Preparation, characterization and thermal studies of some transition metal ternary complexes

Article abstract of DOI:10.1007/s10973-008-9224-7

Ternary complexes of Co(II), Ni(II), Cu(II) and Zn(II) with nitrilotriacetic acid as a primary ligand and glycine as secondary ligand were prepared in slightly acid medium. Their molecular masses were determined by acid-base titration against standard pot

Full text of DOI:10.1007/s10973-008-9224-7

Journal of Thermal Analysis and Calorimetry, Vol. 95 (2009) 1, 253–258
PREPARATION, CHARACTERIZATION AND THERMAL STUDIES OF
SOME TRANSITION METAL TERNARY COMPLEXES
E. R. Souaya*, E. H. Ismail, A. A.Mohamed and N. E. Milad
Chemistry Department, College of Science, Ain Shams University, 11566 Cairo, Egypt
Ternary complexes of Co(II), Ni(II), Cu(II) and Zn(II) with nitrilotriacetic acid as a primary ligand and glycine as secondary ligand
were prepared in slightly acid medium. Their molecular masses were determined by acid-base titration against standard potassium
hydroxide solution. Their molecular structures were found to be [M (HNTA)(glyH)(2H2O) ]. Thermogravimetric analysis
confirmed this structure and that the water present is coordinated to the central metal atom. UV-Vis spectra showed that the
complexes have octahedral symmetry. IR spectra suggested the presence of intermolecular hydrogen bonding. This phenomenon
was supported by mass spectra. The ionization constants of these complexes, as diprotic acids, were determined.
Keywords: glycine, nitrilotriacetic acid, ternary complexes, thermogravimetric analysis
Introduction
studies of mixed ligand NTA is characterization of
thorium–ethylenediaminetetraacetic acid complex
which was investigated by electro spray ionization-
mass spectrometry (ESI-MS) [20].
Many authors investigated coordination compounds
due to their chemical, biological, environmental,
ion-exchange and catalytic importance [1–5]. Recently,
considerable attention has been devoted to the study
of mixed-ligands complexes of metal(II) containing
nitrogen donor ligands [6–9]. While structural analysis
of ternary complexes containing amino acids or peptides
is difficult, X-ray diffraction has been used to structurally
characterize a limited number of ternary complexes
containing NTA [10]. Only a few structures of the
general form [M(NTA) (amino acid)] have been
reported [11–13].
The formation constants of the four ternary
complexes of the present work were determined by
Hopgood and Augelici [21] and were found to be
3.65, 4.95, 5.42 and 3.64 for the Co, Ni, Cu and Zn
complexes, respectively.
In an effort to improve our understanding of the
interaction between NTA, metal ions used in IMAC
and glycine, the present study on the synthesis and the
characterization of ternary complexes of Co(II),
Ni(II), Cu(II) and Zn(II) with NTA as a primary
ligand and the biologically important ligand glycine,
as a secondary ligand is conducted.
Ternary metal complexes have been recently
studied due to their ability as metal systems for
metal–protein complexes such as metallo-enzymes.
They received particular attention and have been
employed in mapping protein surfaces [14], as probes
for biological redox centers [15] and in protein
capture for both purification [16] and study [17, 18].
Study of the structure of model ternary complexes
provides information about how biological systems
achieve their specificity and stability, as well as
strategies to improve these features for biotechnol-
ogical applications.
Experimental
Materials
All chemicals used in the present study are reagent
grade. Nitrilotriacetic acid, Merk product was used
without further purification. Glycine, CoCl2·6H2O,
NiSO4·7H2O, CuSO4·5H2O and ZnSO4·7H2O, B.D.H.
products were used.
Ternary metal complexes of nitrilotriacetic
acid (NTA) have been widely adopted in biology and
are gaining increasing uses in biotechnology, parti-
cularly in protein purification techniques known as
immobilized metal ion affinity chromato-
graphy (IMAC) [19]. Nitrilotriacetic acid has gained,
recently, popularity as metal chelator in IMAC. Other
Preparation of the (1:1:1) metal nitrilotriacetate
glycine complexes
Two solutions were prepared, the first contains 10–3
mole of the metal salt; 23.793, 26.250, 24.968 or
24.968 mg of Co, Ni, Cu or Zn salts in 100 mL water
*
Author for correspondence: eglals@yahoo.com
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

