Characterization of transition metal ion chelates with 8-(arylazo) chromones using thermal and spectral techniques

Article abstract of DOI:10.1023/A:1026390105986

Transition metal chelates of TiO^IV, VO^IV, Cr^III, Fe^III, Co^III and Pd^III ions with 8-(arylazo)chromones have been isolated and investigated by elemental analysis, IR, electronic spectra, the

Full text of DOI:10.1023/A:1026390105986

Journal of Thermal Analysis and Calorimetry, Vol. 74 (2003) 181–200
CHARACTERIZATION OF TRANSITION METAL ION
CHELATES WITH 8-(ARYLAZO) CHROMONES USING
THERMAL AND SPECTRAL TECHNIQUES
Omyma E. Sherif *, Hussein M. Abd El-Fattah and Marie M. El-Ajily
Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt
(Received October 27, 2002; in revised form March 15, 2003)
Abstract
Transition metal chelates of TiOIV, VOIV, CrIII, FeIII, CoIII and PdII ions with 8-(arylazo)chromones
have been isolated and investigated by elemental analysis, IR, electronic spectra,
thermogravimetric analysis, conductance and magnetic moment measurements. Chelates of gen-
eral formula [ML or M2LXm⋅nH2O]·yH2O, where, X=OH– or Cl– or SO2– ion, m=1, 2 or 4, n=1, 2, 4
4
or 5, y=1–6, M=TiOIV, VOIV, CrIII, FeIII, CoIII and PdII ions and L=8-(arylazo) chromones have been
synthesized. IR spectra indicate that the chromone carbonyl in position four and oxygen anion in
position five participate in chelation in the 1:1 (M:L) chelates, whereas, for 2:1 (M:L) chelates the
interaction of the metal ion takes place via the chromone carbonyl in position four and oxygen an-
ion in position five as first chelation site, for the second metal ion chelation takes place through the
formyl carbonyl in position six and oxygen anion in position seven for some chelates or through
the azo group and any acidic substituent in ortho position to it. Octahedral or square planar struc-
tures were proposed for the prepared chelates based on their electronic spectra and magnetic mo-
ments. Thermogravimetric analysis suggests the presence of hydrated and coordinated water mole-
cules. Non electrolytic nature was assigned based on molar conductance values.
Keywords: 8-(arylazo)chromones, chelates, Co(III), Cr(III), Fe(III), Pd(II), TiO(IV), VO(IV)
Introduction
Chromones are those compounds which contain γ-pyrone nucleus fused to benzene ring
at the 5- and 6-positions. Chromones are important compounds having pharmacological
activity. Some chromones have marked analgesic anti-inflammatory and antipyretic ac-
tivities [1]. The interest in the chemistry of hydroxychromones is based on their ability to
function as determination of a large number of transition metals [2–7].
The metal chelates of vanadium(IV), chromium(III), manganese(II), iron(II), co-
balt(II), nickel(II), copper(II) and zinc(II) ions of 8-(arylazo)-6-formyl-7-hydroxy-5-
methoxy-2-methylchromones were prepared and investigated using several techniques
[8]. Divalent metal ion chelates of 6-formyl-7-hydroxy-5-methoxy-2-methylchromone;
5,7-dihydroxy-6-formyl-2-methylchromone and 5,7-dihydroxy-2,6-dimethylchromone
have been prepared and studied [9].
*
Author for correspondence: E-mail: salatif_1@yahoo.com
1388–3150/2003/ $ 20.00
Akadémiai Kiadó, Budapest
© 2003 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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Several 8-(arylazo)chromones were prepared and investigated by using elemen-
tal analysis, IR, 1H NMR and mass spectra by Issa et al. [10].
Metal chelates of the divalent metal ions Mn(II), Fe(II), Co(II), Ni(II), Cu(II),
Zn(II) and Pd(II) with 8-(arylazo) derivatives of 5,7-dihydroxy-2,6-dimethyl-
chromones and 5,7-dihydroxy-6-formyl-2-methylchromones were synthesized and
characterized by elemental analyses, IR and electronic spectra, thermogravimetric
analyses, magnetic and conductance measurements [11].
The preparation and structure elucidation of the metal chelates of Ce(III),
Th(IV) and U(VI) with some Schiff bases derived from 5,7-dihydroxy-6-formyl-2-
methylbenzopyran-4-one have been reported [12].
