Spectroscopic investigations of new binuclear transition metal complexes of Schiff bases derived from 4,6-diacetylresorcinol and 3-amino-1-propanol or 1,3-diamino-propane

Article abstract of DOI:10.1016/j.saa.2006.12.034

The bifunctional carbonyl compound; 4,6-diacetylresorcinol (DAR) serves as precursor for the formation of different Schiff base ligands, which are either di- or tetra-basic with two symmetrical sets of either O₂N or N₂O tridentate chelating sites. The condensation of 4,6-diacetylresorcinol with 3-amino-1-propanol (3-AP) or 1,3-diaminopropane (DAP), yields the corresponding hexadentate Schiff base ligands, abbreviated as H₄L_a and H₂L_b, respectively. The structures of these ligands were elucidated by elemental analyses, IR, mass, ¹H NMR and electronic spectra. Reaction of the Schiff base ligands with copper(II), nickel(II), cobalt(II), zinc(II), cadmium(II), iron(III), chromium(III), vanadyl(IV) and uranyl(VI) ions in 1:2 molar ratio afforded the corresponding transition metal complexes. A variety of binuclear complexes for the metal complexes were obtained with the ligands in its di- or tetra-deprotonated forms. The structures of the newly prepared complexes were identified by elemental analyses, infrared, electronic, mass, ¹H NMR and ESR spectra as well as magnetic susceptibility measurements and thermal gravimetric analysis (TGA). The bonding sites are the azomethine and amino nitrogen atoms, and phenolic and alcoholic oxygen atoms. The metal complexes exhibit different geometrical arrangements such as square planar, tetrahedral, square pyramid and octahedral arrangement.

Full text of DOI:10.1016/j.saa.2006.12.034

Spectrochimica Acta Part A 68 (2007) 592–604
Spectroscopic investigations of new binuclear transition metal
complexes of Schiff bases derived from 4,6-diacetylresorcinol
and 3-amino-1-propanol or 1,3-diamino-propane
Adel A.A. Emara∗, Akila A. Saleh, Omima M.I. Adly
Department of Chemistry, Faculty of Education, Ain Shams University, Roxy, Cairo 11341, Egypt
Received 25 November 2006; accepted 17 December 2006
Abstract
The bifunctional carbonyl compound; 4,6-diacetylresorcinol (DAR) serves as precursor for the formation of different Schiff base ligands, which
are either di- or tetra-basic with two symmetrical sets of either O2N or N2O tridentate chelating sites. The condensation of 4,6-diacetylresorcinol
with 3-amino-1-propanol (3-AP) or 1,3-diaminopropane (DAP), yields the corresponding hexadentate Schiff base ligands, abbreviated as H4La
and H2Lb, respectively. The structures of these ligands were elucidated by elemental analyses, IR, mass, 1H NMR and electronic spectra. Reaction
of the Schiff base ligands with copper(II), nickel(II), cobalt(II), zinc(II), cadmium(II), iron(III), chromium(III), vanadyl(IV) and uranyl(VI) ions
in 1:2 molar ratio afforded the corresponding transition metal complexes. A variety of binuclear complexes for the metal complexes were obtained
with the ligands in its di- or tetra-deprotonated forms. The structures of the newly prepared complexes were identified by elemental analyses,
infrared, electronic, mass, 1H NMR and ESR spectra as well as magnetic susceptibility measurements and thermal gravimetric analysis (TGA).
The bonding sites are the azomethine and amino nitrogen atoms, and phenolic and alcoholic oxygen atoms. The metal complexes exhibit different
geometrical arrangements such as square planar, tetrahedral, square pyramid and octahedral arrangement.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Schiff base ligands; Binuclear complexes; 4,6-Diacetylresorcinol; 3-Amino-1-propanol and 1,3-diaminopropane
1. Introduction
4,6-diacetylresorcinol, as starting material, with 3-amino-1-
propanol (3-AP) and 1,3-diaminopropane (DAP) to afford the
corresponding Schiff base, H4La and H2Lb ligands, respectively.
The reactions of these ligands with some transition metal ions
in molar ratio (1:2, ligand:metal ion) were studied. The newly
prepared metal complexes of these ligands were identified by
different physicochemical and spectroscopic techniques.
The Schiff base, H4La and H2Lb ligands were allowed to react
with copper(II), nickel(II), cobalt(II), cadmium(II), zinc(II),
iron(III), chromium(III), vanadyl(IV) and uranyl(VI) ions. All
reactions afforded binuclear complexes except cadmium(II) and
zinc(II) for H4La ligand, and zinc(II) and iron(III) for H2Lb lig-
and, which gave oily products which were difficult to isolate. For
the former ligand, the copper(II) complex exhibits a square pla-
nargeometry, whilenickel(II), cobalt(II)andiron(III)complexes
exhibit an octahedral geometry which contain nitrate ions acting
as unidentate ligands, also, the chromium(III) complex showed
an octahedral geometry but the nitrate ions acting as bidentate
ligand. The vanadyl(IV) complex showed a square pyramidal
geometry and the uranyl(VI) complex has a coordination num-
ber = 8. The bonding sites are the azomethine nitrogen atoms and
A large number of Schiff bases and their metal complexes
have been studied because of their interesting and important
properties such as their ability to reversibly bind oxygen [1],
and their use in catalyses for: oxygenation and oxidation reac-
tions of organic compounds [2], redox systems in biological
processes [3], Aldol reactions [4], degradation of dyes through
decomposition of hydrogen peroxide and other reagents, in tex-
tile industries [5], reduction of thionyl chloride [6] and oxidation
of DNA [7]. Also Schiff bases can be used in degradation of
organic compounds [8] and in radiopharmaceuticals [9]. For
these applications, we are extending this field in synthesis of
novel binuclear Schiff base complexes.
The present study, is extension to our work [10,11]
where the ligands were prepared by the condensation of
∗
Corresponding author. Tel.: +20 2 4551530; fax: +20 2 4530271.
E-mail addresses: adelaemara@yahoo.com,
adelaemara@gmail.com (A.A.A. Emara).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.12.034