SOUAYA et al.
and the second one containing a mixture of 10–3 mole
Jasco model V-550 UV/Vis spectrophotometer. Mass
spectra were recorded at 350°C and 70 eV on a GL/MS
finnigan mat SSQ 7000 apparatus.
glycine (7.511 mg) and 10–3 mole nitrilotriacetic acid
(19.1 mg) in 100 mL water. The two solutions were
mixed and heated nearly to boiling to ensure complete
reaction. The solution was left on a water bath until
the volume was reduced to about 50 mL. The solution
was then left to cool to room temperature. Crystals of
the 1:1:1 complexes were separated, filtered and
washed with ethyl alcohol followed by ether and then
kept in a vacuum desiccator over anhydrous CaCl2.
Solubility
The solubility of the prepared complexes was determined
by shaking few mg of the complex with about 50 mL
aliquots of distilled water in a water thermostat at
25°C±0.2 for about 3 h. The suspension was rapidly
filtered and measured aliquots of the filtrate titrated
against standard potassium hydroxide solution using
phenolphthalein as indicator.
Instrumentation
All measurements were carried out at the microanalytical
laboratories of Cairo University, Ain Shams University
and the National Research Center, Cairo. C, H and N
were determined by Vario El Elementar. Co, Ni, Cu
and Zn percentages were determined by atomic
absorption spectrometry (AAS), using a Perkin
Elmer AAS 3100. IR spectra of the solid complexes
were recorded on a Jasco FTIR-300 E Fourier
Transform Infrared Spectrometer, using KBr discs in
the range 400–4000 cm–1 and CsI technique in the
range 200–500 cm–1.
Potentiometric and conductometric measurements
Determination of the ionization constants of the prepared
complexes was carried out by a potentiometric titration
of a given mass of the acid against standard potassium
hydroxide solution at 25±0.2°C using a Fisher pH
meter according to the method described by Albert
and Serjeant [22].
Conductivity measurements of 10–3 M solutions
in bidistilled water thermostated at 25°C were carried
out using WTW D-812 Weilheium conductivity meter;
model LBR, fitted with a cell model LTA 100.
Thermogravimetric analysis was carried out using a
Perkin-Elmer
7
series thermal analyzer. The
measurements were carried out under nitrogen
atmosphere at a heating rate 10°C min–1. Magnetic
susceptibilities of the paramagnetic metal complexes
were measured by using a magnetic susceptibility
balance Johnson Mtthy, Alfa products; model No
MKI at room temperature. The electronic UV-Vis
spectra were measured at room temperature on a
Results and discussion
Elemental analysis and physical properties
Elemental analyses and physical and chemical properties
of the four complexes are given in Table 1 The four
Table 1 Elemental analysis, physical and chemical properties of the acid complexes
Co(HNTA)(glyH) Ni(HNTA)(glyH) Cu(HNTA)(glyH) Zn(HNTA)(glyH)
(H2O)2
(H2O)2
(H2O)2
(H2O)2
Cal.
16.400
16.000
26.740
27.000
4.460
4.180
7.800
7.240
3.657
245.0
359.0
5.384
3.000
pink
16.360
16.000
26.780
26.700
4.460
4.270
7.810
7.230
5.256
17.400
17.000
26.460
26.470
4.400
4.460
7.700
7.690
2.468
265.0
363.5
1.883
3.100
blue
17.810
17.400
26.300
26.280
4.380
Metal/%
C/%
Found
Cal.
Found
Cal.
H/%
Found
Cal.
4.690
7.670
N/%
Found
7.500
Solubility in H2O/g L–1
Decomposition temperature/°C
Molecular mass
19.754
210.0
260.0
358.5
3.342
365.0
Magnetic susceptibility/B.M.
Conductivity of 10–3 M solution/mS
Colour
diamagnetic
2.600
2.800
green
2.489
6.810
colourless
2.257
pka1
pka2
2.407
4.423
2.499
5.326
Ionization constants/
5.172
254
J. Therm. Anal. Cal., 95, 2009