The thermal decomposition of the complexes of N,N-dialkyl-N’-benzoylthiourea
with Pd(II) and Fe(III) were studied by TG and DTA techniques. These metal complexes
decompose in two stages. The influence of the alkyl substituents in these benzoylthiourea
chelates on the thermal behaviour of the metal complexes was investigated [13]. New
complexes of CuL2⋅2H2O, CoL2⋅3H2O, MnL2⋅2H2O and FeL3⋅2H2O, L=γ-L-glutamyl-5-
(p-nitroanilide) were synthesized [14] and characterized via investigating their spectral,
magnetic and thermal properties. The thermal stabilities of the synthesized complexes
were examined in the temperature range 20–500°C. The ligand acted as a bidentate one
and its coordination involving the carbonyl oxygen and the nitrogen atom of the second
amino group. The metal ion is in the high-spin form with octahedral stereochemistry.
Fe(III) complexes of o-vanilin oxime have been synthesized and characterized via ele-
mental analysis, NMR, IR and thermal analyses [15]. Investigation of the influence of the
Fe(III) and other metal ions on thermal decomposition of humic acids were carried out by
using TG and DTA [16].
The aim of the present study is to prepare the metal chelates formed between tita-
nium(IV), vanadium(IV), chromium(III), iron(III), cobalt(III) and palladium(II) with
8-(arylazo)-5,7-dihydroxy-6-formyl-2-methylchromone (Ia–d) and 8-(arylazo)-5,7-di-
hydroxy-2,6-dimethylchromone (IIa–d). Also, to elucidate the geometrical structures of
the obtained chelates.
Scheme 1 Structure of the ligands used
Experimental
Preparation and analysis of the chelates
The chromone derivative, 6-formyl-7-hydroxy-5-methoxy-2-methylchromone was
prepared by oxidation of visnagin with chromic acid while 5,7-dihydroxy-6-formyl-
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2-methylchromone(I) was prepared by hydrolysis of the previously prepared
chromone with HCl and 5,7-dihydroxy-2,6-dimethylchromone(II) was prepared by
reduction of compound (I) with Zn [10].
The 8-(arylazo)chromones (Ia–d and IIa–d) were prepared by coupling equimolar
amounts of the diazotized corresponding amines (aniline, 2-aminophenol, 2-anthranilic-
and 2-arsanilic acids) with the chromones I and II in presence of NaOH [10]. The metal
chelates of 8-(arylazo)chromones were synthesized by mixing a hot saturated EtOH solu-
tion of the metal salt [Ti2(SO4)3, VOSO4⋅H2O, CrCl3⋅2H2O, FeSO4⋅7H2O, CoCl2⋅6H2O
and PdCl2] (0.01 mole) with the requisite amount of a saturated ethanol solution of the
azo dye to form 1:1 or 2:1 (M:L) chelates [8]. The metal ion content of the chelates was
determined, after wet decomposition of the chelates [17], as previously described [18].
IR spectra of the chelates were obtained by applying the KBr disc technique us-
ing Perkin Elmer 1430 infrared spectrometer. The electronic spectra of the
8-(arylazo)chromones and their chelates were measured by applying the Nujol mull
technique using a Perkin Elmer lambda 4B spectrometer with 1 cm matched silica
cells. TG for some chelates containing water molecules were achieved using
Schimadzu-50 thermal analyzer (Japan). The mass loss was measured from ambient
temperature up to 1000°C at a rate of 10°C min–1. The conductivity measurements
were carried out in DMF solution by using conductivity bridge model CM-IK, TOA
Company (Japan). Magnetic moments were measured using a Johnson–Mathey
susceptometer devised by D. F. Evans.
The chemicals used were reagent grade (BDH or Aldrich). All organic solvents
used in this work were either obtained as pure materials from BDH or purified by rec-
ommended methods [19]. Double distilled water was always used.
Results and discussion
Table 1 comprises the elemental analysis and thermogravimetric data of the transition
metal ion chelates of 8-(arylazo)chromones (Ia–d and IIa–d). The obtained data are in
good agreement with those calculated for the proposed tentative formulae of the che-
lates. The suggested formula is [ML] – or [M2LXm⋅nH2O]⋅yH2O where, X=OH– or Cl–
or SO24– ion, m=1, 2 or 4, n=1, 2, 4 or 5, y=1–6, M=TiOIV, VOIV, CrIII, FeIII, CoIII and
PdII) ions, L is the 8-(arylazo)chromones (Ia–d and IIa–d) molecule.
