored in the corresponding hydrazone form.
∗
A bathochromic shift of the longest wavelength ␲−␲
Fig. 2. Azo form.
transition is expected due to N → M coordination and elec-
tron withdrawal from the N atom [11] relative to the com-
pound in which not much interaction is present.
moiety hinders this reaction, presumably due to the hydra-
zone form [1] (Fig. 1) which destroys the planarity of the
molecule in the azo form. This is expected to affect the sta-
bility of the chelated structure adversely.
3. Results and discussion
All the complexes are stable and decompose at elevated
temperatures and are insoluble in common organic solvents
but soluble in dimethylformamide and dimethylsulphoxide.
The insolubility of the complexes in common organic sol-
vents suggests polymeric character. Non-electrolytic nature
of the complexes is indicated by their low conductance val-
ues (6–10 cm2 ꢁ−1 mol−1). The analytical data (Table 1) are
in agreement with 1:2 metal–ligand stoichiometry.
2.7. Electronic absorption spectra
LH2 and complexes in which the 8-OH group is present
show two absorption bands in the visible region at ∼25,000
and ∼21,739–20,833 cm−1 due to azo-hydrazone tau-
tomerism, well known in such systems [2,10].∗The lower
wavelength band must be assigned to the ␲−␲ transition
of the azo form since all compounds in which the –OH
group is methylated or complexes, thus, exist only in the
azo form, show a single intense absorption at ∼25,000
The electronic spectrum data of the Ni(II) complex
show three bands 9090 cm−1 [2A2g
→
3T2g (F)] (ν1),
14,850 cm−1 [3A2g
→
3T1g (F)] (ν2) and 25,000 cm−1
-1
cm . The ∼21,740–20,830 cm−1 had must, therefore, be
3
[3A2g (F) → T1g (P)] (ν3) transition, respectively [12,13].
The relative strength of the ligand and geometry of the
complex can be illustrated by calculating the parameters
of ligand field splitting (10Dq) and of interelectronic re-
pulse (B) by different methods. The values of 10Dq and B
are used to calculate ν2 and/or ν3 and the data (Table 2)
together with the magnetic moment values suggest an octa-
hedral geometry for the Ni(II) complex [13,14]. Results are
given in Table 1 leading to the following conclusions:
attributed to the hydrazone form in agreement with the ear-
lier observations that the hydrazone form generally absorbs
at a longer wavelength [9].
5-(2ꢀ-Carbomethoxyphenylazo)-8-methoxyquinoline
(Fig. 2), a model for the azo form, shows a single absorp-
tion maximum at 25,974 cm−1 in polar as well as non-polar
solvents.
In the o-carboxy compound, intramolecular H-bonding as
shown in Fig. 3 is possible. Esterification of the –COOH
group is then expected to result in a large hypsochromic
(1) 10Dq and B values for the Ni(II) complex indicate that
∗
the ligand give rise to reasonably strong ligand fields
shift of the absorption maximum because the ␲ level will
Fig. 3. Azohydrazone tautomerism and intramolecular H-bonding for o-carboxy compound(LH2).

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A.Z. El-Sonbati et al. / Spectrochimica Acta Part A 60 (2004) 965–972
Table 1
Analytical data of the isolated complexes
Complexa
Experimental (Calcd., %)
b
C
H
N
M
µeff
[CuL2]n
54.1 (54.2)
50.0 (49.8)
50.0 (49.8)
34.0 (34.2)
2.6 (2.5)
3.3 (3.4)
3.5 (3.4)
1.7 (1.6)
12.1 (11.9)
11.1 (10.9)
11.2 (10.9)
7.7 (7.5)
17.7 (17.9)
15.0 (15.3)
15.1 (15.2)
42.0 (42.4)
1.75
4.35
2.90
0
[CoL2(H2O)2]n
[NiL2(H2O)2]n
[UO2L2]n
a
The abbreviation L represents the deprotonated from of the ligand.
Value per metal ion and measured at room temperature.
b
and lead to reasonably strong covalent bonds. The high
values of 10Dq and B are also consistent with the nitro-
copper(II) is regarded as covalent. The evaluation of EPR
covalency parameters (vide infra) also supports this. Fur-
ther, the value is consistent with the mixed Cu–N and Cu–O
=
gen coordination of C N quinoline azo compound.
