Structural, spectroscopic and thermal characterization of 2-tert-butylaminomethylpyridine-6-carboxylic acid methylester and its Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and UO2(II) complexes

Article abstract of DOI:10.1016/j.saa.2004.06.025

Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and UO₂(II) complexes with the ligand 2-tert-butylaminomethylpyridine-6-carboxylic acid methylester (HL²) have been prepared and characterized by elemental analyses, molar conductance, magnetic moment, thermal analysis and spectral data. 1:1 M:HL² complexes, with the general formula [M(HL²)X ₂]·nH₂O (where M = Co(II) (X = Cl, n = 0), Ni(II) (X = Cl, n = 3), Cu(II) (grey colour, X = AcO, n = 1), Cu(II) (yellow colour, X = Cl, n = 0) and Zn(II) (X = Br, n = 0). In addition, the Fe(III) and UO ₂(II) complexes of the type 1:2 M:HL² and with the formulae [Fe(L²)₂]Cl and [UO₂(HL ²)₂](NO₃)₂ are prepared. From the IR data, it is seen that HL² ligand behaves as a terdentate ligand coordinated to the metal ions via the pyridyl N, carboxylate O and protonated NH group; except the Fe(III) complex, it coordinates via the deprotonated NH group. This is supported by the molar conductance data, which show that all the complexes are non-electrolytes, while the Fe(III) and UO₂(II) complexes are 1:1 electrolytes. IR and H1-NMR spectral studies suggest a similar behaviour of the Zn(II) complex in solid and solution states. From the solid reflectance spectral data and magnetic moment measurements, the complexes have a trigonal bipyramidal (Co(II), Ni(II), Cu(II) and Zn(II) complexes) and octahedral (Fe(III), UO₂(II) complexes) geometrical structures. The thermal behaviour of the complexes is studied and the different dynamic parameters are calculated applying Coats-Redfern equation.

Full text of DOI:10.1016/j.saa.2004.06.025

Spectrochimica Acta Part A 61 (2005) 1089–1096
Structural, spectroscopic and thermal characterization of
2-tert-butylaminomethylpyridine-6-carboxylic acid methylester
and its Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and UO2(II) complexes
Gehad G. Mohamed∗, Nadia E.A. El-Gamel
Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt
Received 12 April 2004; accepted 8 June 2004
Abstract
Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and UO2(II) complexes with the ligand 2-tert-butylaminomethylpyridine-6-carboxylic acid methylester
(HL2) have been prepared and characterized by elemental analyses, molar conductance, magnetic moment, thermal analysis and spectral data.
1:1 M:HL2 complexes, with the general formula [M(HL2)X2]·nH2O (where M = Co(II) (X = Cl, n = 0), Ni(II) (X = Cl, n = 3), Cu(II) (grey
colour, X = AcO, n = 1), Cu(II) (yellow colour, X = Cl, n = 0) and Zn(II) (X = Br, n = 0). In addition, the Fe(III) and UO2(II) complexes of
the type 1:2 M:HL2 and with the formulae [Fe(L2)2]Cl and [UO2(HL2)2](NO3)2 are prepared. From the IR data, it is seen that HL2 ligand
behaves as a terdentate ligand coordinated to the metal ions via the pyridyl N, carboxylate O and protonated NH group; except the Fe(III)
complex, it coordinates via the deprotonated NH group. This is supported by the molar conductance data, which show that all the complexes
are non-electrolytes, while the Fe(III) and UO2(II) complexes are 1:1 electrolytes. IR and 1H-NMR spectral studies suggest a similar behaviour
of the Zn(II) complex in solid and solution states. From the solid reflectance spectral data and magnetic moment measurements, the complexes
have a trigonal bipyramidal (Co(II), Ni(II), Cu(II) and Zn(II) complexes) and octahedral (Fe(III), UO2(II) complexes) geometrical structures.
The thermal behaviour of the complexes is studied and the different dynamic parameters are calculated applying Coats–Redfern equation.
© 2004 Elsevier B.V. All rights reserved.