Thermogravimetric analysis
Thermal studies have been carried out using a thermogravimetric TG and a derivative
thermogravimetric DTG techniques. TG and DTG curves are given in Figs 1, 2 and 3.
From TG curves (Figs 1, 2 and 3), the mass loss of the metal chelate was calculated
using the change produced as a result of the removal of water molecules. Table 2 shows
the calculated and the found mass losses of the chelates. The found mass losses were cal-
culated from the TG curves and the calculated ones were obtained from the suggested
tentative molecular formulae based on the obtained data of the elemental analyses.
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The initial mass loss occurring within the 55–160°C range is interpreted as being
due to a loss of moisture and hydrated water molecules during the chelate drying pro-
cess, whereas the second mass loss within the 160–310°C range is due to coordinated
water molecules.
Fig. 1 Thermogravimetric and derivative thermogravimetric analysis curves of Ti che-
late of 8-(phenylazo)-5,7-dihydroxy-2,6-dimethylchromone (IIa) and V chelate
of 8-(2-arsonophenylazo)-5,7-dihydroxy-2,6-dimethylchromone (IId)
Fig. 2 Thermogravimetric and derivative thermogravimetric analysis curves of Pd and Fe
chelates of 8-(2-hydroxyphenylazo)-5,7-dihydroxy-2,6-dimethylchromone (IIb)
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Fig. 3 Thermogravimetric and derivative thermogravimetric analysis curve of Ti chelate
of 8-(carboxyphenylazo)-5,7-dihydroxy-6-formyl 2-methylchromone (Ic)
For 2:1 (M:L) TiO–IIa and VO–IId chelates, the mass losses occurring at 15.20
and 11.20%, respectively, correspond to the loss of five water molecules of hydration
in the 66–160°C range. For the TiO–Ic and Fe–IIb chelates, the percentages of mass
losses (up to 140°C) are 14.50 and 16.00%, respectively suggesting the presence of six
hydrated water molecules. On the other hand, the mass losses are 10.50, 8.5, 12.80 and
9.80% for TiO–Ic, TiO–IIa, Fe–IIb and VO–IId chelates within the 140–310°C range
may be due to the existence of four coordinated water molecules. The 2:1 Pd–IIb che-
late exhibits mass loss of 5.00% showing the existence of two coordinated water mole-
cules. The results of the TG analyses suggest the formation of the metal oxide as the fi-
nal product in the temperature range from 500 to 1000°C. The percentages of the metal
oxide found experimentally for TiO–Ic, TiO–IIa, Fe–IIb,VO–IId and Pd–IIb chelates
amounted to 22.00, 25.71, 23.00, 24.60 and 33.40%, respectively which are in good
agreement with the calculated ones for the suggested formulae (Table 1).
The organic part of the complexes (the ligand moiety attached to the metal atom)
may decompose in one or more steps with the possibility of the formation of one or
more intermediates. These intermediates finally decompose to the stable metal oxides.
The 2:1 (M:L) complexes of TiO–IIa and Pd–IIb were found to decompose in one step
(Table 2). The decomposition of (2:1) TiO–Ic, Fe–IIb and VO–IId was completed
through more than one decomposition step (Table 2).
In case of 2:1 (M:L) TiO–Ic, TiO–IIa, VO–IId, Pd–IIb and Fe–IIb chelates, their
thermal decomposition may be illustrated using the following scheme:
55–160°C
[M2LXm⋅nH2O]⋅yH2O→[M2LXm⋅nH2O]
160–310°C
[M2LXm⋅nH2O]→[M2LXm]
300–466°C
[M2LXm]→Intermediate (unstable)
>500°C
Intermediate→Metal oxide. (TiO2, V2O5, PdO and Fe2O3).
(X=OH– or Cl–, m=2, 4 n=2–4 and y=5 or 6).
The molar conductance of the 1:1 and 2:1 (M:L) chelates lie in the
3.50–22.60 ohm–1 cm2 mol1 range, suggesting their non-electrolytic nature [20].
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IR spectra of ligands and metal chelates
The chelation between the ligands and metal ions can be inferred by comparing the IR
spectra of the metal chelates with those of the organic ligands. IR-band assignments
of the chelates are depicted in Tables 3 and 4. The presence of water molecules in the
solid complex may lead to confusion in assignment of the νOH group. The participa-
tion of the hydroxyl groups in the chelation is confirmed by the shift of δOH and γOH
bands of the free ligands on chelation. Meanwhile, bands due to different modes of
water vibration and M–O are observed (Tables 3 and 4).
