Supramolecular structures and stereochemical versatility of azoquinoline containing novel rare earth metal complexes

Article abstract of DOI:10.1016/j.saa.2006.10.056

Rare earth complexes of 5-(phenylazo)-8-hydroxyquinoline (HL) of composition [M(L)₂X·H₂O] [where M = La, Ce, Pr, Nd and X = NO₃^- or NCS^-] have been prepared and characterized on the basis of their che

Full text of DOI:10.1016/j.saa.2006.10.056

Spectrochimica Acta Part A 68 (2007) 134–138
Supramolecular structures and stereochemical versatility of azoquinoline
containing novel rare earth metal complexes
A.Z. El-Sonbatia,∗, R.M. Issab, A.M. Abd El-Gawadb,1
a Department of Chemistry, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
b Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt
Received 31 July 2006; accepted 3 October 2006
Abstract
−
Rare earth complexes of 5-(phenylazo)-8-hydroxyquinoline (HL) of composition [M(L)2X·H2O] [where M = La, Ce, Pr, Nd and X = NO3 or
NCS−] have been prepared and characterized on the basis of their chemical analyses, 1H NMR, magnetic measurements, conductance, and visible
and IR spectral data. Composition, conductance and IR spectral data of the complexes show that the HL acts as a bidentate monobasic ligand. The
visible spectra of Pr3+ and Nd3+ show characteristic f–f transitions, and the nephelauxetic effect (1 − β) of these transitions has been evaluated.
These data indicate the weak involvement of the 4f orbitals in complex formation.
© 2006 Elsevier B.V. All rights reserved.
1. Introduction
8-hydroxyquinoline (HL). The structure and bonding of these
compounds of lanthanides of high coordination numbers were
established with the help of various physicochemical studies.
Much research on azo ligand complexes has been focused
on species containing a first-row transition metal ion and a
bidentate ligand [1–3]. The majority of the ligands contain nitro-
gen donor atoms and are coordinated in a planar manner to
the metal ions [4–9]. The formation of oxin azo complexes
depends significantly on the dimension of the internal cavity,
on the rigidity of the quinoline nucleus, on the nature of its
donor atoms and on the coordination [10]. These ligands are
also of theoretical interest since they are capable of furnishing
an environment of controlled geometry and ligand field strength
[11].
2. Experimental
All the chemicals used were of BDH quality. 5-(Phenylazo)-
8-hydroxyquinoline (HL) and all of the physicochemical
techniques were done as reported earlier [2–8].
2.1. Preparation of 5-phenylazo-8-hydroxyquinoline (HL)
In a typical preparation, 25 ml of distilled water containing
hydrochloric acid (12 M, 2.68 ml, 32.19 mmol) was added to ani-
line (0.979 ml, 10.73 mmol). To the resulting mixture stirred and
cooled to 0 ◦C, a solution of sodium nitrite (740 mg, 10.73 mmol,
in 20 ml of water) was added dropwise. The so-formed dia-
zonium chloride was consecutively coupled with an alkaline
solution of 8-hydroxyquinoline (1.557 g, 10.73 mmol) in 20 ml
of ethanol containing 602 mg (10.73 mmol) of potassium
hydroxide. The orange precipitate, which formed immediately
filtered and was washed several times with water. The crude
product was purified by recrystallization from hot ethanol, yield
65–75%. The analytical data confirmed the expected composi-
tion (Table 1). The ligands were also characterized by 1H NMR
and IR spectroscopy.
Recently, the structural chemistry of the lanthanide elements
has undergone considerable development, and a wide variety of
coordination numbers and geometries have been observed. This
structural versatility arises from lack of strong crystal effects
for the 4f electronic configurations as well as from the large
ionic radii of these metal ions, which change markedly with
oxidation number or atomic number of the lanthanides [12]. To
confirm this view, in the present work, we have isolated six-
coordinated complexes of lanthanides(III) with 5-(phenylazo)-
∗
Corresponding author.
E-mail address: elsonbatisch@yahoo.com (A.Z. El-Sonbati).
Abstracted from his Ph.D.
1
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.10.056

A.Z. El-Sonbati et al. / Spectrochimica Acta Part A 68 (2007) 134–138
135
(a) Preparation of [M(L)2NO3·H2O]. A solution (25 ml) of
3.1. Infrared spectra and nature of coordination
lanthanide nitrate hexahydrate (1.0 mmol) in ethanol was
mixed with 60 ml of an ethanolic solution of the ligand
(1.20 mmol) and refluxed for about 20 min in a water bath.
