Dissociation kinetics of cobalt and zinc β-octabromo-meso-tetraphenyl- and β-octaethyl-meso-tetraphenylporphyrin complexes

Article abstract of DOI:10.1134/S0023158407020024

Kinetics of solvoprotolytic dissociation of zinc and cobalt porphyrin complexes with a porphyrin macrocycle distorted to different extents (β-octaethyl-meso-tetraphenylporphyrin and β-octabromo-meso- tetraphenylporphyrin) have been studied in binary proton-donating solvents. The main factor decreasing the stability of the porphyrin complexes is the distortion of the planar structure of the macrocycle. The reactivity of the cobalt porphyrins is governed by both structural and electronic effects.

Full text of DOI:10.1134/S0023158407020024

ISSN 0023-1584, Kinetics and Catalysis, 2007, Vol. 48, No. 2, pp. 190–193. © MAIK “Nauka /Interperiodica” (Russia), 2007.
Original Russian Text © S.G. Pukhovskaya, L.Zh. Guseva, A.S. Semeikin, O.A. Golubchikov, 2007, published in Kinetika i Kataliz, 2007, Vol. 48, No. 2, pp. 205–209.
Dissociation Kinetics of Cobalt
and Zinc b-Octabromo-meso-Tetraphenyl-
and b-Octaethyl-meso-Tetraphenylporphyrin Complexes
S. G. Pukhovskayaa, L. Zh. Gusevab, A. S. Semeikina, and O. A. Golubchikova
a Ivanovo State University of Chemical Technology, Ivanovo, Russia
b Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, 153045 Russia
e-mail: sgp@isc-ras.ru
Received July 1, 2005
Abstract—Kinetics of solvoprotolytic dissociation of zinc and cobalt porphyrin complexes with a porphyrin
macrocycle distorted to different extents (β-octaethyl-meso-tetraphenylporphyrin and β-octabromo-meso-tet-
raphenylporphyrin) have been studied in binary proton-donating solvents. The main factor decreasing the sta-
bility of the porphyrin complexes is the distortion of the planar structure of the macrocycle. The reactivity of
the cobalt porphyrins is governed by both structural and electronic effects.
DOI: 10.1134/S0023158407020024
INTRODUCTION
EXPERIMENTAL
The solvents were purified using standard proce-
dures [6–8]. Acetic acid (reagent grade) was dehy-
drated by fractional freezing and was then distilled. Tri-
fluoroacetic acid (pure grade) was mixed with concen-
trated sulfuric acid in a ratio of 10 : 1. Dehydrated
CF3COOH was distilled from the mixture, and the frac-
tion with the boiling point 72–73°C was again distilled.
Benzene (reagent grade) was dehydrated with calcium
chloride and distilled. The residual water content,
determined by Fischer titration, did not exceed 0.03%
for acetic acid and 0.01% for benzene.
The electronic absorption spectra and the solvopro-
tolytic dissociation rates of the metal porphyrins were
measured on a Specord M-400 spectrophotometer in
temperature-controlled ground-glass-stoppered cells at
T = 288–338 K. The temperature changes did not
exceed 0.1 K. The experimental procedure and kinetic
calculations are described elsewhere [9].
The initial concentration of a metal porphyrin in
kinetic experiments was (1–2) × 10–5 mol/l. The con-
centration of trifluoroacetic acid in acetic acid solutions
was varied between 0.02 and 0.2 mol/l for the cobalt
complex of porphyrin I and between 2 and 7 mol/l for
the cobalt complex of porphyrin II. The concentration
of acetic acid in its binary mixtures with benzene was
varied between 10 and 100 vol %.
The porphyrins were synthesized by standard proce-
dures [3, 10]. The zinc and cobalt complexes were syn-
thesized by reacting a porphyrin with excess metal ace-
tate in boiling dimethylformamide. The porphyrins
were purified by chromatography on alumina (activity
grades II and III). The eluent was a chloroform–ben-
zene mixture, whose composition was chosen with con-
Porphine, the progenitor of the porphyrin family,
has the rigid planar structure of a tetrapyrrole macrocy-
cle [1]. However, numerous porphyrins have recently
been synthesized in which the pyrrole macrocycle is
distorted (in particular, by many bulky substituents at
the periphery of the molecule) [2]. The studies of coor-
dination properties of these compounds revealed a
strong correlation between the degree of distortion of
the porphyrin macrocycle and the formation rate and
stability of metal porphyrins in proton-donating media
[3, 4]. It is of interest to study the stability of spatially
distorted porphyrin complexes in proton-donating
media because the cobalt and iron complexes are active
in hydrocarbon oxidation yielding a carboxylic acid as
the major product or as a by-product [5].
In the present work, the dissociation kinetics of
cobalt and zinc complexes with spatially distorted por-
phyrins, namely, β-octaethyl-meso-tetraphenylporphy-
rin (I) and β-octabromo-meso-tetraphenylporphyrin
(II), was studied in binary proton-donating solvents.
R
NH
R
R
NH
R
N
N
R
R
R
R
R = C2H5 (I), Br (II)
190