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593
the phenolic and alcoholic oxygen atoms. For the H2Lb ligand,
all complexes having octahedral environment around Cu(II),
Ni(II), Co(II), Fe(III) Cr(III), VO(IV) and UO2(VI) ions. The
bonding sites are the azomethine and amino nitrogen atoms and
the phenolic oxygen atoms, except uranyl(VI) complex, where
the amino nitrogen atoms are not involved in the coordination.
Most of the complexes of the H2Lb ligand contain nitrate ions
which acting as unidentate ligands, also, in case of vanadyl(IV)
complex, the SO42− ions is involved in the coordination and acts
as a bridged ligand.
2. Experimental
2.1. Materials
Resorcinol, acetic anhydride, zinc chloride, 3-amino-1-
propanol and 1,3-diaminopropane were either Analar or Merck.
Copper(II), nickel(II), cobalt(II), cadmium(II), zinc(II), iron(III)
and chromium(III) were used as nitrate salts and were Merck or
BDH. Uranyl(VI) acetate dihydrate and vanadyl(IV) sulphate
monohydrate were Merck or BDH. Organic solvents (ethanol,
absolute ethanol, methanol, diethylether, acetone, dimethylfor-
mamide (DMF) and dimethylsulfoxide (DMSO)) were reagent
grade.
Scheme 1. Schematic representation for the formation of the Schiff base, H4La
and H2Lb ligands. Structure 1. Representative structures of the Schiff base, H4La
and H2Lb ligands.
from ethanol. The ligands were kept in a desiccator until used.
The yield was 5.00 g (81.0%), and 4.80 g (78.1%), for H4La and
H2Lb, respectively. The melting point (mp) for both ligands were
>280 ◦C.
2.2. Synthesis of the Schiff base ligands
Schiff base ligands were prepared in two steps. The first step
was the formation of 4,6-diacetylresorcinol (DAR) by acety-
lation of resorcinol. The second step was the condensation of
4,6-diacetylresorciol (DAR) stoichiometrically in a molar ratio
1:2 with 3-amino-1-propanol or 1,3-diaminopropane in abso-
lute ethanol. The Schiff base ligands were formed by stirring
the reaction for 1 h at room temperature. If the ligand did not
precipitate, the reaction was heated to reflux for 1–3 h, where
the solid was formed.
The Schiff base, H4La and H2Lb ligands were characterized
by elemental analysis, 1H NMR spectroscopy and UV–vis spec-
trophotometer in DMF or DMSO and in the solid state by FT-IR
and mass spectroscopy. The synthetic steps are given in details
as follows.
The reaction for the formation of 4,6-diacetylresorcinol and
their corresponding H4La and H2Lb ligands are illustrated in
Scheme 1.
2.3. Synthesis of the Schiff base metal complexes
Methanolic solutions of the metal salts (30 mL) were added
gradually to methanolic solutions of the deprotonated ligand
(60 mL) in the molar ratio 2:1. Triethylamine, Et3N, was used
to affect the deprotonation of the ligands. The reaction mixture
of the deprotonated H4La ligand with metal salts was stirred on
cold for 1–3 h, while the reaction mixture of the other ligand,
H2Lb, was heated to reflux for 2–3 h. The time of reflux depends
on the formation of the solid products on either hot or cold. The
resulting precipitates were filtered off, washed with methanol
then ether. Most of the complexes are insoluble in most common
organic solvents, but some of them are partially soluble in DMF
and/or DMSO.
In each case, the ligand is deprotonated using Et3N. In
the case of H4La ligand, Et3N (1.212 g, 12 mmol) dissolved
in methanol (20 mL) was added to 3 mmol of the ligand in
30 mL methanol, i.e., in the molar ratio 4:1 (Et3N:ligand) and
heated to reflux for 30 min. The appropriate weight (6 mmol)
of the metal salts was added to the ionic deprotonated ligand
to form the metal complexes. In the case of H2Lb, the same
procedure was used, however, the molar ratio was 2:1 (Et3N:
ligand).
2.2.1. Synthesis of 4,6-diacetylresorcinol
The method of preparation was the same as mentioned in
Ref. [10]. The yield was 10.0 g (28.4%), mp 178–180 ◦C. 4,6-
Diacetylresorcinol was characterized by elemental analysis and
infrared spectrum.
2.2.2. Synthesis of the Schiff base, H4La and H2Lb ligands
The H4La and H2Lb ligands were synthesized by adding
4,6-diacetylresorcinol, DAR (3.88 g, 20.0 mmol) dissolved in
hot absolute ethanol (30 mL) to 3-amino-1-propanol (3.00 g,
40.0 mmol) or 1,3-diaminopropane (2.96 g, 40.0 mmol) in abso-
lute ethanol (30 mL). The reaction mixtures were heated to
reflux for 1–3 h. The yellow products obtained were filtered
off and washed with few amounts of ethanol then diethylether
and air dried. Fine crystals were obtained by recrystallization
The following detailed preparations are given as examples
and the other complexes were obtained similarly.

