Synthesis, IR, magnetic, solid reflectance, and thermal characterization of transition metal chelates with 2-(5-acetylamino-2-hydroxyphenylazo)-benzoic acid

Article abstract of DOI:10.1081/SIM-200026299

The azo ligand 2-(5-acetylamino-2-hydroxyphenylazo)benzoic acid (H ₂L) is derived from the diazotization of 4-acetamidophenol with anthranilic acid. Metal(II) and (III) chelates were prepared and characterized by elemental and thermal analyses, conductance, IR, mass, magnetic, and diffuse reflectance spectra. The complexes were found to have the formulae [Cr(HL) ₂]Cl·H₂O; [Cd(HL)₂]·2H ₂O; [M(HL)(H₂O)_z]X·nH₂O for M = Co(II) (X = Cl, z = 1, n = 2), M = Ni(II) (X = Cl, z = 1, n = 2.5); M = Cu(II) (X = AcO, z = 2, n = 2), and M = Zn(II) (X = AcO, z = 1, n = 2); and [Fe(HL)Cl₂(H₂O)]·2H₂O. The molar conductance data reveal that the Cd(II) and Fe(III) chelates are non-electrolytes, while the Cr(III), Co(II), Ni(II), Cu(II), and Zn(II) chelates are 1:1 electrolytes. IR spectra show that the ligand is coordinated to the metal ions in a terdentate manner with ONO donor sites of the phenolic OH, azo-N, and carboxylic OH. Magnetic and solid reflectance spectra were used to infer the coordinating capacity of the ligand and the geometrical structure of these complexes. The mass spectra give the possible molecular ion fragments resulting from the fragmentation of the chelates. The thermal behavior of these chelates shows that water molecules (coordinated and uncoordinated) and anions are removed in two successive steps followed immediately by decomposition of the ligand molecule in the subsequent steps. The relative thermal stability of the anhydrous chelates is evaluated. The final decomposition products are found to be the corresponding metal oxides. The thermodynamic activation parameters, such as, E*, ΔH*, ΔS*, and ΔG* are calculated from the TG curves and discussed.

Full text of DOI:10.1081/SIM-200026299

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Synthesis, IR, Magnetic,
Solid Reflectance, and
Thermal Characterization
of Transition Metal
Chelates with
2‐(5‐Acetylamino‐2‐hydroxyphen
Acid
Dr. Gehad G. Mohamed a & Zenat M. Zaki b
a Chemistry Department, Faculty of Science ,
Cairo University , Giza, Egypt
b Chemistry Department, Faculty of Science,
Beni‐Suef Branch , Cairo University , Egypt
Published online: 16 Nov 2010.
To cite this article: Dr. Gehad G. Mohamed & Zenat M. Zaki (2004) Synthesis, IR,
Magnetic, Solid Reflectance, and Thermal Characterization of Transition Metal
Chelates with 2‐(5‐Acetylamino‐2‐hydroxyphenylazo)‐benzoic Acid, Synthesis
and Reactivity in Inorganic and Metal-Organic Chemistry, 34:8, 1497-1516, DOI:
10.1081/SIM-200026299
To link to this article: http://dx.doi.org/10.1081/SIM-200026299
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 34, No. 8, pp. 1497–1516, 2004
Synthesis, IR, Magnetic, Solid Reflectance,
and Thermal Characterization of
Transition Metal Chelates with 2-(5-
Acetylamino-2-hydroxyphenylazo)-
benzoic Acid
2
Gehad G. Mohamed1, and Zenat M. Zaki
*
1Chemistry Department, Faculty of Science, Cairo University,
Giza, Egypt
2Chemistry Department, Faculty of Science, Beni-Suef Branch,
Cairo University, Egypt
ABSTRACT
The azo ligand 2-(5-acetylamino-2-hydroxyphenylazo)benzoic acid (H2L)
is derived from the diazotization of 4-acetamidophenol with anthranilic
acid. Metal(II) and (III) chelates were prepared and characterized by
elemental and thermal analyses, conductance, IR, mass, magnetic, and
diffuse reflectance spectra. The complexes were found to have the
.
.
.
formulae [Cr(HL)2]Cl H2O; [Cd(HL)2] 2H2O; [M(HL)(H2O)z]X nH2O
for M ¼ Co(II) (X ¼ Cl, z ¼ 1, n ¼ 2), M ¼ Ni(II) (X ¼ Cl, z ¼ 1,
*
Correspondence: Dr. Gehad G. Mohamed, Chemistry Department, Faculty of
Science, Cairo University, Giza, Egypt; E-mail: ggenidy@hotmail.com.
1497
DOI: 10.1081/SIM-200026299
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