Keywords: 2-tert-Butylaminomethylpyridine-6-carboxylic acid methylester (HL2) complexes; 1H-NMR; IR; Molar conductance; Solid reflectance; Magnetic
moment; Thermal analysis
phosphorous donor and Eꢀ can be either a pyridine nitrogen
or ring sp2 carbon as shown in Fig. 1. This ligand is most
often found as a meridional ligand (i.e., Eꢀ and two E
will be approximately coplanar in the metal coordination
sphere).
It is our purpose here to report progress on a related goal:
pincer ligand chemistry of transition metals with a secondary
amine donor at site E. Moreover, we have decided to make the
donors E secondary (i.e., NHtBu) rather than tertiary amines
for the specific purpose of having a reactive hydrogen accom-
panied by a bulky but three-fold symmetric substituent [1].
This substitution pattern (ligand abbreviation N2py when Eꢀ
= N) will give rise to both mirror and C2 asymmetric isomers
3, which are not anticipated to interconvert on the synthetic
time scale when the nitrogens are coordinated. It is shown
from Fig. 2 that isomer 3 (m) clearly has a crowded (“up”)
1. Introduction
In many catalytic reactions, coordinatively unsaturated
complexes appear as reactive intermediates at some stage.
Such species initiate catalytic processes (catalytic precursor)
and, of course, are part of catalytic cycles themselves. Thus,
investigations of the factors controlling and affecting the
reactivity and stability of unsaturated compounds are very
important, not only for intrinsic reasons, but also for the
understanding of virtually all homogeneously catalyzed
processes. Recently, the pincer ligand class (1) has expe-
rienced a renaissance [1] from its earliest incarnations in
mono-metal coordination chemistry. Here, E is a nitrogen or
∗
Corresponding author. Fax: +20 25727556.
E-mail address: ggenidy@hotmail.com (G.G. Mohamed).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.06.025

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G.G. Mohamed, N.E.A. El-Gamel / Spectrochimica Acta Part A 61 (2005) 1089–1096
carried out at the Microanalytical Center at Cairo Uni-
versity. Infrared spectra were recorded on a Perkin–Elmer
FT–IR type 1650 spectrophotometer in wave number
region 4000–200 cm−1. The spectra were recorded as KBr
disc. The solid reflectance spectra were measured on a
Shimadzu 3101pc spectrophotometer. The molar magnetic
susceptibility was measured on powdered samples using
the Faraday method. The thermogravimetric analysis (TGA
and DrTG) was carried out in dynamic nitrogen atmosphere
(20 mL min−1), with a heating rate of 10 ◦C min−1 using
Shimadzu TGA-50H thermal analyzer. The 1H-NMR
spectra were recorded using a Bruker ARX-300 instrument.
Chemical shifts are reported in parts per million downfield
from tetramethylsilane. Deuterated chlorofirm and DMSO
were used as a solvent in a concentric tube containing d6-
acetone.
Fig. 1. Pincer ligands with E=N or P and Eꢀ=pyridine N or sp2 ring carbon.
2.1. 2,6-Pyridinedicaboxylic acid dimethyl ester
(C9H9NO4)
Fig. 2. N2Py ligands with mirror and C2 asymmetric isomers.
2,6-pyridinedicarboxylic acid (26.0 g, 155.7 mmol) and
H2SO4 concentrated (0.2 mL) in methanol (200 mL) were
heated to reflux for 100 h. After evaporation of the solvent,
the residue was redissolved in CH2Cl2 (200 mL), extracted
twice with saturated aqueous solutions of NaHCO3 (100 mL)
followed by H2O (100 mL). The organic layer was dried over
Na2SO4, filtered and the solvent removed. The white prod-
uct was dried in vacuo. Yield: 26.0 g (85%); IR (KBr disc):
1730 sh (␯C=O), 1595 sh (␯C=N), 1H-NMR (25 ◦C, CDCl3):
δ = 8.33 (d, 2H, J = 7.8 Hz), 8.02 (t, 1H, J = 7.8 Hz), 4.02
(s, 6H).
and an uncrowded (“down”) side, while in 3 (C2), these are
equivalent. The Ru(II)/Cl−, NCMe, PPh3 chemistry of the
related (but C2␯ symmetric) ligand 4 was recently reported
[2,3].