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2.3.1. Reaction of Cu(II) with H4La to form complex (1)
Copper(II) nitrate tetrahydrate, Cu(NO3)2·4H2O (1.45 g,
6 mmol) in methanol (30 mL) was added gradually with constant
stirring to a solution of the deprotonated ligand, H4La (0.918 g,
3 mmol)inmethanol(60 mL). Thestoichiometryofthemetalion
to ligand was 2:1. The solution was stirred for 1 h. A green pre-
cipitate was formed and washed with small amount of methanol
then ether. The yield was 0.96 g (51.6%), mp > 280 ◦C.
netic corrections were calculated from Pascal’s constants for
all atoms in the compounds [15]. Thermal gravimetric analysis
(TGA) data was measured from room temperature up to 800 ◦C
at a heating rate of 20 ◦C/min. The data were obtained using
a Shimadzu TGA-50H instrument. Melting points reported in
this work were not corrected. ESR spectra of compounds were
recorded on the Bruker, Model: EMX, X-band spectrometer.
Mass spectra of the compounds were recorded on a Hewlett
Packard mass spectrometer model MS 5988. Samples were
introduced directly to the probe, and the fragmentations were
carried out at 300 ◦C and 70 eV.
2.3.2. Reaction of VO(IV) with H2Lb to form complex (13)
Vanadyl(IV) sulphate monohydrate, VOSO4·H2O (1.06 g,
6 mmol)inmethanol(30 mL)andsmallamountofdistilledwater
was added gradually with constant stirring to a solution of the
deprotonated ligand (0.918 g, 3 mmol) in methanol (60 mL). The
solution was heated to reflux for 3 h. A green precipitate was
obtained and washed with small amount of methanol then ether.
The yield was 0.95 g (52.8%), mp > 280 ◦C.
3. Results and discussion
3.1. The Schiff base, H4La and H2Lb ligands
The Schiff base, H4La, and H2Lb ligands were prepared by
the condensation of 4,6-diacetylresorcinol (DAR) with 3-amino-
1-propanol (3-AP) or 1,3-diaminopropane (DAP) in molar ratio
1:2. The structures are identified by elemental analyses, infrared,
The effect of the triethylamine as deprotonating agent was
investigated, and did not coordinate as a ligand with the metal
ions.
1
UV–vis, H NMR and mass spectra. Table 1 lists the physical
2.3.3. Unsuccessful trials
and analytical data of the Schiff base, H4La, and H2Lb ligands
and their transition metal complexes. From the investigation, the
H4La, and H2Lb ligands can be represented in Structure 1.
The infrared frequencies of the Schiff base, H4La and H2Lb
ligands along with 4,6-diacetylresorcinol (DAR), 3-amino-1-
propanol (3-AP) and 1,3-diaminopropane (DAP), and their
assignments are listed in Table 2. The infrared spectra are
consistent with the formation of H4La and H2Lb ligands.
The vibrational assignments were aided by comparison with
the vibrational frequencies of the related compounds, such
as, the Schiff bases of salicylaldehyde [16]. The fundamen-
tal stretching mode of the azomethine moiety, ν(–C N–),
is readily assigned by comparison with the infrared spec-
tra of 4,6-diacetylresorcinol (DAR), 3-amino-1-propanol(3-AP)
and 1,3-diaminopropane (DAP). This intense band at 1578
and 1575 cm−1 for H4La and H2Lb ligands, respectively, are
assigned to the –C N– stretching frequency of both ligands
and are characterized for the azomethine moiety of most Schiff
base compounds. The absorption band of the C O in the 4,6-
diacetylresorcinol disappeared in the infrared spectrum of the
ligand, which indicate that the condensation has occurred. The
ν(NH2) stretching frequencies of DAP which lies at 3357 and
3283 cm−1 still persist even after the formation of the H2Lb
ligand. This could be due to one of the two NH2 groups of
DAP is participating in the condensation with DAR, while the
other NH2 group still free in H2Lb ligand and did not dis-
appear in the infrared spectrum of H2Lb. On the other hand,
other fundamental bands were assigned in the infrared spectra
of H4La and H2Lb ligands. Of these, the fundamental stretch-
ing bands which are the ν(C–C) aliphatic, ν(OH) aliphatic,
ν(OH) aromatic, ν(CH3), ν(C–C) aromatic, ν(–C N–), ν(C–O),
ν(ArC–H), ν(C–N), ν(CH2) and the bending bands which are
the δ(H–OC) aliphatic, δ(H–OC) aromatic, δ(CCC) aliphatic,
δ(CH2) aliphatic, δ(C–C N), δ(N C–C), δ(CH3), δ(ArC–H), in
plane, (ArC–H) out of plane, ρr(CH3), ρr(CH2), ρw(CH3) and
ρw(CH2) are identified (Table 2).
Trials to prepare Cd(II) and Zn(II) complexes of the Schiff
base, H4La, ligand and Zn(II) and Fe(III) complexes of the Schiff
base, H2Lb, ligand were unsuccessful, which gave oily products
which were not isolated in their pure forms.
2.4. Physical measurements
The analyses of carbon, hydrogen and nitrogen were carried
out at the Microanalytical Center, Cairo University, Giza, Egypt.
Analyses of metal ions after the dissolution of the solid complex
in hot concentrated nitric acid, HNO3, then diluting with dis-
tilled water and filtering to remove the precipitated ligand. The
solution was neutralized with ammonia solution and the metal
ions were then titrated with EDTA [12–14]. The FT-IR spec-
tra (4000–400 cm−1) of the compounds were recorded as KBr
discs using FT-IR (Shimadzu) spectrophotometer model 4000.
Absorption frequencies are given in wavenumbers (cm−1). 1H
NMR spectra were recorded using a Varian spectrometer, EM-
390, 90 MHz. Dimethyl-sulphoxide, DMSO-d6, was used as a
solvent and tetramethylsilane (TMS) as an internal reference.
The spectra were extended from 0 to 18 ppm. The chemical shifts
(δ) are given down field relative to TMS. D2O was added to every
sample to test for the deuteration of the samples. Reflectance
spectra of the compounds were recorded as NaCl discs using
a Shimadzu UV–vis spectrophotometer 1601 provided with a
diffuse reflectance attachment. Reflectance was carried out at 5◦
incidence angle in the range 400–1200 nm. The solution spec-
tra of the ligands were also carried out in 10−3 M of either
DMF or DMSO solution on a JASCO model V-550 UV–vis
spectrophotometer in the range 200–500 nm. Magnetic suscep-
tibilities of the complexes were measured by the Gouy method at
room temperature using Johnson Matthey, Alfa product, Model
No. (MKI), MagneticSusceptibilityBalance. Effectivemagnetic
moments were calculated from the expression μeff (μB) = 2.828
(χM × T)1/2, where χM is the molar susceptibility [15]. Diamag-