1498
Mohamed and Zaki
n ¼ 2.5); M ¼ Cu(II) (X ¼ AcO, z ¼ 2, n ¼ 2), and M ¼ Zn(II)
.
(X ¼ AcO, z ¼ 1, n ¼ 2); and [Fe(HL)Cl2(H2O)] 2H2O. The molar
conductance data reveal that the Cd(II) and Fe(III) chelates are non-
electrolytes, while the Cr(III), Co(II), Ni(II), Cu(II), and Zn(II) chelates
are 1 : 1 electrolytes. IR spectra show that the ligand is coordinated
to the metal ions in a terdentate manner with ONO donor sites of the phe-
nolic OH, azo–N, and carboxylic OH. Magnetic and solid reflectance
spectra were used to infer the coordinating capacity of the ligand and
the geometrical structure of these complexes. The mass spectra give the
possible molecular ion fragments resulting from the fragmentation of
the chelates. The thermal behavior of these chelates shows that water
molecules (coordinated and uncoordinated) and anions are removed in
two successive steps followed immediately by decomposition of the
ligand molecule in the subsequent steps. The relative thermal stability
of the anhydrous chelates is evaluated. The final decomposition products
are found to be the corresponding metal oxides. The thermodynamic
activation parameters, such as, Eꢀ, DHꢀ, DSꢀ, and DGꢀ are calculated
from the TG curves and discussed.
Key Words: 2-(5-Acetylamino-2-hydroxyphenylazo)benzoic acid
complexes; IR; Conductance; Solid reflectance; Mass spectra; Thermal
analyses.
INTRODUCTION
Azo compounds are known to be involved in a number of biological reac-
tions such as inhibition of DNA, RNA, and protein synthesis, carcinogenesis,
and nitrogen fixation.[1,2] Furthermore, they were proved to have biological
activity against bacteria and fungi.[3,4] Metal chelates of Schiff bases contain-
ing anthranilic acid have been prepared previously.[5–10] Anthranilic acid has
been also involved in mixed-ligand complex formation with different
ligands.[11–13] Macrocyclic aza complexes of anthranilic acid have been
reported.[14–18] In continuation of our interest in azo ligands and their metal
chelates,[19–23] the main target of the present study is to synthesize new
azo-metal chelates and determine the coordination capacity of the highly
colored 2-(5-acetylamino-2-hydroxyphenylazo)benzoic acid (H2L) that incor-
porates several binding sites. The coordination behavior of H2L towards
transition metal ions is investigated by IR, molar conductance, magnetic
moment, solid reflectance, and mass spectral measurements. The thermal
decomposition of the complexes was also used to infer the structure and the
various thermodynamic activation parameters are calculated. The structure
of H2L is shown in Fig. 1.