In 1:1 (M:L) chelates of Cr(III) with the ligands Ia–c, the band due to the
chromone carbonyl in position four (1662–1680 cm–1) disappeared while in case of
1:1 chelates of Cr(III)–IIa–c and Co(III)–IIa, this band is shifted to lower frequency
(1651–1658 cm–1). This suggests the participation of this group in coordination.
The 1409–1440 cm–1 band due to –N=N–, 1636–1650 cm–1 band attributed to
the formyl carbonyl in position six and 1700 cm–1 band of the carboxylic carbonyl of
free azo dyes are slightly affected on chelation in case of 1:1 chelates.
In the IR spectra of 2:1 (M:L) chelates, the band due to chromone carbonyl in po-
sition four (1662–1680 cm–1) disappeared for the chelates of titanium(IV), vana-
dium(IV), chromium(III), iron(III) ions and palladium(II) with ligands Ia–d. On the
other hand, this band is shifted to lower frequency for chelates of the above men-
tioned ions with ligands IIa–d. This indicates the participation of the chromone car-
bonyl group in coordination with the metal ion.
The 1636–1650 cm–1 band due to stretching vibration of the formyl carbonyl in
position six for 8-(arylazo)-5,7-dihydroxy-6-formyl-2-methylchromones, (Ia–d) is
shifted to lower frequency in the 2:1 (M:L) chelates of titanium(IV), vanadium(IV),
chromium(III) and iron(III) ions with ligands Ia, Ib, Id and Pd(II)–Id chelates sug-
gesting for this stoichiometric ratio, that the aldehydic group is involved in chelation.
On the other hand, the band due to aldehydic group is not affected by chelation in
case of 2:1 VO–Ic chelate, meanwhile, the bands assigned to the carboxylic carbonyl
in ortho position to the azo group and that due to the azo group disappeared.
For ligands IIa–d in which the position six is substituted by methyl group rather than
formyl group. The IR spectra of 1:1 and 2:1 (M:L) chelates display a shift in C=O of the
chromone moiety (1651–1658 cm–1) to lower frequency (1630–1648 cm–1). The band
due to N=N is shifted to lower frequency in case of 2:1 (M:L) chelates. In the IR spectra
of 2:1 (M:L) chelates of ligand IIc with metal ions, titanium(IV), vanadium(IV), iron III)
and palladium(II), the band due to the carboxylic carbonyl at 1700 cm–1 disappeared. This
indicates the participation of these groups in chelation. The IR spectra of 1:1 chelates of
Cr(III)–IIb and 2:1 Cr(III) with the azo dyes Id and IId and Pd(II) chelates with Id and
IIb–c exhibit a band at 340–360 cm–1 which may be attributed to the M–Cl stretching
[21]. The IR spectra of the chelates show a single band at 420–565 cm–1 which is as-
signed to M–O [22].
In case of the 2:1 chelates of VO(IV)–Ic and TiO(IV), VO(IV), Cr(III), Fe(III)
and Pd(II) with 8-(arylazo)-5,7-dihyroxy-2,6-dimethylchromones (IIa–d) the IR
spectra display a band at 270–390 cm–1 due to the M–N vibration [23]. The new band
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occurring at 1114 cm–1 in the IR spectrum of Fe(III) chelate with the ligand IId can be
assigned to the normal mode of vibration of the SO24– [24].
Electronic spectra and magnetic moments
The electronic spectra of the organic ligands and their metal chelates are depicted in
Table 5. The 8-(arylazo)chromones (Ia–d and IIa–d) display a shoulder in the
320–330 nm (31250–30303 cm–1) and 300–314 nm (33333–31847 cm–1) range, re-
spectively. This band is attributed to electronic transition within the chromone ring.
On the other hand, the bands in the 352–450 nm (28392–22222 cm–1) range may be
due to the π–π transition of the azo moiety influenced by the intramolecular charge
transfer within the ligand molecules.