DISSOCIATION KINETICS OF COBALT
191
–logkapp[s–1]
ln(c0/ci)
1.8
2
1
4
3
2
1
3
2.0
2.2
1.3
1.2
1.1
1.0
0.9
–logCCF COOH[mol/l]
3
Fig. 2. log k versus logc
for the cobalt β-octa-
app
CF3COOH
ethyl-meso-tetraphenylporphyrin complex at 308 K in the
acetic acid–trifluoroacetic acid system. The slope of the
straight line is 1.02 0.05; the correlation coefficient is
0.995.
imental error and show no systematic trend. This is
clear from the plots of ln(c0/ci) versus t (where ci and c0
are the current and initial concentrations of the metal
porphyrin, respectively, and t is time) for the zinc com-
plex of porphyrin II in the C6H6–CH3COOH medium,
which are shown as a typical example in Fig. 1.
0
4
8
12
16
20
t, min
Fig. 1. Time dependences of ln(c /c ) for the dissociation of
0
i
the zinc β-octabromo-meso-tetraphenylporphyrin complex
The plot of ln kapp versus lncCF COOH is presented in
–5
3
(c
= 1.2 × 10 mol/l) at (1) 288, (2) 298, and (3) 308 K
ZnP
Fig. 2. These data suggest that the dissociation of the
cobalt complex of porphyrin I is first-order with respect
to trifluoroacetic acid:
in the acetic acid–benzene binary solvent (1 : 4 vol/vol).
sideration for the individual features of the porphyrins.
The purity of the porphyrins was checked by electronic
absorption spectroscopy against earlier reported data
[3, 10, 11].
–dcZnP/dt = kcZnPcCF COOH
.
3
The dissociation of the cobalt complex of porphy-
rin II in binary mixtures of acetic and trifluoroacetic
acids proceeds at a measurable rate at much higher
CF3COOH concentrations of 2 to 7 mol/l, when the sys-
tem is essentially nonideal. The empirical equation
relating the apparent rate constant to the CF3COOH
concentration has the form
RESULTS AND DISCUSSION
As a rule, the dissociation of porphyrin complexes
in individual proton-donating solvents cannot be stud-
ied by kinetic methods. The use of mixed solvents (an
acetic acid–benzene mixture for the Zn complexes and
an acetic acid–trifluoroacetic mixture for the cobalt
porphyrins) and the variation of their composition
make it possible to establish the acidity necessary for
dissociation to occur at the rate optimum for kinetic
measurements. The porphyrin complexes with d-metals
dissociate in proton-donating media to form a solvated
metal cation (or a solvated salt) and a metal porphyrin
(MP) in free or protonated form [1, 12–14]
logkapp = (8.7 0.8) + (4.9 0.5)logcCF COOH
.
3
Numerous studies of the kinetics of dissociation of
coordination compounds of phthalocyanine and por-
phyrins in proton-donating media [1] demonstrated that
the transition state of the reacting system is reached
upon the dissociation of two M–N bonds of the metal
porphyrin.
Because the acetic acid–benzene system is nonideal
toward the dissociation of the zinc complexes of por-
phyrins I and II at all temperatures examined, the
2+
åê + 4Hs+olv
M
+ ç4ê2+.
solv
The dissociation of the complexes was studied in at dependences of ln kapp on lncAcOH are nonlinear. Using
least 1000-fold excess of the acid over the metal por- the activity coefficients of acetic acid in its binary mix-
phyrin. This made it possible to determine the partial tures with benzene, which were calculated from data
order of the dissociation reaction with respect to MP, available from the literature [15], we constructed
which was equal to unity in all cases. The first-order lnkapp–lnaAcOH plots. Such a plot for the reaction of the
apparent rate constants are invariable within the exper- zinc complex of porphyrin II at 298 K is shown in
KINETICS AND CATALYSIS Vol. 48 No. 2 2007