Transition Metal Chelates
1499
Figure 1. Structure of the ligand (H2L).
EXPERIMENTAL
Material and Methods
All chemicals used were of analytical reagent grade. They include
4-acetamidophenol (Sigma), 2-aminobenzoic acid (Aldrich), sodium nitrite
and hydroxide (BDH), ethyl alcohol (Adwic), and the disodium salt of
ethylenediaminetetraacetic acid (Adwic). Cupric acetate dihydrate
(Prolabo), cobalt, chromium, and nickel chloride hexahydrates (BDH),
zinc and cadmium chlorides (Adwic), and ferric chloride hexadydrate
¨
(Riedel-de Haen) were used as received. The elemental analyses (C, H,
N) were made at the Microanalytical Center at Cairo University. IR
spectra were recorded on a Perkin–Elmer FT-IR type 1650 spectropho-
tometer. The solid reflectance spectra were measured on a Shimadzu
3101 PC spectrophotometer. The molar magnetic susceptibilities were
measured on powdered samples using the Faraday method. The conduc-
tance measurements were carried out using a Sybron–Barnstead conduct-
ometer. A Shimadzu TGA-50H thermal analyzer was used to record
simultaneously thermogravimetric (TG) and DTG curves. The experiments
were carried out in a dynamic nitrogen atmosphere (20 mL min21) with a
heating rate of 10 8C min21 in the temperature range of 20–1000 8C
using platinum crucibles. The sample sizes ranged in mass from 1.2 to
3.5 mg. Highly sintered a-Al2O3 was used as a reference. The mass
spectra were recorded by the EI technique at 70 eV on a Hewlett–
Packard Model MS-5988 GS–MS instrument in the Microanalytical
Center of Cairo University. Metal contents were determined by titration
against standard EDTA after complete decomposition of the complexes
with aqua regia in a Kjeldahl flask several times.

1500
Mohamed and Zaki
Synthesis of 2-(5-Acetylamino-2-hydroxyphenylazo)benzoic
Acid
2-Aminobenzoic acid (4.11 g, 10 mmol) was mixed with HCl (11.5 M,
5 mL) and diazotized below 5 8C with NaNO2 (2.07 g, 10 mmol). The resulting
diazonium chloride was coupled with an alcoholic NaOH solution (3 g,
25 mL) of 4-acetamidophenol (4.53 g, 10 mmol) below 5 8C. The formed
solid product was separated by filtration, purified by crystallization from
diethyl ether and dried in a vacuum over anhydrous calcium chloride. The
red azo product is produced in 80% yield (6.92 g). Anal. Calcd. for
C15H13N3O4 (299 g/mol): C, 60.20; H, 4.35; N, 14.05%. Found: C, 60.45;
H, 4.50; N, 14.35%, m.p., 165 + 2 8C.
Synthesis of the Complexes
The metal complexes of H2L were prepared by the addition of the appro-
priate metal chloride or acetate (1 mmol) in an ethanol–water mixture (1 : 1,
25 mL) to the solution of the azo compound (0.299 g, 2 mmol) in the same
solvent (50 mL). The resulting mixture was stirred under reflux for one half
hour whereupon the complexes precipitated. They were collected by filtration,
washed with a 1 : 1 ethanol : water mixture and diethyl ether. The analytical
data for C, H, and N were repeated twice.
RESULTS AND DISCUSSION
The general reaction for the preparation of the metal complexes of 2-(5-
acetylamino-2-hydroxyphenylazo)benzoic acid is shown below
CrCl3 þ 2H2L þ H2O ꢁ! ½CrðHLÞ2ꢂCl ꢃ H2O þ 2HCl
CdCl2 þ 2H2L þ 2H2O ꢁ! ½CdðHLÞ2ꢂ ꢃ H2O þ 2HCl
FeCl3 þ H2L þ 3H2O ꢁ! ½FeðHLÞCl2 ꢃ H2OÞꢂ2 ꢃ 2H2O þ HCl
MX2 þ H2L þ zH2O þ nH2O ꢁ! ½MðHLÞðH2OÞzꢂX ꢃ nH2O þ HX
ð1Þ
ð2Þ
ð3Þ
ð4Þ
(where M ¼ Co(II) (X ¼ Cl, z ¼ 1, n ¼ 2), Ni(II) (X ¼ Cl, z ¼ 1, n ¼ 2.5);
Cu(II) (X ¼ AcO, z ¼ 2, n ¼ 2), and Zn(II) (X ¼ AcO, z ¼ 1, n ¼ 2).
The results of the elemental analyses of the metal chelates of 2-(5-acetyl-
amino-2-hydroxyphenylazo)benzoic acid, which are recorded in Table 1, are
in good agreement with those required by the proposed formulae. In most
cases 1 : 1 (M : L) solid complexes were isolated and found to have the
.
general formulae [M(HL)(H2O)z]X nH2O, where M ¼ Co(II) (X ¼ Cl,