The electronic spectra of the 1:1 Cr(III) chelates with ligands Ia, Ib and IIa–c
show a band in the 20665–22794 cm–1, which can be assigned to the 4A2g(F)→4T1g(F)
transition. The magnetic moment values for these chelates, given in Table 4, indicate
the presence of three unpaired electrons in each chelate molecule. The electronic
spectrum of 1:1 Co(III)–IIa chelate exhibits a band at 22237 cm–1 corresponding to
1
the A1g→1T2g transition, and it is diamagnetic indicating the presence of Co ion in
the trivalent state. It is to be mentioned that in the other cobalt chelates with the lig-
ands Ia–d and IIb–d the cobalt is formed in oxidation state II [11].
The 2:1 (M:L) TiO(IV) chelates with the 8-(arylazo)chromones (IIa–d) display a
band in the 22016–22614 cm–1 range and they are diamagnetic. Whereas, the VO(IV)
chelates with the ligands (Ia–d and IIa–d) exhibit bands in the 19747–22675 and
20312–23866 cm–1 ranges, respectively which may be due to the 2T2g→2Eg transition.
The magnetic moment values per VO(IV) ion (Table 4) indicate the presence of one
unpaired electron confirming the quadrivalent state of vanadium ion (VO2+). The
electronic spectra of Cr(III) chelates with the ligands Id and IId reveal an absorption
4
band at 21958 and 22060 cm–1, respectively, corresponding to the A2g(F)→4T1g(F)
transition. The magnetic moments per Cr(III) ion are 3.04 and 3.44 B.M, indicating
the presence of high spin octahedral structure [19]. The Fe(III) chelates with the
chromone azo dyes Ia, Id and IIb–d display a band in the 21834–24509 cm–1 range,
which can be attributed to the 2A2g→2T1g transition. The obtained magnetic moment
values per Fe(III) ion are given in Table 4 suggesting the presence of Fe(III) ion in
each chelate. From the foregoing observations, the octahedral geometry was sug-
gested for all chelates [25, 26].
The electronic spectra of the 1:1 (M:L) Pd(II) chelates with the ligands (Ia–c)
show bands in the 19230–22629 and 19948–22472 cm–1 ranges, respectively which
1
can be assigned to A1g→1B1g transition. Whereas, the electronic spectrum of the
2:1 (M:L) Pd–Id chelate exhibits a band at 22614 cm–1, corresponding to the same
transition. The electronic spectra of the Pd–IIa–d chelates display a band in the
19853–22614 cm–1 range, due to the 1A1g→1B1g transition. A square planar geometry
was thus suggested for all chelates [27].
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Conclusions
From the previous investigation [elemental analysis, thermal analysis, spectral and
magnetic properties of metal chelates], the following conclusions can be drawn con-
cerning the chromone azo dyes as well as the stereochemistry of the corresponding
chelates. The infrared spectral data reveal two different modes of chelation. In the 1:1
(M:L) chelates of Cr(III) with ligands (Ia–c and IIa–c), Co(III)–IIa and Pd(II)–Ia–c
the ligands behave as monobasic bidentate and the coordination sites are via the
chromone carbonyl in position four and hydroxyl oxygen anion in position five. The
electronic spectral data are in agreement with an octahedral geometry, formula [1] in
case of Cr(III) and Co(III) chelates whereas in case of Pd(II) chelates a square planar
geometry was proposed, formula [2]. In the binuclear chelates, the first metal ion
bound via the same positions of the 1:1 (M:L) chelates, whereas, the second metal ion
chelation takes place through the formyl carbonyl in position six and oxygen anion in
position seven for the chelates of VO(IV) with ligands Ia, Ib, Id, Cr(III) and Pd(II)
with Id and Fe(III) with Ia, d, formulae [3 and 4]. On the other hand, the chelates of
Formula [1]
Formula [2]
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Formula [3]
Formula [4]
Formula [5]
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VO(IV) with ligand Ic and TiO(IV), VO(IV), Fe(III) and Pd(II) with IIc, the second
metal ion combines through the nitrogen azo and the carboxylic carbonyl in ortho po-
sition to it, formulae [5 and 6]. In the 2:1 chelates of Cr(III) with IId, Fe(III) with IIb,
and Pd(II) chelates with IIa,b,d the first chelation is through the same position of 1:1
chelates, while the second chelation is via the azo group and oxygen anion
(chromone) in position seven, formulae [7 and 8].
Formula [6]
Formula [7]
Formula [8]
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Formula [9]
It is to be mentioned that in most 2:1 (M:L) chelates the coordination sphere is
completed with chloride or hydroxyl ion except Fe(III)-chelate with ligand IId the
sulphate ion takes place in chelation as shown from the IR spectrum, formula [9].
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