SOME TRANSITION METAL TERNARY COMPLEXES
complexes prepared have some common features
of Co, Ni and Cu octahedral complexes respectively,
it was concluded that the octahedral structure is
recommended for the ternary complexes of this study.
The pH values at which all the complexes under
study were crystallized ranged from 3 to 4.5. In this
pH range HNTA2– and glyH predominate. This is
shown in the distribution curves of the two ligands
concerned, Fig. 2 HNTA2– has three coordination
sites while glycine coordinates in slightly acid medium
via its carboxylic oxygen after being converted into
the zwitter ion form (H3N+CH2COO–). The structure
of the 1:1:1 complex is thus as shown in Fig. 1b.
Different tools were utilized to support the above
conclusions regarding the octahedral structure of
these complexes. These tools are thermal analysis, IR
and mass spectra and conductivity measurements.
such as low solubility in water, thermal decomposition
before melting, effervescence and evolution of carbon
dioxide on reaction with sodium bicarbonate. Use
was made from their quantitative reaction with
sodium hydroxide in determining their molecular
masses. The molecular masses of the four complexes
suggest the presence of two water molecules. If these
two molecules are crystal water, the complex should
have a tetrahedral structure, Fig. 1a, while if the two
water molecules are coordinated to the central atom,
then the complex should have an octahedral structure,
Fig. 1b. On comparing the 10 Dq values of the
complexes of this study which were found to be
20400, 12615 and 12200 cm–1 with literature values
Thermal analysis
Thermogravimetric analysis of the four complexes were
carried out Figs 3–6. The results are shown in Table 2.
Interpretation of the thermal mass losses shows that the
complexes thermally decompose, in the same pattern
Fig. 1 The two expected structures of the four complexes;
a – tetrahedral structure, b – octahedral structure
Fig. 3 TG and DTG of cobalt complex
Fig. 2 Distribution curves of the two ligands; a – H3NTA and
Fig. 4 TG and DTG of nickel complex
b – glycine
J. Therm. Anal. Cal., 95, 2009
255

SOUAYA et al.
Table 2 Temperature values for the decomposition along with the species lost in each step
Mass loss/%
Complex
Temperature/°C
Mol. mass
358.900
Significance
Cal.
Found
177
370
10.000
74.100
16.430
11.509
76.665
11.826
2H2O
Co(HNTA)(Hgly)(2H2O)
Ni(HNTA)(Hgly)(2H2O)
Cu(HNTA)(Hgly)(2H2O)
2glycine+maleic acid
mixture Co and CoO
Above 370
210
379
379
358.700
363.500
650.000
10.020
74.090
46.400
9.434
78.124
12.442
2H2O
2glycine+maleic acid
mixture Ni and NiO
206
570
9.900
73.100
17.460
10.308
71.879
17.813
2H2O
2glycine+maleic acid
residue Cu
Above 570
2H2O+glycine
glycine+maleic acid
residue Zn
206
650
30.410
52.320
17.800
30.609
52.520
16.871
Zn(HNTA)(Hgly)(2H2O)
Above 650
exhibited by the parent ligands. It was shown that
H3NTA thermally decompose in two overlapping
steps, [23] into glycine and maleic acid. Both thermal
products, on further heating, decompose giving
CH3NH2, CO2, C2H4, C2H2 and CO. The first mass loss
in the four complexes was found to be equivalent to two
water molecules which supported our suggested struc-
ture for these prepared complexes.
It may be observed from Table 2 that the final
thermal decomposition residue is the metal alone as in
case of Cu and Zn complexes. It may be noted that
thermal decomposition was made under nitrogen
atmosphere and that almost all gaseous products were
reducing gases, (CO, C2H4 and C2H2). In case of Co
and Ni complexes the percentage of the residue is less
than the calculated value corresponding to the full
decomposition to the metal so there must be some
oxide left.
IR spectra
IR spectra of the two ligands and those for the four
complexes are given in Figs 7 and 8. Assignments of
the frequencies of the vibrational bands are given in
Table 3. It may be observed that one can discuss the
structure of these complexes in the light of the
infrared spectra obtained for the carboxylate group
both in nitrilotriacetic acid and in metal complexes.
For the four complexes, two very strong bands are
observed in the region 1739–1560 cm–1. The first
band is observed at 1734, 1739, 1737 and 1731 cm–1
for Co, Ni, Cu and Zn complexes, respectively. This
Fig. 5 TG and DTG of copper complex
Fig. 7 IR spectra of cobalt and nickel complexes
Fig. 6 TG and DTG of zinc complex
256
J. Therm. Anal. Cal., 95, 2009