Transition Metal Chelates
1501
m

1502
Mohamed and Zaki
z ¼ 1, n ¼ 2), Ni(II) (X ¼ Cl, z ¼ 1, n ¼ 2.5), Cu(II) (X ¼ AcO, z ¼ 2,
n ¼ 2) and Zn(II) (X ¼ AcO, z ¼ 1, n ¼ 2). While the Cr(III) and Cd(II) che-
.
lates are of the type 1 : 2 (M : L) and have the formulae [Cr(HL)2]Cl H2O and
.
[Cd(HL)2] 2H2O. In addition, Fe(III) forms a 1 : 1 (M : L) chelate of the
.
formula [Fe(HL)Cl2(H2O)] 2H2O. The H2L ligand offers several alternatives
to coordination to metals. The results shown in Table 1 prove that H2L is
indeed monoanionic, where the carboxylate proton is lost when coordinated
to the metal ions, and thus reducing the charge on the complex ion. The pre-
sence of chloride or acetate ions in the inner or outer sphere of the chelates
neutralizes the charge on the complex. Thus H2L is expected to fill three
coordination positions and contribute a charge of 21. The non-involvement
of chloride or acetate ions in the coordination to the metal ions is also
confirmed by the conductance measurements.
IR Spectra and Mode of Bonding
A detailed interpretation of the IR spectra of H2L and the effect of binding
with Cr(III), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) ions on the
vibrational frequencies of the free H2L ligand are discussed. The IR spectra
of the free ligand and its metal chelates were carried out in the 4000–
200 cm21 range (Table 2). The IR spectrum of the ligand shows a broad
band at 3500–3050 cm21, which can be attributed to the phenolic OH, acet-
amide NH, and carboxylic OH groups. This band is still broad in all com-
plexes, which renders it difficult to attribute to the involvement of phenolic
OH and carboxylic OH groups in coordination. The involvement of the phe-
nolic OH group in chelation is confirmed by the blue-shift of the n(C–O)
stretching band, observed at 1266 cm21 in the free ligand, to the extent of
3–29 cm21 in the complexes.[21] However, the n(N55N) stretching band in
the free ligand is observed at 1506 cm21 [19,21]
This band is shifted to
.
higher or lower frequency values upon complexation suggesting coordination
via the azo group (M N). If the diazo nitrogen bonded to the pohenol is
coordinated to the metal ion, 7- and 5-membered chelate rings are formed,
with 7-membered chelate rings not being favored. However, if the diazo nitro-
gen bonded to the benzoic acid coordinated to the metal ion, two 6-membered
chelate rings, which, energetically, should be more favored.[24]
In addition, the IR spectrum of H2L reveals bands at 1604 and 1374 cm21
due to nasym(COO2) and nsym(COO2).[25] These bands decrease in intensity
and shift to lower frequency on complexation, suggesting coordination via
the carboxylate OH group.[22] In the far-IR spectra of all the complexes, the
non-ligand bands observed at the 435–414 cm21 region can be assigned to the
n(M–N) stretching vibration.[22] Conclusive evidence regarding the bonding