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595
The mass spectra of the Schiff base, H4La and H2Lb ligands,
revealed the molecular ion peaks at m/e 308 and 306, respec-
tively, which are coincident with the formula weights (308.4
and 306.4) for H4La and H2Lb ligands and supports the iden-
tity of the structures. The fragmentation patterns of the mass
spectrum of the H4La ligand is depicted in Scheme 2.
1H NMR spectra of H4La and H2Lb ligands in deuterated
dimethyl-sulphoxide, DMSO-d6, without and with D2O are
shown in Fig. 1. The chemical shifts of the proton signals in the
spectrum of the Schiff base, H4La and H2Lb ligands with their
assignments are listed in Table 3. The proton signals of the –OH
for the alcoholic (4.62 ppm) and phenolic (17.28 ppm) protons
in the former ligand and the proton signals of the –NH2 group
at 3.31 ppm and –OH group at 17.28 ppm, in the later ligand
disappeared in the presence of D2O, which indicate that these
protons are acidic and the hydroxyl groups can participate in the
1
coordination with the metal ions. The H NMR spectra of the
both ligands in DMSO-d6 showed different tautomeric species
for the same ligands. The proposed tautomeric forms I–IV is
illustrated in Scheme 3. It is expected that the tautomeric form I
is more favorable and the tautomeric form IV is less favorable.
On the other hand, it is clear that the tautomeric forms II and
III are in a similar chemical environment and expected to show
the same signals. The aromatic proton signals, which are cen-
tered at 5.86 and 8.17 ppm, in the H4La spectra, and at 5.65 and
8.00 ppm for the H2Lb spectra, can be used as a probe to detect
the ratio of these tautomeric forms. The integrated ratio of I:(II
and III):IV tautomeric forms are 56:40:4 for the H4La ligand
and 50:48:2 for the H2Lb ligand, in DMSO-d6 solvent.
Electronic spectral data of the H4La and H2Lb ligands were
recorded in DMF solution. In the electronic spectra of the lig-
ands, four absorption bands at 220, 315, 365 and 390 nm for
the former ligand and 265, 278, 339 and 371 nm for the later
ligand were characterized. In both cases, the first two bands cor-
respond to 1La → A1 and 1Lb → A1 transitions of the phenyl
1
1
*
ring [17]. The third band corresponds to the ␲ → ␲ transition
of the azomethine group, and the last band corresponds to the
*
n → ␲ transition which is overlapped with the intermolecular
CT from the phenyl ring [18,19].
3.2. Schiff base complexes of H4La and H2Lb ligands
Although, the Schiff base, H4La, ligand behaves as a di- or
tetra-basic ligand with two O2N tridentate sites, the Schiff base,
H2Lb, ligand behaves as dibasic ligand with two N2O triden-
tate sites. The two ligands reacted with Cu(II), Ni(II), Co(II),
Cd(II), Cr(III), Fe(III), VO(IV) and UO2(VI) ions to yield the
corresponding binuclear transition metal complexes. The iso-
lated transition metal complexes were identified by different
techniques such as elemental analyses, FT-IR, 1H NMR, mass,
UV–vis, ESR spectroscopy and thermal gravimetric analysis
(TGA), beside, the magnetic susceptibility measurements.
3.2.1. Infrared spectra
IR spectra of the complexes were recorded to confirm their
structures. Thevibrationalfrequenciesandtheir tentativeassign-
ments for the H4La and H2Lb ligands and their transition metal

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Table 2
Characteristic infrared frequencies (cm−1) of the Schiff base, H4La and H2Lb ligands, 4,6-diacetylresorcinol (DAR), 3-amino-1-porpanol (3-AP) and 1,3-
diaminopropane (DAP) and their assignments
H4La
H2Lb
DAR
3-AP
DAP
Assignments
3250 s, sh
–
3422 s, br
3250 s, sh
3222 w, br
–
–
ν(OH) aromatic
ν(NH2) aliph.
3348 vs,
3300 vs, br
–
3357 vs, br,
3283 vs, br
–
2926 vs,
2856 vs
–
1600 s
–
–
1472 m
1391 m
–
–
–
3082 w
2924 w,
2850 w
1667 vs
–
1624 s, sh, 1588 s
–
1490 m
1370 s
ν(ArC–H)
ν(CH3) and ν(CH2)
2942 m
2929 w,
2850 w
–
1625 s, sh
–
1575 s, br
1476 m
1382 m
1262 s
1200 w
2931 vs
–
–
–
–
ν(C O)
δ(NH2)
ν(C C)
ν(–C N–)
δ(CH3) asym. and δ(CH2) asym.
δ(CH3) sym., δ(CH2) sym.
δ(OH) in plane
ν(C–N) and ν(C–C)
1575 vs
–
1578 vs
1457 m
1370 s
1297 s
1268 vs
1476 s
1387 m
1324 m
1146 w
1258 vs
1180 m
1319 w,
1057 m
1071 s
1050 s
890 w
845 m
745 w
665 w
583 w
421 w
–
1050 m
1030 w, sh
951 w, 903 w
–
1071 s
–
966 w, 910 w
–
–
–
–
–
ν(C–O)
1042 m
957 m, 887 w
854 m
751 w
664 w
δ(ArC–H) in plane
ρr(CH3) and ρr(CH2)
δ(C–N C)
888 s, br
–
–
–
520 w
–
–
δ(N C–C)
666 w
579 s
423 w
δ(ArC–H) out of plane
ρw(CH3) and ρw(CH2)
δ(OH) out of plane
572 w, 449 w
426 w
527 w
–
s: strong, m: medium, w: weak, v: very, vs: very strong, sh: shoulder, and br: broad.
Scheme 2. Fragmentation pattern of the Schiff base, H4La, ligand.

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597
Fig. 1. 1H NMR spectra (δ, ppm) in DMSO-d6 solvent of the (a) Schiff base, H4La, ligand, (b) expansion of the range 5.5–8.5 ppm from the spectrum (a), (c) Schiff
base, H4La, ligand after the addition of D2O, (d) Schiff base, H2Lb, ligand, (e) expansion of the range 1.0–3.4 ppm from the spectrum (d) and (f) Schiff base, H2Lb,
ligand after the addition of the D2O; *suppressed solvent.
complexes are listed in Table 4. The assignments were aided by
comparison with the vibrational frequencies of the free ligands
and their related compounds [20].
of ν(–C N–) did not change compared to the free H2Lb ligand.
This may be due to the interference of the deformation vibra-
tion of H2O molecules, δ(H2O), with the stretching vibration
of the azomethine group, ν(–C N–) [19]. The second feature
is the broad bands in the range of 3333–3445 cm−1 which
can be assigned to the stretching frequencies of the ν(OH) of
methanol, OH-phenolic and/or the water molecules associated to
the complexes which are also confirmed by the elemental anal-
There are four conceptual features in the infrared spectra
of the complexes. The first feature is the shift of the stretch-
ing frequencies of azomethine ν(–C N–) group of the metal
complexes which are in lower frequencies and lie in the range
of 1538–1559 cm−1, for the former ligand complexes, and
1530–1575 for the later ligand complexes compared with the
yses. The third feature is the weak bands at (419–522 cm−1
)
free H4La and H2Lb ligand bands at 1578 and 1575 cm−1
,
and (561–615 cm−1), ranges which could be assigned to the
stretching frequencies of the ν(M–O) and ν(M–N) bands for
the transition metal complexes of both H4La and H2Lb lig-
ands, respectively [23], supporting that the bonding of the ligand
respectively, which may be due to the coordination of the two
azomethine groups to metal ions [21,22]. In the case of Cu(II)
complex (8) and Ni(II) complex (9), it was found that, the value