The main idea of this article is to prepare an asymmetric
pincer ligand type with Eꢀ is a nitrogen and E as NHtBu in
one arm and as COOMe in the second arm. The ligand and
its complexes were characterized using different physico-
chemical tools like elemental and thermal analyses, mag-
1
netic moment, molar conductance, IR, H-NMR and solid
reflectance spectral techniques.
2.2. 2-Hydroxymethylpyridine-6-carboxylic acid methyl
ester (C8H9NO3) (HL1)
To 2,6-pyridinedicarboxylic acid dimethylester (20.9 g,
107.1 mmol) in MeOH (200 mL) was slowly added NaBH4
(10.5 g, 55 mol), whereupon the exothermic reaction warmed
the reaction mixture to reflux. The solution was then stirred at
2. Experimental
All reagents and solvents were obtained from commercial
sources and used as received. Elemental analyses were
Fig. 3. Preparation of HL2 ligand.

G.G. Mohamed, N.E.A. El-Gamel / Spectrochimica Acta Part A 61 (2005) 1089–1096
1091
room temperature for 12 h. After evaporation of the solvent,
the residue was dissolved in saturated aqueous NaHCO3 so-
lution (200 mL) and extracted with CHCl3 (300 mL) by con-
tinuous liquid–liquid extraction for 15 h. After evaporation
of the solvent, the white product was washed with Et2O and
dried in vacuo. Yield: 15.0 g (72%); IR (KBr disc): 3384 br
1
(␯OH), 1732 sh (␯C=O), 1590 sh (␯C=N), H-NMR (25 ◦C,
DMSO-d6): δ = 7.75 (t, 1H, J = 7.3 Hz), 7.35 (d, 2H, J =
7.3 Hz), 4.65 (s, 2H), 2.96 (bs, 1H), 3.94 (5, 3H).
2.3. 2-Toulenesulfonylmethylpyridine-6-carboxylic acid
methyl ester (C15H15NO5S)
A solution of HL1 (2.24 g, 13.41 mmol) and KOH (2.58 g,
46.0 mmol) in 50 mL of THF was cooled in an ice bath
and toluenesulfonyl chloride (3.5 g, 18.4 mmol) in 150 mL
of THF was added dropwise. The temperature was not al-
lowed to rise over 0 ◦C. The mixture was stirred for 5 h at
0 ◦C and then for another 12 h at room temperature. The white
residue (KOH, KCl) was filtered off and washed twice with
THF (30 mL). The THF fractions were combined, and after
evaporation of the solvent, the product was dried in vacuo.
Yield: 6.10 g (80%); IR (KBr disc): 1732 sh (␯C=O), 1593 m
(␯C=N), 1H-NMR (25 ◦C, CDCl3): δ = 7.83 (m, 4H), 7.71 (t,
1H), 7.43 (d, 2H), 5.14 (s, 2H), 3.82 (5, 3H), 2.44 (s, 3H).
2.4. 2-tert-Butylaminomethylpyridine-6-carboxylic acid
methylester (C12H18N2O2) (HL2)
2-Toluenesulphonylmethylpyridine-6-carboylic
acid
methylester (5.19 g, 16.17 mmol) and tert-butylamine
(36 mL, 350 mmol) were dissolved in benzene and heated to
reflux for 15 h. After evaporation of the solvent, the residue
was redissolved in CH2Cl2, extracted twice with saturated
aqueous Na2CO3 (100 mL) and H2O (100 mL), and dried
over Na2SO4. The solvent was removed and the product
dried in vacuo. Yield: 2.86 g (93%); IR (KBr disc): 3269 sh
1
(␯NH), 1726 sh (␯C=O), 1591 sh (␯C=N), H-NMR (25 ◦C,
CDCl3): δ = 7.67 (t, 1H, J = 8.1 Hz), 7.26 (d, 2H, J = 8.1 Hz),
3.99 (s, 3H), 1.85 (bs, 1H), 1.45 (5, 9H).
3. Results and discussion
The pincer ligand class (1) is found as a meridional ligand
where the tBuNH and ester groups are approximately copla-
nar in the metal coordination sphere. HL2 can be synthesized
according to the reaction given below in Fig. 3.