Transition Metal Chelates
1503
d
d

1504
Mohamed and Zaki
of oxygen to the metal ions is provided by the occurrence of bands at 505–
496 cm21 as the result of n(M–O).[21,22]
Therefore, the IR spectra indicate that H2L behaves as monobasic acid
and the coordination sites being Ar–OH, –N55N– and the O atom of the
carboxylic OH group.
Molar Conductivity Measurements
The solubility of the complexes in DMF permitted determination of the
molar conductivity (Lm) of 1023 M solutions at 25 8C[26] and, by comparison,
the electrolytic nature for each complex (Table 1). The molar conductivity for
the Cr(III) complex is 135 Ohm21 cm2 mol21 indicating a 1 : 1 electrolyte and
suggests under these circumstances the non-involvement of Cl in coordination
to the Cr(III) ion. On the other hand, the low conductivity values of the Fe(III)
and Cd(II) complexes prove their weak ionic nature (non-electrolytes). The
conductance data in Table 1 also indicate that the Co(II), Ni(II), Cu(II), and
Zn(II) complexes are 1 : 1 electrolytes indicating their ionic nature.
Magnetic Susceptibility and Electronic Spectral
Measurements
The diffuse reflectance spectra of the Fe(III), Co(II), Ni(II), and Cu(II)
chelates show two bands at 252–256 nm and 352–356 nm which are attribu-
ted to the p ! pꢀ and n ! pꢀ transitions, respectively, within the H2L
ligand. For the hexa-coordinated Cr(III) complex there are three spin-
4
4
allowed transitions in the electronic spectrum, n1: A2g ! T2g, n2:
4
4
4A2g ! T2g(F), and n3: 4A2g ! T1g(P).[27] The diffuse reflectance spectrum
of the Cr(III) chelate shows three absorption bands at 19,050 (n1), 28,410 (n2),
and 39,060 cm21 (n3), where n1 corresponds to 10 Dq. The electronic spec-
trum of the chelate reported here is in reasonable agreement with those in
the literature.[28] The magnetic moment at room temperature is 3.95 BM
which corresponds to the expected value for Cr(III) compounds.[28] From
the diffuse reflectance spectra, it is observed that the Fe(III)-chelate exhibits
6
5
a band at 20,830 cm21, which may be assigned to the A1g ! T2g(G) tran-
5
sition in octahedral geometry of the complex.[19] The 6A1g ! T1g transition
appears to be split into two bands at 11,110 and 8960 cm21. The observed
magnetic moment of the Fe(III) complex is 5.37 BM. Thus, the complex
formed has octahedral geometry involving d2sp3 hybridization for the
Fe(III) ion.[19]

Transition Metal Chelates
1505
The reflectance spectrum of the Ni(II) chelate show two bands at 16,030
1
1
1
1
and 25,000 cm21 which are assigned to A1g ! A2g and A1g ! B1g tran-
sitions, respectively, in a square-planar geometry.[29] The diamagnetic
nature of the complex lends support to the above geometry. The solid state
electronic spectrum of the Co(II) chelate with H2L shows three d–d transitions
of the type expected for distorted tetrahedral Co(II) compounds,[27,30] centered
at 23,810 (n3), 9520 (n2), and 5710 cm21 (n1). The band in the visible region
4
has been assigned to 4A2 ! T1(P) transitions and the other two bands in the
4
4
near-IR region to 4A2 ! T1(F) 4A2 ! T2 transitions, respectively. The
magnetic moment for the complex, 3.6 BM, is in the range expected for tetra-
hedral Co(II) compounds.[30] The Cu(II) chelate exhibits a broad band at
10,960 and 15,380 cm21. These peaks are generally consistent with a five-
coordinate geometry for Cu(II) complexes.[31] The magnetic susceptibility
measurements of 2.1 BM is indicative of the appearance of the copper
complex in trigonal bipyramidal geometry.[31]
The Zn(II) chelate is diamagnetic and is likely to be tetrahedral. In analogy
with described Zn(II) and Cd(II) complexes containing N–O donor Schiff
bases[32,33] and according to the empirical formulae of these complexes, we
propose an octahedral geometry for the Cd(II) chelate with the six axial
positions occupied by two ligand molecules.
Mass Spectra
The electron impact mass spectra of H2L and its chelates were recorded
and investigated at 70 eV of electron energy. The important peaks and their
relative intensities for the molecular ion fragments down to m/z ¼ 63 (only
predominant and important peaks are recorded for simplicity) are listed in
Table 3. The mass spectra of the studied chelates are characterized by moder-
ate to high relative intensity molecular ion peaks, except the Fe(III) chelate,
which has a low abundance at 70 eV.
The formula weight of these chelates appear inþtheir mass spectra at
m/z ¼ 692.5 [Mþ (9%)], 481 [MH2þ (1%)], 449 [MH2.5 (33%)], 457 [MHþ1.5
(15%)], 493 [Mþ (17%)], and 475 [M2 (6%)] for the Cr(III), Fe(III),
Co(II), Ni(II), Cu(II), and Zn(II) chelates, respectively, whereas the peak of
the Cd(II)-complex molecular ion at m/z ¼ 708 is significantly lower than
the calculated value, m/z ¼ 744, which is attributed to the loss of two
hydrated water molecules as it is subjected to ionization.
Mass spectra of the Cr(III), Fe(III), Co(II), and Cd(II), chelates show a
fragment with m/z ¼ 299 (1–42%) that corresponds to the C15H13N3O4 ion
which is the molecular ion of the ligand, indicating that the H2L molecule
binds to the metal ions in the same manner. The mass spectra of the Cr(III),