SOME TRANSITION METAL TERNARY COMPLEXES
The infrared spectra of the four complexes exhibit
a crowded region between 3600–2900 cm–1, where
H2O, –OH and –NH are expected to absorb. The broad
band in the range 3300–3400 cm–1 appearing in all the
spectra of the four complexes is assigned to the probable
intermolecular hydrogen bonding (O…H–O, O…H–N,
O–H…N) which explains the low solubility of these
complexes in water (Table 2). Such probable
hydrogen bonding is shown in Fig. 9. Intermolecular
hydrogen bonding will result into the formation of
different structures including mononuclear or
polynuclear molecules, one dimensional chain, two
dimensional and three dimensional networks.
Fig. 8 IR spectra of copper and zinc complexes
Mass spectra
band is also observed at 1730 cm–1 in the spectrum of
the free acid where the absorption band of the free
carboxylic acid group is expected [24, 25]. The other
band which appeared as very strong absorption at
1671, 1560, 1593 and 1614 cm–1 for the Co, Ni, Cu
and Zn complexes, respectively is not observed in the
spectrum of the free H3NTA and can be assigned to
stretching vibration nCO of the coordinated carboxylate
group, COOM [26]. From these observations, it is
clear that in these four complexes, one or more of the
free carboxyl groups do not coordinate with the central
metal ion and remain as free carboxylic acid and the
suggested structure, [M(HNTA) (glyH)], is justified.
The nMO frequencies of copper and nickel
The mass spectra of the four complexes were recorded.
It may be observed that all the spectra contain
molecular ion beams exceeding the molecular masses
of the complexes, Table 3. Such a phenomenon strongly
supports our assumption that all these complexes
contain intermolecular hydrogen bonds, Fig. 9.
Ionization constants
Potentiometric titrations of the four acid complexes were
carried out for the determination of ionization constant
of such complexes. The metal salt, nitrilotriacetic acid
and glycine were mixed in 1:1:1 molar ratio in about
20 mL bidistilled water then the solution was warmed
complexes with glycine as a ligand are at 360, 290 cm–1,
respectively [27]. In the prepared complexes these two
bands appeared at 343 and 285 cm–1. Thus one may
assign the bands at 279 cm–1 for the copper complex
and at 274 cm–1 for the nickel complex to the M–O
vibrational frequencies between these two metals and
oxygen of nitrilotriacetic acid. In a similar way, the
M–N frequencies for Cu and Ni glycinate, in basic
medium, were found at 439 cm–1 [27]. In the present
study the bands at 469 and 403 cm–1 may be assigned
to the vibrational frequencies of Cu–N and Ni–N
bands. By analogy the M–O and M–N bands for the
Co and Zn complexes are those at 365 and 565 cm–1
for cobalt and at 257 and 593 cm–1 for zinc.
Fig. 9 Probale H-bond sites in the ternary complexes
Table 3 Assignments of the IR bands in cm–1 and m/e values of the mass spectra exceeding the molecular masses of the pre-
pared complexes
Complex
nOOH
nOOM
nM–O
nM–N
m/e
Co(HNTA)(Hgly)(2H2O)
1734
1671
1490
365
565
497
Ni(HNTA)(Hgly)(2H2O)
Cu(HNTA)(Hgly)(2H2O)
Zn(HNTA)(Hgly)(2H2O)
1739
1737
1730
1560
1460
285
343
257
403
469
593
512, 408
491
1593
1450
1614
1435
378, 428, 505
J. Therm. Anal. Cal., 95, 2009
257

SOUAYA et al.
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Received: May 14, 2008
Accepted: June 11, 2008
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DOI: 10.1007/s10973-008-9224-7
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