After cooling, the pH (∼7.0) of the solution was adjusted by
dropwise addition of dilute ammonia with constant stirring.
The reaction mixture was concentrated in a water bath until
a coloured precipitate was obtained, which was filtered and
washed thoroughly with ethanol and ether and dried over
P2O5 in a vacuum desicator.
A study and comparison of infrared spectra of free ligands
and their rare earth metal complexes imply that these ligands
behave as monobasic bidentate and the metal ions also coor-
dinated through two coplanar N,O metal-chelate rings in an
N,N(O,O) trans geometry [7–9].
The infrared absorption of HL has been assigned by a com-
parison of its spectra with those of the five-membered quinoline
ring system [2,3,9] and pyrazoline [8].
(b) Preparation of [M(L)2NCS·H2O]. A warm ethanolic solu-
tion (15 ml) of lanthanide nitrate hexahydrate (10 mmol)
was added to a warm ethanolic solution (30 ml) of potas-
sium thiocyanate (3.0 mmol). The solution, evaporated to
half volume, was left overnight for complete precipitation
of potassium nitrate, which was removed by filtration. The
filtrate was treated the same way with the ligand as in pro-
cedure (a). The precipitate was purified and dried as usual.
By tracing the IR spectra of the azo compounds, no NH2
stretching vibrations are apparent. The structurally important IR
bands like υ(C O), υ(C N), υ(–N N–), υ(M–O) and υ(M–N),
which throw light on the structural features of the new azodye
ligand and its mode of bonding to the metal ions have been
assigned. The broad bands at 3450, 2109 and 925 cm−1 due
to OH and hydrogen bonds, as they are not present in the
complexes where the deprotonated O− atom is coordinated
to metal. El-Sonbati and co-workers [1–9] found three kinds
of bonded OH structures on the basis of the IR frequen-
cies:
3. Results and discussion
−
The reaction of non-aqueous solution of MX3 (X = NO3
or NCS− and M = La, Pr, Nd and Sm) with 5-(phenylazo)-
8-hydroxyquinoline (HL) resulted in the formation of
the complexes of the general composition [ML2·X·OH2]
(X = NO3− or NCS−) according to the following equation:
(i) Only oxygen is in the bridge while the hydrogen is free.
(ii) Polymer chains are formed in which both hydrogen and
oxygen atoms are precipitating in the hydrogen bond.
(iii) Dimer associated structures are formed.
MX3 + nHL → [ML2·X·OH2]
On the basis of these, HL exists on a five-membered chelate
skeleton and the hydrogen bonding could be classified into two
types:
Analytical data show 1:2 metal–ligand stoichiometry in all
the rare earth complexes (Table 1). All the complexes behave as
non-electrolyte in DMF, as revealed by their molar conductance
data.
The complexes are fairly stable and could be stored for a long
time in a vacuum desicator and are non-hygroscopic. Most of
the complexes do not have a sharp melting point and decom-
pose at higher temperature. The complexes are also soluble in
coordinating solvents such as DMF and DMSO. The molar con-
ductance values of the complexes are too low to account for any
dissociation; therefore, the complexes are non-electrolytes.
(a) Intramolecular hydrogen bonding O–H· · ·N (Fig. 1A)
resulting from hydrogen (C8–OH) and nitrogen of azome-
thine group (CNpy).
(b) Intermolecular hydrogen bonding O–H· · ·N (Fig. 1B) and
(O–H· · ·O) (Fig. 1C) resulting from the C8–OH group
through two molecules and/or C8–OH with CN through
two molecules. The 945 cm−1 is a strong indication of the
existenceoftheseligandsinadimericskeleton. So, theselig-
Table 1
Analytical and magnetic moment data of rare earth complexesa (for molecular structure see Fig. 1)b
Complexc
Yield (%)
Found (calculated) %
C
μeff.d (B.M.)
H
N
HL
85
80
74
80
76
70
65
72
68
72.22 (72.3)
43.2 (43.3)
43.0 (43.2)
43.0 (43.1)
42.7 (42.8)
42.7 (42.5)
42.1 (42.3)
24.3 (42.3)
42.0 (42.0)
4.4 (4.5)
3.8 (4.0)
3.5 (4.0)
3.8 (4.0)
3.9 (4.0)
3.8 (3.9)
3.8 (3.9)
4.0 (3.9)
4.0 (3.9)
16.6 (16.9)
5.9 (5.6)
5.9 (5.6)
6.0 (5.6)
5.6 (5.4)
5.8 (5.5)
5.5 (5.8)
5.8 (5.5)
5.4 (5.5)
–
[LaL2·NO3·OH2] (1)
[CeL2·NO3·OH2] (2)
[PrL2·NO3·OH2] (3)
[NdL2·NO3·OH2] (4)
[LaL2·NCS·OH2] (5)
[CeL2·NCS·OH2] (6)
[PrL2·NCS·OH2] (7)
[NdL2·NCS·OH2] (8)
dia.
