Thermodynamic and thermal investigation of Co(II), Ni(II) and Cu(II) complexes with adenine

Article abstract of DOI:10.1023/B:JTAN.0000027834.34263.48

Complexes of adenine, AdH, with cobalt, nickel and copper chlorides were prepared and their thermodynamic functions were determined. The complexing processes are endothermic in nature. The thermal behaviour of complexes was followed up by using TG and DTA

Full text of DOI:10.1023/B:JTAN.0000027834.34263.48

Journal of Thermal Analysis and Calorimetry, Vol. 76 (2004) 343–348
THERMODYNAMIC AND THERMAL
INVESTIGATION OF Co(II), Ni(II) AND Cu(II)
COMPLEXES WITH ADENINE
*
M. G. Abd El Wahed, E. M. Nour, S. Teleb and S. Fahim
Faculty of Science, Zagazig University, Zagazig, Egypt
(Received June 4, 2003; in revised form October 11, 2003)
Abstract
Complexes of adenine, AdH, with cobalt, nickel and copper chlorides were prepared and their ther-
modynamic functions were determined. The complexing processes are endothermic in nature. The
thermal behaviour of complexes was followed up by using TG and DTA analyses. The stoichio-
metry of thermal decomposition of the investigated complexes was suggested.
Keywords: thermodynamic parameters, TG and DTA
Introduction
The chemistry of adenine and its metal complexes is of much current attention due to
its contribution in many biological systems. Adenine show various probabilities of
coordination with transition metal ions due to its potential donor sites. It coordinates
with nickel, cobalt and palladium halides as a mono- or bidentate ligand and chloride
bridging was found in the dehydrated complexes [1–3]. Binary and ternary com-
plexes of Cu(II) with adenine and amino acids were prepared and characterized using
spectral, thermal and magnetic measurements [4]. The electrical behaviour of ade-
nine complexes was studied [5] which exhibited a semiconducting character.
Thermodynamic and thermal analyses play an important role in studying the
structure and properties of metal complexes [6–13]. Consequently, in the present
work, which has originated from our interest in studying the physical properties of
metal complexes, the thermodynamic and thermal analyses of some adenine com-
plexes have been investigated.
Experimental
All chemicals used were of pure laboratory grade. A digital pH-meter 5800-05 solu-
tion analyzer, Cole Parmer, was used for pH measurements. The method of Irving
*
Author for correspondence: E-mail: sfsalem2000@yahoo.com
1388–6150/2004/ $ 20.00
© 2004 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

344
ABD EL WAHED et al.: Co(II), Ni(II) AND Cu(II) COMPLEXES
and Rossotti was used to determine the stability constants and related thermodynamic
parameters of the complexes as described previously [14–16]. The measurements
were repeated at 295, 305, 315 and 325 K.
The solid complexes were prepared in 50% (v/v) aqueous ethanol medium with
a molar ratio of M:2L. In case of cobalt and nickel complexes 0.568 g of adenine dis-
solved in a small amount of bi-distilled water was mixed with a solution containing
0.5 g of CoCl2×6H2O or NiCl2×6H2O, respectively. An equal volume of ethanol and
water was added to the reacting mixture. The reaction mixtures were allowed to re-
flux for four days. For copper complex a solution of 0.793 g of adenine was mixed
with a solution of 0.5 g CuCl2×2H2O with continuous stirring. A blue precipitate was
appeared at once. The prepared complexes were filtered, washed several times with
water then ethanol and finally dried over anhydrous CaCl2. The analytical data of the
formed complexes are summarized in Table 1. Determination of chloride ions was
carried out using Mohr’s and Volhard’s methods. Atomic absorption Perkin Elmer
spectrophotometer A 100, was used for the determination of the metal ions.
Table 1 Analytical data of solid complexes
Analysis/%; found/(calc.)
Compound
Colour
(MW)
m.p./°C
C
H
N
M
Cl
[Co(AdH)2Cl2]×H2O
28.6
3.20
35.80
14.01
15.90
violet
335
(418.04)
(28.70)
(2.87)
(33.48)
(14.09)
(16.96)
[Ni(AdH)(Ad)Cl×3H2O]×H2O
28.50
4.40
30.80
13.87
9.71
green
300
(435.5)
(27.56)
(3.90)
(32.15)
(13.48)
(8.14)
[Cu(AdH)2Cl2]×2H2O
26.50
3.20
30.40
14.80
16.40
blue
275
(440.65)
(27.23)
(3.18)
(31.77)
(14.42)
(16.09)
The thermal analysis of the complexes was performed using a Shimadzu DTA-
–1
TGA in the range 20–800°C with a uniform heating rate of 10°C min . Platinum cell
was used and mass of samples were ca. 3.5 mg. Nitrogen was used as a dynamic atmo-
3
–1
sphere (flow rate is 30 cm min ). Gravimetric analysis were carried out for solid com-
plexes in a very clean, dried and carefully weighed platinum crucible. The samples
were heated gradually from 20 to 800°C in an automatically controlled electric furnace
for a period ranging from 3 to 4 h till constant mass at each temperature.
Results and discussion
The protonation and stability constants of adenine and its complexes were deter-
mined potentiometrically using the usual manner [14–16]. The calculated data are re-
ported in Table 2. It is clear that the formation of the complexes takes place in two
steps and the formation of M:L is more stable than M:2L. This finding may be attrib-
uted to the presence of pyrimidine ring and NH2 group which showed electron –
withdrawing and/or the electron – repelling character between adenine molecules.
J. Therm. Anal. Cal., 76, 2004

