Specific features of the formation of zinc and cadmium complexes with tetraphenylporphine in amphiprotic media

Article abstract of DOI:10.1134/S0036023609030140

The kinetics of the complex formation reactions of tetraphenylporphine with zinc and cadmium acetates in mixed solvents containing alcohols and inert diluents was studied by electronic absorption spectroscopy. The dependences of the rate constant on the b

Full text of DOI:10.1134/S0036023609030140

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2009, Vol. 54, No. 3, pp. 413–416. © Pleiades Publishing, Inc., 2009.
Original Russian Text © V.D. Kononov, I.P. Trifonova, V.V. Aleksandriiskii, V.A. Burmistrov, O.I. Koifman, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol.
5
4, No. 3, pp. 460–463.
COORDINATION
COMPOUNDS
Specific Features of the Formation of Zinc and Cadmium
Complexes with Tetraphenylporphine in Amphiprotic Media
V. D. Kononov, I. P. Trifonova, V. V. Aleksandriiskii, V. A. Burmistrov, and O. I. Koifman
Ivanovo State University of Chemistry and Technology, pr. Engel’sa 7, Ivanovo, 153000 Russia
Received March 11, 2008
Abstract—The kinetics of the complex formation reactions of tetraphenylporphine with zinc and cadmium
acetates in mixed solvents containing alcohols and inert diluents was studied by electronic absorption spectros-
copy. The dependences of the rate constant on the binary solvent composition have an extreme character, which
is universal in the temperature range 298–313 K for different salts, alcohols, and weakly solvating diluents. The
electric conductivity of zinc acetate solutions was measured in a wide range of compositions of alcohol–chlo-
roform binary solvents. The electric conductivity increased with an increase in the content of alcohols.
DOI: 10.1134/S0036023609030140
Alcohols as polar solvents with amphiprotic (donor the process under consideration. The assumption that
and acceptor) properties are a suitable low-molecular- changes in the solvation shells of the salt and porphyrin
weight model of the inner region of biomolecules, such caused by varying the composition of the solvent have
as hemoglobin, nucleic acids, and enzymes [1]. This considerable effect on the reaction kinetics was not
fact is responsible for the importance of studying the confirmed by experimental methods.
reactivity of macrocycles both in neat alcohol and its
various mixtures with inert diluents, which makes it
possible to control the solvation power of a medium.
EXPERIMENTAL
Tetraphenylporphine H TPP, synthesized as
2
Previously, the kinetics of complex formation reac-
described in [5], was additionally purified on Al O of
2
3
tions of etioporphyrin [2–4] and tetraphenylporphine
activity IV. Cadmium and zinc acetates (Cd(AcO) and
2
(
H TPP) [7] with metal salts was studied in different
2
Zn(AcO) ) were preliminarily calcined. To study the
2
solvents: alcohols with different structures, acetic acid,
benzene, and ethanol and their mixtures with carbon
tetrachloride. Of special interest are studies dealing
with the effect of the binary solvent on the kinetics of
formation of metalloporphyrins where it was attempted
to determine the contribution of solvation effects to the
complexation reaction, aliphatic alcohols (methanol,
ethanol, propanol) and weakly solvating solvents (dilu-
ents) (chloroform and dichloroethane) were used. The
solvents were purified by common procedures [8].
Electronic absorption spectra were recorded on a
kinetic parameters of the reaction. For the binary sol- PerkinElmer Lambda 20 UV/VIS scanning spectropho-
vent ethanol–CCl , a smooth decrease in the reaction tometer with a wavelength setting accuracy of 0.1 nm.
4
The wavelength setting reproducibility was ±0.05 nm.
The photometric accuracy was ±0.003. For taking mea-
surements at different temperatures, a Peltier tempera-
ture control accessory was used, which allows one to
record spectra in the temperature range 15–45°ë with
an accuracy of ±0.15°ë. All measurements were carried
out in standard quartz cells with an optical pathlength
of 1 cm.
Conductometric measurements were carried out on
an INSTEK LCR-817 instrument in a thermostated cell
with platinum electrodes at 298 ± 0.02 K. The error of
determination of electric conductivity was no more
than 3%.
rate constant with an increase in the concentration of
the nonpolar component (the maximal CCl content
4
was 90 vol %) was observed [2, 3]. This is accompanied
by an increase in the reaction activation energy and
entropy in the range of CCl concentrations of 40–
4
9
0 vol %. This fact was explained from the standpoint
of resolvation or a change in the composition of the sol-
vation shells of both the metal salt and porphyrin. In
addition, experimental data were obtained on the kinet-
ics of formation of cadmium etioporphyrin in the etha-
nol–benzene binary solvent (the maximal benzene con-
tent was 0.6 mol fractions). A smooth decrease in the
reaction rate constant was observed as the fraction of
the nonpolar component of the solvent mixture
increased. Unfortunately, in these works, no experi-
mental data were reported for the range of high contents
of the nonpolar component of the binary solvent, which
RESULTS AND DISCUSSION
The complex formation of H TPP with zinc and cad-
2
prevents us from assessing the general kinetic pattern of mium acetates was studied in mixed solvents contain-
4
13