2.47
3.65
3.53
dia.
2.43
3.61
3.57
a
Microanalytical data as well as metal estimations are in good agreement with the stoichiometry of the proposed complexes.
The excellent agreement between calculated and experimental data supports the assignment suggested in the present work.
L is the anion of the ligand HL as given in Fig. 1.
b
c
d
Per metal ion and measured at room temperature; dia. = diamagnetic.

136
A.Z. El-Sonbati et al. / Spectrochimica Acta Part A 68 (2007) 134–138
Fig. 1. General formula and proton numbering scheme of 5-(4ꢀ-R-phenylazo)-8-hydroxylquinoline ligand HL. R− = P–OCH3PhN N–; P–CH3PhN N–;
P–HPhN N–; P–ClPhN N– and P–NO2PhN N–.
ands exist in an associated structure through intermolecular
hydrogen bond (Fig. 1).
tentatively assigned to the υ(M–O)/υ(M–N) modes [1]. The lig-
and shows a band at 1540 cm−1 due to the υ(–N N–) mode
remains more or less at the same position in complexation. The
pyridine υ(C N) (1580–1570 cm−1) group is characteristic of
the very strong band which upon complex formation shifted to
lower frequency with a reduction in its intensity. Furthermore,
δPy appeared at ∼640 cm−1 in the free ligands, exhibiting a neg-
ative shift in the complexes. This indicates that pyridine ring is
involved in coordination through its CN donor.
In the nitrate complexes described herein, bands observed
at ∼965, ∼820 and ∼1400 cm−1 have been assigned to sym-
metric stretch of NO2 (υ1), out of plane deformation (υ2) and
υ3, respectively [14]. In case of monodentate coordination of
nitrate group, the generacy of the υ3 and υ4 bands is shifted.
Thus, one expects six bands for this mode of nitrate coordina-
tion [14]. A characteristic coupled band due to (υ1 + υ4) in the
region 1770–1720 cm−1 has been split into two, the magnitude
of splitting (∼25 cm−1) of this band indicates the monodentate
mode of coordination of nitrate ion. The characteristic band due
to M–O (nitrate) vibration has been observed in the far IR spec-
tra of the nitrato complexes which further indicates the presence
of ONO2 group in coordination sphere [16] (Table 2).
The assignment of the CN and CC stretching modes on our
previouswork[2,3]andinvestigationsbyHenryetal. [13]. Com-
paring the position of these bands for the different ligands, it is
obvious that ring substitution results in a shift to lower frequen-
cies. This behaviour can probably be explained by the lowering
in the electronic density caused by electronegative groups.
υ(C N) always occurs at higher frequencies in the higher
complexes than in the respective complexes. In this case, the
decrease in the electronic density of the ring may be correlated
with the donor character of the nitrogen atom in the N–M bond.
There is also another correlation observable when comparing
the position of the υ(C N) band in the free ligand and in the
corresponding complexes to higher electronegativity. Evidently,
the presence of electronegative groups in the ring affects the
donor capacity of the N-atom.
These comparisons show that the electronegativity of the ring
substituents produces not only a decrease in the electronic den-
sity over the ring, geometrating a diminution in the υ(C N)
stretching frequency, but also causes a plane deformation, dou-
bly degenerate stretch and doubly degenerate in plane bending
in D3␯ symmetry [14].
Much has been written about the –OH stretching frequency
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 [15] wrote a review on the relationship between
the hydrogen bond band and the –OH stretching frequency, but
there have been many other research workers in the 25 years
since the publication of this study, who examined the problem
in finer and more quantitative data on turning our attention to
the IR spectrum.