(2) The ratio ν2/ν1 lie in the range expected for octahedral
Ni(II) complexes [15].
(3) The best fit is obtained using method (c).
bonded copper complex. Since G = (g − 2)/(g − 2)
|| ⊥
value is higher than 4 in this complex, exchange interactions
are considered to be absent.
The EPR data indicate that g > g , which are accord-
ing to the expectations (not more than two g values) and
the occurrence of an electronic spectrum only one band at
||
⊥
The electronic spectrum of the Co(II) polymer complex
show two bands 15,880 cm−1 [4T1g (F) → A2g (F)] (ν2)
4
and 18,780 cm−1 [4T1g (F) → T1g (P)] (ν3), in an octahe-
4
14,830 cm−1 (2Eg
→
2T2g) of the octahedral copper(II)
dral field [13]. The parameter 10Dq and B have been calcu-
lated by using ν2 and ν3 according to method (c). Method (c)
used to calculate 10Dq, B and ν1 is encouraging owing to:
complex. Jørgensen [18] has used the ratio νꢀ(Cu)/νꢀ(Ni) be-
tween the wave number of the principle band of a Cu(II)
complex and wave number of the first spin allowed band of
the high spin octahedral Ni(II) complex, with the same lig-
and, as a measure of tetragonality of the Cu(II) complexes.
The ratio of νꢀ(Cu)/νꢀ(Ni) is (1.68). So the complex under
(a) The ratio ν2/ν1 is found to be 2.13 as required for the
octahedral Co(II) complexes [16].
(b) It is found that method (d) affords the best procedure.
(c) 10Dq determined by method (c) and that determined by
ν2 − ν1 are found to be in good agreement.
study is equally tetragonally distorted. The ratio g /A is
|| ||
an empirical factor for the tetrahedral distortion of a tetrag-
onally arranged complex [19]. Square planar arrangements
can be assigned between 105 and 135 cm−1. The progressive
rise up to 250 cm−1 can be attributed to a tetrahedrally dis-
The EPR spectrum of a polycrystalline sample of
bis[5-(2-carboxyphenylazo)-8-hydroxyquinoline)] copper
(II) exhibits two g values (g = 2.08, g = 2.21) indicating
torted arrangement. The g /A (cm−1) value lie just within
⊥
||
|| ||
the overall symmetry to be tetragonal around the copper(II)
ion. Kivelson and Neiman [17] have demonstrated that for
the range expected for square planar and tetragonal distorted
complex [5]. Approximate metal–ligand ␴-bond coefficients
(α2), which is defined as the fraction of unpaired electron
density located on the copper ion, for this complex under
study was calculated, neglecting the ␲-bonding, with the
covalent environments g is less than 2.3 and for ionic envi-
||
ronment this is usually ≥2.3. Thus, the metal–ligand bond-
ing in bis[5-(2ꢀ-carboxyphenylazo)-8-hydroxyquinoline)]
Table 2
Electronic spectral data and ligand field parameters of the complexes
Method of evaluation
ν1
ν2
ν3
B
βc
␦νb
10Dq
ν2 − ν1
ν2/ν1
Complex: Ni
Observed values
9090
10Dq
10Dq
10Dq
14845
Fitteda
14825
14840
14810
25000
25070
Fitted
24995
24965
–
–
70
–
9090
9090
9090
9090
9090
5755
–
5735
5750
5720
1.63
–
1.63
1.63
1.63
(a)
(b)
(c)
(d)
843
837
838
0.78
0.78
0.78
0.77
−20
−5
−35
Complex: Co
Observed values
–
15848
Fitted
15812
Fitted
15854
18783
18966
Fitted
Fitted
18785
–
841
830
830
831
–
186
−33
−
–
–
8461
8435
8450
8458
8461
2.13
(a)
(b)
(c)
(d)
Fitted
Fitted
7389
7397
0.85
0.84
0.84
0.84
6
a
The observed values of the frequency is fitted in the corresponding method in order to calculated other frequency.
Difference in the calculated and observed values of the frequency.