The purpose of preparing such ligand is to have a reactive
hydrogen accompanied by a bulky but three-fold symmet-
t
ric Bu-NH group in one arm of pyridine and a C=O group
in the second arm. The analytical and physical data for the
metal complexes are listed in Table 1. The complexes are
coloured, quite stable in air and insoluble in common or-
ganic solvents, but soluble in DMF and DMSO. Analyses
reveal the metal to ligand ratio to be 1:2 in case of UO2(II)

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G.G. Mohamed, N.E.A. El-Gamel / Spectrochimica Acta Part A 61 (2005) 1089–1096
Table 2
IR assignment (4000–200 cm−1) of HL2 and its complexes
␯(NH)
␯
asym(COO)
␯
sym(COO) ␦(NH)
␯(C–N)
␯(C=N) ␯(C=O) ␯(M–O) ␯(M–N) (NH) ␯(M–N) (py)
HL2
3269 sh 1555 sh
3269 sh 1598 sh
3163 sh 1565 s
1392 m
1369 sh
1373 sh
1381sh
1375 m
1390 m
1371 sh
1370 sh
765 s
767 m
1138 sh 1591 sh 1726 sh
1140 sh 1658 sh 1737 m
–
590 m
–
–
[Fe(L2)2]Cl
[Co(HL2)Cl2]
[Ni(HL2)Cl2]·3H2O
[Cu(HL2)Cl2]
493 m
450 w
472 w
470 w
494 m
640 s
423 m
553 m
480 m
500 s
505 s
554 m
510 s
492 m
770 sh 1145 m
761 m 1150 m
772 sh 1145 m
769 sh 1140 sh 1657 m
769 sh 1148 m 1605 sh 1732 sh 530 s
770 m 1140 sh 1624 m 1666 m 553 sh
1605 sh 1713 sh 520 s
3087 m
1597 sh
1630 m
1607 sh 1688 sh 530 s
1703 m 590 sh
1674 sh 596 s
3089 sh 1560 m
[Cu(HL2)(AcO)2]·H2O 3269 sh 1598 m
[Zn(HL2)Br2]
3173 sh 1565 s
3569 sh 1598 sh
[UO2(HL2)2](NO3)2
s = small; w = weak; m = medium; br = broad, sh = sharp.
and Fe(III) complexes and 1:1 in all other complexes. The
Co(II), Ni(II) and Zn(II) complexes were found to have the
formula [M(HL2)X2]·yH2O, where M = Co(II) (X = Cl, y =
0), Ni(II) (X = Cl, y = 3), Zn(II) (X = Br, y = 0). The Fe(III)
complex has the formula [Fe(L2)2]Cl while UO2(II) com-
plex has the formula [UO2(HL2)2](NO3)2. The grey Cu(II)
complex has the formula [Cu(HL2)(AcO)2]·H2O, while the
yellow Cu(II) complex has the formula [Cu(HL2)Cl2].
regions in the free ligand are assigned to the asymmetric and
symmetric stretching vibrations of the carboxylate group, re-
spectively. The νas(COO) band is shifted to higher wave-
lengths (1598–1560 cm−1), while νsym(COO) band is shifted
to lower wavelengths (1390–1369 cm−1). The shift of these
two bands to higher or lower frequencies suggest the partic-
ipation of –COOMe group in complex formation [8,9]. New
bands observed in the complexes at 596–520, 510–480 and
494–423 cm−1 are tentatively assigned to the ν(M–O) [10],
ν(M–N) of the pyridine [10] and ν(M–N) of the secondary
amine [11].
3.1. IR spectra
Therefore, from the IR spectra, it is obvious that HL2
ligand behaves as a neutral terdentate ligand coordinated
to the metal ions through C=O, pyridyl-N and protonated
secondary aliphatic amine-NH groups; in case of UO2(II),
CO(II), Ni(II), Cu(II) and Zn(II) complexes, except Fe(III)
complex, it behaves as a uni-negative terdentate ligand where
deprotonation of NH proton takes place. This is supported by
the conductance and 1H-NMR spectral data.
The IR spectra of the ligand as well as its complexes with
Fe(III), Co(II), Ni(II), Cu(II)andZn(II)andUO2(II)arelisted
in Table 2, together with assignments for most of the major
peaks.