192
PUKHOVSKAYA et al.
lnkapp[s–1]
In the acetic acid–benzene composition range exam-
ined, the zinc tetraphenylporphyrin complex without
β-substituents dissociates very slowly. The extrapola-
tion of the data plotted in Fig. 3 to 100% acetic acid
leads to kapp = 9 × 10–3 s–1 for the dissociation of the zinc
complex of porphyrin II. This value is 50 times larger
than that of the same constant for the zinc tetraphe-
nylporphyrin complex [1]. Thus, the introduction of
either eight electron-donating ethyl groups or eight
electron-withdrawing bromine atoms into the β-posi-
tions of tetraphenylporphyrin destabilizes the zinc
complexes of I and II. Therefore, the effect of these
substituents is structural rather than electronic. It
should be kept in mind that, in the series of undistorted
Zn porphyrins, electron-withdrawing substituents sta-
bilize the complex and electron-donating substituents
destabilize the complex [1].
–5.2
–5.6
–6.0
–6.4
0.8
0.6
0.4
0.2
–lnaAcOH
Fig. 3. ln k
versus lna
at 298 K for the zinc β-
ÄÒéç
app
Tetraphenylporphyrin and its complexes with cat-
ions matching the size of the coordination cavity N4
have a nearly planar tetrapyrrole macrocycle [16, 17].
Porphyrin I and its complexes have a saddlelike confor-
mation in which one pair of opposite pyrrole rings is
below the mean plane of the macrocycle and the other
pair is above this plane [17, 18]. The deviation of the
β-carbon atoms from the mean plane of 24 atoms of the
macrocycle is 110–130 nm. The calculations of the
geometric parameters of porphyrins I and II performed
using molecular mechanics methods (force field MM+)
[4, 19, 20] also showed that these porphyrins have a
saddlelike distorted macrocycle and the macrocycle of
porphyrin II is more distorted due to the large covalent
radius of the bromine atom. Therefore, it is not surpris-
ing that the Zn complexes of porphyrin II, which con-
tains electron-withdrawing substituents (Br atoms),
dissociate more rapidly than the complexes of porphy-
rin I, which contains eight electron-donating ethyl
groups (Table 1).
octabromo-meso-tetraphenylporphyrin complex in the ben-
zene–acetic acid system.
Fig. 3. The slope of the straight line is unity at acetic
acid concentrations of 0.1 to 0.7. The break in the plot
is likely due to a change in the solvating power of the
medium, since acetic acid in this system is not only a
proton-donating species but also a solvating agent.
Thus, the dissociation of the zinc complexes of porphy-
rins I and II is first-order with respect to the acetic acid
activity. Therefore, the labile zinc complexes of por-
phyrins I and II dissociate under the action of
ëç3ëééç molecules and the transition state of this
reaction is reached already upon the dissociation of one
M–N bond of the metal porphyrin.
The dissociation of this bond results in substantial
spatial strains in the coordination center MN4H+ caused
by steric and electrostatic effects. Evidently, the mono-
The substitution of Br atoms for the eight ethyl
groups in the complexes of the d6 cation Co2+ (passing
from I to II), as distinct from the complexes of the d10
cation Zn2+, results in a sharp decrease in the solvopro-
tolytic dissociation rate: the rate constant decreases by
a factor of several thousands and the activation energy
increases by 66 kJ/mol (Table 2). Evidently, in this
case, reactivity is governed by both structural and elec-
tronic effects and the conjugation effect dominates over
the I effect of the bromine atoms.
protonated complex MHPs+olv has a distorted macrocy-
clic structure. This facilitates the dissociation of a sec-
ond M–N bond even if the metal porphyrin is initially
planar. In the case of low-stability porphyrin com-
plexes, which are spatially distorted even in the initial
state, the distortion of the MHPs+olv fragment increases
to such an extent that the other M–N bonds dissociate,
obviously beyond the peak of the activation barrier.
Table 1. Kinetic parameters of dissociation of the zinc(II) complexes of porphyrins I and II in the benzene–acetic acid binary
solvent
288
app
298
app
308
app
× 103, s–1
k
× 103, s–1
k
× 103, s–1
Porphyrin
aAcOH
E, kJ/mol
k
I*
II**
0.855
0.855
1.59 0.04
1.88 0.03
2.31 0.04
3.30 0.10
3.24 0.07
5.64 0.12
26.1 0.2
40.4 0.3
–5
*c = 1 × 10 mol/l; the measurements were carried out at λ = 570 and 692 nm.
II
I
–5
**c = 1 × 10 mol/l; the measurements were carried out at λ = 613 and 755 nm.
KINETICS AND CATALYSIS Vol. 48 No. 2 2007