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Table 3
1H NMR chemical shifts (ppm) of the Schiff base, H4La, and H2Lb ligands in DMSO-d6 and their assignments
Chemical shifts (ppm)
Assignments
H4La
H2Lb
1.59
1.58
[m, 4H, 2CH2] (a)
1.79
1.80
[t, 4H, 2CH2] (b)
2.41
2.41
[s, 6H, 2CH3] (c)
2.69
2.67
[t, 4H, 2CH2] (d)
4.62*
3.31*
[s, br, 2H, 2OH] (e) or [s, br, 4H, 2NH2] (e)
[s, 1H, Ar–H] (f) and [s, 1H, Ar–H] (g)
[s, 2H, 2OH] (h)
5.86 and 8.17
17.28*
(5.44, 5.70 and 5.80 centered at 5.65) and (7.78, 8.03 and 8.18 centered at 8.00)
17.28*
(1) s: singlet, t: triplet, br: broad, m: multiplet. (2) Data given from the spectra depicted in Fig. 1. (3) Chemical shifts (ppm) were referenced internally at 25 ◦C with
respect to TMS. (4) Asterisks (*) denote the bands disappeared after the addition of D2O.
themetalion. Incomplex(5), twoNO3− ionsbehaveasbidentate
to the metal ions is achieved by the alcoholic and/or pheno-
lic oxygen atoms, the azomethine and amino nitrogen atoms of
the ligands. The fourth feature is the coordination behavior of
nitrate and sulfate anions. The NO3− ions are coordinated to the
metal center as unidentate ligands for the complexes (1), (2),
(3) and (4) with C2v symmetry. Each unidentate nitrate group
possesses three non-degenerated modes of the vibrations (νs, νs‘
and νas), which appeared at the ranges 1381–1434, 1317–1356
and 802–810 cm−1, respectively, for the metal complexes which
contain the nitrate groups, the middle band is further splits in
the case of Co(II) an−d Cr(III) complexes [23]. The νs (NO3−) of
the unidentate NO3 is markedly shifted to lower frequencies
compared to that of the free nitrate (1700–1800 cm−1) [23]. This
could be a factor measuring the covalent bond strength which is
formed due to the transfer of an electron density from NO3− to
ligand. These nitrate groups possesses three non-degenerated
modes of the vibrations (ν, νa and νs), which appeared at
−
1542, 1325, and 811 cm−1. Beside, there are two ionic NO3
groups in the Cr(III) complex, their vibrations appeared at
2−
1354 and 1054 cm−1. The SO4 anion is also coordinated as
bridging bidentate ligand in the [(VO)2(Lb)(SO4)(H2O)2]·2H2O
complex (13). Generally, the free sulfate ion belongs to the high-
symmetry point group Td [23]. Of the four fundamentals, only
ν3 and ν4 are infrared active. If the symmetry of the SO42− anion
is lowered by complex formation, the degenerate vibrations split
and also ν1 and ν2 appear in the infrared spectrum [23]. In case
of [(VO)2(Lb)(SO4)(H2O)2]·2H2O complex (13), also ν1 and
ν2 appear with medium intensity at 1056 and 455 cm−1, respec-
tively, moreover, ν3 and ν4 each splits, where ν3 splits into 1124
Scheme 3. Tautomeric structures of the Schiff base, H4La and H2Lb ligands.

Table 4
Characteristic infrared frequencies (cm−1)a of the Schiff base, H4La and H2Lb ligands and their transition metal complexes
Ligand/complex
ν(OH)
νas(NH2)
νs(NH2)
δ(H2O)
ν(C C)
ν(C N)
ν(M–N) ν(M–O)
Other bands
H2O and/or Phenolic and/or
CH3OH
alcoholic
H4La
–
3215 s, br
–
–
–
–
–
–
–
–
–
1615 vs, br 1578 vs, br
–
–
–
–
–
H2Lb
3422 s, br
3225 s, br
3255 s, sh
3242 s, br
3250 s, br
3233 s, br
3250 s, sh
1625 vs, sh 1575 vs, br
–
(1) Cu2(H2La)(NO3)2]·2CH3OH
(2) [Ni2(H2La)(NO3)2(CH3OH)2(H2O)2]
(3) [Co2(H2La)(H2O)2(CH3OH)2(NO3)2]
(4) [Fe2(La)(H2O)4(NO3)2]·2H2O
3412 s, br
3333 s, br
3389 s, br
3390 s, br
3404 s, br
–
–
–
–
–
–
–
–
–
–
1596 vs, br 1543 vs, br 615m
1583 vs, br 1559 vs, br 602m
1562 vs, br 1557 vs, br 590m
1597 vs, br 1538 vs, br 602m
1601 vs, br 1542 vs, br 613m
1594 vs, br 1540 vs, br 613w
504 w
522 w
500 w
513 w
510m
1404 s, 1326 s, 805 w,
unidentate NO3 groups
1413 s, 1326 s, 804 w,
unidentate NO3 groups
1417 s, 1323s, 802 w,
unidentate NO3 groups
1382 s, 1317 s, 807 w,
unidentate NO3 groups
1542 vs, 1325 sh, 811 w,
bidentate NO3 groups
986 w, ν(V O)
(5) [Cr2(H2La)(H2O)2(NO3)2](NO3)2·
2H2O·3CH3OH
(6) [(VO)2(La)(H2O)2]·2H2O
(7) [(UO2)2(H2La)2]·6H2O
(8) [Cu2(Lb)(NO3)2(H2O)4]·H2O
3442 m, br
3421 m, br
3423 s, br
3233 m, br
3250 m, br
–
–
–
–
–
–
–
502 w
487 w
510 w
1555 vs, br 593 w
1575 s, br
–
905 s, ν(UO2)
3250 s, sh
596w
1421 s, 1321 s, 808 w;
unidentate NO3 groups
1434 m, 1329 s, 810 w,
sh; unidentate NO3
groups
1426 m, 1352 vs, 1297 m,
sh, 810 w, sh,; unidentate
NO3 groups
1420 s, sh, 1311 s, br, 810
w, sh; unidentate NO3
groups
1381 vs, 1356 s, 1297 s,
810 w, sh; unidentate
NO3 groups
(9) [Ni2(Lb)(NO3)2(H2O)4]·2CH3OH
(10) [Co2(Lb)(NO3)2(H2O)4]·3H2O
(11) [Cd2(Lb)(NO3)2(H2O)4]·3H2O
(12) [Cr2(Lb)(NO3)4(H2O)2]·4.5H2O
(13) [(VO)2(Lb)(SO4)(H2O)2]·2H2O
3418 s, br
3250 s, sh
3250 s, sh
3250 s, sh
3250 m, sh
3208 s, sh
–
1577 vs, br
1577 vs, br
1546 vs
601w
598w
580w
561m
600sh
496 w
519 w
492 vw
484 m
3422 vs, br
3445 s, br
3415 s, br
3419 s, br
3146 s, sh 1577 vs, br
–
–
1578 vs, br
1599 s, br
1556 s, sh
1540 s, br
1564 vs
3144 s, sh 1580 vs, sh
500 w, sh 1124 s, 1095 s, sh, 1056 s,
sh, 625 m, 612 m, 455 w,
sh; bridging SO4 groups,
942 m, sh; ν(V O)
(14) [(UO2)2(Lb)2(H2O)4]·4H2O
3420 m, br
3250 s, sh
–
1578 vs
1530 s, sh
587vw
419 vw
897 m; ν3(UO2)
a
s: strong, m: medium, w: weak, v: very, vs: very strong, sh: shoulder, and br: broad.