The absorption bands in the region 3269–3087 cm−1 in
the free ligand and its complexes are assigned to the NH
stretching frequencies. The IR spectrum of the ligand shows
two bands at 765 and 1138 cm−1, which are assigned to the
out-of-plane δ(NH) and ν(C–N) aliphatic secondary amine
stretching frequencies. The coordination of the NH group to
the metal ions is confirmed by the blue shift (4 cm−1) or red
shift (2–12 cm−1) in the frequencies of out-of-plane δ(NH)
and ν(C–N) bands [4].
3.2. Molar conductance data
The conductance data of the complexes discussed herein
indicate that Co(II), Ni(II), Cu(II) and Zn(II) complexes
are non-electrloytes suggesting that the anions [Cl or Br]
are covalenty bonded to the metal ions. While Fe(III) and
UO2(II) complexes have conductance values of 69 and
120 ꢀ−1 cm2 mol−1, respectively. This indicates that Fe(III)
and UO2(II) complexes are 1:1 electrolytes.
The band located at 1591 cm−1 is assigned to the ν(C=N)
stretching vibration of pyridyl nitrogen. Coordination of the
pyridyl nitrogen is indicated by a 14–66 cm−1 shift to longer
wave numbers in all complexes [5,6]. In the wave number
region 1726 cm−1 range, some of the most characteristic
bands of these systems are found. The so-called amide I
band, which represents mainly the ν(C=O) stretching mode
in the chelates. This band is shifted to higher (6–11 cm−1) or
lower (23–60 cm−1) frequencies, supporting the participation
of the carbonyl group of this ligand in the metal coordina-
tion [7]. The bands at the wave number 1555 and 1392 cm−1
Table 4
1H-NMR data of HL2 and its Zn complex
Compound
HL2
Chemical shift (␦) (ppm)
Assignment
7.67
7.26
5.05
3.99
1.90
1.15
8.17
7.89
4.15
3.93
2.10
1.28
(t, 1H, py)
(d, 2H, py)
(s, 2H, –CH2)
(s, 3H, –COOH3)
(bs, 1H, –NH)
(s, 9H, –C(CH3)3)
(t, 1H, py)
Table 3
The solid reflectance spectra of HL2 complexes
Complex
Absorption maximum (cm−1
)
[Zn(H L2)Br2]
Fe(III)
Co(II)
Ni(II)
Cu(II) (yellow)
Cu(II) (green)
15,552
12,870
12,903
12,850
12,500
17,482
15,873
15,923
15,600
15,037
25,000
20,000
21,786
20,161
21,276
–
(d, 2H, py)
22,779
27,777
–
–
(s, 2H, –CH2)
(s, 3H, –COOCH3)
(s, 1H, –NH)
(s, 9H, –C(CH3)3)

G.G. Mohamed, N.E.A. El-Gamel / Spectrochimica Acta Part A 61 (2005) 1089–1096
1093
moment of 5.69 B.M., where Fe(III) has the ground state 6S.
Reflectance spectrum shows three bands at 25,000, 17,482
3.3. Electronic spectra and magnetic moment data
6
5
The solid reflectance spectra of the complexes show bands
in the r∗ange 35,∗714–40,160 cm−1, which may be attributed
to ␲–␲ or n–␲ transition within the HL2 ligand.
In the high spin octahedral Fe(III) complexes, the mag-
netic moments are always very close to spin-only value
of 5.9 B.M. because the ground state has no orbital angu-
lar momentum. The complex under study has a magnetic
and 15,552 cm−1, which may be assigned to A1g → T2g
(G) transition in octahedral geometry [12]. The 6A1g→ T1g
5
transition appears to be split into two bands at 15,552 and
17.482 cm−1 [13].
As the result of failure to obtain a single crystal
for X-ray analyses to confirm the trigonal bipyramidal
structure for these complexes, solid reflectance spectra
Fig. 4. 1H-NMR spectra of HL2 and its Zn(II) complex.

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G.G. Mohamed, N.E.A. El-Gamel / Spectrochimica Acta Part A 61 (2005) 1089–1096
and magnetic moment measurements are used for this
purpose.
The spectra of Ni(II) and Co(II) complexes (Table 3) ap-
pear to be five coordinate as do the corresponding NS2 and
N3 complexes [14–16], for which similar arguments apply.