1506
Mohamed and Zaki
Table 3. Mass spectral data of H2L chelates.
m/e Found
(calcd)
RIa
(%)
Chelates
Fragment
.
[Cr(HL)2]Cl H2O
692.5 (692.5)
593 (592)
474 (474)
368 (368)
300 (299)
235 (236)
207 (207)
70 (70)
9
25
C30H26ClCrN6O9
C28H20CrN6O7
C22H17CrN4O6
C16H13CrN3O5
C15H13N3O4
C8H5CrN2O4
C8H5CrO4
27
100
17
82
70
62
C3H2O2
.
[Fe(HL)Cl2(H2O)] 2H2O
481 (479)
426 (425)
313 (311)
300 (299)
256 (254)
149 (147)
105 (104)
70 (70)
1
1
C15H18Cl2FeN3O7
C15H12Cl2FeN3O4
C13H9FeN3O3
C15H13N3O4
C14H12N3O2
C8H7N2O
100
1
10
9
9
C6H4N2
C3H2O2
8
.
[Co(HL)(H2O)]Cl 2H2O
449 (446.5)
367 (366)
299 (299)
299 (299)
236 (236)
163 (161)
135 (134)
121 (120)
69 (70)
33
31
C15H18ClCoN3O7
C15H13CoN3O4.5
C13H8CoN2O3
C15H13N3O4
C8H5CoN2O3
C8H5N2O2
42
42
75
16
77
C7H4NO2
C7H4O2
C3H2O2
100
16
.
[Ni(HL)(H2O)]Cl 2.5H2O
457 (455.5)
376 (375)
313 (313)
239 (238)
194 (194)
135 (134)
109 (108)
71 (70)
15
12
C15H19ClNiN3O7.5
C15H14NiN3O5
C13H8NiN3O3
C13H8N3O2
C12H8N3
30
48
100
11
C7H4NO2
C6H4O2
C3H2O2
22
19
.
[Cu(HL)(H2O)2](AcO) 2H2O
493 (492.5)
459 (457)
406 (405)
359 (362)
17
6
C17H23CuN3O10
C17H19CuN3O8
C17H15CuN3O5
C15H12CuN3O4
47
29
(continued)

Transition Metal Chelates
1507
Table 3. Continued.
m/e Found
(calcd)
RIa
(%)
Chelates
Fragment
207 (209)
117 (117.5)
72 (70)
40
100
12
C8H3CuNO2
C7H3NO
C3H2O2
68 (68)
19
C3H2NO
.
[Zn(HL)(H2O)](AcO) 2H2O
475 (476)
362 (363)
305 (305)
207 (208)
154 (154)
84 (84)
6
8
C17H21N3O9Zn
C15H12N3O4Zn
C13H8N2O3Zn
C13H8N2O
C10H7N2
10
19
28
100
5
C3H4N2O
C3H2O2
C5H3
72 (70)
63 (63)
24
.
[Cd(HL)2] 2H2O
711 (744)
663 (664)
620 (620)
567 (568)
441 (440)
476 (474)
384 (384)
298 (299)
258 (256)
83 (82)
6
7
C30H28CdN6O10
C28H20CdN6O7
C26H17CdN6O6
C24H17CdN4O6
C24H17N4O5
C19H13N3O4
C13H9N2O4
11
24
15
22
40
4
C15H13N3O4
C11H10N3O3
C3NO2
100
70
aRelative intensity.
Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) chelates show a fragment at
þ
;;
m/z ¼ 70 that may corresponds to the [HO–C C–CHO] ion, with relative
intensity values in the range 5–62%.
TG Analysis
The importance of this study of the H2L ligand and its complexes stems
from their possible biological activities. Therefore, they were subjected to
thermal analyses and other physico-chemical methods. Thermal data of the
complexes are given in Table 4, and the thermodynamic activation data
associated with the loss of water of crystallization (dehydration process)
and ligand molecules are listed in Table 5.