ABD EL WAHED et al.: Co(II), Ni(II) AND Cu(II) COMPLEXES
345
The increase in temperature of heat treatment of the investigated complexes leads to
an increase in the values of the stability constants suggesting an endothermic behav-
iour for the complexing process.
Table 2 Stability constants of adenine and its complexes
Temperatures
305 K 315 K
8.90 8.85
Compounds
195 K
9.50
325 K
9.40
Adenine
pK
logK1
logK2
3.20
2.95
3.60
3.30
4.20
3.80
4.50
4.00
Cobalt complex
logK1
logK2
3.50
3.26
4.10
3.65
4.50
3.85
4.80
4.05
Nickel complex
logK1
logK2
4.80
4.40
5.75
5.30
6.23
5.90
6.75
6.50
Copper complex
0
0
0
The thermodynamic functions, DG , DH and DS , associated with the formation
of complexes are computed using the well known relations:
0
0
0
DG =–2.303RTlgK=DH –TDS
0
K
T -T
2
DH
1
2
1
and
lg
= -
K
2.303R T T
1
2
The results obtained are collected and given in Table 3. The positive values of the
enthalpy changes demonstrate that the metal–adenine bonds are weak and the
complexing process is endothermic in nature. However, the complexes are stabilized by
relatively big increase in DS values, i.e., increase of the randomness of the complexes
studied. At a given temperature, the more closely spaced the energy levels, the greater the
number of levels that can be populated and consequently the greater the entropy, i.e., dis-
order. An increase in temperature provides energy for the population of even more en-
ergy levels and the accessibility of energy levels increases, therefore the disorder in-
0
creases and higher stability values are obtained [17]. In other words, the big values of DS
means that the entropy is the main driving force for the formation of adenine complexes
in aqueous medium.
Table 3 Thermodynamic data of adenine complexes at 295 K
0
–DG /kcal mol
–1
0
DH /kcal mol
–1
0
DS /cal mol K
–1 –1
Compounds
4.32
3.98
19.76
14.18
81.62
61.57
Cobalt complex
4.73
4.40
18.94
11.32
80.22
53.30
Nickel complex
6.48
5.94
27.88
20.24
116.47
112.98
Copper complex
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ABD EL WAHED et al.: Co(II), Ni(II) AND Cu(II) COMPLEXES
To clarify the nature of interaction between the metal ions and adenine molecule,
covalent or ionic, the calculated thermodynamic data can be analyzed into electrostatic
and non-electrostatic using the procedure suggested by Murakami and Yoshino [18].
0
0
0
T/ q
DG =DG +DG =nRTlnM+RC(a+e
non el
)
0
0
0
T/ q
DH =DH +DH =RC[a+(1–T/q)e
non el
]
0
0
0
T/q
DS =DS +DS =–nRlnM–(RC/q)e
non el
where q is the characteristic temperature of the solvent and M is its molality. The
terms C and a can be determined by solving the above equations. The obtained data
0
are given in Table 4. It is obvious that the negative values of DG compared to those
el
0
of DG suggest that the electrostatic forces are stronger than non-electrostatic ones,
non
0
0
i.e., the ionic character is more predominant. Whilst, DH on and DH values are
non el
0
0
positive, however, the values of DH are higher than those of DH indicating that
non el
the electrostatic nature of bond formation is predominant. The ratio of electrostatic
values to non-electrostatic values are approximately constant for all complexes indi-
cating that the complexing processes take place by a similar stoichiometry.
Table 4 Electrostatic and non-electrostatic components of thermodynamic parameters
0
0
0
0
Compounds
DG
–DG
DH
DH
non
el
non
el
14.67
12.45
18.99
16.43
12.31
7.74
7.45
6.44
Cobalt complex
13.96
10.28
18.69
14.68
11.61
6.57
7.33
4.76
Nickel complex
19.89
23.50
26.37
29.44
17.54
12.53
10.34
7.70
Copper complex
The thermal decomposition of complexes was followed using TG and DTA
techniques in the temperature range 20–800°C. The thermogrevimetric curves of ad-
enine and its cobalt complex are given in Figs 1a and b. The thermal degradation of
the complex shows three decomposition steps. The first peak at maximum tempera-
ture of 57°C which was not associated with any mass loss may be ascribed to the
cleavage of hydrogen bonds or van der Waals bonds. The second stage occurs at
maximum temperature of 120°C and associated with a mass loss of 4.065% corre-
sponding to the loss of one uncoordinated water molecule (calc.=4.305%). The com-
plete thermal decomposition of the complex occurred at 361.8°C with a mass loss of
76.423% (theoretically equals to 77.74%) giving tricobalt tetraoxide as a final prod-
uct. Therefore, the stoichiometry of the thermal analysis can be represented as:
120°C
[Co(AdH)2Cl2]×H2O¾¾®[Co(AdH)2Cl2]+H2O
361.8°C
3[Co(AdH)2Cl2]+2O2 ¾¾¾®Co3O4+decomposition products
J. Therm. Anal. Cal., 76, 2004