4
14
KONONOV et al.
kapp, s–1
acetate concentrations met the pseudo-first-order con-
(a)
ditions. The plots of the rate constant of the complex
formation reaction versus the binary solvent composi-
tion have an extreme character, which is universal since
it is observed in the temperature range 298–313 K for
different salts (Zn, Cd), alcohols (methanol, ethanol, n-
propanol), and low-polarity diluents (chloroform,
dichloroethane) (Fig. 1).
0
0
0
0
0
0
0
0
0
0
.0019
.0017
.0015
.0013
.0011
.0009
.0007
.0005
.0003
0
0
0
0
0
.0006
.0005
.0004
.0003
.0002
4
0.0001
It is worth noting that, despite the universal charac-
ter of the kinetic parameters of the porphyrin complex
formation reaction, we failed to find literature data
obtained for the entire range of compositions of alco-
hol-containing binary solvents. Therefore, the available
literature data on the kinetics of the formation of metal-
loporphyrins in mixed solvents [2–4, 7, 8] prevent us
from formulating the reasons for the extreme character
of the curves.
0
2
4 6 8 10 12 14
calcohol, mol/L
2
1
The activation parameters of the complex formation
reaction were obtained from the plots of the logarithm
of the rate constant for the reaction of coordination of
3
.0001
0
ç íêê to zinc and cadmium acetates in binary solvents
2
.5
2.5
4.5
6.5
8.5
10.5 12.5
ethanol–chloroform of various compositions (Fig. 2).
The data in the table show that, at low alcohol contents
in the mixture, the parameters for the zinc and cadmium
salts are the same. With an increase in the alcohol con-
centration, the kinetic parameters become different,
which is accompanied by the compensation effect of
the enthalpy and entropy contribution to the rate con-
stant of the process. However, it is evident that the
acceleration of the complex formation reaction at a
high alcohol concentration is mainly caused by a
decrease in the activation enthalpy.
(
b)
0
0
0
0
0
0
0
0
0
.00083
.00073
.00063
.00053
.00043
.00033
.00023
.00013
.00003
1
Taking into account the composition of the partici-
pants of the reaction, we may assume that the reasons
for the extreme character of the curves (Fig. 1) can be
very different and determined by the specific features of
the solvation of the porphyrin, cadmium and zinc salts,
and transition state; salt dissociation in mixed solvents;
and the associative state of the solvents. It is evident
that the possible salt dissociation, which radically
changes the nature of the complex-forming compound,
from a neutral molecule to a ionic one, is expected to
have the strongest disturbing effect on the reaction sys-
tem [8].
3
2
0
.3
2.3
4.3
6.3
8.3
10.3 12.3
calcohol, mol/L
Taking into account the high sensitivity of the elec-
tric conductivity of solutions to the appearance of
charged particles, we measured the electric conductiv-
ity of zinc acetate solutions in a wide range of compo-
sitions. The salt concentration corresponded to its con-
tent in the kinetic experiment. The data in Fig. 3 show
that the conductivity of the solution monotonically
Fig. 1. Apparent rate constant of the complex formation
reaction between (a) Zn(OAc) , (b) Cd(OAc) , and (c)
2
2
H TPP vs. the alcohol content in the binary solvent. (1)
2
methanol–chloroform, (2) ethanol–chloroform, (3) pro-
panol–chloroform, and (4) methanol–dichloroethane.
ing alcohols and inert diluents (chloroform and dichlo- increases with an increase in the alcohol content and in
roethane) in the entire concentration range.
H TPP + Zn(AcO) ZnTPP + 2AcOH
The apparent rate constants kapp for the pseudo-first-
order reaction were calculated for the initial ç TPP
its dielectric constant (methanol > ethanol > n-pro-
panol). At the same time, the monotonic increase in the
conductivity allows us to assume that salt dissociation
can have an effect on the acceleration of complex for-
mation at high alcohol concentrations; however, this in
no way explains the existence of the descending branch
2
2
2
–5
2+
2+
concentration 5 × 10 mol/L, and the Zn and Cd
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 3 2009