Ratio of the free ion and the complex.
b
c

A.Z. El-Sonbati et al. / Spectrochimica Acta Part A 60 (2004) 965–972
969
help of the optical absorption data in the solid state using
the relation:
4λα2
gav = 2.0023 −
,
ꢀE
where λ = −829 cm−1. The α2 value (0.54) for this complex
indicates considerable covalency in the bonding between
the Cu(II) ion and the ligand, comparable to that obtained
by El-Sonbati and co-workers [2,5,7]. The super-exchange
Fig. 4. Ionization of the carboxylic acid and existence of a Zwitter ion.
(Scheme 1B) and/or · · · N type between C8–OH of one
molecule and CNPy of another (Scheme 1C).
splitting constant A was obtained semi-empirically, accord-
||
In general, hydrogen bonding involving both COOH and
OH groups are proton donors and both O atoms are proton
acceptors. Both intra- and intermolecular OH–N may form
leading to a number of structures in simultaneous equilib-
rium.
The data observed in the IR spectrum of LH2 commen-
surate to the structures shown on Scheme 1.
The spectrum of the ligand shows a broad band centered
ing to:
A
3
||
α2Cu
=
+ (g − 2.0023) + (g − 2.0023) + 0.04,
||
⊥
P
7
where P = 0.036 cm−1. The αC2 u value represents the frac-
tion of unpaired electron density located on the copper(II)
ion and is 0.54 for bis[5-(21-carboxyphenylazo)-8-hydroxyquinoline]
copper(II). The small g value can be attributed to the
||
to ν(OH) (intramolecular hydrogen bonded) [5]. There are
−1
−1
decreased dipolar interaction.
=
=
other bands (ν(N N): 1540 cm and ν(C N): 1555 cm
(quinoline ring)) [2–5,21]. The broadness of the OH stretch-
ing band at 3130 cm−1 and the out-of-plane OH deformation
band at 940 cm−1 in the ligand is indicative of a hydrogen
bonded carboxylic acid group.
3.1. Stereochemistry and mode of bonding of the prepared
complexes
The IR spectroscopy is a powerful tool for structural de-
terminations of the ligand and metal chelates. The assign-
ments of fundamental functional groups are basic for such
purposes.
The strong band observed at 1655 cm−1 in the free lig-
and is assigned to the carbonyl stretch of the carboxylic acid
group [21]. Ionization of the carboxylic acid results in equi-
libration of the two C–O bands of the carboxylate group and
disappearance of the characteristic carbonyl absorption.
In a recent study El-Sonbati and co-workers [2–5] have
calculated from the IR data of 5-(4ꢀ-alkyl-phenylazo)-8-
hydroxy-7-quinolinecarboxaldehyde complexes that the
carbonyl group of ligands are coordinated to the metal ions.
On the other hand, 8-hydroxyquinoline is a ligand whose
reactivity toward metal(II) ions is well known from literature
[22]. The products which form are neutral, with two copla-
nar N,O metal–chelate rings in an N,N(O,O) trans-geometry
[2–5]. bis-(8-Hydroxyquinolinato)metal(II) is scarcely solu-
ble either in water or in common organic solvents. Therefore,
in order to improve solubility, we which here to prepara-
tion of substituted 8-hydroxyquinoline reported in Scheme 1
taking with carboxy aniline and their polymeric complexes.
The ligand was indeed obtained in good yields.
In the ligand under investigation, four functional groups
=
namely, COOH, OH, CN and N N are of great importance
in the infrared spectra.
Much has been written about the –OH stretching fre-
quency and intensity of phenol derivatives depending on
many factors (e.g. substituents, medium, etc.). The study
of IR frequencies is especially useful in the analysis of the
properties of hydrogen bonds. Hoyer [20] has reviewed the
relationship between the hydrogen bond band and the –OH
stretching frequency, but there have been many other re-
search workers in the 25 years since the publication of this
study who examined the problem in finer and more quanti-
tative data on turning our attention to the IR spectrum.
Infrared spectrum of ligand show medium broad bands lo-
cated at ∼3450–3500, 2185 and 830 cm−1 due to the stretch-
ing frequency of the phenolic and carboxylic OH and the
stretching vibration of hydrogen bonding. El-Sonbati and
co-workers [2–7] made detailed studies for the different
types of hydrogen bonding which are favorable to exist in
the hydrogen under investigation.