1508
Mohamed and Zaki

Transition Metal Chelates
1509

1510
Mohamed and Zaki
The thermal analysis of the chelates under study results in the following
weight losses:
(1) 4.83–9.82%: loss of water molecules of hydration over the range
25–180 8C interval (calculated 4.84–9.88%), except the Cr(III)
chelate, where the loss of water is accompanied by a loss of HCl.
(2) 11.5–19.29%: elimination of coordinated water and anions (Cl or
AcO) molecules over the 150–350 8C interval (calculated 11.75–
19.29%).
(3) 57.37–81.09%: elimination of the ligand molecules over the temp-
erature range 150–1000 8C (calculated 57.20–81.44%).
(4) 11.43–17.53%: residues of metal oxides remain over the 550–
1000 8C interval (calculated 10.83–17.20%).
Water loss at low temperature (25–150 8C) indicated that these H2O mole-
cules are in the lattice network, while water loss at higher temperatures up to
350 8C indicated that H2O is coordinated to the metal ions.[34] The thermal
stability of the chelates, after removal of hydrated and coordinated water
and anions, follows the order Cd(II) . Cr(II) . Fe(III)55Co(II) . Cu(II)55
Zn(II) . Ni(II).
In general, the stages of thermal decomposition of the complexes may be
written as shown below.
For Co(II), Ni(II), Cu(II), and Zn(II) chelates:
25–150 8C
½MðHLÞðH2OÞzꢂX ꢃ nH2O ꢁꢁꢁꢁꢁ! ½MðHLÞðH2OÞzꢂX þ nH2O
ð5Þ
dehydration
150–350 8C
½MðHLÞðH2OÞzꢂX ꢁꢁꢁꢁꢁꢁ! ½MðHLÞꢂ þ zH2O þ HX
ð6Þ
ð7Þ
150–350 8C
½MðHLÞ ꢂ ꢁꢁꢁꢁꢁꢁꢁꢁꢁ! Metal oxides þ HL
final decomposition
where X ¼ Cl or AcO, z ¼ 1 or 2, n ¼ 2 or 2.5.
For Cr(III) and Cd(II) chelates:
25–160 8C
½CrðHLÞ2ꢂCl ꢃ H2O ꢁꢁꢁꢁꢁ! ½CrðHLÞ2ꢂ þ HCl þ H2O
dehydration
30–150 8C
½Cd½ðHLÞ2ꢂ ꢃ 2H2O ꢁꢁꢁꢁꢁ! ½CdðHLÞ2ꢂ þ 2H2O
dehydration
ð8Þ
ð9Þ
160–1000 8C
½MðHLÞ2ꢂ ꢁꢁꢁꢁꢁꢁꢁꢁꢁ! Metal oxide þ 2HL
ð10Þ
final decomposition