The Ni(II) complex shows bands at 15,923 and 21,786 cm−1
,
which correspond to the 1A1 → E transitions from the two
e levels to the a1 level [17]. The spectrum of Co(II) complex
resembles those of other five coordinate Co(II) complexes
[15,16], where three bands are observed at 12,870, 15,873
and 20,000 cm−1. The relatively low values of µeff of Co(II)
and Ni(II) complexes compared with those of octahedral or
tetrahedral geometry may support the five-coordinate geom-
etry of these complexes. The bands observed at 27,777 and
22,779 cm−1 are attributed to L → Ni and L → Co charge
transfer bands, respectively.
1
The Cu chelates [Cu(HL2)Cl2] and [Cu(HL2)(AcO)2]
·H2O exhibit broad bands at 12,820–12,500, 15,037–15,600
and 21,276–20,161 cm−1. These bands are generally con-
sistent with five coordinate geometry for Cu(II) complexes
[14,16,18]. The reflectance spectra also show a band at
30.769–30.487 cm−1, which may be attributed to L →
Cu charge transfer band. The magnetic moment values of
2.03–2.1 B.M. are indicative of the existence of Cu(II) com-
plexes in trigonal bipyramidal structure [14,16,18–20].
According to the previously reported data of Zn complex
with similar N3 or NS2 ligand [14–16], the Zn complex re-
ported herein is considered to be trigonal bipyramidal.
3.4. 1H-NMR spectra
1
The H-NMR spectra of the diamagnetic Zn (1:1) com-
plex was compared with that of the free ligand as shown
in Fig. 4 (Table 4). The NH singlet, which appears at
1.90 ppm in the free ligand, is deshielded at 2.1 ppm in the
Zn complex, indicating the participation of the NH group in
1
chelation without proton displacement. Therefore, the H-
NMR data are consistent with IR and molar conductance
data.
3.5. Thermogravimetric analysis
Thermal analysis (TGA) data of the complexes are given
in Table 5 and the activation thermodynamic data are listed
in Table 6.
The Fe(III), Cu(II) and UO2(II) complexes give pattern
one step of decomposition within the temperature range
100–400 ◦C. The found mass losses of 86.0% (calculated
mass loss 85.0%), 78.24% (calculated mass loss 77.70%)
and 70.24% (calculated mass loss 69.69%) for Fe(III), Cu(II)
and UO2(II) complexes, respectively, are attributed to the
decomposition of the anions and ligand molecules leaving
1/2Fe2O3, CuO and UO as a residue. The energy of acti-
vation of the thermal decomposition of the complexes was
98.35, 39.39 and 56.13 kJ mol−1 for the Fe(III), Cu(II) and
UO2(II) complexes (Table 6), respectively.

G.G. Mohamed, N.E.A. El-Gamel / Spectrochimica Acta Part A 61 (2005) 1089–1096
1095
Table 6
Thermodynamic data of the thermal decomposition of HL2 chelates
Complex
Decomposition range (◦C) E* (kJ mol−1
)
A (S−1
)
ꢁS* (J K−1 mol−1
)
ꢁH* (kJ mol−1
)
ꢁG* (kJ mol−1
)
r2
[Fe(L2)2]Cl
130–400
50–110
98.35
144.8
114.2
132.20
109.60
39.39
23.82
90.10
56.13
2.15 × 106
4.01 × 108
5.08 × 104
2.28 × 1010
3.88 × 1011
2.85 × 106
1.14 × 105
2.07 × 109
1.09 × 108
−39.14
−26.85
−14.93
−36.25
−68.16
−22.05
−19.93
−28.11
−42.65
43.92
56.06
118.7
81.55
135.7
43.17
89.54
64.80
88.31
43.92
59.38
164.80
119.60
87.62
49.10
64.94
106.20
93.17
0.973
0.929
0.993
0.992
0.974
0.968
0.993
0.994
0.987
[Ni(HL2)Cl2]·3H2O
110–240
240–320
320–650
130–350
220–350
350–660
100–350
[Cu(HL2)Cl2]
[Zn(HL2)Br2]
[UO2(HL2)2](NO3)2
On the other hand, the Ni(II) complex of the general for-
mula [Ni(C12H18N2O2)Cl2]·3H2O exhibits four successive
decomposition steps. The first decomposition step in the tem-
perature range 50–110 ◦C, with an estimated mass loss of
13.40% (calculated mass loss 13.27%), may be attributed to
the loss of three water molecules of hydration. The activa-
tion energy of the thermal dehydration of the complex was
144.8 kJ mol−1. The 2nd, 3rd and 4th steps within the temper-
ature range of 110–650 ◦C correspond to the removal of 2HCl
and ligand molecules as deduced from mass loss calculations
(estimated mass loss 68.60%, calculated mass loss 68.06%),
leaving NiO residue. The activation energies of these steps
were 114.2, 132.2 and 109.6 kJ mol−1 for the 2nd, 3rd and
4th steps, respectively.