Two new bands are generated in the ranges (symmetric)
1615–1550 cm−1 and (asymmetric) 1435–1320 cm−1 due to
vibrations of the COO− structure [23]. This points to the
probable existence of a Zwitter ion as in the structure of
Fig. 4.
Intramolecular hydrogen bond between the nitrogen
Further proof to the azo moiety is the appearance of
two bands 1408 cm−1 (C–N) and 1255 cm−1 (C–O). In
all the complexes, the strong carbonyl band disappears
and is replaced by a strong and broad absorption at
1390 cm−1 (5 cm−1 which is assigned to the symmetric
=
atom of the –N N– system and hydrogen atom of the
carboxylic hydroxy hydrogen atom (six membered) and hy-
drogen (C8–OH) and nitrogen of azomethine group (CNPy)
(Scheme 1A).
=
Intermolecular hydrogen bonding is possible form-
ing cyclic dimmer through the O–H· · · OH type between
C8–OH of one molecule and C8–OH group of another one
vibration of the carboxylate group) [24]. Pyridine ν(C N)
(1583–1570 cm−1) group is characteristic of the very strong
band which upon complex formation shifted to lower

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A.Z. El-Sonbati et al. / Spectrochimica Acta Part A 60 (2004) 965–972
Scheme 1. General formula of the 5-(21-carboxyphenylazo)-8-hydroxyquinoline and metal chelates.
frequency with a reduction in its intensity. Furthermore,
Py appeared at ∼645 cm−1 in the free ligand, exhibiting a
negative shift in the complexes. This indicates pyridine ring
is involved in coordination through its CN donor.
The difference between νasym(COO) and νsym(COO) (ꢀν)
can be regarded as a measure of the type of M–COO bond.
If ꢀν > 225 cm−1 it is mainly ionic. The ꢀν value for
LH2 is 265 cm−1, confirming the covalent character of the
–COOH group. The ꢀν values are larger than 225 cm−1 for
the complexes, indicating a covalent bond.
decreases that of the metal azo link increases, leading to
=
lower ν(–N N–).
Also, in all the complexes the 3400 cm−1 band disap-
peared. However, a broad band characteristic band at –OH
of coordinated water is observed in the 3500–3000 cm−1 re-
gion in the spectra of the complexes [2–7]. The presence of
coordinated water is further confirmed by the appearance of
a non-ligand band in the 820–730 cm−1 region assignable to
a rocking mode of water [5]. Also, the complex does not lose
weight on heating at 100 ◦C for 1 h, so water is coordinated.
= =
The absence of both the OH stretch and deformation ab-
The characteristic ν(O U O) stretching mode is found
at 918 cm−1 in the case of [UO2L2]n in agreement with
previous data [3,4,25]. This band shifts to lower frequen-
cies in the other compounds, and this situation renews and
old discussion, about which exists a wealth of conflicting
and contradictory reports [26,27]. This shift has usually
been explained in terms of polymerization occurring by in-
teraction of the uranyl oxygen in one molecule with the
open axial site of uranyl in another molecule. In many of
the investigated cases, in which also magnetic susceptibil-
ity measurements have been performed, the polymerization
apparently generates no appreciable ferromagnetic interac-
tions, at least at room temperature [28]. Our own magnetic
susceptibility measurements clearly demonstrate a similar
behavior.
=
sorption bands and the replacement of the C O vibration by
a band assigned to the carboxylate group in the spectra of
the metal complexes is indicative of the metal attachment
as an anionic ligand via the carboxylate oxygen atom. The
absence of –OH (phenolic) at 3400 cm−1 in the spectra of
the metal complexes support the cleavage of intramolecu-
lar hydrogen bonded [2–7] OH and subsequent deprotona-
tion of the phenolic group and confirmed by shift to higher
frequency in ν(C–O) (phenolic) by 25–30 cm−1. A shift to
=
=
lower frequency ν(C N) and ν(–N N–) in the spectra of the
complexes reveals their involvement in complex formation.