Transition Metal Chelates
1511
For Fe(III) chelate:
25–150 8C
½FeðHLÞðClÞ2ðH2OÞꢂ ꢃ 2H2O ꢁꢁꢁꢁꢁ! ½FeðHLÞðClÞ2ðH2OÞꢂ þ 2H2O ð11Þ
dehydration
150–300 8C
½FeðHLÞðClÞ2ðH2OÞꢂ ꢁꢁꢁꢁ! ½FeðHLÞꢂ þ 2HCl þ H2O
ð12Þ
ð13Þ
300–600 8C
1
½FeðHLÞ ꢂ ꢁꢁꢁꢁꢁꢁꢁꢁꢁ! Fe2O3 þ HL
final decomposition
2
Kinetic Analysis
The first stage of dehydration of the complexes was studied in more detail.
The kinetic parameters such as activation energy (DEꢀ), enthalpy (DHꢀ),
entropy (DSꢀ) and free energy change of decomposition (DGꢀ) were evaluated
graphically by employing the Coats–Redfern relation:[35]
ꢀ
ꢁ
ꢀ
ꢂ
ꢃꢁ
logfWf=ðWf ꢁ WÞg
AR
uEꢀ
2RT
Eꢀ
Eꢀ
log
¼ log
1 ꢁ
ꢁ
ð14Þ
T2
2:303RT
where Wf is the mass loss at the completion of the reaction, W is the mass loss
up to the temperature T; R is the gas constant, E is the activation energy in
kJ mol21, u is the heating rate and (1 2 (2RT/Eꢀ)) ffi 1. A plot of the left-
hand side of Eq. (14) against 1/T gives a slope from which Eꢀ was calculated
and A (Arrhenius constant) was determined from the intercept. The entropy of
activation (DSꢀ), enthalpy of activation (DHꢀ) and the free energy change of
activation (DGꢀ) were calculated using the following equations:
ꢀ
ꢂ
ꢃꢁ
Ah
kT
DSꢀ ¼ 2:303 log
DHꢀ ¼ Eꢀ ꢁ RT
R
ð15Þ
ð16Þ
ð17Þ
DGꢀ ¼ Hꢀ ꢁ TSꢀ
where k and h are the Boltzmann and Plank constants, respectively. The cal-
culated values of Eꢀ, A, DSꢀ, DHꢀ, and DGꢀ for the decomposition steps are
given in Table 5.
According to the kinetic data obtained from the DTG curves, all the com-
plexes have a negative entropy, which indicates that the complexes are formed
spontaneously. From Table 5 it is obvious that, the nickel complex shows a
higher thermal stability than that of copper. This can be interpreted in terms
of repulsion among electron pairs in the valence shell of the central ion.
The higher electronegativity of copper compared with that of nickel leads to
a higher repulsion between bonding pairs in the valence shell of copper ion

1512
Mohamed and Zaki
giving lower stability.[36] Based on the activation energy values, the Zn(II)
complex shows a lower thermal stability than the Cd(II) complex. This can
be explained in terms of repulsion among electron pairs in the valence shell
of the central ion. Both zinc and cadmium ions in the investigated complexes
have multiple bonds (four bonds) in their valence shells. But due to the smaller
ionic size of zinc compared with that of cadmium, this leads to a higher repul-
sion between the bonding pairs in the valence shell of zinc which in turn alters
Figure 2. Suggested structural formulae of H2L complexes.

Transition Metal Chelates
1513
the bond angles from those of ideal tetrahedral, giving a lower stability.[36,37]
The lower thermal stability of the cobalt complex may be attributed to the
higher repulsion between the multiple-bonding electron pairs (six bonds) in
the valence shell of cobalt, as well as to the higher repulsion obtained by
the non-bonded pairs of electrons on the axial donating oxygen of the water
molecule and the bonded electrons.[36]
Structural Interpretation
The design and synthesis of a new tridentate azo ligand derived from
4-acetamidophenol and anthranilic acid for use in tetrahedral, square-planar,
trigonal-bipyramidal or octahedral molecular templates have been success-
fully demonstrated. The synthesis of the ligand and its complexes proved to
be as straightforward as expected, giving high yields of either the free
ligand and its complexes in simple, one-pot reactions. As anticipated, the
ligand coordinates equatorially to four- or six-coordinate transition metal
ions to give square-planar [Ni(II)], tetrahedral [Co(II) and Zn(II)], octahedral
[Cr(III), Fe(III), and Cd(II)] and trigonal-bipyramidal [Cu(II)] environments,
around the metal ion anchor. The proposed general structures of the com-
plexes are shown in Fig. 2.
There are additional factors which might affect the structure of these che-
lates such as the possibility of various isomeric structures and intermolecular
hydrogen bonds, however, due to instrumental limitations, we are not able to
explore this further.
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Received July 14, 2003
Accepted April 17, 2004