DISSOCIATION KINETICS OF COBALT
193
Table 2. Kinetic parameters of dissociation of the cobalt(II) complexes of porphyrins I and II in acetic acid with trifluoro-
acetic acid additives at 298 K
kapp × 104, s–1
E, kJ/mol
cCF COOH , mol/l
Porphyrin
λa, nm
3
I*
II**
0.069
5.2
550
65.8 0.1
15.7 0.7
81.8 0.5
446, 487
0.59 0.04
*c = 1.1 × 10–5 mol/l.
I
**c = 1 × 10–5 mol/l.
II
7. Gyenes, I., Titration in Non-Aqueous Media, Budapest:
Bromine atoms, which exert a +C effect, increase
the electron density on the dz2 orbital of the central
Akadémiai Kiádó, 1967.
8. Weisberger, A., Proskauer, E., Riddick, J., and Toops, E.,
Organic Solvents: Physical Properties and Methods of
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9. Kuvshinova, E.M., Pukhovskaya, S.G., Golubchikov, O.A.,
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10. Shatunov, P.A., Cand. Sci. (Chem.) Dissertation,
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cobalt cation, enhancing the reverse, π-dative Co–N
interaction, resulting in a decrease in the effective neg-
ative charge on the nitrogen atoms of the ligands. This
charge is attacked by a solvated proton, which causes a
sharp decrease in the solvoprotolytic dissociation rate
and an increase in the activation energy.
It should be mentioned in conclusion that, from the
standpoint of stability in proton-donating media, the
cobalt complexes of spatially distorted porphyrins can
be used as catalysts of hydrocarbon oxidation.
12. Berezin, B.D. and Enikolopyan, N.S., Metalloporphy-
riny (Metalloporphyrins), Moscow: Nauka, 1988.
ACKNOWLEDGMENTS
13. Lomova, T.N., Shormanova, L.P., and Klyueva, M.E., in
Uspekhi khimii porfirinov (Advances in Porphyrin
Chemistry), Golubchikov, O.A., Ed., St. Petersburg:
Nauchno-Issled. Inst. Khim., S.-Peterburg Gos. Univ.,
1997, vol. 1.
This work was supported by the Russian Foundation
for Basic Research, project no. 03-03-96458-
r2003tschr-a.
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