=
A bathochromic shift of the –N N– band located at
1535 cm−1 in the free ligand, is observed on complexation
(1540–1525 cm−1). As the strength the metal–carboxy bond

A.Z. El-Sonbati et al. / Spectrochimica Acta Part A 60 (2004) 965–972
971
ligand under investigation in d6-DMSO also show two sig-
nals at δ ∼ 8.75 [OH(phenolic and carboxylic)] protons
which are absent in uranyl complex, indicating the depro-
tonation of the phenolic group. The existence of the OH
signal at high values downfield of TMS and commensurates
strong hydrogen bonding as in Scheme 1. The signal due
to the OH proton disappears in D2O solution. This multi-
ples at 8.15 ppm can be assigned to the proton of pyridine
ring [4]. This signal shifts downfield in the complex, and
is probably due to lone pair donation by the azomethine
nitrogen to uranyl.
3.2. Thermal decomposition studies
The thermal decomposition data of the complexes exhibit
wide range of stability from ambient to ∼265 ◦C. Three
main decomposition steps characterized the thermal behav-
ior of these compounds. Resolution improved with lower
heating rates. Only slight differences could be observed in
the characteristic temperatures corresponding to the second
and third steps, which can be accounted for by the formation
of a common decomposition product at the end of the first
step. The first step gave strong evidence for the release of
water crystallization molecules. The first and second steps
are exothermic, while the third is endothermic.
Finally, the thermal curves of all compounds show an
exothermic effect in the temperature range 165–350 ◦C fol-
lowed by an endothermic effect characterized partially by
overlapping in the same temperature range. To verify these
processes, the IR spectra of the heated samples have been
registered. These spectra show that the bands due to wa-
ter have disappeared, while the bands corresponding to the
ligand remain. Therefore, the former exothermic effect is
probably due to a structural change. The exothermic effect
begins at about 580 ◦C and must be attributed to combustion
of organic matter. As residue at the end of pyrolysis metallic
oxides remain.
Fig. 5. Molecular structure proposed for [UO2L2]n.
Finally, it is worth commenting that the spectroscopic
behavior of oxine complex is different, a fact that suggests
that it remains monomeric, whereas all the other species are
probably polymeric. However, as stated by El-Sonbati and
co-workers [4,21,29], the most obvious difference between
the ligands that form monomeric complexes and those that
give rise to extended structures is that the latter processes
electron withdrawing groups. The electron density at the
uranyl and metal atoms may be reduced by these groups
to such an extent that the increased Lewis acidity of the
=
=
atoms makes U O· · · U O bridging possible (Fig. 5). These
arguments support the generation of polymeric species in all
investigated metal complexes containing 5-azo substituted
ligands.
The ν(UO) mode of [UO2L2]n are located at lower fre-
quencies than those in the other complexes. This fact is con-
sistent with the increasing Lewis acidity at the uranyl atom,
caused by the presence of azo in the ring, inducing a stringer
␲-retro donation from uranyl to oxygen and reinforcing the
U–O bands.
In conclusion, the results arising from the present investi-
gations confirms that the selected 5-(4ꢀcarboxyphenylazo)-8-
hydroxyquinoline ligand is suitable for building a supramo-
El-Sonbati [30] has assigned the region between 500
and 400 cm−1 to ν(M–O) in the case of the transition
metal of poly(5-vinylsalycilidene) semi-carbazone and
poly(ethylene). In this work the ν(M–O) bands has been
located in the vicinity of 440 cm−1. The force constant
FU–O has been calculated by the method of McGlynn et al.
[31] and the obtained (6.985 mdyn Å−1) agree well with
those for similar dioxouranium(VI) complexes. The U–O
bond length (R) has been calculated using the equation:
R = 1.03F−1/3 +1.17 and the value obtained (1.734 Å) lies
satisfactory in the expected range [2–4,32] (1.60–1.92 Å).
El-Sonbati and co-workers [2–7] investigated the 1H
NMR spectra of quinoline and its alkyl derivatives with
UO2(II) and Pd(II) salts and found two signals in the
9.10–13.00 and 8.89–9.28 ppm regions assigned to ν(OH)
=
=
lecular U O· · · U O based structure. Moreover, since
the azo compounds experience photochemical isomer-
ization and are therefore of interest for applicative pur-
poses [33], dioxouranium(VI) complex containing the
5-phenylazo-8-hydroxyquinoline moiety combine features
which could be useful in molecular materials. Work is
underway on the synthesis and characterization of further
uranyl compounds of this family of ligands and toward the
development of the materials they produce.
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