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A.A.A. Emara et al. / Spectrochimica Acta Part A 68 (2007) 592–604
Table 5
Characteristic electronic spectral bands (nm) and magnetic moments of the Schiff base, H4La and H2Lb ligands and their transition metal complexes
Ligand/complex
λmax (nm)a
Assignments
Magnetic moments (μB)
b
c
μcompl.
μeff.
(1) [Cu2(H2La)(NO3)2]·2CH3OH
675
480,
693,
1080
605
500, 533, 691
3T2g(G) ← Eg
2.43
4.89
1.72
3.11
3
(2) [Ni2(H2La)(NO3)2(CH3OH)2(H2O)2]
3T1g(P) ← A2g(F),
3
3T1g(F) ← A2g(F),
3
3T2g(F) ← A2g(F)
4
(3) [Co2(H2La)(H2O)2(CH3OH)2(NO3)2]
(4) [Fe2(La)(H2O)4(NO3)2]·2H2O
4A2g(F) ← T1g(F)
5.85
6.70
4.81
4.74
Charge transfer tailing from UV to
visible region
4
(5) [Cr2(H2La)(H2O)2(NO3)2](NO3)2·2H2O·3CH3OH
485
605
652
509
668
444,
526
631
–
615,
745
674
485
4T1g(F) ← A2g(F)
5.47
3.87
4
4T2g(F) ← A2g(F)
(6) [(VO)2(La)(H2O)2]·2H2O
(7) [(UO2)2(H2La)2]·6H2O
b1 ← b2
1.70
–
3.07
4.39
1.20
–
2.17
3.10
Charge transfer
3T2g(G) ← Eg
(8) [Cu2(Lb)(NO3)2(H2O)4]·H2O
(9) [Ni2(Lb)(NO3)2(H2O)4]·2CH3OH
3
3T1g(P) ← A2g(F),
3
3T1g(F) ← A2g(F)
4
(10) [Co2(Lb)(NO3)2(H2O)4]·3H2O
(11) [Cd2(Lb)(NO3)2(H2O)4]·3H2O
(12) [Cr2(Lb)(NO3)4(H2O)2]·4.5H2O
4A2g(F) ← T1g(F)
7.19
–
5.38
5.08
–
3.81
–
4
4T1g(F) ← A2g(F),
4T2g(F) ← A2g(F)
4
(13) [VO)2(Lb)(SO4)(H2O)2]·2H2O
(14) [(UO2)2(Lb)2(H2O)4]·4H2O
b1 ← b2
–
–
–
–
Charge transfer
a
Reflectance spectra of all complexes were recorded with disks made by compressing a mixture of the complex sample with suitable amount of NaCl (Analar).
Also, NaCl compressed disk was used as a reference sample.
b
μcompl. is the magnetic moment of all metal ions present in the complex.
μeff. is the magnetic moment of one metal ion in the complex.
c
and 1095 cm−1, and ν4 splits into 625 and 612 cm−1. These
splitting can be explained by a lowering of symmetry, where
the bridging bidentate sulfato group belongs to C2v point group
[23].
The IR spectra of VO(IV) complexes display a band at
986 cm−1 for complex (6) and a shoulder band at 942 cm−1
for complex (13) which have no counterpart in the spectrum of
the H4La and H2Lb ligands and are assigned to the stretching
frequency of the ν(V O). Also, The IR spectra of the UO2(VI)
complexes display a characteristic band for complexes (7) and
(14) at 905 and 897 cm−1, respectively, which are assigned to
the stretching frequency of the ν(UO2).
complex (1) and octahedral structure for complex (8) [24].
X-Band ESR spectra of [Cu2(H2La)(NO3)2]·2CH3OH (1) and
[Cu2(Lb)(NO3)2(H2O)4]·H2O (8), Fig. 2a and b, were recorded
in the solid state at 25 ◦C. The spectrum of the former complex
exhibits one broad band with g = 2.1081. The shape of the spec-
trum is consistent with the square planar geometry around each
Cu(II) ion in the complex [24,25]. The spectrum for complex
(8) exhibits two bands, one of them is sharp with g = 2.1003 and
the other is very weak at g = 2.0023. The shape of the spectrum
is consistent with the octahedral geometry around each Cu(II)
center in the complex.
The electronic spectrum of the green Ni(II) complexes (2) and
(9) showed several bands. Generally, three spin-allowed transi-
tionsareexpected, becauseofthesplittingofthefree-ion, ground
3
3F term and the presence of the P-term [25–27]. Usually, the
3.2.2. Electronic, mass and ESR spectra and magnetic
moment measurements
spectra of octahedral Ni(II) consist of three bands which are
3
3
accordinglyassignedas3T2g(F) ← A2g(F), 3T1g(F) ← A2g(F)
The metal complexes are insoluble in most common solvents
and were not possible to get electronic spectra in solution. The
electronic spectra were obtained by reflectance within the range
400–1200 nm. It is possible to draw up the electronic transitions
and predict the geometry with the aid of magnetic moments of
most metal ions. Table 5 lists the electronic spectral bands and
the magnetic moments of the metal ions in their complexes in
the solid state.
and 3T1g(P) ← A2g(F). The 3T2g(F) ← A2g(F) transition was
observed at 1080 nm, in complex (2) and was not observed in
complex (9), as it occurs in the near infrared and thus out of
3
3
the instrument. The 3T1g(F) ← A2g(F) transition was observed
3
at 693 nm and 526 nm, for both complex (2) and complex (9),
respectively. The third band due to 3T1g(P) ← A2g(F) transition
3
was observed at 480 nm for the former complex, while in the
second complex this transition is obscured by ligand absorption
bands. The magnetic moments were measured which gave 3.11
and 3.10μB, for complexes (2) and (9), respectively, which lies
in therange(2.9–3.3μB) of theNi(II) octahedralcomplexes [23].
The mass spectrum of the, [Ni2(Lb)(NO3)2(H2O)4]·2CH3OH,
complex (9) showed the parent peak at m/e 617 which compares
The electronic spectrum of the green Cu(II) complexes (1)
3
and (8) showed one band due to T2g(G) ← Eg transition,
which was observed at 675 and 668 nm, respectively. The mea-
sured value of the magnetic moment for Cu(II) complex was
1.72μB, for the former complex (1) and 2.17μB for the later
complex (8), which confirms the square planar structure for