On the basis of the previous works on a large number of
thiocyanate complexes, Bailey et al. [17] and Burmeister [18]
indicated that the region near or above 2100 cm−1 for is due to
Table 2
IR spectral data of nitrato complexes
Compounda
(υ1 + υ2)
υ1
υ2
υ3
M–O (nitrate)
322
(1)
1755
1740
955
815
1425
(2)
(3)
1758
1745
967
963
952
812
815
812
1425
1400
1428
328
330
336
1762
1750
There is no υ(OH) of quinoline and a new band at ∼1350
cm−1, υ(C–O) (∼1340 cm−1 in ligands) in metal chelates is
indicative of deprotonation of phenolic hydroxyl groups and
consequent monobasic nature of the ligand. Some new medium
or weak bands are observed in the range 440–330 cm−1 and are
(4)
1770
1743
a
The serial number corresponds to that used in Table 1.

A.Z. El-Sonbati et al. / Spectrochimica Acta Part A 68 (2007) 134–138
137
(Ph–Ph*, ␲–␲*) [2,3,8,9] and ∼29340 cm−1, and transition of
phenyl rings overlapped by composite broad (␲–␲*) of azo
structure.
Table 3
IR spectral data of thiocyanato complexes
Compounda
υ(CN) of
NCS
υ(CS) of
NCS
υ(NCS) of
NCS
υ(M–N)
(thiocyanate)
The data are summarized in Table 4. These data clearly
indicate that the energy of f–f transitions in the complexes
is slightly reduced from the corresponding aquo ions, either
because of the slight covalent interaction of the 4f orbital with
vacant ligand orbitals, leading to some delocalization with con-
sequent reduction in interelectronic repulsion or more normally
byincreasednuclearshieldingofthef-orbitalsduetoslightcova-
lent ligand–metal electron drift. There are small differences in
the frequencies or molar absorptions except that the hypersitive
(5)
(6)
(7)
(8)
2110
2105
2100
2107
785
780
790
785
470
468
473
465
295
280
290
280
a
The serial number corresponds to that used in Table 1.
S-bonding, below this for N-bonding. The C–S stretching fre-
quency (υ2) was assigned at 860–780 cm−1 for M–NCS and
at 720–690 cm−1 for the M–SCN group. The NCS frequency
(υ3) is also different for the two isomers at 480–440 cm−1 for
the M–NCS and 435–395 cm−1 for the M–SCN group. Bridging
thiocyanato usually gives higher coordination number stretching
frequencies than a terminal-NCS group. In the present com-
plexes the IR absorption frequencies of the C–N stretching
(υ1), C–S stretching (υ3) and N–C–S bending (υ2) occur in
the 2025–2020, 835–825 and 875–465 cm−1 regions, respec-
tively. These frequencies are associated with terminal N-bonded
isothiocyanate ions [19] (Table 3).
4
transition 4I9/2 → G5/2 in the neodymium complex increase in
intensity by from ε = 6.10 in the salt to ε = 19.30 in nitrato com-
plex (Table 4). The nephelauxetic for the rare earth complexes
has been determined by the relation:
ν−aquo − ν−complex
1 − β =
ν−aquo
where ν− stands for the wave number of the absorption band
of the rare earth ion. Assuming that each J of 4fn configurate
ion is linearly dependent on the radial integrals. On the basis of
this assumption ν−/ν− aquo of all bands of the same rare earth
complex should identical; however, earlier observation [22] as
well as our present results show that the ν−/ν− aquo values for
ꢀν−/ν−aquo values for different J levels of a rare earth complex
differ from each other considerable.
The magnetic moment values (Table 1) of the present com-
plexes show that the lanthanum complexes are diamagnetic,
as expected from closed-shell electronic configuration and the
absence of unpaired electrons. All other tripositive lanthanide
ions are paramagnetic due to the presence of 4f-electrons, which
are effectively shielded by 5s55p6 electrons. The comparison
of these observed values with those observed for octa-hydrate
sulfates [20] and those calculated for uncomplexed ions [21]
indicates that the 4f-electrons do not participate in any bond
formation in these complexes.
3.2. 1H NMR spectra
Mubarak and co-workers [1–9] investigated the NMR spec-
tra 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 δ(OH) and δ(CN), respectively. Our 1H NMR
studies for the free ligands under investigation in DMSO-d6 also
show two signals at δ 9.3 and 8.95 ppm. Those signals are also
assigned to the protons of OH and CN, respectively, in the spec-
tra of the free ligands under investigation. The existence of the
OH signal at high values downfield of TMS and commensurates
strong hydrogen bonding. The signal due to the OH proton dis-
appears in D2O solution. This bond occurs between OH and the
azomethine nitrogen of quinoline as in Fig. 1.