ABD EL WAHED et al.: Co(II), Ni(II) AND Cu(II) COMPLEXES
347
Fig. 1 TG and DTA curves of a – free adenine and its b – cobalt, c – nickel and d – copper complexes
To establish the proposed formula and structure of nickel complex, the thermal
analysis was studied as shown in Fig. 1c. Four degradation steps were observed. The first
decomposition step at 76.05°C has the same meaning as discussed earlier. The second
step of degradation occurs at 120°C and is accompanied by a mass loss of 5.238% sug-
gesting the loss of one uncoordinated water molecule (calc.=4.14%). The third step takes
place at 208.2°C and is associated with the loss of three water molecules (obs.=12.11 and
calc.=12.40%). The last degradation step was observed in the range 380–630°C with a
mass loss of 66.67% corresponds to the loss of chloride and adenine molecules (neutral
and anionic). Theoretically, the mass loss is 67.65%. In this step, the nickel ion will inter-
act with the oxygen produced from the degradation of water molecules in the above steps
yielding nickel oxide as a final product. Consequently, the proposed stoichiometry for
the thermal decomposition of nickel–adenine complex may be written as:
120°C
[Ni(AdH)(Ad)Cl×3H2O]×H2O ¾¾® [Ni(AdH)(Ad)Cl×3H2O]+H2O
208.2°C
[Ni(AdH)(Ad)Cl×3H2O] ¾¾¾® [Ni(AdH)(Ad)Cl]+3H2O
380°C
[Ni(AdH)(Ad)Cl]+0.5O2 ¾¾® NiO+decomposition products
Finally, the stoichiometry of thermal decomposition of copper complex was
studied and the thermogravimetric curves are demonstrated in Fig. 1d. The TG
curves exhibit three main decomposition steps. The first step with no loss in mass
was observed at 53.62°C. The second step appeared at 120°C and related to loss of
J. Therm. Anal. Cal., 76, 2004

348
ABD EL WAHED et al.: Co(II), Ni(II) AND Cu(II) COMPLEXES
two uncoordinated water molecules (obs.=8.695% and calc.=8.168%). The remain-
ing part of ligands in the coordination sphere and chloride ions from the metal com-
plex break in three steps at 262.5, 292.01 and 352°C. The total mass loss is 72.98%
(cal.=74.81%) which was attributed to the loss of two chloride and two adenine mol-
ecules. The final decomposition product was found to be copper oxide. Accordingly,
the stoichiometry of thermal decomposition can be suggested as:
120°C
[Cu(AdH)2Cl2]×2H2O ¾¾® [Cu(AdH)2Cl2]+2H2O
352°C
[Cu(AdH)2Cl2]+0.5O2 ¾¾® CuO+decomposition products
The above findings were supported by the infrared spectra of the final thermal
decomposition products. The spectra show the absence of all bands associated with
the adenine molecule.
Conclusions
The formation of cobalt, nickel and copper adenine complexes is an endothermic pro-
cess, i.e., the complexes are stable at elevated temperature. An electrostatic nature of
the bond formation is observed. The thermal investigation of complexes illustrates
that a complete decomposition occurred at nearly 360°C giving the metal oxide as a
final product.
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