A.A.A. Emara et al. / Spectrochimica Acta Part A 68 (2007) 592–604
601
Fig. 2. X-Band ESR spectra of: (a) [Cu2(H2La)(NO3)2]·2CH3OH (1), (b) [Cu2(Lb)(NO3)2(H2O)4]·H2O (8), (c) [(VO)2(La)(H2O)2]·2H2O (6) and (d)
[(VO)2(Lb)(H2O)2]·2H2O (13) complexes.
well with the formula weight of the complex after loss of the
two uncoordinated methanol molecules.
(10), showed the molecular ion peak at m/e 672 which agrees
well with the formula weight of the complex (FW 672.4).
The electronic spectrum of the red Fe(III) complex (4)
showed strong bands at 500, 533 nm and a weak band at 961 nm.
It was not possible to identify the type of the d–d transition. This
is due to a strong charge transfer (CT) band tailing from the UV-
region to the visible region. Generally, from the elemental and
the infrared spectrum which gives a significant proof for the
nitrate anion to act as a unidentate ligand, it is expected that the
Fe(III) complex has octahedral arrangement. Magnetic moment
of the Fe(III) complex was measured and gives 4.74μB. This
value is lower than the magnetic moment of the high spin octa-
hedral, t32g e2g arrangement, which amounts to 5.92 B.M [27]. A
complete interpretation of the lower value than normal was not
possible without knowing the crystal structure of the complex.
On the other hand, the experimental value is too far higher than
the low spin octahedral, (t25ge0g) arrangement, where its magnetic
moment amounts to 2.3 B.M [26]. Generally, a tentative inter-
pretation expects the structure of Fe(III) as high-spin octahedral
geometry, with antiferromagnetic interaction.
The electronic spectra of the green Co(II) complexes ((3) and
(10)) showed that, both complexes have an octahedral geome-
try. The transitions can be interpreted by using Tanab-Sugano
diagram. In the Co(II) octahedral complexes the spectra usu-
ally consist of three bands [25]. The first band, which is due to
4T2g(F) ← T1g(F) transition, this band was not observed due to
4
the fact that it occurs in the near infrared region, and out of the
range of the used instrument. The other two bands were observed
4
in the visible region, the former one is due to 4A2g(F) ← T1g(F)
transition, which is observed at 605 and 631 nm for com-
plexes ((3) and (10)), respectively. The second band due to
4
4T1g(P) ← T1g(F) transition is expected at 450 nm was not dis-
tinguished as it may be overlapped by charge transfer band or
it would be obscured by ligand transition. The Co(II) complex
showed the magnetic moment 4.81 and 5.08μB for complexes
(3) and (10), respectively, at room temperature where that of
the usual octahedral complexes are 4.8–5.2 B.M [26]. The mass
spectrumoftheCo(II)complex, [Co2(Lb)(NO3)2(H2O)4]·3H2O

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A.A.A. Emara et al. / Spectrochimica Acta Part A 68 (2007) 592–604
The Cd(II) complex (11) is diamagnetic as expected and
its geometry is most probably octahedral similar to the Cu(II),
Ni(II) and Co(II) complexes of the H2Lb ligand.
The electronic spectrum of the dark green Cr(III) complexes
(5) and (12) showed an octahedral geometry. In the Cr(III)
octahedral complexes, the splitting of the free ion ground F
term along with the presence of the excited P-term of the
same multiplicity provides the possibility of three spin allowed
d–d transitions [28]. The first two bands which are due to
which would be octahedral structure. Although most oxovana-
dium(IV) complexes are magnetically simple which having
virtually ‘spin-only’ moments of 1.73μB corresponding to one
unpaired electron. In this study, the actual magnetic moments
were 1.20 and 1.22μB for complexes (6) and (13), respectively.
These values are less than the expected and could be due to
antiferromagnetic effect between vanadium atoms of adjacent
molecules linked together through oxygen atoms.
The X-band ESR spectra of VO(IV) complexes (6) and (13),
Fig. 2c and d, were recorded in the solid state at 25 ◦C. The ESR
spectra of the oxovanadium, [VO2-sq. py.], (6), and [VO2-oct.]
(13) complexes, exhibit one broad band centered on g = 2.1081
and g = 2.1085, respectively, without resolved hyperfine struc-
ture. In particular, the hyperfine coupling with nearly 51V
(I = 7/2, S = 1/2) nucleus is not observable. The absence of vana-
dium’s hyperfine coupling is common in the solid state [29] and
is attributed to the simultaneous flipping of neighboring electron
spin [30] or due to strong exchange interactions, which average
out the interaction with the nuclei. For [VO2-sq. py.], upon pair-
ing the two vanadium ions, the two electron spins may combined
to give a non-magnetic spin singlet (S = 0) or a paramagnetic spin
4
4
4
4T2g(F) ← A2g(F) and T1g(F) ← A2g(F) transitions, were
observed at 605 and 485 nm, for the complex (5) and 745 and
615 nm, for the complex (12). The third band which is due to
4
4T1g(P) ← A2g(F) transition lies in the range of the ligand tran-
sitions and is not possible to be identified. The magnetic moment
is expected to be very close to the spin-only value for three
unpaired electrons, due to the absence of any orbital contribu-
tion. In the present work, the magnetic moments of the Cr(III)
complexes (5) and (12) are 3.87 and 3.81 B.M, respectively.
The electronic spectrum of the green VO(IV) complexes
(6) and (13) showed one band at 652 and 674 nm, respec-
tively, which corresponds to b1 ← b2 electronic transition [27].
With the aid of the elemental analysis and the infrared spec-
tra, the VO(IV) complexes (6) is 5-coordinate which would
be square pyramidal and the complex (13) is 6-coordinate
triplet (S = 1). Only the latter ESR is detectable ꢀms
[31].
2
In the present complex, there is a slight interaction between
the two magnetically vanadium centers which is consistent with
Fig. 3. TGA–DrTGA curves of: (a) [Fe2(HLa)(H2O)4(NO3)2]·2H2O (4) and (b) [Ni2(Lb)(NO3)2(H2O)4]·2CH3OH (9) complexes.