SPECIFIC FEATURES OF THE FORMATION OF ZINC
415
(
a)
0.0033
(b)
0.0032
1
/í, K–1
0.0034
1/í, K–1
0.0034
0
.0031
0.0032
0.0031
–6.5
0.0033
–
–
–
–
–
–
5.8
6.3
6.8
7.3
7.8
8.3
–
–
–
–
–
–
7.0
7.5
8.0
8.5
9.0
9.5
1
1
2
2
–
10.0
–10.5
(
c)
/í, K–1
1
0
.0031
0.0032
0.0033
0.0034
–
–
–
–
–
–
–
–
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
2
1
Fig. 2. Logarithm of the rate constant vs. inverse temperature for the reaction of metallization of ç íêê with (1) Zn(éAc) and (2)
2
2
Cd(éAc) in the binary solvent ethanol–chloroform. The ethanol content: (a) 0.94, (b) 5.20, and (c) 12.0 mol/L. The concentration
2
–4
–4
of Zn(éAc) 1.4 × 10 , Cd(éAc) – 9.9 × 10 mol/L, respectively.
2
2
Activation parameters of the metallization reaction of H TPP in ethanol at 25°C
2
Alcohol concen- Reaction rate con- Activation ener-
∆S,
J/(mol K)
∆G,
kJ/mol
Salt
–1 –1
∆H, kJ/mol
tration, mol/L
stant, L mol
s
gy E , kJ/mol
a
Zn(OAc)2
0.94
0.35
0.13
0.14
0.34
0.04
0.46
57.7
58.7
46.0
56.5
49.3
33.7
55.2
56.2
43.5
54.0
49.3
31.2
–68.23
–73.45
–115.3
–72.57
–113.95
–146.49
75.6
78.1
77.9
75.7
80.9
74.9
5
.2
1
1
2.0
Cd(OAc)2
0.94
5
.2
2.0
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 3 2009

4
16
KONONOV et al.
1
0
8
6
4
2
1
2
3
0
2
4
6
8
10
12
calcohol, mol/L
–3
Fig. 3. Electric conductivity of Zn(OAc) ( cZn(OAc) = 1.548 × 10 mol/L) vs. the alcohol concentration in the binary solvent (1)
2
2
methanol–chloroform, (2) ethanol–chloroform, and (3) n-propanol–chloroform.
of the curves (Fig. 1) and, hence, their extreme
character.
2. B. D. Berezin, O. A. Golubchikov, and O. I. Koifman,
Zh. Fiz. Khim. 48 (12), 2913 (1974).
3
. B. D. Berezin, O. A. Golubchikov, and O. I. Koifman,
Thus, the dependence kapp = f(calcohol) is presumably
related to the specific features of either the solvation
states of porphyrin and Zn and Cd acetates or the tran-
sition state of the complexation reaction.
Izv. Vyssh. Uchebr. Zaved., Khim. Khim. Tekhnol. 17
(8), 1265 (1974).
4
5
. B. D. Berezin, O. A. Golubchikov, and O. I. Koifman,
Zh. Fiz. Khim. 47 (11), 2817 (1973).
. Porphyrins: Structure, Properties, Synthesis, Ed. by
K. A. Askarov, B. D. Berezin, R. P. Evstigneeva, et al.
ACKNOWLEDGMENTS
(Nauka, Moscow, 1985) [in Russian].
This work was supported by the Russian Foundation
for Basic Research, project no. 05-03-32738a.
6
7
. W. L. F. Armarego and C. L. L. Chai, Purification of
Laboratory Chemicals, 5th ed. (Butterworth Heine-
mann, Elsevier, London, 2003).
. B. D. Berezin, O. A. Golubchikov, L. P. Shormanova,
and E. M. Kuvshinova, Izv. Vyssh. Uchebn. Zaved.,
Khim. Khim. Tekhnol. 21 (2), 208 (1978).
REFERENCES
1
. A. Lehninger, Principles of Biochemistry (Worth, New
York, 1982; Mir, Moscow, 1985).
8. B. D. Berezin, Teor. Eksp. Khim. 9 (4), 500 (1973).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 3 2009