The first estimated mass loss of 36.65% within the temper-
ature range 220–350 ◦C may be attributed to the liberation
of 2HBr gases (calculated mass loss 36.24%). The energy of
activation of this step was 32.82 kJ mol−1. The second step
occurs within the temperature range 280–650 ◦C with an
estimated mass loss 46.20% (calculated mass loss 45.65%)
might be reasonably accounted for the decomposition
of the ligand molecule as CO, CO2, NO gases leaving
ZnO residue. The activation energy of this step was 90.10
kJ mol−1
.
3.6. Kinetic data
The kinetic parameters namely activation energy (E*), en-
thalpy (ꢁH*), entropy (ꢁS*) and free energy change of de-
composition(ꢁG*)wereevaluatedgraphicallybyemploying
The Zn(II) complex, [Zn(C12H18N2O2)Br2], was ther-
mally decomposed in two successive decomposition steps.
Fig. 5. Suggested formulas of HL2 complexes.

1096
G.G. Mohamed, N.E.A. El-Gamel / Spectrochimica Acta Part A 61 (2005) 1089–1096
the Coats–Redfern relation [21]
ligand, where deprotonation of NH proton takes place. From
the reflectance spectra and magnetic moment measurements,
the geometrical structure of the chelates were proposed and
found to be octahedral or trigonal bipyramidal.
ꢀ
ꢁ
ꢀ
ꢁ
[Wf /(Wf − W)]
AR
θE∗(1 − 2RT/E∗)
log log
= log
T2
E∗
−
(1)
2.303RT
References
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 mol−1, θ is the heating rate
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∼
and (1−(2RT)E*)) 1. A plot of the left-hand side of Eq. (1)
=
against 1/T gives a slope from which E* was calculated and A
(Arrhenius constant) was determined from the intercept. The
entropy of activation (ꢁS*), enthalpy of activation (ꢁH*)
and free energy change of activation (ꢁG*) were calculated
using the following equations
ꢀ
ꢂ
ꢃꢁ
Ah
kT
∆S∗ = 2.303 log
R
(2)
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30 (8) (1991) 1893.
∆H∗ = E∗ − RT
(3)
(4)
∆G∗ = ∆H∗ − T∆S∗
where k and h are the Boltzman and Plank constants, re-
spectively. The thermodynamic parameter data are listed in
Table 6, from which it is obvious that all the complexes have
negative entropy values, which indicates that the activated
complexes are formed spontaneously.
3.7. Structural interpretation
From the elemental analyses, molar conductance, mag-
netic moment and spectral studies, the structure of HL2 com-
plexes is shown below in Fig. 5.
[17] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced
Inorganic Chemistry, sixth ed., Wiley, New York, 1999.
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(2004), in press.
4. Conclusion
[19] G. Park, E. Dadanchova, A. Przyborowska, S.J. Lai, D. Ma, G.
Broker, R.D. Rogers, R.P. Planalp, M.W. Berchbiel, Polyhedron 20
(2001) 3155.
[20] A.A. Soliman, G.G. Mohamed, Thermochimica Acta (2004), in
press.
Asageneralconclusion, HL2 ligandbehavesasneutralter-
dentate ligand through ester C=O, pyridyl-N and protonated
secondary aliphatic amine-NH groups in case of UO2(II),
Co(II), Ni(II), Cu(II) and Zn(II) complexes, while in case
of Fe(III) complex, it behaves as a uni-negative terdentate
[21] A.W. Coats, J.P. Redfern, Nature 20 (1964) 68.