A.A.A. Emara et al. / Spectrochimica Acta Part A 68 (2007) 592–604
603
normal–effective magnetic moment. Square–pyramid geometry
is proposed for the binuclear complexes. The value of the mag-
netic moment (1.20μB) agrees well with the proposed structures.
For (VO2-oct.) complex the shape of the spectrum as well as the
spectral studies agree with the proposed structures.
The electronic spectrum of the yellow UO2(VI) com-
plexes (7) and (14) arises from the electronic transition of
metal → ligand charge transfer. This is an allowed transition and
produces a broad, intense absorption band at 509 and 485 nm
for complexes (7) and (14), respectively, tailing into the visi-
ble region. This produces the intense yellow color, where the
UO2(VI) complexes are diamagnetic as expected [32].
Scheme 4. Thermal degradation pattern of Fe(III) complex, [Fe2(La)(H2O)4
(NO3)2]·2H2O (4).
molecules in complex (4). DrTGA curve of the complex exhibits
two endothermic peaks. The first peak was at 74 ◦C which is
ascribed to the loss of crystalline water molecules while the
second peak at 204 ◦C is due to the loss of coordinated water
molecules. Also, there are two exothermic peaks, the first one
was at 381.5 ◦C which is due to the loss of N2O5 molecules
and the second peak was at 440 ◦C which is due to the decom-
position and phase transfer of the complex. Fig. 3a shows the
TGA–DrTGA curves of the [Fe2(La)(H2O)4(NO3)2]·2H2O (4)
complex.
Thermal gravimetric analysis (TGA) for complex (9),
[Ni2(Lb)(NO3)2–(H2O)4]·2CH3OH, shows two stages in the
range 30–310 ◦C. The former one at 35–120 ◦C, is due
to the loss of two uncoordinated methanol molecules
(weight loss; Calc./Found%; 9.4/9.1%). The second stage
at 140–310 ◦C and is due to the loss of four coordinated
water molecules (weight loss; Calc./Found%; 10.5/10.4%).
Fig. 3b shows the TGA–DrTGA curves of complex (9).
3.2.3. Thermal gravimetric analysis
Thermal gravimetric analysis (TGA) was used as a probe
to proof the associated water or solvent molecules to be in the
coordination sphere or in the crystalline form [33]. TGA curve of
[Fe2(La)(H2O)4(NO3)2]·2H2O, complex (4), shows four stages,
Scheme 4. The former one is at 42.5–140 ◦C with the loss of two
hydrated water molecules (Calc./Found%; 5.55/ 6.18%) of the
total weight of the complex. The second stage at 142–254 ◦C
corresponds to the loss of four coordinated water molecules
(Calc./Found%; 11.76/13.91%). The third stage at 254–381.5 ◦C
corresponds to the loss of N2O5 gas molecules (Calc./Found%;
20.00/ 21.34%). The later stage was at 381.5–800 ◦C and is
due to the gradual decomposition of the complex. The residue
was 37.63% of the total weight which may be ascribed to the
formation of iron(III) oxide. The first two stages are the impor-
tant stages to identify the coordinated and crystalline water
Fig. 4. Representative structures of the transition metal complexes of the Schiff base (A) H4La ligand and (B) H2Lb ligand.

604
A.A.A. Emara et al. / Spectrochimica Acta Part A 68 (2007) 592–604
[11] H.S. Seleem, A.A.A. Emara, M. Shebl, J. Coord. Chem. 58 (2005) 1003.
[12] H.A. Flaschka, EDTA Titration, 2nd ed., Pergamon Press, 1964, p. 81.
[13] A.I. Vogel, Textbook of Quantitative Inorganic Analysis, 4th ed., Longman,
London, 1978.
[14] T.S. West, Complexometry with EDTA and Related Reagents, 3rd ed., DBH
Ltd., Pools, 1969.
[15] F.E. Mabbs, D.I. Machin, Magnitism and Transition Metal Complexes,
Chapman and Hall, London, 1973.
[16] E.C. Alyea, A. Malek, Can. J. Chem. 53 (1975) 939.
[17] R.J. Platt, J. Chem. Phys. 17 (1949) 484.
Scheme 5. Thermal degradation pattern of complex (9) in the range of
30–310 ◦C.
[18] E. Pretsch, J. Seibl, Tables of Spectral Data for Structure Determination of
Organic Compounds, Springer-Verlag, Berlin, 1983.
[19] A.A.A. Abou-Hussen, A.A.A. Emara, J. Coord. Chem. 57 (2004)
973.
[20] B. Jazowska, J. Lisowshi, A. Vogt, P. Chemielewski, Polyhedron 7 (1988)
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[21] S.M.E. Khalil, A.A.A. Emara, J. Coord. Chem. 55 (2002) 17.
[22] B.A. El-Sayed, M.M. Abo Aly, A.A.A. Emara, S.M.E. Khalil, Vibr. Spec-
trosc. 93 (2002) 30.
Scheme 5 is the thermal degradation pattern of complex (9),
[Ni2(Lb)(NO3)2(H2O)4]·2CH3OH, in the range of 30–310 ◦C.
Finally, from the interpretation of elemental analysis and
infrared, mass, electronic spectra, ESR for Cu(II) and VO(IV)
complexes and TGA for Ni(II) complex, it is possible to draw up
the tentative structures of the transition metal complexes. Fig. 4
depicts the proposed structures of the metal complexes.
[23] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds,
5th ed., John Wiley and Sons, New York, 1997.
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