3.3. Electronic spectra and magnetic moment
3.4. Stereochemistry
HL exhibit bands at ∼32500 cm−1 (CN) (␲–␲*), ∼33450
cm−1 (H-bonding and association), ∼40038 cm−1 (phenyl)
For the [ML2·X·OH2] complexes (X = NO3− or NCS− ions),
the electrolytic conductivity data indicate that these complexes
Table 4
Electronic absorption spectra (cm−1) of Pr(NO3)3, Nd(NO3)3, [PrL2·NO3·OH2] and [NdL2·NO3·OH2] in DMF
Energy level
M(NO3)3
εaquo
ML2(NO3)(OH2)
εcomplex
εcomplex/εaquo
(1 − β) × 10−2
1
3H4 → D2
16,945
20,825
21,275
22,470
1.87
3.08
4.34
9.96
16,875
20,615
21,160
22,345
2.85
4.65
7.07
1.51
1.50
1.61
1.45
0.40
1.02
0.55
0.51
3
3H4 → p0
3
3H4 → p1
3
3H4 → p2
14.65
4I9/2 → F5/2, 2H9/2
12,500
13,410
14,595
17,385
19,045
19,415
6.40
5.55
0.41
6.08
3.50
1.63
12,415
13,330
14,555
17,165
18,965
19,225
9.80
8.66
0.58
19.25
5.50
2.55
1.50
1.55
1.48
3.01
1.55
1.54
0.62
0.66
0.25
1.25
0.92
0.96
4
4
4I9/2 → F6/2, 4S3/2
4
4I9/2 → F9/2
4I9/2 → G5/2, 2G7/2
4
4
4I9/2 → G7/2
4
4I9/2 → G9/2

138
A.Z. El-Sonbati et al. / Spectrochimica Acta Part A 68 (2007) 134–138
−
vanished. The characteristic proton resonance of the substituted
and unsubstituted benzene rings of the ligand and complex are
almost unchanged.
behave as non-electrolytes. Thus, one of the NO3 and NCS
ions is present in the coordination sphere. IR data reveal
the monodentate nature of NO3−, NCS− and bidentate in
HL in the complexes and hence, a coordination number of
six has been suggested for all of the lanthanide ions in the
complexes. In case of [ML2·X·OH2], the conductance and
spectral data indicate that the lanthanide ions are surrounded
by three nitrogen atoms (one of isothiocyanate and/or nitrate
ions and two from the azomethine quinoline group of HL)
and three oxygen atoms (two of quinoline ions and one from
the water molecule). Hence, a coordination number of six
for all rare earth metals has been suggested for the com-
plexes.
References
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Chem. 36 (1974) 3751.
4. Conclusion
It can be seen from Table 1 that the characteristic absorp-
tion peaks in the IR spectra of all the complexes are similar,
which indicates that the complexes have similar structures. In
the IR spectra of the ligand, characteristic absorption peaks for
υ(C N) and υ(OH) groups appeared. In the IR spectra of the
eight complexes (1–8), the absorption peak for the υ(C N) red-
shifted, while υ(OH) disappeared with deprotonation of H-atom
of C8–OH and υ(C–O) group appeared, showing that ligand
coordination to rare earth metals involves the nitrogen atom of
imineandoxygenatomofC8–OHgroups. Characteristicabsorp-
[15] H. Hoyer, Z. Electrochem. 49 (1943) 97.
[16] R.J. Foster, D.G. Hendricker, Inorg. Chim. Acta 6 (1972) 371.
[17] R.A. Bailey, J.W. Michelsen, W.N. Mills, J. Inorg. Nucl. Chem. 33 (1971)
3206.
[18] J.L. Burmeister, Coord. Chem. Rev. 1 (1996) 205;
J.L. Burmeister, Coord. Chem. Rev. 3 (1968) 225.
[19] R.A. Bailey, S.L. Kzak, J.W. Michelsen, W.N. Mills, Coord. Chem. Rev. 6
(1971) 407.
[20] N.K. Dutt, S. Rahut, J. Inotg. Nucl. Chem. 32 (1970) 210.
[21] R.L. Dutt, A. Syanal, Elements of Magnetochemistry, 2nd ed., Affiliated
East West Press Pvt., New Delhi, 1992.
−
tion peaks of inner sphere NO3 appeared, which suggests
NO3− coordination to the rare earth metal ions in a monodentate
manner (see Table 1).
1H NMR spectra of the ligand showed singlet peaks of the
HC N and OH protons. The singlet peak of the HC N proton
of complex (1) is shifted and singlet peak of the OH proton is
[22] S.P. Sinha, Spectrochim. Acta 22 (1966) 57.